Fabian D. J. Brunner

Linear and nonlinear optical properties of the organic crystal DSTMS

We report on the linear and nonlinear optical properties of the novel organic crystal 4-N, N-dimethylamino-4′-N′-methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate (DSTMS). The principal refractive indices n1, n2, and n3 have been determined by a direct measurement of the group index dispersion over a broad wavelength range from 0.6 to 1.6 μm. The linear absorption coefficients for light polarized along the dielectric axes have been measured in the transparency range from 600 to 2000 nm. DSTMS crystals show a large birefringence of Δn=0.5 and low absorption α<0.7 cm−1 in the telecommunication wavelength range at 1.55 μm. Furthermore, the nonlinear optical tensor elements d111, d122, and d212 have been determined by the Maker-fringe technique at the fundamental wavelength of 1.9 μm. DSTMS crystals exhibit a very large nonlinear activity with a nonlinear optical coefficient d111=214±20 pm/V. Phase-matching curves determined from our optical data showed that parametric oscillation can be achieved in the wavelength range from 1 to 2.2 μm with effective nonlinear optical coefficients larger than 25 pm/V.

Influence of phenolic hydroxyl groups on second-order optical nonlinearity at an example of 2,4- and 3,4-dihydroxyl hydrazone isomorphic crystals

We investigate the crystal structure and physical properties of 2,4- and 3,4-dihydroxybenzaldehyde-4-nitrophenylhydrazone (DHNPH) isomer crystals to understand the relation between molecular ordering with noncovalent interactions based on phenolic OH groups. The microscopic and macroscopic optical nonlinearities of 2,4- and 3,4-DHNPH crystals are investigated experimentally and theoretically by using density functional theory calculations. Although the two isomer crystals possess a very similar molecular orientation based on a similar supramolecular synthon, 2,4-DHNPH exhibits a 1.7 times larger powder second harmonic generation efficiency than 3,4-DHNPH, which is attributed to their different intermolecular interactions involving phenolic OH groups. We show that the microscopic nonlinearity of the DHNPH molecules is particularly sensitive to variations in phenolic OH characteristics such as the orientation and intermolecular interactions.

A terahertz time-domain spectrometer for simultaneous transmission and reflection measurements at normal incidence

We present a versatile terahertz time-domain spectrometer which allows reflection measurements at normal incidence and double pass transmission measurements in a single experimental setup. Two different modes for transmission measurements are demonstrated for precise measurements of transparent high or low refractive index materials, respectively. The refractive indices and absorption coefficients of cesium iodide, potassium bromide, sodium chloride, polytetrafluoroethylene (PTFE, Teflon), and silicon have been measured in the frequency range between 1.4 and 4.7 THz. The parameters of the Lorentz oscillator functions describing the phonon polariton dispersions of CsI and KBr have been determined.

Velocity-matched terahertz generation by optical rectification in an organic nonlinear optical crystal using a Ti:sapphire laser

Broadband terahertz pulses have been generated in 2-cyclooctylamino-5-nitropyridine (COANP) single crystals by optical rectification of 150 fs laser pulses from an amplified Ti:sapphire laser operating at 776 nm. Due to better velocity-matching and a higher figure of merit, COANP allows more efficient terahertz generation between 0.2 and 2.4 THz than the benchmark electro-optic crystal ZnTe at this laser wavelength. The peak amplitude of the terahertz signal obtained in COANP was 2.5 times larger in the time-domain and 2.1 times larger in the frequency-domain compared to ZnTe. We achieved a 4.3 times higher energy conversion efficiency in COANP than in ZnTe.

Improved emission and coherent detection of few-cycle terahertz transients using laser pulses at 1.5 μm

We present a comparative study of nonlinear optical materials for terahertz pulse generation by optical rectification and detection by electro-optic sampling, with the aim to find a material the is best suited for the use together with femtosecond lasers operating at a wavelength of 1.5 μm. Three crystals are identified that fulfill the condition that the optical group velocity matches the terahertz phase velocity: the organic salt DAST and the inorganic semiconductors GaAs and ZnTe, the latter showing velocity-matching with the second harmonic of 1.5 μm. A combined figure of merit for optical rectification and electro-optic sampling is derived. For the three materials under consideration, we find numerical values of 4300 (DAST):110 (GaAs):370 (ZnTe), in units of (pm/V)2. We also present experimental evidence that the performance of DAST is an order of magnitude better than those of the inorganic semiconductors, and we present spectroscopic data in the terahertz range of DSMOS, a chemical derivative of DAST that may in the future also be used for terahertz applications.

High-efficiency terahertz generation in hydrogen-bonded organic nonlinear optical crystals

We demonstrate that the hydrogen-bonded organic nonlinear optical crystals COANP, OH1, and DANPH allow a highly efficient generation of a gap-free terahertz spectrum. COANP at the Ti:sapphire laser wavelength of 776 nm and OH1 at a wavelength of 1460 nm show a higher conversion efficiency than the respective benchmark materials ZnTe and DAST. An unprecedented photon conversion efficiency of 11 percent has been achieved in OH1. First results in DANPH indicate that this material holds an equal potential as OH1.

Velocity-matched THz generation by optical rectification in the organic nonlinear optical crystal COANP using a Ti:sapphire laser

Optical rectification of sub-picosecond laser pulses in a nonlinear optical crystal is a widely used method to generate broadbandTHz pulses [1]. The standard crystal used with Ti:sapphire lasers is the inorganic semiconductor ZnTe [2]. We show that the organic hydrogen-bonded crystal 2-cyclooctylamino-5-nitropyridine (COANP) is a more efficient source of THz radiation than ZnTe at the Ti:sapphire laser wavelength of 776 nm [3].