Semiconductor nanolasers (A Tutorial)

This tutorial focuses on miniaturization of semiconductor lasers down to sub-micron scale based on both bottom-approach such as nanowires and nanosheets and top-down approach such as etched nano-pillars encased in metallic or plasmonic shells. Continuing size reduction of semiconductor lasers is both the natural tread of technology development over the last 50 plus years and the requirement of applications of lasers in many technological areas. Various novel concepts in active region designs such as double heterostructures, quantum wells, quantum wires, and quantum dots have contributed to both size reduction and performance improvement of semiconductor lasers. Concurrently novel light confinement mechanisms or cavity design concepts have played essential roles in this process, including Fabry-Perot cavity, distributed feedback structures, distributed Bragg reflectors (with both horizontal and vertical cavities), surfacegratings, micro-disk structures, photonic crystals, external cavities, and semiconductor nanowires. This development had led to the reduction of semiconductor lasers from millimeter scales of 50 years ago down to a few microns in characteristic length scales until a few years ago. The renaissance of metal optics or now more fashionably known as plasmonics a decade or so ago raised several interesting questions [1][2]: Can semiconductor lasers be made even smaller than those micron-scale lasers? Is it possible to realize a laser that is smaller than the optical wavelength (in vacuum) in all three dimensions, instead of only in one or two dimensions as in the case of some of the micron-scale lasers known then such as micro-disk lasers and nanowire lasers? These questions were most directly stimulated by the novel concept of SPASER by Bergman and Stockman [4], who proposed the stimulated emission of surface plasmons. A question of more fundamental importance was: is there a fundamental limit to the size reduction for a laser? For more practical applications, can such nanoscale lasers operate at room temperature, and under electrical injection? These questions were either raised directly by the DARPA NACHOS program or stimulated during the process. It is important to notice that there questions were not purely academic. Lager scale integration of photonic devices, especially with electronic components ultimately requires nanoscale lasers. Specifically future optical interconnects require semiconductor light sources as small as a few hundred nanometers [Miller, Ding] due to the limit of power budget for any on-chip applications. In 2007, we proposed [2] a semiconductor-core metal-shell structure as a mechanism to achieve a nanolaser with diameter as small as ~100 nm in the wavelength range as long as near-infrared. The simulation using conventional semiconductor as a gain medium and silver as a shell cavity showed that it was indeed possible in several situations for the optical gain in the core to overcome the loss in the metallic shell. The same idea was simultaneously studied by Hill and his colleagues and the first such core-shell nanolasers were soon demonstrated [3] towards later in 2007. Since then a flourish of nanolaser designs have been proposed [5]-[18], all with metallic structures as essence of light confinement or cavities. Looking back now (see [19]-[22] for a summary review), it seemed that 2007 represented a paradigm shift, whereas all the micro-lasers were based purely on semiconductor or dielectric structures to confine light before, all the nanolaser designs after rely on metallic structures. In this tutorial, I will start with a brief introduction to background of semiconductor nanolasers, with specific discussions about light confinement at nanoscale. The important concept of confinement factor will be discussed to illustrate the counter-intuitive nature [29][23] of confinement factor in the case of strongly confined nanoscale cavities. I will then overview the development of semiconductor nanowire lasers [30] based on the bottom-up approach over the last 10 years. One of the greatest advantages of such nanowire technology is the possibility of growing semiconductor materials with much larger lattice mismatch than possible in more conventional planar epitaxial growth. Examples will be presented to show how we can take advantage of such new possibilities to achieve multiwavelength or multi-color lasers[25]-[28], or widely tunable lasers from a single substrate [25] or from a single monolithic structure[32][31], including lasing from a single substrate with over 200-nm wavelength tunability, simultaneous green and red lasing, and dynamically color tuning. In the process, we will also highlight the challenges of further shrinking semiconductor nanowire lasers and the need of metallic cavities. In the last part of the presentation, our progress over the last 7 years [19][21][29] on metallic cavity nanolasers will be presented including the demonstration of first sub-diffraction lasers under electrical injection and the eventual demonstration of a continuous wave operation of a sub-wavelength scale laser at room temperature. Remaining challenges and future directions will be discussed to conclude the tutorial.