H. M. Milchberg

Loyola University Maryland, Baltimore, Maryland, United States

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Publications (232)479.13 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We demonstrate laser-plasma acceleration of high charge electron beams to the ~10 MeV scale using ultrashort laser pulses with as little energy as 10 mJ. This result is made possible by an extremely dense and thin hydrogen gas jet. Total charge up to ~0.5 nC is measured for energies >1 MeV. Acceleration is correlated to the presence of a relativistically self-focused laser filament accompanied by an intense coherent broadband light flash, associated with wavebreaking, which can radiate more than ~3% of the laser energy in a sub-femtosecond bandwidth consistent with half-cycle optical emission. Our results enable truly portable applications of laser-driven acceleration, such as low dose radiography, ultrafast probing of matter, and isotope production.
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    ABSTRACT: The axial dependence of femtosecond filamentation in air is measured under conditions of varying laser pulsewidth, energy, and focusing f-number. Filaments are characterized by the ultrafast z-dependent absorption of energy from the laser pulse and diagnosed by measuring the local single cycle acoustic wave generated. Results are compared to 2D + 1 simulations of pulse propagation, whose results are highly sensitive to the instantaneous (electronic) part of the nonlinear response of N2 and O2. We find that recent measurements of the nonlinear refractive index (n2) in Wahlstrand et al (2012 Phys. Rev. A 85 043820) provide the best match and an excellent fit between experiments and simulations.
    Journal of Physics B Atomic and Molecular Physics 04/2015; 48:094011. DOI:10.1088/0953-4075/48/9/094011
  • J. P. Palastro, T. M. Antonsen, H. M. Milchberg
    Physical Review A 12/2014; 90(6). DOI:10.1103/PhysRevA.90.069902 · 2.99 Impact Factor
  • N. Jhajj, J. K. Wahlstrand, H. M. Milchberg
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    ABSTRACT: Analysis is performed on propagation of light in long-lived optical waveguides in air generated by arrays of femtosecond filaments. Mode structure, leakage losses, and coupling efficiency are studied analytically and numerically as a function of wavelength and time delay after the waveguide-initiating filaments.
    Optics Letters 11/2014; 39(21). DOI:10.1364/OL.39.006312 · 3.18 Impact Factor
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    ABSTRACT: Under certain conditions, powerful ultrashort laser pulses can form greatly extended, propagating filaments of concentrated high intensity in gases, leaving behind a very long trail of plasma. Such filaments can be much longer than the longitudinal scale over which a laser beam typically diverges by diffraction, with possible applications ranging from laser-guided electrical discharges to high power laser propagation in the atmosphere. Understanding in detail the microscopic proc-esses leading to filamentation requires ultrafast measurements of the strong field nonlinear response of gas phase atoms and molecules, including absolute measurements of nonlinear laser-induced polarization and high field ionization. Such measurements enable the assessment of filamentation models and make possible the design of experiments pursuing applications. In this paper, we review filamentation in gases and some applications, and discuss results from diagnostics devel-oped at Maryland for ultrafast measurements of laser-gas interactions. V C 2014 AIP Publishing LLC.
    Physics of Plasmas 10/2014; 21(10):100901. DOI:10.1063/1.4896722] · 2.25 Impact Factor
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    D G Jang, Y S You, H M Milchberg, H Suk, K Y Kim
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    ABSTRACT: We present an all-optical method for measuring the average size, density, and mass fraction of large (>50 nm) clusters produced in cryogenically cooled, continuous gas jets. The technique combines three optical diagnostics—forward/backward Mie scattering detection, 90� scattering imaging, and neutral gas interferometry. Our measurement shows that cryogenic cooling and high backing gas pressure greatly enhance both cluster size and clustering ratio in continuous gas flow. Such a cluster source can be used as a potential target for intense, high-repetition-rate (>kHz) laser pulses.
    Applied Physics Letters 07/2014; 105(2):021906. DOI:10.1063/1.4890596 · 3.52 Impact Factor
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    ABSTRACT: We examine the interaction of relativistic laser pulses with plasma channels formed in a nitrogen cluster jet. We observe creation of nearly pure N5+ plasma channels and ionization injected wakefield beams with energies >100 MeV.
    CLEO: Applications and Technology; 06/2014
  • Jared K. Wahlstrand, Sina Zahedpour, Howard Milchberg
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    ABSTRACT: The plasma-induced phase shift of a probe is measured in 2D spatially with ~3 μm resolution and temporally with ~5 fs resolution. From this ionization rates for the noble gases are found for a 40 fs, 800 nm pulse, for intensities up to nearly full depletion of the neutral population.
    CLEO: QELS_Fundamental Science; 06/2014
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    ABSTRACT: Interferometric measurements of submicrosecond gas density dynamics following passage of an intense ultrashort pulse are presented for single- and multi-lobed beams at 10 Hz and 1 kHz. Results are in excellent agreement with hydrodynamic simulations.
    CLEO: Science and Innovations; 06/2014
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    ABSTRACT: Filaments in air, induced by pulse trains of four ultrashort pulses, each separated by the rotational revival time of nitrogen exhibit an increased degree of spectral broadening and collimation over the single pulse filament case.
    CLEO: QELS_Fundamental Science; 06/2014
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    ABSTRACT: Ionization injection-assisted laser wakefield acceleration of electrons up to 120 MeV is demonstrated in a 1.5 mm long pure helium-like nitrogen plasma waveguide. The guiding structure stabilizes the high energy electron beam pointing and reduces the beam divergence. Our results are confirmed by 3D particle-in-cell simulations.
    Applied Physics Letters 05/2014; 104(21):214105-214105-4. DOI:10.1063/1.4880102 · 3.52 Impact Factor
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    ABSTRACT: Collection of weak signals from remote locations is the primary goal and the primary hurdle of optical stand-off detection schemes. Typically, the measured signal is enhanced using large numerical aperture collection optics and high gain detectors. We show that the signal in remote detection techniques can be enhanced by using a long-lived air waveguide generated by an array of femtosecond filaments. We present a proof of principle experiment using an air plasma spark source and a ~1 m air waveguide showing an increase in collected signal of ~50%. For standoff distances of 100 m, this implies that the signal-to-noise ratio can be increased by a factor ~10$^4$.
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    ABSTRACT: We demonstrate that strong impulsive gas heating or heating suppression at standard temperature and pressure can occur from coherent rotational excitation or de-excitation of molecular gases using a sequence of non-ionizing laser pulses. For the case of excitation, subsequent collisional decoherence of the ensemble leads to gas heating significantly exceeding that from plasma absorption under the same laser focusing conditions. In both cases, the macroscopic hydrodynamics of the gas can be finely controlled with ~40 fs temporal sensitivity.
    Physical Review Letters 04/2014; 112(14):143601. DOI:10.1103/PhysRevLett.112.143601 · 7.73 Impact Factor
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    S. J. Yoon, J. P. Palastro, H. M. Milchberg
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    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.
    Physical Review Letters 04/2014; 112:134803. DOI:10.1103/PhysRevLett.112.134803 · 7.73 Impact Factor
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    ABSTRACT: Collection of weak signals from remote locations is the primary goal and the primary hurdle of optical stand-off detection schemes. Typically, the measured signal is enhanced using large numerical aperture collection optics and high gain detectors. We show that the signal in remote detection techniques can be enhanced by using a long-lived air waveguide generated by an array of femtosecond filaments. We present a proof of principle experiment using an air plasma spark source and a ~1 m air waveguide showing an increase in collected signal of ~50%. For standoff distances of 100 m, this implies that the signal-to-noise ratio can be increased by a factor ~10$^4$.
    Optica 03/2014; 1(1). DOI:10.1364/CLEO_SI.2014.STh5C.4
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    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.
    Optics Letters 03/2014; 39(5):1290-3. DOI:10.1364/OL.39.001290 · 3.18 Impact Factor
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    ABSTRACT: Plasma guiding structures are generated with programmable axial density modulations using a Spatial Light Modulator (SLM). A coherent beam-combining scheme enables the sculpting of high power beams with modest energy passing through the SLM.
    CLEO: Applications and Technology; 01/2014
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    ABSTRACT: This paper presents a new concept for the remote detection of radioactive materials. The concept is based on the detection of electromagnetic signatures in the vicinity of radioactive material and can enable stand-off detection at distances greater than 100 m. Radioactive materials emit gamma rays, which ionize the surrounding air. The ionized electrons rapidly attach to oxygen molecules forming O2- ions. The density of O2- around radioactive material can be several orders of magnitude greater than background levels. The elevated population of O2- extends several meters around the radioactive material. Electrons are easily photo-detached from O2- ions by laser radiation. The photo-detached electrons, in the presence of laser radiation, initiate avalanche ionization which results in a rapid increase in electron density. The rise in electron density induces a frequency modulation on a probe beam, which becomes a direct spectral signature for the presence of radioactive material.
    SPIENewsroom 12/2013; 21(1). DOI:10.1063/1.4861633
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    N Jhajj, Y-H Cheng, J K Wahlstrand, H M Milchberg
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    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.
    Optics Express 11/2013; 21(23):28980-6. DOI:10.1364/OE.21.028980 · 3.53 Impact Factor
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    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.

Publication Stats

4k Citations
479.13 Total Impact Points

Institutions

  • 1995–2014
    • Loyola University Maryland
      Baltimore, Maryland, United States
  • 1989–2014
    • University of Maryland, College Park
      • • Institute for Research in Electronics and Applied Physics (IREAP)
      • • Institute for Physical Science and Technology
      Maryland, United States
  • 2004
    • University of Texas at Austin
      Austin, Texas, United States
  • 1988
    • Lawrence Livermore National Laboratory
      Livermore, California, United States