Optical: systems and elements – Optical modulator – Light wave directional modulation
Patent
1992-02-26
1996-01-02
Moskowitz, Nelson
Optical: systems and elements
Optical modulator
Light wave directional modulation
359240, 359318, 257 9, 257 14, G02F 136, G02F 1015
Patent
active
054813972
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to quantum well structures, and in particular but not exclusively to such structures configured for use as optical modulators.
2. Related Art
In the broad field of optical signal processing there are many applications for high performance optical signal encoding and processing elements. For example, in high speed optical fiber communications systems, direct modulation of laser sources leads to undesirable wavelength shifts, "chirp", in the optical output of the laser. One way in which chirp may be avoided is to cease modulating the laser directly, optical modulation being achieved through use of a modulator in the optical path of the laser's output. In the generally less well developed area of optical signal processing, components such as logic gates, latches, and signal encoders are required. The bandwidth which optical signal processing and optical communications potentially offer means that there is a desire for components which operate at high speed, typically switchable at GHz rates, and preferably switchable at tens of GHz.
The present invention is concerned with such signal processing components, and in particular with modulators, which comprise quantum well structures. A quantum well is, in its simplest form, a double heterostructure, with a layer of low band-gap material sandwiched between two layers of higher band-gap material. Typically the layers all comprise semiconductors, for example the double heterostructure may consist of GaAs sandwiched between identical layers of AlGaAs. If the layer of low band-gap material is sufficiently thin, of the order of 100.ANG. or less, the energy levels in the valence and conduction bands becomes quantised, and the structure is referred to as a "quantum well".
While single quantum wells do exhibit measurable quantum effects, the intensity or strength of the effects can be increased by increasing the number of quantum wells. Several, typically tens or many tens or hundreds of quantum wells are formed in a multilayer structure, which structures are referred to as "multiple quantum wells", or "multiple quantum well" ("mqw") structures.
The basis behind the use of quantum well structures as modulators is that they can exhibit large changes in their optical absorption coefficient on the application of an electric field.
In devices such as QW modulators, utilising excitonic effects, the exciton of most significance is that involving the n=1 heavy hole. In this specification, unless the context clearly requires otherwise, we refer to the n=1 heavy hole exciton.
Our own interpretation of the accepted explanation of this phenomenon will now be given with reference to FIG. 19 which shows, schematically, the behaviour of a conventional quantum well structure. The structure will be assumed to consist of a pair of GaAlAs layers 2, 2' with a GaAs layer 1 therebetween. Thin solid lines 3 and 4 indicate respectively the valence band maximum in bulk GaAlAs and GaAs. Thin solid lines 5 and 6 similarly indicate the conduction band minimum in bulk GaAlAs and GaAs respectively. However because the GaAs layer is thin enough to provide quantum confinement of the electrons and holes there is an increase in minimum energy for them both. The new minima, the quantum well minima, for the electrons and holes are shown as broken lines 7 and 8 respectively. Note that in FIG. 19 electron energy increases towards the top of the Figure and hence hole energy increases as one moves down the Figure. With no applied field the resultant energy gap 9 is greater than that of the equivalent bulk GaAs. Typical probability density distributions of electrons and holes in the well are indicated by 10 and 11. The probability density distributions are pseudo-Gaussian and centered on the mid-point of the well.
FIG. 1b shows, schematically, the effect of applying an electric field across the layers of the well of FIG. 1a. With the field applied, the shape of the potential energy well seen by the electrons and holes changes dramatic
REFERENCES:
patent: 4686550 (1987-08-01), Capasso et al.
patent: 4826295 (1989-05-01), Burt
patent: 4950044 (1990-08-01), Marita
patent: 5008717 (1991-04-01), Joseph et al.
patent: 5121181 (1992-06-01), Smith et al.
Hiroshima et al, Phys. Rev. B. Condens. Matter, vol. 38, #2, pp. 1241-1245, pp. Jul. 15, 1988; abst. only herewith.
Walmsley et al, Semicond. Sci. Technol., vol. 8, #2, pp. 268-275, Feb. 1993; abst. only provided herewith.
Enhanced Bandgap Resonant Nonlinear Susceptibility in Quantum-Well Heterostructures, M. G. Burt, Electronic Letters, Feb. 17, 1983, vol. 19, No. 4, pp. 132-133.
Auger Recombination in a Quantum Well Heterostructure, C. Smith, R. A. Abram & M. G. Burt, J. Phys. C: Solid State Phys., 16 (1983) pp. L171-175.
Gain Spectra of Quantum-Well Lasers, M. G. Burt, Electronics Letters, Mar. 17, 1983, vol. 19, No. 6, pp. 210-211.
Linewidth Enhancement Factor for Quantum-Well Lasers, Electronics Letters, Jan. 5, 1984, vol. 20, No. 1, pp. 27-28.
Auger Recombination in Long-Wavelength Quantum-Well Lasers, Electronics Letters, Oct. 11, 1984, vol. 20, No. 21, pp. 893-894.
Radiative Efficiency In Low Dimensional Semiconductor Structure, Electronics Letters, Aug. 15, 1985, vol. 21, No. 17, pp. 733-734.
Auger Recombination in a Quantum-Well-Heterostructure Laser, R. I. Taylor, R. A. Abram, M. G. Burt, C. Smith, IEE Proceedings, vol. 132, No. 6, Dec. 1985, pp. 364-369.
Erratum, M. G. Burt, Semiconductor Sci & Technology 2, 1987, p. 701.
Continuity Conditions for Envelope Functions at Semiconductor Interfaces Derived from the Bastard Model, R. I. Taylor & M. G. Burt, Semicond. Sci. Technol. 2, 1987, pp. 485-490.
An Exact Formulation of the Envelope Function Method for the Determination of Electronic States in Semiconductor Microstructure, M. G. Burt, Semicond. Sci. Technol. 3, 1988, pp. 739-753.
A New Effective-Mass Equation for Microstructures, M. G. Burt, Semicond. Sci. Technol. 3, 1988, pp. 1224-1226.
Comments on "Comparison of Multiquantum Well, Graded Barrier, and Doped Quantum Well GaInAs/AIInAs Avalanche Photodiodes: A Theoretical Approach", M. G. Burt, IEEE Journal of Quantum Electronics, vol. 25, No. 5, May 1989, pp. 1126-1128.
Journal of Applied Physics, vol. 62, No. 8, 15 Oct. 1987, American Institute of Physics, T. Hiroshima et al: "Quantum-confined start effect in graded-gap quantum wells"--pp. 3360-3365.
Journal of Applied Physics Lett 45 (10), 15 Nov. 1984--"Staggered-lineup heterojunctions as sources of tunable below-gap radiation: Experimental verification" by E. J. Caine et al. pp. 1123-1125.
IEEE Electron Device Letters, vol. EDL-4, No. 1, Jan. 1983--"Staggered-Lineup Heterojunctions as Sources of Tunable Below-Gap Radiation: Operating Principle and Semiconductor Selection" by H. Kroemer et al--pp. 20-22.
British Telecommunications
Moskowitz Nelson
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