We introduce and investigate a special regime of terahertz generation in electro-optic crystals with ultrashort laser pulses, in which neither phase-matching nor Cherenkov radiation mechanism are efficient. This regime occurs under the conditions that the optical group refractive index is larger than the low-frequency phase refractive index of the crystal, and the spectral bandwidth of the laser pulse is smaller than the frequency of the phase-matched terahertz wave. In this regime, that can be ...

Reflectance anisotropy spectroscopy (RAS) has proved itself to be extremely sensitive to both surface reconstruction and ultrathin coverages of material on semiconductor surfaces. This in situ technique therefore lends itself to monitoring the formation of low-dimensional systems in a wide range of growth environments. The following systems have been studied under molecular beam epitaxy (MBE) conditions: (i) the deposition of sub-monolayer coverages of Si, to form Si [delta]-layers within GaAs; (ii) the As/P exchange reaction which leads to the formation of InAs surface quantum wells on the InP(001) surface; (iii) the self-organised growth of InAs quantum dots via strain relaxation of ultrathin layers (<2 ML) of InAs deposited on the GaAs(001) surface; (iv) the kinetics of hydrogen desorption from vicinal Si(001) surfaces. In our studies of Si/GaAs, it has been possible to deduce both that RAS is sensitive to coverages >=0.005 ML Si/GaAs, and to estimate the level of activity of Si donors within a single [delta]-plane. Analysis of time-resolved RAS measurements has led to: an activation energy of 1.23+/-0.05 eV being determined for the As/P exchange reaction on InP(001); a detailed study of the conditions which influence InAs island formation on GaAs; and the suggestion of a new desorption pathway for H on vicinal Si(001) surfaces.