Large laser facilities such as the National Ignition Facility (NIF) are typically limited in performance and physical scale (and thus cost) by optics damage. In this project, we investigated a radically new way to manipulate light at extreme powers and energies, where "traditional" (crystal-based) optical elements are replaced by a medium that is already "broken" and thus does not suffer from optics damage: a plasma. Our method consisted in applying multiple lasers to plasmas to imprint refractive micro-structures with optical properties designed to be similar to those of crystals or dielectric structures used in optics. In particular, we focused our efforts on two elements used to manipulate the polarization of lasers (i.e. the orientation of the light’s electric field vector): (1) a polarizer, which only lets a given polarization direction pass while blocking all others, and (2) a Pockels cell, which can, effectively, rotate the polarization direction or convert it from linear to elliptical or circular. These two elements are essential building blocks in almost all laser systems. For example, they can be combined to design optical gates. Here, we introduced the new concepts of a plasma polarizer and a plasma Pockels cell. Both concepts were demonstrated in proof-of-principle laboratory experiments during this project. We also demonstrated that such laser-plasma systems could be used to provide full control of the refractive index of plasmas as well as their dispersion (variation of the index versus the light wavelength), which constituted the basis for an ongoing experiment aimed at demonstrating the feasibility of slow light in plasmas, i.e. the capability to slow down a light pulse almost to a full stop.
Optics damage is the main limiting factor currently determining the physical scale and cost of large laser facilities. Indeed, when the laser power becomes high enough to damage the optical elements (lenses, mirrors, etc.), beams need to be enlarged in space in order to reduce their fluence (energy per unit surface). For example, on the NIF, reaching the 2 MJ of laser energy presumably required to achieve thermonuclear ignition necessitated increasing the beam size to approximately 30 m2 (192 beams of 40 x 40 cm each), in order to stay below the optics damage threshold of ~7 J/cm2 (since [2 MJ]/[7 J/cm2] = ~ 30 m2).
Our goal was to investigate the potential of plasmas to manipulate intense lasers at fluences many orders of magnitude beyond what traditional optics could endure. Plasmas are traditionally known as an optically isotropic medium (in the absence of an external magnetic field), due to their intrinsically disorganized nature. (They are sometimes described as ionized gases). However, by applying intense lasers to plasmas, it is possible to imprint modulations in the electron density at the wavelength-scale via the ponderomotive force, which pushes charged particles away from regions of high intensity. These modulations can act in the same way as the periodic refractive structures naturally found in some crystals or produced in periodic dielectric coatings, such as those used in modern optical elements (see figure below).
This project was instigated by the observations made while taking measurements during inertial confinement fusion (ICF) experiments. Specifically, researchers noticed that overlapping multiple lasers in plasma was affecting their polarizations (i.e. the orientation of the electric field carried by the light waves). A theoretical investigation of this process (Michel 2014) led to the realization that laser-plasma systems could be used to modify the polarization of a light wave propagating through them. This led us to propose the new concepts of a plasma polarizer and a plasma Pockels cell. We also realized that plasmas may be able to accomplish slow light, the ability to slow down a light pulse almost to a full stop by using an optical medium or manipulating the optical properties of plasmas.
Early in this project, an opportunity presented itself for us to run experimental campaigns at the Jupiter Laser Facility at Lawrence Livermore National Laboratory. While the initial goal of this project was to pursue theoretical studies to investigate new plasma optics concepts, we decided to seize the opportunity to actually test these ideas in experiments and shifted our focus towards a more experimental effort. During this project, we were able to experimentally test our concepts of the Pockels cell and the plasma polarizer. The Pockels cell experiments validated our theoretical prediction concerning the possibility of making a plasma birefringent (i.e. optically anisotropic) by using lasers and provided the first proof-of-principle demonstration of a plasma-based Pockels cell. Later experiments not only demonstrated the plasma polarizer concept, but also enabled us to map out the refractive index of our plasma-optics system. These results led to ongoing experiments attempting to achieve slow light using plasmas.
Using plasmas to manipulate light has an enormous potential for raising the fluence tolerable by such optical elements. Indeed, the physical mechanisms that constitute damage (thermal effects leading to melting and re-solidification or even ionization for the most intense laser pulses) do not exist in the plasma state, where matter is already ionized. The only limit remaining in terms of fluence is the development of non-linearities in plasma waves, which can complicate the control and stability of optical structures; still, these processes should not be a concern until one reaches fluences on the order of millions of Joules per square centimeter (i.e. at least six orders of magnitude beyond what the current technology allows). This research has the potential to lead to transformational technologies that could enable us to achieve unprecedented laser intensities, thus opening a new era of fundamental and applied laser science.
Our research, while radically novel and promising, is still at the stage of proof-of-principle demonstrations and relatively far from a potential transfer to industry or implementation in actual laser systems (5–10 years). In particular, one obstacle is the fact that our experiments required more laser energy to condition the plasma and create the optical structure in it than the energy of the probe beam that was being manipulated by these structures. An important step towards the real application of these concepts will be to demonstrate a favorable energy budget. Follow-up studies should also focus on the optical elements that are most critically prone to optics damage in laser systems, as well as the development of plasma-based concepts to alleviate the damage problem for these particular elements (e.g. the last grating in a laser pulse compressor system).
Michel, P., et al. 2014. "Dynamic Control of the Polarization of Intense Laser Beams via Optical Wave Mixing in Plasmas." Phys. Rev. Lett. 113, 205001.
Turnbull, D., et al. 2016. "High Power Dynamic Polarization Control Using Plasma Photonics." Phys. Rev. Lett. 114, 125001.
Turnbull, D., et al. 2017. "Refractive Index Seen by a Probe Beam Interacting with a Laser-Plasma System," Phys. Rev. Lett. 118, 015001.
Michel, P. 2016. "Multi-Beams Laser-Plasma Interactions: From ICF to 'Plasma Photonics' Applications." Invited seminar. Bordeaux University, France (and other locations) 2016. LLNL-PRES-690151.
Shrauth S. 2016. "Experimental Investigation of Self-Diffraction from Laser Generated Plasma Gratings." Anomalous Absorption Conference, Old Saybrook, CT, May 2016. LLNL-PRES-690579.
Turnbull, D., et al. 2016. "High Power Dynamic Polarization Control Using Plasma Photonics." Phys. Rev. Lett. 114, 125001. LLNL-CONF-683478.