[show abstract][hide abstract] ABSTRACT: We present direct measurements of the gas acoustic dynamics following
interaction of spatial single- and multi-mode 50 fs, 800 nm pulses in air at 10
Hz and 1 kHz repetition rates. Results are in excellent agreement with
hydrodynamic simulations. Under no conditions for single filaments do we find
on-axis enhancement of gas density; this occurs only with multi-filaments. We
also investigate the propagation of probe beams in the gas density profile
induced by a single extended filament. We find that light trapping in the
expanding annular acoustic wave can create the impression of on-axis guiding in
a limited temporal window.
[show abstract][hide abstract] ABSTRACT: We investigate beam pointing dynamics in filamentation in gases driven by high repetition rate femtosecond laser pulses. Upon sudden exposure of a gas to a kilohertz train of filamenting pulses, successive filaments are steered from their original direction to a new stable direction whose equilibrium is determined by a balance among buoyant, viscous, and diffusive processes in the gas. The beam mode is preserved. Results are shown for Xe and air, but are broadly applicable to all configurations employing intense, high repetition rate femtosecond laser pulses in gases.
[show abstract][hide abstract] ABSTRACT: We demonstrate that femtosecond filaments can set up an extended and robust
thermal waveguide structure in air with a lifetime of several milliseconds,
making possible the very long range guiding and distant projection of high
energy laser pulses and high average power beams. As a proof of principle, we
demonstrate guiding of 110 mJ, 7 ns, 532 nm pulses with 90% throughput over ~15
Rayleigh lengths in a 70 cm long air waveguide generated by the long timescale
thermal relaxation of an array of femtosecond filaments. The guided pulse was
limited only by our available laser energy. In general, these waveguides should
be robust against the effects of thermal blooming of extremely high average
power laser beams.
[show abstract][hide abstract] ABSTRACT: The energy gain in laser wakefield acceleration (LWFA) is ultimately limited
by dephasing, occurring when accelerated electrons outrun the accelerating
phase of the wakefield. We apply quasi-phasematching, enabled by axially
modulated plasma channels, to overcome this limitation. By matching the
modulation period to the dephasing length, a relativistic electron can undergo
energy gain over several dephasing lengths. Theory and simulations are
presented showing that at a weakly relativistic laser intensity, ~10^17 W/cm2,
and millijoule level pulse energies, quasi-phasematched LWFA results in energy
gains of 50 MeV larger than standard LWFA.
[show abstract][hide abstract] ABSTRACT: We investigate high-power terahertz (THz) generation in two-color laser
filamentation using terawatt (TW) lasers including a 0.5 TW, 1 kHz
system, as well as 2 and 30 TW systems both operating at 10 Hz. With
these lasers, we study the macroscopic effect in filamentation that
governs THz output energy yields and radiation profiles in the far
field. We also characterize the radiation spectra at a broad range of
frequencies covering radio-micro-waves to infrared frequencies. In
particular, our 1 kHz THz source can provide high-energy (>1 μJ),
high average power (>1 mW), intense (>1 MV cm-1)
and broadband (0.01-60 THz) THz radiation via two-color
filamentation in air. Based on our scaling law, an ˜30 TW laser can
produce >0.1 mJ of THz radiation with multi-gigawatt peak power in
˜1.5 m long filamentation.
New Journal of Physics 07/2013; 15(7):5002-. · 4.06 Impact Factor
[show abstract][hide abstract] ABSTRACT: We examine the generation of axially modulated plasmas produced from cluster jets whose supersonic flow is intersected by thin wires. Such plasmas have application to modulated plasma waveguides. By appropriately limiting shock waves from the wires, plasma axial modulation periods can be as small as 70 μm, with plasma structures as narrow as 45 µm. The effect of shocks is eliminated with increased cluster size accompanied by a reduced monomer component of the flow.
[show abstract][hide abstract] ABSTRACT: Broadband terahertz generation via two-color femtosecond laser filamentation is studied with laser input energies up to 60 mJ. In the small f-number focusing regime, the output THz energy strongly saturates, which is attributed to ionization-induced laser defocusing in filamentation. This saturation effect can be minimized by elongating the plasma filament with weak focusing. A conversion efficiency of >10−4 is achieved in elongated filamentation.
[show abstract][hide abstract] ABSTRACT: Femtosecond laser pulses filamenting in various gases are shown to generate long- lived quasi-stationary cylindrical depressions or 'holes' in the gas density. For our experimental conditions, these holes range up to several hundred microns in diameter with gas density depressions up to ~20%. The holes decay by thermal diffusion on millisecond timescales. We show that high repetition rate filamentation and supercontinuum generation can be strongly affected by these holes, which should also affect all other experiments employing intense high repetition rate laser pulses interacting with gases.
[show abstract][hide abstract] ABSTRACT: Nonlinear optics experiments measuring phase shifts induced in a weak probe
pulse by a strong pump pulse must account for coherent effects that only occur
when the pump and probe pulses are temporally overlapped. It is well known that
a weak probe beam experiences a greater phase shift from a strong pump beam
than the pump beam induces on itself. The physical mechanism behind the
enhanced phase shift is diffraction of pump light into the probe direction by a
nonlinear refractive index grating produced by interference between the two
beams. For an instantaneous third-order response, the effect of the grating is
to simply double the probe phase shift, but when delayed nonlinearities are
considered, the effect is more complex. A comprehensive treatment is given for
both degenerate and nondegenerate pump-probe experiments in noble and diatomic
gases. Results of numerical calculations are compared to a recent transient
birefringence measurement [Loriot et al., Opt. Express 17, 13429 (2009)] and a
recent spectral interferometry experiment [Wahlstrand et al., Phys. Rev. A 85,
043820 (2012)]. We also present results from two new experiments using
spectrally-resolved transient birefringence with 800 nm pulses in Ar and air
and degenerate chirped pulse spectral interferometry in Ar. Both experiments
support the interpretation of the negative birefringence at high intensity as
arising from a plasma grating.
Physical Review A 02/2013; 87(5). · 3.04 Impact Factor
[show abstract][hide abstract] ABSTRACT: A laser pulse propagating in an axially corrugated plasma channel is
composed of spatial harmonics whose phase velocities can be subluminal.
The phase velocity of a spatial harmonic can be matched to the speed of
relativistic electrons, resulting in direct acceleration by the guided
laser field and linear energy gain over the interaction length. Here we
examine the fully self-consistent interaction of the laser pulse and
electron beam using particle-in-cell (PIC) simulations. For low electron
beam densities, we find that the ponderomotive force of the laser pulse
pushes plasma channel electrons toward the propagation axis, which can
deflect the beam electrons. When the beam density is high, the space
charge force of the beam drives the channel electrons off axis,
providing collimation of the beam. In addition, we consider a ramped
density profile for lowering the threshold energy for trapping by a
subluminal spatial harmonic. By using a density ramp, the trapping
energy for a normalized vector potential of a0 = 0.1 is reduced from a
relativistic factor γ0 = 170 to γ0 = 20.
[show abstract][hide abstract] ABSTRACT: We examine the generation of plasma waveguides generated from cluster
jets whose supersonic flow is intersected by a periodic array of wires.
We find that the minimum modulation period is dictated by shock waves
launched off the wires by residual monomer gas. This effect is reduced
with increased cluster size accompanied by a reduced monomer component
of the flow. By appropriately limiting shockwaves, the modulation period
can be less than 100 μm.
[show abstract][hide abstract] ABSTRACT: A sequence of femtosecond laser pulses propagating through atmosphere and delayed near the rotational recurrence period of N2 can resonantly drive molecular alignment. Through the polarization density, the molecular alignment provides an index of refraction contribution that acts as a lens copropagating with each laser pulse. Each pulse enhances this contribution to the index, modifying the propagation of subsequent pulses. Here we present propagation simulations of femtosecond pulse sequences in which we have implemented a self-consistent calculation of the rotational polarization density using linearized density matrix theory. We find that a femtosecond pulse sequence can enhance pulse compression or collimation in atmosphere. In particular, when the pulses are delayed by exactly the rotational recurrence period, each subsequent pulse is increasingly compressed due to a combination of spectral broadening and negative dispersion. Alternatively, when the intensity peak of each pulse is centered on the maximum index generated by the preceding pulses, each pulse is increasingly collimated.
Physical Review A 09/2012; 86(3). · 3.04 Impact Factor
[show abstract][hide abstract] ABSTRACT: The nonlinear optical response to high fields is absolutely measured for the noble gas atoms He, Ne, Ar, Kr, and Xe. We find that the response is quadratic in the laser field magnitude up to the ionization threshold of each gas. Its size and quadratic dependence are well predicted by a Kramers-Kronig analysis employing known ionization probabilities, and the results are consistent with calculations using the time-dependent Schrödinger equation.
[show abstract][hide abstract] ABSTRACT: Two-pulse excitation of a molecular quantum wake in air during filamentary propagation is shown to controllably shape an intense femtosecond probe pulse and to significantly extend the filament compared to single-pulse excitation. The effect is sensitive to pump-probe delay on a ∼10-fs time scale.
Physical Review A 08/2012; 86(2). · 3.04 Impact Factor
[show abstract][hide abstract] ABSTRACT: A laser pulse propagating in a corrugated plasma channel is composed of spatial harmonics whose phase velocities can be subluminal. The phase velocity of a spatial harmonic can be matched to the speed of a relativistic electron resulting in direct acceleration by the guided laser field in a plasma waveguide and linear energy gain over the interaction length. Here we examine the fully self-consistent interaction of the laser pulse and electron beam using particle-in-cell (PIC) simulations. For low electron beam densities, we find that the ponderomotive force of the laser pulse pushes plasma channel electrons towards the propagation axis, which deflects the beam electrons. When the beam density is high, the space charge force of the beam drives the channel electrons off axis, providing collimation of the beam. In addition, we consider a ramped density profile for lowering the threshold energy for trapping in a subluminal spatial harmonic. By using a density ramp, the trapping energy for a normalized vector potential of a0=0.1 is reduced from a relativistic factor γ0=170 to γ0=20.
Physical Review Special Topics - Accelerators and Beams 08/2012; 15(8). · 1.57 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report experimental confirmation of the ionization-grating-induced transient birefringence predicted by Wahlstrand and Milchberg [Opt. Lett. 36, 3822 (2011)] and discuss its impact on the higher-order Kerr effect interpretation by Loriot et al. of pump-probe transient birefringence measurements made at 800 nm [Opt. Express 17, 13429 (2009)]. Measurement of the transient birefringence in air at 400 nm shows a negative contribution to the index of refraction at zero delay for frequencies within the pump bandwidth, the degenerate case, and no negative contribution for frequencies exceeding the pump bandwidth, the nondegenerate case. Our findings suggest that a reevaluation of the higher-order Kerr effect hypothesis of Loriot et al. is necessary.
[show abstract][hide abstract] ABSTRACT: Graphene is an attractive material for use in optical detectors because it absorbs light from mid-infrared to ultraviolet wavelengths with nearly equal strength. Graphene is particularly well suited for bolometers-devices that detect temperature-induced changes in electrical conductivity caused by the absorption of light-because its small electron heat capacity and weak electron-phonon coupling lead to large light-induced changes in electron temperature. Here, we demonstrate a hot-electron bolometer made of bilayer graphene that is dual-gated to create a tunable bandgap and electron-temperature-dependent conductivity. The bolometer exhibits a noise-equivalent power (33 fW Hz(-1/2) at 5 K) that is several times lower, and intrinsic speed (>1 GHz at 10 K) three to five orders of magnitude higher than commercial silicon bolometers and superconducting transition-edge sensors at similar temperatures.
[show abstract][hide abstract] ABSTRACT: We discuss recent experiments and calculations of the high-intensity optical nonlinearity in gases. Spectral interferometry measurements of the nonlinear optical response of air constituents to laser intensities near the ionization threshold are performed. A calculation of the phase shift caused by a plasma grating created by interference between the pump and probe beams in a transient birefringence measurement suggests that experimental techniques measuring cross phase modulation of a probe pulse by a strong pump pulse are unreliable for studying the optical nonlinearity when the pump and probe pulses are of the same wavelength. An interferometric measurement of the electron density in a filament is also performed. The peak elec-tron density measured is consistent with a model that includes plasma defocusing, but not higher-order Kerr terms. These tech-niques promise to improve the quantitative understanding of nonlinear optics near the ionization threshold and filamentation.
[show abstract][hide abstract] ABSTRACT: A laser pulse propagating through the atmosphere self-focuses due to the nonlinear index of refraction modifications from the instantaneous electronic and delayed rotational responses of the air molecules. If the pulse power is sufficient, the focused pulse intensity can surpass the ionization threshold, resulting in a plasma filament. The balance between defocusing due to plasma refraction and focusing due to the instantaneous and delayed responses results in extended propagation at high intensities. Because the rotational response induced by the first pulse (the pump pulse) is periodic in time, owing to quantum-mechanical discreteness of the rotational eigenfrequencies of the molecules, a subsequent laser pulse (the probe pulse), delayed at the recurrence period, experiences a propagating wake of index modification left behind by the previous pulse. Here, we present propagation simulations based on a recent experiment [Varma et al. (unpublished)] showing that axial extension of the plasma filament and probe pulse shaping imposed by the molecular alignment wake are sensitive to probe delay changes of as little as 10 fs.
Physical Review A 04/2012; 85(4). · 3.04 Impact Factor