Terahertz time-domain spectroscopy

Terahertz time domain spectroscopic imaging (THz-TDSI) is a non-ionizing, non-contact and nondestructive measurement technique that has been recently utilized to study cultural heritage artifacts. We will present this technique and the results of non-contact measurements of papyrus texts, including images of hidden papyri. Inks for modern papyrus specimens were prepared using the historical binder, Arabic gum, and two common pigments used to write ancient texts, carbon black and red ochre. The samples were scanned in reflection at normal incidence with a pulse with a spectral range between 0.1 and 1.5 THz. The temporal analysis of the signals provides the depth of the layer, and their frequency spectra give information about the inks.

Terahertz time-domain transmission spectroscopy was used to obtain the absorption coefficients and refractive indices of polycrystalline quartz, amorphous silica, Pyrex and BK7 glasses. The results were analyzed in terms of the power-law model of far-infrared absorption. Evidence of the Boson peak was seen in the absorption spectra. Relationships were observed between THz absorption and refractive indices on the one hand, and glass structure and properties on the other. THz TDS is demonstrated to be a useful tool in the study of far-infrared transmission properties of glasses, producing low-noise, high resolution measurements of absorption coefficients and refractive indices.

Phase sensitive measurement techniques, such as THz time-domain spectroscopy or dispersive Fourier transform spectroscopy, are very useful tools to obtain a complete set of optical material parameters. When recording the electric field as a function of time delay between THz and optical pulse, the absorption coefficient and the index of refraction can be extracted. However, the analysis shows ambiguity. Here, we describe an analysis which yields a complete set of mathematical solutions and show how the physically relevant can be deduced. We present a comprehensive mathematical survey for parameter extraction. We have recorded the THz spectra of anthracene and the fatty acid capric acid as examples for weakly absorbing solid samples, and an ionic liquid as an example for a strongly absorbing liquid sample. Finally, we discuss the uncertainty of the obtained optical parameters using error propagation of the Fourier transformation with a simple model and a rigorous mathematical procedure.

Goniometric time-domain spectroscopy (GTDS), employing an ultrashort electromagnetic (EM) pulse technique, has been developed for measuring the dielectric constant of thin films in a broad band of gigahertz to terahertz. An ultrafast optoelectronic system, including an emitter and a detector unit, is constructed with a -2 goniometer. A silicon wafer was analyzed as the reference substrate material. A sharp phase-shift of the reflected EM wave was observed at the Brewster angle of 73.5 for a bare silicon wafer. The phase shift for a film on the Si substrate is relatively smooth due to its two surfaces' providing a complex reflectance. The dielectric constant of the film on Si, related with angular dependency of the phase shift, can be extracted by means of fitting the curve or measuring slope of the curve near the Brewster angle. The measured dielectric constants of FLARE, TiO x , and PZT film are reported.

We compare the generation and decay dynamics of charges and excitons in a model polymer semiconductor ͑MEH-PPV͒ in solution and drop-cast thin films, by recording the sub-ps transient complex conductivity using THz time-domain spectroscopy. The results show that the quantum efficiency of charge generation is two orders of magnitude smaller in solution ͑ϳ10 −5 ͒ than in the solid film ͑ϳ10 −3 ͒. The proximity of neighboring chains in the films apparently facilitates ͑hot͒ exciton dissociation, presumably by allowing the electron and hole to separate on different polymer strands. For both samples, photoexcitation leads to the predominant formation of bound charge pairs ͑excitons͒ that can be detected through their polarizability. Surprisingly, the polarizability per absorbed photon is a factor of 3 larger in solution than in the film, suggesting that interchain interactions in the film do not result in a substantial delocalization of the exciton wave function.

Time-resolved, pulsed terahertz spectroscopy has developed into a powerful tool to study charge carrier dynamics in semiconductors and semiconductor structures over the past decades. Covering the energy range from a few to about 100 meV, terahertz radiation is sensitive to the response of charge quasiparticles, e.g., free carriers, polarons, and excitons. The distinct spectral signatures of these different quasiparticles in the THz range allow their discrimination and characterization using pulsed THz radiation. This frequency region is also well suited for the study of phonon resonances and intraband transitions in low-dimensional systems. Moreover, using a pump-probe scheme, it is possible to monitor the nonequilibrium time evolution of carriers and low-energy excitations with sub-ps time resolution. Being an all-optical technique, terahertz time-domain spectroscopy is contact-free and noninvasive and hence suited to probe the conductivity of, particularly, nanostructured materials that are difficult or impossible to access with other methods. The latest developments in the application of terahertz time-domain spectroscopy to bulk and nanostructured semiconductors are reviewed.

Abstract This paper reports on the pre- and post- conservation diagnostics of a 14th century icon, the Virgin with the Child and a Saint, by an unknown author, belonging to the Library of Taormina, Sicily. A set of non-­invasive imaging techniques for the investigation of the small painting was applied before and after the restoration treatment. The methods used included technical photography (UV Fluorescence, UV Reflected, Infrared, Infrared False Color, and Infrared Fluorescence), Infrared Reflectography (IRR), Reflectance Transformation Imaging (RTI) and two spectroscopic imaging techniques, Terahertz Time Domain Imaging (THz­TDI) and Reflectance Imaging spectroscopy using a Multispectral Imaging system (MSI). At the end of the investigation, an identification of the palette has been attempted, crossing the results obtained by reflectance spectra analysis with those obtained by the other imaging techniques. THz-­TDI examination gave a substantial contribution to the understanding of the gilding technique. An accurate examination of the surface has been performed with Reflectance Transformation Imaging (RTI) and with THz­-TDI to locate cracks, flacking, incisions and material losses. Subsurface layers have been tested by THz-­TDI, imaging the support/­primer/paint interfaces. This study is a valid example of the application of noninvasive imaging methods before and after the conservation treatment of a work of art.

In this work we report on the fluorescence lifetime characterization of the experimentally prepared Tm-doped silica optical fibers with increased quantum conversion efficiency (QE). Optical fibers were drawn from preforms prepared by conventional solution-doping of thulium and aluminium chlorides and by deposition of dispersed alumina nanoparticles with thulium chloride. Prepared preforms and optical fibers were characterized by means of thulium and aluminium concentrations, refractive index profiles, optical spectral attenuations (absorptions) and fluorescence lifetimes. Highly aluminium-codoped optical fiber prepared from alumina nanoparticles exhibited fluorescence lifetime of about 690 μs, which is about 40% higher compared to the conventionally prepared Tm-doped silica fiber. Downloaded From: https://proceedings.spiedigitallibrary.org/ on 01/16/2015 Terms of Use: https://spiedl.org/terms Proc. of SPIE Vol. 9450 945017-2 Downloaded From: https://proceedings.spiedigitallibrary.org/ on 01/16/2015 Terms of Use: https://spiedl.org/terms Proc. of SPIE Vol. 9450 945017-6 Downloaded From: https://proceedings.spiedigitallibrary.org/ on 01/16/2015 Terms of Use: https://spiedl.org/terms

The dielectric constants and loss tangents of KRS-5 and KRS-6 thallium halide mixed crystals have been measured at 95 GHz using both the shorted wavegnide (SWG) reflection method and the Fabry-Perot (F-P) transmission method on samples filfing standard WR-10 waveguide.

Using terahertz time domain spectroscopy we investigate the normal incidence transmission through periodically and randomly arranged planar split ring resonators (SRRs). Introduction of positional disorder in metamaterials has no effect on the quality factor of the fundamental Inductive-Capacitive (LC) resonance. The dipole resonances undergo broadening and shift in their resonance frequencies. The experiment reveals that the randomly distributed SRR structures interact incoherently at LC resonance but couple coherently at the higher frequency dipole resonance.

Using terahertz time domain spectroscopy we investigate the normal incidence transmission through periodically and randomly arranged planar split ring resonators (SRRs). Introduction of positional disorder in metamaterials has no effect on the quality factor of the fundamental Inductive-Capacitive (LC) resonance. The dipole resonances undergo broadening and shift in their resonance frequencies. The experiment reveals that the randomly distributed SRR structures interact incoherently at LC resonance but couple coherently at the higher frequency dipole resonance.

We have measured the frequency dependence and temperature dependence of the optical constants of a liquid crystal 4Ј-n-pentyl-4-cyanobiphenyl ͑5CB͒ in both nematic and isotropic phases by using terahertz time-domain spectroscopy. The extinction coefficient of 5CB is less than 0.02 and without sharp absorption features in the frequency range of 0.2-1.0 THz. The extraordinary and ordinary indices of refraction at 25°C are around 1.77 and 1.58, respectively, giving rise to a birefringence of 0.20Ϯ 0.02 in this frequency range. The temperature-dependent order parameter extracted from birefringence agrees with the results from the visible region quite well.

We study the far infrared electromagnetic response of grating devices by THz time-domain spectroscopy. We first show that THz waves are efficiently injected into silicon waveguides using grating couplers. Moreover, changing the waveguide material parameters by white-light illumination allows us to strongly modify the coupling efficiency. Then we demonstrate resonant effects in segmented grating structures that act as perfect mirrors at selected wavelengths even for focused beams. About 10 periods of the grating participate in the phenomenon, nevertheless the resonance frequency width of the device remains narrow. This collection of experiments shows that millimeter-size mock-ups and THz waves can be effectively used to extrapolate the optical response of micron-size actual devices.

Terahertz time-domain spectroscopy and standoff spectral imaging for detection of explosives and cold and fi re arms
hidden, for example, under clothing, were reviewed. Special attention was paid to different schemes for practical
implementation of these methods. Progress in this direction and existing problems and the prospects for their solution
were discussed. Issues related to sources and receivers of terahertz radiation were briefl y discussed. It was noted
that interest in quantum-cascade lasers as compact sources of terahertz radiation and the potential of using them at
room temperature were increasing.

In the last decades, many research teams working at Terahertz frequencies focused their efforts on surpassing the diffraction limit. Numerous techniques have been investigated, combining methods existing at optic wavelength with THz system such as Time Domain Spectroscopy. The actual development led on one side to a resolution as high as λ/3000 and one the other side to a video-rate recording. The purpose of this paper is to give an overview of the history of the field, to describe the different approaches, to give examples of existing applications and to draw the perspective for this research area.

Terahertz time-domain spectroscopy measurements were made for vertically aligned multi-walled carbon nanotube (VACNT) films. We obtained the frequency dependent complex permittivity and conductivity (on the assumption that permeability l = 1) of several samples exhibiting Drude behaviour for lossy metals. The obtained material properties of VACNT films provide information for potential microwave and terahertz applications. (A. Katsounaros). C A R B O N 5 0 ( 2 0 1 2 ) 9 3 9 -9 4 2 A v ai l abl e a t w w w . s c i e n c e d i r e c t. c o m j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c a r b o n

In this paper, measurements of dielectric properties of fluid-saturated cortical and trabecular bovine bones are presented. Results are reported for native and demineralized states from 80MHz to 1GHz. A non invasive technique using open-ended coaxial lines is proposed, and compared to invasive configurations. Measurements are performed in time domain and data are processed using system identification techniques in continuous and discrete domains. These are very interesting tools for signal processing when working with time domain spectroscopy data. As a validation test for the system identification technique, data are also obtained in the frequency domain. A clear evidence of relaxation processes around 0.2 to 0.4GHz is shown, which may be due to the movement of polar side chains of the collagen fibers. A strong difference between the dielectric properties of native and demineralized bones was found. The experiments here reported aim to contribute to a more in-depth knowledge of the relaxation processes due to the fully hydrated collagen matrix and its relation to the mineralized phase.

We have used terahertz time-domain spectroscopy to investigate photoinduced charge generation in conjugated polymer field-effect transistors. Our measurements show that excitons dissociate in the accumulation layer under the application of a gate voltage, with a quantum efficiency of ϳ0.1 for an average gate field of ϳ1 ϫ 10 8 V m −1 . The transistor history is found to affect the exciton dissociation efficiency, which decreases as holes are increasingly trapped in the accumulation layer. The quantum efficiency of charge formation from excitons is compared with the two contrasting models proposed by Onsager and Arkhipov based on the assumption that field-induced exciton dissociation is assisted by the Brownian diffusive motion or an initial excess energy supplied by excited vibrational modes, respectively.

Using terahertz time-domain spectroscopy (THz-TDS) the complex dielectric response of mixtures of the prototypical ionic liquid [1-butyl-3-methylimidazolium] + :[BF 4 ] À ([bmin]BF 4 ) with water is investigated. The dielectric response can be described well using a two-term Debye model, and these two relaxation processes in the THz region are followed as a function of volume fraction of the constituent liquids. For the mixture, slow and fast Debye relaxation times are larger than the values expected from interpolation of pure water and pure ionic liquid values, pointing to increased structural coherence within the mixture.

Exciton formation and carrier-carrier interactions in semiconductor quantum dots have been investigated by means of terahertz time-domain spectroscopy. The formation of excitons leads to a blueshift of the terahertz absorption as compared to the bare electronic intersublevel absorption. We show that this is a consequence of Coulomb interactions among carriers in the quantum dots. Time-resolved studies reveal the time scale on which photogenerated electron-hole pairs in the GaAs barriers transform into confined excitons in the dots.

Two paintings on metal support have been imaged by terahertz time-domain imaging (THz-TDI) in a reflection setup and the X-ray radiographs were also recorded. The study was performed for testing the terahertz radiation (THz) as an imaging method alternative to X-ray radiography, which suffers several limitations in imaging paint layers on metal support. While the information regarding the paint layers of the paintings was almost lost in the records provided by the X-ray radiography, THz-TDI demonstrates the ability to provide important information about them, despite the presence of the underlying metal.

A simple and precise method based on fixed-point iteration is used to estimate dielectric parameters using terahertz time-domain spectroscopy (THz-TDS). The method converges and gives correct parameters when the sample thickness is greater than 200 mm at a frequency of 0.1 THz or 20 mm at a frequency of 1.0 THz. The technique in validated using measured terahertz data, obtained by probing a sample of high-resistivity silicon.

THz time domain spectroscopy can be used to determine the refractive index and absorption coefficient of materials. The technique uses short THz pulses generated by photo conductive antenna which is switched by a femtosecond laser pulse to produce a short burst of THz radiation. This THz pulse is imaged onto a second photo-conductive antenna, triggered by a delayed femtosecond laser pulse, which acts as receiver in a pump-probe geometry. Hence the electric field (of the THz pulse) is sampled in time by varying the time delay. Since material generally affects the amplitude (absorption) and phase
(refractive index) of the THz waves, the time domain pulse is modified by transmission through the material. The THz waves are also reflected from the interfaces of the material sample, hence the time pulses also carry information about the refractive index of the material (reflection coefficient and phase
changes). Spectral information can be extracted by studying the Fourier transforms of the time pulse. Simple models describing material properties can be used to test the analysis of the recorded data. In the essay the basic concepts of the THz TDS technique will be uncovered and simple model of dielectric material be discussed.

While there are no novel experimental techniques used in this project, it seeks to do complex analysis of the available data using simple tools, namely, Microsoft Excel. New methods were developed and existing methods were employed on Excel to extract data on the spectrum, refractive index and absorption coefficient of samples of KBr.

In this paper, measurements of dielectric properties of fluid-saturated cortical and trabecular bovine bones are presented. Results are reported for native and demineralized states from 80 MHz to 1 GHz. A non invasive technique using open-ended coaxial lines is proposed, and compared to invasive configurations. Measurements are performed in time domain and data are processed using system identification techniques in continuous and discrete domains. These are very interesting tools for signal processing when working with time domain spectroscopy data. As a validation test for the system identification technique, data are also obtained in the frequency domain. A clear evidence of relaxation processes around 0.2 to 0.4 GHz is shown, which may be due to the movement of polar side chains of the collagen fibers. A strong difference between the dielectric properties of native and demineralized bones was found. The experiments here reported aim to contribute to a more in-depth knowledge of the relaxation processes due to the fully hydrated collagen matrix and its relation to the mineralized phase.

We present a comprehensive study of the characteristics of carrier dynamics using temperature dependent terahertz time domain spectroscopy. By utilizing this technique in combination with numerical calculations, the complex refractive index, dielectric function, and conductivity of n-GaN, undoped ZnO NWs, and Al-doped ZnO NWs were obtained. The unique temperature dependent behaviors of major material parameters were studied at THz frequencies, including plasma frequency, relaxation time, carrier concentration and mobility. Frequency and temperature dependent carrier dynamics were subsequently analyzed in these materials through the use of the Drude and the Drude-Smith models.

We design, fabricate, and characterize split-ring resonator ͑SRR͒ based planar terahertz metamaterials ͑MMs͒ on ultrathin silicon nitride substrates for biosensing applications. Proof-of-principle demonstration of increased sensitivity in thin substrate SRR-MMs is shown by detection of doped and undoped protein thin films ͑silk fibroin͒ of various thicknesses and by monitoring transmission changes using terahertz time-domain spectroscopy. SRR-MMs fabricated on thin film substrates show significantly better performance than identical SRR-MMs fabricated on bulk silicon substrates paving the way for improved biological and chemical sensing applications.

Abstract—In this study a European lacquerware replica has been investigated by terahertz (THz) reflectometric imaging. The inspected lacquerware is a wooden panel covered by multiple complex layers of lacquers and plaster. Utilizing pulsed Terahertz Time-Domain Imaging (THz-TDI) in reflection mode, we observe non-invasively buried layers of the lacquerware replica, including the internal structure of the wooden panel itself. We find that non-invasive terahertz reflectometric imaging analysis of lacquerware is can provide conservators with important information about the condition of the compositional layers, potentially aiding in the development of appropriate conservation treatments. With the same technique we have performed a surface material mapping. The material distribution has been enhanced through reflected THz composite RGB false color rendering, where RGB mapping allows distinction between different materials and textures on the surface of the lacquerware. The contrast between different textures is enabled by wavelength-dependent scattering from the surface, as well as differences in the composition of the surface layer.

The potential of terahertz technology has been clearly demonstrated by its large applications in security and defence (remote detection of object). A flexible alternative monochromatic millimeter wave system coupled with an original infrared temperature sensor has been developed to visualize large size 3D manufactured opaque phantoms with different refractive index contrasts. The results clearly illustrate applied terahertz tomography particularities such as boundary effects, refraction and diffraction losses that must be prevented for efficient inspection and detection.

It is shown theoretically and experimentally that for the specific case of an equidistant frequency spacing of semiconductor laser modes, signals similar to terahertz (THz) time domain spectroscopy (TDS) can be detected in a standard photomixer setup. This quasi TDS system approach enables for both, time and frequency domain data processing. Measurements with a THz system which is based on a low cost multimode laser diode are presented. The system exhibits a bandwidth of 600 GHz and can be applied to the classical THz TDS application scenarios.

The ability to assess the quality properties of each pharmaceutical tablet, in the production line during manufacturing, is one of the burning desires of pharmaceutical scientists as well as patients. We have therefore developed and proposed a suit of nondestructive and non-invasive methods based on terahertz timedomain (THz-TD) measurement techniques, allowing us to predict important tablet quality parameters such as porosity, strain, structural parameter, weight and height of training sets of pharmaceutical compacts. A close match was observed between the predicted tablet parameters (i.e. porosity, weight, strain and height) and the experimental data. For the first time, we have introduced and demonstrated a novel structure parameter and an optical strain concept which can offer valuable information on the dissolution rate and mechanical behaviour of pharmaceutical tablets. It is therefore suggested that this preliminary study could be the onset of in-depth future studies geared towards the realisation of a multifunctional terahertz sensor for the screening of each tablet during production.

Pulsed THz (T-ray) spectroscopy is an increasingly common tool for studying materials from physics to biology. Liquid transmission T-ray studies are valuable for understanding solvation dynamics of salts, exploring longrange structure in mixtures and probing biomolecules in suspension. In this paper the uncertainty in parameter estimation of liquid samples is shown to be dependent on the thickness change of the sample, and on the noise of the T-ray spectrometer. For many important liquids, such as water, with high THz absorption, we show that measurement uncertainty can be greatly decreased using a dynamically-modulated liquid sample, using differential T-ray time-domain spectroscopy (DTDS). Preliminary experiments with a novel sample holder, based on a loud speaker, support these calculations. Amplitude and mean detection is used to simultaneously measure two waveforms in DTDS, potentially decreasing the noise by 10 2 .

The side panels of the Franciscan Triptych (St. Jerome, St. John the Baptist, and the Archangel Gabriel and St. Francis, St. Onofrio, and the Virgin Annunciate, by Fra Angelico, before 1429) were scanned by means of terahertz time-domain imaging (THz-TDI). THz analysis supplied information on the stratigraphy of the panel paintings and the associated construction, Bgessoing^ and gilding techniques. Furthermore, THz-TDI provided information regarding the location of restoration materials within the painting stratigraphy on St. Jerome, St. John the Baptist, and the Archangel Gabriel, as well as on the extension and nature of subsurface cracks in the panel painting of St. Francis, St. Onofrio, and the Virgin Annunciate.

Low-frequency torsional vibration modes of the conjugated polyene chain of the biologically important chromophore retinal are of major relevance when studying its photoisomerization dynamics. We present absorptivity and dispersion spectra of three dierent retinal isomers in the far-infrared region between 10 and 100 cm À1 (0.3±3.0 THz) measured by terahertz time-domain spectroscopy at 298 and 10 K. At low temperatures the observed broad absorption bands resolve into narrow peaks correlating to torsional modes of the molecule. Comparison of the absorption spectra of dierent retinal isomers enables the approximate localization of the modes within the molecule. Experimental linewidths and transition strengths are reported. Ó

from crude oils. One of these techniques is coalescence of The electrically induced coalescence of water-in-oil emulsions the water droplets induced by the application of high electric stabilized by interfacially active fractions from crude oils has been field. The application of high electric fields to emulsified studied by means of time domain dielectric spectroscopy at high systems has been utilized since the pioneering work by Cotelectric fields. The experiments were designed with a 2 7-3 reduced trell in 1907 (1). The first commercial electrical coalescers factorial design. Regression analysis clearly shows that the choice were established at a production field in 1909, and Cottrell's of organic solvent and the amount of asphaltenes, as well as the first patents were granted in 1911 (2, 3). Ever since then interplay between these variables, are the most significant paramenumerous improvements have been presented, resulting in a ters for determining the stability of these emulsions. It should be pointed out that the asphaltenes were the only surface active frac-multitude of patents. Much effort has been focused on the tion tested. No interplay between, for instance, asphaltenes and design of new coalescers, in order to obtain optimal condiresins was investigated. The nonlinearity found in the regression tions for the resolution of the dispersed water, with respect analysis is explained by different aggregation states of asphaltenes to the electric field, such as pulse frequency and electric in aliphatic and aromatic solvents. The influence of the variables field intensity. However, few works are directly related to upon the emulsion stability is discussed. ᭧ 1996 Academic Press, Inc.

Terahertz time-domain spectroscopy measurements were made for vertically aligned multi-walled carbon nanotube (VACNT) films. We obtained the frequency dependent complex permittivity and conductivity (on the assumption that permeability l = 1) of several samples exhibiting Drude behaviour for lossy metals. The obtained material properties of VACNT films provide information for potential microwave and terahertz applications. (A. Katsounaros). C A R B O N 5 0 ( 2 0 1 2 ) 9 3 9 -9 4 2 A v ai l abl e a t w w w . s c i e n c e d i r e c t. c o m j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c a r b o n