Xiaohui Gao

Shaoxing University · Physics

Manipulation of intense pulse propagation in gas-filled capillaries is desirable for various high-field applications. Tuning the parameters of the driving laser pulse and the working gas is the conventional approach, and it provides limited capability of control. Here we demonstrate through numerical simulations a practical scheme to control the pr...

Ionization is one of the most fundamental processes in intense laser-matter interaction. It is extremely efficient for clusters in laser fields and often leads to surprisingly high charge states at moderate laser intensities. Here we show an interesting ionization mechanism in laser-cluster interaction through particle-in-cell simulations. As the l...

Sub-micrometer-sized targets are found in intense laser–cluster interaction experiments and laser-based material processing. Here, we investigate the internal field localization due to Mie scattering and its effect on ionization dynamics in sub-micrometer-sized clusters using Mie calculation and particle-in-cell simulations. As a result of intertwi...

Strong mid-infrared light–matter interactions have attracted extensive attention as they open up new frontiers in nonlinear optics. Here we observe through simulations a novel, to the best of our knowledge, aspect of mid-infrared pulse dynamics in a high-pressure gas-filled capillary, where a pulse with a power well below the critical power for Ker...

Ionization is a fundamental process in intense laser–matter interactions and is known to cause plasma defocusing and intensity clamping. Here, we investigate theoretically the propagation dynamics of an intense laser pulse in a helium gas jet in the ionization saturation regime, and we find that the pulse undergoes self-focusing and self-compressio...

Ionization occurs ubiquitously in intense laser–matter interaction and often leads to rapid decrease in laser intensity via plasma defocusing, shortening the effective interaction length of desired high-field processes. Refocusing of pulses may compensate for this adverse effect. However, it typically relies on Kerr-induced self-focusing and requir...

We show theoretically and demonstrate experimentally that collapsing elliptically polarized laser beams experience a nonlinear ellipse rotation that is highly sensitive to small fluctuations in the input power. For arbitrarily small fluctuations in the input power and after a sufficiently large propagation distance, the polarization angle becomes u...

We study theoretically harmonic generation from ionizing nano-clusters irradiated by intense few-cycle laser pulses and identify a Brunel-type harmonic generation mechanism that originates from subcycle ionization dynamics in clusters. Compared to Brunel harmonics in gases, the spectra are shifted toward odd-order harmonics of Mie frequency ω M due...

We numerically investigate the propagation dynamics of intense long-wavelength infrared pulses in a weakly ionized argon gas and show that the pulses undergo self-focusing due to the transverse variations of electron-impact ionization caused by quivering electrons. We demonstrate this plasma-induced self-focusing at a power much lower than the crit...

We theoretically demonstrate Brunel-type harmonic generation from ionizing nano-clusters irradiated by intense few-cycle laser pulses. Mie oscillations strongly blue-shift and enhance the internal field. The resulting subcycle ionization dynamics efficiently produce broadband VUV radiation.

We show theoretically and demonstrate experimentally that collapsing elliptically-polarized laser beams experience a nonlinear ellipse rotation that is highly sensitive to small fluctuations in the input power. For arbitrarily small fluctuations in the input power and after a sufficiently large propagation distance, the polarization angle becomes u...

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 P cr as a result of ionization-induced refraction that excites higher-order modes. The constructive i...

The processes of energy gain and redistribution in a dense gas subject to an intense ultrashort laser pulse are investigated theoretically for the case of high-pressure argon. The electrons released via strong-field ionization and driven by an oscillating laser field collide with neutral neighbor atoms, thus effecting the energy gain in the emergin...

We report experimental measurements of nonlinear refractive index of solids in the mid-infrared. We present results for fused silica and for silicon, and our results are consistent with theoretical predictions.

We investigate the plasma dynamics inside a femtosecond-pulse-induced filament generated in an argon gas for a wide range of pressures up to 60 bar. At higher pressures, we observe ionization immediately following a pulse, with up to a threefold increase in the electron density within 30 ps after the filamentary propagation of a femtosecond pulse....

We experimentally demonstrate the control and complete elimination of multi-filamentation in condensed matter by varying the focusing geometry. In particular, increasing the input beam power enables the extension of the filament length without generating multi-filaments up to 1400 times the critical power in fused silica at an 800 nm wavelength. Fu...

We investigate pulse propagation from the ultraviolet to the mid-infrared in gas-filled capillaries. For shorter wavelength pulses and smaller capillary diameters, ionization-induced refraction excites higher-order modes, and their constructive interference leads to spatio-temporal localization.

We theoretically demonstrate stable nonlinear propagation of ultrashort pulses over several centimeters in high-pressure, gas-filled waveguides with intensities approaching 1014 W/cm2. This intensity stabilization is achieved by self-compression to a single cycle combined with modal loss.

Variable focusing geometries are used to eliminate multi-filamentation in favor of a stable single filament propagating as a spatial solitary wave at input powers up to 1400 times the critical power in normally dispersive media.

High-harmonic generation is a universal response of matter to strong femtosecond laser fields, coherently upconverting light to much shorter wavelengths. Optimizing the conversion of laser light into soft x-rays typically demands a trade-off between two competing factors. Because of reduced quantum diffusion of the radiating electron wave function,...

We observe a 3-fold increase in the electron density within 30 picoseconds after the filamentary propagation of femtosecond pulses in 60-bar argon gases. This suggests that electron-impact ionization dominates on this time scale.

We report on harmonic generation experiments and calculations in air to investigate the theoretical prediction of Kolesik et al. [Opt. Lett. 35, 2550 (2010)] for testing the recently proposed higher-order Kerr effect model. Our observations show that although the fifth-order nonlinearity is non-negligible, the overall defocusing effect via the high...

We present a new regime of UV-driven high harmonic generation, where the larger refractive index of ions counterbalances the plasma dispersion. Bright harmonic emission from ions is observed for the first time that extends up to 280eV.

We measure the ratio between fifth- and third-harmonic signals from air as a function of the laser intensity determined by mode measurements. Our results do not support the presence of a higher-order Kerr nonlinearity.

We present an in-depth study of a rapid, noninvasive, single-shot optical method of determining cluster mass fraction fc(r, t) at specified positions r within, and at time t after opening the valve of, a pulsed high-pressure pulsed supersonic gas jet. A ∼2 mJ, 40 fs pump pulse ionizes the monomers, causing an immediate drop in the jet's refractive...

Laser schlieren deflectometry for temperature analysis of filamentary non-thermal atmospheric pressure plasma Rev. Sci. Instrum. 83, 103506 (2012) Reconstruction of polar magnetic field from single axis tomography of Faraday rotation in plasmas Phys. Plasmas 19, 103107 (2012) Study of the plasma wave excited by intense femtosecond laser pulses in a...

Accurate interpretation and control of laser-cluster experiments require complete characterization of the cluster gas jet, of which the cluster mass fraction is usually difficult to measure. Here we present an in situ optical measurement of the cluster mass fraction. We measured the fs-time-resolved phase shifts from the gas jet after ionization an...

Laser interactions with a mixture of a gaseous plasma and microclusters depend strongly on the cluster-size distribution, which is usually difficult to measure directly. We present a new method for recovering the cluster-size distribution and cluster mass fraction from measurements of a time-dependent refractive index of the medium in a two-pulse p...

Noble gas clusters irradiated by intense ultrafast laser expand quickly and become typical plasma in picosecond time scale. During the expansion, the clustered plasma demonstrates unique optical properties such as strong absorption and positive contribution to the refractive index. Here we studied cluster expansion dynamics by fs-time-resolved refr...

Laser-irradiated microclusters can generate energetic ions that produce fusion reactions. The amount and spectrum of these ions depend on the cluster-size distribution, electron heating mechanism, and cluster expansion dynamics. This paper describes recent physics results pertinent to the items listed. It is shown that the size distribution of larg...

Characterization of supersonic gas jets is important for accurate interpretation and control of laser-cluster experiments. While average size and total atomic density can be found by standard Rayleigh scatter and interferometry, cluster mass fraction and size distribution are usually difficult to measure. Here we determine the cluster fraction and...

We report femtosecond-time-resolved enhancement and anisotropy of thirdharmonic generation in highly ionized clustered argon at intensities exceeding 1015 W/cm2.Results suggest a path to phase-match high-order harmonic generation in dense plasmas with ultrahigh intensity.

We present experiments showing that optical third-harmonic generation (THG) from expanding argon nanoclusters by time-delayed 80 fs probe pulses exhibits strong transient polarization anisotropy for several hundred femtoseconds after cluster ionization and heating by a linearly polarized pump, even though linear optical properties remain isotropic....