We review our recent efforts on power scaling of THz pulses generated by several nonlinear crystals. By
using a single GaP crystal and stacking three GaP plates, we have significantly increased the output peak
power to as high as 722 W and 2.36 kW, respectively. On the other hand, by using CO2 laser pulses, we have
obtained the average output power of 260 μW. We have also used these laser pulses to scale up the output
power for the THz pulses to 29.8 μW by stacking GaAs wafers. Indeed, by stacking up to ten wafers, we
have increased the output power by a factor of 160. Finally, by using ultrafast laser pulses, we have achieved
record-high output powers for the THz pulses generated from multi-period periodically-poled LiNbO3
crystals based on a backward configuration. The highest output power obtained by us so far is 10.7 μW.
We have observed that the temperature of the electrons drifting under a relatively-high electric field in an
AlN/GaN-based high-electron-mobility transistor is significantly higher than the lattice temperature (i.e. the
hot electrons are generated). These hot electrons are produced through the Fröhlich interaction between the
drifting electrons and long-lived longitudinal-optical phonons. By fitting electric field vs. electron
temperature deduced from the measurements of photoluminescence spectra to a theoretical model, we have
deduced the longitudinal-optical-phonon emission time for each electron is to be on the order of 100 fs. This
value is consistent with the value measured previously from Raman scattering.
We studied several crystals of Yb-doped LuVO4 with different orientations (a-cut and c-cut) in order to evaluate the
potential of this new laser material for high power continuous-wave operation using simple hemispherical cavities,
longitudinally pumped by a fiber coupled diode laser. We achieved substantial improvement with respect to previous
results in terms of output power and slope efficiency. The highest output power and optical efficiency were obtained for
the &pgr;-polarization using a-cut samples. Bistability of the input-output power characteristics in terms of a hysteresis loop
was also observed. Significant intensity fluctuations were found existing in a small operational region near the critical
point (up-threshold) of the bistability region. The heating of the crystal is reduced in the lasing state when stimulated
emission keeps the part of the radiative relaxation high in comparison to the nonradiative relaxation processes.
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