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Annual Meeting Optical Society of America

Electrooptical switching and bistability in coupled quantum wells

Jacob B. Khurgin

Published Jan 1, 1989• 1 citation
Publisher

Abstract

Modulation-doped system of asymmetric coupled quantum wells is shown theoretically to possess a large optical nonlinearity due to the charge transfer associated with intersubband transition. The range of parameters leading to optical bistability has been determined. A novel triple quantum well system is suggested, allowing a few orders of magnitude reduction in switching power density, to ~50 W/cm2, while having a response time of <100 ps. Optically assisted transfer of up to 90% of all carriers between wells can become an operating principle of novel hybrid optoelectronic devices, incorporating the triple quantum well system into the channel of modulation-doped field-effect transistor with potential applications as fast optoelectronic switches and detectors in the 10-µm range.

DOI
10.1364/oam.1989.mb4

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Modulation-doped system of asymmetric coupled quantum wells is shown theoretically to possess a large optical nonlinearity due to the charge transfer associated with intersubband transition. The range of parameters leading to optical bistability has been determined. A novel triple quantum well system is suggested, allowing a few orders of magnitude reduction in switching power density, to ~50 W/cm2, while having a response time of <100 ps. Optically assisted transfer of up to 90% of all carriers between wells can become an operating principle of novel hybrid optoelectronic devices, incorporating the triple quantum well system into the channel of modulation-doped field-effect transistor with potential applications as fast optoelectronic switches and detectors in the 10-µm range.
A modulation-doped system of asymmetric-coupled quantum wells is shown theoretically to possess a large optical nonlinearity due to the change transfer associated with intersubband transition. The range of parameters leading to optical bistability is detetermined. A novel triple quantum well system is suggested, allowing a few orders of magnitude reduction in switching power density. Novel hybrid optoelectronic devices, incorporating the triple quantum well system into the channel of a modulation-doped field-effect transistor, are proposed with potential applications as optoelectronic switches and detectors.
In multiple quantum wells system, a large dipole strength and a narrow band width were observed. This suggests that the intersubband optical transitions in a quantum well may have very large optical nonlinearities. The intersubband transition have potential for device applications in far-infrared laser amplifiers, photodetectors and high-speed electrooptical modulators. In device design, the region of input optical density for linear response is a very important parameter. And the optical saturation is related with tunnelling effects and the intersubband relaxations. So the intersubband nonlinear absorption is very valuble for a fundamental and a practical point of views. Recently, there are some works on this region [1], [2].
We propose the use of a novel doped, stepped semiconductor quantum well structure as an optical nonlinear element. The large nonlinearity results from the change of many-body effects on resonant excitation of electrons from the ground to the first excited subband. Self-consistent calculations showed that the intersubband energy separation can be increased by up to 7 meV on intense illumination, compared to 1 meV for a conventional, square quantum well. We estimate that optical switching at powers below 1 W are feasible in an optimized structure, with switching times of about 1 ps.
We present self-consistent calculations of the energy levels in an asymmetric coupled quantum-well structure coupled to a CO2 laser source which is nearly resonant with an intersubband transition. Solving Schrödinger’s and Poisson’s equations concurrently with a set of rate equations, it is shown that intrinsic optical bistability can be achieved. The bistability arises from charge transfer and a subsequent, self-consistent shift of the energy levels which feeds back to the absorption rate. Using realistic parameters, an on-off intensity ratio of 4:1 at switching powers of a few 100 kW/cm2 is predicted at about 10-μm wavelength.
In the past few years advance in nanofabrication techniques have made it possible to make well-like compound semiconductor structures of low nanometer size.In these structures,the state of the electron in the direction perpendicular to the length of the well is quantized with sufficiently large separation in energy levels and the motion of the electron parallel to the length is that of a quasi-free electron.One of the most remarkable properties of these one-dimensional electron system is that the optical transitions between the size-quantized subbands are feasible.The result shows there is a large optical nonlinearities in asymmetric quantum wells.The problem of optical saturation of intersubband absorption has recently attracted considerable attention.The problem is very important to the development of superlattice devices such as fast infrared detectors and(modulators) because one needs to know the range of input optical intensities for which linear operation is(possible.) The analytical expression of the linear and the third-order nonlinear optical intersubband absorption coefficients are obtained in Pschl-Teller wells using the density matrix formalism taking into account the(intrasubband) relaxation.Since there are two adjustable parameters κ and λ in Pschl-Teller wells,the shapes of the wells will change with the change of the parameters κ and λ,accordingly the linear and the third-order nonlinear optical absorption coefficients will change with the parameters κ and λ.It is shown that the optical absorption coefficient depends on the parameter,and the optical intensity strongly. The absorption saturation will occur with I increase.Numerical results are presented for a typical AlGaAs/GaAs Pschl-Teller well.