Prospects of semiconductor terahertz pulse sources

Summary form only given. In the low-frequency THz range optical rectification (OR) in LiNbO<;sub>3<;/sub> (LN) has been providing the highest THz pulse energies, but limitations became apparent [1]. The potential of semiconductor nonlinear optical materials for high-energy high-field THz pulse generation by OR has been recently demonstrated [2-4]. Whereas pumping OR in ZnTe at 0.8 μm, near its collinear phase-matching wavelength, resulted in maximum 1.5 μJ THz pulse energy at 3.1×10<;sup>5<;/sup> efficiency [5], pumping at 1.7 μm wavelength recently resulted in more than two orders of magnitude higher efficiency, as high as 0.7%, and 14 μJ THz pulse energy [4]. The reason for the enormous increase in efficiency was the elimination of lower-order (2nd- and 3rd-order) multiphoton pump absorption at the longer pump wavelength and the associated free-carrier absorption in the THz range. An important advantage of semiconductors over LN is the much smaller required pulse-front tilt angle of <;30°, in contrast to 63° in LN. This enabled the realization of a monolithic contact-grating THz source easily scalable to high energies [3].Here, we summarize our experimental findings for infrared-pumped semiconductor THz sources [3,4] and present a numerical study to give practical guidelines for the selection of the semiconductor nonlinear material, crystal thickness, pump wavelength, pulse duration and intensity, as well as THz phase matching frequency. Both the measurements and simulations for ZnTe, pumped at 1.7 μm wavelength, showed a maximum of the THz generation efficiency due to the onset of four-photon absorption (Fig. 1a). The calculations show an optimal pump pulse duration and intensity giving the highest peak electric field (Fig. 1b). Whereas ZnTe is better suited to generate THz radiation around 1 THz, GaP has the potential of becoming an efficient source up to 5 THz (Fig. 1c) or even beyond.