The fact that light carries spin angular momentum (SAM) of $\sigma \hslash$ ($\sigma =\pm 1$) was known as early as 1909, when Poynting demonstrated that a circularly polarized beam of light could cause a piece of birefringent material suspended on a thin wire to rotate about its center.¹ However, it was not until 1992 that Allen et al.² proved that an optical beam with azimuthal varying phase $\exp (il\varphi )$ carries orbital angular momentum (OAM) of $l\hslash$ per photon, with $l$ being the topological charge of the beam. The past three decades have seen tremendous growth of such optical beams from being a mere scientific curiosity to being a key enabler in a wide variety of cutting-edge applications, such as super-resolution imaging, optical trapping, and classical and quantum optical communications. In all these applications, the first step is the generation of an OAM mode. Experimentally, there are numerous methods to generate the desired OAM modes.^{3 – 5} One of the first reports on the generation of OAM mode was simply using two cylindrical lenses.⁶ Since then, there has been tremendous growth in this field, and the current state of the art includes several different design techniques such as binary holographic optical designs, spiral phase plate (SPP) designs, and planar metasurface designs. In general, OAM mode generation is achieved by modulation of one or more of the incident light beam’s characteristics through an optical element fabricated lithographically using electron beam, ion beam, and/or photolithography, or by using active devices, such as liquid crystal on silicon (LCoS)-based spatial light modulators (SLMs) and digital micromirror devices (DMDs).

Metasurfaces are engineered thin surfaces comprising two‐dimensional (2D) arrays of sub‐wavelength‐spaced and sub‐wavelength‐sized resonators. Metasurfaces can locally manipulate the amplitude, phase, and polarization of light with high spatial resolution. In this paper, we report numerical and experimental results of a vortex‐beam‐generating metasurface fabricated specifically for infrared (IR) and terahertz (THz) wavelengths. The designed metasurface consists of a 2D array of dielectric cross‐shaped resonators with spatially varying length, thereby providing the desired spatially varying phase shift to the incident light. The metasurface was found to be insensitive to the polarization of the incident light. The dimensions of the cross‐resonators were calculated using rigorous finite‐difference time‐domain analysis. The spectral scalability via physical scaling of the meta‐resonators is demonstrated using two vortex‐generating optical elements operating at 8.8 μm (IR) and 0.78 THz. The vortex beam generated in the mid‐IR spectral range was imaged using a Fourier transform IR (FTIR) imaging miscroscope equipped with a focal plane array detector. This design could be used for efficient wavefront shaping and various optical imaging applications in the mid‐IR spectral range, where polarization insensitivity is desired.

The realization of spiral phase optical elements on the cleaved end of an optical fiber by focused ion beam milling is presented. A focused ${\mathrm{Ga}}^{+}$ ion beam with an acceleration voltage of 30 keV is used to etch continuous spiral phase plates and fork gratings directly on the tip of the fiber. The phase characteristics of the output beam generated by the fabricated structures measured via an interference experiment confirmed the presence of phase singularity in the output beam. The devices are expected to be promising candidates for all-fiber beam shaping and optical trapping applications.

Stokes polarimetry is widely used to extract the polarization structure of optical fields, typically from six measurements, although it can be extracted from only four. To measure the required intensities, most approaches are based on optical polarization components. In this work, we present an all-digital approach that enables a rapid measure of all four intensities without any moving components. Our method employs a polarization grating (PG) to simultaneously project the incoming mode into left- and right-circular polarized states, followed by a polarization-insensitive digital micromirror device (DMD), which digitally introduces a phase retardance for the acquisition of the remaining two polarization states. We demonstrate how this technique can be applied to measuring the SoP, vectorness, and intramodal phase of optical fields, without any moving components, and shows excellent agreement with theory, illustrating fast, real-time polarimetry.