Study of thin, achromatic diffractive structures to focus terahertz radiation on a detector

Abstract

Thin and lightweight achromatic focusing elements with F-number close to 1 are desirable in many practical applications. We present the idea to use diffractive structures designed to work for the substantially increased THz frequency range. The paper analyses mono- and multi-focal lenses forming point-like foci as well as axicon and light sword optical elements focusing THz radiation into line segments located along the optical axis. We consider diffractive elements in a form of the first and the second order kinoforms having various thicknesses. Designed and fabricated elements were numerically and experimentally examined to verify their achromatic functioning. We present point spread functions (XY scans) and 2D energy maps (XZ scans) for different THz frequencies. Moreover, a diagram of chromatic aberration is created by registering energy distribution along the optical axis for different frequencies. The distance corresponding to the highest energy is chosen for each frequency. Therefore, we can compare broadband working of designed structures. The spherical lens coded as kinoform of the second order provides the best broadband functioning, however it is two times thicker than structures providing extended depth of focus (light sword and axicon) working with slightly smaller efficiency but being much thinner.

High-throughput terahertz imaging: progress and challenges

“…In terahertz imaging, diffractive lenses find their most widespread application as Fresnel lenses incorporated within raster scanning systems, primarily for reducing the size of the focused light spots. Additionally, some applications utilize multifocal diffractive lenses to achieve wide-field, broadband, or extended-depth-of-focus imaging [166][167][168][169][170] . Other diffractive lens designs offer capabilities of beam shaping 171 and generation of intricate patterns, such as Airy 172,173 , Bessel [174][175][176] , and vortex beams 177,178 , expanding their utility in diverse terahertz imaging applications.…”

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

“…It is an interesting idea to use diffractive structures with extended depth of focus as optical elements focusing at a particular distance the radiation with a broader spectral range and maintaining small thickness [ 84 , 85 ]. However, in recent years, a vast majority of optical elements working broadband have been designed using metamaterials [ 76 , 86 , 87 ].…”