We review highlights of our recent contributions to understanding the propagation dynamics and transverse orbital angular momentum of optical pulses carrying spatiotemporal optical vortices (STOVs). STOVs, which were first observed as an emergent phenomenon in nonlinear self-focusing, were first linearly generated using a 4𝑓 pulse shaper and measured using transient-grating single-shot supercontinuum spectral interferometry (TG-SSSI). That STOV-based transverse orbital angular momentum (OAM) is carried at the single photon level was then confirmed in measurements of OAM conservation in second harmonic generation. Our recent theory for the electromagnetic mode structure and transverse OAM of STOV-carrying pulses in dispersive media predicts half-integer OAM and the existence of a transverse OAM-carrying quasiparticle: the bulk medium STOV polariton.
I will present recent results from 2 sets of laser plasma acceleration experiments spanning 4 orders of magnitude in plasma density. In near critical density hydrogen plasmas using 5 fs, < 3mJ laser pulses , we have demonstrated acceleration of few pC monoenergetic electron bunches up to 15 MeV at 1 kHz, at a record low beam divergence <10 mrad [1]. Mitigation of carrier envelope phase slip is key to this result. At the other extreme of plasma density, we have demonstrated 2 techniques [2,3] for generation of metre-scale low density plasma waveguides up to several hundred Rayleigh ranges in length, with recent preliminary results showing guiding of up to several hundred terawatts.
[1] Laser-accelerated, low divergence 15 MeV quasi-monoenergetic electron bunches at 1 kHz, F. Salehi, M. Le, L. Railing, and H. M. Milchberg, submitted for publication
[2] Optical Guiding in Meter-Scale Plasma Waveguides, B. Miao, L. Feder, J. E. Shrock, A. Goffin, and H. M. Milchberg, PHYSICAL REVIEW LETTERS 125, 074801 (2020)
[3] Self-waveguiding of relativistic laser pulses in neutral gas channels, L. Feder, B. Miao, J. E. Shrock, A. Goffin, and H. M. Milchberg, PHYSICAL REVIEW RESEARCH 2, 043173 (2020)
In a multi-GeV laser-driven plasma accelerator the driving laser pulse must remain focused as it propagates through tens of centimetres of plasma of density 1017 cm-3. This distance is orders of magnitude greater than the Rayleigh range, and hence the laser pulse must be guided with low losses. Since many applications of laser-plasma accelerators will require that the pulse repetition rate is in the kilohertz range, methods for guiding relativistically-intense laser pulses at high repetition rates must be developed.
We describe the development of hydrodynamic optical-field-ionized (HOFI) plasma channels and conditioned HOFI channels, which can meet all of these challenging requirements. We present experiments and numerical simulations that show that hydrodynamic expansion of optical-field-ionized plasma columns can generate channels at low plasma densities. We show that guiding a conditioning pulse in a HOFI channel leads to the formation of long, very low loss plasma channels via ionization of the collar of neutral gas which surrounds the HOFI channel.
We describe proof-of-principle experiments in which we generated conditioned HOFI (CHOFI) waveguides with axial electron densities of ne0 ≈ 1×1017 cm−3 and a matched spot size of approximately 30 μm. We present hydrodynamic and particle-in-cell simulations which demonstrate that meter-scale, low-loss CHOFI waveguides could be generated with a total laser pulse energy of about 1 J per meter of channel.
Several recently proposed methods for detecting radioactivity at range involve driving laser induced avalanche breakdown seeded by electrons or negative ions whose density are elevated in the vicinity of a radioactive source. Using a chirped, mid-IR laser, we drive breakdowns at 1 meter standoff distances and monitor the breakdown timing using the backscattered spectrum. In addition to the on/off radiation detection based on the increased probability of finding a seed electron in the focal volume, we also can determine the spatial distribution of these seed electrons in the focal volume through temporal information encoded in this backscatter spectrum. We demonstrate that the backscatter spectrum is a superior detection method relative to visible plasma fluorescence, total pump backscatter, or absolute backscatter timing in its ability to determine the relative radiation level. We discuss scaling to longer focal geometries inherent in remote sensing and possible limitations to the technique, supported by modeling
Laser-driven plasma accelerators operating in a quasi-linear regime require external guiding of the laser driver. We describe our work to develop hydrodynamic optical-field-ionized (HOFI) channels with properties which are well suited to all types of laser-driven plasma accelerator. In this approach a plasma channel is formed by hydrodynamic expansion of a plasma column formed by OFI with elliptically-polarized laser pulses; since the electron energies generated with OFI are independent of the gas density, channels can be formed with much lower axial densities than is possible with collisional heating. An attractive feature of HOFI channels is that they are free-standing, and hence they could operate at high-repetition rates for extended periods.
We present simulations which demonstrate the possibility of forming plasma channels 100s of millimetres long, with axial densities of order 10^{17} cm^{-3} and lowest-order modes of spot size of order 40 um. We also present recent experimental results which confirm the formation of HOFI channels with properties similar to those predicted by simulations.
High-resolution X-Ray spectromicroscopy methods were used for investigations of fs laser interaction with N2O cluster media. Elongated (up to 8.5 mm) femtosecond laser self-channeling in N2O cluster media under sufficiently low laser intensity (0.5-4x1017 W/cm2) was observed. Results are revealing a strong macroscopic effect on laser beams owing to their interaction with a gas of clusters. This has occurred at moderate pulse intensities, so the effect is unrelated to either relativistic self-focusing or ponderomotive filamentation. Enough homogeneous multicharged ions plasma with bulk electron temperature around 100 eV was observed along the plasma channel. The spectral lines shapes of the H- and He-like Oxygen ions demonstrated the presence of strong "blue wings", which are caused by Doppler-shifted lines radiation from the essential fraction of ions (~10-2 - 10-3) with energies 0.1 - 1.5 MeV. The slope of Doppler-shifted lines radiation is good approximated by ~300 KeV ion temperatures.
A series of experiments has been performed to investigate the interaction of intense laser pulses with cryogenic noble gas droplets. Understanding of the time scales for this interaction is important for optimization of extreme ultraviolet (EUV) sources for next-generation lithography that utilize laser-produced plasmas. The temporal character of the plasma formed by the irradiation of micron-sized argon and krypton droplets with intense 200-mJ, 100-ps laser pulses was investigated using a pump-probe scheme. The evolution of the droplet plasma was assessed by monitoring delay-dependent x-ray and EUV emission, and by imaging frequency-doubled probe light scattered from the interaction region. Depending on the spectral region of interest and the droplet characteristics, the effective plasma lifetime extends from a few hundred picoseconds to several nanoseconds. These results are explained in terms of the plasma expansion, excitation emission, and recombination emission time scales.
Theoretical analysis and preliminary experiment on ionization instability of intense laser pulses in ionizing plasmas are presented. The ionization instability is due to the dependence of the ionization rate on the laser intensity and scatters the laser energy off the original propagation direction.
A recently developed laser-produced plasma channel is shown to be a promising means to produce an efficient, compact soft x-ray laser. The channel provides a route for efficient high power laser pumping through optical waveguiding of the pump. The channel also acts as a waveguide for generated soft x-rays, since it has wavelength independent mode structure. Channel creation and guided laser pulses of moderate duration and energy can be highly effective in driving nonequilibrium behavior of these plasmas to generate substantial population inversions.
We present results from a 1-D plasma dynamics calculation, describing the evolution of strongly heated material in the vicinity of a solid-vacuum interface. We find that the radiation emitted by the hot material in the range hν > kTe, where Te is the initial peak plasma temperature, comes primarily from the region of the original step function interface. This emission is dominated by recombination radiation. The emitted radiation pulse is extremely short; the cooling at the interface is dominated by expansion. It is seen that thermal conduction minimally affects the radiation pulse intensity and duration.
In these proceedings, we report on a time-resolved investigation of the hydrodynamics of a laser-produced plasma. A sub-100ps pulse is focused into a chamber filled with xenon for various pulse energies and pressures. This pulse (the pump pulse) forms a plasma, which is probed by a second pulse (the probe pulse) with a variable delay of up to 2.5 ns. The gradients in the plasma density profile produce a lensing effect on the probe pulse. The beam transmitted through the plasma is viewed with a CCD camera. The diffraction pattern of the probe pulse can be seen by subtracting the image of the first pulse beam from the image produced by the two-pulse beam.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.