Optical shock-enhanced self-photon acceleration

P. Franke, D. Ramsey, T. T. Simpson, D. Turnbull, D. H. Froula, and J. P. Palastro

Phys. Rev. A 104, 043520 – Published 22 October 2021

Abstract

Photon accelerators can spectrally broaden laser pulses with high efficiency in moving electron density gradients driven in a rapidly ionizing plasma. When driven by a conventional laser pulse, the group velocity walk-off experienced by the accelerated photons and deterioration of the gradient from diffraction and plasma refraction limit the extent of spectral broadening. Here we show that a laser pulse with a shaped space-time and transverse intensity profile overcomes these limitations by creating a guiding density profile at a tunable velocity. Self-photon acceleration in this profile leads to dramatic spectral broadening and intensity steepening, forming an optical shock that further enhances the rate of spectral broadening. In this new regime, multi-octave spectra extending from 400 to 60 nm wavelengths, which support near-transform-limited $< 400$ as pulses, are generated over $< 100 μ m$ of interaction length.

Received 19 July 2021
Revised 9 September 2021
Accepted 7 October 2021

DOI:https://doi.org/10.1103/PhysRevA.104.043520

Physics Subject Headings (PhySH)

Laser-plasma interactions Nonlinear optics Photonics Radiation & particle generation in plasmas Strong electromagnetic field effects Ultrafast optics

Atomic, Molecular & OpticalPlasma Physics

Authors & Affiliations

P. Franke ^*, D. Ramsey, T. T. Simpson, D. Turnbull, D. H. Froula, and J. P. Palastro

University of Rochester, Laboratory for Laser Energetics, Rochester, New York 14623, USA

^*pfranke@ur.rochester.edu

Article Text (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 104, Iss. 4 — October 2021

Reuse & Permissions

Access Options

Article Available via CHORUS

Download Accepted Manuscript

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
(a) A Gaussian beam drives a (gray) radially convex ionization front at the group velocity of light over approximately a Rayleigh length. Photons diverge from the optical axis due to diffraction and plasma refraction. Frequency upshifted photons dephase from the ionization front due to group velocity walk-off, limiting the system to relatively small frequency shifts. (b) A SFF pulse drives a concave ionization front at a tunable focal velocity $v_{f} ≳ v_{g 0}$ over a distance much greater than a Rayleigh length. Photons are concentrated near the optical axis and stay in phase with the ionization front, resulting in many photons undergoing a large frequency shift. Transverse intensity profiles of a typical (c) Gaussian pulse and (d) SFF pulse used in two-dimensional (2D) simulations.
Reuse & Permissions
Figure 2
The outside pulse intensity profiles (color bar) at (a) 96, (b) 192, and (c) 288 fs are shown with the corresponding transverse intensity profile (black) at the maximum intensity in each frame. The inside pulse intensity profiles (color bar) at (d) 96, (e) 192, and (f) 288 fs are shown with the corresponding on-axis electron density (red) and intensity (black) profiles. The color scales were normalized to the peak intensity at each time. (g) The electron density driven by the full SFF pulse after propagating for 288 fs. A guiding channel has formed over $5 Z_{R}$ .
Reuse & Permissions
Figure 3
(a) Evolution of the power spectral density (PSD) on the optical axis with interaction length $z$ , where $ω_{0}$ is the frequency corresponding to the peak wavelength of the incident pulse, $λ_{0} = 400 nm$ . The black line indicates the position of the short-pass filter applied to all light passing through $z = 90 μ m$ to obtain the pulse shown in (c). (b) The inside pulse at the output of the accelerator ( $z = 90 μ m$ ) with no filtering. (c) The inside pulse at $z = 90 μ m$ after filtering out all wavelengths $> 124$ nm.
Reuse & Permissions
Figure 4
Spectrograms obtained by wavelet transform of the inside pulse electric field on the optical axis for a SFF pulse at (a) the source input plane, (b) $15 μ m$ , (c) $45 μ m$ , and (d) $90 μ m$ . For comparison, the spectrograms at higher density ( $n_{0} = 6.5 \times 10^{20} {cm}^{- 3}$ ) are shown for (e) a SFF pulse with $v_{f} = 1.030 v_{g 0} = 0.8740 c$ and (f) a standard Gaussian pulse. The color scale in each plot is normalized to the peak intensity. On-axis intensity profiles (black) are normalized to $10^{17} W {cm}^{- 2}$ .
Reuse & Permissions
Figure 5
Parameter scaling of focal velocity $v_{f}$ and initial density $n_{0}$ for a $90 μ m$ accelerator. $η$ is the energy efficiency to produce all light at frequencies above the frequency indicated on the horizontal axis. For all data shown, $I_{in} = 4.0 \times 10^{16} W {cm}^{- 2}$ and $I_{out} = 4.8 \times 10^{16} W {cm}^{- 2}$ .
Reuse & Permissions

Physical Review A

covering atomic, molecular, and optical physics and quantum information