The electronic and optical properties and the recent progress in applications of 2D semiconductor transition metal dichalcogenides with emphasis on strong excitonic effects, and spin- and valley-dependent properties are reviewed. Recent advances in the development of atomically thin layers of van der Waals bonded solids have opened up new possibilities for the exploration of 2D physics as well as for materials for applications. Among them, semiconductor transition metal dichalcogenides, MX2 (M = Mo, W; X = S, Se), have bandgaps in the near-infrared to the visible region, in contrast to the zero bandgap of graphene. In the monolayer limit, these materials have been shown to possess direct bandgaps, a property well suited for photonics and optoelectronics applications. Here, we review the electronic and optical properties and the recent progress in applications of 2D semiconductor transition metal dichalcogenides with emphasis on strong excitonic effects, and spin- and valley-dependent properties.

Photonics and optoelectronics of 2D semiconductor transition metal dichalcogenides

A laser cavity formed from a single defect in a two-dimensional photonic crystal is demonstrated. The optical microcavity consists of a half wavelength–thick waveguide for vertical confinement and a two-dimensional photonic crystal mirror for lateral localization. A defect in the photonic crystal is introduced to trap photons inside a volume of 2.5 cubic half-wavelengths, approximately 0.03 cubic micrometers. The laser is fabricated in the indium gallium arsenic phosphide material system, and optical gain is provided by strained quantum wells designed for a peak emission wavelength of 1.55 micrometers at room temperature. Pulsed lasing action has been observed at a wavelength of 1.5 micrometers from optically pumped devices with a substrate temperature of 143 kelvin.

https://science.sciencemag.org/content/sci/284/5421/1819.full.pdf

Two-Dimensional Photonic Band-Gap Defect Mode Laser

Inorganic semiconductor light-emitting diodes (LEDs) are environmentally benign and have already found widespread use as indicator lights, large-area displays, and signage applications. In addition, LEDs are very promising candidates for future energy-saving light sources suitable for office and home lighting applications. Today, the entire visible spectrum can be covered by light-emitting semiconductors: AlGaInP and AlGaInN compound semiconductors are capable of emission in the red to yellow wavelength range and ultraviolet (uv) to green wavelength range, respectively. Currently, two basic approaches exist for white light sources: The combination of one or more phosphorescent materials with a semiconductor LED and the use of multiple LEDs emitting at complementary wavelengths. Both approaches are suitable for high efficiency sources that have the potential to replace incandescent and fluorescent lights. In this article, the properties of inorganic LEDs will be presented, including emission spectra, electrical characteristics, and current-flow patterns. Structures providing high internal quantum efficiency, namely, heterostructures and multiple quantum well structures, will be discussed. Advanced techniques enhancing the external quantum efficiency will be reviewed, including resonant-cavities, die shaping (chip shaping), omnidirectional reflectors, and photonic crystals. Different approaches to white LEDs will be presented and figures-of-merit such as the color rendering index, luminous efficacy, and luminous efficiency will be explained. Finally, the packaging of low power and high power LED dies will be discussed.


Keywords:

light-emitting diodes (LEDs);
solid-state lighting;
compound semiconductors;
device physics;
reflectors;
resonant cavity LEDs;
white LEDs;
packaging

Light Emitting Diodes

Carbon nanotubes possess unique properties that make them potentially useful in many applications in optoelectronics. This review describes the fundamental optical behaviour of carbon nanotubes as well as their opportunities for light generation and detection, and photovoltaic energy generation.

Carbon-nanotube photonics and optoelectronics

We report the experimental demonstration of an electrically driven, single-mode, low threshold current (∼260 μA) photonic band gap laser operating at room temperature. The electrical current pulse is injected through a sub-micrometer-sized semiconductor wire at the center of the mode with minimal degradation of the quality factor. The actual mode of interest operates in a nondegenerate monopole mode, as evidenced through the comparison of the measurement with the computation based on the actual fabricated structural parameters. As a small step toward a thresholdless laser or a single photon source, this wavelength-size photonic crystal laser may be of interest to photonic crystals, cavity quantum electrodynamics, and quantum information communities.

http://science.sciencemag.org/content/sci/305/5689/1444.full.pdf

Electrically Driven Single-Cell Photonic Crystal Laser

Optical microcavities are resonators that have at least one dimension on the order of a single optical wavelength. These structures enable one to control the optical emission properties of materials placed inside them. They can, for example, modify the spatial distribution of radiation power, change the spectral width of the emitted light, and enhance or suppress the spontaneous emission rate. In addition to being attractive for studying the fundamental physics of the interaction between materials and vacuum field fluctuations, optical microcavities hold technological promise for constructing novel kinds of light-emitting devices. One of their most dramatic potential features is thresholdless lasing. In this way and others, controlled spontaneous emission is expected to play a key role in a new generation of optical devices.

https://science.sciencemag.org/content/sci/256/5053/66.full.pdf

Physics and Device Applications of Optical Microcavities

Enhanced spontaneous emission has been observed with wavelength‐sized monolithic Fabry–Perot cavities containing GaAs quantum wells. With an on‐resonance cavity structure, the photoluminescence intensity increases in the cavity axis direction, and the spontaneous emission lifetime is experimentally found to decrease.

Enhanced spontaneous emission from GaAs quantum wells in monolithic microcavities

Spontaneous Emission in Optical Cavities: A Tutorial Review, E.V. Goldstein and P. Meystre Introduction Free Space Spontaneous Emission Spontaneous Emission in Cavities Velocity-Dependent Spontaneous Emission Conclusion A Simple Theory on the Effect of Dephasing of Vacuum Fields on Spontaneous Emission in a Microcavity, Y. Lee Introduction Theoretical Model I Theoretical Model II Summary Appendicies A-C Effects of Atomic Broadening on Spontaneous Emission in an Optical Microcavity, K. Ujihara Introduction Analysis of Spontaneous Emission Discussion and Conclusion Microcavities and Semiconductors: The Strong-Coupling Regime, C. Weisbuch, R. Houdre, and R.P. Stanley Introduction The Fabry-Perot Resonator: A Planar Microcavity Models of Strong Light-Matter Coupling Optics of Semiconductors Conclusion Electromagnetic Field Mode Density Calculated via Mode Counting, S.D. Brorson Introduction No Confinement: A Dipole in Free Space One Dimension of Confinement: The Planar Mirror Cavity Two Dimensions of Confinement: The Waveguide Cavity Three Dimensions of Confinement: The Box Microcavity Discussion Spontaneous Emission in Dielectric Planar Microcavities, G. Bjork and Y. Yamamoto Introduction The Ideal Planar Cavity Fundamentals of Dielectric Bragg Mirrors Dielectric Cavity Spontaneous Emission Pattern Dielectric Cavity Spontaneous Emission Lifetime Dielectric Cavity Stimulated Emission Conclusions and Outlooks Spontaneous Emission in Microcavity Surface Emitting Lasers, T. Baba and K. Iga Introduction Spontaneous Emission in a Microcavity Expression of Radiation Energy Modes in Microcavity SELS Spontaneous Emission Factor Effects of Electron Quantum Confinement Lasing Characteristics Summary Spontaneous and Stimulated Emission in the Microcavity Laser, H. Yokoyama Introduction Photon Emission in Microcavities Microcavity Lasers Microcavity Semiconductor Lasers Prospects for Device Applications Summary Recent Progress in Optical Microcavity Experiments, H. Yokoyama Introduction Cavity Configurations Alterations in Spontaneous Emission Properties Laser Oscillation Summary Application of Microcavities: New Photoelectronic Integrated Systems, I. Hayashi Introduction Photoelectronic Integrated Systems Micro-Photoelectronic Devices Summary and Future Prospects Index

Spontaneous Emission and Laser Oscillation in Microcavities

We describe the alteration of spontaneous emission of materials in optical microcavities having dimensions on the order of the emitted wavelength. Particular attention is paid to one-dimensional optical confinement structures with pairs of planar reflectors (planar microcavities). The presence of the cavity causes great modifications in the emission spectrum and spatial emission intensity distribution accompanied by changes in the spontaneous emission lifetime. Experimental results are shown for planar microcavities containing GaAs quantum wells or organic dye-embedded Langmuir-Brodgett films as light emitting layers. Also discussed are the laser oscillation properties of microcavities. A remarkable increase in the spontaneous emission coupling into the laser oscillation mode is expected in microcavity lasers. A rate equation analysis shows that increasing the coupling of spontaneous emission into the cavity mode causes the disappearance of the lasing threshold in the input-output curve. Experimentally verification is presented using planar optical microcavities confining an organic dye solution. The coupling ratio of spontaneous emission into a laser mode increases to be as large as 0.2 for a cavity having a half wavelength distance between a pair of mirrors. At this point, the threshold becomes quite fuzzy. Differences between the spontaneous emission dominant regime and the stimulated emission dominant regime are examined with emission spectra and emission lifetime analyses.

Controlling spontaneous emission and threshold-less laser oscillation with optical microcavities

We have demonstrated successful two-photon excitation fluorescence bioimaging using a high-power pulsed all-semiconductor laser. Toward this purpose, we developed a pulsed light source consisting of a mode-locked laser diode and a two-stage diode laser amplifier. This pulsed light source provided optical pulses of 5 ps duration and having a maximum peak power of over 100 W at a wavelength of 800 nm and a repetition frequency of 500 MHz.

Hiroyuki Yokoyama

Papers

Physics and Device Applications of Optical Microcavities

Enhanced spontaneous emission from GaAs quantum wells in monolithic microcavities

Spontaneous Emission and Laser Oscillation in Microcavities

Controlling spontaneous emission and threshold-less laser oscillation with optical microcavities

Two-photon fluorescence bioimaging with an all-semiconductor laser picosecond pulse source