Abstract-Focusing of Terahertz Radiation With Laser-Ablated Antireflective Structures

 Milda Tamošiūnaitė,  Simonas Indrišiūnas, Vincas Tamošiūnas,   Linas Minkevičius,  Andrzej Urbanowicz,  Gediminas Račiukaitis,   Irmantas Kašalynas,  Gintaras Valuši

https://ieeexplore.ieee.org/document/8419328/

Numerical simulations and experimental characterization of laser-ablated focusing antireflective and phase-shifting structures for terahertz frequencies are presented. More than 10% shift of reflectance minimum to lower frequencies was predicted by simulations for relatively coarse structures with the period of 100  μ m in comparison with that of a substantially smaller period and with results of the model used for the design of antireflective surfaces in the terahertz range. Such a shift of the resonance frequency can be employed to optimize the thickness of antireflective layers simultaneously obtaining additional means of more precise control of layer properties due to ablation of larger structures. Nearly 90% transmittance of silicon wafers within 0.5–0.6 THz frequencies was confirmed experimentally. Optical path differences equivalent to a half period at 0.53 THz, suitable for applications in high-efficiency zone plates, were demonstrated with high transmittance simultaneously. Possibilities of delay adjustment up to one wavelength were illustrated by numerical simulations. A focusing binary zone plate for 0.6 THz was produced employing phase-shift differences of the dual-function antireflective layer. Its close to diffraction-limited focusing performance was evaluated, further confirming sufficient uniformity of the structured layer.

Abstract-Compact solutions for spectroscopic solid-state-based terahertz imaging systems

Rimvydas Venckevičius,  Linas Minkevičius,  Antanas Reklaitis, Vincas Tamošiūnas,  Irmantas Kašalyns,  Domas Jokubauskis,  Dalius Seliuta,  Bogdan Voisiat,  Gediminas Račiukaitis,  Gintaras Valušis,

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10383/103830S/Compact-solutions-for-spectroscopic-solid-state-based-terahertz-imaging-systems/10.1117/12.2273353.short

Convenience in use of room-temperature terahertz (THz) imaging systems, reduction of their dimensions and presence of on-chip solutions remains one of prime interests for direct implementation aims. Solid-state-based solutions in miniaturization of spectroscopic THz imaging systems including novel semiconductor nanostructures bias-free emitters, diffractive THz optics components and their on-chip integration with THz detectors are discussed. In particular, pulsed optoelectronic terahertz emitter based on a δ-doped p-i-n-i GaAs/AlxGa1−xAs heterostructure was studied and it is demonstrated that the heterostructure can serve as efficient antenna- and bias-free surface emitter. Diffractive optics elements – Fresnel zone plates –with integrated band-pass filters were simulated employing Finite-difference time domain method. Structures were fabricated using the laser direct writing and investigated using electronic THz sources and an optically pumped terahertz laser. Advantages of on-chip integration of diffractive optics and bow-tie-shaped InGaAs-based terahertz detectors are revealed via detection enhancement. Bow-tie diodes properties in frequency scale and detection sensitivity are considered and compared for different materials. Homodyne detection and imaging of low-absorbing objects at 0.6 THz are demonstrated and discussed.

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Abstract-On-chip integration solutions of compact optics and detectors in room-temperature terahertz imaging systems

 Linas Minkevičius; Vincas Tamošiūnas; Irmantas Kašalynas; Rimvydas Venckevičius; Karolis Madeikis; Bogdan Voisiat; Dalius Seliuta; Gediminas Račiukaitis; Gintaras Valušis

http://spie.org/Publications/Proceedings/Paper/10.1117/12.2187921

On-chip integrated solutions employing properties of Fresnel zone plates with integrated band-pass filters for the room temperature terahertz imaging systems are discussed. Finite-difference time-domain simulations were used to predict properties of conventional zone plates and ones with resonant filter areas as flat optics components. They are produced employing the laser direct writing and characterized by electronic THz sources and an optically pumped terahertz laser. It was shown that more than one order of magnitude detection enhancement can be observed of bow-tie-shaped InGaAs-based terahertz detectors by on-chip incorporation of the secondary diffractive optics.