Self-Referencing Spectral Interferometric Probing of the Onset Time of Relativistic Transparency in Intense Laser-Foil Interactions

S.D.R. Williamson, R. Wilson, M. King, M. Duff, B. Gonzalez-Izquierdo, Z.E. Davidson, A. Higginson, N. Booth, S. Hawkes, D. Neely, R.J. Gray, and P. McKenna

Phys. Rev. Applied 14, 034018 – Published 8 September 2020

Abstract

Irradiation of an ultrathin foil target by a high-intensity laser pulse drives collective electron motion and the generation of strong electrostatic fields, resulting in ultrabright sources of high-order harmonics and energetic ions. The ion energies can be significantly enhanced if the foil undergoes relativistic self-induced transparency during the interaction, with the degree of enhancement depending in part on the onset time of transparency. We report on a simple and effective approach to diagnose the time during the interaction at which the foil becomes transparent to the laser light, providing a route to optically controlling and optimizing ion acceleration and radiation generation. The scheme involves a self-referencing approach to spectral interferometry, in which coherent transition radiation produced at the foil rear interferes with laser light transmitted through the foil. The relative timing of the onset of transmission with respect to the transition-radiation generation is determined from spectral fringe spacing and compared to simultaneous frequency-resolved optical-gating measurements. The results are in excellent agreement, and are discussed with reference to particle-in-cell simulations of the interaction physics and an analytical model for the onset time of transparency in ultrathin foils.

Received 11 December 2019
Revised 4 August 2020
Accepted 10 August 2020

DOI:https://doi.org/10.1103/PhysRevApplied.14.034018

Physics Subject Headings (PhySH)

Electromagnetically induced transparency Laser driven ion acceleration Laser-plasma interactions Light propagation, transmission & absorption Relativistic & quantum electrodynamic effects in atoms, molecules,& ions

Laser ablation Optical interferometry Optical spectroscopy

Plasma Physics

Authors & Affiliations

S.D.R. Williamson¹, R. Wilson¹, M. King¹, M. Duff¹, B. Gonzalez-Izquierdo¹, Z.E. Davidson¹, A. Higginson¹, N. Booth², S. Hawkes², D. Neely^2,1, R.J. Gray¹, and P. McKenna ^1,*

¹SUPA Department of Physics, University of Strathclyde, Glasgow G4 0NG, United Kingdom
²Central Laser Facility, STFC Rutherford Appleton Laboratory, Oxfordshire OX11 0QX, United Kingdom

^*paul.mckenna@strath.ac.uk

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Vol. 14, Iss. 3 — September 2020

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Images

Figure 1
(a) Schematic of the experiment showing the laser-focusing geometry and diagnostic channel. The drive laser pulse is focused using off-axis parabola OAP 1 and the light collected at the target rear, using OAP 2, is directed for simultaneous measurement using a GRENOUILLE and an optical spectrometer. (b) Schematic illustrating the timing of the CTR generation and transmitted part of the laser pulse, where the temporal separation ( $Δ t$ ) relates to the onset time of RSIT relative to the peak of the laser pulse interacting and fast electron generation. (c,d) Schematic of the laser-plasma interaction before, (c), and after, (d), RSIT, resulting in a generated pulse of CTR light and the transmitted laser pulse, with a temporal delay defined by the onset time for RSIT.
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Figure 2
FROG measurements of the transmitted and/or generated light for (a) no target, (b) $d = 37$ nm, (c) $d = 30$ nm; all for $τ_{L} = 40$ fs. (d) Temporal-intensity profile of light measured at the target rear side extracted from example GRENOUILLE measurements for $d = 30$ nm, for three given durations of the drive laser pulse. (e) Corresponding results from three-dimensional (3D) epoch PIC simulations, including a plot of the total number of electrons (with energy $> 1$ MeV) as a function of time, for the 80-fs pulse-duration case.
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Figure 3
Measured light level at the target rear as a percentage of the incident laser-pulse energy as a function of (a) target thickness, for $τ_{L} = 40$ fs, and (b) pulse duration, for $d = 30 nm$ . The gray band corresponds to calculations of the upper and lower bounds using the model reported in Yan et al. [31], with the limits defined by the uncertainty in intensity in the experiment in (a) and the uncertainties in intensity and target thickness in (b).
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Figure 4
(a) Temporal-intensity profile extracted from GRENOUILLE measurements for three given target thicknesses, for fixed drive laser pulse duration $τ_{L} = 40$ fs. (b) Representative measurements of the input laser spectrum and the generated CTR spectrum. (c)–(e) Spectral measurements for (c) $d = 28$ nm, (d) $d = 35$ nm, and (e) $d = 37$ nm targets.
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Figure 5
(a) Model spectrum for the superposition of two identical pulses with $τ_{L} = 40$ fs and $Δ t = 150$ fs, illustrating spectral fringes. (b) Spectral fringe pattern as a function of $Δ t$ , from $- 200$ to 200 fs. (c) Calculated delay from a Fourier transform of the spectrum, as a function of the input delay. (d) Delay determined from a Fourier transform of the measured spectra in the experiment, as a function of target thickness (red points) and the points measured using the GRENOUILLE (for comparison), for $τ_{L} = 40$ fs. Calculated time for the target to become transparent to the laser using the plasma-expansion model reported in Yan et al. [31] (blue).
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