Beyond quantum dot LEDs: Optical gain and laser action in red, green, and blue colors
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Introduction II−VI compound colloidal quantum dots (CQDs), or semiconductor nanocrystals, have advanced from being laboratory curiosities to technologically viable, competitive materials in full-color solid-state displays, as evidenced by the articles in this issue as well as from recent literature.1–10 These successes beg the question of the possibility to further expand the role of CQDs to other optical device technologies. In this article, we examine their status across the visible spectrum as possible candidates for active devices such as lasers. Beyond the requirements for current display materials, additional challenges for CQD lasers rapidly emerge, including fundamental physics questions (e.g., the nature of excited electronic states) and practical considerations (e.g., robustness of the material to withstand high excitation levels). While significant research into the physics of optical gain in II–VI CQDs has been investigated actively for more than a decade,11–14 we show in this review how recent developments may have advanced the landscape to gauge the technological feasibility of a singlematerial-based laser across the red, green, and blue (RGB) wavelengths. If eventually realized, such devices could replace existing (and ubiquitous) visible semiconductor lasers, fabricated from epitaxially grown single crystals, which presently typically require three different materials to cover the visible wavelengths (such as InGaN, GaAsP, InGaAs), each with distinct materials science and device architectures.15–17 The green
spectral region is especially difficult to access by either III-nitride or III-phosphides and still carries the appellation “valley of death.”16–18 In the following, we first examine key material requirements for a CQD laser, focusing specifically on the benefits of very densely packed solid CQD thin films. We then address fundamental questions regarding the physics of optical gain and stimulated emission from photoexcited cadmium selenidebased II–VI CQDs. We point out some of the fundamental inhibiting mechanisms that only recently appear to have become surmountable, to enable electron–hole excitation (injection) levels that are practical and reasonable. Finally, early proof-of-concept demonstrations of stimulated emission and lasing in the RGB are illustrated, with some emphasis on recent advances in the authors’ laboratories.19
Material considerations for potential colloidal quantum dot lasers Any candidate material for a laser must possess superior intrinsic optical properties. In addition to the obvious requirements of high radiative quantum efficiency and suitable electronic structure, other attributes such as optical homogeneity, suitability to adapt to an optical resonator for optical feedback, plus a host of other material demands11,21 place a high bar to entry for a new material. In recent years, striking progress has been made to increase the radiative efficiency of II–VI CQDs
Cuong Dang, EEE Luminous! Center of Excellence for Semiconductor Lighting and Displays, Nanyang Technological Univer
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