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Herein, the lasing threshold and gain characteristics of ultraviolet‐C optically pumped edge‐emitting lasers with thick single‐quantum‐well (SQW) active regions are investigated by the variable‐stripe length method. Positive net modal gain is observed in lasers with AlGaN‐based SQWs with thicknesses up to 12 nm. The lasers show a reduction of the threshold power density with increasing SQW thickness, with the lowest threshold of 1.3 MW cm−2 achieved for a 9 nm SQW laser. The high gain and low threshold in lasers with thick quantum wells are attributed to lasing from excited states, where the polarization fields are screened by the carriers in the fundamental state and the barriers, thus recovering larger electron and hole wavefunctions overlap. These findings are supported by k · p simulations, and for a 9 nm SQW, the calculation predicts a contribution of the fundamental transition to the gain of 75% for non‐resonant optical excitation and below 1% for resonant optical or electrical excitation. [ABSTRACT FROM AUTHOR]

With their exceptional optoelectronic properties, CsPbBr3 perovskite nanocrystals (NCs) are promising materials for light‐emitting devices. Elucidating their stimulated emission mechanisms is fundamental to grasp the limitations hindering their use as electrically pumped lasers. In particular, two questions remain open: why the Amplified Spontaneous Emission (ASE) band is significantly shifted from the fluorescence one, and why the former seems to suddenly emerge from, and coexist with, the latter. Here, these questions are addressed through experimental ASE measurements, combined with numerical simulations and a comprehensive assessment of the performance of different analytical expressions used in the literature to retrieve the optical gain from these experiments. This multi‐facet study shows that the ASE behavior in CsPbBr3 NCs thin films stems from four distinctive processes: reabsorption due to a large overlap between the absorption and fluorescence spectra, a strong contribution of excited state absorption within the fluorescence window, the excitation of differently polarized waveguide modes, and the coexistence of short‐ and long‐lived localized excitons. The results in this work delineate the best practices to analyze the optical gain in perovskite samples, help to understand their ASE mechanisms, and provide insights to boost their lasing efficiency. [ABSTRACT FROM AUTHOR]

Halide perovskite materials are considered a promising optical gain medium for stimulated emission and have aroused tremendous interest in the field of laser research. The development of low‐threshold optically pumped amplified spontaneous emission (ASE) is critical for the advancement of continuous wave (CW) and electrically pumped micro‐nano lasers. In this work, an efficient ZnS additive is incorporated into the FA0.85MA0.15PbI2.55Br0.45 perovskite films to diminish the ASE threshold and extend the photostability by the dual‐site synergistic passivation. The experimental measurements and first‐principles calculations based on the density functional theory (DFT) are employed to elucidate the doping mechanism. The designed ZnS‐based perovskite films yield a mitigated ASE threshold of 8.5 µJ cm−2 with an elevated optical gain coefficient of 109.06 cm−1, which outperform those of the control films (12.7 µJ cm−2 and 74.10 cm−1). The in‐depth experimental characterizations and theoretical calculations have corroborated that the improvement of ASE performance can be ascribed to the dual‐site synergistic passivation effects of Zn2+ and S2−, which reduces the surface defects of perovskite films, suppresses the nonradiative charge recombination and improves the optical stability. This work provides a resultful strategy to construct the solution‐processed perovskite lasers with low thresholds and superior stability. [ABSTRACT FROM AUTHOR]

Lead chalcogenide quantum dots (QDs) have been considered promising gain media in the near‐infrared region. Precipitation of lead chalcogenide QDs into glasses can significantly improve their stabilities and processabilities using mature fiber‐optic technologies and has great potential for application as a broad‐band optical amplification in optical telecommunication. However, the net optical gain of lead chalcogenide QD‐doped glasses has not been realized upon continuous‐wave (CW) pumping, hindering their application as fiber‐optic amplifiers. Here, the study reports a heterostructured PbS/NaCl QD‐doped glass and realizes a net optical gain in the near‐infrared region upon CW pumping. The photoluminescence quantum efficiencies of these QD‐doped glasses are boosted to 43–75% with significantly improved photo/thermal stabilities, and the gain threshold is reduced to ∼single exciton level due to the passivation and charge doping of the PbS QDs by the NaCl wrapping. As a result, net optical gain in the conventional band of telecommunication is realized from the 1 mm thick PbS QD‐doped glasses under CW pumping. [ABSTRACT FROM AUTHOR]

In this study, a novel metal-dielectric film mode filter structure that can flexibly regulate the transverse mode inside vertical-cavity surface-emitting lasers (VCSELs) is proposed. The number, volume, and stability of transverse modes inside the VCSEL can be adjusted according to three key parameters—the oxide aperture, the metal aperture, and the distance between the oxide aperture and the metal aperture—to form a flexible window, and a new parameter is defined to describe the mode identification. This study provides a complete simulation theory basis and calculation method, which is of great significance for the optical mode control in VCSELs. [ABSTRACT FROM AUTHOR]

A near‐infrared luminescent complex Er(DBTTA)3(DBFDPO) [where DBTTA = dibenzotetrathienoacene; DBFDPO = 4,6‐bis (diphenylphosphoryl) dibenzofuran] is synthesized. Based on the intramolecular energy transfer between organic ligands and Er3+ ions, optical gains at 1535 nm are demonstrated in Er(DBTTA)3(DBFDPO)‐doped polymer waveguides under the excitation of light‐emitting diodes (LEDs) instead of laser pumping. Relative gains of 6.4, 8.2, and 10.6 dB are obtained in 1 cm long waveguides with cross‐sectional dimensions of 6 × 4, 4 × 4, and 2 × 3 µm2 respectively, using the vertical top‐pumping mode of a 365 nm LED with 462 mW. Incorporating an ≈100 nm thick aluminum reflector grown under the lower cladding, enhanced the optical gain to 11.6 dB cm−1 in a waveguide with a cross‐section of 4 × 4 µm2, and an internal gain of ≈7.4 dB cm−1 is achieved. By relying on the intramolecular energy transfer and LED top‐pumping technology, the upconversion luminescence of Er3+ ions and thermal damage to polymer waveguides caused by the high power density of laser pumping can be effectively reduced. The complex Er(DBTTA)3(DBFDPO)‐doped polymer can be spin‐coated on different types of waveguides to compensate for optical losses at 1.5 µm and is expected to have a critical role in planar photonic integration. [ABSTRACT FROM AUTHOR]

The cylindrical surface with Bragg-enhanced Fresnel reflectivity constructed from non-close-packed photonic crystals (NPCs) was designed for spectral selectivity and optical gain, which broke through the limitations of traditional Fresnel curved mirror with lowest central reflectivity and inability to perform spectral selectivity, and was conducive to expending the application of Fresnel mirror in the fields of signal transmission, optical measurement and instrument design. [Display omitted] According to the Fresnel theory, the reflectivity intensity of spherical and cylindrical convex surfaces decreases from their edge to center, and it is noteworthy and interesting for optical gain to study the enhancement of center reflectance. In this paper, a polydimethylsiloxane (PDMS) − encapsulated cylindrical non-closed-packed photonic crystals (NPCs) composite with Bragg-enhanced Fresnel reflectance was designed for spectral selectivity and optical gain. Theoretically and experimentally, the periodically ordered structure of NPCs achieved high-reflection of light in photonic bandgap and high-transmission in other bands, which enhanced Fresnel reflectivity of the convex center to specific bands. Furtherly, the cylindrical NPCs hydrogel with stretchability was applied for the dynamic tuning of optical signals. The reflection peak of the PDMS-encapsulated cylindrical NPCs composite blue-shifted from 608 nm to 413 nm with 50 % tensile strain and achieved a rapid transition of structural color from orange to blue-violet in 60 cycles. The new kind of photonic crystals composite for optical gain and spectral selection broke through the limitations of traditional Fresnel curved mirrors with the lowest central reflectivity and inability to perform spectral selectivity, and have great significance and application prospects in fields of signal transmission, optical measurement, and instrument design. [ABSTRACT FROM AUTHOR]

A simulation procedure for analyzing the electrical and optical characteristics of an AlGaN/GaN intersubbandquantum well middle-wavelength infrared photodetector is presented. The photoconductive gain spectrumwas simulated by coupling the drift-diffusion and capture-escape models in the active region of the devicestructure and by ignoring the contribution of radiative emission. It was shown that the photodetector at zero biasis sensitive over a spectral range from 4 to 6 μm, with the peak absorption occurring at 4.64 μm. The dependenceof the available photocurrent on both the wavelength and the angle of incidence of an unpolarized monochromaticbeam of light was also evaluated. An assessment of the dark current characteristics was estimated at varioustemperatures.

The optical gain parameters in the six heavily doped AlxGa1 – xN:Si structures with x = 0.56, 0.62, 0.65, 0.68, 0.74, were experimentally studied at room temperature. Under optical excitation by pulsed radiation with λ = 266 nm, the mechanisms of stimulated emission of radiative recombination of nonequilibrium charge carriers, leading to the appearance of broadband radiation in the wide range (350–650 nm) of the spectrum with a high luminescence quantum yield are studied. High optical gain (>103 cm–1) are realized due to the good optical quality of the structures, large donor-acceptor recombination cross sections (~10–15 cm2) and the high density (up to 1020 cm–3) of radiative recombination centers. [ABSTRACT FROM AUTHOR]

Relying on a reverse energy transfer mechanism between erbium (Er3+) and ytterbium (Yb3+) ions, optical gains at wavelengths of 1550, 980, and 1067 nm are achieved in NaYF4: Er3+, and Yb3+ nanoparticles‐doped polymer waveguides under excitation of low‐power light‐emitting diodes (LEDs) instead of 976 or 1480 nm semiconductor laser as pump sources. The optical gains at 1550, 980, and 1067 nm are demonstrated in five waveguide structures: active‐rectangular, evanescent‐field with and without an aluminium (Al) reflector, and dual‐active hybrid with and without an Al reflector. The maximum relative optical gains of 6.6, 4.0, and 3.8 dBcm−1 are realized at 1550, 1067, and 980 nm, respectively, in a 5 mm long dual‐active hybrid waveguide with cross‐section of 4 × 10 µm, and a ≈100 nm‐thick Al reflector grown under the lower cladding layer under excitation of a 395 nm LED. This work proves the feasibility of using a low‐cost LED to realize multi‐wavelength amplification in a dense wavelength‐division multiplexing system and is thus valuable for the development of complex multi‐functional photonic‐integrated‐module mixed‐band optical communication systems. [ABSTRACT FROM AUTHOR]

It is well-known that thermal detuning between the gain peak and the cavity mode always occurs and leads to considerable gain loss in single-frequency laser diodes. For this issue, scientists have not found an effective solution so far. In this article, we propose an approach to addressing this problem effectively. It is associated with a fundamental change of the quantum structure to acquire a supergain spectrum (i.e., both wide and uniform gain distribution) by means of the self-assembled quantum well–wire hybrid nanostructure. With the supergain spectrum, the cavity mode can constantly obtain the max gain over a wide temperature range with a very small gain fluctuation of <4 cm–1. The simulation of an InGaAs vertical-cavity surface-emitting laser (VCSEL) model shows that the effective gain that the cavity mode can obtain is increased by >202% and the threshold pump power is reduced by >38.9% accordingly when the device temperature is higher than 360 K, in comparison to the performance of a pure quantum well. Therefore, this demonstrates great superiority for removing the gain–cavity detuning effect and improving the high-temperature performance of single-frequency laser diodes. [ABSTRACT FROM AUTHOR]

Based upon the 10-band kp approach, the electronic properties, namely the electronic band structure, effective mass, band offset, their ratio, and optical gain of AlGaAsN/GaAs, have been studied. An extensive range of electronic bandgaps spanning from 0.866 eV and 0.942 eV are observed. Considering strain, 16.7 meV/N% (keep 2%Al) splitting rates were observed due to the compressive strain. While under tensile strain, we observed a changeover from type II to type I heterostructure with a valence band offset ratio (Qv) of 0.21 < 1, which is the definition for type I heterostructure. However, a conduction band offset ratio (Qc) of 0.795, a higher value, is advantageous as it establishes better electron confinement. It is shown that (x = 0.05, y = 0.04) is the more suitable couple of contents for developing Al x Ga 1 - x As 1 - y N y / GaAs single quantum well compounds are emitting in the telecommunications windows range. Increasing nitrogen concentration and an active region's dimension leads to a red shift in the gain spectra. While under the influence of compressive and tensile strain, lateral variation by 214 nm is observed in the optical gain spectra. We also studied the variation of threshold current density with a density of injected carriers. These findings may help future optical device applications. [ABSTRACT FROM AUTHOR]

Abstract Bismuth(Bi)‐doped optical fibers are widely used in various aspects such as fiber lasers and amplifiers. Despite extensive research efforts to explore high‐gain Bi‐doped fibers, the pursuit of Bi‐doped fibers with emission over an extremely wide wavelength range remains unfulfilled. Here, the strategy of cluster control via regulating glass structure is proposed. It is applied to construct Bi‐doped photonic glass with ultra‐broadband near‐infrared (NIR) emission and a full width at half maximum (FWHM) of 343 nm. The strategy is further combined with the modified chemical vapor deposition (MCVD) technique with solution doping. The fiber exhibits ultra‐broadband amplified spontaneous emission (ASE) covering the entire fiber communication band (1260–1625 nm). Finally, the fiber exhibits higher optical gain and bandwidth compared with the fiber without cluster control. This device can be utilized to potentially enhance the capacity of telecommunication systems.

In this study, a novel metal-dielectric film mode filter structure that can flexibly regulate the transverse mode inside vertical-cavity surface-emitting lasers (VCSELs) is proposed. The number, volume, and stability of transverse modes inside the VCSEL can be adjusted according to three key parameters—the oxide aperture, the metal aperture, and the distance between the oxide aperture and the metal aperture—to form a flexible window, and a new parameter is defined to describe the mode identification. This study provides a complete simulation theory basis and calculation method, which is of great significance for the optical mode control in VCSELs.

A systematic study of a series of InGaAs/GaAs lasers in the 1–1.3 μm optical range based on quantum wells (2D), quantum dots (0D), and quantum well-dots of transitional (0D/2D) dimensionality is presented. In a wide range of pump currents, the dependences of the lasing wavelength on the layer gain constant, a parameter which allows comparing lasers with different types of active region and various waveguide designs, are measured and analyzed. It is shown that the maximum optical gain of the quantum well-dots is significantly higher, and the range of lasing rawavelengths achievable in edge-emitting lasers without external resonators is wider than in lasers based on quantum wells and quantum dots. [ABSTRACT FROM AUTHOR]

X-ray structural analysis and photoluminescence spectroscopy techniques were used to study heterostructures based on InGaAs/InAlGaAs superlattice for active regions of 1300 nm range lasers grown by molecular beam epitaxy. It is shown that the grown heterostructures have a high crystal quality. The perpendicular lattice mismatch of the average crystal lattice constant of the InGaAs/InAlGaAs superlattice with respect to the crystal lattice constant of the InP substrate is estimated at ~ +0.01%. An analysis of the photoluminescence spectra made it possible to conclude that the contribution of Auger recombination is insignificant in the studied range of excitation power density. Studies of vertical-cavity surface-emitting lasers with an active region based on the InGaAs/InAlGaAs superlattice made it possible to estimate the gain coefficient at a level of 650 cm–1 for the standard logarithmic approximation of the dependence of the gain on the current density. The transparency current density of the laser was 400–630 A/cm2, which is comparable to the record low values for the case of highly strained InGaAs–GaAs and InGaAsN–GaAs quantum wells in the spectral ranges of 1300 nm, respectively. [ABSTRACT FROM AUTHOR]

Weak fluorescence signals, which are important in research and applications, are often masked by the background. Different amplification techniques are actively investigated. Here, a broadband, geometry-independent and flexible feedback scheme based on the random scattering of dielectric nanoparticles allows the amplification of a fluorescence signal by partial trapping of the radiation within the sample volume. Amplification of up to a factor of 40 is experimentally demonstrated in ultrapure water with dispersed TiO 2 nanoparticles (30 to 50 nm in diameter) and fluorescein dye at 200 μ mol concentration (pumped with 5 ns long, 3 mJ laser pulses at 490 nm). The measurements show a measurable reduction in linewidth at the emission peak, indicating that feedback-induced stimulated emission contributes to the large gain observed. [ABSTRACT FROM AUTHOR]

Fiber laser field is typically dominated by rare earth ions as gain material in the core of a silica optical waveguide. Due to their specific emission wavelengths, rare‐earth‐doped fiber lasers are available only at few predefined wavelengths. However, quantum dots (QDs) are materials which show tunable emission with change in size and composition. Due to such tunability, QDs seem to be promising candidates for obtaining fiber lasers at a spectrum of wavelengths which is not possible using rare earth ions. To replace rare earth ions with QDs, it is of paramount importance that QDs show signatures of optical gain. Herein, the synthesis of asymmetric pod‐shaped CdSe/CdS QDs is reported, which demonstrate efficient gain on pumping. The intrinsic gain properties of the QDs are evaluated through transient absorption (TA) spectroscopy. Later, the exquisite QDs are used to fabricate specialty fibers from which amplified spontaneous emission (ASE) is obtained using continuous wave laser pump at room temperature. Finally, emission signal stability is checked by studying photobleaching and controlling the concentration of the QDs. [ABSTRACT FROM AUTHOR]

Fiber laser field is typically dominated by rare earth ions as gain material in the core of a silica optical waveguide. Due to their specific emission wavelengths, rare‐earth‐doped fiber lasers are available only at few predefined wavelengths. However, quantum dots (QDs) are materials which show tunable emission with change in size and composition. Due to such tunability, QDs seem to be promising candidates for obtaining fiber lasers at a spectrum of wavelengths which is not possible using rare earth ions. To replace rare earth ions with QDs, it is of paramount importance that QDs show signatures of optical gain. Herein, the synthesis of asymmetric pod‐shaped CdSe/CdS QDs is reported, which demonstrate efficient gain on pumping. The intrinsic gain properties of the QDs are evaluated through transient absorption (TA) spectroscopy. Later, the exquisite QDs are used to fabricate specialty fibers from which amplified spontaneous emission (ASE) is obtained using continuous wave laser pump at room temperature. Finally, emission signal stability is checked by studying photobleaching and controlling the concentration of the QDs.

Dyes with aggregation‐induced emission (AIE) properties have gained interest due to their bright luminescence in solid‐state aggregates. While fluorescence from AIE dyes is widely exploited, relatively little is known about aggregation‐induced stimulated emission. Here, stimulated emission of tetraphenylethene (TPE)‐based organoboron AIE dyes, TPEQBN, in thin films and in microcavity lasers is investigated. Using femtosecond pump–probe spectroscopy, gain coefficients up to 230 cm−1 at 500 nm are measured. Using rate equations, concentration‐ and pump‐dependent gain dynamics as well as laser build‐up dynamics are analyzed. During laser oscillation, radiative stimulated emission allows high instantaneous quantum yield greater than 90% to be achieved. Solid‐state microspheres made of 100% AIE dyes are fabricated via microfluidic emulsion and solvent evaporation method. Coupled with high gain and high refractive index of 1.76, microspheres as small as 2 µm in diameter show lasing by nanosecond pumping with a threshold of ≈10 pJ µm–2. Polymer coated, but not bare, microspheres are internalized by live cells and generate narrowband cavity‐mode emission from within the cytoplasm. This work shows the potential of AIE dyes as laser materials. [ABSTRACT FROM AUTHOR]

The development of intelligent photonic systems made of stimuli‐responsive materials, i.e., with features tunable and switchable by environmental signals, is gaining increasing attention. Here, the study reports on switchable optical gain based on complex arrays of nanofibers made of thermo‐responsive poly(2‐n‐propyl‐2‐oxazoline), incorporating a blue‐emitting chromophore. The fluorescent component endows the nanofibers with optical gain in addition to the moisture absorption capability of the polymer. Light amplification is found with temperature‐ and humidity‐dependent excitation threshold. The threshold value is halved close to the polymer cloud point temperature, enabling reversible switching of the emission intensity upon temperature change. Waveguiding analysis by back‐focal plane imaging on individual fibers allows the switching mechanisms to be rationalized, in terms of moisture sorption swelling‐induced morphological changes. These responsive light‐emitting nanofibers may find application in a novel class of lasers with dynamically‐controlled properties, environmentally‐switchable optoelectronics, and smart sensors. [ABSTRACT FROM AUTHOR]

Although traditional quantum-confined nanostructures e.g. regular quantum wells or quantum dots have achieved huge success in the field of semiconductor lasers for past decades, these traditional nanostructures are encountering the difficulty of enhancing device performance to a higher level due to their inherent gain bottleneck. In this paper, we are proposing a new super-gain nanostructure based on self-assembled well-wire complex energy-band engineering with InGaAs-based materials to break through the existing bottleneck. The nanostructure is constructed by utilizing the special strain-driven indium (In)-segregation and the growth orientation-dependent on-GaAs multi-atomic step effects to achieve the distinguished ultra-wide and uniform super-gain spectra. The structural details and its luminescence mechanism are investigated by multiple measurement means and theoretical modeling. The polarized gain spectra with the max fluctuation of

Optical gain and resistance against photo-bleaching are key material parameters for practical applications in microlasers and microphotonics. Here, we present studies of optical amplification and photo-bleaching in a wide range of optical gain materials, including fluorescent dyes, quantum dots and rods, organic and inorganic room temperature polaritons, and nanodiamonds with emission range from 430 to 680 nm. We used amplified spontaneous emission (ASE) for each material to measure the optical gain. Robustness against photo-bleaching was determined by measuring the intensity of spontaneous emission of the material as a function of the number of excitation femtosecond pulses (106–1010) at various excitation energy densities. We show that pyrromethene laser dyes are the best organic emitters in terms of low excitation energy and good optical gain, whereas solid-state polariton materials are better in terms of high optical gain and stability. We found that all organic and inorganic optical gain materials bleach completely after 106–109 illumination pulses with typical 0.1–0.6 GW/cm2 peak power density, with an exception of nanodiamonds. We show that nanodiamonds are the only optical gain material that shows no photo-bleaching beyond the 1010 excitation pulses of 300 fs pulse duration and with 0.16 GW/cm2 peak power density. [ABSTRACT FROM AUTHOR]

In quantum dots, where confinement is strong, interactions between charge carriers play an essential role in the performance of semiconductor materials for optical gain. Therefore, understanding this phenomenon is critical for achieving new devices with enhanced features. In this context, the current study examines the optical properties of an exciton confined in a spherical InAs quantum dot under the influence of magnetic and intense laser fields. We investigate the oscillator strength, exciton lifetime, and optical gain, considering the effects of both external fields. We also pay particular attention to the influence of quantum dot size on the results. Our calculations show that the two external fields have opposite effects on our findings. Specifically, the applied magnetic field increases the oscillator strength while the intense laser reduces it. In addition, the optical gain peaks are redshifted under the application of the intense laser, whereas the magnetic field causes a blueshift of the peak threshold. We also find that both external perturbations significantly influence the exciton lifetime. Our study considers the outcomes of both the exciton's ground ( 1 s ) and first excited ( 1 p ) states. The theoretical results obtained in this study have promising implications for optoelectronic devices in the ∼3–4 μ m wavelength range only through the control of quantum dot sizes and external perturbations. [ABSTRACT FROM AUTHOR]

Halide perovskite lasers based on CsPbBr3 micro‐ and nanoscale crystals have demonstrated fascinating performance owing to their low‐threshold lasing at room temperature and cost‐effective fabrication. However, chemically synthesized thin films of CsPbBr3 usually have rough polycrystalline morphology along with a large amount of crystal lattice defects and, thus, are mostly utilized for the engineering of light‐emitting devices. This obstacle prevents their usage in many photonic applications. Here, a protocol to deposit large‐grain and smooth CsPbBr3 thin films is developed. Their high quality and large scale allow to demonstrate a maximum optical gain up to 12 900 cm−1 in the spectral range of 530–540 nm, which is a record‐high value among all previously reported halide perovskites and bulk semiconductors (e.g., GaAs, GaN, etc.) at room temperature. Moreover, femtosecond laser ablation technique is employed to create high‐quality microdisc lasers on glass from these films to obtain excellent lasing characteristics. The revealed critical roles of thickness and grain size for the CsPbBr3 films with extremely high optical gain pave the way for development of low‐threshold lasers or ultimately small nanolasers, as well as to apply them for polaritonic logical elements and integrated photonic chips. [ABSTRACT FROM AUTHOR]

Theoretical analysis of the effect of quantum well thickness on the energy band dispersion curve, matrix elements, transition energy, optical gain, and transition wavelength of the type-II In0.7Ga0.3As/GaAs0.4Sb0.6 heterostructure based on InP substrate using six bands k.p method has been done. A decrease in the optical gain and an increase in the transition wavelength are reported with an increase in the quantum well thickness. The transition wavelength is located in the short wavelength infrared region (SWIR). An increase in optical gain is observed with an increase in the injected carrier density. In conclusion, the results clarified that In0.7Ga0.3As/GaAs0.4Sb0.6 based heterostructures are suitable for making optoelectronic devices in the SWIR region. [ABSTRACT FROM AUTHOR]

The electronic structures of Ge nanowires under the [110] direction stress are calculated via effective-mass k · p theory, and the results manifest eight equivalent L -valleys will be split into fourfold degenerate L _1 -valleys and L _2 -valleys. With increasing stress, the electron levels at the L _1 -valleys and L _2 -valleys can be pushed close to and away from those at the Γ-valley, respectively, which causes the appearance of a rising inflection point in the Γ-valley filling ratio and gain peak intensity at around 2.5 GPa stress. Moreover, we prove the positive net peak gain with small diameters is apt to be obtained considering the free-carrier absorption loss.

The tunability of the optical gain characteristics of the AlN/GaN/InAlN quantum well heterostructure under uniaxial and biaxial pressures is reported. To study the tunability, pressures on the designed heterostructure are applied in the range of 1–4 GPa along different orientations, i.e., (001), (100), and (110). Under these pressures, the gain characteristics are computed. To perform the calculations for the gain characteristics, the well‐known k⋅p method is adopted. The optical gain is computed under uniaxial and biaxial pressures and the results are analyzed thoroughly. Quantitatively speaking, the amount of pressure (1–4 GPa) along z‐direction (i.e., 001) is suitable to make significant enhancement in the peak gain (i.e., from ≈7000 to ≈8000 cm−1) with the variation in transition wavelength from ≈1347 to ≈1512 nm. While the (100) and (110) oriented pressures having the range 1–4 GPa reduce the peak gain from ≈7100 to ≈5800 cm−1, and from ≈3550 to ≈3410 cm−1, respectively. On comparison of the results, it is concluded that only z‐direction pressure is useful for enhancing the gain characteristics with fine tuning of the transition wavelengths. Consequently, the designed heterostructure can be tuned with enhanced peak gain having red shift wavelength. [ABSTRACT FROM AUTHOR]

With the development of optofluidic technology, liquid lasers have attracted intense interest but still face a formidable challenge due to the lack of qualified gain media and creative device design. Compared to the organic fluorescent dyes and traditional CdSe‐based nanocrystals (NCs), the lead‐halide perovskite (LHP) NCs feature larger gain coefficient and higher robustness, which renders LHP NCs a promising unexploited liquid gain medium. Herein, for the first time, the hidden principle governing the solution‐based light amplification in LHP semiconductor NCs is demystified and it is demonstrated that the LHP NCs are superior solution‐phase gain media showing a giant (≈2600 cm−1) optical gain. On this basis, a novel microfluidic laser device is designed and realized that exhibits a record‐low pump threshold (≈22.7 µJ cm−2), high Q‐factor (≈7480), large output polarization (≈0.91), and long‐time robustness over high‐intensity operation, which is readily applicable to practical applications. The findings represent a significant step toward the technologically important liquid lasers of the new generation and can advance optofluidic applications. [ABSTRACT FROM AUTHOR]

Colloidal quantum dots (CQDs) have emerged as promising candidates for solution‐processed optical gain materials. Despite the substantial progress made with Cd or Pb‐contained CQDs, lasing from the environmentally friendly CQDs remains underexplored. Here, the highly emissive ZnSeTe/ZnSe/ZnS and ZnSe/ZnS CQDs is developed. The optical gain and lasing action from such Te‐alloyed and intrinsic ZnSe/ZnS CQDs across the blue spectral region is achieved. Te‐alloyed ZnSe core manifests a quasitype‐II band alignment with the ZnSe intermediate layer, favorable for suppressing the Auger recombination and also lasing wavelength tuning. The transient absorption spectroscopy reveals evidence of a blue optical gain with a threshold of one exciton per dot and a lifetime of 171 ps in the alloyed CQDs. Under a quasicontinuous‐wave pumping, lasing is observed from the blue CQDs in Fabry–Pérot cavities with a lower threshold fluence of 12.5 µJ cm−2 as compared with the traditional counterparts with heavy metals. The demonstration of heavy‐metal‐free CQD lasing opens up new opportunities in the application of CQDs as efficient and environmental‐friendly gain media. [ABSTRACT FROM AUTHOR]

Investigation of the signal enhancement of Nd3+ codoped TeO 2 -ZnO pedestal waveguides, at 1064 nm, due to Au nanoparticles deposited over the core is presented for the first time. Nd3+ doped TeO 2 -ZnO thin film was obtained by RF Magnetron Sputtering deposition. The resulting core with 500 nm height and widths in the 4–40 μm range, exhibited low roughness average in all area measured (0.48 ± 0.04) nm. Minimum propagation losses of 2.2 dB/cm were observed for waveguide width of 40 μm whereas an increase took place for smaller ones. Scanning electron microscopy (SEM) allowed the waveguide structure inspection and transmission electronic microscopy (TEM) the Au nanoparticles evaluation. The results showed that the Au nanoparticles contributed up to 75 % of relative gain enhancement, under 808 nm excitation. This increase was due to the local field growth in the proximity of the nanoparticles that enhances the density of excited Nd3+. The internal gain that considers the propagation losses reached positive values for larger core widths (above 8 μm). The present study opens possibilities for optical amplifiers with low propagation losses based on different metal-dielectric composites, as well as other waveguide-based devices. • Enhanced optical gain in Nd3+ doped TeO 2 -ZnO waveguides due to Au nanoparticles. • New possibilities for integrated photonics assisted by plasmonics effects. • Au nanoparticles contribution: relative gain enhancement of up to 75 %, at 1064 nm. • Relative gain of about 12.0 dB/cm at 1064 nm: waveguide widths of 8, 6 and 40 μm. • Au nanoparticles influence: Nd3+ luminescence enhancement of 37 % (at 1.064 μm). [ABSTRACT FROM AUTHOR]

The numerical simulations of optical and electronic properties of BGaAsBi/GaAs quantum wells (QWs) are obtained by diagonalizing the 16-band kp Hamiltonian with distinct boron and bismuth compositions that exhibit multifaceted potential in 1.3–1.5 μm. It has been observed that under strain-relaxed conditions for a doping concentration of 12.5 % (B, Bi), the anticipated values of spin-orbit (SO) coupling energy (Δ SO) and the bandgap (E g) are 302 meV and 0.952 eV, respectively. Additionally, strain interaction on band structure decreases the band gap from (E g = 0.92eV–0.85eV), increasing dopant concentration from 8 % to 12.5 %. The consequence of incorporating both Bi and B impurity is a reduction in electron effective mass of BGaAsBi by ∼1.3 times concerning the host GaAs, enhancing the optical properties of BGaAsBi/GaAs quantum-confined heterostructures. Moreover, an increase in well-width introduced a red shift and reduced the peak amplitude of the gain spectra. In addition, for solar cell application, the onset of the fundamental absorption edge is depicted at an energy of about 0.96 eV. For laser applications, the optimum threshold current density and quality factor of BGaAsBi/GaAs quantum well achieved at room temperature are 1089 A/cm2 and 3653, respectively, at well width (w = 2.0 nm), cavity length (L = 0.5 mm), and average thickness of the active region (d = 45 nm). The variation in power density is studied with injected current density, while the quality factor is calculated with well width. The outcomes could be beneficial for future use in optical devices. • Band structure of BGaAsBi/GaAs studied using 16 band kp model. • Observing an extensive range of electronic bandgaps and corresponding operating wavelengths spanning from 0.92 eV (∼1.34 μm) to 0.85 eV (∼1.5 μm) is observed for B and Bi impurity concentrations up to 8 % and 12.5 %, respectively. • Incorporation of 12.5 % (B, Bi) into the host GaAs leads to a decrease in electron effective mass by 1.3 times. • The onset of the fundamental absorption edge is depicted at an energy of about 0.96 eV. • Increasing well width leads to a red shift in the gain spectra and a reduction in peak amplitude. • Variation of threshold current density is studied with temperature, cavity length, the average thickness of region, and well-width conditions. (The lowest threshold current density value at 77 K is 142 A/cm2, but at 300 K is 1089 A/cm2.) • The power density variation was studied with injected current density and the quality factor variation with well width. [ABSTRACT FROM AUTHOR]

Weak fluorescence signals, which are important in research and applications, are often masked by the background. Different amplification techniques are actively investigated. Here, a broadband, geometry-independent and flexible feedback scheme based on the random scattering of dielectric nanoparticles allows the amplification of a fluorescence signal by partial trapping of the radiation within the sample volume. Amplification of up to a factor of 40 is experimentally demonstrated in ultrapure water with dispersed TiO2 nanoparticles (30 to 50 nm in diameter) and fluorescein dye at 200 μmol concentration (pumped with 5 ns long, 3 mJ laser pulses at 490 nm). The measurements show a measurable reduction in linewidth at the emission peak, indicating that feedback-induced stimulated emission contributes to the large gain observed.

All‐inorganic CsPbBr3 perovskite thin films are regarded as promising gain media to realize electrically pumped green laser diodes due to their good stability. However, the amplified spontaneous emission (ASE) of CsPbBr3 perovskite thin films suffers from a relatively high threshold due to their high defect density and large scattering loss. This work demonstrates high‐quality CsPbBr3/CsPb2Br5 and CsPbBr3/Cs4PbBr6 thin films using a vapor deposition method. Through rational design and modification on the compositions of thin films, it is found that the existence of CsPb2Br5 or Cs4PbBr6 phase in CsPbBr3 can dramatically improve photoluminescence and lengthen carrier lifetime. The mixed phase films show good crystalline quality with lower trap density and enhanced spatial confinement. As a result, the ASE threshold of CsPbBr3 perovskite thin film is reduced from 12.0 ± 0.2 to 10.1 ± 0.3 µJ cm−2 (with CsPb2Br5) and 10.2 ± 0.2 µJ cm−2 (with Cs4PbBr6). Furthermore, the optical gain is increased from 10.4 ± 0.5 to 21.6 ± 0.9 cm−1 (19.5 ± 0.8 cm−1) with the participation of CsPb2Br5 (Cs4PbBr6) phase. The findings provide an effective approach to lower the threshold of CsPbBr3 lasers and shed light on the potential of stable electrically pumped perovskite lasers. [ABSTRACT FROM AUTHOR]

Whispering gallery mode (WGM) ZnO microlasers gain attention due to their high Q-factors and ability to provide low-threshold near-UV lasing. However, a detailed understanding of the optical gain mechanisms in such structures has not yet been achieved. In this work, we study the mechanisms of stimulated emission (SE) in hexagonal ZnO microrods, demonstrating high-performance WGM lasing with thresholds down to 10–20 kW/cm2 and Q-factors up to ~3500. The observed SE with a maximum in the range of 3.11–3.17 eV at room temperature exhibits a characteristic redshift upon increasing photoexcitation intensity, which is often attributed to direct recombination in the inverted electron-hole plasma (EHP). We show that the main contribution to room-temperature SE in the microrods studied, at least for near-threshold excitation intensities, is made by inelastic exciton-electron scattering rather than EHP. The shape and perfection of crystals play an important role in the excitation of this emission. At lower temperatures, two competing gain mechanisms take place: exciton-electron scattering and two-phonon assisted exciton recombination. The latter forms emission with a maximum in the region near ~3.17 eV at room temperature without a significant spectral shift, which was observed only from weakly faceted ZnO microcrystals in this study. [ABSTRACT FROM AUTHOR]

HgCdTe-based heterostructures with quantum wells (QWs) are a promising material for semiconductor lasers in the atmospheric transparency window (3–5 μm) thanks to the possibility of suppressing Auger recombination due to the no-parabolic law of carrier dispersion. In this work, we analyze the thresholds of stimulated emission (SE) under optical pumping from heterostructures with a different number of QWs in the active region of the structure. Total losses in structures are determined from the comparison of thresholds for the different number of QWs in the active region. It is shown that, thanks to the increased modal gain, a higher number of QWs results in lower threshold pumping intensity and, consequently, higher temperature of SE. These results indicate that improvements to the modal gain can result in a moderate uplift in the temperature of SE from mid-infrared HgCdTe-based heterostructures. On the other hand, at a high enough QW count threshold, the intensity no longer depends on the number of the QWs and is determined by the transparency concentration of a single QW. [ABSTRACT FROM AUTHOR]

All‐inorganic perovskite nanocrystals (NCs) have received extensive attention for next‐generation thin film devices due to their excellent optical properties, such as strong light absorption, high carrier mobility, and defect tolerance. However, significant challenges remain to obtain high‐quality perovskite thin films. Herein, a simple but effective post‐treatment by laser irradiation for CsPbBr3 NCs thin films is reported. Laser‐induced secondary crystallization is observed in CsPbBr3 NCs thin films after treatment. In addition, amplified spontaneous emission (ASE) with a low threshold (5.6 µJ cm−2) and a high gain value (743 cm–1) is achieved. Based on optical measurements, it is attributed to the low defect density, reduced Auger recombination, and weak exciton–phonon interactions, which greatly suppress the nonradiative recombination channels. The ASE from the film after treatment has a high characteristic temperature (134 K), showing a stable optical gain performance that maintains its intensity for 35 h at room temperature (and 12 h at 40 °C). Finally, the proof‐of‐concept demonstration of graphic coding is shown. This study deepens the understanding of the optical gain mechanism of CsPbBr3 perovskite films and provides a simple and convenient laser treatment that enables the fabrication of high‐quality CsPbBr3 perovskite thin films. [ABSTRACT FROM AUTHOR]

In this research work, a theoretical approach has been adopted to study GaAs-based InAlAs/InGaAs/GaAsSb type-II heterostructures. The 6 × 6 k·p Hamiltonian matrix with self-consistent calculations has been carried out to calculate the wavefunctions, localization of the charge carriers (probability density of charge carriers) and their dispersed energy states (Discrete energy level) under variable quantum well width and external strain of the double quantum well heterostructure. Bulk band structures of InAlAs/GaA for different aluminium compositions have been reported. The quantum well size and external strain effect on optical gain have also been explored. Computational analysis showed the generated light has a 1.93 μm NIR lasing wavelength. At 12 GPa, optical gain reaches 7450 cm−1 and wavelength 1.93 μm. Peak gain is 5931 cm−1 for 2 nm well width. The developed type-II structure can be employed in a semiconductor laser diode operating in the NIR range as per the observed results. [ABSTRACT FROM AUTHOR]

This paper reports quantum mechanical study to optimize the type-I AlN/GaN/InAlN QW (quantum well) heterostructure and investigates the optical gain characteristics. The heterostructure studied here has single QW of InAlN (~ 10 nm well width) with barrier as GaN (~ 20 nm width) and grown with the claddings of AlN binary semiconductor material (~ 100 nm width). The optical gain spectra have been calculated by solving the 6 × 6 Luttinger–Kohn Hamiltonian considering the effective mass approximation. The optical characteristics of the designed QW heterostructure have been studied for the different well width of the QW heterostructure. For the InGaN/GaN QW heterostructure, by reducing the well width, the peak gain has been improved with blue shift in wavelength, which can be considered as significant increase in the peak gain. This study may be advantageous in designing the tunable III-nitride optoelectronic devices. [ABSTRACT FROM AUTHOR]

Conjugated polymer blends coupled by Förster resonance energy transfer (FRET) have been widely exploited to achieve optically pumped lasers operating at very low pumping thresholds. Among the plaid of conjugated polymers and molecules exploited for optical gain, fluorene‐based polymers are considered front‐runners, based on their high photoluminescence quantum yields, large optical gain coefficients, and their processability assets in films of high optical quality. Two archetypes of polymers with these properties are poly(9,9‐dioctyl‐fluorene) and its green‐emitting relative poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole) that work as excellent FRET‐coupled blends but unexpectedly do not exhibit stimulated emission. In this study, light is shed upon the optical gain limiting factors of these blends. Upon investigating a series of poly(diarylfluorene‐co‐N‐phenyl) polymers bearing a different number of phenyl units inserted between the fluorenes (NPhs, N = 1, 2, 3), important improvements are revealed in the photoluminescence quantum yields and stimulated emission properties, as well as harnessing of exciton‐exciton annihilation. These effects are ascribed to disruption of exciton transport in NPhs motivated by changes in chain conformation and molecular packing upon phenyl‐insertion. [ABSTRACT FROM AUTHOR]

A laser device based on InGaAsN quantum well active regions emitting at 1.26 mm is reported. The performances of the laser under the effect of thermal energy are investigated in terms of threshold current, Ith, gain parameter, gt, photon energy, hn, and cavity length, Lc. Four structures with one, two, and three quantum wells along with a structure that have three quantum wells with tensile strained barriers are proposed to study the relation between the peak gain, gpeak, and photon energy. The founds show that structures with one and two quantum wells operating at room temperature and under pulse wave condition, exhibit a linear dependence of gpeak on both Lc and photon energy. It is shown that the threshold current density, Jth, increases at any temperature with the cavity length Lc ranging from 250 nm to 1000 nm. Also, the investigation of the proposed structures shows that gt decreases with increasing temperature, while the ratio of the current density parameter to internal efficiency, Jt/ηi per well increases with the quantum well number. A comparison was carried out for two particular structures with three quantum wells and GaAsP barriers, the results show a decrease in the threshold current per well. [ABSTRACT FROM AUTHOR]

Excellent high-temperature performance (298–523 K) of the 4H-SiC n-p-n bipolar phototransistor detector (PTD) was explored. It was fabricated using the 4H-SiC epitaxial wafer grown by high-temperature chemical vapor deposition (HTCVD). The static and dynamic properties of the device were tested and analyzed. The results show that the device still has an optical gain of up to 103, a wide linear response range, and good repeatability at the high temperatures up to 523 K under a 3-V bias with a wavelength of 360 nm. The temperature coefficient of the optical gain is extracted, the value of which is 1.02/K. Although the response time of the device increases with the temperature, the increased percentage in responsivity is significantly greater than that of the response time. The results provide an important experimental reference for the application of high-temperature detection and suggest that the PTD has the potential to be used in ultraviolet (UV) detecting systems at high temperatures. [ABSTRACT FROM AUTHOR]