There are a number
of advantages to terahertz (THz) waves by Cherenkov
phase matching using a nonlinear optical (NLO) crystal; however, a
thin crystal of micrometer-scale must be grown to satisfy the optimal
phase matching conditions. Inorganic crystals with well-developed
crystal growth and processing techniques have been widely used to
date; however, THz wave generation with improved efficiency in the
broadband region could be achieved with a single-crystalline thin
film of an organic NLO material with a large NLO coefficient. Here,
the growth of a crystalline thin film with a size of several microns
using a very simple method is reported. Physical vapor deposition
is used to grow a crystal of the organic nonlinear material 2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene)
malononitrile (OH1). This method is much simpler than the conventional
crystal growth method. Additionally, the OH1 single-crystalline thin
film grown by physical vapor deposition has a better crystalline character
than a bulk OH1 crystal grown by the solution method. The OH1 single
crystal grown by this technique demonstrates efficient THz wave generation
and low absorption of THz waves by the crystal.
This is the first report of single-cycle terahertz (THz)-wave pulse generation with a wide band (over 6 THz) and a high dynamic range (over 70 dB) using an organic nonlinear optical crystal, 4-dimethylamino-N′-methyl-4′-stilbazolium tosylate (DAST), and the prism-coupled Cherenkov phase-matching method. The prism-coupled approach allowed the use of a DAST crystal as a Cherenkov-type emitter for reduced light absorption by the crystal, resulting in the generation of single- and half-cycle THz pulse shapes, which were detected using dipole and bow-tie antennas, respectively. The resulting THz pulse generation is suitable for various THz-wave application technologies, such as tomography.
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