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.

Hot-Casting-Assisted Liquid Additive Engineering for Efficient and Stable Perovskite Solar Cells

High-performance inorganic-organic lead halide perovskite solar cells (PSCs) are often fabricated with a liquid additive such as dimethyl sulfoxide (DMSO), which retards crystallization and reduces roughness and pinholes in the perovskite layers. However, DMSO can be trapped during perovskite film formation and induce voids and undesired reaction byproducts upon later processing steps. Here, it is shown that the amount of residual DMSO can be reduced in as-spin-coated films significantly through use of preheated substrates, or a so-called hot-casting method. Hot casting increases the perovskite film thickness given the same concentration of solutions, which allows for reducing the perovskite solution concentration. By reducing the amount of DMSO in proportion to the concentration of perovskite precursors and using hot casting, it is possible to fabricate perovskite layers with improved perovskite-substrate interfaces by suppressing the formation of byproducts, which increase trap density and accelerate degradation of the perovskite layers. The best-performing PSCs exhibit a power conversion efficiency (PCE) of 23.4% (23.0% stabilized efficiency) under simulated solar illumination. Furthermore, encapsulated devices show considerably reduced post-burn-in decay, retaining 75% and 90% of their initial and post-burn-in efficiencies after 3000 h of operation with maximum power point tracking (MPPT) under high power of ultraviolet (UV)-containing continuous light exposure.

Nanosecond-Pulsed Perovskite Light-Emitting Diodes at High Current Density

While metal-halide perovskite light-emitting diodes (PeLEDs) hold the potential for a new generation of display and lighting technology, their slow operation speed and response time limit their application scope. Here, high-speed PeLEDs driven by nanosecond electrical pulses with a rise time of 1.2 ns are reported with a maximum radiance of approximately 480 kW sr^-1  m^-2 at 8.3 kA cm^-2 , and an external quantum efficiency (EQE) of 1% at approximately 10 kA cm^-2 , through improved device configuration designs and material considerations. Enabled by the fast operation of PeLEDs, the temporal response provides access to transient charge carrier dynamics under electrical excitation, revealing several new electroluminescence quenching pathways. Finally, integrated distributed feedback (DFB) gratings are explored, which facilitate more directional light emission with a maximum radiance of approximately 1200 kW sr^-1 m^-2 at 8.5 kA cm^-2 , a more than two-fold enhancement to forward radiation output.

Development of a density-tapered capillary gas cell for laser wakefield acceleration

A capillary gas cell for laser wakefield acceleration was developed with the aid of three-dimensional computational fluid dynamics simulations. The gas cell was specially designed to provide upward density tapering in the longitudinal direction, which is expected to suppress the dephasing problem in laser wakefield acceleration by keeping the accelerated electrons in the acceleration phase of the wake wave. The density-tapered capillary gas cell was fabricated by sapphire plates, and its performance characteristics were tested. The capillary gas cell was filled with a few hundred millibars of hydrogen gas, and a Ti:sapphire laser pulse with a peak power of 3.8 TW and a pulse duration of 40 fs (full width at half maximum) was sent through the capillary hole, which has a length of 7 mm and a square cross section of 350 × 350 µm². The laser-produced hydrogen plasma in the capillary hole was then diagnosed two-dimensionally by using a transverse Mach-Zehnder interferometer. The capillary gas cell was found to provide an upward plasma density tapering in the range of 10¹⁸ cm^-3-10¹⁹ cm^-3, which has a potential to enhance the electron beam energy in laser wakefield acceleration experiments.

Thermal Management Enables Bright and Stable Perovskite Light-Emitting Diodes

The performance of lead-halide perovskite light-emitting diodes (LEDs) has increased rapidly in recent years. However, most reports feature devices operated at relatively small current densities (<500 mA cm^-2 ) with moderate radiance (<400 W sr^-1 m^-2 ). Here, Joule heating and inefficient thermal dissipation are shown to be major obstacles toward high radiance and long lifetime. Several thermal management strategies are proposed in this work, such as doping charge-transport layers, optimizing device geometry, and attaching heat spreaders and sinks. Combining these strategies, high-performance perovskite LEDs are demonstrated with maximum radiance of 2555 W sr^-1 m^-2 , peak external quantum efficiency (EQE) of 17%, considerably reduced EQE roll-off (EQE > 10% to current densities as high as 2000 mA cm^-2 ), and tenfold increase in operational lifetime (when driven at 100 mA cm^-2 ). Furthermore, with proper thermal management, a maximum current density of 2.5 kA cm^-2 and an EQE of ≈1% at 1 kA cm^-2 are shown using electrical pulses, which represents an important milestone toward electrically driven perovskite lasers.

Improved Outcoupling Efficiency and Stability of Perovskite Light-Emitting Diodes using Thin Emitting Layers

Hybrid organic-inorganic perovskite semiconductors have shown potential to develop into a new generation of light-emitting diode (LED) technology. Herein, an important design principle for perovskite LEDs is elucidated regarding optimal perovskite thickness. Adopting a thin perovskite layer in the range of 35-40 nm is shown to be critical for both device efficiency and stability improvements. Maximum external quantum efficiencies (EQEs) of 17.6% for Cs_0.2 FA_0.8 PbI_2.8 Br_0.2 , 14.3% for CH₃ NH₃ PbI₃ (MAPbI₃ ), 10.1% for formamidinium lead iodide (FAPbI₃ ), and 11.3% for formamidinium lead bromide (FAPbBr₃ )-based LEDs are demonstrated with optimized perovskite layer thickness. Optical simulations show that the improved EQEs source from improved light outcoupling. Furthermore, elevated device temperature caused by Joule heating is shown as an important factor contributing to device degradation, and that thin perovskite emitting layers maintain lower junction temperature during operation and thus demonstrate increased stability.

Mixed Lead-Tin Halide Perovskites for Efficient and Wavelength-Tunable Near-Infrared Light-Emitting Diodes

Near-infrared (NIR) light-emitting diodes (LEDs), with emission wavelengths between 800 and 950 nm, are useful for various applications, e.g., night-vision devices, optical communication, and medical treatments. Yet, devices using thin film materials like organic semiconductors and lead based colloidal quantum dots face certain fundamental challenges that limit the improvement of external quantum efficiency (EQE), making the search of alternative NIR emitters important for the community. In this work, efficient NIR LEDs with tunable emission from 850 to 950 nm, using lead-tin (Pb-Sn) halide perovskite as emitters are demonstrated. The best performing device exhibits an EQE of 5.0% with a peak emission wavelength of 917 nm, a turn-on voltage of 1.65 V, and a radiance of 2.7 W Sr^-1 m^-2 when driven at 4.5 V. The emission spectra of mixed Pb-Sn perovskites are tuned either by changing the Pb:Sn ratio or by incorporating bromide, and notably exhibit no phase separation during device operation. The work demonstrates that mixed Pb-Sn perovskites are promising next generation NIR emitters.

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.

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.

Pigmentation in Koreans: study of the differences from caucasians in age, gender and seasonal variations

BACKGROUND

Human skin colour shows variations throughout life, and many extrinsic and intrinsic factors influence melanogenesis. Facultative pigmentation of sun-exposed skin has been suggested to reflect cumulative lifetime ultraviolet (UV) exposure in caucasians. However, pigmentary changes due to various regulatory factors may be different in dark-skinned peoples.

OBJECTIVES

To observe the variations in skin colour due to ageing, gender differences and seasonal changes in Koreans with skin type IV or V.

METHODS

Skin pigmentation was measured at five body sites (buttock, glabella, the V of the neck area, inner arm and dorsal forearm) using skin reflectance spectroscopy in 497 subjects (age range 0-87 years) in winter and 311 subjects (age range 0-84 years) in summer. Among these subjects, 110 were assessed in both seasons. Three independent measurements at each site were done and the average value was used as the pigmentation level.

RESULTS

Constitutive pigmentation of the buttock was highest in the first decade of life. It then decreased during the second decade and this decreased level was maintained after the third decade. In contrast to caucasians, facultative pigmentation and sun exposure index did not increase with ageing. Gender differences were significant at all body sites after the first decade. Seasonal changes were apparent in dorsal forearm pigmentation. Little difference was seen in forehead pigmentation between summer and winter.

CONCLUSIONS

Basal melanogenic regulation might not be different between Asians and caucasians. However, the sun exposure index may not represent lifelong cumulative UV exposure in Koreans. Age-, gender- and season-related characteristics of skin pigmentation in Koreans imply that genetically determined basal skin colour plays an important part in characterizing later responsiveness to UV radiation and sex hormones. Understanding differences between races will be helpful in studying the regulatory mechanisms of melanogenesis.

Genomic cloning and characterization of mitochondrial elongation factor Tu (EF-Tu) gene (tufM) from maize (Zea mays L.)

We have cloned and characterized a mitochondrial elongation factor Tu (EF-Tu) gene (tufM) in maize (Zea mays L.). This maize tufM gene encoded a polypeptide of 452 amino acid residues, consisting of a putative transit peptide of 55 residues and a mature EF-Tu of 397 residues. The coding region was composed of 12 exons and 11 introns that ranged from 76 to 1673bp in length. The deduced amino acid sequence showed 85.9% and 61.2% identity with Arabidopsis mitochondrial EF-Tu and Arabidopsis chloroplast EF-Tu sequence respectively. The transcription initiation site was determined to be 165bp upstream of the AUG initiation codon by primer extension analysis. Southern blot analysis revealed that the cloned EF-Tu gene was encoded by the members of small gene family in maize. Although this gene does not resemble the Arabidopsis nuclear tufA gene, which encodes the plastid EF-Tu, and does not contain sequence elements found in all cyanobacterial and plastid tufA genes, the predicted amino acid sequence includes an N-terminal extension that resembles a mitochondrial targeting sequence, and shares three unique sequence elements with mitochondrial EF-Tu's from Arabidopsis thaliana, Saccharomyces cerevisiae, and Homo sapiens. Therefore, we concluded that this gene encodes the maize mitochondrial EF-Tu.