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.

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.

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

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.

High efficiency generation and detection of terahertz pulses using laser pulses at telecommunication wavelengths.

We report on the generation and detection of few-cycle THz pulses in the organic crystal DAST (4-N,N-dimethylamino-4'-N'-methyl stilbazolium tosylate) using the telecommunication wavelength 1.5 mum. A continuous spectrum from 1.3 to 4.8 THz was generated with a very high conversion efficiency (E(THz,max) ~ 50 kV/cm at an optical pump pulse energy of 25 muJ in a 0.6 mm thick crystal) using the high nonlinear optical susceptibility of DAST (chi (2)111 = 490 pm/V at lambda = 1535 nm) and velocity-matching observed in this material. Using a frequency-doubled probe beam at lambda = 750 nm, nearly velocity-matched detection can also be realized in ZnTe crystals, resulting in a continuous spectrum from 1.3 to 4 THz.

Beam-size effects in electro-optic sampling of terahertz pulses.

In electro-optic sampling of terahertz pulses with a finite diameter, the probe pulse is subject to a lensing effect. The altered diffraction leads to an additional phase shift Deltaeta in the far field compared to the case of a plane wave. It is demonstrated that Deltaeta is in the same order of magnitude as the direct electro-optic phase shift at the exit of the detection crystal and thus also gives a significant contribution to the measured amplitude.

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.