Enhancement in the spectral irradiance of photoconducting terahertz emitters by chirped-pulse mixing

We describe the use of mixing linearly chirped optical pulses in biased photoconductors to generate tunable narrow-band terahertz (THz) radiation with enhanced spectral brightness. The increase in conversion efficiency from optical to THz radiation at a given THz frequency arises from the improved saturation characteristics of the photoconductor for chirped-pulse mixing compared with the usual case of excitation by an ultrafast optical pulse. In the weak saturation limit, the enhancement in the saturation fluence scales with the ratio of the duration of the chirped optical pulse to the photocurrent relaxation time in the emitter and is essentially independent of the beat frequency generated by the chirped-pulse mixing technique. This dependence allows for substantial enhancements in the saturation fluence and, hence, in the THz spectral brightness. We demonstrate enhanced saturation fluences experimentally for dipole emitters fabricated on radiation-damaged Si on sapphire. © 1999 Optical Society of America [S0740-3224(99)00608-6] OCIS codes: 320.7160, 320.5540, 260.5150, 230.0250.