Deflection of a reflected intense circularly polarized light beam induced by asymmetric radiation pressure

Deflection of a reflected intense spatiotemporal optical vortex beam

In this Letter, we reveal a new deflection effect in the reflection of an intense spatiotemporal optical vortex (STOV) beam. When a STOV beam with relativistic intensities (>10¹⁸ W cm^-2) impacts on an overdense plasma target, the reflected beam deviates from the specular reflection direction in the incident plane. Using two-dimensional (2D) particle-in-cell simulations, we demonstrated that the typical deflection angle is of a few milliradians and can be enhanced by using a stronger STOV beam with tightly focused size and higher topological charge. Though similar to the angular Goos-Hänchen effect, however, it is worth emphasizing that the deviation induced by a STOV beam exists, even in normal incidence, revealing an essentially nonlinear effect. This novel effect is explained from the viewpoint of angular momentum conservation, as well as the Maxwell stress tensor. It is shown that an asymmetrical light pressure of the STOV beam breaks the rotational symmetry of the target surface and leads to nonspecular reflection. Unlike the shear press of an Laguerre-Gaussian beam, which only acts in oblique incidence, the deflection caused by the STOV beam exists more widely, including in normal incidence.

Reflection and absorption of electromagnetic radiation by inhomogeneous photoionized plasma, produced by multiphoton ionization of inert gas atoms

The interaction of electromagnetic radiation with inhomogeneous plasma formed by multiphoton ionization of inert gas atoms has been studied. In high-frequency and normal skin effects modes the field structure in plasma is described and reflection and absorption coefficients are found. It is shown that as the thickness of the plasma region, in which the photoelectron density grows linearly, increases, both the depth of field penetration and the absorption coefficient increase, too. It is found that, due to the Ramzauer-Townsend effect, there is a relative increase in the effective frequency of photoelectron collisions with atoms, which is accompanied by a significant increase in the absorption coefficient.

High-Harmonic Generation and Spin-Orbit Interaction of Light in a Relativistic Oscillating Window

When a high power laser beam irradiates a small aperture on a solid foil target, the strong laser field drives surface plasma oscillation at the periphery of this aperture, which acts as a "relativistic oscillating window." The diffracted light that travels though such an aperture contains high-harmonics of the fundamental laser frequency. When the driving laser beam is circularly polarized, the high-harmonic generation (HHG) process facilitates a conversion of the spin angular momentum of the fundamental light into the intrinsic orbital angular momentum of the harmonics. By means of theoretical modeling and fully 3D particle-in-cell simulations, it is shown the harmonic beams of order n are optical vortices with topological charge |l|=n-1, and a power-law spectrum I_{n}∝n^{-3.5} is produced for sufficiently intense laser beams, where I_{n} is the intensity of the nth harmonic. This work opens up a new realm of possibilities for producing intense extreme ultraviolet vortices, and diffraction-based HHG studies at relativistic intensities.

Orbital Angular Momentum Reversal and Asymmetry in Acoustic Vortex Beam Reflection

We present acoustic modeling, measurements, and interpretation of the angular momentum carried in an ultrasonic vortex beam that is obliquely reflected off a flat water-air interface. The experimental measurements observe the theoretically predicted reversals of phase rotation, topological charge, and orbital angular momentum in a reflected vortex beam in direct analogy to optical phenomena. The spatial and temporal evolution of the linear and angular momentum during the reflection are determined by calculating the velocity field from two-dimensional scanned pressure fields. A conversion of the angular momentum indicates a radiation torque along the oblique reflecting surface. We understand this radiation torque originates from the break of rotational symmetry with respect to the incident plane for normal components of the energy flux and linear momentum density at the reflecting surface. Our study provides mechanical evidence on the effect of a flat surface on the reflection of vortex beams and gains insight into the underlying physics, impacting non-contact manipulation of objects and communication.