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The pressure of radiation in a refractive medium has been a matter of theoretical controversy for many years, though relatively few experiments have been performed. We have measured the photon drag effect in germanium and silicon in the far-infrared, up to a wavelength of 1.2 mm. At sufficiently long wavelengths the effect is independent of the semiconductor band structure and depends on the radiation pressure in the dielectric. We find that the expression originally deduced by Minkowski correctly describes our results.

For the first time optically detected cyclotron resonance (ODCR) has been demonstrated using a CO, pumped far-infrared (FIR) laser instead of mircowaves. Both the electron and the light-hole cyclotron resonances have been observed in GaAs, as well as the 1 S to 2p + impurity transitions. Valence band quantum effects, well known in Ge, are resolved directly for the first time in GaAs and the electron cyclotron resonances show strong spin doublet in the highest quality MBE samples. The technique has remarkable resolution and sensitivity at low temperatures and, by constrast with other techniques that have been reported, we also observe the n = 1 to 2 (polaron shifted) and higher spin doublet split resonances at helium temperatures and with low FIR laser power. The conduction band results are analysed on the five-band model and the implications of this model on the valence band results are discussed. We have determined the valence band inverse mass parameters to be: ;' , = 7.5, 7, = 2.6, y3 = 3.1, K = 1.0.

Novel far-infrared optically detected cyclotron resonance (FIR-ODCR) techniques are used to investigate GaAs epilayers and t h e resuits are compared with conventional cyclotron resonance performed at far-infrared frequencies and ODCR at microwave frequencies. The FIROOCR technique shows remarkable resolution and sensitivity and has been applied to investigations of t h e electronic structure of low-dimensional systems. In particular. cyclotron resonance has been optically detected in a GaAsIGaAIAs multiple quantum well ( M a w ) sample and compared with ODCR results performed at microwave frequency. Multi-single quantum wells (MSOW) in an MBE GaAsIGaAIAs structure with different well thicknesses have also been investigated, and by detecting cyclotron resonance via the.FlR-induced changes in the luminescence of the separate wells, the power of the technique to investigate the cyclotron resonance mass versus well thickness in a single sample has been demonstrated. Finally. t h e experimentally determined values of effective mass for different well widths are compared with the theoretical results, showing good agreement.

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