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.

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.

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The kinetics of electrogenic events associated with the different steps of the light-induced proton pump of bacteriorhodopsin is well studied in a wide range of time scales by direct electric methods. However, the investigation of the fundamental primary charge translocation phenomena taking place in the functional energy conversion process of this protein, and in other biomolecular assemblies using light energy, has remained experimentally unfeasible because of the lack of proper detection technique operating in the 0.1-to 20-THz region. Here, we show that extending the concept of the familiar Hertzian dipole emission into the extreme spatial and temporal range of intramolecular polarization processes provides an alternative way to study ultrafast electrogenic events on naturally ordered biological systems. Applying a relatively simple experimental arrangement based on this idea, we were able to observe light-induced coherent terahertz radiation from bacteriorhodopsin with femtosecond time resolution. The detected terahertz signal was analyzed by numerical simulation in the framework of different models for the elementary polarization processes. It was found that the principal component of the terahertz emission can be well described by excited-state intramolecular electron transfer within the retinal chromophore. An additional slower process is attributed to the earliest phase of the proton pump, probably occurring by the redistribution of a H bond near the retinal. The correlated electron and proton translocation supports the concept, assigning a functional role to the light-induced sudden polarization in retinal proteins. coherent terahertz emission ͉ excited-state kinetics ͉ nonlinear spectroscopy ͉ sudden polarization ͉ ultrafast charge separation

A novel scheme for the generation of UV pulses in the 295 - 450 nm range is presented. Sum frequency mixing of the chirped visible pulses from a noncollinear optical parametric amplifier with deliberately chirped pulses from the Ti:sapphire amplifier ensures efficient energy conversion and easy tunability. Pulse energies as high as 5.5 J at 295 nm, and >2 J in most of the tuning range are obtained with highly symmetric and smooth spectra. They are compressed to sub-30 fs throughout the entire tuning range (20 fs at 348 nm) with a newly designed prism compressor.

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A compact rotating-mirror autocorrelator design for femtosecond and picosecond laser pulses Rev.An optical pulse autocorrelator is described which allows online measurement of femtosecond pulse profiles of tunable light sources having 100 Hz-10 kHz repetition rates. The device is capable of dispersion-free noninterferometric measurement of pulse durations ranging from about 100 to sub-10 fs. The applicability of the device is demonstrated in the wavelength range from 420 to 1460 nm. The time delay is scanned with a simple open-loop piezoceramic translator, which is interferometrically calibrated. The accuracy of the device is verified by comparing an autocorrelation trace of a 10 fs pulse with the autocorrelation computed from a zero-additional-phase-spectral phase interferometry for direct electric field reconstruction ͑ZAP-SPIDER͒ measurement in the visible region. Its compact design and high sensitivity make the autocorrelator an ideal tool for the diagnostics of tunable ultrashort pulses.

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