By a convolution approach for the spectra generated by photoconductive and photomixing THz emitters, the intrinsic response of the material can be separated from the spectrum of the optical pulse and that of any attached antenna. The spectrum turns out to be simply a product of the spectra of the individual components and can be calculated analytically. This result has impact on materials design and derivation of the optimum excitation conditions for photoconductors and photodiodes. Spectra of three technologically important devices, namely low-lifetime materials, semi-insulating materials and devices supporting ballistic transport, are calculated. The derived pulse shape is compared to experimental data.
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