Compact Semiconductor Lasers

Richard De La Rue, Jean-Michel Lourtioz, Siyuan Yu

• Publisher: Wiley-VCH
• Year: 2014
• Print ISBN: 9783527410934
• eBook ISBN: 9783527655366
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

This book brings together in a single volume a unique contribution by the top experts around the world in the field of compact semiconductor lasers to provide a comprehensive description and analysis of the current status as well as future directions in the field of micro- and nano-scale semiconductor lasers.
It is organized according to the various forms of micro- or nano-laser cavity configurations with each chapter discussing key technical issues, including semiconductor carrier recombination processes and optical gain dynamics, photonic confinement behavior and output coupling mechanisms, carrier transport considerations relevant to the injection process, and emission mode control.
Required reading for those working in and researching the area of semiconductors lasers and micro-electronics.

1
Nanoscale Metallo-Dielectric Coherent Light Sources

Maziar P. Nezhad, Aleksandar Simic, Amit Mizrahi, Jin-Hyoung Lee, Michael Kats, Olesya Bondarenko, Qing Gu, Vitaliy Lomakin, Boris Slutsky and Yeshaiahu Fainman

1.1 Introduction

Compact photonic components are important for the design and fabrication of integrated optical devices and circuits. In the case of light sources, reducing the size can result in improved metrics such as higher packing density and reduced power consumption and also may enhance cavity–emitter interactions such as the Purcell effect. Until recently it was commonly known that the minimum size for a laser is ultimately determined by the free-space wavelength, λ₀. For example, as the size of a conventional Fabry–Perot semiconductor laser is scaled down in all three dimensions toward λ₀, three effects adversely influence the lasing process. Firstly, the roundtrip path of the optical wave in the gain medium is shortened. Secondly, radiative losses from the end mirrors have an increased effect. Thirdly, the lateral field confinement in the resonator waveguide is reduced, resulting in a smaller overlap of the optical mode with the gain medium. All these effects lead to a significant increase in the lasing threshold. As a result lasing cannot be achieved below a certain size limit. By allowing the laser size to increase in one or two dimensions, it is possible to reduce the physical size of the laser in the remaining dimension(s) to values below this limit. For example, the disk thickness in whispering-gallery-mode (WGM) lasers [1] can be reduced to a fraction of the free-space wavelength [2] but, to compensate for the small thickness, the disk diameter must be increased. It should be noted that, in addition to the optical mechanisms noted above, nonradiative surface recombination can have a non-negligible negative effect on the emitter efficiency and thus needs to be accounted for in the design and analysis of such sources. The ultimate challenge in this respect is concurrent reduction of the resonator size in all three dimensions, and, at the same time, satisfying the requirements for lasing action.

The size of an optical cavity can be defined using different metrics, for example, the physical dimensions of the cavity or the size of the optical mode. However, if the goal of size reduction is to increase the integration density (for example, in a laser array), the effective cavity size should account for both the overall physical dimensions of the resonator and the spread of the optical mode beyond the physical boundary of the resonator. By this token, most conventional dielectric laser cavities are not amenable to dense integration because they have either a large physical footprint or a large effective mode. For example, distributed Bragg resonators [3] and photonic-crystal cavities [4] (both of which can be designed to have very small mode volumes) have physical footprints that are many wavelengths in size, due to the several Bragg layers or lattice periods that are required for maintaining high finesse. On the other hand, it has been demonstrated that the diameter of thick (λ₀/n) micro-disk lasers can be reduced below their free-space emission wavelength [5]; however, the spatial spread of the resultant modes (which have low azimuthal numbers owing to the small disk diameters) into the surrounding space beyond the physical boundaries of the disks may lead to mode coupling and formation of “photonic molecules” in closely spaced disks [6]. For illustration purposes, an M = 4 WGM for a semiconductor disk with radius r_c = 460 nm and height h_c = 480 nm (Figure 1.1a) is shown in Figure 1.1b, clearly indicating the radiative nature of the mode and its spatial spread, which, as mentioned, can lead to mode coupling with nearby structures. (M is the azimuthal order of the resonance, corresponding to half the number of lobes in the modal plot of |E|.)

Figure 1.1 The M = 4 whispering gallery resonance for a thick semiconductor disk (a) is shown in (b) (r_c = 460 nm, h_c = 480 nm, and n_semi = 3.4). Note the spatial spread of the mode compared to the actual disk size. (c) The same disk encased in an optically thick (d_m = 100 nm) gold shield will have well-confined reflective (d) and plasmonic (e) modes but with much higher mode losses. |E| is shown in all cases and the section plane is horizontal and through the middle of the cylinder. (From [7].). (Please find a color version of this figure on the color plates.)

One approach to alleviate these issues is to incorporate metals into the structure of dielectric cavities, because metals can suppress leaky optical modes and effectively isolate them from their neighboring devices. The modes in these metallo-dielectric cavities can be grouped into two main categories: (i) surface bound (that is, surface plasmon polariton (SPP)) resonant modes and (ii) conventional resonant modes (called photonic modes), resulting purely from reflections within the metal cavity. Although they are highly confined, the disadvantage of plasmonic modes is their high loss, which is caused by the relatively large mode overlap of the optical field with the metal (compared to the reflective case). Owing to the high Joule loss at telecommunication and visible wavelengths, the lasing gain threshold for such cavities can be very large. On the other hand, the negative permittivity of metals not only allows them to support SPP modes, but also enables them to act as efficient mirrors. This leads to the second class of metallo-dielectric cavity modes, which can be viewed as lossy versions of the modes in a perfectly conducting metal cavity. Because the mode volume overlap with the metal is usually smaller than in the plasmonic case, in a cavity supporting this type of mode it is possible to achieve higher resonance quality-factors (Q-factors) and lower lasing gain thresholds, albeit at the expense of reduced mode confinement (compared to plasmonic modes). In general, both types of modes can exist in a metal cavity. Embedding the gain disk mentioned earlier in a gold shield (Figure 1.1c) effec...

Table of contents

• Cover
• Related Titles
• Title Page
• Copyright
• Preface and Introduction
• List of Contributors
• 1 Nanoscale Metallo-Dielectric Coherent Light Sources
• 2 Optically Pumped Semiconductor Photonic Crystal Lasers
• 3 Electrically Pumped Photonic Crystal Lasers: Laser Diodes and Quantum Cascade Lasers
• 4 Photonic-Crystal VCSELs
• 5 III–V Compact Lasers Integrated onto Silicon (SOI)
• 6 Semiconductor Micro-Ring Lasers
• 7 Nonlinearity in Semiconductor Micro-Ring Lasers
• Index
• End User License Agreement