Liquid crystal lasers: the last decade and the future

2.1 Lasers – principles of operation

The LASER acronym can be expanded as light amplification by stimulated emission of radiation. Therefore, all lasers will utilize the STE phenomenon to obtain light amplification (see paragraph (a) in Section 2.1). Here, it should be noted that the acronym, as mentioned above, is specifically useful for a class of devices representing the self-excited amplifiers. Frequently, lasers are also called light generators. This is because they generate, filtrate, and amplify the optical signal. Thus, they require a feedback mechanism to operate. Optical amplifiers are another class of devices crucial for STE. Those types of devices use an optical medium sustained in the population inversion state to provide light amplification. However, they do not utilize the feedback mechanism, and thus, they are not considered lasers. Erbium amplifiers are devices that use STE for light amplification but not generation. They are frequently used in telecommunication to restore the optical signal after losses experienced during the propagation at long distances in optical fibers [28]. The difference between lasers and optical amplifiers is similar to the difference between OPO and OPA; however, in this case, STE plays the leading role in gain delivery. The first laser – the ruby laser, was constructed in 1960 by Theodore Maiman [22]. Since then, many types and classes of lasers have been built, including gas, dye, and semiconductor lasers. The typical laser consists of three main parts: the gain medium, the pump, and the optical cavity:

4.1 Cholesteric

The main features that make cholesteric liquid crystal lasers (ChLCL) very attractive for photonic applications are the ease of fabrication and tuning the emission wavelength by using different angle pitch gradients, by changing the composition [43], [44], via external stimuli such as electric field [45], temperature [46], or mechanical strain [47].

In 2010, Uchimura et al. [48] reported how the proper design of laser dyes is important to realize low-threshold dye-doped ChLCL to achieve continuous-wave (cw) operation, the ultimate goal of LC lasers. The lasing spectra of pyrene-derivative-doped ChLCL were observed at the high energy edge of the photonic band-gap, which is an unusual phenomenon and is attributed to the orientation of the transition dipole moment of the dye molecules. These results should serve as guidelines for designing new dyes to lower the threshold of LC lasers. Aggregation-induced-emission (AIE) dyes have also been used as a gain material. Wang et al. [49] have shown that such AIE dye-doped ChLC is capable of lasing action with huge Stokes shift (250 nm) at a moderate threshold (600 μJ/mm²), which could be further reduced with optimization in AIE dye concentration, sample thickness, input optics, and selection of other AIE materials. Furthermore, the sample is highly photostable without noticeable degradation of performance. To conclude, AIE dyes are promising to gain media for a variety of laser hosts.

The novel BODIPY based red emitter (4,4-difluoro-2,6-di(4-hexylthiopen-2-yl)-1,3,5,7,8-pentamethyl-4-bora-3a,4a-diaza-sindacene) was also employed as a ChLC laser dye [50]. A new construction method in which a flat capillary filled with an optically isotropic dye solution is sandwiched between two dye-free planar ChLC cells was described. Such construction provides optically pumped lasing at the same wavelength as in the classical scheme of a dye-doped planar ChLC cell. Facile assembly/disassembly of the capillary laser can be useful for optimizing laser characteristics using small amounts of dye and ChLC, which can be used as a simple method to explore new active laser media and optimize the concentration dependencies of the lasing parameters.

In 2014, authors first described an optically stable and tunable laser using a quantum-dot (QD) embedded cholesteric liquid crystal (QD-ChLC) microresonator with an added chiral-azobenzene moiety [51], which possesses optical stability and tunability. The tunable spectral ranges in the PBG and lasing wavelength of the QD-ChLC composite cell are over 60 and 40 nm, respectively. Because of the high stability and optical and electrical dual-excitability of the QDs, the QD-ChLC has a high potential to develop multiway drivable coherent QD light sources or lasers with highly flexible tunability, high stability, and reliability.

An azo-chiral dopant was also used by Lin et al. [52]. This study first demonstrates a spherical microlaser with optical tunability and switchability based on a dye-doped cholesteric liquid crystal (DDChLC) microdroplet with an azo-dye. The spherical microlaser’s lasing wavelength can be tuned from 563 nm up to 586 nm by increasing the irradiation time of a weak UV beam. This tunability is attributed to the slow trans-cis isomerization of the azo-chiral dopant induced by weak UV irradiation. The microlaser can be switched on by the blue beam irradiation-induced cis-trans back-isomerization of the azo-chiral dopant following the intense UV irradiation. The three-dimensional DDChLC spherical microlaser is a highly promising controllable 3D microlight source or microlaser for applications of 3D all-optical integrated photonics, laser displays, biomedical imaging, and therapy, and as a 3D UV microdosagemeter or microsensor.

In 2010, Humar and Musevic [53] published pioneering work about 3D lasing from dye-doped cholesteric microdroplets with a Bragg-onion configuration of the refractive index. The lasing wavelength was determined solely by the cholesteric pitch. The laser light was emitted from the center of the ChLC microdroplet in all directions. Thus, the laser is acting as a coherent, point-like, and omnidirectional light source, where the emission wavelength is tunable by changing the temperature. The tuning range is several tens of nanometers. Since then, many other interesting papers have been published about the 3D ChLC omnidirectional photonic structures and their applications in lasing, including core–shell structures [54], [55], [56], [57], [58], [59], [60].

A wide-range electrically tunable laser based on dye-doped polymer-stabilized cholesteric LCs (PSChLCs) with negative dielectric anisotropy was described by Lu et al. [60]. The laser’s wavelength could be tuned reversibly in a wide spectral range, from 558 to 673 nm, by controlling the pitch gradient through DC electric fields (Figure 5).

Figure 5:

The electric-field-dependent broadening of the selective reflection band and the electrical tunability of the laser’s emission wavelength.

(a) The microphotographs observed under a polarized optical microscope and photographs of the dye-doped PSChLC under various voltages, (b) the transmission and corresponding laser emission spectra of the dye-doped PSChLC under various voltages with excited pump energy of ∼1.4 μJ, and (c) the schematic illustration of the pitch distribution under different electric fields [60].

The electrically controlled PSChLC laser’s principal advantage is that the electric field is applied parallel to the helical axis and changes the pitch gradient instead of rotating this helix axis, thus preserving the heliconical structure during lasing [45]. The PSChLC laser, with advantages including a wide tunable range, self-restoration, and rapid response and stability, coupled with its miniature size and narrow linewidths, has diverse applications in intelligent optoelectronic devices.

In 2017, Lin et al. [61] reported that PBG reflectivity could be tuned over nearly the full visible region based on a gradient-pitched enantiomorphic ChLC polymer template refilled with a nematic LC (NLC) or dye-doped nematic LC (DDNLC) (Figure 6). The sample is formed by suitably merging two oppositely-handed gradient-pitched templates and then refilling the DDNLC with a fluorescence distribution covering almost the entire visible region. The experimental results show that the high reflective PBG tuning range is spanning from 450 to 750 nm. The lasing output with simultaneous right- and left-circular polarizations can be tuned from the blue to the red region (483.2–633.7 nm).

Figure 6:

(a) Lasing emission spectra of the DDNLC-refilled gradient-pitched enantiomorphic template sample at E = 4.56 mJ per pulse, measured at positions from x = 4 mm to x = 11 mm, at T = 48 °C.

The inset in (a) shows the reflection bands with the associated normalized lasing lines measured at five selected positions in the DDNLC-refilled gradient-pitched enantiomorphic template sample. The grey and blue dotted curves in (a) are the measured fluorescence emission and absorption spectra of the DDNLC, respectively, in the isotropic state for reference. (b) Corresponding reflection spectra of the sample measured at positions from x = 4 mm to x = 11 mm at 48 °C [61].

The same research group has shown that the PBG can be spatially tuned over the entire visible region, and the lasing is thermally convertible between single-mode and multimode nature [62].

Finally, lasing from cholesteric liquid crystal elastomer (CLCE) films, both in glassy and rubbery states, with a dramatic reversible blue-shift of the selective reflection band from near IR to near UV ranges of the spectrum, was achieved by a novel mechanical compression of the CLCE film using uniaxial strain along the helical axis. Using an accurate measurement scheme based on volume conservation of the CLCE film, it was shown, for the first time, that both the helical pitch and the wavelength of laser emission from the CLCE film in the rubbery state are linearly dependent on the mechanical strain applied to the film [47].

4.2 Blue phase

The liquid-crystalline BPs appearing between the Ch and isotropic (Iso) phases have attracted particular attention owing to their unusual self-assembled cubic structures and unique optical characteristics. These structures comprise double-twist cylinders (DTCs) and vary depending on how the DTCs are assembled in three-dimensional space. These phases are known as cubic BP (BPI, BPII) and amorphous BP (BPIII), numbered in order of increasing temperature (Chapter 3). The cubic BP has a three-dimensional cubic symmetry with lattice parameters ranging from 200 to 300 nm, while the amorphous BP has an isotropic symmetry [63], [64]. BPs are optically active and isotropic, exhibiting a Bragg reflection due to their periodic structures. These characteristics, combined with the capability to shift the PBG in response to external perturbations, make BPs fascinating materials for applications such as tunable lasers, where laser emission can be realized in three orthogonal directions and can be obtained simultaneously using the three-dimensional PBG.

A spatially tunable PBG of wide spectral range and lasing emission based on BP and dye-doped blue phases (DDBP) wedge cells, respectively, was demonstrated for the first time in 2014 [65]. The wedge cell’s gradient thickness provides different boundary forces to the LC molecules, hence achieving the continuously spatial shifting reflections of BP in the wedge cell. The continuously shifting reflection was more than 130 nm, nearly covering the range of blue, green, and red regions. Based on the tunable PBG of the wedge cell, a spatially tunable laser based on a DDBP wedge cell was also demonstrated, where the lasing wavelength is continuously tunable within a spectral range of about 68 nm (Figure 7). Both the tuning ranges of PBG and lasing emission are broader than those of CLC and DDCLC wedge cells.

Figure 7:

(a) Measured reflection spectra and corresponding R-POM BP images for the DDBP wedge cell at y = 2.5–28.5 mm (t = 20–238 μm) at 54.5 °C. (b) Measured lasing spectra (solid curves with sharp peaks) and corresponding reflection spectra (dotted curves, same as those shown in (a)) for the DDBP wedge cell measured at y = 2.5–28.5 mm (t = 20–238 μm) at 54.5 °C. The black solid curve is the fluorescence emission spectrum of the laser dye [65].

Numerous efforts have been put to develop new BP materials that exist over an inherently wide range of temperatures [64], [66]. In 2015, highly tunable 3D liquid photonic crystals were demonstrated using low-dc-field-driven polymer-stabilized blue phase LCs [46]. The photonic band-gap’s central wavelength can be reversibly shifted to more than 200 nm away from the original position (Figure 8). Besides, by controlling the polymerization-induced morphology variations, the band-gap can also be expanded from a bandwidth of around 30 nm to at least 310 nm. Finally, in such a system, a “white” blue phase was observed for the first time, where shifting and expansion of PBG can be independently manipulated in any crystal axis without affecting the lattice spacings in the other dimensions (Figure 8).

Figure 8:

Red-shift of the photonic band-gap.

(a) normalized reflection spectra of a 12-μm thick PSBP film in various DC fields (0–2.67 V/μm) and an AC field (5.83 V/μm). (b) Time-resolved microscopic images (top, scale bars: 100 μm) and Kössel diagrams (bottom) captured under a DC field of 1.33 V/μm. (c) Schematics of the underlying mechanism for the DC-induced band-gap shifting in PSBP. The spectra were normalized to their peak intensities [46].

Highly appealing work was published by Wang et al., where they report on an electrically tuning of the PBG that exhibits the dependence on the polarity of bias voltage in the polymer-stabilized blue phase I system through fabricating polymer network, with nonhomogeneous density distribution from top to bottom across the cell gap. A tunable PBG [67] across the entire visible spectrum was achieved with a hypsochromic shift under negative bias and a bathochromic shift under positive bias.

Another example of a polymer-stabilized blue phase (PSBP) photonic band-gap (PBG) device with a wide-band spatial tunability in the nearly entire visible region within a wide blue phase (BP) temperature range, including room temperature, was published by Lin et al. [68]. Such a device was fabricated based on the reverse diffusion of two injected BP-monomer mixtures with low and high chiral concentrations and, afterward, through UV-curing. This gradient-pitched PSBP can show a rainbow-like reflection appearance in which the peak wavelength of the PBG can be spatially tuned from the blue to the red regions (165 nm) at room temperature.

Yan et al. have shown that freestanding, large-domain blue phase (BP) films based on self-assembly technology can be obtained [69]. This fabrication method enables the formation of BP domains with large-scale lateral dimensions of 230 nm, which exhibit sharp photonic band-gaps with high reflectivity. Additionally, they have shown that dual-wavelength lasing can be achieved by doping the large-domain BP films with fluorescent dyes, which was first observed in BP materials.

In 2018, Petriashvili et al. [70] demonstrated the laser emissions from BPII and BPI microspheres and the temperature-controlled laser tuning from the BPI microspheres. They have prepared BPs microspheres formed and emulsified in a Polyvinyl alcohol/water (PVA/water) environment. Due to the temperature-dependent selective reflection of BPI, the temperature-controlled lasing tuning of about 55 nm was obtained.

Finally, in 2018, Gandi et al. presented the first photonic device based on amorphous BPIII by demonstrating that a three-dimensional BPIII polymer scaffold or template, when infiltrated with liquid crystal and laser dye, forms a system where random lasing action is generated due to multiple scattering events occurring in the nanoporous and disordered polymer replica of BPIII [71]. This study represents a facile approach for developing photonic devices, favorably exploiting unique polymer network morphologies for laser emission. Information about blue phase liquid crystalline lasers is gathered and presented in Table 1.

4.3 DFB

The distributed feedback (DFB) based lasers were constructed for the first time 10 years after the ruby laser development. What is typical for the DFB lasers, and what distinguishes them from the other type of lasers, is a specific resonator shape. Instead of the classical system with two external mirrors, the feedback is provided by a periodic structure with modulation of refractive index and/or gain coefficient. In the last decade, tunable DFB lasers have attracted significant attention because of their great promise in applying modern technologies such as tunable optical sources, biosensors, and optical communication. According to the Bragg condition, the emission wavelength can be tuned by adjusting the periodicity or effective refractive index of the grating. Several DFB lasers that are tunable by changing the grating period have been demonstrated using methods of electrically or mechanically stretching the grating periodicity [74], [75], two-beam holography [76], and in a spatial gradient of film thickness or grating period [77]. Because of the high efficiency of the refractive index modulation of LCs, incorporating LC material into the DFB resonant cavity is one of the most promising ways to effectively tune the emission wavelength of the DFB laser [78].

As an efficient method to realize tunable lasers, holographic polymer dispersed liquid crystal (HPDLC) configurations have been developed. The HPDLC grating is fabricated by a mixture of LC and monomers exposed to two or multiple interfering laser beams. The monomers undergo a fast free-radical photopolymerization in the bright regions of the interference pattern; counter-diffusions of the LC and monomers are initialized due to the resulting concentration gradient and alternating layers of the polymer in the bright fringes, and the LCs are formed in the dark fringes [79]. The formed periodic structure can be regarded as a one-dimensional photonic crystal that an optical resonator for emitted lasers. In 2014, Diao et al. proposed an anisotropic waveguide theory for electrically tunable distributed feedback laser from dye-doped holographic polymer dispersed liquid crystal grating, where the period grating structure, optical anisotropy of the liquid crystal, and practical light propagation path in the HPDLC have been considered [80].

The same research group has demonstrated the electrical control of the DFB organic semiconductor laser based on an HPDLC [81] and giving a full optimization and characterization of the lasing performance of MEH-PPV activated HPDLC lasers [82].

As DFB resonator nanoimprinted ITO transparent electrode was also used, where together with LCs, successful tuning of lasing wavelength from an organic dye-doped LC laser by applying external voltage was observed (Figure 9) [83].

Figure 9:

(a) Schematic illustration, (b) transmission and emission spectra at different pumping energies, (c) threshold behavior of lasing emission intensity, (d) polarized lasing emission spectra at the pumping energy of 11.6 μJ/pulse, and (e) photograph of far-field emission pattern of the surface-emitting DFB laser with the nanoimprinted ITO electrode and dye-doped LCs [83].

Since the reorientation of LCs by applying a voltage across the DFB resonator brought changes of the effective refractive indices of the guided mode, lasing emission from the LC laser with the nanoimprinted ITO electrode was tuned by 6 nm at a low voltage of 8.0 V. From these results; it is shown that the nanoimprinted ITO film simultaneously acts as a DFB resonator, a transparent electrode, and an alignment layer for LCs. The same research group described surface-emitting DFB lasers with the 2D nanostructured ITO films and dye-doped LCs. From the optical measurements for the LC lasers, it was proven that the energy transfer process between two organic dyes and highly ordered dye molecules in the LC host, together with strong optical feedback in 2D DFB resonators, reduces the lasing threshold. Additionally, it was shown that wide spectral tuning of laser emissions of ∼26 nm in a single LC laser with a wedge-shaped ITO resonator could be obtained [84]. Such a tunable lasing device with a functional electrode gives rise to various opportunities and advances in tunable laser technology.

Nanoimprint UV lithography was also used to manufacture temperature sensors based on thermally tunable dye-doped LC (DDLC)/polymer-grating DFB laser based on an LC-covered polymer grating with nano grooves [85]. The surface-emitting DFB laser’s lasing wavelength can be tuned from 625.1 to 606.35 nm by varying the temperature from 10 to 50 °C. This thermal tunability is attributable to the temperature sensitivity of the effective refractive index of the LCs in the laser. Several other studies on lasing for dye-doped 2D HPDLC quasicrystals have been reported. Li et al. reported multimode lasing from the microcavity of an octagonal quasicrystal [86]. Luo et al. have investigated the lasing from Penrose quasicrystal with excellent linear polarization and low threshold [87], as well as the temperature effects [88]. They have also shown the studies on lasing from organic nanostructure quasicrystals formed by seven- or nine-beam interference using low contrast material. The wavelengths of the lasing peak were determined by both the local structure of the quasicrystal that the pumping light experienced and the photoluminescence of laser dye-doped [89].

In 2012, Diao et al. described an organic dual-wavelength DFB laser empowered by a dye-doped HPDLC grating [90]. The dual-wavelength laser operates at 586.6 and 670.2 nm simultaneously in one laser beam via the seventh and eighth Bragg orders of a one-dimensional (1D) HPDLC grating with a special period (Figure 10).

Figure 10:

(a) The lasing spectrum of the dual-wavelength laser and (b) the calculated lasing wavelength positions of the DCM-doped HPDLC grating with a period of 1520 nm for different Bragg orders. The inset in (a) is the SEM image of the grating, and the inset in (b) is an amplification of the data from the 6 to 9 Bragg orders [90].

Stable tri-wavelength laser emission (625.5, 617.4, and 614.3 nm) at six different directions was observed by Huang et al. A continuously tunable multiwavelength polymer laser is based on a triangular lattice photonic crystal cavity, which was initially fabricated through multiple interference exposure and was then replicated into a low refractive index polymer via UV-nanoimprinting. The blend of a blue-emitting conjugated polymer and a red-emitting polymer was used as the gain medium. Working behaviors are explained by the grating diffraction theory as the triangular-lattice photonic crystal cavity could be regarded as a combination of three 1D DFB lasers.

Such a mirrorless laser with a simple fabrication process, low cost, and possible tunability due to the excellent optoelectronic response of LCs make holographic polymer dispersed liquid crystal (HPDLC) photonic quasicrystal promising for a new type of all miniature organic lasers.

4.4 VCSEL and other types

Vertical Cavity Surface Emitting Lasers (VCSELs) have been used as light sources in optoelectronic devices in the past decades, mostly in optical interconnects and sensing applications. It has been demonstrated that the properties of the emitted light can be controlled, in both wavelength [91], [92] and polarization [93], by using an LC layer in combination with a VCSEL. An applied voltage controls the LC’s orientation in the layer and this, in turn, changes the optical path length of a beam propagating through the LC layer. The main advantage of the tunable LC-VCSELs is the use of liquid crystal technology, which is well established and, therefore, cost-effective. Moreover, LC being enclosed within the structure makes the device robust and reliable, in contrast to MEMS technology requiring moving mirrors suspended in the air.

In 2018, Frasunkiewicz et al., by performing one-dimensional transfer-matrix optical calculations supplemented with a fully-vectorial three-dimensional self-consistent modeling of thermal, electrical, and optical phenomena, have optimized the design of VCSELs and show the dependency of the emitted wavelength on LC cell thickness and the LC director orientation. The analysis is concluded with a possibility of achieving a 68.5-nm tuning range in a continuous-wave operation regime, without polarization or transverse mode switching [94].

Neyts et al. published a series of work showing that the laser emission and emission wavelength polarization can be controlled by applying an appropriate voltage over the liquid crystal layer [95], [96], [97]. A schematic illustration of VCSEL is shown in Figure 11 (left).

Figure 11:

(left) Schematic of VCSEL with CLC overlay: the LC cell is closed from the top by a glass plate with a deposited LC alignment layer [83]. (right) Schematic of the LC-polymer composite laser with a symmetric sandwich structure in which a dye-doped nematic liquid crystal (DDNLC) layer is sandwiched between two polymer-stabilized cholesteric liquid-crystal (PSCLC) layers [98].

The experimental results show that the CLC-VCSEL has several advantages: (i) its circular polarization has a higher purity than the linear polarization of a stand-alone VCSEL; (ii) the threshold current is lowered; and (iii) the temperature-dependent emission wavelength can be tuned over a broader range within the same temperature interval. These properties can provide applications, which require higher polarization purity, lower energy consumption, and broader wavelength tuning [97]. In 2020, it was shown that CW operation at room temperature and 23.5 nm wavelength tunability could be obtained in 1.55 μm InP-VCSEL [99]. This device combines InP-based materials with LC microcells collectively fabricated and integrated on the surface of a half-cavity VCSEL.

Finally, Lin et al. demonstrated, for the first time, an electrically tunable LC-polymer composite laser with a symmetric sandwich structure. This novel device consists of two identical polymer-stabilized cholesteric liquid-crystal layers as a distributed Bragg reflector (DBR) cavity and a dye-doped nematic liquid crystal (DDNLC) layer as the gain medium and half-wave plate sandwiched between the PSCLC layers (Figure 11 (right)) [98].

By simultaneous application of an identical DC voltage on the two PSCLC layers, the helical pitch within PSCLCs is elongated and compressed at opposite sides to form a pitch gradient (because of the electrically induced ion concentration gradient), thereby leading to a widened PBG. Given the electrically induced broadening of the PSCLC reflection band, the amount of the resonant modes and lasing wavelength of the sandwich LC-polymer composite laser can be controlled by the DC voltage. This device provides new insights into tunable LC lasers and can be potentially applied in sensors, medical imaging, displays, and light sources, among others. Given the proper designs of the gain profile and resonant modes and simultaneous generation of red, green, and blue lasing emissions, even an electrically controllable white-light laser can be anticipated.

5.1 Nematic

Thanks to the unique properties of the LCs in the nematic phase, they constitute great potential if we consider the laser action in disordered organic systems [100]. The main structural difference among typical anisotropic media, which provides constructive light diffusion, is the molecular arrangement. Rod-like architectures, oriented along with the director n(r,t), make this uniaxial liquid as the particular medium, which can fluctuate and provide light diffusion on about 10⁶-times higher than other, well-known isotropic liquids. Spontaneous director fluctuations represented by [100]:

leads to further fluctuation in the local dielectric tensor, consequently [100]:

Such an effect is responsible for multiple light reflections and diffusion, which provides optical feedback for stimulated emission.

Temperature is another parameter that can noticeably affect the random lasing action in NLCs [101]. This issue is strictly connected with a dedicated application. Because the nematic phase (as the other ones) is involved with the well-working temperature regime, it is important to maintain the proper environment. The nematic phase can be easily modulated by adjusting the heat flow [102]. A luminescent dye has to be implemented in such a system as well to achieve laser action. Then, one of the most critical parameters is LC and dye compatibility (chemical structures resemblance) [103]. In such a case, since the considered dye can be well-mixed with the branched matrix, it prevents additional light diffraction, which is an essential parameter in the random lasing phenomenon. Such compatibility is advantageous because only one component provides and, at the same time, limits all kinds of light diffusion contribution, which is easily controllable [104]. Furthermore, also electric [105] or magnetic [106] fields can change NLC architecture, and the effect provides various light diffusion [4]. However, the LC cell construction type (planar or homeotropic) influences the optical response coming from the material. If we consider nematic LC, the planar texture is preferred. Strangi et al. have recently presented the first observation of random lasing action from NLC (BL001) doped with pyrromethene 597 luminescent dye [107] (Figure 12(a)).

Figure 12:

Fluorescence and lasing spectra are reported. Discrete sharp peaks emerge from the residual spontaneous emission for pump energy of about 1.2 μJ/pulse.

The inset shows the lasing intensity’s dependence on the angle θ between the linearly polarized light and the local nematic director orientation [94] (a). The output intensity of random lasing versus the input pump energy at different incident angles θ with a fixed lateral section of the pump pulse on the NLC film S′ = 0.040 mm² (b); Transmitted laser spots of the pump pulse with pump energy of about 30 μJ/pulse obtained at θ of (c) 15°, (d) 30°, (e) 45°, and (f) 60° [108].

Above the lasing energy threshold, which in that case was very low (∼900 nJ/pulse), the discrete sharp peaks (FWHM = 0.5 nm) typical for RL were obtained. As the experimental proof that the observed emission has STE character, the authors performed a lasing intensity vs polarization angle investigation (inset of Figure 12(a)). It is seen that NLC doped with luminescent dye can provide efficient and polarization-sensitive RL, where the orthogonal direction causes an almost diminishing of the lasing action. In the same contribution, the authors have compared the NLC random laser energy threshold with two other, well-known lasing systems. The cholesteric liquid crystal (ChLC) [8, 109, 110], as an example of a well-ordered dye medium (thanks to the periodic helical structure of the matrix), characterizing the lowest energy threshold to obtain random lasing (∼1 μJ/pulse). Disordered media was represented by a methanol solution with dispersed ZnO nanoparticles [111], which achieved the RL energy threshold a few times higher (∼8 μJ/pulse). A partially ordered system contributed by nematic LC placed the energy threshold values close to the ChLC one, at ∼2 μJ/pulse. The authors concluded that random, partially, and fully ordered systems provide various light scattering mechanisms leading to the random lasing modes.

Fengfeng et al. have investigated the pump beam intensity and angle influence on the RL nature, efficiency, and energy threshold values in highly concentrated DDNLC systems [108]. Due to the NLC domain’s mobility controlled by an electric field direction from electromagnetic wave, a strong correlation between incident beam angle and LC cell positioning was proven, which resulted in different RL intensity and energy threshold values (Figure 12(b)). The authors also noticed that changing the incident laser line angle from the normal of the LC cell surface by 15, 30, 45, and 60° results in various energy threshold values, namely 3.7, 2.1, 1.4, and 2.5 μJ/cm² respectively. This means that the emission properties depend on the NLC director reorientation caused by the delivered electric field to the highly sensitive and easily controllable liquid crystalline domains. In other words, the photo-induced molecular fluctuations change the scattering conditions of the optically active NLC-based systems, which directly influence the random lasing feedback and loss parameter.

It was also proven that independently of the dye concentration, after a gradual increase of the pump beam intensity, there is a particular I_pump value, where output RL signal drops. However, the random lasing intensity corresponds to the various incidence angle, which is the highest for θ = 30 and 45° (abrupt slope, the lowest energy threshold values), then lower for θ = 30 and 60°, respectively. The last-mentioned laser configuration gives about a six-times lower random lasing feedback than marked in blue in Figure 12(e): θ = 45° configuration.

Huanting et al. have proven that using a very low DC voltage makes it feasible to modulate random lasing intensity due to the long-range NLC fluctuation and dielectric tensor changes [112]. Chia-Rong et al. have shown the possibility of RL intensity modulation in DDNLC even below the Fréedericksz transition threshold, which paved the way for ultra-low energy consumption for random laser emission control [113]. The polarization influence of the generated RL in DDNLC has also been reported in the literature [114]. Ji-Liang et al. have shown that sphere-phase nematic liquid crystal in the shape of 3D twist architectures are more efficient (due to the fewer values of the RL energy threshold and lack of any pretreatment processing) than the typical chiral NLC structures [115], [116]. However, the dye-doped (PM597) chiral NLC generating discrete and narrow (FWHM = 0.3 nm) random laser modes, without rubbing procedure of the utilized soft mater, was recently reported [117]. The authors have proven that LC cell thickness has a significant impact on the RL energy threshold value. To conclude, the NLC-based random lasers constitute an easier and cheaper alternative for the classical lasers, where microcavities have to be prepared with ultra-high precision [118], [119].

5.2 PDLC

Polymer-dispersed LCs (PDLCs) are a class of organic materials that shows interesting electro-optical features (reversible switching processes) and highly optical anisotropic light diffusion [120]. In other words, PDLC materials characterize at the same moment both solid-state properties typical for polymers and specific fluid-like behavior related to the soft matter (LCs). A typical photo-initiated polymerization is used to create such bi-component architecture. A homogenous mixture of monomers and LCs react together under controlled conditions and create polymer-dispersed LCs. The polymerization environment has a significant impact on the final PDLC features [121]. PDLCs have many practical applications. However, they are mainly known as components for active walls [122]. There are two types of electrically controlled wall transparency. The first is called direct-mode PDLC. When the electric field is applied, the refractive index from polymer and LC is equal, and a match condition is achieved. In that situation, such material is fully transparent for visible light [123]. The second kind works exactly the opposite. The reverse-mode polymer-dispersed LCs have a milky structure when the applied electric field is provided. So, in the stationary state, the reverse-mode PDLCs are transparent and do not need any stimuli to sustain such conditions [124]. However, the first working random laser based on the dye-doped polymer-dispersed LC material (DDPDLC) was reported by a group of Wiersma in 2004 [125]. After that, the optical gain observed in disordered media based on the PDLC materials has attracted attention from many research groups [126].

Lihua Ye et al. have presented randomly arranged (nonoriented) and one-side aligned (oriented) dye-doped polymer-dispersed liquid crystal (N-DDPDLC, P-DDPDLC, respectively) systems [127]. These authors investigated macromolecular reorientation influence on the optical feedback parameters in the manner of random lasing emission. The authors found that the N-DDPDLC system characterizes the RL energy threshold on about 4.1 μJ/pulse; however, the achieved light amplification had a multimode (noncoherent) nature (FWHM ∼0.5 nm). Whereas, for the P-DDPDLC system, several sharp laser spikes were observed (FWHM ∼0.2 nm) and E_th = 3.5 μJ/pulse. The signal-to-noise ratio in the second case was huge, and distinct laser modes were well-separated. The results discussed above relate to an LC cell with an estimated thickness of 20 μm. When the samples were prepared according to the same protocol, although the thickness parameter has changed (d = 10 μm), the difference between nonoriented and oriented PDLC systems was even higher. The N-DDPDLC characterized similar behavior as previous LC cell constructions (multimode RL with FWHM ∼0.5 nm and E_th = 3.0 μJ/pulse). Interestingly, the P-DDPDLC system gave even better results. Namely, the RL energy threshold was estimated at ∼1.9 μJ/pulse. The optical cavity length was determined for the set of thinner samples due to the methodology (PFT analysis) introduced by Polson et al. [128]. The results gave the following numbers: L = 16.07 and 26.21 μm for nonoriented and oriented structures, respectively, which means that the optical path length in both cases was longer than the active layer thickness. The P-DDPDLC sample, at about 2.5-times higher, indicates that molecular alignment in the PDLC systems has a significant impact on the RL optical feedback mechanism [127].

According to the typical experimental approach, when LCs-based systems had been successfully enhanced (an increase of light diffusion and amplification) by nanoparticles (NPs), the same issue was concerned with the PDLCs materials. Zhen Zhen and co-workers showed a random laser based on the PM597 laser dye combined with manganese (II) chloride nanoparticles embedded in the same PDLC system [129]. As the reference material, the authors used pyrromethene 597 embedded in PDLC (without MnCl₂ nanoparticles). The effect of such an approach was as follows: RL energy threshold was estimated at about 7.57 μJ/pulse (FWHM = 0.23 nm) and 13.02 μJ/pulse (FWHM = 0.15 nm) for PDLC system with and without NPs, respectively. Also, the optical cavity length for the doped system is about 50% of the case without any MnCl₂ nanoparticles. The authors provided evidence that the addition of NPs to the PDLC system provided an extended lifetime of photons, which are present in the volume of the considered active and disordered architecture. Moreover, the energy transfer between nanoparticles and laser dye, which enhanced RL intensity, decrease its energy threshold, and allowed emission tunability.

A similar contribution is given by Long-Wu and Luo Gen, where a silver nanoparticle-doped PDLC system was recently reported [130]. PDLC-DCM doped with Ag NPs characterizes 20-times stronger light amplification in the manner of random lasing emission (and FWHM of the distinct laser modes less than 0.2 nm) than in the case where only DCM dye was introduced. This confirmed the already known mechanism that liquid crystal domains deliver light diffusion and metallic nanoparticles provide electric field enhancement.

Long-Wu et al. have shown an influence on the zinc oxide nanoparticles on the RL abilities of the PDLC systems [131]. The authors have considered three types of the investigated systems: DDPDLC solution with (i) small-sized LC droplets, (ii) ZnO NPs, and (iii) only dye solution with ZnO capillaries (without macromolecular matrix and LC). The spectral characterization of the mentioned systems is presented in Figure 13(a).

Figure 13:

Emission spectra of DCM + PDLCs + ZnO nanoparticles (b), DCM + PDLCs, (c) and DCM + ZnO nanoparticles (d), respectively, excited by the pump energy above threshold [131] (a). Schematic illustration of the experimental setup of the random laser from the capillary tube infilling of the DD-PDLCs and photograph showed on the right (b), and the description of the multiple recurrent light scattering in the capillary tube comprised of the random alignment of LCs, monomer (NOA65), and laser dye (PM597) [132] (c). The spectra of random lasing from samples with MNP doping concentration of 0 wt.% (d), 0.01 wt.% (e), 0.02 wt.% (f), and 0.03 wt.% (g) in the PDLC with various pump energies [133].

It is visible that the emission band is red-shifted for the systems where ZnO NPs were added due to the PDLC-DCM system. Moreover, the nature of light amplification becomes more coherent, and a higher number of selected and favored laser modes is observed. The spectral narrowing is substantial above the RL energy threshold, which is also visible as an increase of the signal-to-noise ratio. The general conclusion from the above-discussed comparison indicates that metallic NPs always enhance light amplification phenomena independently of the used environment. Such an approach gave an insight into emission wavelength, RL energy threshold, and coherence of light modulation by the addition of ZnO nanoparticles. It originates from scatterers’ permittivity and system geometrical structure [131].

Additionally, the approach concerning magnetic nanoparticles (MNPs) utilization in the working PDLC systems was also performed recently [133]. Dai et al. proposed a tunable random laser localized inside a tube, where the external magnetic field effectively influences the RL energy threshold, laser modes localization, and emission profile in general. Subsequently, it was proven that the magnetic field orientation tunes the light amplification in a meaningful way. The MNPs various concentration gave significant change in the output signal, what was experimentally proven (Figure 17(d–g)).

Ja-Hon and Ying-Li have presented a simple and effective DDPDLC random laser constructed in a capillary tube [132]. The idea of the experiment and the generated phenomenon with the proposed mechanism is shown in Figure 13(b). As it is visible from the sketch and the inset photo, the output emission was collected from the edge of the sample using a fiber spectrometer. The emission directionality is forced by liquid crystalline domains orientation as the response on the electric field coming from pump beam intensity and the internal reflection present inside internal curvature leading to waveguide phenomenon. However, the random lasing coherence and laser mode positioning can be easily tuned by polymer cluster shapes due to the various monomer concentration and morphological textures (grain size). The emission parameters, like FWHM, laser energy threshold, or a number of RL spikes from DDPDLC systems, can be easily changed, which was experimentally proved by the authors (Figure 14) [132].

Figure 14:

(a) The RT-PL of laser dye (PM597) and the RT stimulated emission spectra from the capillary tube through exciting the Q-switched laser that contains the DD-LC mixtures (b) without monomer and with (c) 10 wt.%, (d) 15 wt.%, (e) 20 wt.%, (f) 25 wt.%, (g) 35 wt.%, and (h) 35 wt.% monomer doping concentrations. The POM pictures of the DD-PDLCs are shown on the right of each diagram [132].

As shown in Figure 14(a), the fluorescence band measured for PM597 laser dye is typically broad when a CW laser serves as the pump. Then, by stimulating the LC dye-doped sample without monomer, the emission band becomes narrower (down to FWHM ∼11.4 nm). However, since the NOA65 monomer is gradually added, the discrete laser modes with FWHM equal to or lower than 1.0 nm start to arise and achieve the highest Q factor ∼1135 and the lowest spectral broadness (0.52 nm). Above 20% of NOA65 concentration, the number of laser modes and their intensity decrease, especially those observed at 600 nm (red-shifted). It was explained that scattering losses started to dominate over optical gain.

Interestingly, the investigation also assumed spectroscopic experiments in the lower temperature conditions. The investigated range of temperature was considered between 11 and 50 °C. Since the heat flow was supplied to the DDPDLC system, the emission nature began to transform from coherent random lasing to the ASE. The laser modes were suppressed due to the decrease of birefringence coming from the liquid crystalline domains with the higher temperatures. At around 11 °C, the LC-based system characterizes a much higher difference between ordinary and extraordinary refractive index than at 40 or 50 °C. Significant optical birefringence below room temperature for the considered DDPDLC system provides stronger light diffusion related to the more effective light scattering.

5.3 Quantum dots

Another family of LC-based random lasers is established on the utilization of quantum dots (QDs) as a luminescent dopant. Namely, in the systems where metallic QDs were applied due to surface plasmon resonance (SPR), the lower energy thresholds were acquired [134]. This kind of approach characterizes two main advantages. The first one comes from the NLC structure, where the applied external electric field easily modulates emitted laser light intensity. On the other hand, the implemented QDs cause significant amplification of the generated light (Q factor increases) and decrease the energy threshold to obtain RL. Such a solution also has limitations related to the QDs concentration, which was shown in the literature by Ye et al. [134]. Too high quantum dots addition might quench light amplification and hurt the effective light diffusion. Also, the mutual arrangement between active molecules (dyes) and scatter units (QDs) is not trivial and always needs to be optimized during NLC random laser construction (Figure 15).

Figure 15:

(a) Emission spectra of fluorescence from the toluene solution of PM597 laser dyes containing different Ag NP concentrations under the pump energy of 32.4 µJ/pulse. (b) Emission spectra as a function of pump energy for the NP-free DDNLC. (c) Emission spectra of RL from the NPDDNLC with different Ag NP concentrations under the pump energy of 7.2 µJ/pulse. (d) The threshold energy curves of RLs from the DDNLC with the addition of different Ag NP concentrations [134].

Also, a titanium nitride (TiN) was used to improve lasing action in the LC-based systems [120], [121]. Here, nanomaterials created plasmonic effects in the visible and near-infrared regions. The plasma frequency (ω_p) can be described as:

where n represents electron density in the material’s volume; e and m are charge and electron mass, respectively [135]. For the higher carrier concentrations, the plasmon frequency can exist for the visible range of the spectrum. In comparison with noble metals, the TiN nanoparticles have significant advantages. For instance, their optical features can be modulated during the synthesis procedure environment. Then, the titanium nitride is compatible with the standard fabrication CMOS processing. Finally, the most important issue, TiN quantum dots, are much cheaper and can provide the same effect as the noble metals (Ag or Au). RL obtained from the NLC system doped with TiN NPs characterizes the FWHM parameter less than 0.3 nm for defined modes, which gives a Q factor (λ/Δλ) more than 2000 [136].

Moreover, if we compare RL with the PL spectrum, there is a significant red-shift by about 20 nm. This is caused by the increased dye re-absorption abilities by the TiN presence. The light amplification issue coming from the implemented nanoparticles to the active NLC sample comes from the localized SPR discussed before and the enhanced light diffusion, especially for the particular selected wavelength, which is in resonance.

Furthermore, the sample architecture significantly impacts the stimulated emission efficiency, as was shown recently by Wan et al., when NLC-doped with TiN nanoparticles was placed in the capillary [136]. Interestingly, the emitted light intensity is related to the various directions from the active medium (Figure 16).

Figure 16:

(a) A schematic illustration of the emission spectrum is detected at different angles. (b) The emission spectrum detected at different angle θ from the NPDDNLC sample with 1.186 × 1011/ml in TiN NPs number density, when L = 0 mm, ψ = 0°. (c) The peak intensity of the emission spectrum as a function of the angle θ, when L = 0, 5, and 10 mm. (d) The emission spectrum of the NPDDNLC sample with 5.314 × 1011/ml in TiN NPs number density as a function of the polarization direction ψ of the pump beam, when L = 0 mm, θ = 0°. The inset in Figure 22(d) shows the peak intensity of the emission spectrum as a function of the angle ψ, when L = 0 mm, θ = 0° [138].

In the above example, it was found that maximum intensity was noticed in the long capillary direction when the pumping laser polarization is rotated in the same horizontal direction. There are a few possible mechanisms that indicate such a phenomenon. First, the capillary can be treated as a one-dimensional object, where the light diffusion takes place along the structure and is also reflected from the internal walls surrounding active material. Second, the light emission is related to the pump beam’s polarization direction and dipole moment transition of the luminescent dye. For the same reason, the highest lasing intensity will be along the capillary length when the initial laser polarization is parallel to that direction. The next reason is related to the gain coefficient, which is connected with the electric field enhancement by localized surface plasmon resonance of TiN NPs, which is mainly present along the initial laser light polarization direction. Lastly, thanks to the line-shape pumping irradiation on the sample, the other emitting directions are not favored due to geometrical reasons [136].

5.4 Semiconductor

Semi-conductivity can be realized differently, in various types of materials, like π-conjugated polymers, nanoparticles, or quantum dots. In the organic systems, the maximum value of the valence band is replaced by the highest occupied molecular orbital (HOMO) π-type. Whereas the minimum value of the conduction band is represented by the lowest unoccupied molecular orbital (LUMO) π^*-type [137]. Between these two energy levels, an electronic bandgap is localized. The most practical advantage related to organic-based structures is the almost unlimited possibility of designing and creating various molecular architectures, which is also easily implemented in photonic devices [4, 102, 107, 138]. Organic semi-conductors found many applications not only in the organic light-emitting diodes (oLEDs), solar cells, or field-effect transistors (FETs) but also in the random laser constructions [137]. Random laser intensity can be easily controlled by the different exposure times of the constant-wave laser influence, in green or red (Figure 17(a)). The initial laser beam (in the pulsed working regime) generates the optical gain; the other cw ones control LC phase transition (nematic/isotropic). Consequently, refractive index changes influence the scattering level. Thus, the output power is modulated efficiently.

Figure 17:

Top view of the experimental setup for examining the all-optically controllable random lasing emission of the DDPDLC cell (6 μm thick). The green and red beams are circularly polarized (λ/2, half waveplate for 532 nm; PBS, polarizing beam splitter) (a). The SEM photograph of the DDPDLC cell, in which the length of the white bar is 100 nm, and the black regions are places occupied by nano-sized LC droplets in the polymer matrix (grey regions) [137] (b). Schematic diagram of light propagating in the QD-PDLCs with LCs in the anisotropic phase and the isotropic phase [139] (c).

ZnO [140] or CdS [141] semi-conductor nanoparticles can serve as examples of their use in LC-based random lasers. Such constructed devices might be easily modulated when concerning their working parameters, like output wavelength or current threshold, making them appealing applications. The origin of such a possibility given by nanoparticles comes from their thermal and electrical stability. Moreover, since the NPs concentration increases, the number of scattering centers is higher; the emission wavelength is red-shifted according to the effective refractive index value changes in the considered disordered systems [141]. In summary, the most significant advantage of the semi-conductor NPs combined with LC is an increase of the light absorption, and secondly, more effective (up to 85%) photon delivery to the active medium.

The next approach to improve LC-based random lasers is to utilize quantum dots [142]. Despite the lowering of the energy threshold, a significant increase in the system’s photostability constitutes another advantage. Wang and co-workers have proven high RL stability even after 15 days of laser pumping (approximately 3 h/day), which was estimated to be around 92% of the initial output laser intensity [142]. Nevertheless, another report [139] pointed out that even QDs additives can be insufficient without an LC matrix. The critical issue is to control the liquid crystalline phase transition related to the environment temperature. When the LC goes to the isotropic phase, the light-scattering ability of the considered system dramatically decreases, which results in emission band broadening and intensity decreasing (Figure 17(c)). In other words, LCs play a pivotal role in disordered media dedicated to the random lasing phenomena. However, the predominance of QDs under the NPs is significant. They provide a broader absorption band, the higher photoluminescence quantum yields, and give luminescence, where the output wavelength is easily controlled by synthesis duration and temperature.

5.5 Perovskites

3D Perovskites, nanoparticles, and nanoplates create a natural resonance cavity, which serves to achieve the population inversion needed for amplified spontaneous emission or lasing action [143]. Thanks to the wide modulation range of bandpass in Perovskite-based lasers, it allows their utilization in many kinds of applications, which will be described in this review. Imposingly, such constructed organic-inorganic systems achieved the same efficiency and brightness in two years, whereas organic LEDs technology has improved these parameters in two decades [143]. The first demonstrated device achieved effective biomolecular recombination by spatial electron and holes closing in the thin active layer (∼15 nm). In that way, low exciton Perovskite energy is overpassed [143].

In 2015, Stranks et al. presented enhanced ASE based on the cholesteric LCs, where Perovskites were also applied [144]. The authors improved the mirror-less LC-based system so far that it could achieve single-mode and flexible laser structures.

In the above-cited contribution, the LC layer was deposited by spin-coating technique on the previously rubbed substrate (polyimide glass plate) (Figure 18(a)). Thanks to the transparent and reflective range of the liquid crystalline layer, it was feasible to combine CH₃NH₃PbI₃ Perovskite active medium to be the most effective. Moreover, for the same reason, additional sandwich-like spacer layers were applied (SEM cross-sectional photo is presented in Figure 18(b)). Interestingly, the authors noticed an unexpectedly strong light amplification and emission spectrum narrowing effect featured with the addition of CLC, which excludes superfluorescence or excitonic effects [145], [146], [147]. The effect was experimentally proven and shown in Figure 18(d). The authors postulated that only thanks to the CLC utilization, it was possible to decrease the energy threshold to observe single-mode RL on the mirrorless lasing systems lower (∼7 μJ/cm²/pulse) than already presented in the literature [148], [149]. Then, also wavelength tunability and whole architecture flexibility are provided by CLC as the reflector to the demonstrated organic-inorganic systems (Figure 19(a)).

Figure 18:

(a) Schematic of the device stack. (b) The transmission spectrum of the CLC reflector on glass (left axis) with PL spectrum from the perovskite thin-film overlaid (black circles, right axis). The blue line shows the modeled transmission spectrum of the device stack using the 4 × 4 Berreman matrix approach. (c) SEM cross-sectional image of the full device structure on the glass. The alumina layer has been shaded for contrast. (d) Emission from full device stack (red), stack without CLC (black), and stack without CLC and alumina (blue dashed), with pulsed excitation (530 nm, 4 ns pulses, 10-Hz repetition rate, ∼60 μJ/cm²/pulse) [144].

Figure 19:

(a) Extracted emission intensity from a device stack fabricated on a flexible 80% CLC reflector following photoexcitation at a range of fluences (532 nm, 5 ns pulses, 100-Hz repetition rate). The ASE transition fluence is determined to be 7.6 μJ/cm²/pulse. Inset: photograph of the flexible device [144]. (b) The laser spectra of liquid crystal-covered perovskite microdisks at different temperatures. (b) The peak positions of modes in (c) as a function of temperature. A dramatic wavelength shift can be observed at T = 32 °C [150].

Wenzhao et al. have presented Perovskite microdisk lasers doped with 5CB liquid crystal, where the output signal was easily tuned and switched by environmental temperature changes [150]. The authors found two types of RL tunability: a small type (∼1 nm) due to the LC refractive index changes and a much larger type (∼6 nm) related to the scattering media modulation. In the just-cited contribution, the authors showed an LC-based Perovskite-type nanolaser, based on the nanowire-shaped active medium embedded in a well-aligned environment (Figure 19(b) and (c)). The system was characterized, and at the same time, the output laser wavelength was tuned in the temperature range of 24–34 °C. The separated modes of a random laser were shifted by about 4 nm. This effect was explained by the scattering medium modulation being more efficient when external heat flow is provided and due to the unique LC features (transition from nematic up to the isotropic phase) [150].

Another proof of the obtained highly efficient (RL energy threshold ∼150 nJ/pulse) and narrow linewidth effect (FWHM ∼0.20 nm) in CLC-based laser with lead-free cesium tin halide (Perovskite) active material was recently given by Chen et al. [151]. Moreover, these authors reported a low-cost manufacturing method to achieve multicomponent Perovskite-based lasers. From the future applications point of view, the long spectral stability of the induced lasing action is the most important feature (Figure 20).

Figure 20:

(a) Lasing spectra of the AIPQD−CLC (cell 1) at E = 0.6 μJ/pulse after 1 day and 6 months of storage (black and red peaks, respectively) under room temperature (23 °C) and high humidity of 60% (±5%). (b) Variation of the measured lasing intensity of the AIPQD−CLC device under continuing excitation of the pumped pulses for 10 min. The measurement was performed under room temperature (23 °C) and high humidity of 60% (±5%) [151].

Also, Arumugam et al. have shown that thanks to the application CLC as the reflective layer, Al₂O₃ and P₃HT working as spacers, and finally, due to the active Perovskite medium (C(NH₂)₃PbI₃:Tm), it was possible to achieve a high Q-factor equal to ∼2500 [152]. The CLC layer dramatically reduced emission bandwidth to 0.28 nm with a pump laser equal to 0.70 μJ/cm². These results paved the way for using Perovskite materials doped with LCs as efficient photovoltaic and electro-optic applications.

6.1 Whispering gallery mode (WGM)

Whispering gallery mode (WGM) resonators are optical cavities in which light can be guided around and circulate, based on the phenomenon of total internal reflection. Such a situation takes place on the curved surfaces of the cavity. Therefore, WGM appears in a sphere or ring, and they are characterized by a higher refractive index value compared to the environment. The Q factor in such an example can reach the value of 10¹⁰, which is considered a great advantage [153], [154], [155], [156], [157].

As the main configuration of LC supporting WGM operation, a liquid crystalline matrix with introduced luminescent dye is used, finally excited by the use of an external source of light. By embedding the cholesteric droplet into the water, the cavity can reach a size of about 30 µm diameter. Such a prepared microresonator enables the fabrication of liquid-phase exhibiting a very low lasing threshold. According to the reference, the resolution of the CLC microdroplet-based temperature sensor is 4.17 × 10⁻² °C. These constructions are perfect for integration in optoelectronic devices. It is now worth considering the sensor’s applications, which can be achieved thanks to the small size and high Q factors. For example, WGM sensors can be made of cholesteric mesophase, known as sensitive for temperature changes (Figure 21(a) and (b)) [157].

Figure 21:

(a) Lasing spectrum of 22 μm CLC microdroplet as a function of temperature from 24 to 28 °C. Inset: temperature dependence of the lasing wavelengths of the representative mode TM₁₇₅. The wavelengths exhibited an almost linear relationship with temperature (0.95 nm/°C); (b) Micrograph of a 20 μm dye-doped CLC microdroplet confined at the tip of a capillary microtube. Insets: close-up POM image of the microdroplet (upside); SEM image of the microtube (underside) [157].

An interesting aspect of operating WGMs microcavities is the possibility to tune the resonant frequencies (even in real-time) by changing different stimuli. One can be the optical path of the light, while the second acts on the resonator refractive index changes. It is also possible by varying the resonator physical dimensions. Among the major ways and stimuli influencing the mentioned factors, it is worth mentioning temperature, mechanical stress, and electric or magnetic fields [158]. The effects have been referred to in the literature as obtained on ferronematic LCs. The investigated liquid crystalline nematic mesophase is named 6CHBT (1-(trans-4-Hexylcyclohexyl)-4-isothiocyanatobenzene) doped with spherical or rod-like magnetic particles (Figure 22). The results present the shift towards red under the influence of the magnetic field. Moreover, the sensor’s measured sensitivity was increased by changing the WGM diameter size (from −39.6 pm/mT and −37.3 pm/mT to −61.86 pm/mT and −49.88 pm/mT for rod-like and spherical magnetic dopants, respectively).

Figure 22:

TEM images of (a) spherical magnetic nanoparticles, (b) rod-like magnetic nanoparticles, (c) polarizing microscope images of empty and infiltrated PCF in crossed polarizers, and (d) SEM image of the PM-1550- 01-PCF cross-section [158].

LC microlasers working as WGMs can also be realized for the nematic mixtures, for example, E7. Moreover, they can be easily tuned by an external magnetic field. When the applied field is considered as in-plane geometry of the WGM circulation, the lasing modes shift towards the blue. However, perpendicular to the circulation plan of WGM, the spectrum shifts towards red. Almost the same magnitude characterizes both cases, and the effect is fully reversible [159].

The electric field tuning in the LC matter is especially interesting since it is usually fast and can be easily integrated into, for example, current electro-optical circuits [156].

It is worth recalling the LC-based constructions combined with polymers. Embedded in the polymer, matrix LC-dye-doped droplets from the phase-separated microcavities make lasing possible. These kinds of devices have been created and are still developed as previously described PDLCs. To achieve this construction, the proper methods for homogenization must be applied, such as the use of ultra-sounds or the evaporation of used matrix solvents. The main advantage of PDLCs is the possibility to create the required and expected microresonator size [160]. Liquid crystalline droplets embedded in a polymer matrix remain in the droplet-shape and, therefore, can be interesting for the WGM applications, size modulations, and lasing performance. An advantage of these devices is that they allow for the investigation of more than one luminescent dye dopant. With the mix of three luminescent dyes emitting in the fundamental colors (green, blue, red), it is possible to obtain a white-lasing device, which can be easily tuned by the applied external fields [103].

6.2 Plasmonic

Another approach involves plasmon-associated issues. Plasmon-based lasers have many advantages, including ultra-compact, small sizes reflecting in easy integration in optoelectronic devices, low threshold, generation of coherent light in nanometer-scale beyond the limit of diffraction, short response time, and light localization providing the strong and efficient output [161]. It is worth noting the remarkable tunability and sensing properties of LC mesophases. Surface plasmon-LC combined systems can find application in QDs laser devices or optical planar circuits switches [162]. It can be realized via the Tamm-Plasmon Mode, formed between a metal and dielectric Bragg mirror. Therefore, it can be connected with the photonic crystal substrates. The presented method bases on the preparation of a small LC gap layer (in the range of 200 nm), placed between the photonic crystals and metal surfaces (Figure 23(a)).

Figure 23:

(a) An apparatus for changing the LC gap by using fine screws and PZT tubes [147]; (b) Stopband reflection of CLCs (blue curve) and fluorescence spectra for undoped (black curve) and nano gold-doped (red curve) CLCs excited by a CW 532-nm laser [163].

By changing the temperature of LC, affecting phase evolution (between isotropic and nematic), it is possible to obtain different resonance wavelengths (a shift about 15 nm). Combining the LC mesophase with the plasmon technologies is very interesting for future, novel display ideas [164]. With the investigation of LC-display technologies, the aluminum plasmonic nanostructures can provide low cost, good quality colors, and resistant to fast bleaching. Such technology could be combined with the idea of novel displays pumped by the laser source (e.g., UV range), ensuring an excellent quality color gamut and easy tunability. Surface plasmons (SPs) can be combined with LC mesophase and metallic nanoparticles [165] in the form of micro, PDLC droplets. The evidence of plasmonic excitation in this context leads to the further possibility of creating plasmon-LC-hybrid lasing devices for coherent, efficient, directional, intense lasing from well-defined microresonators. Chiral liquid crystalline molecules (CLCs) are well known for the existence of the photonic bandgap in their structure. The novel studies conclude that CLCs doped with gain medium and gold nanoparticles can strongly enhance the fluorescence signal via localized surface plasmon effect [163]. For referred studies, the defect of the investigated layer was prepared by the use of commercially available materials, like an E7 mixture doped with pyrromethene (PM597) laser dye. The effect of enhanced fluorescence is presented in Figure 23(b).

The photonic crystal characteristic of dye-doped CLCs is very promising for further studies of Surface-Plasmon lasing under the influence of electric, magnetic field-tuning, or mechanical stress.

Another example of tunable nanophotonic structures formed by LC and metallic nanostructures (based on spherical, gold nanodots, nanodiscs, nanorods) is localized surface plasmon resonance (LSPR). LSPR is a particular type of SPs, occurring with the use of nanoparticles. The LSPR is observed when the electromagnetic field remains localized in a nanoscale region around the nanoparticle-dielectric interface [166]. These constructions are very appealing for novel tunable devices, like LC-based displays, light emitters, and biosensors, which will be discussed in the next section of this chapter [148], [167].

6.3 Biological

In the context of biological applications, it is worth recalling the WGM resonators, which can easily interact with the environment. When the particle or even the single-molecule settles on the WGM ring or can be found near its surface, the optical path of light, likewise the loss factor, changes significantly, and the object can be detected [168]. As mentioned before, a high Q-factor is a reason for the great sensitivity of these devices. Therefore, in the context of liquid crystalline microlasers, they became precious tools for detecting lipids [169], responses for pH variations [170], viruses, or proteins [171].

Here we cite an interesting example of a WGMLC-based resonator [172]. As with the LC mesophase, it has become a very popular and widely reported nematic liquid crystal, named 5CB (full name: 4-cyano-4′-pentylbiphenyl), doped with PBA (4′-pentyl-biphenyl-4-carboxylic acid) in a buffer solution (PBS). The first great advantage is selecting materials that are widely available, well tested, easy to use, and relatively cheap. The authors present the lasing signal and the droplet configuration for the pH changes (Figure 24).

Figure 24:

(a) POM image of lasing PBA-doped 5CB microdroplet in PBS with a pH of 6.0. The microdroplet exhibits a radial configuration. The corresponding micrograph of the radial microdroplet (top right inset). (b) POM image of the same microdroplet 3 min after adding 10 μL hydrochloric acid solution: the microdroplet undergoes ordering transition from radial to the bipolar configuration as the pH decreases. Scale bars 50 μm. (c) WGM lasing spectra of PBA-doped 5CB microdroplet with radial (black line) and bipolar (red line) configurations [175].

Such an example can be considered valuable in oceanography, clinical, or environmental studies.

One of the latest discoveries shows that LC microdroplets can be useful in detecting negatively charged biomolecules [173]. This result opens new possibilities for ultra-sensitive bio-sensing in the scope of LC matter for novel diagnosis and biomedical science.

The LC microlasers are especially promising in the context of environmental monitoring, industrial drain control, drinking water quality testing, and waste treatment according to the possibility of heavy metal toxic ions detection [174]. For these effects, researchers have employed a commercially-available 5CB-nematic LC mesophase doped with stearic acid in the form of WGM microdroplets (Figure 25), thus forming a biosensor.

Figure 25:

Schematic illustration of the orientational transition of stearic acid-doped 5CB microdroplets induced by changes of 5CB anchoring at the LC/aqueous solution interface.

(a) without HM ions at pH = 6.5; (b) without HM ions at pH = 8.5; and (c) with HM ions at pH = 8.5 [174].

The sensing properties could be achieved according to the heavy metal ions adsorption, monitored by polarized pattern evolution, the observation of resonance shift, and spectral response. Because of the many tests and repetitions made on this device, this biosensor can be considered reliable.

7.1 Sensors

Since the heat sensitivity is a typical property for the NLC, these can be used as remote thermal sensors, where by using a telescope, the response signal can be analyzed [118]. Because of the synergic effect of the NPs and QDs combination, it was possible to achieve polymeric fiber optic random lasers, sensors, and displays [142]. The LC-based sensor, recently proposed by Duan et al., has a great sensitivity to urine molecules and can detect a molecular concentration as low as 0.1 mM [175]. That system has many advantages. Namely, quantitative and sensitive detection takes place in real-time processing, and its resolution (detection limit) is much higher than previously described urea detectors. These advantages are a result of the acid-doped 5CB liquid crystal and WGM lasing technology. It revealed in a dual light amplification and emission band shift, which comes from LC microdroplets configuration transition. This feature makes such systems excellent candidates for monitoring enzymatic reactions (Figure 26).

Figure 26:

POM images of 65-μm-diameter acid-doped 5CB microdroplets in PBS (pH = 7) containing 0.1 mg/ml urease after addition of 40 μL of 10 mM (a), 1 mM (b), 0.1 mM (c) urea. Scale bars 20 μm. (d) Temporal dependence of WGM wavelength shift with different urea concentrations. (e) Portion of WGM lasing spectra as a function of time. Spectra were collected from a 65-μm-diameter acid-doped 5CB microdroplet in PBS containing 0.1 mg/ml urease after adding 40 μL 0.1 mM urea [175].

There are many more examples of such applications. For instance, LC-based sensors were introduced to detect various biological molecules (i.e., lipids [176], glucose [177], proteins [178]), but also heavy-metal ions [179], [180] or synthetic polymers [179]. Then, using the synergetic effect of the up-conversion nanoparticles (UCNPs) and cellulose LC, it was possible to construct a real microfiber humidity (RH) sensor (Figure 27) [181]. The mirrorless low threshold lasing provided by the LC-based part, together with UCNPs fluorescence molecules, provides a highly sensitive RH detector (284.75 pm/%RH independently of the environment temperature), which is shown in Figure 27.

Figure 27:

(a) ×20 micrographs of the planar CLC film at different RH levels. (b) Schematic diagram of the experimental setup for RH testing. (c) and (d) show the output spectra and the laser emission wavelengths at RH levels of 38, 54, 68, 80, and 98% [181].

The biosensor for heavy metals (HM) detection based on the LCs, which gives various lasing behavior, was recently introduced in the literature [174]. Again, the WGM mechanism was implemented to achieve a significant signal in the LC microdroplet sensor. The results indicate the discussed device as pioneering among currently known systems dedicated to the detection of the HM ions. It was experimentally proven that the achieved sensitivity level is equal to 40 pM for Cu²⁺ ions, which is six orders of magnitude better (concentration threshold, when copper ions are detected) than that defined by the World Health Organization (WHO). Notably, the WGM LC-based sensor is selective as well when the light metal ions are considered, which helps to avoid any artificial signals. That example can be straightforwardly utilized to analyze drink water quality, or in general, to monitor the environmental condition (Figure 28).

Figure 28:

(a) Evolution of the POM images of stearic acid-doped 5CB microdroplets incubated with 200 pM copper(II) chloride in PBS solution (pH = 8.5). Scale bar = 20 μm. (b) Lasing spectra of stearic acid-doped 5CB microdroplet incubated with 200 pM copper(II) chloride in PBS solution (pH = 8.5) as a function of time. Black dots indicate the position of the WGM selected to monitor the spectral shifting of the droplet lasing emission [174].

7.2 Display technology

Due to the possibility of molecular re-arrangement in NLC random lasers using the external electric field, they started to be implemented in modern type LC displays (LCDs) [118], [182]. By adjusting the NPs concentration, which is embedded in LC-based laser displays, it is feasible to modulate the generated screen brightness [140]. LC-established random lasers doped with CdS semiconductor nanoparticles can be implemented as the optical element into the photonic structures or utilized in the holographic displays [141].

A full-color LC-based laser display was recently shown [183]. The adopted idea was based on inkjet technology, which leads to the construction of a flat-panel, microtemplated pixelated laser array. Single-mode RGB LC-based arrays were highly ordered on the well-defined microtemplates. In this way, the self-emissive panels provided a full-color laser (Figure 29). These results paved the way for constructing the other LC-based light sources, covering the full range of the visible spectrum.

Figure 29:

Full-color lasing combinations from RGB subpixels. (a) Lasing spectra of different combinations of RGB subpixels.

Insets: Corresponding PL images and CIE1931 coordinates calculated from respective spectra. Scale bars = 20 μm. (b) Chromaticity distribution of the lasing peaks on the CIE1931 color diagram. The peaks were extracted from seven spectra in (a). (c) Far-field photograph of the “LCLD” patterns composed of different RGB pixel arrays. The photograph was captured by a built-in digital camera in a cell phone. The scale bar is 150 μm [183].

7.3 Others

Due to the high flexibility of the NLC-based random lasers, they can be utilized as flexible displays or in various military applications, such as identifying friendly or enemy units [144]. Thanks to the NPs utilization, LC-based random lasers can be applied as the new light sources and optical communication technologies [140]. Whispering gallery mode texture based on the liquid crystal can serve as an electrically tunable microresonator [184]. It has been proven [185] that liquid crystal cells can work as dynamic write-and-read media, thanks to the opportunity to create a diffraction pattern. Such a device can be easily adopted as an optically addressed spatial light modulator in technology’s commonly known multimedia field. LC-based random lasers also have applications in spectroscopy and medicine [4]. Finally, a white laser based on the nematic LC was proposed very recently [103]. Interestingly, the presented NLC-based white random lasing intensity was also controlled by low external electric field application values. A temperature-controlled LC-based random laser was recently introduced by Wiersma and co-workers [186]. Such a device can be easily introduced for remote control sensing and in temperature-dependent displays or screens. The report given by W. Huang and co-workers paved the way for using LCs in distributed feedback lasers [82].

Supplementarily, the nematic LCs offer interesting applications in nonlinear optics [187]. DDNLC can also interact as the NLO medium in the manner of passive response (beam self-focusing and self-trapping), similar to the Kerr-medium in all-optical switching [188] phenomena. It can act together efficiently with the active part of the system (optical gain system) and, in this way, impart many advantages (e.g., all-optical control in the disordered systems generating single- and multimode random lasing). NLO-induced light sources or processors can be constructed based on the LC materials as well [188].

Lastly, information encryption and decryption have become alternative fields for the application of NLC [118].