Arno Schneider

Generation of terahertz pulses through optical rectification in organic DAST crystals: theory and experiment

We present a combined theoretical and experimental investigation of the generation of few-cycle terahertz (THz) pulses via the nonlinear effect of optical rectification and of their coherent detection via electro-optic sampling. The effects of dispersive velocity matching, absorption of the optical and the THz waves, crystal thickness, pulse diameter, pump pulse duration, and two-photon absorption are discussed. The theoretical calculations are compared with the measured spectra of THz pulses that have been generated and detected in crystals of the highly nonlinear organic salt 4-N,N-dimethylamino-4′-N′-methyl stilbazolium tosylate (DAST). The results are found to be in agreement with the theory. By the selection of the optical pump wavelength between 700 and 1600 nm, we achieved several maxima of the overall generation and detection efficiency in the spectral range between 0.4 and 6.7 THz, with an optimum at 2 THz generated with 1500 nm laser pulses.

Terahertz-induced lensing and its use for the detection of terahertz pulses in a birefringent crystal

We demonstrate terahertz-induced lensing (TIL), the focusing and defocusing of an ultrashort “probe” laser pulse induced by a collinearly traveling terahertz pulse in an electro-optic crystal. The intensity change on the axis of the probe beam after the crystal is shown to be linear with the terahertz (THz) electric field and can thus be used to measure the THz wave form. We show that the sensitivity of TIL is comparable to conventional electro-optic sampling, with the advantage of being applicable to all classes of electro-optic crystals, including strongly birefringent ones. The better sensitivity of TIL is demonstrated using the highly birefringent organic salt DAST (4-N,N-dimethylamino-4′-N′-methyl stilbazolium tosylate).

Optical properties of 4-N,N-dimethylamino-4 ′ -N ′ -methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate crystals at terahertz frequencies

We have measured the optical properties of 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate (DSTMS), a highly nonlinear organic crystal, in the terahertz (THz) spectral range. The conditions for efficient THz pulse generation through optical rectification of near-infrared pulses have been calculated and confirmed experimentally. The results show that DSTMS as a source of THz radiation is superior to 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate (DAST), a well-investigated nonlinear material with one of the highest nonlinear susceptibilities known so far.

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.

Spectrum of Terahertz Pulses from Organic DAST Crystals

Few-cycle electromagnetic pulses with a spectral content between 0.1 and 10 THz (terahertz pulses) are commonly generated through the optical rectification of ultrashort laser pulses in nonlinear optical crystals. We describe how the THz spectrum emitted from the polar organic salt DAST (4-N,N-dimethylamino-4′-N′-methyl stilbazolium tosylate) can be controlled by varying experimental parameters such as wavelength and duration of the laser pulse, and crystal thickness.

Measurement of the terahertz-induced phase shift in electro-optic sampling for an arbitrary biasing phase

We present a simple method to correct for nonlinearities in the detection of terahertz (THz) transients by electro-optic sampling. The THz-induced phase shift of the optical probe beam is calibrated with an additional biasing phase from a variable wave plate. All the parameters that are needed for the determination of this phase shift can be directly measured, which makes the method insensitive to errors resulting from residual birefringences of the electro-optic material or imperfections of the wave plate. We show that the THz-induced phase shift is correctly revealed for a wide range of the biasing phase.

Epitaxial K 1−x Na x Ta 0.66 Nb 0.34 O 3 thin films for optical waveguiding applications

Monomodal slab waveguides in lattice-matched epitaxial thin films of paraelectric potassium sodium tantalate niobate (x≈0.02) on paraelectric potassium tantalate substrates without surface polishing have been fabricated for the first time, to the best of our knowledge. A surface roughness of 21 nm (rms) has been measured by atomic force microscopy. The film thickness, d=1.4 μm, and refractive index contrast, Δn=8×10−3, have been deduced from the intensity profile of the guided mode at λ=633 nm.

Coherent detection of terahertz pulses based on two-photon absorption in a photodiode

The authors present a simple and robust method for the measurement of the terahertz-induced lensing effect that may be used for the coherent detection of few-cycle terahertz pulses. It is based on two-photon absorption of a 1.5μm probe pulse in a standard silicon photodiode. The change in the spatial probe pulse profile caused by terahertz-induced lensing leads to a variation of the diode signal that is induced by two-photon absorption even though the total pulse energy has remained constant. This variation is shown to be proportional to the terahertz electric field.

Terahertz characterization of semiconductor alloy AlInN: negative imaginary conductivity and its meaning: comment

In a recent article [Opt. Lett.34, 2507 (2009)], the authors concluded that the imaginary part of the complex conductivity sigma of an AlInN film was negative. However, this result was obtained using an approximation that is not applicable in this case. Using the correct equation, the data can be well described with a positive imaginary part of sigma.