Loss-free shaping of few-cycle terawatt laser pulses.

We demonstrate loss-free generation of 3 mJ, 1 kHz, few-cycle (5 fs at 750 nm central wavelength) double pulses with a pulse peak separation from 10 to 100 fs, using a helium-filled hollow core fiber (HCF) and chirped mirror compressor. Crucial to our scheme are simulation-based modifications to the spectral phase and amplitude of the oscillator seed pulse to eliminate the deleterious effects of self-focusing and nonlinear phase pickup in the chirped pulse amplifier. The shortest pulse separations are enabled by tunable nonlinear pulse splitting in the HCF compressor.

Atmospheric effects on the laser-driven avalanche-based remote detection of radiation.

The effect of realistic atmospheric conditions on mid-IR (λ = 3.9 µm) and long-wave-IR (λ = 10 µm) laser-induced avalanche breakdown for the remote detection of radioactive material is examined experimentally and with propagation simulations. Our short-range in-lab mid-IR laser experiments show a correlation between increasing turbulence level and a reduced number of breakdown sites associated with a reduction in the portion of the focal volume above the breakdown threshold. Simulations of propagation through turbulence are in excellent agreement with these measurements and provide code validation. We then simulate propagation through realistic atmospheric turbulence over a long range (0.1-1 km) in the long-wave-IR regime (λ = 10 µm). The avalanche threshold focal volume is found to be robust even in the presence of strong turbulence, only dropping by ∼50% over a propagation length of ∼0.6 km. We also experimentally assess the impact of aerosols on avalanche-based detection, finding that, while background counts increase, a useful signal is extractable even at aerosol concentrations 105 times greater than what is typically observed in atmospheric conditions. Our results show promise for the long-range detection of radioactive sources under realistic atmospheric conditions.

Phase front retrieval and correction of Bessel beams.

Bessel beams generated with non-ideal axicons are affected by aberrations. We introduce a method to retrieve the complex amplitude of a Bessel beam from intensity measurements alone, and then use this information to correct the wavefront and intensity profile using a deformable mirror.

Transient-grating single-shot supercontinuum spectral interferometry (TG-SSSI): publisher's note.

This publisher's note contains corrections to Opt. Lett.46, 1013 (2021)OPLEDP0146-959210.1364/OL.417803.

Transient-grating single-shot supercontinuum spectral interferometry (TG-SSSI).

We present a technique for the single-shot measurement of the spatiotemporal (1D space+time) amplitude and phase of an ultrashort laser pulse. The method, transient grating single-shot supercontinuum spectral interferometry (TG-SSSI), is demonstrated by the space-time imaging of short pulses carrying spatiotemporal optical vortices. TG-SSSI is well suited for characterizing ultrashort laser pulses that contain singularities associated with spin/orbital angular momentum or polarization.

Wake dynamics of air filaments generated by high-energy picosecond laser pulses at 1 kHz repetition rate.

We investigated the filamentation in air of 7 ps laser pulses of up to 200 mJ energy from a 1.03 µm-wavelength Yb:YAG laser at repetition rates up to f=1kHz. Interferograms of the wake generated show that while pulses in a train of repetition rate f=0.1kHz encounter a nearly unperturbed environment, at f=1kHz, a channel with an axial air density hole of ∼20% is generated and maintained at all times by the cumulative effect of preceding laser pulses. Measurements at f=1kHz show that the energy deposited decreases proportional to the air channel density depletion, becoming more pronounced as the repetition rate and pulse energy increase. Numerical simulations indicate that contrary to filaments generated by shorter duration pulses, the electron avalanche is the dominant energy loss mechanism during filamentation with 7 ps pulses. The results are of interest for the atmospheric propagation of joule-level picosecond pulses from Yb:YAG lasers, of which average powers now surpass 1 kW, and for channeling other directed energy beams.

Mode Structure and Orbital Angular Momentum of Spatiotemporal Optical Vortex Pulses.

We identify a class of modal solutions for spatiotemporal optical vortex (STOV) electromagnetic pulses propagating in dispersive media with orbital angular momentum (OAM) orthogonal to propagation. We find that symmetric STOVs in vacuum can carry half-integer intrinsic OAM; for general asymmetric STOVs in a dispersive medium, the OAM is quantized in integer multiples of a parameter that depends on the STOV symmetry and the group velocity dispersion. Our results suggest that STOVs propagating in dispersive media are accompanied by a polaritonlike quasiparticle. The modal theory is in excellent agreement with measurements of free space propagation of STOVs.

Dynamics of the femtosecond laser-triggered spark gap.

We present space and time resolved measurements of the air hydrodynamics induced by femtosecond laser pulse excitation of the air gap between two electrodes at high potential difference. We explore both plasma-based and plasma-free gap excitation. The former uses the plasma left in the wake of femtosecond filamentation, while the latter exploits air heating by multiple-pulse resonant excitation of quantum molecular wavepackets. We find that the cumulative electrode-driven air density depression channel plays the dominant role in the gap evolution leading to breakdown. Femtosecond laser heating serves mainly to initiate the depression channel; the presence of filament plasma only augments the early heating.

Nonlinearity and ionization in Xe: experiment-based calibration of a numerical model.

Recently proposed universality of the nonlinear response is put to the test and used to improve a previously designed model for xenon. Utilizing accurate measurements resolving the nonlinear polarization and ionization in time and space, we calibrate the scaling parameters of the model and demonstrate agreement with several experiments spanning the intensity range relevant for applications in nonlinear optics at near-infrared and mid-infrared wavelengths. Applications to other species including small molecules are discussed, suggesting a self-consistent way to calibrate light-matter interaction models.

Full path single-shot imaging of femtosecond pulse collapse in air turbulence.

In a single shot, we measure the full propagation path, including the evolution to pulse collapse, of a high power femtosecond laser pulse propagating in air. Our technique enables examination of the effect of parameters that fluctuate on a shot-to-shot basis, such as pulse energy, pulse duration, and air turbulence-induced refractive index perturbations. We find that even in lab air over relatively short propagation distances, turbulence plays a significant role in determining the location of pulse collapse.

Optical Guiding in Meter-Scale Plasma Waveguides.

We demonstrate a new highly tunable technique for generating meter-scale low density plasma waveguides. Such guides can enable laser-driven electron acceleration to tens of GeV in a single stage. Plasma waveguides are imprinted in hydrogen gas by optical field ionization induced by two time-separated Bessel beam pulses: The first pulse, a J_{0} beam, generates the core of the waveguide, while the delayed second pulse, here a J_{8} or J_{16} beam, generates the waveguide cladding, enabling wide control of the guide's density, depth, and mode confinement. We demonstrate guiding of intense laser pulses over hundreds of Rayleigh lengths with on-axis plasma densities as low as N_{e0}∼5×10^{16} cm^{-3}.

Self-Guiding of Long-Wave Infrared Laser Pulses Mediated by Avalanche Ionization.

Nonlinear self-guided propagation of intense long-wave infrared (LWIR) laser pulses is of significant recent interest, as it promises high power transmission without beam breakup and multifilamentation. Central to self-guiding is the mechanism for the arrest of self-focusing collapse. Here, we show that discrete avalanche sites centered on submicron aerosols can arrest self-focusing, providing a new mechanism for self-guided propagation of moderate intensity LWIR pulses in outdoor environments. Our conclusions are supported by simulations of LWIR pulse propagation using an effective index approach that incorporates the time-resolved plasma dynamics of discrete avalanche breakdown sites.

Absolute Measurement of Laser Ionization Yield in Atmospheric Pressure Range Gases over 14 Decades.

Strong-field ionization is central to intense laser-matter interactions. However, standard ionization measurements have been limited to extremely low density gas samples, ignoring potential high density effects. Here, we measure strong-field ionization in atmospheric pressure range air, N_{2}, and Ar over 14 decades of absolute yield, using mid-IR picosecond avalanche multiplication of single electrons. Our results are consistent with theoretical rates for isolated atoms and molecules and quantify the ubiquitous presence of ultralow concentration gas contaminants that can significantly affect laser-gas interactions.

Ultrashort infrared 2.5-11 µm pulses: spatiotemporal profiles and absolute nonlinear response of air constituents.

We measure the detailed spatiotemporal profiles of femtosecond laser pulses in the infrared wavelength range of λ=2.5-11 µm and the absolute nonlinear response of major air constituents (N2, O2, and Ar) over this range. The spatiotemporal measurements reveal wavelength-dependent pulse front tilt and temporal stretching in the infrared pulses.

Controlling femtosecond filament propagation using externally driven gas motion.

The thermal density depression (or "density hole") produced by a high-repetition-rate femtosecond filament in air acts as a negative lens, altering the propagation of the filament. We demonstrate the effects of externally driven gas motion on these density holes and the resulting filament steering, and we derive an expression for the gas velocity that maximizes the effect. At gas velocities more than ∼3 times this value, the density hole is displaced from the filament, and it no longer affects filament propagation. We demonstrate density hole displacement using an audio speaker-driven sound wave, leading to a controllable, repeatable deflection of the filament. Applications are discussed, including quasi-phase matching in gas-based nonlinear optics. To the best of our knowledge, this is the first demonstration of femtosecond filament propagation control through controlled motion of the nonlinear medium.

Energy deposition of single femtosecond filaments in the atmosphere: erratum.

In this erratum the funding section of Opt. Lett.41, 3908 (2016)OPLEDP0146-959210.1364/OL.41.003908 has been updated.

Laser wakefield acceleration with mid-IR laser pulses.

We report on, to the best of our knowledge, the first results of laser plasma wakefield acceleration driven by ultrashort mid-infrared (IR) laser pulses (λ=3.9 µm, 100 fs, 0.25 TW), which enable near- and above-critical density interactions with moderate-density gas jets. Relativistic electron acceleration up to ∼12 MeV occurs when the jet width exceeds the threshold scale length for relativistic self-focusing. We present scaling trends in the accelerated beam profiles, charge, and spectra, which are supported by particle-in-cell simulations and time-resolved images of the interaction. For similarly scaled conditions, we observe significant increases in the accelerated charge, compared to previous experiments with near-infrared (λ=800 nm) pulses.

MeV electron acceleration at 1 kHz with <10 mJ laser pulses: erratum.

In this erratum the funding section of Opt. Lett.42, 215 (2017)OPLEDP0146-959210.1364/OL.42.000215 has been updated.

Bound-Electron Nonlinearity Beyond the Ionization Threshold.

We present absolute space- and time-resolved measurements of the ultrafast laser-driven nonlinear polarizability in argon, krypton, xenon, nitrogen, and oxygen up to ionization fractions of a few percent. These measurements enable determination of the strongly nonperturbative bound-electron nonlinear polarizability well beyond the ionization threshold, where it is found to remain approximately quadratic in the laser field, a result normally expected at much lower intensities where perturbation theory applies.

MeV electron acceleration at 1 kHz with <10 mJ laser pulses.

We demonstrate laser-driven acceleration of electrons to MeV-scale energies at 1 kHz repetition rate using <10 mJ pulses focused on near-critical density He and H2 gas jets. Using the H2 gas jet, electron acceleration to ∼0.5 MeV in ∼10 fC bunches was observed with laser pulse energy as low as 1.3 mJ. Increasing the pulse energy to 10 mJ, we measure ∼1 pC charge bunches with >1 MeV energy for both He and H2 gas jets.