Time-resolved study of third harmonic generation from anisotropically expanding clusters

textTime-resolved Third Harmonic Generation (THG) from expanding argon gas clusters has been studied. A 400 nm pump (Ipump ∼ 1 x 1015 W/cm2 ) beam ionizes a gas jet composed of atomic clusters and residual gases. An 800 nm, 100 fs probe then generates third harmonic radiation from expanding clusters with controlled delays. The measured THG is sharply peaked at earlier delays than broad absorption resonances. Simulations show that the nonlinear susceptibility χ (3) of the individual clusters and the THG coherence length of the clustered plasma medium are optimized nearly simultaneously as the pre-heated clusters expand, and both contribute to the observed THG enhancement. We also measured THG anisotropy from expanding clusters. When resovii nantly enhanced, THG becomes temporarily anisotropic – i.e. a probe polarized perpendicular to the pump generates third-harmonic more efficiently than one polarized parallel – thereby characterizing the anisotropy of cluster expansion. By contrast, the linear optical response was isotropic. The physical mechanisms contributing to enhanced THG are scalable to relativistic probe intensity (limited only by pre-pulses in the laser system) and to high-order harmonic generation extending to the soft x-ray regime.Physic

Michael F. BeckerTime-Resolved Study of Third Harmonic Generation from Anisotropically Expanding Clusters

loved me so much and are in my heart. Acknowledgments This work was possible due to many people. Most significantly, I thank my research supervisor Dr. Michael Downer, who has supported me for my graduate work and has guided me in developing my research skills and insight of physics. When I struggled, he gave me nice suggestions (most of his suggestions turned out to be major breakthroughs), waiting patiently. When I had good results, he discussed the results with me for several hours and motivated me to do more. Without his patience, creativity and enthusiasm, this work would be absolutely impossible. I thank Dr. Todd Ditmire and his student, Gregory Hays. Dr. Ditmire gave me guidance in this research as a true pioneer of the atomic cluster science and his numerous journals about cluster-laser interaction motivated me to do third harmonic generation simulation. I enjoyed a lot working with Gregory Hays. His knowledge and experience about physics, laser science and humor are incorporated in this dissertation and were crucial to our experimental success

Multi-octave-spanning supercontinuum generation through high-energy laser filaments in YAG and ZnSe pumped by a 2.4 μm femtosecond Cr:ZnSe laser

Abstract We present experimental and numerical investigations of high-energy mid-infrared filamentation with multi-octave-spanning supercontinuum generation (SCG), pumped by a 2.4 μm, 250 fs Cr:ZnSe chirped-pulse laser amplifier. The SCG is demonstrated in both anomalous and normal dispersion regimes with YAG and polycrystalline ZnSe, respectively. The formation of stable and robust single filaments along with the visible-to-mid-infrared SCG is obtained with a pump energy of up to 100 μJ in a 6-mm-long YAG medium. To the best of the authors’ knowledge, this is the highest-energy multi-octave-spanning SCG from a laser filament in a solid. On the other hand, the SCG and even-harmonic generation based on random quasi-phase matching (RQPM) are simultaneously observed from the single filaments in a 6-mm-long polycrystalline ZnSe medium with a pump energy of up to 15 μJ. The numerical simulations based on unidirectional pulse propagation equation and RQPM show excellent agreement with the measured multi-octave-spanning SCG and even-harmonic generation. They also reveal the temporal structure of mid-infrared filaments, such as soliton-like self-compression in YAG and pulse broadening in ZnSe.</jats:p

Ionization-assisted spatiotemporal localization in gas-filled capillaries.

We demonstrate numerically and experimentally that intense pulses propagating in gas-filled capillaries can undergo localization in space and time due to strong plasma defocusing. This phenomenon can occur below or above the self-focusing threshold Pcr as a result of ionization-induced refraction that excites higher-order modes. The constructive interference of higher-order modes leads to spatiotemporal localization and resurgence of the intensity. Simulations show that this confinement is more prominent at shorter wavelength pulses and for smaller capillary diameters. Experiments with ultraviolet pulses show evidence that this ionization-induced refocusing appears below Pcr and thus represents a mechanism for spatiotemporal confinement without self-focusing

Mid-IR Laser Filamentation in air at a kHz Repetition Rate

We present 2.1-μm-pumped, self-guided laser filamentation in air at kHz repetition rate. Efficient odd-harmonic generations to 9th order at ultraviolet and further extension to the mid-infrared are observed. Absorption spectroscopy of atmospheric CO2 is demonstrated

Mid-infrared laser filaments in air at a kilohertz repetition rate

Laser filamentation overcomes diffraction over a highly extended distance, making itself a powerful tool for long-range stand-off detection and light detection and ranging (LIDAR)applications. Mid-infrared (mid-IR) wavelengths are optimal for detecting biochemicals and air pollutants due to molecular fingerprints. Here, we demonstrate mid-IR laser filamenta-tions in ambient air at a kilohertz repetition rate for the first time. Laser filaments significantly longer than the linear con-focal parameter are generated with a pump power exceedingthe critical power in air using a kilohertz, 2.1μm, femtosecond, multi-millijoule optical parametric chirped-pulse amplifier. Odd-harmonic generation up to the ninth order at ultravioletand the mid-IR spectral extensions up to 3.5μm are observed. The highest third and fifth harmonic efficiencies from ambient air are obtained, to our knowledge, thanks to the extended in-teraction length within the filaments. Numerical simulations reproduce the harmonic generation with good agreement and confirm that the plasma effect dominates over the higher-order Kerr effect as the main defocusing mechanism of laser filamentation in our experiment. The detection of atmospheric CO$_2$ is demonstrated via mid-IR absorption spectroscopy. High-flux ultrabroadband mid-IR filaments are useful for the fast and sensitive detection of multiple chemical species in air

Three-octave-spanning supercontinuum generation and sub-two-cycle self-compression of mid-infrared filaments in dielectrics

We experimentally and numerically investigate the spectral and temporal structure of mid-infrared (mid-IR) filaments in bulk dielectrics with normal and anomalous group velocity dispersion (GVD) pumped by a 2.1 μm optical parametric chirped-pulse amplifier (OPCPA). The formation of stable and robust filaments with several microjoules of pulse energy is observed. We demonstrate a supercontinuum that spans more than three octaves from ZnS in the normal GVD regime and self-compression of the mid-IR pulse to sub-two-cycle duration in CaF2 in the anomalous GVD regime. The experimental observations quantitatively agree well with the numerical simulations based on a three-dimensional nonlinear wave equation that reveals the detailed spatio-temporal dynamics of mid-IR filaments in dielectrics