All-optical wavelength conversion is demonstrated, based on cross-bias modulation induced by the photocurrent that results from absorbing the incoming optical carrier directly in an electroabsorption modulator connected to a series resistor. The wavelength conversion was achieved with an efficiency /spl ges/0.25 over a probe wavelength range of 1540-1560 nm.

Calculations of electroabsorption in extremely shallow quantum wells are performed, accurately incorporating mixing of different subband pairs due to the Coulombic interaction. As the AlAs mole fraction is varied in the barriers of a 100 A wide AlxGa1−xAs/GaAs square quantum well, a transition from red to blueshift of the absorption edge with applied electric field occurs at x ~ 0.003. In a 20 A wide square well, which more strongly confines the excitons, the redshift of the absorption edge is still observable at low electric field strength in the simulated absorption spectra, even for AlAs mole fractions as low as ~0.001. However, on increasing the strength of the applied electric field the blueshift becomes apparent. It is demonstrated that inclusion of Coulombic coupling between different subband pairs, particularly between confined states and quasi-continuum states, is essential for a correct prediction of the absorption edge behaviour in extremely shallow quantum wells.

By device simulation, it is shown that non-square quantum well growth (well shaping) provides a means for reducing the threshold current of bipolar quantum well diode lasers. Calculations of subband structure, optical matrix elements and laser gain are performed based on a 4-band (electron, heavy-hole, light-hole, split-off-hole) Hamiltonian with Burt-Foreman Hermitianization. A non-optimized, compressively strained, InGaAs-AlGaAs (on GaAs) shaped well laser, operating at 0.97 µm is predicted to show improvements in both radiative and non-radiative current performance compared to a device based on an optimal square quantum well of the same well width and emission wavelength. These improvements result from modification of subband structure giving greater subband separation in the shaped well than in the square well.

A new hydrodynamic microelectrochemical reactor design is presented for the voltammetric sensing of chemical species contained within two immiscible liquid streams flowing within rectangular ducts, in direct contact. This article describes the design, fabrication and experimental characterisation of the device. A microfabricated rectangular duct (of typical dimensions: height 75μm, width 500μm and length 3 cm) was constructed using FOTURAN glass and standard photolithographic procedures. Microelectrode sensors were positioned on one internal duct wall with a geometry to permit separate voltammetric monitoring of the two solvent phases. Reagent solutions containing N,N,N′,N′-tetramethyl-1,4-phenylene diamine in 1,2-dichloroethane and hexaamineruthenium(III)chloride in water were pumped through the device under laminar flow conditions. Linear sweep voltammetric measurements were performed separately on the two electrolyte streams and the variation of the transport limited current as a function of volume flow rate through the cell monitored. Under conditions where stable flow was obtained the current flow rate relationship was observed to follow analogous voltammetric behaviour to that observed in macroscopic flow cell devices.

It is shown theoretically that selective perturbation by electrorefractive effect of the even-order modes in III-V semiconductor multimode interference couplers gives rise to high-performance optical switching. The calculated performance of the proposed switch is 3.4 GHz/Vmm, compared to 0.85 GHz/Vmm for an equivalent Mach-Zehnder interferometer (MZI) switch. The effects of electroabsorption and dimension variations are analyzed to demonstrate the feasibility of the proposed device.

The linear and quadratic electrooptic coefficients in narrow single and strongly coupled GaAs-Al/sub x/Ga/sub 1-x/As quantum wells have been measured. The quadratic electrooptic effect is enhanced over that of conventional square quantum wells for both TE and TM polarization in all the structures considered, by up to six times in the case of 2-nm-wide GaAs-Al/sub 0.2/Ga/sub 0.8/As strongly coupled quantum wells. The origin of the enhanced quadratic electrooptic effect was found to correlate with a larger red shift in the absorption edge exciton and strong Coulombic coupling of the bound exciton states with the quasi-continua.

A novel approach to the visualisation and quantification of mass transfer in liquid flow is presented. The technique utilises an array of microstrip electrodes sited within a duct through which the electrolyte solution is pumped. The electrolyte solution contains a reagent that may be oxidised and in the process undergo a colour change. The electrolysis products are then swept through the cell and the pathway imaged via a digital video. Results are presented for an experimental geometry where a rectangular obstruction is deliberately introduced into the duct. Transport rates within the cell are restricted such that Stokes flow conditions are maintained throughout and the streamlines generated from the electrolysis used to map the flow profiles through the devices. The merits of the approach are discussed and the potential of numerical modelling to provide quantitative analysis are highlighted.

Wannier-Stark localization of heavy holes and the associated refractive index changes in a strongly coupled GaAs-Al0.75Ga0.25As three-quantum-well structure have been investigated. Electroabsorption has been measured for TE polarization and the results compared with simulations performed by the exciton Green's function method to reveal the dominant contributions to the differential absorption at low applied electric Field. The refractive index changes calculated by Kramers-Kronig transformation are large compared with those arising from the quantum-confined Stark effect in conventional square quantum wells and are shown to derive from the emergence of only first-order ladder states due to the strong localization of heavy holes. Preliminary experimental confirmation of strong electrorefraction associated with heavy-hole state localization is obtained at 80 meV detuning. This effect is potentially useful for electrooptic device applications.

The quadratic electrooptic coefficient of narrow symmetric-coupled GaAs-Al/sub 0.2/Ga/sub 0.8/As quantum wells has been measured for light having a range of detuned wavelengths with TE polarization and is found to be 2-3 times larger than that of single quantum wells equivalent overall width.

The linear and quadratic electro-optic coefficients of 6nm wide GaAs/Al0.2Ga0.8As quantum wells have been measured over a wide range of detuned energies. The quadratic coefficient is found to be about twice that reported for wider and deeper quantum wells for both TE and TM polarisations.

It is shown, via detailed comparison between measured electroabsorption spectra and electroabsorption spectra calculated by a full excitonic Green’s function method, a decoupled excitonic Green’s function method and a variational method, that accurate description of the Coulombic coupling between different subband pairs, including unbound quantum well states above the quantum well edge, is crucial for a correct prediction of electroabsorption in extremely shallow quantum well structures.