Qing Gu - Semiconductor Nanolasers

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First published 2017

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ISBN 978-1-107-11048-9 Hardback

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The infrastructure that supports modern society, including health care, education, transportation, finance, and scientific and technological research, has become inextricably tied to the continuous progress in the ability to generate, transmit, receive, and process information. While electronic devices integrated in highly complex circuits have enabled this progress, electronic devices and circuits suffer from inherent limitations, namely resistor-capacitor]. Like the first transistors, the first lasers were macroscopic devices, with footprints on the order of centimeters to decimeters.

As the cavity size is reduced with respect to the emission wavelength, interesting physical effects, unique to electromagnetic cavities, arise. Experiments in the radio and microwave frequencies first demonstrated that the spontaneous emission rate of atoms in a cavity could be enhanced or inhibited, relative to their rate of emission in free space. The change in the spontaneous emission rate was found to depend on the geometry of the cavity as well as the orientation and spectra of the atoms [].

1.1 The History of Laser Minimization

To complement micro- and nano-electronics, optical components have undergone a process of miniaturization over the past several decades. Thus, it is not an exaggeration to state that the maintenance and improvement of modern society is directly related to research advancements in photonic devices and circuits. Similar to transistors, the reduction of the size of lasers would enable a higher packing density of devices and lower power consumption].

outlines the progress in laser miniaturization over the past few decades. We see from the development time line of small lasers that the evolution of a new laser device usually takes 1020 years: from new laser concept to first proof-of-concept optically pumped demonstration, then to electrically pumped, and in some cases to commercial applications. For practical and commercial insertion of lasers, the devices need to be continuous-wave (CW) current injected at room temperature, ideally with stable emission, reasonably long lifetime and particular properties that the already established types of lasers cannot offer.

Figure 1.1

Development time line of small lasers, from first demonstration to electrical, continuous-wave, and room-temperature operation, and in some cases to commercial applications. Also shown is a size comparison of various types of small lasers. The electron microscopy pictures are scaled to the free-space emission wavelength 0 of each laser: (a) VCSEL, (b) microdisk laser, (c) photonic crystal laser, (d) metallic non-plasmon mode laser; (e) metallic propagating plasmon mode laser; (f) localized plasmon mode laser. The free-space wavelength scale bar of the metal-cavitybased lasers (df) is twice that of the dielectric lasers (ac) to permit details to be seen. The metal-cavity lasers are typically smaller than 0 and dramatically smaller than corresponding dielectric-cavity lasers.

Reprinted from reference [] with permission from Macmillan Publishers Ltd.

. Each type of cavity or laser design has its advantage: for example, photonic crystal dielectric lasers with few-quantum-dot gain show extremely low threshold at cryogenic temperatures; dielectric cavities generally have higher Q factors than metal-clad cavities; and metal-clad cavities have much smaller dimensions than dielectric ones. In the context of subwavelength devices, one of the most figure-of-merit is size.