Frequency Conversion and Amplification Using Doppler-Shift Methods in Solids

摘要

The interaction of electromagnetic and acoustic waves with moving reflectors in solids was investigated. The first part of the study contains theoretical predictions of the frequency shifts and amplitude changes of an electromagnetic or acoustic wave after interacting with a moving interface between regions of different characteristic impedances or a series of such moving interfaces, termed the Moving Reflector Theory. The second part contains experimental results concerning the diffraction of laser light using kilomegacycle acoustic waves in solids. The theory predicts that a high frequency amplification or deamplification of an electromagnetic wave can probably be obtained more easily by reflecting from a moving interface or a series of moving interfaces which are induced by sending an electromagnetic pumping signal into a nonlinear material. The theory also predicts that a frequency shift which occurs in a wave (transmitted through a moving nonabrupt interface) is independent of the width of the interface and the conversion of a longitudinal acoustic wave to a transverse acoustic wave or vice versa in a ferromagnetic material. Various ways of enhancing the amount of diffraction were employed to achieve a large diffraction. Using a suitable crystal (TIO2) and a high-efficiency transducer (ZnO wafer) giving a ribbon-shaped acoustic beam, it was possible to diffract in the first-order 10% of the incident light using 15 watts of cw rf power, and 60% diffraction was achieved with a pulse 60 watts peak power source. In the high power experiments additional diffracted spots of light (second- and third-order) appeared; these are also compared with calculated value.