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This thesis presents studies of novel terahertz photonic devices, including photoconductive optoelectronic devices and guided-wave components, aimed at the development of next-generation terahertz systems. In chapter 2, a scalable interdigitated THz transmitter is designed to increase the output power and compared with a conventional 50 μm coplanar transmitter. In chapter 3, we compare four different receivers with different antenna geometries in terms of bandwidth and sensitivity. Then we describe a photoconductive near-field detector with a subwavelength aperture and its system integration and characterization. In chapter 4, a parallel metal plate waveguide is designed with an integrated step inside the waveguide that can couple to higher order TM modes efficiently from the TEM mode. In this chapter, we also experimentally and numerically study a 2-dimensionally tapered parallel plate waveguide, by which a free-space THz beam can be focused into a deep subwavelength-scale volume. In chapter 5, a parallel thin dielectric film waveguide is used to explore the guiding mechanism of an antiresonant optical reflection waveguide. Cylindrical silica single capillaries and a microstructured capillary, which guide in a similar way, are characterized in terms of mode profiles and attenuation. In chapter 6, we study oblique transmission through freestanding thin nickel films, which are perforated with periodic conical hole arrays. Surface modes can be supported by both metallic surfaces with different nonlinear dispersion curves, which results in spectral interferences in a near-field region when the surface modes couple out of the waveguide into free space.

Developments in semiconductor and laser technologies have facilitated development of terahertz (THz)-frequency (˜2-200 cm-1) technologies. Results published in the literature as far back as the early 20th century demonstrate the utility of this frequency range for myriad applications, but the improved performance of modern THz technologies has renewed interest in THz-frequency analysis. Material presented in this dissertation focuses on three applications of terahertz time-domain spectroscopy (THz-TDS): quantitation of gas-phase molecular species, analysis of polymeric materials, and investigation of dental tissue/composite structures. Gas phase species were quantified individually at concentrations ranging from several parts per million to several parts per thousand using various chemometric methods. Quantitative model robustness was evaluated by comparison of model precision, and partial least-squares (PLS) regressions provided the greatest precision. Species were quantified in mixtures using PLS with errors of prediction below the permissible exposure limits (PELs) set by the Occupational Safety and Health Administration. The effect of spectral broadening as a result of overall sample pressure was investigated, and species were analyzed in mixtures at various overall pressures. Errors of prediction were again near or below the PELs, demonstrating the utility of this method for atmospheric analysis. Chemical selectivity available in THz spectral features was evaluated and compared to selectivity available in infrared frequencies. Spectral parameters measured in the THz frequency range also provide insight into structural properties of polymeric materials. In some cases, spectral peaks may be used to identify the temperature at which phase changes occur within these materials. THz refractive index spectra were found to be a sensitive and non-destructive tool for identification of phase transition temperatures. The time-resolved measurement of THz-TDS makes it particularly useful for rapid, non-destructive analysis of layered structures. Ordinarily, the strength of bonds between dental tissues and composite materials are evaluated in the laboratory using destructive failure analyses. Transparency of dental tissues and composite materials used for restorative procedures to THz pulses allows investigation of interfaces between these materials. Refractive index spectra indicate locations in which delamination has occurred between bonded layers. These results provide an overview of unique capabilities of the THz-TDS method in real-life spectral analyses.

In this thesis, we propose and demonstrate a simple and precise method for measuring mm scaled cell gap by using terahertz time domain spectroscopy (THz-TDS) system. This method allows us to measure the cell gap from 15mm to 1.5mm. In addition, the accuracy of measured thickness for the proposed method is also discussed and analyzed. Meanwhile, a nematic liquid crystal BL006 with birefringence as high as 0.27 in THz frequency range and its optical properties of cholesteric liquid crystal (CLC) as mixing chiral materials are investigated and reported. The ordinary refractive index and average effective refractive index at 20oC are from 1.52 to 1.56 and from 1.61 to 1.64, respectively, in THz frequency ranging from 0.2 THz to 1.4THz. In addition, we also demonstrate that cell filled with CLC is with polarization independent property for THz radiation. Through the 5mm cell filled CLC with diluted concentration of the dopant chiral material for decreasing the critical voltage, an electric controlled polarization independent phase shifter with the modulation depth exceeding 2pi is demonstrated. Furthermore, we also investigate the driving field dependence of phase retardation and discuss the reliability.

In this study, suspensions of metallic nanoparticles in base fluids, nanofluids, are investigated by using terahertz time domain spectroscopy (THz-TDS). Nanofluids are used as the working fluid in a variety of applications especially for the purpose of heat transfer enhancement. Polar fluids are being used as the base in nanofluids for their tendency to stop aggregation and sedimentation. Polar fluids highly absorb THz signal. In order to select the best possible host, various polar liquids have been investigated, and isopropanol (99.5%) is selected to be the best candidate for its low THz absorptivity when compared to ethanol (99.5%), ethylene glycol (99%), methanol (95%) and distilled water. Ag, Pd and Cu nanoparticles have been custom-made in isopropanol by laser ablation method, and the size distributions have been characterized by Zeta Potential Analyzer. The nanoparticle diameters are measured to be on average 10 nm, 12 nm and 75 nm for Ag, Cu and Pd, respectively. Nanofluids of 1X, 2X and 3X concentrations of Ag, Cu and Pd nanoparticles have been prepared by diluting with pure (99.5%) isopropanol. Measurements have been repeated after 7 days up to 12 days in order to check for aggregations and sedimentations. THz-TDS is a strong tool to analyze the refractive index and absorption coefficient, but no distinct difference was observed in the frequency domain analysis for the nanofluid samples. On the other hand, in the time domain data analysis, a shift on the time data with a change in transmission was observed. For Ag nanoparticles a positive time shift with a decrease in transmission with increasing concentration was observed. For Cu nanoparticles an interesting negative time shift and an increase in the intensity was observed with increasing concentration. The Pd nanoparticle solution scans showed almost no shift initially, but a negative time shift after a wait period on the order of days. A model of the transmission of the THz pulse through the nanofluid was developed based on transmission/reflection coefficients due to both dielectric and conducting layered media. The model well explains the positive time shift seen with Ag nanoparticle suspensions but fails to explain the shift seen with the Cu nanoparticle suspensions due to the long path length inside the nanofluid. Negative time-shifts can only be explained by decreasing the path length which suggests additional layering inside the nanofluid medium, or assuming that the chemical composition of the isopropanol host has changed with the addition of Cu and/or Pd nanoparticles. The positive time shifts observed with the Ag nanoparticle suspensions allowed for estimating the change in refractive index of the base fluid. From this change, using effective medium theory based on Maxwell-Garnett model, the concentrations of the nanoparticles were estimated. The results agree within an order of magnitude to commercially available nanofluids which are also non-aggregate.

Disertacijos darbo tikslas buvo sukurti ir ištirti puslaidininkinius terahercinių (THz) impulsų emiterius ir detektorius, skirtus sistemoms, naudojančioms 1 μm bangos ilgio femtosekundinę lazerinę spinduliuotę. THz impulsų generavimo ir detektavimo sistema, kurios optoelektroninius puslaidininkinius komponentus aktyvuoja femtosekundiniai lazerio impulsai, yra plačiai taikoma terahercinėje laikinės srities spektroskopijoje. Tradiciškai tokiose sistemose naudojami Ti:safyre femtosekundiniai lazeriai, kurių spinduliuotės bangos ilgis yra ~800 nm. Šios sistemos nėra patogios dėl jų matmenų, nes lazeriai turi sudėtingą kelių pakopų kaupinimo sistemą. Pastaruoju metu THz impulsų generavimui vis dažniau naudojami femtosekundiniai kietakūniai ir šviesolaidiniai lazeriai, kurių spinduliuotės bangos ilgis patenka į artimosios IR spinduliuotės sritį. Tačiau šios sistemos vis dar neturi tinkamos medžiagos fotolaidiems elementams gaminti, kurie būtų žadinami 1 – 1,55 µm bangos ilgio lazeriais. Tokios medžiagos, visų pirmą, turi būti jautrios optinei spinduliuotei, o jų draustinės energijos tarpas turi atitikti žadinamos spinduliuotės fotonų energiją, be to sluoksniai turi pasižymėti didele tamsine varža bei labai trumpomis krūvininkų gyvavimo trukmėmis (~ 1 ps). Šioje disertacijoje yra pateikiami THz impulsų generavimo panaudojus puslaidininkių paviršius ir fotolaidžias antenas rezultatai, žadinant 1 µm bangos ilgio femtosekundiniais lazerio impulsais. / The aim of dissertation was to develop and explore the semiconductor material terahertz (THz) pulse emitters, for Terahertz time–domain spectroscopy (THz–TDS) systems using a 1 μm wavelength femtosecond laser radiation. THz pulse generation and detection using optoelectronic semiconductor components in THz–TDS excited by femtosecond laser pulses become these days a powerful experimental technique. Traditionally, mode-locked Ti:sapphire lasers emitting at the wavelengths ~800 nm are used. However Ti:sapphire lasers require many-stage optical pumping arrangement, the system is quite bulky and complicated. The solution could be the lasers emitting in 1 – 1.55 µm, which can be directly pumped by diode laser bars. Recently, several compact, efficient and cost-effective solid-state and fiber laser systems that generate femtosecond pulses at near-infrared wavelengths have been developed and employed for activating THz–TDS systems. The main obstacle of these systems is the lack of material with appropriate bandgap, high dark resistivity and short (~ ps) carrier lifetimes.

The aim of dissertation was to develop and explore the semiconductor material terahertz (THz) pulse emitters, for Terahertz time–domain spectroscopy (THz–TDS) systems using a 1 μm wavelength femtosecond laser radiation. THz pulse generation and detection using optoelectronic semiconductor components in THz–TDS excited by femtosecond laser pulses become these days a powerful experimental technique. Traditionally, mode-locked Ti:sapphire lasers emitting at the wavelengths ~800 nm are used. However Ti:sapphire lasers require many-stage optical pumping arrangement, the system is quite bulky and complicated. The solution could be the lasers emitting in 1 – 1.55 µm, which can be directly pumped by diode laser bars. Recently, several compact, efficient and cost-effective solid-state and fiber laser systems that generate femtosecond pulses at near-infrared wavelengths have been developed and employed for activating THz–TDS systems. The main obstacle of these systems is the lack of material with appropriate bandgap, high dark resistivity and short (~ ps) carrier lifetimes. / Disertacijos darbo tikslas buvo sukurti ir ištirti puslaidininkinius terahercinių (THz) impulsų emiterius ir detektorius, skirtus sistemoms, naudojančioms 1 μm bangos ilgio femtosekundinę lazerinę spinduliuotę. THz impulsų generavimo ir detektavimo sistema, kurios optoelektroninius puslaidininkinius komponentus aktyvuoja femtosekundiniai lazerio impulsai, yra plačiai taikoma terahercinėje laikinės srities spektroskopijoje. Tradiciškai tokiose sistemose naudojami Ti:safyre femtosekundiniai lazeriai, kurių spinduliuotės bangos ilgis yra ~800 nm. Šios sistemos nėra patogios dėl jų matmenų, nes lazeriai turi sudėtingą kelių pakopų kaupinimo sistemą. Pastaruoju metu THz impulsų generavimui vis dažniau naudojami femtosekundiniai kietakūniai ir šviesolaidiniai lazeriai, kurių spinduliuotės bangos ilgis patenka į artimosios IR spinduliuotės sritį. Tačiau šios sistemos vis dar neturi tinkamos medžiagos fotolaidiems elementams gaminti, kurie būtų žadinami 1 – 1,55 µm bangos ilgio lazeriais. Tokios medžiagos, visų pirmą, turi būti jautrios optinei spinduliuotei, o jų draustinės energijos tarpas turi atitikti žadinamos spinduliuotės fotonų energiją, be to sluoksniai turi pasižymėti didele tamsine varža bei labai trumpomis krūvininkų gyvavimo trukmėmis (~ 1 ps). Šioje disertacijoje yra pateikiami THz impulsų generavimo panaudojus puslaidininkių paviršius ir fotolaidžias antenas rezultatai, žadinant 1 µm bangos ilgio femtosekundiniais lazerio impulsais.

One-dimensional (1D) nanomaterials are of great importance for a number of potential applications. However, in order to realize this potential a thorough understanding of the charge-carrier dynamics in these materials is required, since these largely determine the optoelectronic properties of the materials in question. This thesis investigates the charge-carrier dynamics of two 1D nanomaterials, single-walled carbon nanotubes (CNTs) and tungsten-oxide nanowires (WOxNWs), with the goal of better understanding the nature of their optoelectronic responses, and how nanomaterial geometry and morphology influence these responses. We do this using terahertz time-domain spectroscopy (THz-TDS) and optical pump - terahertz probe time-domain spectroscopy (OPTP). Firstly, we discuss how to properly analyse and interpret the data obtained from these experiments when measuring 1D nanomaterials. While the data obtained from THz-TDS is fairly straight-forward to analyse, OPTP experimental data can be far from trivial. Depending on the relative size of the sample geometry compared to the probe wavelength, various approximations can be used to simplify the extraction of their ultrafast response. We present a general method, based on the transfer matrix method, for evaluating the applicability of these approximations for a given multilayer structure, and show the limitations of the most commonly used approximations. We find that these approximations are only valid in extreme cases where the thickness of the sample is several orders of magnitude smaller or larger than the wavelength, which highlight the danger originating from improper use of these approximations. We then move on to investigate how the charge-carrier dynamics of our CNTs is influenced by nanotube length and density. This is done through studying the nature of the broad THz resonance observed in finite-length CNTs, and how the nanotube length and density affects this resonance. We do this by measuring the conductivity spectra of thin films comprising bundled CNTs of different average lengths in the frequency range 0.3-1000 THz and temperature interval 10-530 K. From this we show that the observed temperature-induced changes in the terahertz conductivity spectra depend strongly on the average CNT length, with a conductivity around 1 THz that increases/decreases as the temperature increases for short/long tubes. This behaviour originates from the temperature dependence of the electron scattering rate, which results in a subsequent broadening of the observed THz conductivity peak at higher temperatures and a shift to lower frequencies for increasing CNT length. Finally, we show that the change in conductivity with temperature depends not only on tube length, but also varies with tube density. We record the effective conductivities of composite films comprising mixtures of WS2 nanotubes and CNTs vs CNT density for frequencies in the range 0.3-1 THz, finding that the conductivity increases/decreases for low/high density films as the temperature increases. This effect arises due to the density dependence of the effective length of conducting pathways in the composite films, which again leads to a shift and temperature dependent broadening of the THz conductivity peak. Next, we investigate the conflicting reports regarding the ultrafast photoconductive response of films of CNTs, which apparently exhibit photoconductivities that can vastly differ, even in sign. Here we observe explicitly that the THz photoconductivity of CNT films is a highly variable quantity which correlates with the length of the CNTs, while the specific type of CNT has little influence. Moreover, by comparing the photo-induced change in THz conductivity with heat-induced changes, we show that both occur primarily due to heat-generated modification of the Drude electron relaxation rate, resulting in a broadening of the plasmonic resonance present in finite-length metallic and doped semiconducting CNTs. This clarifies the nature of the photo-response of CNT films and demonstrates the need to carefully consider the geometry of the CNTs, specifically the length, when considering them for application in optoelectronic devices. We then move on to consider our WOxNWs. We measure the terahertz conductivity and photoconductivity spectra of thin films compromising tungsten-oxide (WOx) nanowires of average diameters 4 nm and 100 nm, and oxygen deficiencies WO2.72 and WO3 using THz-TDS and OPTP. From this we present the first experimental evidence of a metal-to-insulator transition in WOx nanowires, which occurs when the oxygen content is increased from x=2.72 -> 3 and manifests itself as a massive drop in the THz conductivity due to a shift in the Fermi level from the conduction band down into the bandgap. Furthermore we present the first experimental measurements of the photoexcited charge-carrier dynamics of WOx nanowires on a picosecond timescale and map the influence of oxygen-content and nanowire diameter. From this we show that the decay-dynamics of the nanowires is characterized by a fast decay of < 1 ps, followed by slow decay of 3-10 ps, which we attribute to saturable carrier trapping at the surface of the nanowires.

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No / The terahertz frequency spectrum of pentaerythritol tetranitrate (PETN) is calculated using Discover[1] with the COMPASS[2] force field, CASTEP[3] and PWscf.[4] The calculations are compared to each other and to terahertz spectra (0.3-3 THz) of crystalline PETN recorded at 4 K. A number of analysis methods are used to characterise the calculated normal modes.