Howard Milchberg

University of Maryland, College Park | UMD, UMCP, University of Maryland College Park · Department of Physics

The distant projection of high-peak and average-power laser beams in the atmosphere is a long-standing goal with a wide range of applications. Our early proof-of-principle experiments [Phys. Rev. X 4, 011027 (2014)] presented one solution to this problem, employing the energy deposition of femtosecond filaments in air to sculpt millisecond-lifetime...

Structured waves are ubiquitous for all areas of wave physics, both classical and quantum, where the wavefields are inhomogeneous and cannot be approximated by a single plane wave. Even the interference of two plane waves, or a single inhomogeneous (evanescent) wave, provides a number of nontrivial phenomena and additional functionalities as compar...

We present the first demonstration of multi-GeV laser wakefield acceleration in a fully optically formed plasma waveguide, with an acceleration gradient as high as 25 GeV/m. The guide was formed via self-waveguiding of <15 J, 45 fs (<∼ 300 TW) pulses over 20 cm in a low-density hydrogen gas jet, with accelerated electron bunches driven up to 5 GeV...

The distant projection of high peak and average power laser beams in the atmosphere is a longstanding goal with a wide range of applications. Our early proof-of-principle experiments [Phys. Rev. X 4, 011027 (2014)] presented one solution to this problem, employing the energy deposition of femtosecond filaments in air to sculpt millisecond lifetime...

Atmospheric aerosols, such as water droplets in fog, interfere with laser propagation through scattering and absorption. Femtosecond optical filaments have been shown to clear foggy regions, improving the transmission of subsequent pulses. However, the detailed fog-clearing mechanism had yet to be determined. Here, we directly measure and simulate...

We present results from two new techniques for the generation of meter-scale, low density ([Formula: see text] on axis) plasma waveguides, the “two-Bessel” technique, and the “self-waveguiding” technique. Plasma waveguides of this density and length range are needed for demonstration of a ∼10 GeV laser wakefield accelerator module, key for future s...

Atmospheric aerosols, such as water droplets in fog, interfere with laser propagation through scattering and absorption. Femtosecond optical filaments have been shown to clear foggy regions, improving transmission of subsequent pulses. However, the detailed fog clearing mechanism had yet to be determined. Here we directly measure and simulate the d...

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.

Our use of a microphone array has enabled single-shot measurements of femtosecond filament longitudinal structure and energy deposition [ ¹ ]. Here we study the effect of pulse duration on filament length and axial structure.

Femtosecond filaments deposit energy into air, leading to localized density depressions. We employ this effect with a multi-filamenting LG 01 beam to generate long-lived air waveguides that guide high average power beams up to ~50m.

We demonstrate temporal and spectral measurements of the polarization state of supercontinuum generated from elliptically polarized pulses sent through a hollow core fiber differentially pumped with helium.

We present the first demonstration of multi-GeV laser wakefield acceleration in a fully optically formed plasma waveguide, with an acceleration gradient as high as 25 GeV/m. The guide was formed via self-waveguiding of <15 J, 45 fs (<~300 TW) pulses over 20 cm in a low density hydrogen gas jet, with accelerated electron bunches simultaneously drive...

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 i...

We demonstrate laser wakefield acceleration of quasimonoenergetic electron bunches up to 15 MeV at 1-kHz repetition rate with 2.5-pC charge per bunch and a core with <7−mrad beam divergence. Acceleration is driven by 5-fs, <2.7−mJ laser incident on a thin, near-critical-density hydrogen gas jet. Low beam divergence is attributed to reduced sensitiv...

We identify a class of modal solutions for spatio-temporal vortex (STOV) electromagnetic pulses propagating in dispersive media. We find that STOVs can carry half-integral units of intrinsic orbital angular momentum (OAM) in vacuum, with the OAM pointing orthogonal to propagation. Our results also suggest that STOVs propagating in dispersive media...

We experimentally demonstrate for the first time the second harmonic generation of spatiotemporal optical vortices (STOVs), conservation of STOV orbital angular momentum (OAM) and verify STOV photons can have OAM orthogonal to propagation.

We present experiments and simulations of high energy mid-infrared laser-matter applications, including near-critical density laser wakefield acceleration, detection of ultralow electron densities from radiation or strong-field ionization, and novel mechanisms for nonlinear infrared self-guiding.

We find modal solutions for spatio-temporal optical vortex (STOV) pulses propagating in dispersive media. We find that STOVS can support half integer orbital angular momentum and can excite a polariton-like quasiparticle.

Spatiotemporal optical vortices (STOVs) are a new type of optical orbital angular momentum (OAM) structure in which the OAM vector is orthogonal to the propagation direction [Optica 6, 1547, (2019)] and the optical phase circulates in space-time. Here, we experimentally and theoretically demonstrate the generation of the second harmonic of a STOV-c...

We present a technique for the single-shot measurement of the space- and time-resolved spatiotemporal 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 (STOVs...

We demonstrate that an ultrashort high intensity laser pulse can propagate for hundreds of Rayleigh ranges in a prepared neutral hydrogen channel by generating its own plasma waveguide as it propagates; the front of the pulse generates a waveguide that confines the rest of the pulse. A wide range of suitable initial index structures and gas densiti...

We demonstrate laser wakefield acceleration of quasi-monoenergetic electron bunches up to 15 MeV at 1 kHz repetition rate with 2.5 pC charge per bunch and a core with < 7 mrad beam divergence. Acceleration is driven by 5 fs, < 2.7 mJ laser pulses incident on a thin, near-critical density hydrogen gas jet. Low beam divergence is attributed to reduce...

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...

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 submicro...

Hydrodynamic optically-field-ionized (HOFI) plasma channels up to 100 mm long are investigated. Optical guiding is demonstrated of laser pulses with a peak input intensity of 6×1017 W cm−2 through 100 mm long plasma channels with on-axis densities measured interferometrically to be as low as ne0=(1.0±0.3)×1017 cm−3. Guiding is also observed at lowe...

We demonstrate that an ultrashort high intensity laser pulse can propagate for hundreds of Rayleigh ranges in a prepared neutral hydrogen channel by generating its own plasma waveguide as it propagates; the front of the pulse generates a waveguide that confines the rest of the pulse. A wide range of suitable initial index structures will support th...

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 J0 beam,...

Hydrodynamic optically-field-ionized (HOFI) plasma channels up to 100mm long are investigated. Optical guiding is demonstrated of laser pulses with a peak input intensity of $6\times10^{17}$W.cm$^{-2}$ through 100mm long plasma channels with on-axis densities measured interferometrically to be as low as $n_{e0} = (1.0\pm0.3)\times10^{17}$cm$^{-3}$....

We demonstrate a new highly tunable technique for generating meter-scale low density plasma waveguides. Such guides can enable 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 th...

We present space and time resolved measurements of the air hydrodynamics induced by ultrafast 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...

Nonlinear self-guided propagation of intense long-wave infrared (LWIR) laser pulses is of significant recent interest owing to the high critical power for self-focusing collapse at long wavelengths. This promises transmission of very high power in a single filament as opposed to beam breakup and multi-filamentation. Here, using the most current pic...

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. This technique enables single-shot 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...

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, N2, and Ar over 14 decades of absolute yield, using mid-IR pic...

Long wave infrared (LWIR) filamentation enables long range channeling of higher peak power beams before modulation and breakup. We present self-consistent modeling of the effect of avalanche ionization on LWIR filamentation, consistent with recent experiments.

We demonstrate acceleration of quasi mono-energetic electron bunches up to ~15 MeV at 1kHz repetition rate with > pC charge per bunch while using <3mJ laser pulse energy.

Using a helium cell to terminate filaments mid-flight we investigate the phase evolution of structured multi-filaments such as those used in creating air waveguides.

We observe the acceleration of electron bunches with a Lorentzian transverse profile using few-cycle laser pulses focused on the downstream side of a near-critical hydrogen gas jet.

We present results on the modal and quasi-modal linear propagation in a dispersive medium of ultrashort pulses carrying spatiotemporal optical vortices. We isolate contributions of wavefront tilt and dispersion to the optical angular momentum.

Spatiotemporal optical vortices (STOVs) are a new type of optical orbital angular momentum (OAM) residing in the space-time domain. Here we examine, through experiment and simulation, how STOV OAM is transformed in second-harmonic generation.

Spatiotemporal optical vortices (STOVs) are a new type of optical orbital angular momentum (OAM) residing in the space-time domain. Here we examine, through experiment and simulation, how STOV OAM is transformed in second-harmonic generation.

We demonstrate interferometric measurements of plasma formed by a laser pulse above the relativistic self-focusing power for near-critical density plasmas using a third harmonic probe.

We present research relevant to laser self-guiding in the long–wave infrared, including precise measurements of laser-driven ionization in atmosphere over 14 orders of magnitude and simulations of self-guiding mediated by avalanche ionization in aerosols.

We demonstrate a two-pulse Bessel beam scheme for generating plasma waveguides guiding high intensity laser pulses over 30 cm with on-axis plasma densities as low as 5 × 1016cm−3.

We present the first single-shot method for the measurement of the spatiotemporal phase and amplitude of ultrashort structured (e.g. spatiotemporal windings) and unstructured laser pulses by incorporating an interferometric reference into single-shot supercontinuum spectral interferometry.

We demonstrate acceleration of quasi mono-energetic electron bunches up to ~15 MeV at 1kHz repetition rate with > pC charge per bunch while using <3mJ laser pulse energy.

We report experimental results and simulations of multi-octave Brunel-like harmonic and THz coherent generation from the interaction of two-color mid-infrared laser fields in thin gas jets, preferentially enhancing the ionization-driven bound-free transition nonlinearity.

Through scattering and absorption, aerosols can interfere with long-distance laser propagation through the atmosphere. Here we directly measure the dynamics of aerosol particles acoustically cleared by an optical pulse.

Spatiotemporal optical vortices (STOVs) are a new type of optical orbital angular momentum (OAM) with optical phase circulation in space–time. In prior work [Phys. Rev. X.6, 031037 (2016)PRXHAE2160-330810.1103/PhysRevX.6.031037], we demonstrated that a STOV is a universal structure emerging from the arrest of self-focusing collapse leading to nonli...

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, N2 and Ar over 14 decades of absolute yield, using mid-IR pico...

We present the design and characterization of a thin, high density pulsed gas jet for use in the study of near critical laser plasma interactions with ultrashort Ti:sapphire laser pulses. The gas jet uses a range of capillary nozzles with inner diameters between 50 and 150 μm and is operated in the sonic regime. Cryogenic cooling of the gas valve b...

We present single-shot space- and time-resolved phase and amplitude measurements of ultrashort pulse spatio-temporal optical vortices (STOVs) using a new diagnostic, transient grating single-shot supercontinuum spectral interferometry (TG-SSSI). Our measurements and simulations demonstrate STOV mediation of space-time energy flow within the pulse.

We demonstrate that avalanche ionization breakdown of air with picosecond mid-infrared (mid-IR) laser pulses is an exceptionally sensitive and quantitative probe of extremely low concentrations of charged species. By exponentially increasing the electron density in the vicinity of a single seed atom or molecule to detectable levels, mid-IR electron...

An overview from the past and an outlook for the future of fundamental laser-plasma interactions research enabled by emerging laser systems.

We demonstrate optical guiding of high-intensity laser pulses in long, low-density hydrodynamic optical-field-ionized (HOFI) plasma channels. An axicon lens is used to generate HOFI plasma channels with on-axis electron densities as low as ne(0)=1.5×1017 cm-3 and matched spot sizes in the range 20 μ40 μm. Control of these channel parameters via adj...

There has been growing interest both in studying high intensity ultrafast laser plasma interactions with adaptive control systems as well as using long wavelength driver beams. We demonstrate the coherent control of the dynamics of laser-wakefield acceleration driven by ultrashort (∼ 100 fs) mid-infrared (∼ 3.9 μm) laser pulses. The critical densit...

Remote detection of a distant, shielded sample of radioactive material is an important goal, but it is made difficult by the finite spatial range of the decay products. Here, we present a proof-of-principle demonstration of a remote detection scheme using mid-infrared (mid-IR) (l = 3.9 mm) laser–induced avalanche breakdown of air. In the scheme’s m...

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.

We demonstrate optical guiding of high-intensity laser pulses in long, low density hydrodynamic optical-field-ionized (HOFI) plasma channels. An axicon lens is used to generate HOFI plasma channels with on-axis electron densities as low as $n_e(0) = 1.5\times 10^{17}\, \mathrm{cm}^{-3}$ and matched spot sizes in the range $ 20 \mu \mathrm{m} \lesss...

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 t...

We demonstrate the acceleration of quasi-mono-energetic electron bunches to 5 MeV at 1kHz repetition rate using few-cycle laser pulses focused on a near critical density hydrogen gas jet.

Avalanche breakdown with picosecond, mid-IR lasers allows the detection of single free electrons in gases with precise spatial location. Using this method, we measure radiationinduced charge densities (down to 103 cm−3) near a radioactive source.

Spatiotemporal optical vortices (STOVs), arise naturally during nonlinear self-focusing collapse arrest. Here, we use a 4-f pulse shaper to impose STOVs linearly on a Gaussian pulse and directly measure the vortex in spatiotemporal domains.

Spatiotemporal Optical Vortices (STOVs), optical orbital angular momentum structures arising naturally in collapsing laser pulses, are crucial to filament propagation. We characterize STOVs directly in the spatiotemporal domain.

Air turbulence leads to increased spread of filament collapse locations; this is measured by means of a synchronized array of microphones which capture the full energy deposition profile in a single shot.

We measure the detailed spatiotemporal profiles of femtosecond laser pulses in the infrared wavelength range {\lambda}=2.5-11{\mu}m, 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 puls...

The injection of electrons into a laser wakefield accelerator (LWFA) is observed to generate an intense coherent ultra-broadband and ultrashort pulse radiation flash, consistent with the acceleration of electrons from rest to nearly the speed of light in a distance <∼1μm. Under certain conditions, the radiation occurs at harmonics of the local plas...

The injection of electrons into a laser wakefield accelerator (LWFA) is observed to generate an intense coherent ultra-broadband and ultrashort pulse radiation flash, consistent with the acceleration of electrons from rest to nearly the speed of light in a distance < ~ 1 {\mu}m. The flash is sufficiently bright to induce large nonlinear refractive...

We present experiments and numerical simulations which demonstrate that fully-ionized, low-density plasma channels could be formed by hydrodynamic expansion of plasma columns produced by optical field ionization (OFI). Simulations of the hydrodynamic expansion of plasma columns formed in hydrogen by an axicon lens show the generation of \unit[200]{...

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

Although high field laser-induced ionization is a fundamental process underlying many applications, there have been no absolute measurements of the nonlinear polarizability of atoms and molecules in the presence of ionization. Such information is crucial, for example, for understanding the propagation of high intensity ultrashort pulses in matter....

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

DOI:https://doi.org/10.1103/PhysRevA.97.019904

We report on the first results of laser plasma wakefield acceleration driven by ultrashort mid-infrared laser pulses (\lambda= 3.9 \mu m, 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...