THz Spectrometer Research Articles

High-precision measurements of terahertz polarization states with a fiber coupled time-domain THz spectrometer

We present a new method for high precision measurements of polarization rotation in the frequency range from 0.2 to 2.2 THz using a fiber coupled time-domain THz spectrometer. A free standing wire-grid polarizer splits THz light into orthogonal components that are then measured by two separate detectors simultaneously. We theoretically model the uncertainties introduced by optical component non-idealities and predict that we may expect to achieve accuracies of 0.8% when anti-symmetrizing the response with respect to an applied field. Anti-symmetrization improves accuracy by more than four orders of magnitude. We demonstrate this method on a 2D electron gas in magnetic field and show that we achieve a precision of 20 μrad (1.1 mdeg) for small polarization rotation angles. A detailed description of the technique and data analysis procedure is provided, demonstrating its capability to precisely measure polarization states in the 0.2 to 2.2 THz range.

Cavity assisted high-resolution THz spectrometer

The analysis of gases by THz radiation offers a high degree of discrimination due to the narrow linewidths that are observed at low pressure. The sensitivity of existing high-resolution instruments is limited by the availability and performance of critical system components. This study uses two key components with physical structures at the wavelength scale to realise a high finesse THz cavity. The cavity is characterised and incorporated into a spectrometer. Sensitivity limits of the instrument are experimentally demonstrated for trace and pure gases. Both CEAS (Cavity Enhanced Absorption Spectroscopy) and CRDS (Cavity Ring-Down Spectroscopy) configurations are shown to give sub-ppm detection levels. The cavity has also been used to measure the atmospheric losses.

In situ terahertz reflective spectral detection of biological tissues using a transceiver‐microprobe

AbstractTerahertz (THz) reflective spectroscopy is suitable and promising for biomedical applications due to its potential to evolve into an in situ detection modality. Here, we demonstrate this feasibility by using a micrometer‐sized photoconductive transceiver probe, in association with a novel home‐built THz reflection‐mode system to achieve simple optical path adjustment, and to closely approach the biological tissue surface to be probed, without additional measures to ensure the smoothness of the surface under test. Experimentally, a fresh sample consisting of muscle and adipose of porcine tissue was directly detected and successfully distinguished, showing the potential of in situ biomedical detection using this micro transceiver probe‐based THz spectrometer.

On-Chip Time-Domain Terahertz Spectroscopy of Superconducting Films below the Diffraction Limit.

Free-space time domain THz spectroscopy accesses electrodynamic responses in a frequency regime ideally matched to interacting condensed matter systems. However, THz spectroscopy is challenging when samples are physically smaller than the diffraction limit of ∼0.5 mm, as is typical, for example, in van der Waals materials and heterostructures. Here, we present an on-chip, time-domain THz spectrometer based on semiconducting photoconductive switches with a bandwidth of 200 to 750 GHz. We measure the optical conductivity of a 7.5-μm wide NbN film across the superconducting transition, demonstrating spectroscopic signatures of the superconducting gap in a sample smaller than 2% of the Rayleigh diffraction limit. Our spectrometer features an interchangeable sample architecture, making it ideal for probing superconductivity, magnetism, and charge order in strongly correlated van der Waals materials.

ИССЛЕДОВАНИЕ СОСТАВА МЕТАБОЛИТОВ ТКАНЕЙ ЛОР-ОРГАНОВ МЕТОДОМ ТГЦ СПЕКТРОСКОПИИ ВЫСОКОГО РАЗРЕШЕНИЯ

Recently, physical and physicochemical methods of analysis, such as gas chromatography, mass spectrometry, IR and THz spectroscopy, are increasingly used in medical diagnostics. With their help, it is possible to identify substances, the so-called metabolites, characteristic of a particular disease. Compiling a metabolic profile will speed up and facilitate the process of diagnosing a disease, assess the stage and etiology, and predict therapy. It is relevant to study the state of the ear-nose-throat (ENT) organs, as one of the main ways of penetration into the human body of infections, viruses, etc. But in the literature and the database of human metabolomes, there are no data on metabolites in diseases of the ENT organs. The work is devoted to analyzing the metabolites composition of the ENT organs tissues by high-resolution THz spectroscopy and the comparison of the metabolite’s composition, which are formed during thermal decomposition of a relatively healthy mucosa, a sphenoid sinus polyp and a cyst of the maxillary sinus. The studies were carried out using high-resolution THz spectrometers developed by the authors, operating in the range of 118-178 GHz. Substances were identified using electronic databases. A qualitative analysis of the chemical composition of the thermal decomposition products and a comparison of the obtained spectra of substances and the number of absorption lines of samples to evaluate the metabolites were performed. Analysis of the results showed that all samples contained substances such as alcihols, nitriles, sulfur compounds. However, in the spectrum of a relatively healthy mucosa, the number of absorption lines of these substances is much less than in the spectra of a polyp and a cyst, which indicates their lower concentration. In addition, absorption lines appear in the spectra of the polyp and cyst, corresponding to substances such as ketones, nitriles, aldehydes, acids, at that their concentrations differ significantly. Thus, the potential possibility of using high-resolution THz spectroscopy for the analysis of the composition of metabolites of ENT tissues has been shown.

Sub-terahertz silicon-based on-chip absorption spectroscopy using thin-film model for biological applications

Spectroscopy in the sub-terahertz (sub-THz) range of frequencies has been utilized to study the picosecond dynamics and interaction of biomolecules. However, widely used free-space THz spectrometers are typically limited in their functionality due to low signal-to-noise ratio and complex setup. On-chip spectrometers can revolutionize THz spectroscopy allowing integration, compactness, and low-cost fabrication. In this paper, a low-loss silicon-based platform is proposed for on-chip sub-THz spectroscopy. Through functionalization of silicon chip and immobilization of bio-particles, we demonstrate the ability to characterize low-loss nano-scale biomolecules across the G-band (0.14–0.22 THz). We also introduce an electromagnetic thin-film model to account for the loading effect of the immobilized biomolecules, i.e. dehydrated streptavidin and immunoglobulin antibody, as two key molecules in the biosensing discipline. The proposed platform was fabricated using a single mask micro-fabrication process, and then measured by a vector network analyzer (VNA), which offers high dynamic range and high spectral resolution measurements. The proposed planar platform is general and paves the way towards low-loss, cost-effective and integrated sub-THz biosensors for the detection and characterization of biomolecules.

Tunable cryogenic terahertz cavity for strong light-matter coupling in complex materials.

We report here the realization and commissioning of an experiment dedicated to the study of the optical properties of light-matter hybrids constituted of crystalline samples embedded in an optical cavity. The experimental assembly developed offers the unique opportunity to study the weak and strong coupling regimes between a tunable optical cavity in cryogenic environment and low energy degrees of freedom, such as phonons, magnons, or charge fluctuations. We describe here the setup developed that allows for the positioning of crystalline samples in an optical cavity of different quality factors, the tuning of the cavity length at cryogenic temperatures, and its optical characterization with a broadband time domain THz spectrometer (0.2-6 THz). We demonstrate the versatility of the setup by studying the vibrational strong coupling in CuGeO3 single crystal at cryogenic temperatures.

Optical Spectrometer With a Pulse-to-Pulse Resolution for Terahertz and mm-Wave Signals

The development of ultracompact particle accelerators and accelerator-based high-power THz generators are some of the top priorities for research and development programs around the world. These tools require the development of accelerating structures, operating in the mm-Wave and THz range, where diagnostics are not available or are insufficient. This article presents the design of a pulse-to-pulse THz spectrometer, primarily developed for diagnostics of accelerating structures and THz radiation sources. In particular, this spectrometer can be used to detect RF pulse shortening caused by vacuum breakdown and beam misalignment in THz accelerators, as well as for bunch length monitoring and radiation source diagnostics. The spectrometer is based on a diffraction grating and is capable of covering a frequency range where RF-based breakdown measurements are not possible. We have built and tested the first prototype with different sources in a frequency range of 0.1–1.0 THz. In this article, we present the physical design, supported numerical simulations, electronics development, and test results.

Фотопроводящий ТГц детектор на основе новых функциональных слоев в многослойных гетероструктурах

We report on the experimental study of the photoconductive antennas (PCAs) - detectors based on superlattice heterostructures (SLS) InGaAs/InAs/InAlAs with different types of elastic stresses in their functional layers. By using our laboratory time-domain THz spectrometer we measured and compared the detected THz signals, noise characteristics, and signal-to-noise ratios of the developed bow-tie PCA-detectors at different average probe power. We showed that the SLS-based PCA-detector with elastic stresses of both compression and tension in the layers demonstrates increased THz detection bandwidth compared to that for the SLS-based PCA-detector with only compression stresses in the SLS layers in the whole range of the optical probe power. We thus demonstrate that modification of the SLS via introduction of the elastic strain in the crystalline lattice of its layers could become an efficient approach to enhance the PCA-detectors performance leading to implementation of these PCAs to the spectroscopic THz setups.

Reference-free THz-TDS conductivity analysis of thin conducting films.

We present a reference-free method to determine electrical parameters of thin conducting films by steady state transmission-mode terahertz time-domain spectroscopy (THz-TDS). We demonstrate that the frequency-dependent AC conductivity of graphene can be acquired by comparing the directly transmitted THz pulse with a transient internal reflection within the substrate which avoids the need for a standard reference scan. The DC sheet conductivity, scattering time, carrier density, mobility, and Fermi velocity of graphene are retrieved subsequently by fitting the AC conductivity with the Drude model. This reference-free method was investigated with two complementary THz setups: one commercial fibre-coupled THz spectrometer with fast scanning rate (0.2-1.5 THz) and one air-plasma based ultra-broadband THz spectrometer for greatly extended frequency range (2-10 THz). Certain propagation correction terms for more accurate retrieval of electrical parameters are discussed.

A THz Spectrometer Using Band Pass Filters

We describe a THz spectrometer operating between 1.2 and 10.5 THz, consisting of band pass filters made by metasurfaces. The source is made of a 10 W small black body. The detector is a high sensitivity room temperature pyroelectric sensor. Various techniques that are used to prepare samples are described. The spectra obtained are compared with those measured with a Fourier Transformer Infrared Spectrometer on the same samples. Our instrument, which uses commercial technologies available at the present time, can constitute an economical alternative to very expensive spectrometers. It has already been successfully used, obtaining precise spectroscopic measurements on many inorganic powders.

A superconducting flux-flow oscillator of terahertz range

We have elaborated, fabricated and tested a THz source radiating to open space based on the superconducting flux-flow oscillator (FFO). In this concept, the oscillator is integrated with the transmitting lens antenna based on a slot structure in Nb film with a thickness of ~200 nm located on the same chip. The slot planar antenna is matched to the oscillator (by input) and to the semielliptical Si lens with a diameter of 10 mm (by output) providing a narrow output beam of THz emission. A harmonic mixer based on the superconductor-insulator-superconductor junction embedded in the “FFO and antenna” integrated structure has been used for the phase locking of the oscillator. Several designs of antenna coupled with the oscillator by microstrip lines have been numerically simulated, and the batches of experimental samples based on Nb-AlN-NbN superconducting trilayers with Rn ·A ~ 20 Ω·μm2 (jc ~ 10 kA/cm2) have been fabricated and tested. Two different setups were used for experimental study: a THz spectrometer based on the SIS receiver with a high spectral resolution (better than 0.1 MHz) and a Si bolometer. The overall operating range of 250 to 700 GHz is covered by all the developed designs.

Quantum cascade laser based hybrid dual comb spectrometer

Four-wave-mixing-based quantum cascade laser frequency combs (QCL-FC) are a powerful photonic tool, driving a recent revolution in major molecular fingerprint regions, i.e. mid- and far-infrared domains. Their compact and frequency-agile design, together with their high optical power and spectral purity, promise to deliver an all-in-one source for the most challenging spectroscopic applications. Here, we demonstrate a metrological-grade hybrid dual comb spectrometer, combining the advantages of a THz QCL-FC with the accuracy and absolute frequency referencing provided by a free-standing, optically-rectified THz frequency comb. A proof-of-principle application to methanol molecular transitions is presented. The multi-heterodyne molecular spectra retrieved provide state-of-the-art results in line-center determination, achieving the same precision as currently available molecular databases. The devised setup provides a solid platform for a new generation of THz spectrometers, paving the way to more refined and sophisticated systems exploiting full phase control of QCL-FCs, or Doppler-free spectroscopic schemes.

Self and N2 broadening coefficients of H2S probed by submillimeter spectroscopy: Comparison with IR measurements and semi-classical calculations

High-resolution lines of H2S self-perturbed and perturbed by N2 have been recorded at room temperature using a frequency modulated THz spectrometer at pressures of H2S ranging from 0.1 to 1.0 mbar. A non-linear least squares fitting of these spectra has been performed to retrieve self- and N2-broadening coefficients of pure rotational transitions (mainly in the RQ sub-branch) of H2S in the submillimeter region. We show that the lineshapes are well reproduced by the Speed-Dependent Voigt profile accounting for the speed dependence of relaxation rates which display linear pressure dependencies. Using the Robert and Bonamy formalism, these coefficients were calculated, showing the need to account for all contributions of the intermolecular interaction for N2-broadening. The calculated coefficients are consistent with the measured data and the average accuracy of the line parameters has been estimated to be about 5%. The observed J″ and Ka″ rotational dependences have been analyzed using an empirical model and compared to previous rovibrational studies in the ν2 fundamental band. In our study the self- and the N2-broadening coefficients decreases monotonically with Ka″ and J″, respectively.

Plasmonic FET Terahertz Spectrometer

We show that Si MOSFETs, AlGaN/GaN HEMTs, AlGaAs/InGaAs HEMTs, and p-diamond FETs with feature sizes ranging from 20 nm to 130 nm could operate at room temperature as THz spectrometers in the frequency range from 110 GHz to 9.2 THz with different subranges corresponding to the transistors with different features sizes and tunable by the gate bias. The spectrometer uses a symmetrical FET with interchangeable source and drain with the rectified THz voltage between the source and drain being proportional to the sine of the phase shift between the voltages induced by the THz signal between gate-to-drain and gate-to-source. This phase difference could be created by using different antennas for the source-to-gate and drain-to gate contacts or by using a delay line introducing a phase shift or even by manipulating the impinging angle of the two antennas. The spectrometers are simulated using the multi-segment unified charge control model implemented in SPICE and ADS and accounting for the electron inertia effect and the distributed channel resistances, capacitances and Drude inductances.

Terahertz Source Radiating to Open Space Based on the Superconducting Flux-Flow Oscillator: Development and Characterization

We have elaborated, fabricated, and tested a terahertz source based on the Josephson flux-flow oscillator (FFO) integrated with a transmitting lens antenna. The oscillator was coupled to the on-chip double-slot antenna via microstrip lines, and the chip was mounted on the silicon lens providing the continuous terahertz emission output. The oscillator samples were made of superconductor–insulator–superconductor (SIS) trilayers based on Nb/AlN/NbN, with a gap voltage of about 3.6 mV. The output emission was studied using two independent techniques: a THz spectrometer based on the SIS receiver with a high spectral resolution (better than 0.1 MHz) and an Si bolometer. An operating range of the oscillator of 400–580 GHz and a ratio of detected signal to background signal at the receiver output of up to 55 dB are obtained. In addition, a design for the oscillator with an integrated harmonic mixer for FFO locking is developed and fabricated using Nb/AlO x /Nb trilayers, which is better for FFO operation than Nb/AlN/NbN trilayers at some frequencies due to lower surface losses and hence better spectral properties. The pumping of the mixer by the FFO output power was measured and found to be sufficient for phase locking.

Broadband dielectric spectroscopy from sub GHz to THz of hydrated lipid bilayer of DMPC.

In order to study the dynamics of a phospholipid and its hydration water, we measured complex dielectric spectra of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) from the sub-GHz to the THz frequency region with varying the temperature and hydration level of the sample. Spectra obtained from a vector network analyzer and two terahertz time-domain spectrometers are adjusted, which enables us to analyze the dielectric spectra from the sub-GHz region to the THz region by a model function. We confirmed a fast relaxational mode in the sub-THz region, which was suggested by the previous work which only used a THz spectrometer.

Ultra-fast, High-Bandwidth Coherent cw THz Spectrometer for Non-destructive Testing

Continuous wave THz (cw THz) systems define the state-of-the-art in terms of spectral resolution in THz spectroscopy. Hitherto, acquisition of broadband spectra in a cw THz system was always connected with slow operation. Therefore, high update rate applications like inline process monitoring and non-destructive testing are served by time domain spectroscopy (TDS) systems. However, no fundamental restriction prevents cw THz technology from achieving faster update rates and be competitive in this field. In this paper, we present a fully fiber-coupled cw THz spectrometer. Its sweep speed is two orders of magnitude higher compared to commercial state-of-the-art systems and reaches a record performance of 24 spectra per second with a bandwidth of more than 2 THz. In the single-shot mode, the same system reaches a peak dynamic range of 67 dB and exceeds a value of 100 dB with averaging of 7 min, which is among the highest values ever reported. The frequency steps can be as low as 40 MHz. Due to the fully homodyne detection, each spectrum contains full amplitude and phase information. This demonstration of THz-spectroscopy at video-rate is an essential step towards applying cw THz systems in non-destructive, in line testing.

THz spectroscopy for inspection and evaluation of internal structure of sandwich samples

Purpose The purpose of this paper is to discuss the application of THz spectroscopy for the inspection and evaluation of the internal structure of complex samples with honeycomb fillers. Design/methodology/approach Three complex samples with honeycomb fillers are investigated using THz spectrometer in order to determine the applicability of chosen non-destructive method for the analysis of internal structure of structural components. The first analysed sample has aluminium honeycomb filler with some cells filled with water. The aim of the analysis is to distinguish empty and full cells. The other two sandwich samples are made of different non-metallic components and for them the possibility of THz spectroscopy application is analysed. Findings The empty and full cells in metal honeycomb filler were easily distinguished due to different absorption coefficients of electromagnetic waves in THz range for air and water. It was especially visible for frequency domain. The THz spectroscopy was able to inspect the non-metallic samples internal structures and distinguish skins (with layers), honeycomb fillers and adhesive layers between them. It was also possible to detect, localise and determine the size of a local damage of honeycomb walls due to impact influence. Originality/value The present study is an original research work. There are very limited literature papers which present analyses of internal structures of sandwich elements using THz spectroscopy and investigate utility of the method for mechanical damage and contamination (water) detection and localisation.

Continuous bunch-by-bunch spectroscopic investigation of the microbunching instability

Electron accelerators and synchrotrons can be operated to provide short emission pulses due to longitudinally compressed or sub-structured electron bunches. Above a threshold current, the high charge density leads to the micro-bunching instability and the formation of sub-structures on the bunch shape. These time-varying sub-structures on bunches of picoseconds-long duration lead to bursts of coherent synchrotron radiation in the terahertz frequency range. Therefore, the spectral information in this range contains valuable information about the bunch length, shape and sub-structures. Based on the KAPTURE readout system, a 4-channel single-shot THz spectrometer capable of recording 500 million spectra per second and streaming readout is presented. First measurements of time-resolved spectra are compared to simulation results of the Inovesa Vlasov-Fokker-Planck solver. The presented results lead to a better understanding of the bursting dynamics especially above the micro-bunching instability threshold.