A hydrogen-bonded organic nonlinear optical crystal for high-efficiency terahertz generation and detection

ETH Zurich, Institute of Quantum Electronics, Nonlinear Optics Laboratory, 8093 Zurich,Switzerland.
Optics Express (Impact Factor: 3.53). 11/2008; 16(21):16496-508. DOI: 10.1364/OE.16.016496
Source: PubMed

ABSTRACT Broadband THz pulses have been generated in 2-[3-(4- hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene]malononitrile (OH1) by optical rectification of sub-picosecond laser pulses. We show that OH1 crystals allow velocity-matched generation and detection of THz frequencies in the whole range between 0.3 and 2.5 THz for a pump laser wavelength range from 1200 to 1460 nm. OH1 crystals show a higher figure of merit for THz generation and detection in the optimized range compared to the benchmark inorganic semiconductor crystals ZnTe and GaAs and the organic ionic salt crystal 4-N,N-dimethylamino-4'-N'-methyl stilbazolium tosylate (DAST). The material shows a low THz absorption coefficient alpha3 in the range between 0.3 and 2.5 THz, reaching values lower than 0.2mm(-1) between 0.7 and 1.0 THz. This is similar as in ZnTe and GaAs, but much lower than in DAST in the respective optimum frequency range. A peak THz electric field of 100 kV/cm and a photon conversion efficiency of 11 percent have been achieved at a pump pulse energy of 45 microJ.

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    ABSTRACT: We present a compact, room temperature, and narrowband terahertz source, based on difference-frequency generation in the organic nonlinear optical crystals OH1 (2-[3-(4-hydroxystyryl)-5, 5-dimethylcyclohex-2-enylidene] malononitrile). The system employs a specific dual-wavelength infrared laser that emits coaxial, synchronous 10-ns pulses of similar energy and duration at wavelengths of 1064 nm and 1030 nm by using Nd:YAG and Yb:YAG crystals within the split laser cavity. The common part of the laser cavity comprises an acousto-optic Q-switch and an output coupler. The output is frequency-mixed in a stack of several OH1 crystals in a quasi-phase-matching configuration, which is determined on the basis of refractive index and absorption measurements in the 1-11 THz range. The system generates terahertz radiation in pulse trains with 1.0 mu W average power and a near-Gaussian intensity profile.
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    ABSTRACT: Organic electro-optic crystals are efficient THz-wave generators using optical rectification (OR) or difference-frequency generation (DFG) of various fs or ns/ps pump laser sources in the infrared. This is due to their higher second-order susceptibilities compared to inorganics, combined with excellent phase-matching possibilities. However, most of the efforts in the past have focused only on the generator crystal DAST (4-N, N-dimethylamino-4'-N'-methyl-stilbazolium tosylate), because of the lack of alternative crystals with similarly high figures of merit and possibilities for growth of high-quality single crystals. We investigate THz interactions using recently developed materials, stilbazolium salt DSTMS (4-N, N-dimethylamino-4'-N'-methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate), which is a DAST derivative with improved properties for THz-wave generation, as well as phenolic polyene OH1 (2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene)malononitrile), which is hydrogen bonded crystal and an important alternative for stilbazolium salts due to the low THz absorption and even better figures of merit for THz wave generation. THz wave generation using optical rectification has been previously demonstrated using DSTMS and OH1 up to about 3 THz.
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