In this paper, we study THz generation by optical rectification in GaSe crystal pumped under normal incidence below and around the bandgap. We compared the generated THz signal from 3 crystals with different thicknesses (1 mm, 0.1 mm and 0.02 mm) when pumped with femtosecond laser pulse whose wavelength is tuned from 800 down to 540 nm. With the 1-mm thick crystal, we observe a pronounced THz emission peak when the pumping photon energy is 1.77 eV because of better phase matching conditions. We also notice that the efficiency improvement is more pronounced for higher THz frequencies. The experimental data are nicely fitted by our OR generation model. Since the 2 other crystals are thin, we don’t observe phase matching impact on the signal. When pumping around the bandgap, we detect another, smaller, THz emission peak, due to nonlinearity enhancement this time.
In this work, we demonstrate a high-selectivity terahertz (THz) band-stop filter with a wide range of center wavelengths (CWLs) from 150 to 600 μm (2.0 THz to 0.5 THz). The dip transmission is lower than 10 % at the center CWLs, even to 5 % at 1.9 THz. The band-stop terahertz filter is based on periodic metallic Cross Cell (CC) structures deposited on the top of a 50 μm thick polyimide film (Kapton) with low absorption and good mechanical properties, resulting in a large-area, freestanding and flexible membrane with a low intrinsic loss. The excellent tunable terahertz band-stop filter properties are investigated using terahertz time-domain spectroscopy. The measured and simulated results are coming to an excellent agreement. The THz band-stop filter possesses not only a light weight and polarization insensitivity but also a simple structure and high integration.
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