THz generation via Cherenkov radiation
We use different techniques to generate terahertz (THz) radiation in lithium niobate crystals excited by ultrashort femtosecond (fs) laser pulses. One can use either bulk or domain inverted crystals (periodically poled lithium niobate, PPLN). In PPLN each domain emits a THz half cycle which is added to the next one in phase to a pulse train .
If a bulk lithium niobate is used, the point sources in the non-linear crystal interfere constructively to a phase front (Cherenkov cone). This is the electromagnetic equivalent to the Mach cone of a sonic wave. The indices of refraction in lithium niobate for the THz frequencies (nTHz = 5.2) and the NIR femtosecond pulse (NNIR = 2.3) determine the angle of the Cherenkov cone [2,3]:
This cone strikes the crystal's surface at an angle of 25°, which is larger than the angle of total internal reflection at the interface lithium niobate and air. In order to avoid total internal reflection the crystal can be cut exactly in the corresponding angle so that the radiation propagates normal to the surface:
This solution suffers from high losses in the crystal due to the high absorption coefficient of lithium niobate in the THz range. A more effective way is to apply a silicon prism at the emitting surface [4,5]. This reduces the step of refractive index and the path length of THz in the crystal (The focus of the NIR laser beam is close to the surface). Therefore the spectral amplitude, especially in the high frequency part of the spectrum, is considerably increased:
In order to increase the conversion efficiency an enhancement cavity will be applied. An actively controlled length stabilization [6,7,8,9] of a resonator locked to the repetition rate of the oscillator recycles the unused pump photons like in a Fabry-Perot etalon. A much higher THz power and a broader spectrum is expected.
Terahertz (THz), THz generation, Cherenkov Radiation, enhancement cavity, laser pulses, lithium niobate crystals, far-infrared, broadband
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