Tilted pulse front technique in LiNbO3
One of the most used techniques for generation of ultrashort THz pulses is optical rectification in nonlinear crystals. The typical materials employed are ZnTe and GaP, in which a collinear geometry can be used to satisfy the phase matching condition (the near-infrared excitation pulse propagates parallel to the THz radiation). However, the nonlinear susceptibility of these materials is not particularly high, and only THz pulse energies on the order of hundreds of picojoules (with field strengths of ~1 kV/cm) can be achieved.
Lithium Niobate (LiNbO3) has a much larger THz nonlinear susceptibility, but in this case the intensity front of the near infrared radiation has to be tilted by an angle in order to fulfill the phase matching condition. The optimum tilt angle inside the crystal is ~60°, which can be achieved by diffracting the near-infrared radiation off a grating.
The tilted pulse front setup built in our laboratory is shown schematically in Figure 1. Near-infrared pulses from an amplified Ti:Sa laser are incident on a blazed grating. A telescope is then used to image the laser spot onto the LiNbO3 crystal. Figure 2 shows typical high-intensity THz transients and the corresponding spectra (pulse energies of the order of 1-2 µJ and field strengths of ~100 kV/cm are achieved). Through adjustment of the pulse front tilt angle, the central frequency can be tuned to match the experimental requirements.
Fourier amplitude as a function of frequency.
Related experiment
Free-Electron Laser sources
The tilted pulse front technique can generate single-cycle broadband THz pulses using a table-top setup. However, when relative spectral bandwidths of a few percent or less are required, one has to use multi-cycle pulses from Free-Electron Laser sources.
Free-Electron Lasers (FELs) are large-scale facilities capable of providing tunable, spectrally brilliant, and coherent high-power electromagnetic radiation. The wavelengths that are achievable span from the far infrared to hard x-rays, and the pulse duration can be in the ps to fs range.
In a FEL, relativistic electron bunches move freely through a magnetic structure called undulator. Due to the acceleration by the magnetic fields, the charge carriers emit photons. In THz-FELs (like FELBE at Helmholtz Zentrum Dresden-Rossendorf) the undulator is confined in an optical cavity to enhance the interaction of photons and electrons (see Figure 3). A typical spectrum of the FELBE source (150 µm central wavelength, corresponding to 2 THz frequency) is displayed in Figure 4. Remarkaby, the relative spectral width can be as small as a few percent.
In our experiments in Dresden, we use a near-infrared laser oscillator, whose repetition rate is synchronized to the high-intensity FEL pulse train, to generate probe pulses in a photoconductive antenna. In this way, narrowband FEL pump – broadband THz probe experiments can be performed.
References
J. Hebling et al., Optics Letters 10, 1161 (2002); J. Opt. Soc. Am. B, 6 (2008).
M. C. Hoffmann et al., Appl. Phys. Lett. 95, 231105 (2009).
A. Dienst, PhD thesis (2011).
http://www.hzdr.de/FELBE.
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