Slow and Fast Light in Plasma Using Optical Wave Mixing

Slow and fast light, or large changes in the group velocity of light, have been observed in a range of optical media, but the fine optical control necessary to induce an observable effect has not been achieved in a plasma. Here, we describe how the ion-acoustic response in a fully ionized plasma can produce large and measurable changes in the group velocity of light. We show the first experimental demonstration of slow and fast light in a plasma, measuring group velocities between $0.12c$ and $-0.34c$.

Figure 1

Group velocity modification in plasma. (a) Schematic of the wave-mixing arrangement, with the inset graph showing ion-wave response. (b) The real and imaginary components of the nonlinear refractive index as a function of the frequency separation between pump and probe: $\mathrm{\Delta }\omega ={\omega }_1-{\omega }_0$. (c) The corresponding group index and group velocity.

Figure 2

Experiment schematic. The probe and pump beams interact in a plasma formed by the pump. (a) Thomson scattering of the pump beam used to measure the density and temperature. (b) CCD measures the probe focal spot and spatial distribution of gain. (c) The streak camera gives relative timing between probe polarization components and gain. (d) The incident probe is linearly polarized with $H$ and $V$ components. (e) Plasma introduces delay in the probe $H$ component. (f) A Wollaston prism separates the polarization components. (g) The pump beam is polarized in the horizontal plane.

Figure 3

Time-resolved probe envelope. (a) Streak camera traces of the incident probe horizontal and vertical polarization components, measured without plasma or pump. (b) Corresponding traces for both components after transmission through a pump-plasma system with probe detuning near the gain resonance leading to slowing of the horizontal component. A Gaussian curve is fit to all traces.

Figure 4

Experimental measurement of slow and fast light compared to theory. (a) The measured gain (blue circles) compared to linear theory calculations with (blue) and without (black) compensation for the finite $f$ number of the pump ($f/6.7$) and probe ($f/10$) for both the slow light ($\mathrm{\Delta }\lambda <0$) and fast light ($\mathrm{\Delta }\lambda <0$) sides. (b) The measured pulse delay compared to linear theory calculations with (red) and without (black) compensation for the finite $f$ number. For both plots, the theory curves are calculated for $n_e=1.25\times {10}^{19}{\mathrm{cm}}^{-3}$, $T_e=256\mathrm{eV}$, $T_i=45\mathrm{eV}$, and $I_0=3.25\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$.