Lithuanian Journal of Physics article / Lietuvos fizikos žurnalo straipsnis

COMPACT DIFFRACTIVE OPTICS FOR THz IMAGING
Linas Minkevičius, Simonas Indrišiūnas, Ramūnas Šniaukas, Gediminas Račiukaitis, Vytautas Janonis, Vincas Tamošiūnas, Irmantas Kašalynas, and Gintaras Valušis
Center for Physical Sciences and Technology, Saulėtekio 3, 10257 Vilnius, Lithuania
E-mail: linas.minkevicius@ftmc.lt

Received 16 January 2018; accepted 22 March 2018

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.

Keywords: binary optics, diffractive lenses, phase shift, three-dimensional fabrication, lenses, lens system design
PACS: 42.15.Eq, 42.79.Bh

KOMPAKTINĖ DIFRAKCINĖ OPTIKA TERAHERCŲ VAIZDINIMO SISTEMOMS
Linas Minkevičius, Simonas Indrišiūnas, Ramūnas Šniaukas, Gediminas Račiukaitis, Vytautas Janonis, Vincas Tamošiūnas, Irmantas Kašalynas, Gintaras Valušis

Fizinių ir technologijos mokslų centras, Vilnius, Lietuva

Darbe pristatomi kompaktiški silicyje suformuoti difrakciniai Frenelio lęšiai, skirti terahercinio dažnio (THz) vaizdinimo sistemoms. Jie pagaminti naudojant lazerinės abliacijos technologiją. Šiuo metodu silicio paviršiuje suformuota 5 ir 10 mm židinio nuotolio difrakcinė zoninė plokštelė su skirtingais fazinio kvantavimo lygiais pasiekiant beveik tolygų sferinį paviršių, reikalingą efektyviam THz spinduliuotės fokusavimui. Šių lęšių skersmuo siekia iki 50 mm, o skaitinė apertūra neviršija 0,86 mm. Fokusavimo efektyvumas ištirtas matuojant Gauso pluošto intensyvumo pasiskirstymą židinio plokštumoje ir optinės ašies kryptimi. Įvertintas sufokusuoto Gauso pluošto sąsmaukos plotis ir kiekvienos Frenelio lęšio konfigūracijos židinio gylis. Išmatavus fazinio kvantavimo skaičiaus įtaką fokusuotos spinduliuotės amplitudei nustatyta, kad 8 pazonių difrakcinio lęšio fokusavimo efektyvumas sotinasi, kai pralaidumo koeficientas pasiekia daugiau nei 90 %. Atlikti THz vaizdinimo eksperimentai parodė, kad naudojant tvirtą 50 mm Frenelio lęšį galima pasiekti geresnę nei 1 mm skyrą ties 580 GHz dažniu.

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