János Hebling

Generation of 10μJ ultrashort terahertz pulses by optical rectification

Generation of near single-cycle pulses centered at 0.5THz frequency with up to 10μJ energy, 100μW average power, and 5.0MW peak power was demonstrated by tilting the intensity front of a femtosecond optical pump pulse from a 10Hz Ti:sapphire laser to match the phonon-polariton phase velocity to the group velocity of the pump pulses in a lithium niobate crystal. Terahertz pulse intensity as high as 10MW∕cm2 was achieved. The photon conversion efficiency was 45% and the calculated peak electric field strength at the focus of an off-axis parabolic mirror was 250kV∕cm.

Velocity matching by pulse front tilting for large-area THz-pulse generation

We propose a generally applicable velocity matching method for THz-pulse generation by optical rectification in the range below the phonon frequency of the nonlinear material. Velocity matching is based on pulse front tilting of the ultrashort excitation pulse and is able to produce a large area THz beam. Tuning of the THz radiation by changing the tilt angle is experimentally demonstrated for a narrow line in the range between 0.8- 0.97 times the phonon frequency. According to model calculations broadband THz radiation can be generated at lower frequencies. Advantages of the new velocity matching technique in comparison to the electro-optic Cherenkov effect and non-collinear beam mixing are discussed.

Generation of high-power terahertz pulses by tilted-pulse-front excitation and their application possibilities

The principles and most-recent results of high-power THz generation through optical rectification using a tilted optical pulse front are described. Single-cycle THz pulses of multimicrojoule energies are generated at kHz repetition rates, and average THz power levels exceeding 1 mW can be generated at kHz-MHz repetition rates. Applications in nonlinear THz spectroscopy and THz coherent control are discussed.

Temperature dependence of the absorption and refraction of Mg-doped congruent and stoichiometric LiNbO3 in the THz range

The absorption coefficient and the index of refraction of Mg-doped LiNbO/sub 3/ crystals with different compositions are determined in the 30 - 200 cm-1 frequency range. We find that stoichiometric LiNbO/sub 3/ has smaller absorption and index of refraction than congruent samples.

Generation of sub-mJ terahertz pulses by optical rectification

Recent theoretical calculations predicted an order-of-magnitude increase in the efficiency of terahertz pulse generation by optical rectification in lithium niobate when 500 fs long pump pulses are used, rather than the commonly used ~100 fs pulses. Even by using longer than optimal pump pulses of 1.3 ps duration, 2.5× higher THz pulse energy (125 μJ) was measured with 2.5× higher pump-to-THz energy conversion efficiency (0.25%) than reported previously with shorter pulses. These results verify the advantage of longer pump pulses and support the expectation that mJ-level THz pulses will be available by cooling the crystal and using large pumped area.

Design of high-energy terahertz sources based on optical rectification

Detailed analysis of the tilted-pulse-front pumping scheme used for ultrashort THz pulse generation by optical rectification of femtosecond laser pulses is presented. It is shown that imaging errors in a pulse-front-tilting setup consisting of a grating and a lens can lead to a THz beam with strongly asymmetric intensity profile and strong divergence, thereby limiting applications. Optimized setup parameters are given to reduce such distortions. We also show that semiconductors can offer a promising alternative to LiNbO(3) in high-energy THz pulse generation when pumped at longer wavelengths. This requires tilted-pulse-front pumping, however the small tilt angles allow semiconductors to be easily used in such schemes. Semiconductors can be advantageous for generating THz pulses with high spectral intensity at higher THz frequencies, while LiNbO(3) is better suited to generate THz pulses with very large relative spectral width. By using optimized schemes the upscaling of the energy of ultrashort THz pulses is foreseen.

Scaling up the energy of THz pulses created by optical rectification

The possibility for up-scaling the energy of sub-ps THz pulses generated by tilted pulse front excitation is demonstrated. Using 150-fs-long 500 muJ optical pump pulses at 800 nm up to 240 nJ THz pulse energy has been achieved. For a 1.2 mm2 pump spot area, the energy conversion efficiency of pump energy to THz pulse energy had a maximum of 5 x 10-4 at 300 muJ pump pulse energy. The corresponding photon conversion efficiency amounts to 10 %. For comparison, the maximum attainable THz pulse energy was limited to 3.1 nJ if a line focusing excitation geometry was utilized. This limit was reached at 32 muJ pump energy. For the latter configuration the THz energy dropped for larger pump energies. The tilted pulse front excitation allows further up-scaling of the THz pulse energy by using a larger pump spot size and still stronger pump pulses.

Impact ionization in InSb probed by terahertz pump—terahertz probe spectroscopy

Indium antimonide InSb is a model system for the study of hot-electron dynamics due to its low band gap of 170 meV at room temperature 1 and the fact that it has the highest electron mobility and saturation velocity among all known semiconductors. The wealth of nonequilibrium transport phenomena that have been observed in this material 2–4 is of special interest due to the large nonparabolicity of the conduction band, 5 which results in negative differential mobility

High-Power THz Generation, THz Nonlinear Optics, and THz Nonlinear Spectroscopy

It is now possible to generate terahertz (THz) pulses with sufficient energy and field amplitude to enable versatile applications in THz nonlinear optics and spectroscopy. In addition, THz waveform shaping at high intensities promises wide ranging new capabilities in THz coherent control. We review recent progress in generation of high-power THz phonon-polariton waves in lithium niobate that can be coupled into free space THz radiation. A ldquopolaritonicsrdquo toolset for control and processing of the THz waves is also reviewed briefly. Recent demonstrations of THz nonlinear optics and spectroscopy are then presented.

Efficient terahertz generation by optical rectification at 1035 nm

We demonstrate efficient generation of THz pulses by optical rectification of 1.03 um wavelength laser pulses in LiNbO3 using tilted pulse front excitation for velocity matching between the optical and THz fields. Pulse energies of 100 nJ with a spectral bandwidth of up to 2.5 THz were obtained at a pump energy of 400 uJ and 300 fs pulse duration. This conversion efficiency of 2.5x10(-4) was an order of magnitude higher than that obtained with collinear optical recitification in GaP, and far higher still than that measured using ZnTe in an optimized geometry. Using a simple model we demonstrate that two- and three-photon absorption strongly limit the THz generation efficiency at high pump fluences in ZnTe and GaP.