Modeling for laser-induced surface thermal lens in semiconductors

A new three-dimensional theoretical model for the laser-induced surface thermal lens in semiconductor materials has been developed by combining Fresnel diffraction theory with several photothermal effects, that is, photo-displacement effect, air-thermal lens effect, and photoreflectance effect. A characteristic parameter that is different from conventional photothermal deformation techniques is defined to characterize the frequency response of the signals. The influences of the electronic parameters of the semiconductors on the detected signal are discussed. The model indicates that the photo-displacement effect is dominant in the detected signal and diffraction pattern, but the air-thermal lens effect at low frequencies and the photoreflectance effect at high frequencies are significant. It is also shown that the position of the sample can be optimized to achieve higher detection sensitivity and the optimized position is dependent upon the modulation frequency.