Nonlinear Optics and Dynamics of Relativistically Transparent Plasmas

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The fundamental properties of optics are well understood at moderate light intensities. However, at the highest intensities capable of being produced using state-of-the-art lasers, many new and useful optical phenomena arise. When a high intensity laser pulse is focused onto a medium it generates a plasma and can drive extreme temperatures and intense electric and magnetic fields. This results in the production of beams of high energy particles and radiation with unique properties, which are opening up new frontiers in science and new applications. The plasma electrons quiver in the intense laser field at velocities close to the speed of light, which changes fundamental properties of the plasma, such as its refractive index. The fact that the particle motion and nonlinear optical properties dynamically evolve in response to inter-action with the laser pulse means that the plasma can act as an active optical element. If harnessed, this would provide researchers with a tool to dynamically control both the properties of ultraintense laser light and the beams of charged particles and radiation produced.

Great progress has been made in controlling the collective response of electrons to intense laser pulses propagating in low density (transparent) plasma, resulting in the production of high energy, ultrashort bunches of electrons in a low divergence beam. The situation is more complex in the case of solid density plasma, used for example for ion acceleration and high harmonic generation. The dense plasma acts as a mirror (a plasma mirror), which reflects a significant portion of the laser beam. At ultrahigh laser intensities, however, the nonlinear motion of the plasma electrons results in relativistic optical phenomena which can render the dense plasma transparent.

Our proposed research focuses on exploring relativistic plasma optics in ultrathin foils. Such targets initially act as a plasma mirror, reflecting laser light, and evolve over the course of the interaction to become relativistically transparent. This transient behaviour offers a promising route to controlling charged particle acceleration in dense plasma. During the opaque phase of the interaction, strong longitudinal electrostatic fields are generated, resulting in forward-directed electron and ion beams, which can be controlled using relativistic optical effects induced as the laser propagates through the target during transparency. We will investigate this approach as a means of dynamically controlling fundamental properties of the transmitted intense laser light and the resulting high energy particles and radiation.

We will use the complementary capabilities of the new 350 TW laser at the Scottish Centre for the Applications of Plasma Accelerators, in which new techniques can be developed and optimised over time, and the Gemini and Vulcan lasers at the Central Laser Facility, which offer higher power and dual beam capability. We will also perform closely coupled simulations using high performance computers. This will allow us to investigate the potential for developing relativistic plasma optics processes for the dynamic control of the spatial, temporal and polarisation properties of ultraintense laser pulses. We will investigate the use of this approach for controlling the properties of beams of high energy particles and radiation produced in the interaction. Together with our international partners at the next-generation extreme light infrastructure laser facilities in the Czech Republic and Romania, we will also investigate the physics of relativistic optics and plasma dynamics at ultrahigh intensities, for which high field processes will modify the underpinning physics.

We will develop a clear understanding of ultrahigh intensity optical processes, their potential use in developing plasma optical and photonic devices and the dynamic control of particle and radiation production in dense plasma.
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The research outlined in this proposal is a programme of fundamental science with the potential to deliver significant impact on academia and industry in the short term, and society and the economy in the long term.

Plasma optics have the potential to circumvent many of the limitations of conventional solid state optics. They are, for example, much more compact due to their ability to sustain much higher amplitude electromagnetic fields. Importantly, the ultrafast evolution of their optical characteristics enables intra-pulse tailoring of specific properties of high intensity laser light, which opens up new scientific possibilities. The resulting ability to manipulate the properties of high power laser pulses is vital to the development of compact laser-driven high energy sources of particles and radiation. Our proposed project directly addresses this challenge. Its successful realisation could have a profound impact on the development of laser-driven sources in dense plasma. By extension, the research has strong potential for long term, broad societal impact via application of these sources.

Due to their compact nature and the unique properties of the resulting radiation beams (e.g. ultrashort pulse and low emittance), high power laser plasma sources are an enabling technology which, in due course, are likely to be applied in sectors such as healthcare (e.g. solutions to diagnosis and treatment of cancer), energy (e.g. in fusion research) and security (e.g. X-ray probes for penetrative imaging of hidden materials). In healthcare, for example, in addition to reducing the cost and footprint of particle therapy centres, leading to more widespread availability, a laser-driven approach to particle- and radio-therapy is predicted to offer clinical advantages (e.g. via mixed modality for optimised treatment). The development of this approach could contribute to enhancing patient survival rates and could drive inward investment in the health sector. Applications of laser-plasma-driven radiation beams within industry (e.g. for the detection of defects in manufacturing) could increase the competitiveness of UK manufacturing. Thus, the proposed fundamental laser-plasma physics research, which will contribute significantly to the development of high power laser-driven radiation sources in dense plasma (ions, extreme ultraviolet harmonics, terahertz and gamma-radiation), could have significant long-term societal impact.

In the short term, the project will benefit industry and the wider society via the training of potential employees (PDRAs and PhDs) with state-of-the-art experimental and theoretical/numerical expertise. We also plan to explore the potential for commercial exploitation of the plasma optics techniques and plasma photonic devices resulting from this research.

We will proactively engage with academic and non-academic beneficiaries to maximise the impact of the research. The results will be communicated in a variety of formats to relevant researchers within the laser-plasma and accelerator development communities. The uptake of the new nonlinear photonics techniques and mechanisms for controlling particle motion developed within this project forms part of the critical pathway from this research to economic and societal impact. We will also directly engage with potential end-users of the research, in a variety of ways, including two workshops to communicate the results and capture end-user input to the research programme.

We will engage in increasing the public understanding of science via a variety of public lectures and outreach events, along with web-based and social media dissemination. In particular, we aim to raise awareness of how new developments in physics can produce original approaches to addressing key societal challenges (e.g. in the area of healthcare technologies).
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