Temporal effect of the spin-to-orbit conversion in tightly focused femtosecond optical fields

Abstract

Spin and orbital angular momenta are two of the most fundamental physical quantities that describe the complex dynamic behaviors of optical fields. A strong coupling between these two quantities leads to many intriguing spatial topological phenomena, where one remarkable example is the generation of a helicity-dependent optical vortex that converts spin to orbital degrees of freedom. The spin-to-orbit conversion occurs inherently in lots of optical processes and has attracted increasing attention due to its crucial applications in spin–orbit photonics. However, current researches in this area are mainly focused on the monochromatic optical fields whose temporal properties are naturally neglected. In this work, we demonstrate an intriguing temporal evolution of the spin-to-orbit conversion induced by tightly-focused femtosecond optical fields. The results indicate that the conversion in such a polychromatic focused field obviously depends on time. This temporal effect originates from the superposition of local fields at the focus with different frequencies and is sensitive to the settings of pulse width and central wavelength. This work can provide fundamental insights into the spin–orbit dynamics within ultrafast wave packets, and possesses the potential for applications in spin-controlled manipulations of light.

Fundamental symmetry origins in the chiral interactions of optical vortices

“…This identifies various groups of chiral phenomena that can arise according to the presence of circular polarization σ , topological charge l , and a nominal parameter ξ whose value is non‐zero for chiral matter. The diagram also indicates the scope for the many spin‐orbit interactions not involving matter, as, for example, identified in early work by Allen et al, 88 and in numerous other, more recent papers 86,89–92 . The present analysis now focuses such considerations into a simpler and more directly applicable form by considering molecules in their local interaction with the optical beam.…”

“…The diagram also indicates the scope for the many spin-orbit interactions not involving matter, as, for example, identified in early work by Allen et al, 88 and in numerous other, more recent papers. 86,[89][90][91][92] The present CPT analysis now focuses such considerations into a simpler and more directly applicable form by considering molecules in their local interaction with the optical beam.…”

Spatiotemporal modulation of ultrafast plasmonic vortices with spin–orbit coupling

“…In order to gain more insight into the spatiotemporal evolution of OAM and SAM and their interaction in the plasmonic vortices, in this section, we study the local OAM and SAM densities (in unit of h), which are defined as [39,40]: where r = (x, y, z) is the position vector of any point in the plasmonic field, and E = (E x , E y , E z ) denotes the electric field of the SPP vortex field on the gold surface (z = 0). Although L and S calculated from equations ( 14) and ( 15) are 3D vectors, in figures 5 and 6 we focus only on the distributions of z-components of local OAM and SAM densities (L z and S z ), because according to the conservation law of angular momentum, here the net angular momentum of SPP field is contributed by the incident light with only z-directional OAM and SAM [20].…”

“…To further understand the evolutionary relationship between OAM and SAM of the whole plasmonic vortex field, we calculate the averaged OAM per photon ⟨L⟩ and the averaged SAM per photon ⟨S⟩ as [20,39]:…”

“…In recent years, the rapid development of ultrafast lasers has attracted more and more attention [18][19][20][21][22][23]. It has been reported that SPP pulses excited by ultrafast laser can also carry OAM, which could be an important tool for studying chiral nanostructures and materials.…”