Advances in inorganic and hybrid perovskites for miniaturized lasers

2.1 Crystal structure and tunable bandgap

Three-dimensional (3D) metal halide perovskites with the general chemical formula ABX₃ (Figure 1A) have received extensive attention, where A and B are cations, bounding to an anion X [26]. Among them, the A ion could be organic (e.g. CH₃NH₃⁺ (MA⁺), NH₂CH=NH₂⁺ (FA⁺)) and inorganic cations (e.g. Cs⁺, Rb⁺, etc.). The B ion could be a metallic cation, such as Pb²⁺, Sn²⁺, etc., the X ion could be halogen, such as Cl⁻, Br⁻ and I⁻ [28]. The B together with six X atoms consists of an [BX₆]⁴⁻ octahedron, which then form a 3D octahedral network. In addition, the octahedral units could be cut into layers as 2D perovskites and structurally distorted into 0D perovskites [29]. These can affect the physical properties, such as the exciton binding energy and bandgap.

Figure 1:

The optical properties of perovskites.

(A) The perovskite crystal structure. A is usually the MA, FA or Cs. B is Pb or Sn. X is halogen. Reproduced with permission [26]. Copyright © 2014, Springer Nature. (B) Schematics of the bandgaps of MAPbX₃, FAPbX₃, CsPbX₃ and CsSnX₃. Reproduced with permission [15]. Copyright © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (C) Top: the colloidal solutions in toluene under UV lamp (λ=365 nm). Bottom: the PL spectra of CsPbX₃. Reproduced with permission [27]. Copyright © 2015, American Chemical Society. (D) The typical optical absorption and PL spectra. Reproduced with permission [27]. Copyright © 2015, American Chemical Society. (E) The time-resolved PL decays for all samples shown in (D) except CsPbCl₃. Reproduced with permission [27]. Copyright © 2015, American Chemical Society.

Figure 1B describes the emission spectra from perovskites [15], which can be tuned by halogen stoichiometry and serve as substitute of the A or B site. For example, the bandgaps decrease with the increase of effective ionic radius (R) of A site (R_Cs+<R_MA+<R_FA+) due to the lattice expansion [30]. Furthermore, through compositional modulations from either a mixture of Cl⁻ and Br⁻ or Br⁻ and I⁻, the bandgap energies and emission spectra of all-inorganic CsPbX₃ are readily tunable over the entire visible spectral region of 410−700 nm (Figure 1C) [27]. Aside from the facile tuning of the bandgap, the narrow FWHM (12–42 nm), tunable optical absorption and high PLQY (Figure 1D–E) would be beneficial in developing multicolor or multi-wavelength light-emitting devices, including LEDs, optical amplifiers and lasers.

2.2 Optical gain

There are three essential elements of a laser: gain medium, pumping source and the resonant cavity. The gain medium provides transition energy level that induces specific particles transiting from higher energy levels to a lower energy level to form stimulated emission under the excitation of pump source. The generated photons are confined and provided feedback by the resonant cavity, which would output the spontaneous emission with certain frequency, and subsequently form a laser oscillation above a certain pump intensity. For a simple laser resonator that is a Fabry–Pérot (F–P) cavity, the minimum cavity length must satisfy L_min=−ln(R₁R₂)/G_m [31], where R₁, R₂ and G_m are the end mirror reflectivities and the modal gain related to the material gain, respectively. Therefore, the high optical gain is highly beneficial for miniaturized laser.

The high optical gain indicates the high ability of light amplification and lowers the requirement of strong pump to overcome the optical losses for the lasing behaviors. The net optical gain could be acquired by variable stripe length (VSL) technique in which the emission intensity is recorded as a function of the “stripe length” of the excitation laser spot shaped by a cylindrical mirror (Figure 2A) [18]. The current highest net optical gain is as high as ~ 3200±830 cm⁻¹ from MAPbI₃ perovskite thin film via atomic layer deposition, comparable to that of single-crystalline GaAs [11]. Employing the VSL method, the net optical gains are determined to be ~980, ~580, ~502, ~480, ~450, ~250 and ~125 cm⁻¹ for CsPbBr₃ nanorods [36], CsPbBr₃ nanocrystals [32], CsPbBr₃ nanocuboids [33], FAPbBr₃ nanocrystal [37], CsPbBr₃ nanocrystals [18], MAPbI₃ film [10] and MAPbI₃ nanocrystals [17], respectively. Although there is no definite value, the gain of the Cs-based perovskites is expected to be higher than those of the MA-based ones.

Figure 2:

The optical gain and lifetime of stimulated emission process.

(A) The VSL experiment for estimation of modal net gain. Reproduced with permission [18]. Copyright © 2015, Springer Nature. (B) Gain spectra at 3 ps of MAPbI₃ thin film for various pump fluences. Reproduced with permission [11]. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (C) The TA spectroscopic data of CsPbBr₃ nanocrystals film with two-photon excitation at 1.5 mJ/cm². SE: stimulated emission. Inset: the TA kinetics probed at 527 nm with components of the SE and excited state absorption. Reproduced with permission [32]. Copyright © 2016, American Chemical Society. (D) The TA spectroscopic data of CsPbBr₃ nanocuboids film with two-photon excitation below (top) and above (bottom) threshold. Reproduced with permission [33]. Copyright © 2018, American Chemical Society. (E) The TRPL decay kinetics after photoexcitation with pump fluence below and above the threshold. Reproduced with permission [34]. Copyright © 2015, Springer Nature. (F) The TRPL from a CsPbBr₃ nanowire below (red) and above (blue) the lasing threshold. The emergence of a rapid decay component (<10 ps) suggests rapid carrier quenching due to stimulated emission. Reproduced with permission [35]. Copyright © 2016, National Academy of Sciences of the United States of America.

In addition, the gain lifetime is also investigated by ultrafast transient absorption spectroscopy (TAS). In 2015, Sutherland et al. observed the gain of MAPbI₃ perovskite film via TAS, which could sustain as long as 200 ps and large gain bandwidth at high pump fluence (Figure 2B) [11]. Subsequently, Xu et al. found that the lifetime of optical gain was ~35 ps and that the multiexciton effect plays an important role in the development of optical gain in CsPbBr₃ nanocrystals [32], as depicted in Figure 2C. Meanwhile, Liu et al. used the TAS to study the optical gain of CsPbBr₃ nanocubes under different pump intensities and observed the lifetime of 19.7 ps, which was also confirmed by the output pulse width of laser and ascribed to the electron-hole plasma [33]. Furthermore, the time-resolved photoluminescence (TRPL) measurement is another strong tool to study the stimulated emission. In 2015, Zhu et al. reported that the stimulated emission process was less than 20 ps for both MAPbBr₃ and MAPbI₃ NWs (Figure 2E) and was assigned to a correlated electron-hole plasma, which formed in a broad carrier density range of 10¹⁶~10¹⁹ cm⁻³ [34]. Fu et al. also observed the ultrafast decay of stimulated emission of <20 ps in FAPbI₃ NWs [38]. As shown in Figure 2F, similar rapid decay corresponding to the stimulated emission process of <30 ps was measured by TRPL for CsPbX₃ NWs under strong excitation, and the electron-hole plasma mechanism was responsible for the stimulated emission [35]. More recently, Schlaus et al. further reported that the CsPbBr₃ NW lasing originated from the stimulated emission of an electron-hole plasma by TRPL and transient reflectance [39]. They observed an anomalous blue-shifting of the lasing gain profile with time up to 25 ps and assigned this as a signature for lasing involving plasmon emission. The fast stimulated emission process leads to the low threshold for population inversion, which is faster than the Auger process (nonradiative loss), and then avoids being disturbed by the Auger recombination for the lasing process [40]. Therefore, the excellent optical gain characteristics of perovskites offer great promise for light-emitting devices.

2.3 Exciton binding energy

Understanding the nature of excitons is a critical key in the optical emission performance of semiconductors. An exciton consists of an electron and a hole bounded by Coulomb forces. The Coulomb interactions between the electron and hole could be denoted by the exciton binding energy (E_b), which determines the existence or dissociation of excitons and then influences the exciton recombination dynamics. At room temperature, when the E_b is smaller than the thermal fluctuation energy (K_bT~26 meV), the generated exciton is likely to dissociate into free carriers (electron and hole), which is highly desired for photovoltaic device. On the contrary, the large E_b signifies the existence and stability of excitons, which is more favorable for light-emitting devices due to the efficiently radiative recombination achieved through the excitons at relatively lower carrier densities [41].

Considering the importance of E_b, many experimental and theoretical efforts have been carried out in recent years. For example, temperature-dependent PL [42], [43], [44], [45], optical absorption [46], [47] and magnetoabsorption spectra [48], [49], [50] have been widely used to obtain the values in the perovskite family. MAPbI₃ has attracted extensive attention as a common and important light absorption layer in perovskite solar cells. The initial E_b values of MAPbI₃ have been reported in a wide range of 2–50 meV [41], [49], [50]. Furthermore, the E_b of the analogues of MA-based perovskites has been studied and shown to increase with substitution of halide elements from I, Br to Cl. For example, the E_b values of MAPbCl₃ and MAPbBr₃ have been reported as ranging from ~50–106 meV [51], [52] and 21–100 meV [51], [53], [54], respectively, higher than that of MAPbI₃. Similar trends have been shown in FAPbX₃ (~8.1–110 meV) and CsPbX₃ (~20–72 meV) [41], [47]. The perovskite E_b values from several to hundreds of meV are favorable for high-performance and versatile optoelectronic devices. Especially, the large E_b (over 26 meV) values of (MA/FA)PbBr₃, (MA/FA)PbCl₃ and CsPbX₃ have been demonstrated to stabilize excitons at room temperature and boost the developments of miniaturized coherent photonic sources, such as micro/nano lasers.

3.1 Perovskite F–P lasers

Benefitting from the relatively high difference between the refractive indexes of perovskite gain medium and air, the interfaces can be regarded as high reflectivity mirrors for microcavities. The simplest cavity is the F–P cavity geometry, which consists of only two end mirrors. In 2015, the first perovskite NW lasing from hybrid organic-inorganic perovskites (MAPbX₃) was reported by Zhu et al. [34]. The single-crystal NWs from low-temperature solution processing possessed high optical quality and suitable dimension with long carrier lifetimes and low non-radiative recombination rates, which are ideal for F–P lasing. As shown in Figure 3A, the remarkable performance of perovskite NW lasing are demonstrated under optical pumping (402 nm, ~150 fs, 250 kHz) with small size (length: 8.5 μm), low lasing thresholds (0.22 μJ/cm²), high quality factors (~3600) and broad tunability lasing covering the near-infrared to visible wavelength region (~500–790 nm). Soon after that, Xing et al. also demonstrated optical-pumped (400 nm, 50 fs, 1 kHz), room-temperature MAPbI₃ nanowire lasers (~10 μm length and ~200 nm diameter) with wavelength of 777 nm, low threshold of 11 μJ/cm², FWHM of 1.9 nm and a quality factor of 405 (Figure 3B) [55]. The free-standing perovskite nanowires were synthesized by a two-step vapor phase synthesis method with good optical properties and long electron hole diffusion length. Tunable lasing has also been observed from MAPbBr₃ (551 nm) and MAPbI_xCl_3−x (744 nm) with thresholds of 20 and 60 μJ/cm², respectively. Notably, the relatively short lifetimes for the NWs can be attributed to the special morphology that is not compatible with its intrinsic lattice structure and large surface to volume ratio. To improve the thermal and photo stability of hybrid perovskites, Fu et al. substituted MA with FA to prepare the FAPbX₃ NWs with lengths ranging from ~6–30 μm via low-temperature solution growth [38]. Under 402 nm pulsed laser excitation (150 fs, 250 kHz), the FAPbX₃ NWs were demonstrated to have low lasing thresholds of several μJ/cm² (~6–30 μJ/cm²) and high quality factors of about ~1500–2300. As depicted in Figure 3C, through cation and anion substitutions, the NW lasers can be tuned from 490 to 820 nm, and can fill in the gap of lasing wavelength that are previously unavailable with MA-based perovskites. In particular, both FAPbI₃ and MABr-stabilized FAPbI₃ NW lasers demonstrated better photostability than MAPbI₃ NW lasers due to the better hydrogen bonding between FA and the I anions in the former.

Figure 3:

The perovskite NW lasers.

(A) Top: optical image (left) of single NW with a length of 8.5 μm. The middle and right images show the NW emission below and above lasing threshold (scale bar, 10 μm), respectively. Middle: The NW emission spectra around the lasing threshold. Inset: The integrated emission intensity and FWHM as a function of pump intensity. Bottom: The tunable lasing from mixed perovskite NWs. Reproduced with permission [34]. Copyright © 2015, Springer Nature. (B) Top: The evolution from spontaneous emission to lasing in a typical MAPbI₃ nanowire. Inset: The threshold behavior with a knee at ~11 μJ/cm². Bottom: The lasing spectra of MAPbI₃, MAPbBr₃ and MAPBI_xCl_3−x with the corresponding thresholds. Reproduced with permission [55]. Copyright © 2015, American Chemical Society. (C) Top: The broad wavelength-tunable lasing from single-crystal lead perovskite NWs. Bottom: The lasing stability measurement of FAPbI₃, MABr-stabilized FAPbI₃ and MAPbI₃ NW. Reproduced with permission [38]. Copyright © 2016, American Chemical Society. (D) Left: The power-dependent emission spectra from the CsPbBr₃ NW. Inset: The optical image of NW under high pump intensity. Right: The integrated emission intensity for CsPbBr₃ NW under constant pulsed excitation while exposed to N₂ and air. Reproduced with permission [35]. Copyright © 2016, National Academy of Sciences of the United States of America. (E) Top: The schematic diagram of the aligned CsPbBr₃ NW lasers. Bottom: The real-color optical images of the in-plane directional CsPbBr₃ NWs under optical pumping above the lasing threshold. Reproduced with permission [56]. Copyright © 2018, American Chemical Society. (F) Left: The PL spectra of a 20 μm-long NW obtained with increasing excitation power densities. Inset: The integrated intensity plotted against the power density. Right: The fluorescence images above threshold with different lengths. Reproduced with permission [57]. Copyright © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In addition to the hybrid perovskite NWs, the first all-inorganic CsPbX₃ NW lasers (lengths: ~2–40 μm) were reported by Eaton et al. with low-temperature, solution-phase growth in 2016 [35]. The NWs F–P lasing was pumped by 400 nm pulsed source (150–200 fs, 295 kHz) and displayed an excellent performance with a threshold of ~5 μJ/cm² and maximum quality factor of ~1009±5. Remarkably, the lasing can be maintained for over 1 h (~10⁹ excitation cycles) and persist upon exposure to ambient atmosphere, showing much better performance than the hybrid perovskite NW lasers (Figure 3D). Fu et al. also reported the facile solution growth of single-crystal CsPbX₃ NWs for lasing [58]. They examined CsPbBr₃ NWs with different lengths ranging from 5–21 μm and observed either single or multiple lasing with thresholds ranging from 2.8–9 μJ/cm². For example, under optical excitation with 402 nm, 150 fs and 1 kHz, lasing action at a peak wavelength of ~538 nm with low threshold of ~6.2 and a high quality factor of 2069 (FWHM ~0.26 nm) was achieved from an individual CsPbBr₃ NW with a length of 12 μm and width of 0.7 μm. Using continuous pumping, the NW laser can output stable lasing emissions for longer than 8 h or over 7.2×10⁹ pulse excitations.

Due to the large E_b and sub-wavelength scale of NWs, the strong light–matter interactions of NWs were also revealed by the exciton-polariton model with the reduced oscillator strength [40], [59], [60], [61]. Park et al. synthesized the CsPbX₃ NWs with lengths of ~2–15 μm by using the chemical vapor transport method and realized the lasers with low thresholds of 6, 3 and 7 μJ/cm², with high quality factors of 1200, 1300, and 1400 for CsPbI₃, CsPbBr₃ and CsPbCl₃, respectively [40]. They also discovered the nonclassical and nonidentical spacing of the lasing modes, thereby suggesting that the general photonic model was inadequate in explaining the F–P modes in the CsPbBr₃ nanowires. This can be attributed to the strong light−matter interactions in the NWs using the exciton-polariton model. Zhang et al. also comprehensively investigated strong exciton–photon coupling in micro/nanowires system with low-threshold polariton lasing at room temperature [62]. As shown in Figure 3E, large-area CsPbX₃ NW laser arrays (length: ~10–20 μm) were achieved by vapor growth method with low threshold (~4 μJ/cm²) and high quality factor (~2256) under excitation by 400 nm, 100 fs and 1 kHz laser [56]. The room-temperature Rabi splitting energy determined by coupling strength in these NWs were ~210±13, 146±9 and 103±5 meV for the CsPbCl₃, CsPbBr₃, and CsPbI₃ NWs, respectively. Zhu et al. further realized the CW lasing in CsPbBr₃ NWs with a threshold of ~6 kW/cm² (Figure 3F) [57]. A vacuum Rabi splitting of 0.20±0.03 eV was achieved from the polariton modes near the bottleneck region on the lower polariton branch. These findings suggested that lead halide perovskite NWs have great potential in the production of excellent and low-threshold coherent light sources.

Except for the NWs, other nanostructures of perovskites, such as the micro/nanorods and cubes were also naturally F–P cavities with optical gain. Zhou et al. realized the effective F–P cavities in CsPbX₃ micro/nanorods with lengths ranging from ~2–20 μm with low thresholds of (~14.1 μJ/cm²), high quality factor (~3500) and tunable lasing wavelength (428–668 nm) under 360 nm laser with 100 fs and 1 kHz (Figure 4A) [63]. Subsequently, Wang et al. reported the multiphoton-pumped lasing from CsPbX₃ nanorods with thresholds of ~0.6 and 1.7 mJ/cm² under excitations of 800 and 1200 nm with 80 fs and 1 kHz [65]. Meanwhile, Hu et al. demonstrated F–P lasing in CsPbBr₃ microcubes with high quality factor of 1150 under optical excitation (800 nm, 35 fs, 1 kHz), as described in Figure 4B [64]. The microcube cavities were synthesized by a solution process with a side length of ~500 nm; tunable ASE and enhanced stability were observed. Further, Liu et al. succeeded in reducing the laser size to subwavelength scale in three dimensions using an individual CsPbBr₃ perovskite nanocuboid with only ~400 nm³, as depicted in Figure 4C [33]. Although such a small size can introduce relatively large optical losses, the single-mode F–P lasing at room temperature has been achieved with low laser thresholds of 40.2 and 374 μJ/cm² under 400 and 800 nm excitation, respectively, with the corresponding high quality factors of 2075 and 1859. The physical volume of the CsPbBr₃ nanocuboid laser is only ~0.49 λ³. The 3D subwavelength size, excellent stable lasing performance, frequency up-conversion ability and temperature-insensitive properties of nanocuboids were also demonstrated, which may lead to a miniaturized platform for nanolasers and integrated on-chip photonic devices in the nanoscale. Moreover, Mi et al. and Yang et al. demonstrated the F–P lasing in MAPbBr₃ (edge length ~8 μm) and CsPbI₃ (bottom side ~4–10 μm) triangular pyramid with low thresholds (~92 μJ/cm², 53.15 μJ/cm² at 223 K) and narrow FWHM (~0.63 nm, <0.5 nm), respectively [66], [67].

Figure 4:

The Perovskite microrod and microcube lasers.

(A) Left: The triangular rod emission spectra around the lasing threshold. Inset: The optical images of a single triangular rod. Right: The wavelength-tunable lasing at room temperature from the CsPbX₃ nanorod lasers. Reproduced with permission [63]. Copyright © 2017, American Chemical Society. (B) The intensity-dependent emission spectra from CsPbBr₃ microcubes around the lasing threshold. Inset: A photograph of a single CsPbBr₃ microcube above the lasing threshold. Reproduced with permission [64]. Copyright © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (C) Left: The SEM image of the CsPbBr₃ nanocuboids. The scale bar is 500 nm. Middle: Schematics of the crystalline structure (below) and stand-wave in F−P cavity (upper). Right: The pump intensity-dependent emission spectra from a single CsPbBr₃ nanocuboid under two-photon excitation. Reproduced with permission [33]. Copyright © 2018, American Chemical Society.

3.2 Perovskite WGM lasers

Different from the F–P cavity between two mirrors, the WGM laser is another type of cavity in which the light propagates as a ring to form travelling wave amplification. The interface between the gain medium and air can provide total internal reflection due to the large difference between their refractive indexes. In 2014, Zhang et al. first demonstrated the perovskite WGM laser from hybrid organic-inorganic perovskite MAPbI_3−aX_a nanoplatelets, which they fabricated using a two-step vapor-phase deposition method [13]. As shown in Figure 5A, the triangular or hexagonal shapes can naturally serve as WGM cavities. A typical MAPbI₃ (MAPbI_3−aX_a) triangular (hexagonal) nanoplatelet with a thickness of ~150 nm and an edge length of ~32 μm realized the near-infrared lasing with a threshold of ~37 μJ/cm² (~128 μJ/cm²), quality factor of ~650 (~900), and FWHM of ~1.2 nm (~0.9 nm) when they were pumped by a 400 nm, 50 fs and 1 kHz laser. Furthermore, the output lasing mode can be tuned to red-shift by different edge lengths from 28 to 47 μm owing to the intrinsic self-absorption of the excitons. Most remarkably, the WGM nanolaser can be easily integrated onto conductive platforms (Si, Au, ITO and so forth). These results suggested the potential of perovskite WGM nanolasers as suitable on-chip integrated laser sources.

Figure 5:

The perovskite WGM lasers.

(A) Top: The schematic of the hexagonal NP laser. Bottom: The lasing spectrum from NP. Insets: The optical images of NPs. Right: The lasing spectra of the perovskite NP laser devices on mica, Si, ITO and Au from the bottom to top panels. Reproduced with permission [13]. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (B) Left: The emission spectra of a square microdisk with pump intensity. Inset: The locally amplified spectra with distinct blue shift. Right: The optical images of a microdisk under different pump intensity. Reproduced with permission [68]. Copyright © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (C) Left: The SEM image of the big “cross” in MAPbBr₃ microrod. The area under the yellow dashed rectangular box is the optical pumping area. Right: The laser spectrum and fitted FWHM at the threshold. Reproduced with permission [69]. Copyright ©2017, Royal Society of Chemistry. (D) Left: The lasing spectra and emission images of individual CsPbX₃ perovskite nanoplatelets with different halide ions. Right: The zoom-in spectrum of a lasing mode of CsPbBr_xI_3−x with FWHM of 0.14 nm. Reproduced with permission [47]. Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (E) Left: The schematic of an individual CsPbBr₃ microsphere on silicon substrate pumped by a 400 nm laser excitation. Right: The excitation power-dependent lasing spectra from a single CsPbBr₃ microsphere. Inset: The PL image of microsphere above the lasing threshold. Reproduced with permission [70]. Copyright © 2017, American Chemical Society.

In contrast, Liao et al. used the one-step self-assembly method and reported the square microdisk laser from MAPbBr₃ with a side length of ~1–10 μm and thickness of 0.1–0.25 [68]. As shown in Figure 5B, single-mode lasing at 557.5 nm was achieved in a 2.0×2.0×0.6 μm³ square MD with a threshold of ~3.6 μJ/cm² and quality factor of 430, under excitation source of 400 nm, 150 fs and 1 kHz. Based on the side length of the MDs, the lasing mode can be tuned from single- to multi-mode. Figure 5C displays the WGM laser in the cross-sections of the MAPbBr₃ microrods with widths ranging from a few hundred nanometers to a few microns [69]. With pulsed excitation (400 nm, 100 fs, 1 kHz), the smallest lasing FWHM at the threshold is around 0.1 nm, giving a quality factor over 5000. In contrast, due to much better stability than hybrid perovskites, the all-inorganic perovskite CsPbX₃ has gained great research attention as well. In 2016, Zhang et al. prepared high-quality, single-crystalline CsPbX₃ NPs by using the vapor phase van der Waals epitaxy method and first demonstrated the all-inorganic WGM laser [47]. The laser threshold was as low as ~2.0 μJ/cm² under 400 nm excitation with 50 fs and 1 kHz. As shown in Figure 5D, the lasing emission can cover the whole visible regime and the lasing FWHM was narrowed down to ~0.14 nm. The excellent lasing performance can further promote the nanophotonic applications of the WGM.

The optical losses are inevitable due to the field leakage from the cavity facet and edge scattering, which can induce relatively high pump intensity and low quality factor. Changing the types of WGM microcavities is an effective method to improve lasing performance. Similar with NPs, spheres can also naturally form the WGM cavity. Meanwhile, Tang et al. prepared CsPbX₃ submicron spheres (~0.2–10 μm) by dual-source chemical vapor deposition method and realized tunable single-mode lasing from 425 to 715 nm under optical excitation of 400 nm, 40 fs and 1 kHz, as shown in Figure 5E [70]. The lasing was demonstrated in an individual sphere with a diameter of around 1.0 μm and showed a low threshold of ~0.42 μJ/cm² and a high quality factor of ~6100 with narrow FWHM (~0.09 nm). They further improved the quality factor to 15,000 with FWHM of 0.037 nm. They concluded that the ultrahigh quality factor can be attributed to the regular spherical structure with smooth surface, the high gain and high spontaneous emission coupling efficiency [71]. These high-performance perovskite WGM lasers have huge potential to pioneer a new field of ultracompact, low-threshold and high quality factor laser sources for photonic integrated systems.

3.3 Perovskite random laser

Unlike regular lasers, random lasers do not rely on additional resonators, thus ensuring low production cost and simple technological requirement. In a random laser, the optical cavity is absent, but multiple scattering between particles in the disordered gain medium can keep the photons from travelling long enough for the efficient amplification. Figure 6A presented a schematic diagram of the random laser principle [72]. In 2014, Dhanker et al. first reported a random laser under an optical pump (355 nm, 0.8 ns, 1 kHz) in the MAPbI₃ perovskite microcrystal networks [73]. The laser threshold was less than 200 μJ/cm² and the FWHM was less than 0.5 nm (Figure 6B). Moreover, the spectral mode was unevenly distributed under different power excitations and large areas of microcrystals were excited. In the same year, Kao et al. also observed random lasers due to polycrystalline boundaries scattering in the MAPbI₃ perovskite thin film synthesized by the solution [77].

Figure 6:

The perovskite random laser.

(A) The schematic diagram of the random laser principle. Reproduced with permission [72]. Copyright © 2000, Springer Nature. (B) Left: The emission spectra collected below, near and above lasing threshold. Right: The micro-PL images showing the spatial distribution of the emission at a certain pump intensity. Reproduced with permission [73]. Copyright © 2014, AIP Publishing. (C) Left: The tunable ASE via compositional modulation. Right: The random lasing from CsPb(Br/Cl)₃ nanocrystal film. Inset: The path-length distribution averaged over 256 pump laser shots. Reproduced with permission [18]. Copyright © 2015, Springer Nature. (D) Random laser from the CsPbBr₃ QDs/A-SiO₂. Inset: The isolation effect of QDs on the silica sphere surface. Reproduced with permission [74]. Copyright © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (E) The schematic illustration of the excitation and emission of a flexible thin film perovskite laser at a bending condition. Reproduced with permission [75]. Copyright © 2019, American Chemical Society. (F) The intensity-dependent emission spectra of CsPbBr₃ QDs/TiO₂. Reproduced with permission [76]. Copyright © 2019, Optical Society of America Publishing.

In 2016, Liu et al. observed the random laser in self-assembled MAPbBr₃ perovskite nanoparticles with a threshold of 60 μJ/cm² and quality factor of 500 [78]. When the excited position changed slightly, all laser characteristics changed correspondingly, thus confirming that the laser was formed by the scattering between internal nanoparticles rather than the overall boundaries. Further theoretical simulation and experimental results also verified this gain source of multiple scattering. Subsequently, Shi et al. observed random lasers in MAPbI₃ films with a threshold of 102 μJ/cm², thickness of 450 nm and particle size of ~200 nm; the lasing mode of the MAPbI₃ films changed when detected at different angles [79]. This can be attributed to the high optical gain and strong multiple scattering provided by the polycrystalline grain boundaries.

Compared to the hybrid organic-inorganic perovskite, the CsPbX₃ of bulk materials has been studied as early as 1958 [80]. Meanwhile, the development of all-inorganic CsPbX₃ perovskite colloidal quantum dots is relatively later than this. In 2015, Protesescu et al. synthesized for the first time some high-quality CsPbX₃ quantum dots [27] with a quantum efficiency of up to 90% and a spectrum covering around 410–700 nm. Yakunin et al. first realized the all-inorganic CsPbX₃ random laser with narrow line width (0.14 nm) and tunable ASE spectra (440–700 nm) from perovskite nanocrystal thin film under laser excitation of 400 nm, 100 fs and 1 kHz (Figure 6C) [18]. The optical gain coefficient of the perovskite CsPbBr₃ was as high as 450±30 cm⁻¹, and its ASE threshold was less than 5 μJ/cm². Furthermore, the lasing mode was fully stochastic due to the unique and irreproducible path. In 2017, Li et al. reported the random laser formed by amino-mediated anchoring perovskite quantum dots on the surface of monodisperse SiO₂ spheres by one-step synthesis (Figure 6D) [74]. By dispersing the quantum dots on the SiO₂ sphere, the “aggregation induced quenching” of the quantum dots was effectively suppressed and their spatial distribution on the SiO₂ sphere was regulated to form a suitable light scattering loop. This led to the formation of a low threshold (~40 μJ/cm²) and highly stable random emission under the optical pump (400 nm, 100 fs, 1 kHz).

Meanwhile, the use of disordered gain materials has emerged as a strategy for fabricating random laser sources. Recently, Wang et al. demonstrated a random laser in curved perovskite MAPbBr₃ thin films on flexible substrates by using the solvent-engineered method (Figure 6E) [75]. Under 355 nm excitation with 0.5 ns and 1 kHz, the lasing threshold was 2.5 mJ/cm² with emission FWHM of ~1.8 nm, which can be further reduced by increasing the local curvature of the film arising from the variation of the scattering strengths of the bent thin film. Meanwhile, the curved perovskite lasers can be extremely robust with repeated deformations. Moreover, Tang et al. reported a successful room-temperature up-conversion random lasing by distributing uniformly the CsPbBr₃ QDs into TiO₂ nanotubes, as depicted in Figure 6F [76]. Under 800 nm and 35 fs laser excitation, the ASE and random lasing showed thresholds of 2.33 mJ/cm² and 9.54 mJ/cm², respectively. Due to the small diameter of the nanotubes, the lasing emission of the CsPbBr₃/TiO₂ film was assigned to the random laser with FWHM of ~0.49 nm. These results indicated that the random lasers can be expected to be adopted in practical applications in durable speckle-free light sources for various applications, including spectroscopy, bioimaging, medical diagnosis and illumination.

4.1 Perovskite VCSEL and DFB lasers

In 2014, Deschler et al. demonstrated the first perovskite F–P mode vertical laser based on DBR [81]. The laser cavity consisted of a middle gain layer of hybrid perovskite MAPbI_3−xCl_x thin film with 500 nm thickness, a top layer of PMMA and Au, and a bottom layer of DBR (Figure 7A). Under the excitation of 532 nm and 0.4 ns pulse width, the lasing threshold was as low as 0.2 μJ/pulse. Notably, the luminescence in the perovskite MAPbI_3−xCl_x film was ascribed to the bimolecular-free carrier electron−hole recombination by TAS. Chen et al. succeeded in microfabricating a VCSEL by embedding the perovskite MAPbI₃ thin film (~300 nm) into two high-reflectivity (~99.5%) DBRs [19]. The low threshold (~7.6 μJ/cm²) and high quality factor (~1100) vertical laser was achieved under optical excitation at 532 nm with 340 ps and 1 kHz. Further optimization was carried out by using other lead halide precursors or by changing the organic cation. They subsequently used the FAPbBr₃ and Cs_0.17FA_0.83PbBr₃ as gain layers together with the optimized DBR structures, and good lasing performance was demonstrated with the following characteristics: high quality factors of 1420 and 1350; low thresholds of 18.3 and 13.5 μJ/cm², respectively; and the corresponding stable lifetimes of 20 h and >35 h [85], [86]. Room temperature CW pumped VCSEL-based MAPbBr₃ and MAPbI₃ were reported at pump thresholds of 89 and 13 W/cm², respectively [87], [88]. This finding is a huge step towards achieving more efficient CWs and electric pumped perovskite lasers.

Figure 7:

Perovskite VCSEL and DFB lasers.

(A) The emission spectra of MAPbI_3−xCl_x with different pump fluence. Inset: The vertical microcavity structure. Reproduced with permission [81]. Copyright © 2014, American Chemical Society. (B) Left: The structure of the VCSEL with the CsPbBr₃ nanocrystals. Right: The blue (up) and red (bottom) laser emissions from the CsPb(Br/Cl)₃ and CsPb(I/Br)₃ VCSEL, respectively. Reproduced with permission [82]. Copyright © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (C) The CsPbBr₃ thin film VCSEL with a FWHM of 0.07 nm. Inset: The far-field beam characteristics of the VCSEL above threshold. Reproduced with permission [83]. Copyright © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (D) Left: The schematic diagram of a DFB cavity. Right: The emission fluence-dependent PL spectra of the MAPbI₃ DFB cavity. Reproduced with permission [84]. Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (E) Left: The structure of the perovskite CsPbBr₃ DFB laser. Right: The laser spectrum from the DFB structure with a line width of 0.14 nm. Inset: The photograph of far-field emission profile. Reproduced with permission [83]. Copyright © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In 2017, Wang et al. realized the first all-inorganic perovskite VCSEL [82]. As shown in Figure 7B, the high reflective dielectric mirror was composed of 25 pairs of SiO₂/TiO₂ quarter-wave layers, and the all inorganic perovskite CsPbBr₃ nanocrystals gain layer was sandwiched between two DBRs. Under the excitation of the fs laser (400 nm, 100 fs, 1 kHz), the thresholds of the F–P mode laser were as low as ~19, ~9 and ~25.5 μJ/cm² for the stable red, green and blue lasing, respectively. Recently, Pourdavoud et al. implemented the CsPbBr₃ thin film featuring large crystals with micrometer lateral extension as a gain layer to demonstrate the high performance VCSEL [83]. The threshold was only ~2.2 μJ/cm² and the FWHM was narrowed down to 0.07 nm (Figure 7C). In contrast to the conventional photon laser, the lasing in a strong coupling cavity, such as VCSEL, could occur without the need for population inversion due to fact that the coherent light emission originated from the steady-state leakage of an exciton-polariton condensate [89]. Exciton-polaritons are bosonic quasiparticles that exist inside semiconductor microcavities; these consist of a superposition of an exciton and a cavity photon rather than the independent eigenmodes of the system. Su et al. reported the experimental realization of room-temperature polariton lasing with threshold of ~12 μJ/cm² based on an epitaxy-free CsPbCl₃ perovskite nanoplatelet microcavity [90]. A 373 nm-thick nanoplatelet was embedded in the bottom and top DBRs made of 13 and 7 HfO₂/SiO₂ pairs, respectively. The superlinear power dependence, macroscopic ground-state occupation, blueshift of the ground-state emission, narrowing of the line width and the buildup of the long-range spatial coherence verified the polariton lasing. They also demonstrated the capability of long-range nonresonantly excited polariton condensate flow at room temperature in a CsPbBr₃ perovskite microwire microcavity [91]. A microwire with a length of ~30 μm, a thickness of ~120 nm and a width of ~2 μm was embedded in two DBRs to form the microvavity. Under 400 nm laser (100 fs and 1 kHz) excitation, the threshold of the polariton lasing was as low as 0.8 μJ/cm². Such strong coupling polariton lasing can serve as ideal candidates for the commercialization of electrically driven laser devices.

In 2016, Saliba et al. first introduced Bragg gratings as reflective devices in perovskite lasers [84]. They deposited a 120 nm-thick organic-inorganic perovskite MAPbI₃ onto two gratings with different periods (380–420 nm) and achieved low threshold lasing (~0.32–2.11 μJ/cm²) and tunable lasing (770–793 nm) under the excitation of a 532 nm, 1 ns, and 1 kHz laser. The DFB single-mode laser provides a good idea for the development of all-electrically pumped perovskite lasers, thus demonstrating the potential commercial application of perovskite lasers. In 2017, Jia et al. first realized the CW-pumped MAPbI₃ DFB laser with threshold 17 kW/cm² for over an hour and at temperatures below the tetragonal-to-orthorhombic phase transition (T<160 K) [92]. They deposited the MAPbI₃ film onto a period grating etched into an alumina layer on a high-thermal-conductivity sapphire substrate to avoid the thermal damage. They reported the continuous gain originating from tetragonal-phase inclusions, which were photogenerated by the pump within the bulk orthorhombic host matrix on a submicrosecond timescale. Soon, Li et al. reported room-temperature CW lasing action in a surface-emitting MAPbI₃ perovskite DFB laser on a silicon substrate [88]. With the thermal nanoimprint lithography technique, the perovskite thin film was directly patterned into a high-Q cavity with large mode confinement and enhanced emission intensity. Consequently, the ultralow lasing threshold of 13 W/cm² with narrow FWHM of ~0.7 nm was achieved, thus demonstrating a major step toward electrically pumped perovskite lasing. In 2019, Pourdavoud et al. used a DFB structure, which was formed by a recrystallized perovskite CsPbBr₃ thin film, to achieve a low threshold (7.2 μJ/cm²) and narrow FWHM (0.14 nm) laser emission (Figure 7E) [83]. PC refers to an artificial periodic dielectric structure with photonic band gap characteristics and has a wavelength selection function. A typical 1D PC uses the DBR or DFB structure to hold a perovskite in the middle in order to form a simple F–P resonant cavity, thereby achieving a single-mode and tunable laser. In 2016, Chen et al. embedded hybrid perovskite MAPbI₃ thin films, which they synthesized via solution-processed method into the 2D PC [20]. When the pump density reached 68.5±3.0 μJ/cm², a single-mode lasing was achieved at 788.1 nm with a FWHM of 0.24 nm and power conversion efficiency of 13.8%±0.8%. Furthermore, the single-mode laser output can be tuned by changing the PC pitch range. Meanwhile, Schünemann et al. reported 3D PC DFB laser in perovskite MAPbBr₃, which they prepared by using the all-solution process [93]. Under the excitation of pulsed laser (532 nm, 0.5 ns, 2 kHz), they achieved highly stable lasing with a threshold of 1.6 mJ/cm² and FWHM of 0.15 nm.

4.2 Microsphere and microcapillary for perovskite lasers

Apart from the vertical cavity, the perovskites can be coupled with sphere and microcapillary as the WGM laser resonator. In 2014, Sutherland et al. deposited a thin layer of MAPbI₃ film onto a silica sphere via the atomic-layer deposition technique and successfully achieved WGM lasing at 80 K (Figure 8A) [17]. Under 355 nm excitation with 2 ns and 100 Hz, the threshold of ~75 μJ/cm² and quality factor of ~1000 were achieved. Similarly, Yakunin et al. demonstrated the WGM laser from CsPbX₃ nanocrystal with narrow FWHM (~0.15–0.20 nm) using silica microspheres with a diameter of ~15 μm as high-finesse resonators (Figure 8B) [18]. The feasibility of lasing in perovskite films indicates the high-quality optical gain in perovskites. Almost simultaneously, Wang et al. also observed the WGM laser from the CsPbBr₃ QDs, as depicted in Figure 8C [94]. After the evaporation of the solvent, they filled the CsPbBr₃ QDs into a capillary tube with solid film around the inner wall. The capillary tube with the inner diameter of ~50 μm provided optical feedback, after which multi-mode WGM lasing was demonstrated with threshold of ~11 mJ/cm² under excitation at 400 nm with 5 ns and 20 Hz. They also observed the low-threshold, wavelength-tunable and stable ASE from the CsPbX₃ perovskite QDs, which could be ascribed to the biexciton emission.

Figure 8:

The perovskite microsphere and microcapillary laser.

(A) Left: The schematic of the laser emission of MAPbI₃ perovskite-coated microsphere. Right: The spectra of the MAPbI₃ perovskite-coated microsphere laser emission at 80 K for three different pump fluences. Reproduced with permission [17]. Copyright © 2014, American Chemical Society. (B) The emission spectra with different pump intensity. Inset: A microsphere resonator covered by a film of the CsPbBr₃ nanocrystals. Reproduced with permission [18]. Copyright © 2015, Springer Nature. (C) The pump intensity-dependent PL spectra from the CsPbBr₃ QDs microcapillary laser. Left inset: The suddenly increasing emission at the pump threshold. Right inset: The optical images of the CsPbBr₃ QDs capillary tube cavity below and above the lasing threshold. Reproduced with permission [94]. Copyright © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (D) The two-photon-pumped PL spectra of the CsPbBr₃ nanocrystals microcapillary laser. Inset: The emission intensity as a function of excitation fluence and threshold can be calculated to be ~900 μJ/cm². Reproduced with permission [32]. Copyright © 2016, American Chemical Society. (E) Left: The lasing image of a cylindrical microtubule incorporated with CsPbBr₃/SiO₂ QDs. Inset: The schematic of WGM cavity with the micro-ring resonator. Right: The emission spectra with increasing pump intensity. Reproduced with permission [95]. Copyright © 2019, Optical Society of America Publishing. (F) Intensity-dependent emission spectra from CsPbBr₃/CdS. Inset: The 3D schematic representation of core/shell structured CsPbBr₃/CdS QDs. Reproduced with permission [96]. Copyright © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Xu et al. incorporated the perovskite CsPbBr₃ nanocrystals into optical resonators of glass microcapillary tubes with diameter of ~60 μm (Figure 8D) [32]. Under 800 nm fs excitation, the lasing threshold was around 900 μJ/cm² with FWHM of ~0.15–0.3 nm, indicating the quality factors of lasing modes to be in the range of 1700−3500. Such high quality factors could be ascribed to the high refractive index contrast between the glass tube (1.46) and nanocrystal solids (~2.30), which can well-confine the light emission from nanocrystals supporting the WGM oscillation. Liu et al. embedded the CsPbBr₃ QDs into the sub-micro silica sphere to improve the stability and observed the compounded mode of random and WGM under 800 nm excitation with 35 fs and 1 kHz [95]. Then, they incorporated the CsPbBr₃-SiO₂ spheres into a microtubule with a diameter of ~40 μm, as shown in Figure 8E. The multi-mode lasing with a threshold of 430 μJ/cm² and FWHM of ~0.35 nm was sustained for over 140 min under continuous excitation. The lasing mode was analyzed by the WGM model. Furthermore, they also coupled the FAPbBr₃ nanocrystals into a hollow capillary tube and demonstrated the WGM laser with a low threshold of ~310 μJ/cm² under same pump condition [37]. Kurahashi et al. reported the MAPbBr₃ WGM laser in microcapillary with different inner diameters of 2–40 μm [97]. Under excitation at 397 nm with 200 fs and 1 kHz, the mode number and the lasing threshold fluence decreased with reduced inner diameter. Notably, the single mode was realized at a pump threshold at ~4.7 μJ/cm² in a microcapillary cavity with a diameter of 2 μm.

Recently, Tang et al. succeeded in preparing a single core/shell structured perovskite semiconductor QDs with CsPbBr₃/CdS, which exhibited ultrahigh chemical stability and nonblinking photoluminescence with high PLQY due to the reduced electronic traps within the core/shell structure [96]. Lasing action was demonstrated from the core/shell perovskite QDs in a cylindrical microcapillary with a threshold of 87 μJ/cm² and FWHM of ~0.44 nm. These flexible and changeable perovskites lasing can simplify the requirements of microcavities and enable the perovskite to be used in versatile photoelectric integrated devices.

4.3 Perovskite laser array

The high-density laser arrays have great potential for compact on-chip optoelectronic circuit integration. The nanomaterial arrays highly depend on the nanofabrication and template technology. In 2016, Liu et al. utilized a novel bottom-up growth technology for synthesizing high-quality patterned perovskite arrays on Si with pre-patterned single layer hexagonal boron nitride (h-BN) buffer layer [98]. The fabricated MAPbI₃ perovskite microplatelet arrays displayed a hexagon with an edge length of ~15 μm and a natural WGM cavity. Under optical pump by 400 nm laser excitation (~50 fs, 1 kHz), the multi-mode WGM lasing was realized with a pump threshold of ~11 μJ/cm² and FWHM of ~0.64 nm, corresponding to a quality factor of 1210. Notably, modal selectivity for single-mode lasing could be achieved with different cavity sizes or by simply breaking the structural symmetry of the cavity through designing the pattern.

Wang et al. successfully achieved high density nanolaser arrays by positioning a MAPbBr₃ perovskite microwire onto a silicon grating with threshold of ~6 μJ/cm² [99]. The transverse single-crystalline microwire were divided by a silicon grating into tiny subunit with the smallest subunit period of 800 nm. Only the suspended parts could hold high quality resonances and generate laser emissions. They further transferred the MAPbBr₃ nanoribbon onto a gold grating and successfully achieved laser arrays with threshold of 2.5 μJ/cm², as shown in Figure 9B [100]. Notably, the smallest size of the gap for light confinement is optimized to around 350−400nm.

Figure 9:

The perovskite laser array.

(A) Left: The SEM image of the as-prepared MAPbX₃ platelet array. Right: The intensity-dependent emission spectra of microplatelet laser. Inset: The 2D pseudo color plot of emission spectra under different pump fluence. Reproduced with permission [98]. Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (B) Left: The top-view SEM image of the nanoribbon onto a gold grating; the scale bars were 50 and 5 μm, respectively. Right: The laser spectra from nanoribbons with different pumping densities. Reproduced with permission [100]. Copyright © 2017, American Chemical Society. (C) The MAPbBr₃ perovskite nanowire laser emission and laser arrays (insets). Reproduced with permission [101]. Copyright © 2017, American Chemical Society. (D) The intensity-dependent emission spectra of a microdisk laser. Inset: The optical image of the microdisk lasers above the lasing threshold. Reproduced with permission [102]. Copyright © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (E) Left: The schematic of laser printing perovskite microdisks. Middle: The close-up false-color SEM image (top) as well as a photograph of a 1×1 cm² array of perovskite microlasers. Right: The lasing spectra of the MAPbI₃ microdisk with diameter of 3.8 μm. Inset: The optical image of microdisk above the threshold. Reproduced with permission [16]. Copyright © 2019, American Chemical Society

Liu et al. developed a large-scale perovskite MAPbX₃ nanowire array by combining the “top-down” fabricated polydimethylsiloxane rectangular groove template with the “bottom-up” solution self-assembly [101]. As exhibited in Figure 9C, not only the dimensions of individual NWs (width 460−2500 nm; height 80−1000 nm, and length 10−50 μm) but also the interwire distances could be precisely adjusted. Under the excitation of pulsed laser with wavelength of 400 nm (150 fs, 1 kHz), the nanowire laser was achieved with almost identical optical modes and similarly low-lasing thresholds from 9.5 to 12.7 μJ/cm², together with good photostability of an operation duration exceeding 4×10⁷ laser pulses. He et al. adopted the polydimethylsiloxane cylindrical-hole-template confined solution-processed growth method to obtain the large-area CsPbX₃ microdisk laser arrays (up to 1×1 cm²) [102]. The micodisks were rectangle shapes with a side length of 2.5±0.3 μm and a thickness of 0.6±0.2 μm, supporting WGM cavity in Figure 9D. The lasing output wavelengths could be well tuned in the blue-green region from 425 to 540 nm by adjusting their compositions of CsPbCl_3−xBr_x from x=0 to 3, respectively, with similarly low lasing thresholds of 3–12 μJ/cm². Furthermore, they integrated the multicolored lasing arrays on the same substrate.

Duan et al. also fabricated well-controlled and uniform MAPbBr₃ microdisks by a simple solution-based one-step anti-solvent method [103]. The size and mode numbers of the self-assembled microdisk cavities could be well adjusted by modifying the sizes of the SiO₂ microdisks on the substrate or changing the concentration of perovskite precursor. Recently, Zhizhchenko et al. proposed a laser-printed perovskite MAPbBr_xI_y microdisks by laser ablation of glass films directly using a donut-shaped femtosecond laser beam, as shown in Figure 9E [16]. The prepared microdisks were 760 nm thickness and diameters ranging from 2 to 9 μm precisely controlled by a topological charge of the vortex beam. The tunable lasing from different compositions could be realized from 500 to 800 nm under optical excitation at a wavelength of 532 nm (0.56 ns, 1.5 kHz) with a quality factor up to 5500. These results would further push the potential application of these perovskite-based lasers in fully integrated optoelectronic circuitry.

4.4 2D perovskite laser

The inherent poor stability of 3D perovskites resulting from their high moisture and heat sensitivity impedes their commercialization [104]. Perovskites with reduced dimensionality can promote the radiative recombination, improve the stability and enhance the exciton binding energy and PLQYs [28], [105], [106], [107]. Recently, scientists have demonstrated the great potential of quantum well (QW) quasi-2D Ruddlesden–Popper perovskites (RPPs) to enhance the stability and exciton binding energy by adopting the chemical formula of R₂M_n−1Pb_nX_3n+1 with R as a long-chain ligand [106]. As shown in Figure 10A, the inorganic lead-halide layers are sandwiched between two hydrophobic organic layers formed by R bulky cations [108]. The large dielectric constant difference between the two layers can lead to a high exciton binding energy, thus indicating stable excitons at room temperature.

Figure 10:

The 2D perovskite laser.

(A) The crystal structures of the 2D perovskites with n from 1 to 4. Reproduced with permission [108]. Copyright © 2016, American Chemical Society. (B) The tunable ASE spectra from the (NMA)₂(FA)Pb₂Br_yI_7−y thin films. Reproduced with permission [109]. Copyright © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (C) Left: The μ-PL image recorded above lasing threshold with distinct spatial interference patterns. Right: The intensity-dependent PL spectra recorded from microring 8 labeled in Left. Inset: The lasing spectrum above the threshold with a Gaussian fitting. Reproduced with permission [110]. Copyright © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (D) The pump-intensity-dependent PL spectra of a single MPL. Left inset: The optical image with scale bar of 5 μm. Right inset: The optical image of MPL above the lasing threshold. Reproduced with permission [111]. Copyright © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (E) The evolution of spontaneous emission to normalized lasing spectra under various excitation fluences for n=3 RPP. Right panels: The corresponding dark filed images below and above the lasing threshold and simulated electric field distribution (|E|²) from the top to bottom panels, respectively. Scale bar: 5 μm. Reproduced with permission [112]. Copyright © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In 2018, Li et al. reported for the first time the intrinsic gain properties of 2D-RPP thin films by demonstrating the ASE behavior in a cavity-free configuration [109]. The self-organized multiple QWs 2D-RPP thin films were prepared by a solution process with gain coefficients as high as >300 cm⁻¹. Under excitation at 400 nm with 150 fs and 1 kHz, room-temperature ASE was achieved at a low pump threshold of ~8.5 μJ/cm² from 75 nm-thick quasi-2D (NMA)₂(FA)Pb₂Br₇ thin film. The tunable ASE wavelengths from visible to near-infrared spectral range (530–810 nm) by adjusting the halide compositions were presented in Figure 10B. Remarkably, an energy cascade between the QWs in the quasi-2D perovskites was revealed, which enabled an ultrafast energy transfer process from higher energy-bandgap QWs to lower energy-bandgap QWs to build up the population inversion.

Zhang et al. fabricated high-density,large-area microring arrays of 2D (BA)₂(MA)_n−1Pb_nBr_3n+1 RPPs for WGM cavities by using a facile PDMS template confined solution-processed method [110]. The microring exhibited smooth outer surfaces and regular cross-sections with a width of ~2.5 μm and height of ~1.6 μm along the diameter of 19 μm. As a consequence, a multi-mode WGM laser was observed with high-quality factor of ~2600 and low threshold of ~12.2 μJ/cm² under 400 nm, 150 fs and 1k Hz pulsed excitation, as shown in Figure 10C. Using a similar method, they further reported high-density large-areas NW arrays in 2D-RPPs of (BA)₂(FA)_n−1Pb_nBr_3n+1 with width of ~2 μm, height of ~420 nm and length of 17 μm [113]. The NW laser arrays exhibited high photo-stability, high quality factor (~1800) with FWHM of 0.3 nm and excitation threshold of 27.2 μJ/cm² under a 400 nm laser pump.

Raghavan et al. reported the growth of high-quality, millimeter-sized single crystals of (BA)₂(MA)_n−1Pb_nI_3n+1 by a slow evaporation at a constant-temperature solution growth [114]. Under the 374 nm pulse laser excitation with a 40 MHz and 55 ps, the multi-mode lasers were achieved in the inorganic layers 1, 2 and 3 with low thresholds of 2.85, 3.02 and 3.21 μJ/cm², respectively; FWHM values of 0.22, 0.5 and 0.4 nm, respectively; and corresponding lasing wavelengths of 522.3, 577.6, and 623.6 nm. The lasing modes could be assigned to the possible existence of some cavities or microdomains inside the crystals. Li et al. realized the solution-processed room-temperature microplatelet lasers from multilayered (OA)₂(MA)_n−1Pb_nBr_3n+1 RPPs [111]. Figure 10D showed the multi-mode microplatelet laser with threshold of ~8.5 μJ/cm² and narrowed FWHM of ~0.3–0.6 nm pumped by a 400 nm laser excitation (~150 fs, 1 kHz). Simultaneously, highly stable low-threshold (down to ~7.8 μJ/cm²) linearly polarized lasing was achieved from the microplatelets by adjusting the layers. Combining the femtosecond microarea PL with TAS revealed that the mixed lower-dimensional layers of the RPPs can cause enhanced exciton and photon confinement in the higher-dimensional layers.

Liang et al. reported the lasing and loss mechanisms of homologous 2D (BA)₂(MA)_n−1Pb_nI_3n+1 RPP micron-sized thin flakes, which they mechanically exfoliated from the bulk crystal [112]. Wavelength-tunable lasing was achieved from the large-n RPPs (n ≥ 3) but not from small-n RPPs (n≤2) even down to 78 K. Figure 10E displayed the pump dependent PL spectra of an n=3 RPP microflake (length: ~11.2 μm; width: ~11.1 μm; thickness: ~150 nm) at 78 K, exhibiting an excitation threshold at ~2.6 μJ/cm² pumped by 400 nm laser with 80 fs and 1 kHz. Temperature-dependent and time-resolved PL spectroscopy together with TAS revealed that the non-radiative recombination pathways were mainly induced by the Auger recombination and exciton–phonon interaction as a result of the n-dependent lasing behaviors. These 2D-RPPs possessing improved environmental stability, enhanced exciton confinement and solution processable application-level ability have great potential in the electrically driven semiconductors lasers.