Coherent synchrotron radiation by excitation of surface plasmon polariton on near-critical solid microtube surface

Abstract

Coherent synchrotron radiation (CSR) is crucial for the development of powerful ultrashort light sources. We present a mechanism for generating CSR in the form of generalised superradiance, based on surface plasmon polaritons (SPPs), which are resonantly excited on a solid, near-critical-density inner surface of a microtube. A high-intensity, circularly polarised laser pulse, propagating along the microtube axis, efficiently couples the cylindrical SPP modes. This process creates azimuthally structured, rotating electromagnetic fields. These rotating fields subsequently confine, modulate, and directly accelerate surface electrons to emit CSR in the Vavilov-Cherenkov angle. We further demonstrate that by improving the azimuthal symmetry of these electrons, the helical modulation enables CSR emission across all azimuthal directions in the form of isolated harmonics, significantly enhancing radiation intensity even when full coherence is imperfect. Our full 3D Particle-in-Cell simulations indicate this scheme can generate X-rays with coherence enhanced by up to two orders of magnitude compared to incoherent emission. The challenges to experimentally realise this scheme are discussed, including the need for high-contrast lasers to prevent pre-plasma formation and the demanding tolerances for microtube fabrication and alignment, while these challenges are not beyond the scope of existing or near-future experimental capabilities.

Figures (5)

• Figure 1: Schematic: a CP laser pulse (yellow) enters the vacuum channel of a microtube (grey) and excites SPPs while being scattered at the sharp vertical edge. The rotation mode $m=\pm 1$ (green-yellow) can efficiently couple with the laser field and accelerates the trapped electrons(sphere, blue-to-red colour presents the energy) into a spiral motion along the solid surface for the CSR emission (yellow-red cone).
• Figure 2: (a) Electron density distribution $n_e$ (black colour) and $E_z$ (blue-red colourmap) in $yz$-plane sliced at $x=0$ while the laser pulse entering the microtube. (b) Snapshots of $E_z$ in $xy$-plane sliced at $z=5~\mu m$ corresponding to (a). (c)-(e) Snapshots of $E_r$, $E_{\phi}$ and $E_{z}$ in $xy$-plane at the same phase after the laser pulse propagating $z=41~\mu m$ in the tube. The solid black and dashed red lines in each plot present the line plots of the corresponding field along $y=0~\mu m$ from PIC and analytical solutions, respectively.
• Figure 3: PIC results: 3D (a) and front (b) views of the high-energy ($\mathcal{E}>88~MeV$) electron beam modulation after the laser propagates $z=40~\mu m$ in the microtube. The arrows in (b) present the transverse momentum field ($u_x, u_y$). (c): Energy evolution of the high-energy electrons in the beam (grey lines) and the mean value (black line) as a function of propagation distance. (d) Trajectories of these electrons. The red line represents one of the electrons in the beam.
• Figure 4: (a) Radiation spectrum coherently calculated by the trajectories of the electrons from a PIC simulation in Fig. \ref{['fig:e_trajectory']}. Insert (b): Line plot along Vavilov-Cherenkov angle, $\varphi_{vc} \simeq 0.23$, indicated by horizontal yellow dashed line. The dashed white curves are theoretical resonant frequencies of different harmonics.
• Figure 5: (a) Theoretical radiation spectrum calculated coherently (upper) and incoherently (bottom) by using an electron beam of $N=100$ electrons uniformly distributed over six modulation periods, or $4.8~\mu m$ long. The other beam parameters are the same as those from PIC simulations, as shown in Fig. \ref{['fig:e_trajectory']}. Insert (b) Line plot of (a) along axis $\theta=0$ and $\phi=\pi/2$. (c) Line plot of onaxis ($\theta=0$ and $\phi=\pi/2$) spectrum in Fig. \ref{['fig:radiation_spectrum']}(a).