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).

Bessel beams (BBs) appear to be immune to diffraction over finite propagation distances due to the conical nature of light propagation along the optical axis. This offers promising advantages in laser fabrication. However, BBs exhibit a significant intensity variation along the direction of propagation. We present a simple technique to engineer the axial intensity of the BBs over centimeter-long propagation distances without expansion of the incoming laser beam. This method uses two diffractive optical elements (DOEs), one converts the input Gaussian intensity profile to an intermediate intensity distribution, which illuminates the second DOE, a binary axicon. BBs of a desired axial intensity distribution over a few centimeters length can be generated.

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.

Metasurfaces have recently emerged as a promising technology to realize flat and ultra-thin optical elements that can manipulate light at sub-wavelength scale. The typical design flow of a metasurface involves tedious Finite Difference Time Domain (FDTD) simulations followed by creation of a GDSII layout of the metasurface phase profile, the latter being essential for fabrication purposes. Both these steps can be time-consuming and involve the usage of expensive software. To make the design process more straightforward, we have developed an open-source software called MetaOptics built using Python for designing a generic metasurface optical element. MetaOptics uses the FDTD simulated phase response data of a set of meta-atoms and converts the phase profile of any given optical element into a metasurface GDSII layout. MetaOptics comes with in-built FDTD data for most commonly used wavelengths in the visible and infrared spectrum. It also has an option to upload user-specific dimension versus transmission phase data for any choice of wavelength. In this work we describe the software’s framework and provide details to guide users to design a metasurface layout using MetaOptics.

Recently, metasurfaces have gained popularity due to their ability to offer a spatially varying phase response, low intrinsic losses and high transmittance. Here, we demonstrate numerically and experimentally a silicon meta-surface at THz frequencies that converts a Gaussian beam into a Vortex beam independent of the polarization of the incident beam. The metasurface consists of an array of sub-wavelength silicon cross resonators made of a high refractive index material on substrates such as sapphire and CaF₂ that are transparent at IR-THz spectral range. With these substrates, it is possible to create phase elements for a specific spectral range including at the molecular finger printing around 10 μm as well as at longer THz wavelengths where secondary molecular structures can be revealed. This device offers high transmittance and a phase coverage of 0 to 2π. The transmittance phase is tuned by varying the dimensions of the meta-atoms. To demonstrate wavefront engineering, we used a discretized spiraling phase profile to convert the incident Gaussian beam to vortex beam. To realize this, we divided the metasurface surface into eight angular sectors and chose eight different dimensions for the crosses providing successive phase shifts spaced by π/4 radians for each of these sectors. Photolithography and reactive ion etching (RIE) were used to fabricate these silicon crosses as the dimensions of these cylinders range up to few hundreds of micrometers. Large 1-cm-diameter optical elements were successfully fabricated and characterised by optical profilometry.