Compact diffractive optics for THz imaging

Abstract

We present a compact diffractive silicon-based multilevel phase Fresnel lens (MPFL) with up to 50 mm in diameter and a numerical aperture up to 0.86 designed and fabricated for compact terahertz (THz) imaging systems. The laser direct writing technology based on a picosecond laser was used to fabricate diffractive optics on silicon with a different number of phase quantization levels P reaching an almost kinoform spherical surface needed for efficient THz beam focusing. Focusing performance was investigated by measuring Gaussian beam intensity distribution in the focal plane and along the optical axis of the lens. The beam waist and the focal depth for each MPFL were evaluated. The influence of the phase quantization number on the focused beam amplitude was estimated, and the power transmission efficiency reaching more than 90% was revealed. The THz imaging of less than 1 mm using a robust 50 mm diameter multilevel THz lens was achieved and demonstrated at 580 GHz frequency.

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.…”

“…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. Therefore, the THz regime is an ideal frequency band to capture the real significance of this work 44,45 . Second, since the THz band lies in almost the centre of the electromagnetic spectrum, hence the metrics developed in this paper can be scaled up or down to both the microwave as well as optical ranges without any loss of generality.…”

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

“…Here, direct laser writing [ 68 ] (related to ablating the unwanted parts of the substrate layer) ensures proper manufacturing resolution. However, such a technique can be also used to manufacture phase structures with different number of phase steps [ 3 , 69 , 70 ] or even kinoform structures [ 71 ]. Laser ablation is mostly related to using high-resistance silicon plates or metals.…”

“…Moreover, by additional phase corrections resulting from the fact that the structure has a physical thickness (it is not an ideal infinitely thin element) the signal to noise ratio in the focal plane can be significantly augmented [ 74 ]. Diffractive optics gives the remarkable opportunity to increase the numerical aperture (NA) value of the structure (decrease F-number) to values that are not achievable in refractive and reflective approaches remaining compact element architecture [ 70 ]. Making the focal length shorter and element diameter larger leads to extreme off-axis functioning.…”

Terahertz Diffractive Optics—Smart Control over Radiation

“…The last group continued research in applying the diffractive optics for imaging purposes with free electron laser illumination [70] and [71]. The group of Gintaras Valušis described manufacturing of lenses by laser patterning of silicon [72], next used for imaging [73], or used laser ablation to integrate FZP on-chip of detector [74]. They also proposed a manufacturing technique using cross-shaped apertures (being a resonant filter) [75][76][77][78] to select the THz frequencies ( Fig.…”