Kwangdong Roh

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.

Highly efficient, spatially coherent distributed feedback lasers from dense colloidal quantum dot films

Colloidal quantum dots (CQD) are now making their entry to full-color displays, endowed by their brightness and single-material base. By contrast, many obstacles have been encountered in their use towards lasers. We demonstrate here optically pumped distributed feedback (DFB) lasers, based on close-packed, solid films self-assembled from type-I CQDs. Notably, the single mode CQD-DFB lasers could reach such a low threshold as to be pumpable with a compact pulsed source in a quasi-continuous wave regime. Our results show the spatially and temporally coherent laser beam outputs with power of 400 μW and a quantum efficiency of 32%.

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.

Spectroscopy of optical gain in low threshold colloidal quantum dot laser media: dominance of single-exciton states at room temperature

Experimental studies of amplified spontaneous emission (ASE) and lasing from various colloidal II-VI semiconductor nanocrystals have been used as inputs to several microscopic models for underlying optical gain, usually involving permutations of quantum confined multiple excitonic states. Here we focus on particular types of CdSe/ZnCdS and CdSe/ZnS/ZnCdS colloidal quantum dot (CQD) films and elucidate on the discovery of single-exciton states at the fundamental edge as a dominant mechanism for optical gain at room temperature. Pump-probe spectroscopic techniques enable us to measure the onset of gain at ensemble-average exciton occupancy per CQD, = 0.6 and 0.7 for the two types of CQD films at room temperature. Time-resolved measurements, in turn, show how optical gain persists well into the time regime associated with spontaneous emission (nanoseconds), thus providing direct evidence for how the non-radiative Auger recombination processes (~100 ps) can be thwarted. In addition to benefits of the material assets of densely packed CQD films with high luminescence efficiency (quantum yield ~90%) and nanoparticle monodispersity therein, we propose that access to the single-exciton gain regime at room temperature requires a careful spectral balance between the lowest exciton absorption resonance and its corresponding red-shifted spontaneous emission maximum (“Stokes shift”).

Transient gain spectroscopy in the potent single-exciton regime of dense II-VI colloidal quantum dot films

We have reached the long-sought single exciton gain regime in dense colloidal II-VI semiconductor quantum dot films. Transient spectroscopy details their exciton dynamics, informing further development of single material based lasers across the visible.

Red, green, and blue laser action in solid colloidal quantum dot films

We report on wavelength-variable lasing from optical gain media composed of colloidal quantum-dots (CQD) thin films across the visible spectrum. Exploiting single exciton gain enables amplified spontaneous emission (ASE) and vertical cavity lasing at very low optical pump thresholds. The average number of exciton per CQD at the ASE threshold is <;N>;~ 0.8, which significantly reduces losses from enhanced nonradiative multiexciton Auger recombination in nanometer sized semiconductor particle, enabling quasi-continuous-wave CQD laser performance in densely packed nanocomposites.