Cascaded chirped photon acceleration for efficient frequency conversion

Abstract

A cascaded sequence of photon acceleration stages using the instantaneous creation of a plasma density gradient by flash ionization allows the generation of coherent and chirped ultraviolet and x-ray pulses with independently tunable frequency and bandwidth. The efficiency of the cascaded process scales with 1/ω in energy, and multiple stages produce significant frequency up-conversion with gas-density plasmas. Chirping permits subsequent pulse compression to few-cycle durations, and output frequencies are not limited to integer harmonics.

Theory of electromagnetic wave frequency upconversion in dynamic media

“…The energy partitioning in different modes can be verified by the numerical simulation results reported in Ref. [6]. As the plasma frequency increases, the total energy density remains a constant W total = W 1+ + W 1− + W 2 = ε 0 E 2 i .…”

“…Frequency conversion is typically achieved using nonlinear optical methods, which often require high light intensities. As an alternative linear method, frequency conversion by temporally changing the medium refractive index [1][2][3][4][5] has attracted particular attention for its ability to create "tabletop" sources of coherent x rays out of near-infrared lasers at high efficiency and low cost [6,7]. A rapid change in refractive index has also been proposed [8,9] for observing the Unruh effect, which is crucial for testing fundamental quantum field theory.…”

Creating localized plasma waves by ionization of doped semiconductors

“…This method may allow the Raman pump to exceed the ionization limit and hence produce stronger reflection. However, if the plasma frequency continuously increases when the strong peak of the pump pulse arrives, there will be a frequency upshift [44][45][46][47] and the frequency mismatch may or may not be tolerable for Raman amplification.…”

Laser frequency upconversion in plasmas with finite ionization rates

“…If the medium is ionizing plasma, the method can also sustain high intensities above the ionization threshold limit. "Proof-of-principle" numerical simulations predicted [8] that this method may create "tabletop" sources of coherent x rays out of near-infrared lasers at high efficiency and low cost. However, the existing theories only describe either "flash ionization" [3,9], with which the plasma is fully ionized instantaneously, or "adiabatically gradual ionization" [9][10][11][12], with which the plasma is ionized at an infinitely slow rate.…”