Nonlinear free-carrier absorption of intense THz radiation in semiconductors

We calculate the nonlinear free-carrier absorption coefficient of an intens e terahertz (THz) electromagnetic wave propagating in a hulk semiconductor and the absorption percentage when an intense THz radiation passes through a quasi-two-dimensional (2D) sheet, with the help of the balance-equation a pproach to hot-electron transport in semiconductors subject to an intense h igh-frequency field. We find that at frequency around 1 THz, the absorption coefficient in a bulk GaAs system increases with increasing amplitude of t he radiation field from zero and reaches a maximum at around 8 kV cm(-1) be fore decreasing quickly with further increase of the field strength. The ab sorption percentage of a quasi-2D system exhibits an even stronger nonlinea rity than that of a three-dimensional hulk. It is shown that high-order mul tiphoton processes play a major role in determining the absorption of an in tense THz field.