Prof Michael Johnston

Abstract:

The excitation density dependence of magnetic-field-enhanced terahertz (THz = 1012 Hz) emission from (100) GaAs is studied. It is found that THz power saturates at a higher optical-excitation density, when a magnetic field is applied. This observation explains the different magnetic field enhancements that have been reported recently. At low excitation densities the results are shown to be consistent with a simple model of carrier-carrier scattering, whilst at higher densities surface field screening becomes important.

Abstract:

We demonstrate enhancement of terahertz (THz) emission from indium arsenide at 170 K in magnetic fields (B) up to 8 T. An order of magnitude increase in visible to terahertz conversion efficiency was observed, with no suggestion of saturation of the TE polarization at higher magnetic fields. Free-space electro-optic sampling measurements confirmed the coherent nature of this radiation over the field range investigated, and gave an insight into the carrier motion subsequent to photoexcitation, which may be responsible for the observed THz power enhancement. © 2000 American Institute of Physics.

Abstract:

The intermixing technique was used to modify the energy level distribution of GaAs/AlGaAs quantum-wells so as to change the photoelectric characteristics of the photodetector. A large response wavelength shift was achieved by the proton implantation followed by standard annealing procedures (950°C for 30s). The measurement results show that (1) PL and photoresponse spectra are the function of ion dose in the range from 5×1014/cm2 to 2.5×1015/cm2; (2) the peak photoresponse wavelength is tunable between 8.4 μm and 10.2 μm for the infrared; (3) the PL peak for the related material changes from 1.62 eV to 1.645 eV. The influence on responsivity and dark current was analyzed.