Bremsstrahlung from the interaction of short laser pulses with dielectrics

An intense, short laser pulse incident on a transparent dielectric can excite electrons from the valence to the conduction band. As these electrons undergo scattering, both from phonons and ions, they emit bremsstrahlung. Here we present a theory of bremsstrahlung emission appropriate for the interaction of laser pulses with dielectrics. Simulations of the interaction, incorporating this theory, illustrate characteristics of the radiation (power, energy, and spectra) for arbitrary ratios of electron collision frequency to radiation frequency. The conversion efficiency of laser pulse energy into bremsstrahlung depends strongly on both the intensity and duration of the pulse, saturating at values of about ${10}^{-5}$. Depending on whether the intensity is above or below the damage threshold of the material, the emission can originate either from the surface or the bulk of the dielectric, respectively. The bremsstrahlung emission may provide a broadband light source for diagnostics.

Figure 1

Spectral fluence of the bremsstrahlung radiation created during the interaction of a laser pulse with $\mathrm{Si}{\mathrm{O}}_2$ near breakdown. The pulse intensity, duration, and wavelength are $I_L=6\times {10}^{17}\mathrm{W}\mathrm{c}{\mathrm{m}}^{-2}, T_{\mathrm{FWHM}}=60\mathrm{fs}$, and ${\lambda }_L=800\mathrm{nm}$, respectively. The dashed vertical line corresponds to the laser frequency.

Figure 2

Schematics of radiation transport calculations. (a) Conventional radiation transport treatment with a set of paths at various angles; (b) representative path used in the simulations.

Figure 3

Threshold intensity for versus laser pulse duration (solid line). Symbols denote the locations of the runs. Case 1: $I_L=2\times {10}^{17}\mathrm{W}\mathrm{c}{\mathrm{m}}^{-2}, T_{\mathrm{FWHM}}=60\mathrm{fs}$; Case 2: $I_L=2\times {10}^{18}\mathrm{W}\mathrm{c}{\mathrm{m}}^{-2}, T_{\mathrm{FWHM}}=60\mathrm{fs}$; Case 3: $I_L=2\times {10}^{17}\mathrm{W}\mathrm{c}{\mathrm{m}}^{-2}, T_{\mathrm{FWHM}}=1\mathrm{ps}$.

Figure 4

Calculated pulse intensity (top row), electron temperature (middle row), and electron density (bottom row) for case 1.

Figure 5

Radiation intensity (solid line) and laser pulse (dashed line) (a), radiation energy density (b), optical depth (c), and spectral fluence (d) for case 1. The dielectric length is 500 μm.

Figure 6

Calculated pulse intensity (top row), electron temperature (middle row), and electron density (bottom row) for case 2.

Figure 7

Radiation intensity (solid line) and laser pulse (dashed line) (a), radiation energy density (b), optical depth (c), and spectral fluence (d) for case 2. The dielectric length is 500 μm.

Figure 8

Calculated pulse intensity (top row), electron temperature (middle row), and electron density (bottom row) for case 3.

Figure 9

Radiation intensity (solid line) and laser pulse (dashed line) (a), radiation energy density (b), optical depth (c), and spectral fluence (d) for case 3. The dielectric length is 500 μm.

Figure 10

Conversion efficiency of laser pulse energy into bremsstrahlung radiation versus $I_L$ for durations $T_{\mathrm{FWHM}}=60\mathrm{fs}$ and $T_{\mathrm{FWHM}}=1\mathrm{ps}$. The arrows denote the breakdown threshold.