Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
1996-05-14
2001-01-09
Mai, Huy (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S262000, C359S320000
Reexamination Certificate
active
06172793
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an apparatus for controlling an optical transition and has particular but not exclusive application to optical switching performed by producing optically generated resonant excitation of charge carriers in multiple quantum wells (MQWs), and photodetectors.
Optical switching and photodetection using MQWs is attracting much interest because of its potential for ultrafast optical switching for computing, and also for ultrafast optical detection. For example, reference is directed to our co-pending EP-A-0 651 448 which discloses GaAs metal-semiconductor-metal (MSM) photoconductors for photodetection. Reference is also directed to H. M. Gibbs et al, Appl. Phys. Lett. 41, 221 (1982) for a discussion of switchable optical bi-stability in a GaAs etalon.
In such devices, an input, or first optical pulse produces excitation within an optically responsive medium, with the result that it changes its optical or electrical characteristics. In the aforementioned semiconductor devices, the pulse raises charge carriers to an excited state, producing electron-hole-pairs or excitons. The speed of operation of such devices is limited by the dwell time of the optically excited carriers in the devices. High speed devices require a fast mechanism to get rid of these carriers shortly after the optical excitation.
A number of techniques have been proposed previously. It has been proposed to use materials which have a short carrier lifetime for fast carrier trapping, for example GaAs grown at a low substrate temperature. For example Y. Chen et al Appl. Phys. Lett. 59, 1984 (1991) discloses that a high defect density in an optically active material causes a short carrier lifetime. In particular, Chen et al disclose that LT-GaAs grown by molecular beam epitaxy at a substrate temperature of 200° C. results in a carrier lifetime of less than 0.5 ps for a MSM-photodetector. The problem associated with the use of LT-GaAs material is that the quality of the optically responsive layer is reduced and it becomes more difficult to optimise the device parameters when it is integrated with other devices that require material of long carrier lifetime. Reference is also directed to M. Lambsdorff et al, Appl. Phys. Lett. 58, 1881-1883 (1991).
An alternative approach is to sweep charge carriers rapidly out of the optical responsive medium. Reference is directed to M. Klingenstein et al, Appl. Phys. Lett. 58, 2503 (1991). This technique is used in eg P—I—N photodiodes and MSM photodetectors and typically the fastest responsive time is of the order of 10 ps.
A further technique involves tunnelling from quantum wells formed in the optically responsive medium. Reference is directed to M. Tsuchiya et al Phys. Rev. Lett. 59, 2356 (1987) and A. Tackeuchi et al Appl. Phys. Lett. 61, 1892 (1992). This technique has been proposed for fast optical switches. In use, optically generated charge carriers resonantly tunnel out of the MQWs but internal scattering reduces the response time to 2 ps. Furthermore, in practice, the excited carriers are not completely removed from the device, which causes detrimental accumulation effects.
It has been proposed to use a coherent pulse scheme to produce fast optical switching in which first and second coherent pulses are used to switch a transition on and off. Reference is directed to Kobayashi, T. et al, “Coherent push-pull transition for ultrafast optical switching”, Quantum Electronics and Laser Science Conference (QELS) 1991—QELS ′91 Technical Digest, paper QWD21, pages 144-145; and “Ultrafast optoelectronic devices”, Japanese Journal of Applied Physics—Extended Abstracts of the 1991 International Conference on Solid State Devices and Materials, Yokohama, Japan, 27-29 August 1991, pages 378-380.
However, these devices use a bulk resonant absorber and difficulties can arise in achieving satisfactory switching.
STATEMENT OF THE INVENTION
The present invention seeks to provide rapid de-excitation in the optically responsive medium, without the aforementioned disadvantages.
According the invention there is provided apparatus for controlling an optical transition, comprising an optically responsive medium; means for directing a first pulse of optical radiation of a given wavelength characteristic to the medium to produce a resonant excitation therein so as to change characteristics of the medium, and means for directing a second pulse of optical radiation to the medium, the relative timing of the first and second pulses and the relative phase of the radiation thereof being selected so that the second pulse de-excites the resonant excitation produced by the first pulse, wherein the optically responsive medium exhibits first and second energy levels for providing a resonant transition of a relatively narrow linewidth, and a third energy level, the transition between the second and third energy levels having a relatively wide linewidth, both of the transitions being simultaneously excited and de-excited by the first and second pulses.
The invention also provides apparatus for controlling an optical transition, comprising an optically responsive medium; means for directing a first pulse of optical radiation of a given wavelength characteristic to the medium to produce a resonant excitation therein so as to change characteristics of the medium, and means for directing a second pulse of optical radiation to the medium, the relative timing of the first and second pulses and the relative phase of the radiation thereof being selected so that the second pulse de-excites the resonant excitation produced by the first pulse, wherein the optically responsive medium comprises a plurality of nanoscale elements for producing said resonant excitation in response to the first pulse.
The apparatus according to the invention can be used as an optical switch, which includes means for directing an optical beam through the medium to an output path, transmission of the input beam through the medium being switched by the first and second pulses.
The apparatus according to the invention can also be used as a photodetector, in which the optically responsive medium changes its electrical characteristics in response to the resonant excitation produced by the first pulse, the second pulse producing de-excitation in preparation for a subsequent detection of a further input optical pulse.
The optically responsive medium may comprise a semiconductor having valence and conduction bands, with the resonant excitation resulting in a charge carrier transition between the bands.
The invention has the advantage that extremely fast de-excitation of photoexcited carriers in the medium can be achieved. The fastest time resolution that is achievable according to the invention is determined by the duration of the first and second optical pulses and is not limited by internal material properties of the photoresponsive medium. The response time can therefore be optimised separately from the other parameters.
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Physical Review Letters, vol. 73, No. 5, Aug. 1, 1994, J.Y. Marzin et al.: Photoluminescence of Single InAs Quantum Dots Obtained by Self-Organized Growth on GAAs.
Appl. Phys. Lett. 41(3), Aug. 1, 1982, H. M. Gibbs et al.: Room-temperatur excitonic optical bistability in a GaAs-GaAIAs superlattice etalon.
Appl. Phys. Lett. 57 (26), Dec. 24, 1990, Hiroyuki Sakaki et al.: Optical Absorption and Carrier-Induced Bleaching Effect in Quantum Wire and Quantum Box Structures.
Appl. Phys. Lett. 58(17), Apr. 29, 1991, M. Lambsdorff et al.: Subpicosecond carrier lifetimes in radiation-damaged GaAs.
Appl. Phys. Lett. 59(16), Oct. 14, 1991, Yi Chen et al.: 375-GHz-bandwidth photoconductive detector.
Appl. Phys. Lett. 58(22), Jun. 3, 1991, M. Klingenstein et al.: Transit time limited response of
Baumberg Jeremy J.
Heberle Albert P.
Hitachi Europe Limited
Kenyon & Kenyon
Mai Huy
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