Femtosecond 8.5 µm source based on intrapulse difference-frequency generation of 2.1 µm pulses.

We report on a femtosecond ∼8.5 µm, ∼2 µJ source based on the intrapulse difference-frequency generation (DFG) of 2.1 µm pulses in an AgGaSe2 (AGSe) crystal. Compared to the conventional ∼0.8 or 1 µm near-infrared (IR) pulses, a ∼2 µm driver for intrapulse DFG can provide more efficient conversion into the wavelengths longer than 5 µm due to a lower quantum defect and is more suitable for the non-oxide nonlinear crystals that have a relatively low bandgap energy. Using 26 fs, 2.1 µm pulses for type-II intrapulse DFG, we have generated intrinsically carrier-envelope phase-stable idler pulses with a conversion efficiency of 0.8%, which covers the wavelength range of 7-11 µm. Our simulation study shows that the blueshift of intrapulse DFG is assisted by self-phase modulation of the driving pulses in AGSe. The idler pulses are particularly useful for strong-field experiments in nanostructures, as well as for seeding parametric amplifiers in the long-wavelength IR.

Nanostructured ultrafast silicon-tip optical field-emitter arrays.

Femtosecond ultrabright electron sources with spatially structured emission are an enabling technology for free-electron lasers, compact coherent X-ray sources, electron diffractive imaging, and attosecond science. In this work, we report the design, modeling, fabrication, and experimental characterization of a novel ultrafast optical field emission cathode comprised of a large (>100,000 tips), dense (4.6 million tips·cm(-2)), and highly uniform (<1 nm tip radius deviation) array of nanosharp high-aspect-ratio silicon columns. Such field emitters offer an attractive alternative to UV photocathodes while providing a direct means of structuring the emitted electron beam. Detailed measurements and simulations show pC electron bunches can be generated in the multiphoton and tunneling regime within a single optical cycle, enabling significant advances in electron diffractive imaging and coherent X-ray sources on a subfemtosecond time scale, not possible before. At high charge emission yields, a slow rollover in charge is explained as a combination of the onset of tunneling emission and the formation of a virtual cathode.

Octave-spanning coherent mid-IR generation via adiabatic difference frequency conversion.

We demonstrate efficient downconversion of a near-IR broadband optical parametric chirped pulse amplifier (OPCPA) pulse to a 1.1-octave-spanning mid-IR pulse (measured at -10 dB of peak) via a single nonlinearly and adiabatically chirped quasi-phase-matching grating in magnesium oxide doped lithium niobate. We report a spectrum spanning from 2 to 5 µm and obtained by near full photon number conversion of µJ-energy OPCPA pulses spanning 680-870 nm mixed with a narrowband 1047-nm pulse. The conversion process is shown to be robust for various input broadband OPA pulses and suitable for post-amplification conversion for many near-IR systems.