Songtao Chen

A Photonic Crystal Laser from Solution Based Organo-Lead Iodide Perovskite Thin Films

Perovskite semiconductors are actively investigated for high performance solar cells. Their large optical absorption coefficient and facile solution-based, low-temperature synthesis of thin films make perovskites also a candidate for light-emitting devices across the visible and near-infrared. Specific to their potential as optical gain medium for lasers, early work has demonstrated amplified spontaneous emission and lasing at attractively low thresholds of photoexcitation. Here, we take an important step toward practically usable perovskite lasers where a solution-processed thin film is embedded within a two-dimensional photonic crystal resonator. We demonstrate high degree of temporally and spatially coherent lasing whereby well-defined directional emission is achieved near 788 nm wavelength at optical pumping energy density threshold of 68.5 ± 3.0 μJ/cm(2). The measured power conversion efficiency and differential quantum efficiency of the perovskite photonic crystal laser are 13.8 ± 0.8% and 35.8 ± 5.4%, respectively. Importantly, our approach enables scalability of the thin film lasers to a two-dimensional multielement pixelated array of microlasers which we demonstrate as a proof-of-concept for possible projection display applications.

Electrostatic Modeling and Insights Regarding Multigate Lateral Tunneling Transistors

We use pseudo-2-D analytical models to study the electrostatics of multigate tunneling field-effect transistors (TFETs), providing a portable set of equations to simultaneously describe silicon-on-insulator, double gate, and cylindrical nanowire devices. We validate the model via extensive comparisons with numerical simulations and demonstrate its accuracy and general applicability; 2-D tunneling effects are analytically estimated and found to be small for well-scaled devices within the semiclassical model. We also study the impact of source and drain doping on TFET performance, including a seminal analytical treatment of nonabrupt junctions and degeneracy effects. We present a new simple model to explain the adverse effects of excessive source doping, and show for the first time how degeneracy in low density-of-states materials directly degrades not only the tunneling efficiency, but also the device electrostatics, causing particular problems for III-V p-TFETs.

High- Q , Low-Threshold Monolithic Perovskite Thin-Film Vertical-Cavity Lasers

A vertical-cavity surface-emitting perovskite laser is achieved using a microcavity configuration where CH3 NH3 PbI3 thin solid films are embedded within a custom GaN-based high-quality (Q-factor) resonator. This single-mode perovskite laser reaches a low threshold (≈7.6 µJ cm-2 ) at room temperature and emits spatially coherent Gaussian laser beams. The devices allow direct access to the study of perovskite gain dynamics and material robustness.

Surface-emitting red, green, and blue colloidal quantum dot distributed feedback lasers

We demonstrate surface emitting distributed feedback (DFB) lasers across the red, green, and blue from densely packed colloidal quantum dot (CQD) films. The solid CQD films were deposited on periodic grating patterns to enable 2nd-order DFB lasing action at mere 120, 280, and 330 μJ/cm2 of optical pumping energy densities for red, green, and blue DFB lasers, respectively. The lasers operated in single mode operation with less than 1 nm of full-width-half-maximum. We measured far-field patterns showing high degree of spatial beam coherence. Specifically, by taking advantage of single exciton optical gain regime from our engineered CQDs, we can significantly suppress the Auger recombination to reduce lasing threshold and achieve quasi-steady state, optically pumped operation.

Optically pumped distributed feedback laser from organo-lead iodide perovskite thin films

Perovskite (CH3NH3PbI3) films possessing optical quality were prepared by solution-based spin-casting at room temperature. We report near infrared lasing with well-defined spatially coherent output from second-order surface-emitting distributed feedback grating structure with perovskite active media.

Coherent Light Emitters From Solution Chemistry: Inorganic II–VI Nanocrystals and Organometallic Perovskites

Semiconductor lasers are the prime example of compact coherent light-emitters having developed into mature technologies with ubiquitous presence in applications across much of the optical spectrum. Yet there are reasons to explore new materials, both to access those parts of the spectrum not fully accessible by these inorganic single crystal devices, while exploring alternative high throughput methods of material synthesis. In this paper, we focus on solution-processed thin film nanocrystal materials as possible candidates to expand the material repertoire of semiconductor lasers. We review work on inorganic II–VI compound colloidal quantum dots and nanoplatelets and then present recent work in our laboratory on organometallic perovskites, which have gained significant attention quite recently by their promise as high performance photovoltaic solar cells. We concentrate on fundamentals of the optical gain and means of integrating the solution grown nano/microcrystalline thin films into high-Q cavities to identify prime challenges ahead to turn these intriguing materials into viable laser devices.

Hybrid perovskite vertical-cavity surface-emitting laser deploying nanoporous GaN dielectric reflector technology

Microstructure optimized, solution-grown perovskite thin film with high optical gain is sandwiched between two nanoporous GaN-based DBRs forming a vertical cavity. Low threshold spectrally and spatially coherent near infrared lasing is achieved by optical pumping.