Photon Acceleration in a Flying Focus

Abstract

A high-intensity laser pulse propagating through a medium triggers an ionization front that can accelerate and frequency-upshift the photons of a second pulse. The maximum upshift is ultimately limited by the accelerated photons outpacing the ionization front or the ionizing pulse refracting from the plasma. Here we apply the flying focus-a moving focal point resulting from a chirped laser pulse focused by a chromatic lens-to overcome these limitations. Theory and simulations demonstrate that the ionization front produced by a flying focus can frequency-upshift an ultrashort optical pulse to the extreme ultraviolet over a centimeter of propagation. An analytic model of the upshift predicts that this scheme could be scaled to a novel table-top source of spatially coherent x-rays.A growing number of scientific fields rely critically on high intensity, high-repetition rate sources of extreme ultraviolet (XUV) radiation (wavelengths < 120 nm). These sources provide highresolution imaging for high energy density physics and nanotechnology [1,2], fine-scale material ablation for nanomachining, spectrometry, and photolithography [3][4][5], and ultrafast pump/probe techniques for fundamental studies in atomic and molecular physics [6][7][8]. While XUV sources have historically been challenging to produce, methods including nonlinear frequency mixing [9], high harmonic generation [10,11], and XUV lasing or line emission in metal-vapor and noble-gas plasmas [5,12] have demonstrated promising results. Despite their successes, each of these methods introduces tradeoffs in terms of tunability, spatial coherence, divergence, or efficiency [5,[9][10][11][12]. Photon acceleration offers an alternative method for tunable XUV production that could lessen or even eliminate these tradeoffs.Photon acceleration refers to the frequency upshift of light in response to a refractive index that decreases in time [13,14]. In analogy to charged particle acceleration, the increase in photon energy, i.e. frequency, accompanies an increase in group velocity. In the context of an electromagnetic pulse, the leading phase fronts experience a higher index than adjacent, trailing phase fronts, which manifests as a local phase velocity that increases over the duration of the pulse.The trailing phase front, on account of its higher phase velocity, gradually catches up with the

Dephasingless Laser Wakefield Acceleration

“…By stretching the region over which a laser pulse focuses and adjusting the relative timing at which those foci occur, the velocity of an intensity peak can be made to travel at any velocity, independent of the group velocity. This property has already been exploited in proof-of-principle simulations to improve Raman amplification, photon acceleration, and relativistic mirrors, and to generate Cherenkov radiation in a plasma [17][18][19][20][21][22][23] and in experiments to drive ionization waves at any velocity [24]. For LWFA, a spatiotemporally shaped laser pulse could drive a wakefield with a phase velocity equal to the speed of light in vacuum ( v p = c), eliminating dephasing and greatly reducing the accelerator length by allowing for operation at higher densities.…”

Laser frequency upconversion in plasmas with finite ionization rates

“…The strongest transmission and reflection can be obtained when the plasma is fully ionized in a time that is shorter than the pulse duration, which is evidenced by experiment data. The same rule for the maximum intensity also applies to frequency upconversion with a "flying focus" [23][24][25][26].…”

Reciprocating propagation of laser pulse intensity in free space

“…Optical pulse propagation, including velocity and direction, is a very basic characteristic for applications like optical information/communication, laser-matter interaction, and so on [1][2][3][4][5][6]. In linear physics, an optical pulse propagates along a straight-line trajectory at the velocity of c/n, where c is the speed of light in the vacuum and n is the refractive index of the medium.…”