Abstract
Since the new millennium coherent extreme ultra-violet and soft x-ray radiation has revolutionized the understanding of dynamical physical, chemical and biological systems at the electron’s natural timescale. Unfortunately, coherent laser-based upconversion of infrared photons to vacuum-ultraviolet and soft x-ray high-order harmonics in gaseous, liquid and solid targets is notoriously inefficient. In dense nonlinear media, the limiting factor is strong re-absorption of the generated high-energy photons. Here we overcome this limitation by generating high-order harmonics from a periodic array of thin one-dimensional crystalline silicon ridge waveguides. Adding vacuum gaps between the ridges avoids the high absorption loss of the bulk and results in a ~ 100-fold increase of the extraction depth. As the grating period is varied, each high harmonic shows a different and marked modulation, indicating their waveguiding in the vacuum slots with reduced absorption. Looking ahead, our results enable bright on-chip coherent short-wavelength sources and may extend the usable spectral range of traditional nonlinear crystals to their absorption windows. Potential applications include on-chip chemically-sensitive spectro-nanoscopy.
Highlights
Since the new millennium coherent extreme ultra-violet and soft x-ray radiation has revolutionized the understanding of dynamical physical, chemical and biological systems at the electron’s natural timescale
Increasing the guiding distance yields a monotonic buildup of high harmonics over a distance of ~700 nm, indicating a ~100-fold increase of transmission with respect to bulk Si (Labs ~5 nm)
The generated high-harmonics are collected with a Fresnel zone plate (FZP) and their spectrum is recorded with a vacuum-ultraviolet (VUV) spectrometer and a silicon CCD camera
Summary
Since the new millennium coherent extreme ultra-violet and soft x-ray radiation has revolutionized the understanding of dynamical physical, chemical and biological systems at the electron’s natural timescale. The limiting factor is strong re-absorption of the generated high-energy photons We overcome this limitation by generating high-order harmonics from a periodic array of thin one-dimensional crystalline silicon ridge waveguides. Solids provide a suitable platform for photonic applications of strong-field processes, such as for petahertz electronics[16,17,18], electric-field sensing[19], and on-chip extreme ultra-violet microscopy The latter requires efficient generation of high-order harmonics at the nanoscale and low-loss transport on a chip. It consists of a periodic array of thin single-crystal silicon ridge-waveguides that are intended to spatially separate the nonlinear high-harmonic polarization, which travels along the ridge’s sidewalls, from the generated highharmonic waves, which leak and propagate inside the vacuum slots between ridges. An illustration of the emitted high harmonics is superimposed in purple
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