Prospects of semiconductor terahertz pulse sources

In the low-frequency THz range optical rectification (OR) in LiNbOs (LN) has been providing the highest THz pulse energies, but limitations became apparent [I]. 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 5 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].