Extrém nagy hatásfokú félvezető anyagú terahertzes források

Abstract

The widespread availability of table-top laser sources triggered the development of various types of laser-driven pulsed terahertz (THz) sources. This development now enables to routinely provide THz pulses with unprecedented energies and peak electric and magnetic field strengths throughout the entire THz spectral range. Intense pulses at low terahertz frequencies of 0.1‒2 THz are an enabling tool for nonlinear THz spectroscopy, for strong field control of matter [1], and for constructing compact particle accelerators, for enhancement of high-harmonic generation [2, 3], electron undulation [4], electron bunch acceleration [4, 5, 6] and for proton acceleration for hadron therapy [7, 8]. The low-frequency part of the THz spectrum (0.1 to 2 THz) is optimally fitting to such applications mentioned before. Up to now, optical rectification in lithium niobate (LiNbO3, LN) in combination with tilted pulse front pumping has been the most efficient source in this spectral range. However, the large pulse front tilt angle (63°) is disadvantageous for applications and makes the further increase of the THz energy challenging due to the limited interaction length [9], the imaging errors [10] and the nonlinear interaction between the pump and the THz [11], S4]. Contrary to LN, semiconductors, such as ZnTe or GaP, are widely used with collinear phase matching for optical rectification in the low-frequency THz range , although they were considered as less efficient for THz generation [57]. The highest THz energy reported from a semiconductor source was only 1.5 μJ [53]. The reason for the low efficiency was the smaller nonlinear coefficient and the strong two-photon absorption at the pump wavelength, associated with the free-carrier absorption at THz frequencies. Two-photon absorption can be avoided and free carrier absorption can be decreased if we use longer pump wavelengths. Therefore at longer pump wavelengths, typically requiring tilted pulse-front pumping, it is possible to suppress low-order multiphoton absorption. As a result, a higher pump intensity can be used and a higher THz generation efficiency can be expected. My aim is to show that semiconductors pumped above the three-photon absorption edge are competitive with LN in efficiency and pulse energy. I would like to show that the realization of a new-type semiconductor-based THz source, called contact-grating, is possible with better properties for the applications. My aim is to show that the scaling-up of the THz generation efficiency is easier in semiconductors than in LN in the case of optimal pumping conditions.