26 research outputs found
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Nonlinear distortion of intense THz beams
Near- and far-field beam profiles were measured for THz pulses generated in LiNbO3 by optical rectification of 200 fs pulses with a tilted pulse front. The variation of the THz beam size and a dramatically increasing divergence angle with increasing pump fluence were observed in the
(horizontal) plane of the pulse front tilt. No significant variation was observed in the vertical direction.
The reason for the observed nonlinear beam distortion is the shortening of the effective interaction length for THz generation caused by the combined effect of pump spectral broadening and angular dispersion in the tilted pulse front geometry. Our results indicate that nonlinear THz beam distortion effects have to be taken into account when designing intense THz sources and related experiments
High-energy terahertz pulses from semiconductors pumped beyond the three-photon absorption edge
A new route to efficient generation of THz pulses with high-energy was demonstrated using semiconductor materials pumped at an infrared wavelength sufficiently long to suppress both two- and three-photon absorption and associated free-carrier absorption at THz frequencies. For pumping beyond the three-photon absorption edge, the THz generation efficiency for optical rectification of femtosecond laser pulses with tilted intensity front in ZnTe was shown to increase 3.5 times, as compared to pumping below the absorption edge. The four-photon absorption coefficient of ZnTe was estimated to be β₄=(4±1)×10⁻⁵ cm⁵/GW³. THz pulses with 14 μJ energy were generated with as high as 0.7% efficiency in ZnTe pumped at 1.7 µm. It is shown that scaling the THz pulse energy to the mJ level by increasing the pump spot size and pump pulse energy is feasible
Impulzushossz és hőmérséklet hatásai nagyenergiájú lítium-niobát alapú terahertzes forrásokra
Intense terahertz source development in the last decade allowed the growth of new areas
of research. Tilted-pulse-front pumping of inorganic LiNbO3 (LN) shows good
characteristics to achieve the highest possible pulse energies below 1 THz. Ultrafast
carrier dynamics of semiconductors were measured by THz pump - THz probe
measurements, molecule alignment with an intense field and electron wave streaking
were also demonstrated. THz-assisted attosecond pulse generation, would
also benefit from intense terahertz pulses. The above mentioned applications require
peak electric fields on the order of 100 kV/cm. However, newly emerging areas as
acceleration, longitudinal compression, and undulation of relativistic electron bunches, post-acceleration of laser-generated proton and ion beams with potential applications
for hadron therapy, multispectral single-shot imaging, THz-enhanced
attosecond-pulse generation with increased cut-off frequency would benefit from
field strengths higher than currently provided. Multi-mJ and multi-10-MV/cm level
sources are needed for these applications.
Today extremely high field strengths up to 100 MV/cm are available only in the frequency
range above 10 THz. Although the mentioned applications would highly
benefit from low frequency THz sources, so that longer wavelength is preferably matching
typical transversal sizes of particle beams, pulse energies and peak electric fields
necessary for these applications is presently not available. Therefore, it is a priority
to optimize THz pulses generated by optical rectification of fs laser pulses with
tilted-pulse-front pumping in lithium niobate. Theoretical studies predict
an increase in effciency with optimization of pump pulse duration and cryogenic cooling
of LN source crystal