Abstract
Recently a hybrid-type terahertz (THz) pulse source was proposed for high energy terahertz pulse generation. It is the combination of the conventional tilted-pulse-front setup and a nonlinear crystal with a transmission stair-step echelon of period in the hundred-micrometer range etched into the front face. The tilt angle introduced by the conventional tilted-pulse-front setup (pre-tilt) was chosen to be equal to the tilt-angle needed inside the nonlinear crystal (62° for lithium niobate (LN)) in order to fulfill velocity-matching. In this case, plane-parallel nonlinear optical crystals can be used. The possibility of using a plane-parallel nonlinear optical crystal for producing good-quality, symmetric THz beams was considered the most important advantage of this setup. In the present paper, a thorough numerical investigation of a modified version of that setup is presented. In the new version, the tilted pulse-front is created by a transmission grating without any imaging optics, and a wedged nonlinear optical crystal with a small wedge angle is supposed. According to a 1D numerical code, significantly higher THz generation efficiency can be achieved with a transmission stair-step echelon-faced nonlinear crystal having a 5 - 15-degree wedge angle than with a plane-parallel one or with the conventional tilted-pulse-front setup. Because of the spatially-dependent group-delay dispersion introduced by the transmission grating, a small wedge in the nonlinear crystal improves the spatial homogeneity of the THz-generation process, resulting in higher efficiencies and better beam profiles. At 100 K temperature, and by using 800 nm pump pulses with 20 mJ pulse energy, 100 fs pulse length and 8 mm beam spot radius, approximately 4.5% conversion efficiency and close to 1 mJ terahertz pulse energy can be reached with the newly-proposed setup.
Highlights
The study of dynamics and control of materials in the THz frequency range need THz pulses with peak electric fields on the level of 100 kV/cm [1,2]
For γ0 = 62° a plan-parallel nonlinear echelon slab” (NLES) can be used and the setup become similar to the one investigated in [21], with the only difference that the setup does not consist of imaging, just a transmission grating (TG) is used for introducing the pre-tilt
The shape of the generated terahertz pulses and the generation efficiency depend on many parameters
Summary
The study of dynamics and control of materials in the THz frequency range need THz pulses with peak electric fields on the level of 100 kV/cm [1,2]. The large tilt-angle required and the correspondingly large angular dispersion of the pump beam can severely limit efficient generation of high energy THz pulses with good beam quality. The results are poor quality, strongly asymmetric THz beams that drastically hinder many applications, especially particle acceleration In principle, both of these problems can be eliminated by using a contact-grating setup [16] in which the diffraction from an optical grating created directly in the input surface of the NM introduces the TPF of the pump pulse. In order to mitigate this problem, a modified version of the tilted-pulse-front THz generation was introduced [20] In this setup, a discrete TPF is formed by reflecting the pump beam on a stair-step echelon, and this segmented intensity front generates the THz pulse inside the NM. Instead of an optical grating – imaging system, only a transmission grating is used for creating the pre-tilt of the pump intensity front, since such a system is simpler, and scaling it to high pulse energies is more straightforward
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