Theoretical Study of Plasma Photonics with Particle-In-Cell Simulation: Plasma Dipole Oscillator, Radial Plasma Oscillator and Gradient Plasma Photonic Crystal

Abstract

Laser–plasma physics has advanced significantly since the pioneering work of T. Tajima and J. M. Daw- son in 1979, which first demonstrated electron acceleration via laser wakefields [1]. The progress in understanding fundamental laser–plasma phenomena has opened new avenues for diverse applications. The high damage threshold (∼TV/m) and strong electrostatic fields (∼GV/m) inherent to plasmas, when driven by high-power lasers, offer considerable potential for applications such as terahertz (THz) radia- tion sources, attosecond and X-ray pulse generation, and laser wakefield acceleration (LWFA). In recent years, the field of plasma photonics—which explores plasmas as optical media due to their density- dependent refractive index—has attracted renewed interest. With appropriate density modulations, plas- mas can function as optical components such as mirrors [2, 3], waveplates [4], compressors [5–7], grat- ings [8–10], and even amplifiers [11, 12], making them particularly well suited for manipulating ultra- intense laser pulses. This dissertation focuses on three primary categories of plasma photonics: (1) plasma dipole oscillator (PDO), (2) radial plasma oscillator (RPO), and (3) gradient plasma photonic crystal (GPPC). Each of these topics will be carefully discussed based on particle-in-cell (PIC) simula- tions and theoretical analysis. PDO is a type of active plasma photonic structure that can be generated by colliding two detuned laser pulses in plasma. Depending on the laser intensity, the excitation mech- anism can be categorized into a nonlinear current-driven regime or a particle trapping-driven regime, and PDO is primarily utilized as a source of narrowband THz radiation. It can also be considered as a theoretical model for the Langmuir wave collapse occurring in the coronal plasma. RPO is simi- lar to PDO, but it arises in a linearly density-gradient plasma when two co-propagating detuned laser pulses satisfy a resonance condition (∆ω ≈ ωp). Specifically, after the laser pulses pass through the resonance point, the plasma wave number varies over time due to the density gradient, and the plasma wave number approaches zero (kpe ≈ 0). After that, all plasma electrons oscillate in phase. At this point, RPO—resembling a radial plane antenna—is formed, generating a radially polarized THz beam. GPPC is a plasma photonic crystal formed in a density gradient plasma. Such plasma gratings are mainly gen- erated by the beat wave produced through the overlap of incident and reflected laser pulses, and their dynamics are predominantly governed by ponderomotive potential. By tuning parameters such as the grating gap, density amplitude, and density profile, the dispersive properties of the plasma grating can be controlled, making it highly useful for plasma-based pulse compression.