Band 10, Heft 13

Two-dimensional transition metal dichalcogenides (2D-TMDs) are atomically thin semiconductors with a direct bandgap in monolayer thickness, providing ideal platforms for the development of exciton-based optoelectronic devices. Extensive studies on the spectral characteristics of exciton emission have been performed, but spatially resolved optical studies of 2D-TMDs are also critically important because of large variations in the spatial profiles of exciton emissions due to local defects and charge distributions that are intrinsically nonuniform. Because the spatial resolution of conventional optical microscopy and spectroscopy is fundamentally limited by diffraction, near-field optical imaging using apertured or metallic probes has been used to spectrally map the nanoscale profiles of exciton emissions and to study the effects of nanosize local defects and carrier distribution. While these unique approaches have been frequently used, revealing information on the exciton dynamics of 2D-TMDs that is not normally accessible by conventional far-field spectroscopy, a dedicated review of near-field imaging and spectroscopy studies on 2D-TMDs is not available. This review is intended to provide an overview of the current status of near-field optical research on 2D-TMDs and the future direction with regard to developing nanoscale optical imaging and spectroscopy to investigate the exciton characteristics of 2D-TMDs.

A variety of materials such as low dimensional carbon substrates (1D, 2D, and 3D), nanoprisms, nanocubes, proteins, ceramics, and DNA to name a few, have been explored in surface plasmon-coupled emission (SPCE) platform. While these offer new physicochemical insights, investigations have been limited to silver as primary plasmonic material. Although, gold nanoparticles (AuNPs) exhibit robust performance, its intrinsic property to quench the emission from radiating dipoles (at distances < 5 nm) has impeded its utility. Despite the use of metal-dielectric resonances (with Au decorated SiO 2 NPs) and sharp nanotips (from Au nanostars) for dequenching the emission, the enhancements obtained has been less than 200-fold in SPCE platform. To address these long-standing challenges, we demonstrate the utility of gold soret colloids (AuSCs) and photonic crystal-coupled emission (PCCE) platform. The soret nano-assemblies synthesized using adiabatic cooling technique presented integrated hotspots when taken with high refractive index Nd 2 O 3 ‘Huygens sources’. The collective and coherent coupling between localized Mie and delocalized Bragg plasmons (of sorets), dielectric plasmons (of Nd 2 O 3 ), highly confined and intense Bloch surface waves (of PCCE platform) aided in realization of dequenched, as well as amplified > 1500-fold enhancements at the photoplasmonic nanocavity interface, presenting new opportunities for multidisciplinary applications.

Photothermal imaging is useful for detecting individual nanoparticles and obtaining the absorption spectra. This study presents a wide-field photothermal reflectance spectroscopy technique achieved by incorporating a pump beam, a probe beam, and a charge-coupled device (CCD) camera into a commercial microscopic setup. The presented design does not require precise alignment between the pump and the probe beams and enables the observation of numerous individual nanoparticles during image acquisition. Despite the use of a simple imaging processing method, i.e., a four-bucket method using a CCD camera, sufficient sensitivity for the spectral imaging of a single gold nanorod (20 nm diameter and 84 nm length) is demonstrated. Numerous individual nanoparticles within a wide field of view (240 μm × 180 μm) are detected in an image captures at an imaging measurement speed of 0.02 mm 2  min −1 . Furthermore, the proposed photothermal reflectance spectroscopy technique can detect the variation in the absorption peak of the measured spectra depending on the aspect ratio of individual nanoparticles within a spectral resolution of 1 nm.

Improving current injection into r - and m -planes of nanowires (NWs) is essential to realizing efficient GaInN/GaN multiple quantum shell (MQS) NW-based light-emitting diodes (LEDs). Here, we present the effects of different p -GaN shell growth conditions on the emission characteristics of MQS NW-LEDs. Firstly, a comparison between cathodoluminescence (CL) and electroluminescence (EL) spectra indicates that the emission in NW-LEDs originates from the top region of the NWs. By growing thick p -GaN shells, the variable emission peak at around 600 nm and degradation of the light output of the NW-LEDs are elaborated, which is attributable to the localization of current in the c -plane region with various In-rich clusters and deep-level defects. Utilizing a high growth rate of p -GaN shell, an increased r -plane and a reduced c -plane region promote the deposition of indium tin oxide layer over the entire NW. Therefore, the current is effectively injected into both the r - and m -planes of the NW structures. Consequently, the light output and EL peak intensity of the NW-LEDs are enhanced by factors of 4.3 and 13.8, respectively, under an injection current of 100 mA. Furthermore, scanning transmission electron microscope images demonstrate the suppression of dislocations, triangular defects, and stacking faults at the apex of the p -GaN shell with a high growth rate. Therefore, localization of current injection in nonradiative recombination centers near the c -plane was also inhibited. Our results emphasize the possibility of realizing high efficacy in NW-LEDs via optimal p -GaN shell growth conditions, which is quite promising for application in the long-wavelength region.

The reversible insulating-to-conducting phase transition (ICPT) of vanadium dioxide (VO 2 ) makes it a versatile candidate for the implementation of integrated optical devices. In this paper, a bi-functional in-line optical device based on a four-layer stack of PMMA/graphene/VO 2 /graphene deposited on a side-polished fiber (SPF) is proposed. The structure can be employed as an ultra-compact TE modulator or a TM-pass polarizer, operating at 1.55 μm. We show that the ICPT characteristic can be used for polarization-selective mode shaping (PSMS) to manipulate orthogonal modes separately. On the one hand, as an optical modulator, the PSMS is used to modify mode profiles so that the TE mode attenuation is maximized in the off-state (and IL is minimized in the on-state), while the power carried by the TM mode remains unchanged. As a result, a TE modulator with an ultrahigh extinction ratio (ER) of about ER = 165 dB/mm and a very low insertion loss (IL) of IL = 2.3 dB/mm is achieved. On the other hand, the structure can act as a TM-pass polarizer featuring an extremely high polarization extinction ratio (PER) of about PER = 164 dB/mm and a low TM insertion of IL = 3.86 dB/mm. The three-dimensional heat transfer calculation for the ICPT process reveals that the response time of the modulator is in the order of few nanoseconds. Moreover, the required bias voltage of the proposed device is calculated to be as low as 1.1 V. The presented results are promising a key step towards the realization of an integrated high-performance in-line modulator/polarizer.

Dielectric metasurfaces have shown prominent applications in nonlinear optics due to strong field enhancement and low dissipation losses at the nanoscale. Chalcogenide glasses are one of the promising materials for the observation of nonlinear effects thanks to their high intrinsic nonlinearities. Here, we demonstrate, experimentally and theoretically, that significant second harmonic generation (SHG) can be obtained within amorphous Selenium (Se)-based chalcogenide metasurfaces by exploiting the coupling between lattice and particle resonances. We further show that the high-quality factor resonance at the origin of the SHG can be tuned over a wide wavelength range using a simple and versatile fabrication approach. The measured second harmonic intensity is orders of magnitude higher than that from a dewetted Se film consisting of random Se nanoparticles. The achieved conversion efficiency in the resonance region is of the order of 10 −6 which is comparable with direct bandgap materials and at least two orders of magnitude higher than that of conventional plasmonics- and Si-based structures. Fabricated via a simple and scalable technique, these all-dielectric architectures are ideal candidates for the design of flat nonlinear optical components on flexible substrates.

For the widely used vertically pumped (VP) method with a free-space beam, very little pump power is absorbed by the gain materials in microlasers because of the large spatial mismatch of areas between laser modes and free-space pump beams together with small thicknesses of gain materials, resulting in a high pump power threshold. Here, an in-plane-waveguide-pump (IPWP) method with a localized waveguide source is proposed to reduce pump power threshold of perovskite microlasers. Owing to reduced spatial mismatch of areas between laser modes and localized waveguide sources as well as increased absorption distances, the pump power threshold of the IPWP method is decreased to approximately 6% that of the VP method. Moreover, under the same multiple of the pump power threshold, the laser linewidth in the IPWP method is narrowed to approximately 70% that in the VP method. By using the IPWP method, selective pumping two adjacent (separation 2 or 3 μm) parallel-located perovskite microlasers is experimentally demonstrated, and no crosstalk is observed. This IPWP method may have applications in low-energy and high-density microlasers and photonic integrated circuits.

A compact electrical source capable of generating surface plasmon polaritons would represent a crucial step for on-chip plasmonic circuitry. The device fabrication of plasmonic actuator based on Au/SiO 2 /n ++ Si tunnel junction and performance have been reported in [ACS photonics, 2021, 8, 7, 1951–1960]. This work focuses on the underlying mechanisms of electroluminescence. The n-type Si samples were doped with concentrations ranging from 1.6 × 10 15  cm −3 to 1.0 × 10 20  cm −3 . A low voltage of 1.4 V for intense light emission was achieved at the highest concentration. The electrical/spectral characteristics and energy band diagrams calculation show two distinct behaviors indicating two distinct mechanisms of light emission are at work in the heavily doped versus the lightly doped Si. In the heavily doped case, the light output is correlated to tunneling current and the subsequent conversion of surface plasmons to photons, while that for the lightly doped case is due to indirect band-to-band recombination in silicon. The results are validated by numerical simulation which indicates that the heavy doping of the n ++ -Si is necessary to achieve surface plasmon generation via electron tunneling due to the presence of band tail states and their effect on lowering the barrier height.

Topological quantum dots (TQDs) are 3D topological insulator (TI) nanoparticles, displaying symmetry-protected surface states with discretized energies. We present a theoretical proposal to harness these energy levels in a closed lasing scheme operating in the terahertz (THz) frequency range. In this scheme, a single TQD lases from its topological surface states in the THz regime when pumped with low intensity, incoherent THz frequency light. The time scales associated with the system are unusually slow, and we find that lasing occurs with a very low threshold. THz lasers are often bulky or require intricately engineered nanostructures. Topological quantum dots present a new, compact and simple platform for THz lasing. The lasing threshold is so low, we predict that the room-temperature blackbody radiation can substantially contribute to population inversion, providing a route to room-temperature THz lasing pumped via blackbody radiation.

We demonstrate a complementary metal–oxide–semiconductor (CMOS)-compatible optical parametric Bragg amplifier on an ultra-silicon-rich nitride chip. The amplifier design incorporates advantageous group index properties in a nonlinear Bragg grating to circumvent phase matching limitations arising from the bulk material and waveguide dispersion. The grating structure further augments the effective nonlinear parameter of 800 W −1 /m, considerably lowering the power required for the observation of strong parametric gain. On/off optical parametric gain of 20 dB is achieved using a low peak power of 1.6 W, in good agreement with numerical calculations. This represents a 7 dB improvement in the parametric gain compared to the absence of grating enhancement which is attributed to the Bragg grating induced superior phase matching.

It is well-known that a Fourier optical system can be used to perform specific computing tasks, such as image differentiation, with a superior speed and power consumption in comparison with digital computers, despite bulky optical components that are often required. Recently, there has been a surge in the interest to design much more compact nanophotonic structures, such as dielectric and metallic thin films, photonic crystals, and metasurfaces with a tailored angle-dependent (nonlocal) optical response, to directly perform image differentiation without additional lenses for Fourier transformation. Here, we present a straightforward platform, a multilayer dielectric thin film optical filter, fabricated using mature wafer-scale thin film deposition technique, with an optimized nonlocal optical response, for isotropic image differentiation in transmission mode for arbitrary input polarization. The proposed thin film filter may be conveniently coated at various transparent surfaces and inserted in machine vision or microscopy systems for enhanced, real-time image processing.

Recently high-order modes (HOMs) lasers have been extensively investigated due to their potential applications in mode-division multiplexing. In this paper, we present two schemes of generating HOMs from the mode-locked fiber lasers (MLFLs) in the dissipative soliton resonance (DSR) regime. Watt-level HOM outputs are implemented through intra-cavity mode conversion. 3 W average-power HOMs with an efficiency slope of 25% can be obtained based on an MLFL in the DSR regime, which is achieved by incorporating a long-period fiber grating (LPFG) and a dual-resonant acoustically induced fiber grating (AIFG), respectively. Their different spectrum responses enable flexible mode conversion in the MLFLs. Both fiber mode converters are exploited to show their robust capability of efficient mode manipulation. The MLFL with an LPFG inserted in the cavity can achieve wide-bandwidth intra-cavity optical vortex beams (OVBs) near the dispersion turning around point because of the pulses of the fundamental mode and high-order vortex eigenmodes oscillating in the cavity with the same group velocity to form spatiotemporal mode locking. The MLFL based on a dual-resonant AIFG can perform the function of fast switching (∼0.3 ms) in LP 01 , LP 11a , and LP 11b modes with a high modal purity of 96%. These different modes with high-energy pulses can be flexibly switched with programmable radio frequency modulation. Furthermore, a quarter-wave plate and a polarizer are employed at the output of fiber laser to realize the controllability of the mode field, which is possible to generate a controllable mode field of OVBs based on the first-order Poincaré sphere. This control method can be integrated with the MLFLs to extend the flexibility of high-power HOMs generation.

In waveguided nematic liquid crystal random lasers (NLCRLs), we realize polarized random laser (RL) emission and discover that the waveguide effect reduces the transmission loss of the RL whose polarization is parallel to the liquid crystal molecules (LCMs). Compared with the traditional liquid crystal random lasers, the waveguide NLCRLs can achieve the regulation of RLs strength, polarization, and wavelength in the same structure. The electric field can drive the rotation of LCMs to control the RL polarization and intensity. The drop of horizontal polarization laser and the increase of vertical polarization laser prove the role of the waveguide effect. In addition, the disorder of the waveguided NLCRLs is highly sensitive to temperature, which makes it easy to control the wavelength and intensity of the RL. As the temperature rises, the waveguide effect is weakened, resulting in a weakening of the restriction along liquid crystal (LC) cell normal direction. The reduced laser intensity verifies the role of the waveguide effect.

Glucose monitoring sensors with high softness and flexibility are critical for the developments of wearable and implantable healthcare devices that enable diagnosis, prognosis, and management of diabetes. The design and implementation of such sensors have been extensively exploited by electrochemical strategies, which, however, suffer from poor reusability and complex modification procedures, and necessitate frequent calibration or sensor replacement due to enzymatic reaction instability. Here, a soft and plasmonic hydrogel optical sensor is created for quantitative and continuous glucose monitoring under physiological conditions. The optical sensor consists of a flexible optical fiber made from composites of gold nanoparticles and glucose-responsive hydrogels. The reversible binding of glucose to the nanocomposite optical fiber results in dynamic volume expansion of the hydrogel matrix, which modulates the localized surface plasmon resonance effect, enabling glucose to be quantified from the light transmission. To achieve robust readout, a dual-wavelength differential approach is employed to endow the sensor with self calibration capability. We show that the sensor is reversible and reusable for detecting physiological glucose levels with high linearity and negligible hysteresis. The soft and flexible glucose sensor holds great promises of serving as a minimally-invasive probe for point-of-care glucose monitoring in clinics.

Topological edge solitons propagating along the edge of a photonic topological insulator are localized self-sustained hybrid states that are immune to defects/disorders due to the protection of the edge states stemming from the nontrivial topology of the system. Here, we predict that exceptionally robust dark valley Hall edge solitons may form at the domain walls between two honeycomb lattices with broken inversion symmetry. The underlying structure can be created with femtosecond laser inscription, it possesses a large bandgap where well-localized dark edge solitons form, and in contrast to systems with broken time-reversal symmetry, it does not require external magnetic fields or complex longitudinal waveguide modulations for the realization of the topological phase. We present the envelope equation allowing constructing dark valley Hall edge solitons analytically. Such solitons propagate without radiation into the bulk of the lattice and can circumvent sharp corners, which allows observing their persistent circulation along the closed triangular domain wall boundary. They survive over huge distances even in the presence of disorder in the underlying lattice. We also investigate interactions of closely located dark topological valley Hall edge solitons and show that they are repulsive and lead to the formation of two gray edge solitons, moving with different group velocities departing from group velocity of the linear edge state on which initial dark solitons were constructed. Our results illustrate that nonlinear valley Hall systems can support a rich variety of new self-sustained topological states and may inspire their investigation in other nonlinear systems, such as atomic vapors and polariton condensates.