Intense terahertz sources based on tilted pulse-front excitation and their potential applications in imaging, nonlinear THz spectroscopy and attosecond pulse generation

Abstract

The generation of THz pulses having tens of microjoules energy by tilted pulse-front excitation is reviewed. Possibilities of further up-scaling the THz energy as well as existing and future applications of these pulses are analyzed. THz technology is being used in an increasingly wide field of applications ranging from nondestructive evaluation, quality control in the pharmaceutical industry, biological and medical sciences to high-speed communication systems and security control [1]. However, the lack of compact high average power THz sources has limited the practical application of THz radiation. Optical rectification of femtosecond laser pulses is one of the standard methods for generating single cycle THz pulses [2]. The conversion efficiency is determined by the effective nonlinear coefficient, the phase-matching condition and the absorption of the material. In the femtosecond Ti:sapphire era ZnTe was the mostly used nonlinear crystal, since for this material and the 800 nm wavelength of the Ti:sapphire lasers collinear phase matching is fulfilled for THz generation. This technique culminated in generating 1.5 μJ THz pulses with 40 mJ pumping [3]. Lithium niobate (LN) has a significantly higher nonlinearity but it is not phase-matched for THz generation. Achieving phase matching in LN by using tilted pump pulse front (TPF) was suggested [4] and demonstrated [5]. By using this technique single-cycle THz pulses with 0.25 mW average power at 1 MHz repetitions rate [6], and with 3 μJ energy at 1 kHz repetitions rate [7] were generated by Yb-fiber, and Ti:sapphire laser-amplifier pumping, respectively. The first system can be an efficient THz source for real-word applications. The high THz field of about 400 kV/cm achieved with the second system made possible the first observation of self-phase modulation in the THz range [8] and performing the first THz-pump/THz-probe measurements [9,10]. In these measurements it was possible to follow the dynamics of free carrier generation by impact ionization [9] and redistribution of free carriers in the conduction band of semiconductors [10]. The highest THz energy achieved by optical rectification is on the 10 μJ range [11,12]. A few applications are foreseen which need THz pulses with even higher energies or corresponding peak electric field strengths. One of this applications is using the THz pulse as a quasi-dc background electric field in attosecond pulse generation as was suggested very recently [13]. According to model calculations applying 1 MV/cm THz field results in two times increase of the cut-off frequency of high harmonics. This can allow for reducing the attosecond pulse duration by a factor of two. Other possible applications require THz pulses with even higher energies and peak electric field strengths. In order to further increase the THz pulse energy and the electric field strength we have recently proposed a novel, compact scheme for THz pulse generation, the contact grating scheme [14]. This scheme is free of imaging errors and allows using large pump beam diameters. The useful pump intensity in THz generation setups is often limited by THz absorption caused by free carriers generated by absorption at the pump wavelength. In many cases longer pump wavelengths can be used to overcome this limitation (see e.g. [15]). At longer pump wavelengths, only higher order multiphoton absorption will be effective in many semiconductor materials suitable for optical rectification. In order to estimate the performance of various semiconductor materials for high-energy THz pulse generation we have carried out extensive model calculations and compared the results to LN (Fig.1). The results indicate that in some cases semiconductors can provide higher conversion efficiency than LN. With a proper choice of the nonlinear material and by using optimized schemes for pulse-front-tilting it is expected that ultrashort THz pulses approaching the mJ energy level will be available in the foreseeable future.