Semiconductor-based photonic THz sources - II

Recently, THz radiation has found increasing interest because of a wide range of possible applications resulting from its characteristic properties. THz radiation is non-ionizing, and therefore harmless, to biological tissue. Many materials, such as paper and fabrics are transparent for THz radiation. It also covers the spectral range of molecular rotations and of cosmic background radiation. The wavelength range also provides reasonably good spatial resolution for many imaging applications. Therefore, possible applications include such different fields as medicine, security scanning, detection of explosives or astronomy.
As the THz range lies beyond the conventional high-frequency limit of RF sources, but below the low-frequency limit of conventional laser sources, the generation of THz radiation represents a real challenge. This is particular true regarding compact sources for coherent THz radiation of sufficiently high power. During the past years, lasers, in combination with semiconductor devices have emerged as very successful towards reaching these goals. 
In my lectures I will discuss the main laser-based photonic approaches for the generation of THz radiation, with emphasis on the basics. The first approach, photomixing, is based on the heterodyning of two laser beams, differing in photon frequency by the THz frequency. The beating laser field yields a THz-periodic generation of electrons and holes in a semiconductor. With a DC electric field present in the semiconductor, this yields CW-THz emission either directly from the coherent acceleration of the photo-generated charge carriers or via the THz current being fed into a suitably designed THz antenna. The former scheme allows for illuminating a large active semiconductor area with dimensions typically exceeding the THz wavelength (hence the name “large area emitter”s, (LAEs)), whereas the latter scheme (the “antenna emitter”s (AEs)) requires an illumination spot much smaller than the dimensions of the antenna and, hence, much smaller than the THz wavelength. Advantages of the photomixing approach are compactness, narrow bandwidth, easy tuning over an extremely wide THz frequency range, room temperature operation, reasonably high achievable THz power and, in case of LAEs, high directivity of the emitted THz radiation pattern without any optical elements.  
In the second approach fs laser pulses are used for the generation of electrons and holes in similar semiconductor structures, resulting in the emission of THz pulses, either directly, due to the coherent transient acceleration of the photo-generated charge carriers (in LAEs), or, again, due to the transient current pulse fed into an antenna (AEs). Apart from the advantage of compactness, which it shares with the photomixing approach, this time-domain approach yields much higher laser-to-THz conversion efficiency and larger maximum average THz power. The ps-THz pulses, however, are extremely broadband with a Fourier spectrum covering nearly the whole THz range. In this case the time-resolved signal is particularly suitable for imaging, including easily accessible 3-dimensional information.
Apart from presenting the basic concepts of the two approaches, I will also report about our recent theoretical and experimental research in this field.