Deep-Blue Perovskite Single-Mode Lasing through Efficient Vapor-Assisted Chlorination

Low-Threshold Amplified Spontaneous Emission from Quasi-2D Lead-Bromide Perovskites for Lasing Applications

Quasi-two-dimensional (quasi-2D) lead halide perovskite materials have shown great potential as gain media for amplified spontaneous emission (ASE) and lasing. Due to the complexity of the mixed-dimensional perovskite materials, factors influencing their ASE thresholds remain unclear, limiting the pace of development in this emerging area of research. Here, we report exceptionally low ASE thresholds of ∼2.23 μJ cm-2 with high stability in quasi-2D lead-bromide perovskite semiconductors. Improved gain coefficients, suppressed Auger recombination, effective coupling between the optical field and the gain medium, and minimized scattering losses are found to be some of the key contributors to the low-threshold ASE. The optimized materials lead to the demonstration of a low-threshold, single-mode perovskite laser based on a distributed feedback (DFB) optical resonator, yielding a low lasing threshold of 0.69 μJ cm-2. We expect our findings to clarify some of the key design principles of low-threshold ASE in perovskite semiconductors for lasing applications.

Colloidal Synthesis of Blue-Emitting Cs3TmCl6 Nanocrystals via Localized Excitonic Recombination for Down-Conversion White Light-Emitting Diodes

Lead-halide perovskite nanocrystals (NCs) have gained significant attention for their promising applications in lighting and display technologies. However, blue-emitting NCs have struggled to match the high efficiency of their red and green counterparts. Moreover, many reported blue-emitting perovskite NCs contain heavy metal lead (Pb), which poses risks to human health and the environment. In this study, we synthesized rare-earth-based Cs3TmCl6 NCs via the hot injection method, which exhibit a broadband blue emission at 440 nm. Combined experimental and theoretical studies indicate that the broadband emission in Cs3TmCl6 arises from self-trapped excitons due to the excited-state structural distortion of the [TmCl6]3- cluster. Furthermore, the ultrafast dynamics of charge carriers were analyzed using time-resolved photoluminescence and transient absorption measurements. Encouraged by the remarkable thermal, light, and water stabilities of Cs3TmCl6 NCs, as evidenced by experimental and theoretical results, a white light-emitting diode was further designed and fabricated using the Cs3TmCl6 NCs as the color converter. The device exhibits outstanding performance, achieving a long half-lifetime of 336 h and a large color-rendering index of 87.0. Combining eco-friendly features and a facile synthesis method, the rare-earth-based Cs3TmCl6 NCs mark a significant breakthrough as a reliable blue emitter, showcasing their future potential in lighting and display applications.

Pulse-doubling perovskite nanowire lasers enabled by phonon-assisted multistep energy funneling

Laser pulse multiplication from an optical gain medium has shown great potential in miniaturizing integrated optoelectronic devices. Perovskite multiple quantum wells (MQWs) structures have recently been recognized as an effective gain media capable of doubling laser pulses that do not rely on external optical equipment. Although the light amplifications enabled with pulse doubling are reported based on the perovskite MQWs thin films, the micro-nanolasers possessed a specific cavity for laser pulse multiplication and their corresponding intrinsic laser dynamics are still inadequate. Herein, a single-mode double-pulsed nanolaser from self-assembled perovskite MQWs nanowires is realized, exhibiting a pulse duration of 28 ps and pulse interval of 22 ps based on single femtosecond laser pulse excitation. It is established that the continuous energy building up within a certain timescale is essential for the multiple population inversion in the gain medium, which arises from the slowing carrier localization process owning to the stronger exciton-phonon coupling in the smaller-n QWs. Therefore, the double-pulsed lasing is achieved from one fast energy funnel process from the adjacent small-n QWs to gain active region and another slow process from the spatially separated ones. This report may shed new light on the intrinsic energy relaxation mechanism and boost the further development of perovskite multiple-pulse lasers.

Perovskite Quantum Dots Lasing in Double-Heterostructure through Energy Transfer

Planar double heterostructures were initially investigated and have been successfully applied in III-V semiconductor lasers due to their excellent roles in confining both the photons and carriers. Here, we design and fabricate a (PEA)2Csn-1PbnX3n+1 (quasi-2D)/CsPbBr3 QD/quasi-2D double-heterostructure sandwiched in a 3/2 λ DBR microcavity, and then demonstrate a single-mode pure-green lasing with a threshold of 53.7 μJ/cm2 under nanosecond-pulsed optical pumping. The thresholds of these heterostructure devices decrease statistically by about 50% compared to the control group with no energy donor layers, PMMA/QD/PMMA in an identical microcavity. We show that there is efficient energy transfer from the barrier regions of the quasi-2D phases to the QD layer by transient absorption and luminescence lifetime spectra and that such energy transfer leads to marked threshold reduction. This work indicates that the double-heterostructure configurations should play a significant role in the future perovskite electrically pumped laser.

Competition of Carrier Kinetics Contributes to Amplified Spontaneous Emission in Quasi-2D/3D (PBA)2MAn-1PbnBr3n+1 Thin Films under Strip Light Mode

Quasi-2D halide perovskites have potential in lasing due to their amplified spontaneous emission (ASE) properties. The ASE of (PBA)₂MA_n-1Pb_nBr_3n+1 thin films has been confirmed by photoluminescence (PL) testing using stripe light excitation (SLE). The ASE threshold decreases with decreasing environmental temperature (T_E) or increasing number of inorganic layers (n). Using the transient absorption technique, the Auger recombination and the cooling process of the high-activity carrier are accelerated with the decrease of n or T_E. A new ASE mechanism is proposed where high-activity carriers directly emit photons under photon perturbation from adjacent sites, leading to the accumulation and amplification of emitted photons only in the SLE region for ASE to occur. In addition, the reduction of n promotes light scattering between nano-thin layers, which supports a rapid increase in the ASE signal after the ASE threshold is crossed.

Photoluminescence Modulation of Ruddlesden-Popper Perovskite via Phase Distribution Regulation

The intrinsic chaotic phase distribution in Ruddlesden-Popper Perovskite (RPP) hinders its further improvement of photoluminescence (PL) emission and limits its application in optical devices. In this work, we achieve the phase distribution regulation of RPP by varying the composition ratio of organic bulky spacer cations 1-naphthylmethylamine (NMA) and phenylethyl-ammonium (PEA), which is controllable and nondestructive for structures of RPP. By suppressing the small n-phase, the PL intensity emission of RPP is further improved. Through the time-resolved PL (TRPL) measurements, we find the PL lifetime of the sample with 66% PEA concentration increases with the temperature initially and possesses the highest values of τ1 and τ2 at ~255 K, indicating the immediate state assisting exciton radiative recombination, and it can be modulated by phase manipulation in RPP. The immediate state may outcompete other non-radiative decay channels for excited carriers, leading to the PL enhancement in RPP, and broadening its further application.

Amplified Spontaneous Emission Threshold Dependence on Determination Method in Dye-Doped Polymer and Lead Halide Perovskite Waveguides

Nowadays, the search for novel active materials for laser devices is proceeding faster and faster thanks to the development of innovative materials able to combine excellent stimulated emission properties with low-cost synthesis and processing techniques. In this context, amplified spontaneous emission (ASE) properties are typically investigated to characterize the potentiality of a novel material for lasers, and a low ASE threshold is used as the key parameter to select the best candidate. However, several different methods are currently used to define the ASE threshold, hindering meaningful comparisons among various materials. In this work, we quantitatively investigate the ASE threshold dependence on the method used to determine it in thin films of dye-polymer blends and lead halide perovskites. We observe a systematic ASE threshold dependence on the method for all the different tested materials, and demonstrate that the best method choice depends on the kind of information one wants to extract. In particular, the methods that provide the lowest ASE threshold values are able to detect the excitation regime of early-stage ASE, whereas methods that are mostly spread in the literature return higher thresholds, detecting the excitation regime in which ASE becomes the dominant process in the sample emission. Finally, we propose a standard procedure to properly characterize the ASE threshold, in order to allow comparisons between different materials.

Toward Continuous-Wave Pumped Metal Halide Perovskite Lasers: Strategies and Challenges

Reliable and efficient continuous-wave (CW) lasers have been intensively pursued in the field of optoelectronic integrated circuits. Metal perovskites have emerged as promising gain materials for solution-processed laser diodes. Recently, the performance of CW perovskite lasers has been improved with the optimization of material and device levels. Nevertheless, the realization of CW pumped perovskite lasers is still hampered by thermal runaway, unwanted parasitic species, and poor long-term stability. This review starts with the charge carrier recombination dynamics and fundamentals of CW lasing in perovskites. We examine the potential strategies that can be used to improve the performance of perovskite CW lasers from the materials to device levels. We also propose the open challenges and future opportunities in developing high-performance and stable CW pumped perovskite lasers.

Low-Threshold Blue Quasi-2D Perovskite Laser through Domain Distribution Control

Quasi-2D perovskites, composed of self-organized quantum well structures, are emerging as gain materials for laser applications. Here we investigate the influence of domain distribution on the laser emission of CsPbCl_1.5Br_1.5-based quasi-2D perovskites. The use of 2,2-diphenylethylammonium bromide (DPEABr) as a ligand enables the formation of quasi-2D film with a large-n-dominated narrow domain distribution. Due to the reduced content of small-n domains, the incomplete energy transfer from small-n to large-n domains can be greatly addressed. Moreover, the photoinduced carriers can be concentrated on most of the large-n domains to reduce the local carrier density, thereby suppressing the Auger recombination. By controlling the domain distribution, we achieve blue amplified spontaneous emission and single-mode vertical-cavity surface-emitting lasing with low thresholds of 6.5 and 9.2 μJ cm^-2, respectively. This work provides a guideline to design the domain distribution to realize low-threshold multicolor perovskite lasers.

Solvent-Assisted Kinetic Trapping in Quaternary Perovskites

Engineering halide perovskites through alloying allows synthesis of materials having tuned electronic and optical properties; however, synthesizing many of these alloys is hindered by the formation of demixed phases arising due to thermodynamically unstable crystal structures. Methods have been developed to make such alloys, such as solid-phase reactions, chemical vapor deposition, and mechanical grinding; but these are incompatible with low-temperature solution-processing and monolithic integration, precluding a number of important applications of these materials. Here, solvent-phase kinetic trapping (SPKT), an approach that enables the synthesis of novel thermodynamically unfavored perovskite alloys, is developed. SPKT is used to synthesize Cs_{1- x} Rb_x PbCl₃ and report the first instance of ultraviolet emission in polycrystalline perovskite thin films. SPKT leads to materials exhibiting superior thermal and photostability compared to non-kinetically trapped materials of the same precursors. Transient absorption spectroscopy of the kinetically trapped material reveals improved optical properties: greater absorption, and longer ground-state bleach lifetimes. SPKT may be applied to other perovskites to realize improved material properties while benefiting from facile solution-processing.

Energy-Funneling Process in Quasi-2D Perovskite Light-Emitting Diodes

Quasi-two-dimensional (quasi-2D) perovskites, demonstrating excellent radiative efficiency and facile processability, have been considered as next-generation materials for light-emitting applications. Quasi-2D perovskites with a unique energy-funneling process offer an approach to achieve not only high photoluminescence quantum yields at low excitation but also tunable emission induced by dielectric and quantum confinement. In this Perspective, we highlight the mechanism of the energy-funneling process and discuss the salient position of it in quasi-2D perovskite materials for light-emitting applications; we then present the significance of component and molecular engineering strategies for the energy-funneling process to meet the requirements of stable emission and display technologies. Considering present achievements, we also provide promising directions for future advancements of quasi-2D perovskite materials. We hope this Perspective can provide a new viewpoint for researchers to encourage the commercial progress of quasi-2D perovskites for light-emitting applications.