We present a detailed theoretical study on nonlinear transport and optical properties in two-dimensional semiconductor systems ͑2DSS's͒ subjected to intense terahertz electromagnetic fields. By solving the momentum-and energy-balance equations using a steady-state Boltzmann equation approach, where electron interaction with LO phonons is taken into account under the lowest-order approximation for GaAs-based 2DSS's, we have investigated the dependence of electron temperature, momentum relaxation time, and conductivity on the intensity and frequency of THz radiation fields. The results obtained from this study are in line with those obtained from recent experimental measurements ͓N. G. Asmar et al., Appl. Phys. Lett. 68, 829 ͑1996͔͒. ͓S0163-1829͑97͒04308-7͔

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We study light scattering from a periodically modulated two dimensional electron gas in a perpendicular magnetic field. If a subband is partially filled, the imaginary part of the dielectric function as a function of frequency contains additional discontinuities to the case of completely filled subbands. The positions of the discontinuities may be determined from the partial filling factor and the height of the discontinuity can be directly related to the modulation potential. The light scattering cross section contains a new peak which is absent for integer filling. 73.20.Mf, Typeset using REVT E X 1

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A detailed theoretical study is presented for spontaneous optical emission from a twodimensional electron gas (2DEG) in the presence of a unidirectional spatially periodic modulation at low temperatures. The momentum-and energy-balance equations for electron-photon interactions in the device system are solved self-consistently using the Boltzmann equation, from which we can obtain the frequency and the intensity of the electromagnetic radiation generated. The results obtained indicate that: (1) in an electrically modulated 2DEG at low temperatures, a strong electromagnetic radiation emission up to W/cm 2 can be generated by applying a small d.c. electric field; (2) the radiation emission is generated indirectly in the occupied subbands through electronic transitions around the Fermi level; (3) the frequency of the radiation generated is at about 0.1 THz; (4) in the low frequency regime, spontaneous multiphoton emissions can be observed; and (5) this type of optical emission depends strongly on the sample parameters such as the electron density of the 2DEG and the modulation length.