Green Perovskite Distributed Feedback Lasers

Toward Nonepitaxial Laser Diodes

Thin-film organic, colloidal quantum dot, and metal halide perovskite semiconductors are all being pursued in the quest for a wavelength-tunable diode laser technology that does not require epitaxial growth on a traditional semiconductor substrate. Despite promising demonstrations of efficient light-emitting diodes and low-threshold optically pumped lasing in each case, there are still fundamental and practical barriers that must be overcome to reliably achieve injection lasing. This review outlines the historical development and recent advances of each material system on the path to a diode laser. Common challenges in resonator design, electrical injection, and heat dissipation are highlighted, as well as the different optical gain physics that make each system unique. The evidence to date suggests that continued progress for organic and colloidal quantum dot laser diodes will likely hinge on the development of new materials or indirect pumping schemes, while improvements in device architecture and film processing are most critical for perovskite lasers. In all cases, systematic progress will require methods that can quantify how close new devices get with respect to their electrical lasing thresholds. We conclude by discussing the current status of nonepitaxial laser diodes in the historical context of their epitaxial counterparts, which suggests that there is reason to be optimistic for the future.

Distributed Feedback Lasers by Thermal Nanoimprint of Perovskites Using Gelatin Gratings

To date, thermal nanoimprint lithography (NIL) for patterning hybrid perovskites has always involved an intricate etching step of a hard stamp material or its master. Here, we demonstrate for the first time the successful nanopatterning of a perovskite film by NIL with a low-cost polymeric stamp. The stamp consists of a dichromated gelatin grating structured by holographic lithography. The one-dimensional grating is imprinted into a perovskite film at 95 °C and 90 MPa for 10 min, resulting in a high quality second-order distributed feedback (DFB) laser. The laser exhibits an excellent performance with a threshold of 81 μJ/cm2, a line width of 0.32 nm, and a pronounced linear polarization. This novel approach enables cost-effective fabrication of high-quality DFB lasers compatible with different perovskite compositions and photonic nanostructures for a wide range of applications.

Two-Dimensional (PEA)2PbBr4 Perovskites Sensors for Highly Sensitive Ethanol Vapor Detection

Two-dimensional (2D) perovskite have been widely researched for solar cells, light-emitting diodes, photodetectors because of their excellent environmental stability and optoelectronic properties in comparison to three-dimensional (3D) perovskite. In this study, we demonstrate the high response of 2D-(PEA)₂PbBr₄ perovskite of the horizontal vapor sensor was outstandingly more superior than 3D-MAPbBr₃ perovskite. 2D transverse perovskite layer have the large surface-to-volume ratio and reactive surface, with the charge transfer mechanism, which was suitable for vapor sensing and trapping. Thus, 2D perovskite vapor sensors demonstrate the champion current response ratio R of 107.32 under the ethanol vapors, which was much faster than 3D perovskite (R = 2.92).

Self-Aligned Emission of Distributed Feedback Lasers on Optical Fiber Sidewall

This article assembles a distributed feedback (DFB) cavity on the sidewalls of the optical fiber by using very simple fabrication techniques including two-beam interference lithography and dip-coating. The DFB laser structure comprises graduated gratings on the optical fiber sidewalls which are covered with a layer of colloidal quantum dots. Directional DFB lasing is observed from the fiber facet due to the coupling effect between the grating and the optical fiber. The directional lasing from the optical fiber facet exhibits a small solid divergence angle as compared to the conventional laser. It can be attributed to the two-dimensional light confinement in the fiber waveguide. An analytical approach based on the Bragg condition and the coupled-wave theory was developed to explain the characteristics of the laser device. The intensity of the output coupled laser is tuned by the coupling coefficient, which is determined by the angle between the grating vector and the fiber axis. These results afford opportunities to integrate different DFB lasers on the same optical fiber sidewall, achieving multi-wavelength self-aligned DFB lasers for a directional emission. The proposed technique may provide an alternative to integrating DFB lasers for applications in networking, optical sensing, and power delivery.

State of the Art and Prospects for Halide Perovskite Nanocrystals

Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

Amplified spontaneous emission in thin films of quasi-2D BA3MA3Pb5Br16 lead halide perovskites

Quasi-2D (two-dimensional) hybrid perovskites are emerging as a new class of materials with high photoluminescence yield and improved stability compared to their three-dimensional (3D) counterparts. Nevertheless, despite their outstanding emission properties, few studies have been reported on amplified spontaneous emission (ASE) and a thorough understanding of the photophysics of these layered materials is still lacking. In this work, we investigate the ASE properties of multilayered quasi-2D BA₃MA₃Pb₅Br₁₆ films through the dependence of the photoluminescence on temperature and provide a novel insight into the emission processes of quasi-2D lead bromide perovskites. We demonstrate that the PL and ASE properties are strongly affected by the presence, above 190 K, of a minor fraction of the high temperature (HT) phase. This phase dominates the PL spectra at low excitation density and strongly affects the ASE properties. In particular, ASE is only present between 13 K and 230 K, and, at higher temperatures, it is suppressed by absorption of charge transfer states of the HT phase. Our results improve the understanding of the difficulties to obtain ASE at room temperature from these quasi-2D materials and are expected to guide possible materials improvement in order to exploit their excellent emission properties also for the realization of low threshold optically pumped lasers.

Halide Perovskite Semiconductor Lasers: Materials, Cavity Design, and Low Threshold

Solution-processable semiconductor lasers have been a long-standing challenge for next-generation displays, light sources, and communication technologies. Metal halide perovskites, which combine the advantages of inorganic and organic semiconductors, have recently emerged not only as excellent candidates for solution-processable lasers but also as potential complementary gain materials for filling the "green gap" and supplement industrial nanolasers based on classic II-VI/III-V semiconductors. Numerous perovskite lasers have been developed successfully with superior performance in terms of cost-effectiveness, low threshold, high coherence, and multicolor tunability. This mini review surveys the development, current status, and perspectives of perovskite lasers, categorized into thin film lasers, nanocrystals lasers, microlasers, and device concepts including polariton and bound-in-continuum lasers with a focus on material fundamentals, cavity design, and low-threshold devices in addition to critical issues such as mass fabrication and applications.

Micro- and Nanostructured Lead Halide Perovskites: From Materials to Integrations and Devices

In the past decade, lead halide perovskites have been intensively explored due to their promising future in photovoltaics. Owing to their remarkable material properties such as solution processability, nice defect tolerance, broad bandgap tunability, high quantum yields, large refractive index, and strong nonlinear effects, this family of materials has also shown advantages in many other optoelectronic devices including microlasers, photodetectors, waveguides, and metasurfaces. Very recently, the stability of perovskite devices has been improved with the optimization of synthesis methods and device architectures. It is widely accepted that it is the time to integrate all the perovskite devices into a real system. However, for integrated photonic circuits, the shapes and distributions of chemically synthesized perovskites are quite random and not suitable for integration. Consequently, controlled synthesis and the top-down fabrication process are highly desirable to break the barriers. Herein, the developments of patterning and integration techniques for halide perovskites, as well as the structure/function relationships, are systematically reviewed. The recent progress in the study of optical responses originating from nanostructured perovskites is also presented. Lastly, the challenges and perspective for nanostructured-perovskite devices are discussed.

Environment-Induced Reversible Modulation of Optical and Electronic Properties of Lead Halide Perovskites and Possible Applications to Sensor Development: A Review

Lead halide perovskites are currently widely investigated as active materials in photonic and optoelectronic devices. While the lack of long term stability actually limits their application to commercial devices, several experiments demonstrated that beyond the irreversible variation of the material properties due to degradation, several possibilities exist to reversibly modulate the perovskite characteristics by acting on the environmental conditions. These results clear the way to possible applications of lead halide perovskites to resistive and optical sensors. In this review we will describe the current state of the art of the comprehension of the environmental effects on the optical and electronic properties of lead halide perovskites, and of the exploitation of these results for the development of perovskite-based sensors.

Emerging Light-Emitting Materials for Photonic Integration

The arrival of the information explosion era is urging the development of large-bandwidth high-data-rate optical interconnection technology. Up to now, the biggest stumbling block in optical interconnections has been the lack of efficient light sources despite significant progress that has been made in germanium-on-silicon (Ge-on-Si) and III-V-on-silicon (III-V-on-Si) lasers. 2D materials and metal halide perovskites have attracted much attention in recent years, and exhibit distinctive advantages in the application of on-chip light emitters. Herein, this Progress Report reviews the recent progress made in light-emitting materials with a focus on new materials, i.e., 2D materials and metal halide perovskites. The report briefly introduces the current status of Ge-on-Si and III-V-on-Si lasers and discusses the advances of 2D and perovskite light-emitting materials for photonic integration, including their optical properties, preparation methods, as well as the light sources based on these materials. Finally, challenges and perspectives of these emerging materials on the way to the efficient light sources are discussed.

Stable room-temperature continuous-wave lasing in quasi-2D perovskite films

Organic-inorganic lead halide quasi-two-dimensional (2D) perovskites are promising gain media for lasing applications because of their low cost, tunable colour, excellent stability and solution processability^1-3. Optically pumped continuous-wave (CW) lasing is highly desired for practical applications in high-density integrated optoelectronics devices and constitutes a key step towards electrically pumped lasers^4-6. However, CW lasing has not yet been realized at room temperature because of the 'lasing death' phenomenon (the abrupt termination of lasing under CW optical pumping), the cause of which remains unknown. Here we study lead halide-based quasi-2D perovskite films with different organic cations and observe that long-lived triplet excitons considerably impede population inversion during amplified spontaneous emission and optically pumped pulsed and CW lasing. Our results indicate that singlet-triplet exciton annihilation is a possible intrinsic mechanism causing lasing death. By using a distributed-feedback cavity with a high quality factor and applying triplet management strategies, we achieve stable green quasi-2D perovskite lasers under CW optical pumping in air at room temperature. We expect that our findings will pave the way to the realization of future current-injection perovskite lasers.

Robustness to High Temperatures of Al2O3-Coated CsPbBr3 Nanocrystal Thin Films with High-Photoluminescence Quantum Yield for Light Emission

Lead-halide perovskite nanocrystals are a promising material in optical devices due to their high photoluminescence (PL) quantum yield, excellent color purity, and low stimulated emission threshold. However, one problem is the stability of the nanocrystal films under different environmental conditions and under high temperatures. The latter is particularly relevant for device fabrication if further processes that require elevated temperatures are needed after the deposition of the nanocrystal film. In this work, we study the impact of a thin oxide layer of Al₂O₃ on the light emission properties of thin nanocrystal films. We find that nanocrystals passivated with quaternary ammonium bromide ligands maintain their advantageous optical properties in alumina-coated films and do not suffer from degradation at temperatures up to 100 °C. This is manifested by conservation of the PL peak position and line width, PL decay dynamics, and low threshold for amplified spontaneous emission. The PL remains stable for up to 100 h at a temperature of 80 °C, and the ASE intensity decreases by less than 30% under constant pumping at high fluence for 1 h. Our approach outlines that the combination of tailored surface chemistry with additional protective coating of the nanocrystal film is a feasible approach to obtain stable emission at elevated temperatures and under extended operational time scales.

Distributed feedback organic lasing in photonic crystals

Considerable research efforts have been devoted to the investigation of distributed feedback (DFB) organic lasing in photonic crystals in recent decades. It is still a big challenge to realize DFB lasing in complex photonic crystals. This review discusses the recent progress on the DFB organic laser based on one-, two-, and three-dimensional photonic crystals. The photophysics of gain materials and the fabrication of laser cavities are also introduced. At last, future development trends of the lasers are prospected.

Mahan excitons in room-temperature methylammonium lead bromide perovskites

In a seminal paper, Mahan predicted that excitonic bound states can still exist in a semiconductor at electron-hole densities above the insulator-to-metal Mott transition. However, no clear evidence for this exotic quasiparticle, dubbed Mahan exciton, exists to date at room temperature. In this work, we combine ultrafast broadband optical spectroscopy and advanced many-body calculations to reveal that organic-inorganic lead-bromide perovskites host Mahan excitons at room temperature. Persistence of the Wannier exciton peak and the enhancement of the above-bandgap absorption are observed at all achievable photoexcitation densities, well above the Mott density. This is supported by the solution of the semiconductor Bloch equations, which confirms that no sharp transition between the insulating and conductive phase occurs. Our results demonstrate the robustness of the bound states in a regime where exciton dissociation is otherwise expected, and offer promising perspectives in fundamental physics and in room-temperature applications involving high densities of charge carriers.

Tunable polymer lasing in chirped cavities

Continuously tunable polymer lasing was achieved in one-dimensional, two-dimensional, and compound chirped cavities. The chirped cavity was simply fabricated by using interference lithography and spin coating. Two-dimensional and compound chirped cavities were obtained by employing oblique exposure and double exposure, respectively. The tunability range of two-dimensional chirped cavities was much wider than that of one-dimensional chirped cavities, which varied from 557 nm to 582 nm. The interaction between lasing modes was studied in the compound cavity by introducing an additional nanostructure into the two-dimensional chirped cavities. The threshold of the compound chirped cavities changed with the coupling strength between lasing modes. These results may be helpful for designing compact polymer laser sources.

Amplified Spontaneous Emission Threshold Reduction and Operational Stability Improvement in CsPbBr3 Nanocrystals Films by Hydrophobic Functionalization of the Substrate

The use of lead halide perovskites in optoelectronic and photonic devices is mainly limited by insufficient long-term stability of these materials. This issue is receiving growing attention, mainly owing to the operational stability improvement of lead halide perosvkites solar cells. On the contrary, fewer efforts are devoted to the stability improvement of light amplification and lasing. In this report we demonstrate that a simple hydrophobic functionalization of the substrates with hexamethyldisilazane (HMDS) allows to strongly improve the Amplified Spontaneous Emission (ASE) properties of drop cast CsPbBr₃ nanocrystal (NC) thin films. In particular we observe an ASE threshold decrease down to 45% of the value without treatment, an optical gain increase of up to 1.5 times and an ASE operational stability increase of up to 14 times. These results are ascribed to a closer NC packing in the films on HMDS treated substrate, allowing an improved energy transfer towards the larger NCs within the NC ensemble, and to the reduction of the film interaction with moisture. Our results propose hydrophobic functionalization of the substrates as an easy approach to lower the ASE and lasing thresholds, while simultaneously increasing the active material stability.

Amplified Spontaneous Emission and Lasing in Lead Halide Perovskites: State of the Art and Perspectives

Lead halide perovskites are currently receiving increasing attention due to their potential to combine easy active layers fabrication, tunable electronic and optical properties with promising performance of optoelectronic and photonic device prototypes. In this paper, we review the main development steps and the current state of the art of the research on lead halide perovskites amplified spontaneous emission and on optically pumped lasers exploiting them as active materials.

Controlling the emission properties of solution-processed organic distributed feedback lasers through resonator design

Surface-emitting distributed feedback (DFB) lasers with both, resonator and active material based on solution-processable polymers, are attractive light sources for a variety of low-cost applications. Besides, the lasers should have competitive characteristics compared to devices based on high-quality inorganic resonators. Here, we report high performing all-solution-processed organic DFB lasers, consisting of water-processed photoresist layers with surface relief gratings located over the active films, whose emission properties can be finely tuned through resonator design. Their laser threshold and efficiency are simultaneously optimized by proper selection of residual resist thickness and grating depth, d. Lowest thresholds and largest efficiencies are obtained when there is no residual layer, while a trade-off between threshold and efficiency is found in relation to d, because both parameters decrease with decreasing d. This behaviour is successfully explained in terms of an overlap factor r, defined to quantify the interaction strength between the grating and the light emitted by the active film and traveling along it, via the evanescent field. It is found that optimal grating depths are in the range 100–130 nm (r ~ 0.5−0.4). Overall, this study provides comprehensive design rules towards an accurate control of the emission properties of the reported lasers.

Polymer-II-VI Nanocrystals Blends: Basic Physics and Device Applications to Lasers and LEDs

Hybrid thin films that combine organic conjugated molecules and semiconductors nanocrystals (NCs) have been deeply investigated in the previous years, due to their capability to provide an extremely broad tuning of their electronic and optical properties. In this paper we review the main aspects of the basic physics of the organic-inorganic interaction and the actual state of the art of lasers and light emitting diodes based on hybrid active materials.

Controlling the Performance of Polymer Lasers via the Cavity Coupling

The polarization and threshold of distributed feedback (DFB) polymer lasers were controlled by adjusting the cavity coupling. The cavity of DFB polymer lasers consisted of two gratings, which was fabricated by a two-beam multi-exposure holographic technique. The coupling strength of the cavity modes was tuned by changing the angle between the two gratings. The threshold of the polymer lasers decreased with reducing the coupling strength of the cavity modes. A minimum threshold was observed at the lowest coupling strength. Moreover, the azimuthally polarized output of the polymer lasers was modified by changing the cavity coupling. These results may provide additional perspectives to improve the performance of DFB polymer lasers.

Patterning Multicolor Hybrid Perovskite Films via Top-Down Lithography

Lead-halide perovskites have attracted great attention due to their excellent optoelectronic properties, with rapid progress being made in their performance as light-emitting diodes (LEDs), photodiodes, and solar cells. Demonstrating large scale, high-resolution patterning of perovskites is a key enabling step to unlock their full potential for a range of optoelectronic applications. However, the development of a successful top-down lithography fabrication procedure has so far been hampered by the incompatibility of perovskite films with the solvents used during lithographic processes. Here, we perform a study on the effect of different lithographic solvents on perovskite films and use this insight to develop photolithography and electron-beam lithography procedures for patterning perovskite films. This procedure uses standard resists at low temperatures and achieves micron-scale features with flat tops. Furthermore, we expand this platform to produce arrays of multicolor pixels for potential commercial perovskite LED display applications.

Electroluminescence Dynamics in Perovskite Solar Cells Reveals Giant Overshoot Effect

High performance of both photovoltaic and electroluminescent devices requires low nonradiative recombination losses. In perovskites, such loses strongly depend on the carrier traps related to the mobile ions and vacancies, causing I- V hysteresis of solar cells and influencing the performance of other optoelectronic devices, such as photodetectors and LEDs. To address the dynamics of the mobile ions, here we investigate electroluminescence time evolution in perovskite solar cells under constant and pulsed voltage conditions. We propose a model, accounting for the spatial ion accumulation and explaining the complex electroluminescence dynamics both on fast (microseconds) and slow (seconds) time scales. We demonstrate the appearance of a high-intensity short electroluminescence peak (overshoot pulse) immediately after termination of the electrical pulse. The generation of a giant overshoot pulse suggests a simple way to achieve high pulsed luminescence intensity with a low current density, which opens new prospects toward optical gain and implementation of electrically pumped lasers.

Density of bulk trap states of hybrid lead halide perovskite single crystals: temperature modulated space-charge-limited-currents

Temperature-modulated space-charge-limited-current spectroscopy (TMSCLC) is applied to quantitatively evaluate the density of trap states in the band-gap with high energy resolution of semiconducting hybrid lead halide perovskite single crystals. Interestingly multicomponent deep trap states were observed in the pure perovskite crystals, which assumingly caused by the formation of nanodomains due to the presence of the mobile species in the perovskites.

Continuous wave amplified spontaneous emission in phase-stable lead halide perovskites

Sustained stimulated emission under continuous-wave (CW) excitation is a prerequisite for new semiconductor materials being developed for laser gain media. Although hybrid organic-inorganic lead-halide perovskites have attracted much attention as optical gain media, the demonstration of room-temperature CW lasing has still not been realized. Here, we present a critical step towards this goal by demonstrating CW amplified spontaneous emission (ASE) in a phase-stable perovskite at temperatures up to 120 K. The phase-stable perovskite maintains its room-temperature phase while undergoing cryogenic cooling and can potentially support CW lasing also at higher temperatures. We find the threshold level for CW ASE to be 387 W cm^-2 at 80 K. These results indicate that easily-fabricated single-phase perovskite thin films can sustain CW stimulated emission, potential at higher temperatures as well, by further optimization of the material quality in order to extend the carrier lifetimes.

Continuous wave amplified spontaneous emission in phase-stable lead halide perovskites

Sustained stimulated emission under continuous-wave (CW) excitation is a prerequisite for new semiconductor materials being developed for laser gain media. Although hybrid organic-inorganic lead-halide perovskites have attracted much attention as optical gain media, the demonstration of room-temperature CW lasing has still not been realized. Here, we present a critical step towards this goal by demonstrating CW amplified spontaneous emission (ASE) in a phase-stable perovskite at temperatures up to 120 K. The phase-stable perovskite maintains its room-temperature phase while undergoing cryogenic cooling and can potentially support CW lasing also at higher temperatures. We find the threshold level for CW ASE to be 387 W cm^-2 at 80 K. These results indicate that easily-fabricated single-phase perovskite thin films can sustain CW stimulated emission, potential at higher temperatures as well, by further optimization of the material quality in order to extend the carrier lifetimes.

In order to develop perovskite-based lasers and LEDs for applications, their potential as room-temperature CW-pumped gain materials has to be established. Here, Brenner et al. demonstrate cw-pumped amplified spontaneous emission up to 120 K from a hybrid organic-inorganic halide perovskite layer.

Room-Temperature Continuous-Wave Operation of Organometal Halide Perovskite Lasers

Solution-processed organic-inorganic lead halide perovskites have recently emerged as promising gain media for tunable semiconductor lasers. However, optically pumped continuous-wave lasing at room temperature, a prerequisite for a laser diode, has not been realized so far. Here, we report lasing action in a surface-emitting distributed feedback methylammonium lead iodide (MAPbI₃) perovskite laser on a silicon substrate at room temperature under continuous-wave optical pumping. This outstanding performance is achieved because of the ultralow lasing threshold of 13 W/cm², which is enabled by thermal nanoimprint lithography that directly patterns perovskite into a high- Q cavity with large mode confinement, while at the same time, it improves perovskite's emission characteristics. Our results represent a major step toward electrically pumped lasing in organic and thin-film materials as well as the insertion of perovskite lasers into photonic integrated circuits for applications in optical computing, sensing, and on-chip quantum information.

Continuous-wave operation in directly patterned perovskite distributed feedback light source at room temperature

We report a directly patterned perovskite distributed feedback (DFB) resonator and show narrow amplified spontaneous emission (ASE) at pump powers as low as 0.1  W/cm² under continuous-wave (CW) optical pumping conditions at room temperature. Compared to the pristine thin film photoluminescence spectrum, a 16-fold reduction in emission linewidth in the MAPbI₃ DFB cavity was observed. The direct nanostructuring of perovskites was achieved by thermal nanoimprint lithography. Our findings pave the way toward realizing CW pumped perovskite lasers at room temperature and energy-efficient perovskite light sources.

All-inorganic perovskite-based distributed feedback resonator

Halide perovskite materials have rapidly emerged as outstanding optoelectronic materials for solar cells, light-emitting diodes (LEDs), and lasers. Compared to hybrid organic-inorganic perovskites, all-inorganic perovskites have shown unique merits that may contribute to the ultimate goal of developing electrically-pumped lasers. In this paper, we demonstrate a distributed feedback (DFB) resonator using an all-inorganic perovskite thin film as the gain medium. The film has a gain coefficient of 161.1 cm^-1 and a loss coefficient of 30.9 cm^-1. Excited by picosecond pulses, the microstructured all-inorganic perovskite film exhibits a single-mode emission at 654 nm with a threshold of 33 μJ/cm². The facile fabrication process provides a promising route towards low-cost single-mode visible lasers for many practical applications.