Refractive Index Seen by a Probe Beam Interacting with a Laser-Plasma System

We report the first complete set of measurements of a laser-plasma optical system’s refractive index, as seen by a second probe laser beam, as a function of the relative wavelength shift between the two laser beams. Both the imaginary and real refractive index components are found to be in good agreement with linear theory using plasma parameters measured by optical Thomson scattering and interferometry; the former is in contrast to previous work and has implications for crossed-beam energy transfer in indirect-drive inertial confinement fusion, and the latter is measured for the first time. The data include the first demonstration of a laser-plasma polarizer with $85\%–87\%$ extinction for the particular laser and plasma parameters used in this experiment, complementing the existing suite of high-power, tunable, and ultrafast plasma-based photonic devices.

Figure 1

The presence of the pump introduces anisotropy to the plasma as seen by a probe beam. Only the component of the probe’s polarization that is aligned with the pump polarization will have its amplitude and/or phase modified by the interaction, both of which can be measured using polarimetry.

Figure 2

The real (red) and imaginary (blue) refractive index perturbation components are plotted as a function of the wavelength shift between the pump and probe. Experimental measurements (points) are plotted along with the linear theory (solid lines) used to calculate CBET at the NIF. The parameters used in the linear theory calculation are listed in the text and are quite consistent with measured values (where available) and three-dimensional hydra simulations.

Figure 3

The anisotropic laser-plasma system acts like a pure linear polarizer at the negative resonance peak, depleting the probe’s horizontal polarization component. The color scale for each pair is normalized to the vertical polarization. The vertical and horizontal spots appear different because the Wollaston prism slightly affects the imaging. The preshot images are obtained without any plasma, and the horizontal polarization is brighter than the vertical polarization because the polarizer setting the incident polarization was not quite oriented at 45°; $85\%–87\%$ of it is then extinguished by the laser-plasma polarizer, whereas the vertical polarization is minimally perturbed by the system.