Deflection of a monochromatic THz beam by acousto-optic methods

The possibility of controlled deflection of an electromagnetic THz beam of a free-electron laser by acousto-optic (AO) methods has been demonstrated for the first time. The material of the AO deflector was chosen to be single-crystal germanium, which has a fairly large refractive index (n = 4.0) and a relatively low absorption coefficient for electromagnetic waves. The absorption coefficient alpha in germanium is 0.75 +/- 0.02 cm(-1) at a wavelength lambda = 140 mm. The diffracted beam intensity is shown to be maximum at an effective AO interaction length l = 1/alpha. A diffraction efficiency of 0.05 % at a travelling acoustic wave power of 1.0 W is experimentally obtained. It is established that a change in the ultrasonic frequency from 25 to 39 MHz leads to variation in the external Bragg angle in the range from 19.5 degrees to 27.5 degrees. At a fixed Bragg angle theta(B) = 22.4 degrees the frequency band of diffraction is 4.2 +/- 0.1 MHz and the angular range of laser beam scanning reaches 2.5 degrees +/- 0.5 degrees. The results obtained indicate that AO interaction can be used for controlled deflection of electromagnetic THz beams.