High Power Dynamic Polarization Control Using Plasma Photonics

We report the first experimental demonstration of a plasma wave plate based on laser-induced birefringence. An elliptically polarized input was converted into a nearly ideal circularly polarized beam using an optical system composed of a second laser beam and a plasma. The results are in excellent agreement with linear theory and three-dimensional simulations up to phase delays exceeding $\pi /4$, thus establishing the feasibility of laser-plasma photonic devices that are ultrafast, damage-resistant, and easily tunable.

Figure 1

Part (a) shows the real and imaginary components of the refractive index modulation induced by the [$\text{pump}+\text{plasma}$] system. When ${\omega }_1={\omega }_0$, the imaginary component disappears (absence of energy transfer) but the real component is nonzero (induced birefringence). Part (b) illustrates the probe interacting with the plasma wave plate. An ideal quarter or half wave plate requires the probe polarization to have equal components parallel and perpendicular to the pump polarization.

Figure 2

Part (a) shows that the probe is initially elliptical with an orientation close to 45° and a phase delay of 38° between its horizontal and vertical components. (b) With optimized conditions, the laser-plasma wave plate induced an additional $\approx 52°$ delay such that the probe became nearly ideally circularly polarized. (c) This was verified on a subsequent shot by inserting a crystal quarter wave plate into the probe beam path to convert from circular back to linear polarization.

Figure 3

The measured phase delays are compared against the predicted phase delays from linear theory. The data agree quite well with the linear theory at low density and intensity but diverge at predicted phase delays $\mathrm{\Delta }\varphi \gtrsim 50°$. Simulations performed with pF3D indicate that this nonlinearity is due to transient effects when the system is strongly driven.

Figure 4

pF3D simulations of the experiment. (a) The simulation domain is 3.75 mm long, fully containing the $600\mu \mathrm{m}$ diameter pump (with a speckle pattern characteristic of phase plates) and $30\mu \mathrm{m}$ diameter probe [both shown in part (b)]), which overlap with a crossing angle of 27°. (c) The electron density modulation driven by the beat wave is superimposed on the bulk hydrodynamic motion driven by the pump’s speckle pattern, the latter of which is isolated in part (d) showing a region of plasma where the probe beam is not present. (e) Intense speckles prompt discrete jumps in the probe phase shift, which steadily accumulates as the probe interacts with the pump beam and levels off once the pump and seed have separated spatially within the plasma.