Generation of high-order harmonics and attosecond pulses in the water window via nonlinear propagation of a few-cycle laser pulse.

We theoretically and computationally study the generation of high-order harmonics in the water window from a semi-infinite gas cell where a few-cycle, carrier-envelope-phase-controlled 1.7-µm driving laser pulse undergoes nonlinear propagation via optical Kerr effect (self-focusing) and plasma defocusing. Our calculation shows that high harmonic signals are enhanced for extended propagation distances and furthermore, isolated attosecond pulses in the water window can be generated from the semi-infinite gas cell. This enhancement is attributed mainly to better phase matching for extended propagation distances achieved via nonlinear propagation and resulting intensity stabilization.

Study of wavelength-dependent pulse self-compression for high intensity pulse propagation in gas-filled capillaries.

We theoretically investigate the wavelength-dependent pulse self-compression dynamics of intense femtosecond laser pulses in gas-filled capillaries. Simulations with λ = 1, 2, 3 and 4 µm using the multimode carrier-resolved unidirectional pulse propagation equation reveal pulse self-compression or pulse broadening depending on plasma and modal dispersion. Our study shows that the pulse at 1 µm exhibits better pulse self-compression compared with longer wavelengths due to smaller group velocity mismatch between fundamental and higher-order capillary modes.

Self-focusing and self-compression of intense pulses via ionization-induced spatiotemporal reshaping.

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-compression through ionization-induced reshaping, resulting in a manyfold increase in laser intensity. This unconventional behavior is associated with the spatiotemporal frequency variation mediated by ionization and spatiotempral coupling. Our results illustrate a new regime of pulse propagation and open up an optics-less approach for raising laser intensity.

Multioctave supercontinuum generation and frequency conversion based on rotational nonlinearity.

The field of attosecond science was first enabled by nonlinear compression of intense laser pulses to a duration below two optical cycles. Twenty years later, creating such short pulses still requires state-of-the-art few-cycle laser amplifiers to most efficiently exploit "instantaneous" optical nonlinearities in noble gases for spectral broadening and parametric frequency conversion. Here, we show that nonlinear compression can be much more efficient when driven in molecular gases by pulses substantially longer than a few cycles because of enhanced optical nonlinearity associated with rotational alignment. We use 80-cycle pulses from an industrial-grade laser amplifier to simultaneously drive molecular alignment and supercontinuum generation in a gas-filled capillary, producing more than two octaves of coherent bandwidth and achieving >45-fold compression to a duration of 1.6 cycles. As the enhanced nonlinearity is linked to rotational motion, the dynamics can be exploited for long-wavelength frequency conversion and compressing picosecond lasers.

Long-wavelength-infrared laser filamentation in solids in the near-single-cycle regime.

We experimentally demonstrate long-wavelength-infrared (LWIR) femtosecond filamentation in solids. Systematic investigations of supercontinuum (SC) generation and self-compression of the LWIR pulses assisted by laser filamentation are performed in bulk KrS-5 and ZnSe, pumped by ${\sim}{145}\;{\rm fs}$∼145fs, 9 µm, 10 µJ pulses from an optical parametric chirped-pulse amplifier operating at 10 kHz of repetition rate. Multi-octave SC spectra are demonstrated in both materials. While forming stable single filament, 1.5 cycle LWIR pulses with 4.5 µJ output pulse energy are produced via soliton-like self-compression in a 5 mm thick KrS-5. The experimental results quantitatively agree well with the numerical simulation based on the unidirectional pulse propagation equation. This work shows the experimental feasibility of high-energy, near-single-cycle LWIR light bullet generation in solids.

Single-shot ultrafast visualization and measurement of laser-matter interactions in flexible glass using frequency domain holography.

We perform single-shot frequency domain holography to measure the ultrafast spatio-temporal phase change induced by the optical Kerr effect and plasma in flexible Corning Willow Glass during femtosecond laser-matter interactions. We measure the nonlinear index of refraction ($ {n_2} $n2) to be $(3.6 \pm 0.1) \times {10^{ - 16}}\;{{\rm cm}^2}/{\rm W} $(3.6±0.1)×10-16cm2/W and visualize the plasma formation and recombination on femtosecond time scales in a single shot. To compare with the experiment, we carry out numerical simulations by solving the nonlinear envelope equation.

Ionization-assisted refocusing of femtosecond Gaussian beams.

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 requires sufficiently high power. Here, we present simulations showing the refocusing of intense pulses with an initial Gaussian beam profile in atmospheric pressure gases at relatively low power. We attribute this refocusing to the formation of ring-structure plasmas. We find that tighter focusing leads to stronger refocusing, and the initial chirp of the pulse greatly affects its dynamics due to spatiotemporal coupling of focused broadband pulses. Our results highlight a novel aspect of complex pulse dynamics and can be relevant to applications involving tightly focused ultrafast Gaussian beams.

Brunel harmonics generated from ionizing clusters by few-cycle laser pulses.

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 ωM due to efficient excitation of Mie oscillations. Considering the appreciable single-cluster harmonic yield and the relaxed phase-matching condition in overdense clustered plasmas, clusters driven by few-cycle laser pulses can be a promising source of vacuum-ultraviolet radiation.

Impact-ionization mediated self-focusing of long-wavelength infrared pulses in gases.

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 critical power for Kerr self-focusing, and find that it can be controlled by the initial electron density and pulse duration.

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.

Inhibition of multi-filamentation of high-power laser beams.

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. Furthermore, the generated single filament exhibits spatial solitary wave behavior.

Nanoalloy Printed and Pulse-Laser Sintered Flexible Sensor Devices with Enhanced Stability and Materials Compatibility.

While conformal and wearable devices have become one of the most desired formats for printable electronics, it is challenging to establish a scalable process that produces stable conductive patterns but also uses substrates compatible with widely available wearable materials. Here, we describe findings of an investigation of a nanoalloy ink printed and pulsed-laser sintered conductive patterns as flexible functional devices with enhanced stability and materials compatibility. While nanoparticle inks are desired for printable electronics, almost all existing nanoparticle inks are based on single-metal component, which, as an electronic element, is limited by its inherent stabilities of the metal such as propensity of metal oxidation and mobility of metal ions, especially in sintering processes. The work here has demonstrated the first example in exploiting plasmonic coupling of nanoalloys and pulsed-laser energy with controllable thermal penetration. The experimental and theoretical results have revealed clear correlation between the pulsed laser parameters and the nanoalloy structural characteristics. The superior performance of the resulting flexible sensor device, upon imparting nanostructured sensing materials, for detecting volatile organic compounds has significant implications to developing stable and wearable sensors for monitoring environmental pollutants and breath biomarkers. This simple "nanoalloy printing-laser sintering-nanostructure printing" process is entirely general to many different sensor devices and nanostructured sensing materials, enabling the ability to easily construct sophisticated sensor array.

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.

Higher-order nonlinearities revisited and their effect on harmonic generation.

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 higher-order nonlinearities is sufficiently small that plasma formation should be a main defocusing mechanism in high power filamentation. We also explore cross-phase modulation via the optical Kerr effect, and find that the higher-order nonlinearities can significantly alter the phase matching of harmonic generation.

Ultraviolet surprise: Efficient soft x-ray high-harmonic generation in multiply ionized plasmas.

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, the emission from each species is highest when a short-wavelength ultraviolet driving laser is used. However, phase matching--the constructive addition of x-ray waves from a large number of atoms--favors longer-wavelength mid-infrared lasers. We identified a regime of high-harmonic generation driven by 40-cycle ultraviolet lasers in waveguides that can generate bright beams in the soft x-ray region of the spectrum, up to photon energies of 280 electron volts. Surprisingly, the high ultraviolet refractive indices of both neutral atoms and ions enabled effective phase matching, even in a multiply ionized plasma. We observed harmonics with very narrow linewidths, while calculations show that the x-rays emerge as nearly time-bandwidth-limited pulse trains of ~100 attoseconds.

Modelocking and femtosecond pulse generation in chip-based frequency combs.

We investigate simultaneously the temporal and optical and radio-frequency spectral properties of parametric frequency combs generated in silicon-nitride microresonators and observe that the system undergoes a transition to a mode-locked state. We demonstrate the generation of sub-200-fs pulses at a repetition rate of 99 GHz. Our calculations show that pulse generation in this system is consistent with soliton modelocking. Ultimately, such parametric devices offer the potential of producing ultra-short laser pulses from the visible to mid-infrared regime at repetition rates from GHz to THz.

Bright coherent ultrahigh harmonics in the keV x-ray regime from mid-infrared femtosecond lasers.

High-harmonic generation (HHG) traditionally combines ~100 near-infrared laser photons to generate bright, phase-matched, extreme ultraviolet beams when the emission from many atoms adds constructively. Here, we show that by guiding a mid-infrared femtosecond laser in a high-pressure gas, ultrahigh harmonics can be generated, up to orders greater than 5000, that emerge as a bright supercontinuum that spans the entire electromagnetic spectrum from the ultraviolet to more than 1.6 kilo-electron volts, allowing, in principle, the generation of pulses as short as 2.5 attoseconds. The multiatmosphere gas pressures required for bright, phase-matched emission also support laser beam self-confinement, further enhancing the x-ray yield. Finally, the x-ray beam exhibits high spatial coherence, even though at high gas density the recolliding electrons responsible for HHG encounter other atoms during the emission process.

Loss of phase of collapsing beams.

We experimentally investigate the phase of an optical field after it has undergone wave collapse. We confirm the theoretical prediction that it acquires a large cumulative nonlinear phase shift that is highly sensitive to small fluctuations of the laser input power. This results in an effective postcollapse "loss of phase," whereby the phase of the transmitted beam shows a significant increase in sensitivity to the input fluctuations of the pulse energy. We also investigate interactions between two beams that each undergoes collapse and observe large fluctuations in the output mode profiles, which are due to the postcollapse loss of their relative phase difference. Such effects should occur in all systems that exhibit wave collapse.

Filamentation in air with ultrashort mid-infrared pulses.

We theoretically investigate filamentation of ultrashort laser pulses in air in the mid-infrared regime under conditions in which the group-velocity dispersion (GVD) is anomalous. When a high-power, ultra-short mid-infrared laser beam centered at 3.1-µm forms a filament, a spatial solitary wave is stabilized by the plasma formation and propagates several times its diffraction length. Compared with temporal self-compression in gases due to plasma formation and pulse splitting in the normal-GVD regime, the minimum achievable pulse duration (∼70 fs) is limited by the bandwidth of the anomalous-GVD region in air. For the relatively high powers, multiple pulse splitting due to the plasma effect and shock formation is observed, which is similar to that which occurs in solids. Our simulations show that the energy reservoir also plays a critical role for longer propagation of the air filament in the anomalous-GVD regime.

Dynamics of elliptical beams in the anomalous group-velocity dispersion regime.

We investigate 3D spatio-temporal focusing of elliptically-shaped beams in a bulk medium with Kerr nonlinearity and anomalous group-velocity dispersion (GVD). Strong space-time localization of the mode is observed through multi-filamentation with temporal compression by a factor of 3. This behavior is in contrast to the near-zero GVD regime in which minimal pulse temporal compression is observed. Our theoretical simulations qualitatively reproduce the experimental results showing the highly localized spatio-temporal profile in the anomalous-GVD regime, which contrasts to the weakly localized pulse in the normal-GVD regime.