Kerr-type nonlinearities form the basis for our physical understanding of nonlinear optical phenomena in condensed matter, such as self-focusing, solitary waves and wave mixing1–3. In strong fields, ...they are complemented by higher-order nonlinearities that enable high-harmonic generation, which is currently understood as the interplay of light-driven intraband charge dynamics and interband recombination4–6. Remarkably, the nonlinear response emerging from the subcycle injection dynamics of electrons into the conduction band, that is from ionization, has been almost completely overlooked in solids and only partially considered in the gas phase7–10. Here, we reveal this strong-field-induced nonlinearity in a-SiO2 as a typical wide-bandgap dielectric by means of time-resolved, low-order wave-mixing experiments, and show that, close to the material damage threshold, the so far unexplored injection current provides the leading contribution. The sensitivity of the harmonic emission to the subcycle ionization dynamics offers an original approach to characterize the evolution of laser-induced plasma formation in optical microprocessing.Strong-field-induced nonlinearities from the injection of electrons into the conduction band contribute to harmonic generation in amorphous quartz. Close to the damage threshold, they dominate over intraband and interband contributions.
The nonlinear pulse propagation in photonic crystal fibers without slowly varying envelope approximation is studied using an improved variant of first-order wave equation. Supercontinuum generation ...is shown to be caused by a novel mechanism of spectral broadening through fission of higher-order solitons into redshifted fundamental solitons and blueshifted nonsolitonic radiation. Good agreement with experimental observations is found, and subcycle pulse compression is studied.
Broadband ultrashort terahertz (THz) pulses can be produced using plasma generation in a noble gas ionized by femtosecond two-color pulses. Here we demonstrate that, by using multiple-frequency laser ...pulses, one can obtain a waveform which optimizes the free electron trajectories in such a way that they acquire the largest drift velocity. This allows us to increase the THz conversion efficiency to 2%, an unprecedented performance for THz generation in gases. In addition to the analytical study of THz generation using a local current model, we perform comprehensive 3D simulations accounting for propagation effects which confirm this prediction. Our results show that THz conversion via tunnel ionization can be greatly improved with well-designed multicolor pulses.
We numerically study light propagation through a specially designed nonlinear nanoscale metal-dielectric multilayer structure with a linear effective dielectric constant just below zero. The ...calculated dependence of the output intensity on the input intensity shows a steplike behavior. It rests upon an intensity-dependent change of the effective dielectric constant from negative (low-transmission state) to positive (high-transmission state) values, corresponding to a transition of the optical properties from metalliclike to dielectriclike. The study of the transient behavior of the structure demonstrates a switching time of around 1 ps.
We report on damage-free fiber-guidance of milli-Joule energy-level and 600-femtosecond laser pulses into hypocycloid core-contour Kagome hollow-core photonic crystal fibers. Up to 10 meter-long ...fibers were used to successfully deliver Yb-laser pulses in robustly single-mode fashion. Different pulse propagation regimes were demonstrated by simply changing the fiber dispersion and gas. Self-compression to ~50 fs, and intensity-level nearing petawatt/cm(2) were achieved. Finally, free focusing-optics laser-micromachining was also demonstrated on different materials.
Trapping or cooling molecules has rallied a long-standing effort for its impact in exploring new frontiers in physics and in finding new phase of matter for quantum technologies. Here we demonstrate ...a system for light-trapping molecules and stimulated Raman scattering based on optically self-nanostructured molecular hydrogen in hollow-core photonic crystal fibre. A lattice is formed by a periodic and ultra-deep potential caused by a spatially modulated Raman saturation, where Raman-active molecules are strongly localized in a one-dimensional array of nanometre-wide sections. Only these trapped molecules participate in stimulated Raman scattering, generating high-power forward and backward Stokes continuous-wave laser radiation in the Lamb-Dicke regime with sub-Doppler emission spectrum. The spectrum exhibits a central line with a sub-recoil linewidth as low as ∼14 kHz, more than five orders of magnitude narrower than conventional-Raman pressure-broadened linewidth, and sidebands comprising Mollow triplet, motional sidebands and four-wave mixing.
We investigate possibilities to utilize field enhancement by specifically designed metal nanostructures for the generation of single attosecond pulses using the polarization gating technique. We ...predict the generation of isolated 59-attosecond-long pulses using 15-fs pump pulses with only a 0.6 TW/cm2 intensity. Our simulations also indicate the possibility to generate previously inaccessible high-harmonics with circular polarization by using an ensemble of vertically and horizontally orientated bow-tie structures. In the numerical simulation we used an extended Lewenstein model, which includes the spatial inhomogeneity in the hot spots and collisions of electrons with the metal surface.
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