High harmonic generation

Summary

High harmonic generation (HHG) is a non-linear process during which a target (gas, plasma, solid or liquid sample) is illuminated by an intense laser pulse. Under such conditions, the sample will emit the high harmonics of the generation beam (above the fifth harmonic). Due to the coherent nature of the process, high harmonics generation is a prerequisite of attosecond physics. Perturbative harmonic generation Perturbative harmonic generation is a process whereby laser light of frequency ω and photon energy ħω can be used to generate new frequencies of light. The newly generated frequencies are integer multiples nω of the original light's frequency. This process was first discovered in 1961 by Franken et al., using a ruby laser, with crystalline quartz as the nonlinear medium. Harmonic generation in dielectric solids is well understood and extensively used in modern laser physics (see second-harmonic generation). In 1967 New et al. observed the first third harmonic generati

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Probing Rotational and Translational Diffusion of Nanodoublers in Living Cells on Microsecond Time Scales

Lucas Delannoy, Marie Eve Pascale Didier, Fabrizia Dutto, Carlos Alberto Macias Romero, Aleksandra Radenovic, Sylvie Roke, Vitalijs Zubkovs

Nonlinear microscopes have seen an increase in popularity in the life sciences due to their molecular and structural specificity, high resolution, large penetration depth, and volumetric imaging capability. Nonetheless, the inherently weak optical signals demand long exposure times for live cell imaging. Here, by modifying the optical layout and illumination parameters, we can follow the rotation and translation of noncentrosymetric crystalline particles, or nanodoublers, with 50 mu s acquisition times in living cells. The rotational diffusion can be derived from variations in the second harmonic intensity that originates from the rotation of the nanodoubler crystal axis. We envisage that by capitalizing on the biocompatibility, functionalizability, stability, and nondestructive optical response of the nanodoublers, novel insights on cellular dynamics are within reach.

American Chemical Society2014

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The availability of nonlinear parametric processes, such as frequency conversion in photonic integrated circuits is essential. In this contribution, we demonstrate a highly tunable second-harmonic generation in a fully complementary metal–oxide–semiconductor (CMOS)-fabrication-compatible silicon nitride integrated photonic platform. We induce the second-order nonlinearity using an all-optical poling technique with the second-harmonic light generated in the fundamental mode, and a narrow quasi-phase matching (QPM) spectrum by avoiding higher-order mode mixing. We are then able to broadly tune the phase-matched pump wavelength over the entire C-band (1540 nm to 1560 nm) by varying the poling conditions. Fine-tuning of QPM is enabled by thermo-optic effect with the tuning slope Δ𝜆/Δ𝑇 in our device being 113.8 pm/°C. In addition, we exploit the measurable variation of the 3 dB QPM bandwidth to confirm how the length of the all-optically inscribed grating varies with exposure time.

OSA2020

Optically reconfigurable quasi-phase-matching in silicon nitride microresonators

Camille Sophie Brès, Jianqi Hu, Edgars Nitiss, Anton Stroganov

Quasi-phase-matching has long been a widely used approach in nonlinear photonics, enabling efficient parametric frequency conversions such as second-harmonic generation. However, in silicon photonics the task remains challenging, as materials best suited for photonic integration lack second-order susceptibility (χ(2)), and means for achieving momentum conservation are limited. Here we present optically reconfigurable quasi-phase-matching in large-radius silicon nitride microresonators, resulting in up to 12.5-mW on-chip second-harmonic generated power and a conversion efficiency of 47.6% W−1. Most importantly, we show that such all-optical poling can occur unconstrained from intermodal phase-matching, leading to broadly tunable second-harmonic generation. We confirm the phenomenon by two-photon imaging of the inscribed χ(2) grating structures within the microresonators as well as by in situ tracking of both the pump and second-harmonic mode resonances during all-optical poling. These results unambiguously establish that the photogalvanic effect, responsible for all-optical poling, can overcome phase mismatch constraints, even in resonant systems.

2022