Abstract-Numerical investigation of imaging-free terahertz generation setup using segmented tilted-pulse-front excitation

György Tóth, László Pálfalvi, József A. Fülöp, Gergő Krizsán, Nicholas H. Matlis, Gábor Almási, and János Hebling

Fig. 1 The setup of the investigated hybrid NLES THz source. The dark blue lines represent the pump pulse front at different moments. After diffraction of the pump beam on the transmission grating (TG), it propagates horizontally. Consequently, the phase-fronts are vertical. All γ is measured from a vertical line.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-5-7762

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.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement