The generation of THz-frequency radiation via nonlinear parametric frequency down-conversion has long been driven by the spectroscopy and imaging communities. As a result, little efforts have been undertaken toward the generation of high energy THz-frequency pulses. THz-frequency radiation has however recently been identified has a promising driver for strong-field physics and an emerging generation of compact particle accelerators. These accelerators require THzfrequency pulses with energies in the multi-millijoule range therefore demanding orders of magnitude improvements from the current state-of-the-art. <p> </p>Much can be gained by improving the intrinsically low efficiency of the down-conversion process while still resorting to existing state-of-the-art lasers. However, the fundamental Manley-Rowe limit caps the efficiency of parametric downconversion from 1-μm wavelength lasers to sub-THz frequency to the sub-percent range. <p> </p>We present methods that promise boosting the THz radiation yield obtained via parametric down-conversion beyond the Manley-Rowe limit. Our method relies on cascaded nonlinear three-wave mixing between two spectrally neighboring laser pulses in periodically poled Lithium Niobate. Owing to favorable phase-matching, the down-conversion process avalanches, resulting in spectral broadening in the optical domain. This allows <i>in-situ</i> coherent multiplexing of multiple parametric down-conversion stages within a single device and boosting the efficiency of the process beyond the ManleyRowe limit. We experimentally demonstrated the concept using either broadband, spectrally chirped optical pulses from a Joule-class laser or using two narrowband lasers with neighboring wavelengths. Experimental results are backed by numerical simulations that predict conversion efficiencies from 1 μm to sub-THz radiation in the multi-percent range.
We present results from our Ho:YLF regenerative amplifier (RA) producing up to 6.9 mJ at a repetition rate of 1 kHz and up to 12.9 mJ at a repetition rate of 10 Hz. At 1 kHz, we observe strongly bifurcating pulses, starting with certain round trip number, but the measurements identify a highly stable operation point that lies “hidden” beyond the instability region. This operation point allows the extraction of highly stable and high energetic output pulses. Suppression of bifurcation in our system is presented for repetition rates below 750 Hz and Ho:YLF crystal holder temperatures of 2.5 °C. We furthermore present a stability optimization routine for our Ho:YLF RA that was operated close to gain depletion at a repetition rate of 100 Hz. By varying the Ho:YLF crystal holder temperature the gain depletion level could be fine adjusted, resulting in a highly stable RA system with measured pulse fluctuations of only 0.35 %.
We present results from diode-pumped cw and semiconductor saturable absorber mirror (SESAM) mode-locked
resonators containing multiple bulk Yb:KYW crystals. The dual-crystal resonator generated more than 24W of cw-power
at a wavelength of 1042nm in a diffraction limited beam with the maximum power limited by the available pump power.
Two mode-locking regimes were explored. From the soliton mode-locked oscillator we obtained Fourier limited pulses
with a pulsewidth of 450fs at a repetition rate of 79MHz and with an average power of 14.6W. When operating the same
resonator in the positive dispersion regime we achieved an output power of 17W. Using a grating compressor these
pulses could be compressed to a pulse width of 470fs. Both mode-locked lasers were self-starting and operated stably
and in turn key fashion over days and through varying lab conditions. Regarding the power scaling of this type of laser
we anticipate further scalability by once again doubling the number of crystals inside the resonator.