Abstract-Terahertz time-domain spectroscopy of two-dimensional plasmons in AlGaN/GaN heterostructures

Applied Physics Letters

Daniil Pashnev, Tommi Kaplas, Vadym Korotyeyev, Vytautas Janonis, Andrzej Urbanowicz, Justinas Jorudas, Irmantas Kašalynas, 

THz waveforms measured after transmission through a cryostat without (reference) and with (SP1000F50) the sample and (b) corresponding FFT power spectra obtained using the time window tcut2$t_{\mathit{cut}2}$. (c) Transmission power spectrum T1(f)$T_1(f)$ found using the time window tcut2$t_{\mathit{cut}2}$ and tcut1$t_{\mathit{cut}1}$ providing data with (w/) and without (w/o) FP oscillations, respectively.

https://aip.scitation.org/doi/abs/10.1063/5.0014977

Two-dimensional plasmons were investigated by terahertz time domain spectroscopy observing experimentally the distinctive minima and inflection points in the transmission power amplitude and phase spectra, respectively. Gratings of different periods (600, 800, and 1000 nm) and filling factors (50 and 80%) were provided to the two-dimensional electron gas in AlGaN/GaN heterostructures in order to measure the plasmon dispersion and the coupling efficiency with THz radiation. Comparative analysis of experimental data revealed that the resonant plasmon features in the amplitude spectrum are related to those in the phase spectrum by a simple integral relation, paving the way for phase spectroscopy of the plasmon phenomena in fields of THz physics and engineering.

This work was supported by the Research Council of Lithuania (Lietuvos mokslo taryba) under the “TERAGANWIRE” Project (Grant No. S-LL-19-1).

Abstract-Flexible materials for terahertz optics: advantages of graphite-based structures

Rusnė Ivaškevičiūtė-Povilauskienė, Linas Minkevičius, Domas Jokubauskis, Andrzej Urbanowicz, Simonas Indrišiūnas, and Gintaras Valušis

 THz-TDS transmittance spectra and insets depict THz-CW two-dimensional THz beam profiles focused with a) metal TZP; b) graphite foil TZP; c) graphite on paper TZP; d) pure paper TZP. The beam cross sections at the maximum intensity are presented in a linear scale as a solid black line for each case. Maximum signal amplitude of the TZPs is normalized to the maximum amplitude of the unfocused beam.

https://www.osapublishing.org/ome/abstract.cfm?uri=ome-9-11-4438

Flexible materials for applications in terahertz (THz) range imaging systems are investigated in this study. THz time-domain spectroscopy and THz imaging at 0.6 THz frequency are used to analyze optical properties of zone plates (TZP) with integrated cross-shaped filters, which are fabricated using direct laser writing on thin graphite, HB pencil-shaded graphite on paper, as well as reference metal-based and pure paper zone plates. Spectral features and focusing power comparable to the best metal-based TZP is achieved with graphite-based TZP. The pure paper and paper with pencil-shaded graphite TZPs showed increase in focusing power by a factor of ∼1.5, supporting numerical 3D finite-difference time-domain simulations. The findings show that graphite-based TZPs can serve as a flexible, compact, and inexpensive optics elements for emerging THz imaging systems.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Non-destructive inspection of food and technical oils by terahertz spectroscopy

Mindaugas Karaliūnas, Kinan E. Nasser, Andrzej Urbanowicz, Irmantas Kašalynas, Dalia Bražinskienė, Svajus Asadauskas, Gintaras Valušis

https://www.nature.com/articles/s41598-018-36151-3

Quality control and non-destructive monitoring are of notable interest of food and pharmaceutical industries. It relies on the ability of non-invasive inspection which can be employed for manufacturing process control. We hereby apply terahertz (THz) time-domain spectroscopy as non-destructive technique to monitor pure and degraded oils as well as hydrocarbon chemicals. Significant differences in the spectra of refractive index (RI) and absorption coefficient arising from the presence of ester linkages in the edible and technical oils were obtained. Explicit increase from 1.38 to 1.5 of the RI in all THz spectrum range was observed in hydrocarbons and mono-functional esters with the increase of molar mass. This fact is in contrast of RI dependence on molar mass in multi-functional esters, such as Adipate or vegetable oils, where it is around 1.54. Degradation products, Oleic Acid (OA) and water in particular, lead only to some changes in absorption coefficient and RI spectra of vegetable oils. We demonstrate that complex colloidal and supramolecular processes, such as dynamics of inverse micelles and oil hydrolysis, take part during oil degradation and are responsible for non-uniform dependence of optical properties on extent of degradation.

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-Conducting Helical Structures from Celery Decorated with a Metallic Conjugated Polymer Give Resonances in the Terahertz Range

Anders Elfwing, Carlito S. Ponseca Jr.,Liangqi Ouyang, Andrzej Urbanowicz, Aru–nas Krotkus, Deyu Tu, Robert Forchheimer,Olle Inganäs

http://onlinelibrary.wiley.com/doi/10.1002/adfm.201706595/full

A method to decorate cellulose-based helices retrieved from the plant celery with a conductive polymer is proposed. Using a layer-by-layer method, the decoration of the polyanionic conducting polymer poly(4-(2,3-dihydrothieno [3,4-b]-[1,4]dioxin-2-yl-methoxy)-1-butanesulfonic acid (PEDOT-S) is enhanced after coating the negatively charged cellulose helix with a polycationic polyethyleneimine. Microscopy techniques and two-point probe are used to image the structure and measure the conductivity of the helix. Analysis of the optical and electrical properties of the coated helix in the terahertz (THz) frequency range shows a resonance close to 1 THz and a broad shoulder that extends to 3.5 THz, consistent with electromagnetic models. Moreover, as helical antennas, it is shown that both axial and normal modes are present, which are correlated to the orientation and antenna electrical lengths of the coated helices. This work opens the possibility of designing tunable terahertz antennas through simple control of their dimensions and orientation.