Terahertz Diffractive Optics—Smart Control over Radiation

Abstract

Over the last 20 years, thin and lightweight optical elements have become very desirable, especially for the terahertz (THz) range. Reduction of the volume of optical elements alongside an increase in their effective efficiency has begun a new direction of research leading to many practical applications. On top of that, diffractive optical elements can not only focus the incident beam, but also can shape the incoming wavefront into a desirable distribution or can redirect the energy. Starting from theoretical calculations of Fourier optics, diffractive elements have been transformed and nowadays form complicated structures that do not resemble a typical Fresnel lens. The precise control over a phase shift introduced by the designed element creates an opportunity to almost freely transform an incident wavefront. Moreover, the vast diversity of computer-generated holograms (also called synthetic) contributes substantially to this topic. Diffractive elements have a great impact on THz optical systems because their manufacturing is very simple in comparison with any other range of radiation (infrared, visible, ultraviolet, etc.). This review paper underlines developments in evolution of diffractive optics and highlights main principles and technological approaches for fabrication of diffraction optics within the terahertz range, thus serving as a guide to design and production considerations.

Ultrahigh numerical aperture meta-fibre for flexible optical trapping

“…Experimental implementation of the UNM used here with a focal point at an axial distance z = f relies on 3D-DLW, requiring discretization of a hyperbolic phase ( n : refractive index of the medium, λ 0 : vacuum wavelength, r : radial coordinate). The phase profile ϕ is discretized in steps of 2 π into Fresnel zones via ϕ kin = mod( ϕ ,2 π ), leading to a kinoform-type phase distribution 23 – 25 (details in Supplementary Results 1 ). Note that the hyperbolic phase profile exhibits less critical curvature towards the edges than its parabolic counterpart, and is, thus, used throughout this work.…”

Impact of fabrication errors and refractive index on multilevel diffractive lens performance

“…Other metrics like EDOF, FOV magnification, aberrations can also be included; but these are often not the principal criterion that one looks at when designing an optical element. The choice of performing this study on MDLs primarily operating the THz regime is due to two main reasons [42][43][44][45] . First, the ease in modelling larger unit cells ("rings") leads to the total number of elements in the entire structure to be small and tractable; hence, it is easy to capture the trade-off in efficiency with faster simulations and develop accurate prediction metrics.…”

The Magic of Optics—An Overview of Recent Advanced Terahertz Diffractive Optical Elements

“…Thus, the desired phase distribution is coded in the form of the required transmittance of DOE— [ 57 ]. Of course does not have to be binarized, it could be coded into transmittance with different shapes, such as sine-like, step-like or saw-blade (blazed, corresponding to kinoform) [ 21 , 58 , 59 ].…”

“…Therefore, for example, binary amplitude coding will have the maximal equal to 10.1% which is the of the amplitude binary diffraction grating. All maximal values for each coding type are given in Table 1 and in [ 21 , 30 , 60 , 61 , 62 ].…”

“…Although such devices also introduce a particular phase distribution corresponding to the phase delay map, they function according to different phenomena than described in this work diffractive elements. More information about different methods of terahertz beam forming was described in [ 19 , 20 , 21 ].…”