Abstract: An optical switching apparatus comprises an optically resonant system (3, 12) and a pulse source configuration (1, 2) to direct first and second pulses (P1, P2) of optical radiation into the resonant system, the first pulse (P1) being configured to produce a coherent excitation of the resonant optical system so as to change its optical characteristics and the second pulse (P2) being of a phase to thereafter de-excite the coherent excitation produced by the first pulse. According to the invention, a device (13, 34) driveable externally of the resonant system such as an optical amplifier or a laser, maintains the coherence of the excitation produced by the first pulse until de-excited by the second pulse. The optically resonant system may comprise an optically responsive medium (3) which is capable of being switched into a state of coherent resonance or a resonant cavity.

Abstract: A photo-detector comprises a photo-absorptive region (1) which absorbs individual incident photons to produce corresponding electron-hole pairs. A bias (Vb) applied by an electrode (3) to the region 1 separates the oppositely charged electrons and holes such that the individual electrons apply a gate field to an electrometer (4) in the form of a single electron transistor which has a source-drain path (6) along which carrier charged transport is limited Coulomb blockade. The charge of the individual, photo-induced electrons (e) modulate charge carrier transport through the single electron transistor and the resulting current is detected by amplifier (A1) to produce an voltage output (Vout) so as to detect incident photons individually.

Abstract: Semiconductor structures and a method of forming semiconductor structures The avalanche breakdown characteristics, such as breakdown voltage and impact ionisation coefficient, of a semiconductor structure can be controlled by controlling the Brilluin-zone-averaged energy bandgap (<Ec>) of the material forming the structure. Consequently, the avalanche breakdown characteristics of a device may be tailored independently of the bandgap Eg. The Brillouin-zone-averaged energy bandgap (<Ec>) may be controlled by controlling the composition of the semiconductor used or by straining its lattice.

Abstract: An optical switching apparatus comprises an optically resonant system (3, 12) and a pulse source configuration (1, 2) to direct first and second pulses (P1, P2) of optical radiation into the resonant system, the first pulse (P1) being configured to produce a coherent excitation of the resonant optical system so as to change its optical characteristics and the second pulse (P2) being of a phase to thereafter de-excite the coherent excitation produced by the first pulse. According to the invention, a device (13, 34) driveable externally of the resonant system such as an optical amplifier or a laser, maintains the coherence of the excitation produced by the first pulse until de-excited by the second pulse. The optically resonant system may comprise an optically responsive medium (3) which is capable of being switched into a state of coherent resonance or a resonant cavity.

Abstract: Semiconductor structures and a method of forming semiconductor structures The avalanche breakdown characteristics, such as breakdown voltage and impact ionisation coefficient, of a semiconductor structure can be controlled by controlling the Brillouin-zone-averaged energy bandgap (<Ec>) of the material forming the structure. Consequently, the avalanche breakdown characteristics of a device may be tailored independently of the bandgap Eg. The Brillouin-zone-averaged energy bandgap (<Ec>) may be controlled by controlling the composition of the semiconductor used or by straining its lattice.

Abstract: A metal-semiconductor-metal (MSM) device comprises interdigitated metal electrodes (2, 3) on a semiconductor substrate (1). When embodied as a photoconductor, a photoconductive region (4) is bounded by layers (5, 6) which form a resonant cavity for incoming radiation to improve the response. In another embodiment, which can be either a photodiode or photoconductor, the electrodes are arranged to extend into the thickness of the photoresponsive layer (4). To reduce sensitivity to polarization,the electrodes may be arranged in sets extending in mutually transverse directions. Groups of the electrodes may be connected so as to be sensitive to polarization but substantially insensitive to the amplitude of incoming radiation.

Abstract: A nanofabricated logic device, operable at multiple (more than two) logic levels comprises asymmetrically coupled quantum point contacts provided with respective sources and drains and a coupling region between gate electrodes. The quantum mechanical carrier wavefunction in the region of QPC1, 2 is spatially asmmetric with alternate quantised energy levels lying either in QPC1, or QPC2, so that by changing the energy level, the conductance of the device can be switched between multiple stable conductance levels. The device can be used to provide a multilevel output switched in response to terahertz pulses provided by an array of optical detectors.

Abstract: A non-linear optical device utilizes laterally asymmetrical quantum dot structures (D1-D5) that are tunable in terms of their lateral asymmetry by bias potentials (V1, V2) applied to laterally extending electrode structures (13, 14).

Abstract: An avalanche photodetector using a quantum well superlattice in which impact ionization of carriers in the well layers occurs across the band-edge discontinuity is described.