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

Photonics is a promising candidate technology for information processing, communication and data storage. Essential building blocks, such as logic elements and modulators, have been demonstrated. However, because of weak nonlinear light–matter interactions, these components typically require high power densities and large interaction volumes, limiting their application in dense chip-based integration. A solution may be found in surface plasmon polaritons (SPPs), guided electromagnetic waves that propagate with high field confinement along a metal–dielectric interface. We demonstrate an all-optical modulator in which efficient interaction between two light beams at different wavelengths is achieved by converting them into co-propagating SPPs interacting by means of a thin layer of CdSe quantum dots (QDs). The high SPP field confinement and high QD-absorption cross-section enable optical modulation at low power densities (~10^2 W cm^(-2)) in micrometre-scale planar devices.

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All-optical modulation by plasmonic excitation of CdSe quantum dots

The authors have designed, fabricated, and tested vertical-cavity surface-emitting lasers (VCSEL) with diameters ranging from 0.5 mu m to>50 mu m. Design issues, molecular beam epitaxial growth, fabrication, and lasing characteristics are discussed. The topics considered in fabrication of VCSELs are microlaser geometries; ion implementation and masks; ion beam etching packaging and arrays, and ultrasmall devices. >

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Vertical-cavity surface-emitting lasers: Design, growth, fabrication, characterization

We report extended experimental and theoretical results for the quantum well self-electrooptic effect devices. Four modes of operation are demonstrated: 1) optical bistability, 2) electrical bistability, 3) simultaneous optical and electronic self-oscillation, and 4) self-linearized modulation and optical level shifting. All of these can be observed at room-temperature with a CW laser diode as the light source. Bistability can be observed with 18 nW of incident power, or with 30 ns switching time at 1.6 mW with a reciprocal relation between switching power and speed. We also now report bistability with low electrical bias voltages (e.g., 2 V) using a constant current load. Negative resistance self-oscillation is observed with an inductive load; this imposes a self-modulation on the transmitted optical beam. With current bias, self-linearized modulation is obtained, with absorbed optical power linearly proportional to current. This is extended to demonstrate light-by-light modulation and incoherent-to-incoherent conversion using a separate photodiode. The nature of the optoelectronic feedback underlying the operation of the devices is discussed, and the physical mechanisms which give rise to the very low optical switching energy (∼4 fJ/ μm2) are discussed.

The quantum well self-electrooptic effect device: Optoelectronic bistability and oscillation, and self-linearized modulation

The authors studied the effects of changing the barrier design of GaAs-Al/sub x/Ga/sub 1-x/As quantum wells on the electroabsorption, exciton saturation, and carrier sweep-out times. Five samples with x values ranging from 0.2 to 0.4 and barrier thicknesses from 35 to 95 AA were studied. Within this range, the authors find that the electroabsorption is not very sensitive to the barrier thickness, but that the ionization field of the excitons approximately doubles for an increase of x from 0.2 to 0.4. The samples with high, thick barriers have lower internal quantum efficiencies than those with low, thin barriers. It was found that the exciton saturation intensity increases with increasing applied field, and decreasing barrier thickness or height. Time-resolved electroabsorption measurements confirm the variation in sweep-out rates between samples, and indicate that the escape mechanism at low field is probably a thermally-assisted tunneling process. >

Quantum well carrier sweep out: relation to electroabsorption and exciton saturation

Various pulsed logic functions (nor, xor, etc.) are performed in a high‐finesse nonlinear Fabry–Perot etalon containing GaAs‐AlGaAs multiple quantum wells at room temperature. Input pulses with energies less than 3 pJ incident on the device produced contrasts greater than 5 : 1 in the nor gate, and relaxation times of about 5 ns allowed us to operate the gates at 82 MHz. Thermal stability was demonstrated in the nor gate even though the device was poorly heat sunk.

3‐pJ, 82‐MHz optical logic gates in a room‐temperature GaAs‐AlGaAs multiple‐quantum‐well étalon

We present a systematic study of the dependence of the optical nonlinearities on quantum well thickness for GaAs/AlGaAs multiple quantum wells (MQW’s) at room temperature and compare them with bulk GaAs. The maximum change in the refractive index is greatest for the MQW’s with the smallest well size and decreases with increasing well size, reaching a minimum for bulk GaAs. The maximum index change per photoexcited carrier increases by a factor of 3 as the well size decreases from bulk to 76 A MQW.

Measurements of room‐temperature band‐gap‐resonant optical nonlinearities of GaAs/AlGaAs multiple quantum wells and bulk GaAs

Raman experiments on polar phonons in GaAs‐Ga1−xAlxAs superlattices are reported. Other data from the literature, and its interpretation in terms of folding of the phonon Brillouin zone and scattering from q≠0 phonons induced by electronic zone folding, are discussed. An alternative explanation is presented based on optical anisotropy induced by layering. Numerical results show good agreement with experiment.

Raman scattering in superlattices: Anisotropy of polar phonons

Optical bistability is observed using a single‐mode diode laser in a GaAs‐AlGaAs multiple‐quantum‐well etalon with as low as six mW power at 830 nm.

A. C. Gossard

Papers

3‐pJ, 82‐MHz optical logic gates in a room‐temperature GaAs‐AlGaAs multiple‐quantum‐well étalon

Measurements of room‐temperature band‐gap‐resonant optical nonlinearities of GaAs/AlGaAs multiple quantum wells and bulk GaAs

Raman scattering in superlattices: Anisotropy of polar phonons

Use of a diode laser to observe room‐temperature, low‐power optical bistability in a GaAs‐AlGaAs etalon