High-harmonic generation in periodically poled waveguides
Daniel D. Hickstein, David R. Carlson, Abijith S. Kowligy, Matt Kirchner, Scott Domingue, Nima Nader, Henry R. Timmers, Alexander J. Lind, Gabriel G. Ycas, Margaret Murnane, Henry Kapteyn, Scott B. Papp, Scott A. Diddams
Optical waveguides made from periodically poled materials provide high confinement of light and enable the generation of new wavelengths via quasi-phase-matching, making them a key platform for nonlinear optics and photonics. However, such devices are not typically employed for high- harmonic generation. Here, using 200-fs, nJ-level pulses of 4100 nm light at 1 MHz, we generate high harmonics up to the 13th harmonic in a chirped periodically poled lithium niobate (PPLN) waveguide. We observe total conversion efficiencies into the visible--ultraviolet region as high as 10 percent. Additionally, we find that the output spectrum depends on the rate of chirp of the waveguide poling period, indicating that quasi-phase-matching plays a significant role. At low pulse energies, individual harmonic orders are generated, and the conversion efficiency of each harmonic generally follows a perturbative scaling with input power. However, at higher input pulse energies (>10nJ) non-perturbative scaling dominates and the harmonics merge into a supercontinuum that spans the entire visible region and into the ultraviolet. In the future, such periodically poled waveguides may enable compact sources of ultrashort pulses at high repetition rates and provide new methods of probing the electronic structure of solid-state materials.