Optical waveguides made from periodically poled materials provide highconfinement of light and enable the generation of new wavelengths viaquasi-phase-matching, making them a key platform for nonlinear optics andphotonics. However, such devices are not typically employed for high-harmonicgeneration. Here, using 200-fs, 10-nJ-level pulses of 4100 nm light at 1 MHz,we generate high harmonics up to the 13th harmonic (315 nm) in a chirped,periodically poled lithium niobate (PPLN) waveguide. Total conversionefficiencies into the visible--ultraviolet region are as high as 10 percent. Wefind that the output spectrum depends on the waveguide poling period,indicating that quasi-phase-matching plays a significant role. In the future,such periodically poled waveguides may enable compact sources of ultrashortpulses at high repetition rates and provide new methods of probing theelectronic structure of solid-state materials.
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