Radiation generation via a laser-driven oscillating plasma aperture

控制高功率激光脉冲的时空和偏振特性是发展紧凑型激光等离子体粒子加速器和辐射源的核心，它们具有广泛的潜在应用。等离子体可以承受极高的能量密度，其光学性质可以在高功率激光脉冲的超快时间尺度上变化，从而实现对强光的动态操纵。该项目以我们的工作计划为基础，研究强激光-等离子体相互作用中的相对论光子现象，特别是相对论等离子体孔径中的激光衍射和谐波产生。鉴于我们之前用线偏振光在光圈中的固定点产生高能电子束的实验，我们现在建议研究使用圆偏振来驱动围绕等离子体孔边缘的等离子体振荡，以产生基频的高次谐波。理论上预测，当驱动激光被圆偏振时，高次谐波的产生过程有助于将基光的自旋角动量转换为谐波的本征轨道角动量。对这一点的实验验证将打开一个新的领域，可以产生强烈的极端紫外线涡旋，以及在相对论强度下基于衍射的高次谐波产生。这一新的、使粒子和辐射源技术能够发展的核心是控制等离子体中辐射产生的机制的发展。该项目将探索通过振荡等离子体孔径机制产生高次谐波辐射和光学涡旋的物理学。目标是：1.研究线偏振强激光脉冲与超薄金属箔相互作用产生的谐波的频谱；2.研究从线偏振到圆偏振时产生的谐波频谱的变化。研究产生的谐波的光谱和空间强度分布可以在多大程度上通过改变激光偏振中的椭圆度来控制。

Control of the spatial-temporal and polarisation properties of high power laser pulses are central to the development of compact laser-plasma-based particle accelerators and radiation sources, which have wide-ranging potential applications. Plasma can withstand extremely high energy densities and its optical properties can be varied on the ultra fast timescale of a high power laser pulse, enabling dynamic manipulation of intense light. The project builds upon our programme of work investigating relativistic photonic phenomena in intense laser-plasma interactions and specifically laser light diffraction and harmonic generation in a relativistic plasma aperture. Whereas our previous experiments with linearly polarised light generated bunches of high energy electrons at fixed points in the aperture, we now propose to investigate the use of circular polarisation to drive plasma oscillations around the rim of the plasma aperture, to generate high harmonics of the fundamental plasma frequency. It is predicted theoretically that when the driving laser is circularly polarised, the high-harmonic generation process facilitates a conversion of the spin angular momentum of the fundamental light into the intrinsic orbital angular momentum of the harmonics. An experimental verification of this will open up a new realm of possibilities for producing intense extreme ultraviolet vortices, and diffraction-based high harmonic generation at relativistic intensities.Central to the development of this novel and enabling particle and radiation source technology is the development of mechanisms to control radiation generation in plasma. The project will explore the physics of the generation of high harmonic radiation and optical vortices via the oscillating plasma aperture mechanism. The objectives are to:1. Investigate the spectrum of harmonics generated by linearly-polarised intense laser pulses interaction with an ultrathin foil target 2. Investigate changes to the spectrum of harmonics generated when changing from linear to circular polarisation3. Investigate the extent to which the spectral and spatial-intensity distribution of the harmonics generated can be controlled by variation of the degree of ellipticity in the polarisation of the laser light.