Academic literature on the topic 'Terahertz; spectroscopy; beam splitter'

Abstract We propose a terahertz metasurface with chirality induced by surface lattice resonance to create a narrowband circular polarization beam splitter (PBS) with large spectral tunability in both transmission and reflection modes. Results show that strong circular dichroism effects can be observed in two spectrally narrow bands, and thus a dual-band circular PBS can be achieved. We show that surface lattice resonance induces much narrower and stronger circular dichroism effects than localized resonance, resulting in higher polarization extinction ratios, higher quality factors, and more circular polarization states. The narrowband operation frequency of lattice-induced PBS with extinction ratio larger than 10 dB can be tuned over a large spectral range, from 1.6 THz to 2.3 THz, by varying the incidence angle. We expect the proposed strong, narrowband, and spectrally tunable circular PBS will find applications in polarization-dependent systems including imaging, spectroscopy, sensing and telecommunication in the terahertz regime.

Wire-grid polarizers constitute a traditional component for the control of polarization in free-space devices that operate in a broad part of the electromagnetic spectrum. Here, we present an aluminium-based THz wire grid polarizer, fabricated on a sub-wavelength thin flexible and conformal foil of Zeonor polymer having a thickness of 40um. The fabricated device,characterized by means of THz time-domain spectroscopy, exhibitsa high extinction ratio between 30 and 45dB in the 0.3-2.1THz range. The insertion losses oscillate between 0 and 1.1dB andthey stemalmost exclusively from moderate Fabry-Perót reflections and it is engineered forvanishing at 2THz for operation with quantum cascade lasers. Full Text: PDF ReferencesI. F. Akyildiz, J. M. Jornet, C. Han, "Terahertz band: Next frontier for wireless communications", Phys. Commun. 12, 16 (2014). CrossRef M.C. Kemp, P.F. Taday, B.E. Cole, J.A. Cluff, A.J. Fitzgerald, W.R. Tribe, "Security applications of terahertz technology", Proc. SPIE 5070, 44 (2003). CrossRef M. Schirmer, M. Fujio, M. Minami, J. Miura, T. Araki, T. Yasui, "Biomedical applications of a real-time terahertz color scanner", Biomed. Opt. Express 1, 354 (2010). CrossRef R.P. Cogdill, R.N. Forcht, Y. Shen, P.F. Taday, J.R. Creekmore, C.A. Anderson, J.K. Drennen, "Comparison of Terahertz Pulse Imaging and Near-Infrared Spectroscopy for Rapid, Non-Destructive Analysis of Tablet Coating Thickness and Uniformity", J. Pharm. Innov. 2, 29 (2007). CrossRef Y.-C. Shen, "Terahertz pulsed spectroscopy and imaging for pharmaceutical applications: A review", Int. J. Pharm. 417, 48(2011). CrossRef A.G. Davies, A.D. Burnett, W. Fan, E.H. Linfield, J.E. Cunningham, "Terahertz spectroscopy of explosives and drugs", Mater. Today 11, 18 (2008). CrossRef J.F. Federici, B. Schulkin, F. Huang, D. Gary, R. Barat, F. Oliveira, D. Zimdars, "THz imaging and sensing for security applications?explosives, weapons and drugs", Semicond. Sci. Technol. 20, S266 (2005). CrossRef D. Saeedkia, Handbook of Terahertz Technology for Imaging, Sensing and Communications (Elsevier, 2013).N. Born, M. Reuter, M. Koch, M. Scheller, "High-Q terahertz bandpass filters based on coherently interfering metasurface reflections", Opt. Lett. 38, 908 (2013). CrossRef A. Ferraro, D.C. Zografopoulos, R. Caputo, R. Beccherelli, "Periodical Elements as Low-Cost Building Blocks for Tunable Terahertz Filters", IEEE Photonics Technol. Lett. 28, 2459 (2016). CrossRef A. Ferraro, D.C. Zografopoulos, R. Caputo, R. Beccherelli, "Broad- and Narrow-Line Terahertz Filtering in Frequency-Selective Surfaces Patterned on Thin Low-Loss Polymer Substrates", IEEE J. Sel. Top. Quantum Electron. 23 (2017). CrossRef B. S.-Y. Ung, B. Weng, R. Shepherd, D. Abbott, C. Fumeaux, "Inkjet printed conductive polymer-based beam-splitters for terahertz applications", Opt. Mater. Express 3, 1242 (2013). CrossRef J.-S. Li, D. Xu, J. Yao, "Compact terahertz wave polarizing beam splitter", Appl. Opt. 49, 4494 (2010). CrossRef K. Altmann, M. Reuter, K. Garbat, M. Koch, R. Dabrowski, I. Dierking, "Polymer stabilized liquid crystal phase shifter for terahertz waves", Opt. Express 21, 12395 (2013). CrossRef D.C. Zografopoulos, R. Beccherelli, "Tunable terahertz fishnet metamaterials based on thin nematic liquid crystal layers for fast switching", Sci. Rep. 5, 13137 (2015). CrossRef G. Isić, B. Vasić, D. C. Zografopoulos, R. Beccherelli, R. Gajić, "Electrically Tunable Critically Coupled Terahertz Metamaterial Absorber Based on Nematic Liquid Crystals", Phys. Rev. Appl. 3, 064007 (2015). CrossRef K. Iwaszczuk, A.C. Strikwerda, K. Fan, X. Zhang, R.D. Averitt, P.U. Jepsen, "Flexible metamaterial absorbers for stealth applications at terahertz frequencies", Opt. Express 20, 635 (2012). CrossRef F. Yan, C. Yu, H. Park, E.P.J. Parrott, E. Pickwell-MacPherson, "Advances in Polarizer Technology for Terahertz Frequency Applications", J. Infrared Millim. Terahertz Waves 34, 489 (2013). CrossRef http://www.tydexoptics.com DirectLink K. Imakita, T. Kamada, M. Fujii, K. Aoki, M. Mizuhata, S. Hayashi, "Terahertz wire grid polarizer fabricated by imprinting porous silicon", Opt. Lett. 38, 5067 (2013). CrossRef A. Isozaki, et al., "Double-layer wire grid polarizer for improving extinction ratio", Solid-State Sens. Actuators Microsyst. Transducers Eurosensors XXVII 2013 Transducers Eurosensors XXVII 17th Int. Conf. On, IEEE, pp. 530?533 (2013). DirectLink A. Ferraro, D. C. Zografopoulos, M. Missori, M. Peccianti, R. Caputo, R. Beccherelli, "Flexible terahertz wire grid polarizer with high extinction ratio and low loss", Opt. Lett. 41, 2009(2016). CrossRef M.S. Vitiello, G. Scalari, B. Williams, P.D. Natale, "Quantum cascade lasers: 20 years of challenges", Opt. Express 23, 5167(2015). CrossRef A. Podzorov, G. Gallot, "Low-loss polymers for terahertz applications", Appl. Opt. 47, 3254(2008). CrossRef

The beam splitter is an important functional device due to its ability to steer the propagation of electromagnetic waves. The split-ratio-variable splitter is of significance for optical, terahertz and microwave systems. Here, we are the first (to our knowledge) to propose an optically controlled dynamic beam splitter with adjustable split ratio in the terahertz region. Based on the metasurface containing two sets of reversed phase-gradient supercells, we split the terahertz wave into two symmetrical beams. Associated with the reconfigurable pump laser pattern programmed with the spatial light modulator, dynamic modulation of the split ratio varying from 1:1 to 15:1 is achieved. Meanwhile, the beam splitter works at a split angle of 36° for each beam. Additionally, we obtain an exponential relationship between the split ratio and the illumination proportion, which can be used as theoretical guidance for beam splitting with an arbitrary split ratio. Our novel beam splitter shows an outstanding level of performance in terms of the adjustable split ratio and stable split angles and can be used as an advanced method to develop active functional devices applied to terahertz systems and communications.

In dieser Arbeit wird das Wachstum von dünnen quasi-kristallinen Ge-Sb-Te (GST) Schichten mittels Molekularstrahlepitaxie demonstriert, die zu einer geordneten Konfiguration von intrinsischen Kristallgitterfehlstellen führen. Es wird gezeigt, wie es eine Strukturanalyse basierend auf Röntgenstrahlbeugungssimulationen, Dichtefunktionaltheorie und Transmissionselektronenmikroskopie ermöglicht, eine eindeutige Beurteilung der Kristallgitterlückenanordnung in den GST-Proben vorzunehmen. Das Verständnis für die Ordnungsprozesse der Gitterfehlstellen erlaubt eine gezielte Einstellung des Ordnungsgrades selbst, der mit der Zusammensetzung und der Kristallphase des Materials in Zusammenhang steht. Auf dieser Basis wurde ein Phasendiagramm mit verschiedenen Wachstumsfenstern für GST erstellt. Des Weiteren wird gezeigt, dass man eine hohe Ordnung der Gitterfehlstellen in GST auch durch Ausheizprozesse und anhand von Femtosekunden-gepulster Laserkristallisation von amorphem Material erhält, das zuvor auf einem als Kristallisationsgrundlage dienenden Substrat abgeschiedenen wurde. Diese Erkenntnis ist bemerkenswert, da sie zeigt, dass sich kristalline GST Schichten mit geordneten Kristallgitterlücken durch verschiedene Herstellungsprozesse realisieren lassen. Darüber hinaus wurde das Wachstum von GeTe/Sb2Te3 Übergittern durchgeführt, deren Struktur die von GST mit geordneten Gitterfehlstellen widerspiegelt. Die Möglichkeit den Grad der Gitterfehlstellenordung in GST gezielt zu manipulieren wurde mit einer Studie der Transporteigenschaften kombiniert. Die Anwendung von großflächigen Charakterisierungsmethoden wie XRD, Raman und IR-Spektroskopie, erlaubte die Bestimmung der Phase und des Fehlstellenordnungsgrades von GST und zeigte eindeutig, dass die Fehlstellenordnung für den Metall-Isolator-Übergang (MIT) verantwortlich ist. Insbesondere wird durch das Vergleichen von XRD-Messungen mit elektrischen Messungen gezeigt, dass der Übergang von isolierend zu leitend erfolgt, sobald eine Ordnung der Kristallgitterlücken einsetzt. Dieses Phänomen tritt in der kubischen Kristallphase auf, wenn Gitterfehlstellen in GST von einem ungeordneten in einen geordneten Zustand übergehen. Im zweiten Teil des Kapitels wird eine Kombination aus FIR- und Raman-Spektroskopie zur Untersuchung der Vibrationsmoden und des Ladungsträgerverhaltens in der amorphen und der kristallinen Phase angewendet, um Aktivierungsenergien für die Elektronenleitung, sowohl für die kubische, als auch für die trigonale Kristallphase von GST zu bestimmen. Hier ist es wichtig zu erwähnen, dass, in Übereinstimmung mit Ergebnissen aus anderen Untersuchungen, das Auftauchen eines MIT beim Übergang zwischen der ungeordneten und der geordneten kubischen Phase beobachtet wurde. Schlussendlich wurden verschiedene sogenannte Pump/Probe Technik, bei der man das Material mit dem Laser anregt und die Röntgenstrahlung oder Terahertz (THz)-spektroskopie als Sonde nutzt, angewandt. Dies dient um ultra-schnelle Dynamiken zu erfassen, die zum Verständnis der Umschaltmechanismen nötig sind. Die Empfindlichkeit der THz-Messungen hinsichtlich der Leitfähigkeit, sowohl in GST, als auch in GeTe/Sb2Te3 Übergittern zeigte, dass die nicht-thermische Natur der Übergitterumschaltprozesse mit Grenzflächeneffekten zusammenhängt und . Der Ablauf wird mit beeindruckender geringer Laser-Fluenz erreicht. Dieses Ergebnis stimmt mit Berichten aus der Literatur überein, in denen ein Kristall-zu Kristallwechsel von auf Übergittern basierenden Speicherzellen für effizienter gehalten wird als GST Schmelzen, was zu einen ultra-schwachen Energieverbrauch führt.
The growth by molecular beam epitaxy of Ge-Sb-Te (GST) alloys resulting in quasi-single-crystalline films with ordered configuration of intrinsic vacancies is demonstrated. It is shown how a structural characterization based on transmission electron microscopy, X-ray diffraction and density functional theory, allowed to unequivocally assess the vacancy ordering in GST samples, which was so far only predicted. The understanding of the ordering process enabled the realization of a fine tuning of the ordering degree itself, which is linked to composition and crystalline phase. A phase diagram with the different growth windows for GST is obtained. High degree of vacancy ordering in GST is also obtained through annealing and via femtosecond-pulsed laser crystallization of amorphous material deposited on a crystalline substrate, which acts as a template for the crystallization. This finding is remarkable as it demonstrates that it is possible to create a crystalline GST with ordered vacancies by using different fabrication procedures. Growth and structural characterization of GeTe/Sb2Te3 superlattices is also obtained. Their structure resembles that of ordered GST, with exception of the Sb and Ge layers stacking sequence. The possibility to tune the degree of vacancy ordering in GST has been combined with a study of its transport properties. Employing global characterization methods such as XRD, Raman and Far-Infrared spectroscopy, the phase and ordering degree of the GST was assessed, and unequivocally demonstrated that vacancy ordering in GST drives the metal-insulator transition (MIT). In particular, first it is shown that by comparing electrical measurements to XRD, the transition from insulating to metallic behavior is obtained as soon as vacancies start to order. This phenomenon occurs within the cubic phase, when GST evolves from disordered to ordered. In the second part of the chapter, a combination of Far-Infrared and Raman spectroscopy is employed to investigate vibrational modes and the carrier behavior in amorphous and crystalline phases, enabling to extract activation energies for the electron conduction for both cubic and trigonal GST phases. Most important, a MIT is clearly identified to occur at the onset of the transition between the disordered and the ordered cubic phase, consistently with the electrical study. Finally, pump/probe schemes based on optical-pump/X-ray absorption and Terahertz (THz) spectroscopy-probes have been employed to access ultrafast dynamics necessary for the understanding of switching mechanisms. The sensitivity of THz-probe to conductivity in both GST and GeTe/Sb2Te3 superlattices showed that the non-thermal nature of switching in superlattices is related to interface effects, and can be triggered by employing up to one order less laser fluences if compared to GST. Such result agrees with literature, in which a crystal to crystal switching of superlattice based memory cells is expected to be more efficient than GST melting, therefore enabling ultra-low energy consumption.

Terahertz time-domain spectroscopy (THz-TDS) systems have been generally limited to a single mode of operation, either in transmission or reflection geometries. The possibility of systems able to operate simultaneously in both geometries opens new possibilities for material characterisation. This Thesis designs and characterises a novel system able to simultaneously capture spectra from samples at normal incidence transmission and reflection. This enables materials that are opaque and/or partially reflective, as well as materials that exhibit non-unity values of permittivity and/or permeability to be thoroughly investigated. In addition to a dual geometry system, this Thesis presents two novel beam-splitters useable in the terahertz (THz) range of frequencies from 0.1 to 10 THz. Optical components in the THz frequency range have been limited, with ongoing developments being made to fabricate and characterise lenses, polarizers and waveguides, with beam-splitters that are polarization dependent. The presented original contributions include a low-cost beam-splitter fabricated from an ultra-thin polymer substrate and silver paint, and a novel beam-splitter fabricated from conductive polymers. These beam-splitters provide a near frequency and polarization independent response. An introductory background into THz-TDS along with generation and detection methods are also offered as part of this Thesis. Four auxiliary investigations are also described in the appendices: (i) a dual scanning THz-TDS system, to improve acquisition times, (ii) a mini investigation into food quality control using THz-TDS, (iii) an investigation into security applications for THz-TDS and (iv) second harmonic generation (SHG) using a β radiation damaged barium borate (BBO) crystal and a Ti:Sapphire laser.
Thesis (Ph.D.) -- University of Adelaide, School of Electrical and Electronic Engineering, 2013