Quasi-phase-matching of high harmonic EUV radiation

摘要

Nonlinear optics has greatly expanded the utility of the laser. Using nonlinear crystals and frequency-conversion techniques such as frequency-doubling and optical parametric amplification, laser light at one wavelength can be converted to another wavelength. More recently, advances in fabrication techniques have made it possible to make periodically-structured materials, creating a new class of efficient nonlinear-optics devices based on quasi-phase-matching (QPM). The flexibility of frequency conversion of laser light has enabled a variety of new applications in chemical sensing, materials processing, surgery, microscopy, imaging, and lithography. In nonlinear frequency conversion at visible and near-UV wavelengths, phase matching is nearly always accomplished using anisotropic crystals. Most commonly, this is done by finding a configuration where the signal and pump beams propagate with different polarizations, taking advantage of the birefringence of the medium to equalize propagation velocities. When a process cannot be phase matched, the phase between the signal field and the nonlinear response of the medium (whose phase is controlled by the driving field) will eventually reach 180°, resulting in destructive interference and back-conversion into the fundamental field. The propagation distance at which this occurs is defined as the coherence length. In QPM, the relative phase between the output signal field and the nonlinear oscillator is periodically "corrected" with a periodicity corresponding to the coherence length.