In this article we review the experimental and theoretical investigations of the linear and nonlinear optical properties of semiconductor quantum well structures, including the effects of electrostatic fields, extrinsic carriers and real or virtual photocarriers.

Linear and nonlinear optical properties of semiconductor quantum wells

Optics offers unique opportunities for reducing energy in information processing and communications while simultaneously resolving the problem of interconnect bandwidth density inside machines. Such energy dissipation overall is now at environmentally significant levels; the source of that dissipation is progressively shifting from logic operations to interconnect energies. Without the prospect of substantial reduction in energy per bit communicated, we cannot continue the exponential growth of our use of information. The physics of optics and optoelectronics fundamentally addresses both interconnect energy and bandwidth density, and optics may be the only scalable solution to such problems. Here we summarize the corresponding background, status, opportunities, and research directions for optoelectronic technology and novel optics, including subfemtojoule devices in waveguide and novel two-dimensional (2-D) array optical systems. We compare different approaches to low-energy optoelectronic output devices and their scaling, including lasers, modulators and LEDs, optical confinement approaches (such as resonators) to enhance effects, and the benefits of different material choices, including 2-D materials and other quantum-confined structures. With such optoelectronic energy reductions, and the elimination of line charging dissipation by the use optical connections, the next major interconnect dissipations are in the electronic circuits for receiver amplifiers, timing recovery, and multiplexing. We show we can address these through the integration of photodetectors to reduce or eliminate receiver circuit energies, free-space optics to eliminate the need for timing and multiplexing circuits (while also solving bandwidth density problems), and using optics generally to save power by running large synchronous systems. One target concept is interconnects from ∼1 cm to ∼10 m that have the same energy (∼10 fJ/bit) and simplicity as local electrical wires on chip.

Attojoule Optoelectronics for Low-Energy Information Processing and Communications

Optics offers unique opportunities for reducing energy in information processing and communications while resolving the problem of interconnect bandwidth density inside machines. Such energy dissipation overall is now at environmentally significant levels; the source of that dissipation is progressively shifting from logic operations to interconnect energies. Without the prospect of substantial reduction in energy per bit communicated, we cannot continue the exponential growth of our use of information. The physics of optics and optoelectronics fundamentally addresses both interconnect energy and bandwidth density, and optics may be the only scalable solution to such problems. Here we summarize the corresponding background, status, opportunities, and research directions for optoelectronic technology and novel optics, including sub-femtojoule devices in waveguide and novel 2D array optical systems. We compare different approaches to low-energy optoelectronic output devices and their scaling, including lasers, modulators and LEDs, optical confinement approaches (such as resonators) to enhance effects, and the benefits of different material choices, including 2D materials and other quantum-confined structures. Beyond the elimination of line charging by the use optical connections, the next major interconnect dissipations are in the electronic circuits for receiver amplifiers, timing recovery and multiplexing. We can address these through the integration of photodetectors to reduce or eliminate receiver circuit energies, free-space optics to eliminate the need for timing and multiplexing circuits (while solving bandwidth density problems), and using optics generally to save power by running large synchronous systems. One target concept is interconnects from ~ 1 cm to ~ 10 m that have the same energy (~ 10fJ/bit) and simplicity as local electrical wires on chip.

Attojoule Optoelectronics for Low-Energy Information Processing and Communications: a Tutorial Review

We analyze energy consumption in optical modulators operated in depletion and intended for low-power interconnect applications. We include dynamic dissipation from charging modulator capacitance and net energy consumption from absorption and photocurrent, both in reverse and small forward bias. We show that dynamic dissipation can be independent of static bias, though only with specific kinds of bias circuits. We derive simple expressions for the effects of photocurrent on energy consumption, valid in both reverse and small forward bias. Though electroabsorption modulators with large reverse bias have substantial energy penalties from photocurrent dissipation, we argue that modulator diodes with thin depletion regions and operating in small reverse and/or forward bias could have little or no such photocurrent energy penalty, even conceivably being more energy-efficient than an ideal loss-less modulator.

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Energy consumption in optical modulators for interconnects

Indirect excitons (IXs) are explored both for studying quantum Bose gases in semiconductor materials and for the development of excitonic devices. IXs were extensively studied in III-V and II-VI semiconductor heterostructures where IX range of existence has been limited to low temperatures. Here, we present the observation of IXs at room temperature in van der Waals transition metal dichalcogenide (TMD) heterostructures. This is achieved in TMD heterostructures based on monolayers of MoS2 separated by atomically thin hexagonal boron nitride. The IXs we realize in the TMD heterostructure have lifetimes orders of magnitude longer than lifetimes of direct excitons in single-layer TMD and their energy is gate controlled. The realization of IXs at room temperature establishes the TMD heterostructures as a material platform both for a field of high-temperature quantum Bose gases of IXs and for a field of high-temperature excitonic devices.

/pdf/indirect-excitons-in-van-der-waals-heterostructures-at-room-2qtxhfhvii.pdf

Indirect excitons in van der Waals heterostructures at room temperature.

A novel GaAs/AlGaAs coupled quantum well structure, consisting of two 46 A wells separated by a 11.5 A barrier, was embedded in a leaky waveguide. Polarization anisotropy and quantum‐confined Stark effect absorption changes are observed, and a 14:1 modulator is demonstrated operating at the peak of the first light‐hole transition. Although the lowest energy transitions behave as predicted by tunneling resonance calculations, higher energy states exhibit unusual behavior due to valence‐band mixing.

Electroabsorption in GaAs/AlGaAs coupled quantum well waveguides

We present absorption measurements in HgCdTe/CdTe multiple quantum wells as a function of temperature, polarization, and well size in the 3–4 μm wavelength region. The energy levels are calculated using a model that includes band nonparabolicity. The spectra are fitted to a sum of continuous broadened steps including the two‐dimensional Sommerfeld enhancement factor. Polarization measurements confirm the assignment of heavy and light holes. The ratio between heavy and light hole absorption of 2.3/1 agrees well with theory. The fit of the linewidth with temperature shows a homogeneous linewidth of 4.6 meV times the density of longitudinal optical phonons and an inhomogeneous linewidth of 6.4 meV, which is similar to the alloy broadening in the bulk material.

Detailed characterization of HgCdTe/CdTe multiple quantum wells

Most optical studies in quantum wells have dealt with rectangular structures where an a ~100-Ǻ GaAs layer is sandwiched between ~100- Ǻ AlGaAs barriers. In contrast, we present a novel coupled-quantum-well (CQW) structure in which the ~100- Ǻ GaAs is replaced by two narrow GaAs wells (46 Ǻ) coupled by a thin AlGaAs barrier (11.5 Ǻ) between them.

Daniel S Chemla

Papers

Electroabsorption in GaAs/AlGaAs coupled quantum well waveguides

Detailed characterization of HgCdTe/CdTe multiple quantum wells

Electroabsorption in GaAs/AlGaAs coupled-quantum-well waveguides