Spin-to-orbital angular momentum conversion in harmonic generation driven by intense circularly polarized laser

Abstract

Spin-to-orbital angular momentum conversion (STOC) is a very important fundamental phenomenon governed by the conservation of total angular momentum (TAM). In optics, this conversion is usually associated with the vortex light carrying the orbital angular momentum (OAM). In this paper we demonstrate a new mechanism to achieve STOC via the interaction of an intense circularly polarized (CP) laser pulse with a solid density plasma target. We find that when a laser pulse with relativistic intensity is tightly focused, a longitudinal electric field is induced owing to the finite transverse size and profile of the laser field. Therefore, even for the normally incident CP laser, the induced longitudinal electric field can drive an oscillating vortex plasma surface to emit the vortex harmonics when the laser interacts with the plasma target. Based on simulations and theoretical analysis, we verify this harmonic generation mechanism and reveal the STOC process in the harmonic generation. It is shown that the spin angular momenta of multiple fundamental-frequency photons are converted to the OAM of a single harmonic photon because of the TAM conservation. We also discuss the dynamical symmetries in the harmonic generation process, which physically constrains the harmonic orders, as well as the angular momenta. In addition, if a vortex laser beam or a spiral phase plate is used, the OAM of the harmonic photon becomes more tunable and controllable. This study provides a deep insight into the nature of the spin-orbital interaction in optics.

Enhanced radiation of nonlinear Thomson backscattering by a tightly focused Gaussian laser pulse and an external magnetic field

“…The results show that the radiation intensity of the high frequency part can be enhanced by adjusting the strength of the external magnetic field, especially when the cyclotron frequency of the electron in the external magnetic field is close to the laser frequency, the electron can sufficiently obtain energy from the combined field [11]. For the laser pulse with uniform transverse intensity distribution, the influences of different longitudinal laser envelopes, such as Gaussian envelope, cosine envelope and super Gaussian envelope on the Thomson scattering have been studied [18][19][20][21][22][23]. For instance, Tian et al have investigated the dynamics and harmonics emission spectra of a single electron moving in a laser pulse with longitudinal Gaussian envelope and have demonstrated the shifting and broadening of the radiation spectrum, which depend sensitively on the intensity and the pulse width of the laser pulse [18].…”

Isolated intense half-cycle attosecond pulse generation with orbital angular momentum

“…Compared with the vortex attosecond pulse generated by HHG in gaseous and solid target regimes, the attosecond pulse in our scheme is isolated, while most of the others are as pulse trains [28,29,34], which require additional filtering methods [12][13][14][15][16] to obtain a single attosecond pulse. In addition, by using the gas-solid target, our proposal can adopt a relativistic intense laser to get a strong as pulse without the need for a high contrast of the laser pulse and the thin target [17].…”

“…Angular momentum can be separated to spin angular momentum (SAM) of ±ℏ (ℏ is the reduced Plank constant) per photon associated with the circular polarization of light beams [26], and the OAM originates from the helical phasefronts exp (ilϕ) of the vortex beam found by [27], where ϕ is the azimuthal angle, and the topological charge l means one photon carries the OAM of lℏ. Governed by the angular momentum as well as the energy and momentum conservation law, it is indicated that the HHG process driven by the vortex or circular polarized beam will also impart the OAM and/or SAM to the harmonic photons, thus resulting in the attosecond pulse train with OAM [28,29]. Based on the well-developed method to generate an intense/moderate intensity vortex laser [30,31], this has been widely investigated theoretically and experimentally in laser interaction with gaseous and solid target regimes [32][33][34][35].…”

Intense harmonic generation driven by a relativistic spatiotemporal vortex beam

“…Concurrent with the progress in high-power lasers [6,7] , the production of intense spatial vortex beams in the relativistic region (> 10 18 W cm −2 for a laser wavelength of ∼ 1 µm) has been theoretically proposed [8][9][10] and demonstrated in laboratories [11][12][13] . Such advanced light sources provide new possibilities for light-matter interactions, such as donut/twist wakefield acceleration [14][15][16] , attosecond electron bunch generation [17][18][19] , formation of a large magnetic field [20] , high harmonics generation (HHG) [12,21,22] , and the spin-orbital interaction [23][24][25][26][27] . In particular, the SAM and/or OAM of fundamental-frequency photons are transferred into high harmonics during HHG.…”