Continuous-wave Terahertz Research Articles

Viscous terahertz photoconductivity of hydrodynamic electrons in graphene.

Light incident upon materials can induce changes in their electrical conductivity, a phenomenon referred to as photoresistance. In semiconductors, the photoresistance is negative, as light-induced promotion of electrons across the bandgap enhances the number of charge carriers participating in transport. In superconductors and normal metals, the photoresistance is positive because of the destruction of the superconducting state and enhanced momentum-relaxing scattering, respectively. Here we report a qualitative deviation from the standard behaviour in doped metallic graphene. We show that Dirac electrons exposed to continuous-wave terahertz (THz) radiation can be thermally decoupled from the lattice, which activates hydrodynamic electron transport. In this regime, the resistance of graphene constrictions experiences a decrease caused by the THz-driven superballistic flow of correlated electrons. We analyse the dependencies of the negative photoresistance on the carrier density, and the radiation power, and show that our superballistic devices operate as sensitive phonon-cooled bolometers and can thus offer, in principle, a picosecond-scale response time. Beyond their fundamental implications, our findings underscore the practicality of electron hydrodynamics in designing ultra-fast THz sensors and electron thermometers.

Optical parameters extraction of soil and its microplastics contamination using terahertz spectroscopy

The primary focus of this study investigates the potential of continuous-wave terahertz frequency-domain spectroscopy(CW-THz-FDS) as a rapid, non-destructive technique for detecting microplastic contamination in soil. Focusing on low-density polyethylene (LDPE) as a model microplastic, we prepared soil mixtures with varying LDPE concentrations (1 %,5 %,10 %,15 % by weight). We used a THz-FDS system operating between 0.1 and 0.7 THz to analyze the optical properties (transmission coefficient, attenuation and refractive index) of these mixtures and LDPE samples separately. Notably both soil and LDPE particles were of micrometer scale. Our results revealed distinct, concentration-dependent variations in these properties. The refractive index for the higher concentration (15 %) ranged from 2.2 to 2.3, while the attenuation coefficient varies between 10 and 14 cm−1. Crucially, we identified polylactic acid [PLA] as a suitable sample holder material due to its high transmission in the THz range. Consequently, this study suggests that terahertz spectroscopy can be a promising technique for detecting microplastic contamination in soil, offering a non-destructive and sensitive method for environmental monitoring.

Terahertz characterization of organic crystals using paraffin as dilution matrix

Terahertz waves exhibit excellent performance in molecular information detection of substances. Since natural materials exhibit weak absorption of terahertz waves, it's conventional to involve mixing analyte with polymer diluents and pressing them into pellets, or enhance the light-material interaction through coupling systems of natural materials with metamaterials. However, both pressure-induced metastable crystal structure transformations and spatial overlap between the analyte and metasurfaces can impact the absorption of terahertz waves by organic crystals. This paper proposes a novel analytical approach using paraffin as a dilution matrix for qualitatively and quantitatively characterizing organic crystals with THz spectroscopy. The featureless refractive index spectra and negligible absorption demonstrated the feasibility of paraffin as a diluting analyte. The analytical process was further demonstrated with well-studied lactose by being diluted in molten paraffin at different mass-ratios and solidified as pellets under room temperature and ambient pressure. A continuous wave terahertz frequency domain spectroscopy system (THz-FDS) is used for spectral acquisition over 0.4–1.3 THz. The experimental results reveal a significant linear correlation between the absorption coefficient peak areas and molar concentration of lactose, with a correlation coefficient R2 of 0.9678. Our method provides an additional optional dilution matrix for terahertz spectroscopy characterization of organic crystals, enabling non-destructive, rapid, and direct measurement of metastable crystals. Furthermore, this study on paraffin as a binder material holds potential for enhancing spatial overlap between natural materials and metamaterials in the field of metasurface-enhanced sensing.

Continuous wave THz receivers with rhodium-doped InGaAs enabling 132 dB dynamic range

For the first time, we present photoconductive, continuous wave (cw) terahertz (THz) detectors for 1550 nm excitation based on rhodium- (Rh) doped indium gallium arsenide (InGaAs) grown by molecular beam epitaxy. Compared to iron- (Fe) doped material, the Rh-doped InGaAs shows higher carrier mobilities with similar carrier lifetimes. Therefore, these photoconductive antennas outperform InGaAs:Fe-based detectors by a factor of 10 in terms of responsivity and noise-equivalent-power (NEP) while maintaining the same bandwidth. In a homodyne spectrometer configuration, we achieve a record peak dynamic range (DNR) of 132 dB, which constitutes an improvement of 20 dB.

Ultra-Wideband PIN-PD THz Emitter with > 5.5 THz Bandwidth

We present novel PIN photodiode (PD) continuous wave (cw) terahertz (THz) emitters with an increased responsivity and reduced substrate thickness compared to the state-of-the-art. Our improved devices feature up to 4 dB higher output power below 500 GHz with maximum power of -0.53 dBm at 115 GHz and strongly reduced THz absorption of the substrate for frequencies above 3 THz. The latter enables us to measure coherent cw THz spectra with a record bandwidth of 5.5 THz, for the first time, which is 1 THz (22%) more than the state-of-the-art.

Three-dimensional reconstruction method for layered structures based on a frequency modulated continuous wave terahertz radar

The feasibility of employing a continuous-wave terahertz detection system for non-contact and non-destructive testing (NDT) in multi-layered bonding structures is assessed in this study. The paper introduces the detection principle of terahertz frequency modulated continuous wave (FMCW) radar and outlines the two-dimensional (2D) scanning platform, which integrates optical lenses, three linear actuators, a control platform, and data acquisition units. Experimental results on two types of insulation with prefabricated defects demonstrate the capability of terahertz waves for transparent inspection imaging. These results confirm the viability of terahertz FMCW detection technology as an advanced NDT tool for multi-layered bonding structures. However, the inherent limitations of terahertz wavelength and hardware systems pose challenges in discriminating reflection peaks on upper and lower surfaces. To address this issue, a local adaptive empirical wavelet coefficient modal decomposition (LAEWCMD) method is proposed to enhance the longitudinal discrimination ability of terahertz detection. The proposed method involves segmenting the 2D terahertz detection image into regions to differentiate between defective and non-defective areas. Continuous wavelet transforms (CWT) are then applied to the range signals of each region to derive continuous wavelet coefficients (CWCs). Subsequently, empirical mode decomposition (EMD) is performed on the CWCs to decompose them into intrinsic mode functions (IMFs) and residual signals. The 1st IMF is utilized for three-dimensional (3D) reconstruction, and the regions are fused to generate the final output. The effectiveness of the proposed method is validated on aircraft thermal protection structures (TPS), achieving high-precision 3D reconstruction. This offers a novel approach for the application of terahertz computed tomography imaging and NDT.

Classification of non-referenced continuous-wave terahertz reflection spectra for remote material identification

Commonly, terahertz spectra (both continuous-wave and pulsed) are deconvoluted by reference spectra to remove the water vapor absorption lines and other system related responses. However, in real-life applications obtaining reference spectra can be problematic and adds to the complexity of the system. Thus, a reference-free method for classification of terahertz spectra could be a welcomed advance for remote sensing applications. In this paper, we study how simple machine learning algorithms perform as a reference-free method for terahertz stand-off identification of materials. The algorithms are trained using spectra measured under controlled humidity conditions and tested by a completely independent data set measured under ambient conditions. We apply three different classification algorithms; namely a Gaussian Bayes model, the k nearest neighbors, and a support vector machine. We found that, if the terahertz spectra are processed using a supervised algorithm (Regularized Linear Discriminant Analysis), very high classification scores (¿98.6%) can be retained for the non-referenced spectra. Moreover, the high accuracy is obtained meanwhile the dimensionality is reduced by a factor larger than 160, which further reduces the computational requirements. Hence, we have demonstrated that simple supervised machine learning algorithms can serve as a highly accurate reference-free method for THz material identification. This could be of great importance for real-world remote sensing applications based on terahertz spectroscopy.

Discrimination of natural and synthetic forms of azurite: An innovative approach based on high-resolution terahertz continuous wave (THz-CW) spectroscopy for Cultural Heritage

It is not always possible to discriminate natural and synthetic forms of pigments in a non-invasive way. In this work, we demonstrate that THz spectroscopy allows for this selective discrimination of chemically related compounds. In the presented results, we show the spectral characterization of the following copper carbonates: natural azurite and two of its synthetic forms, bice blue and blue verditer. In particular, the physical properties of the two synthetic forms were characterized for the first time in the terahertz spectral range by exploiting a high-resolution coherent terahertz Continuous Wave (THz-CW) spectrometer. The findings reveal distinct absorption spectra for the three chemically related pigments, showing specific fingerprints that can be used to discriminate the natural compound from the synthetic forms.In fact, we show that by THz-CW it was possible to discriminate those pure components in binary mixtures and quantify the single components contribution.This identification is of great interest to assist in dating artworks or evidence restoration and retouching treatments. The results demonstrate that THz-CW spectroscopy can pave the way to an innovative approach to the Cultural Heritage field.

Large‐wavelength Gaussian deconvolution phase‐contrast computed tomography for THz continuous wave (0.11 THz)

AbstractThis letter was to present an attempt of large‐wavelength Gaussian deconvolution phase‐contrast computed tomography (LW‐GD‐PCCT) for promotion of image quality reconstructed in low‐frequency band of terahertz (THz) spectrum at 0.11 THz. The interaction between the imaging samples and the THz Gaussian beam were formulated firstly in this paper, where the unwrapped phase was extracted specifically to portray the spatial structure distribution of the samples. Additionally, a Gaussian deconvolution was employed for the further reduction of spatial distortions. Moreover, an image reconstruction was carried out with the obtained phase sinograms based on phase‐used inverse Radon transform from the different positions on the sample. For an experimental assessment of the concept of LW‐GD‐PCCT, a single ellipsoid reflector‐based THz Gaussian beam generating system was established and samples such as polystyrene (PS) foam cuboid (Sample 1), and cylinder (Sample 2) with hollow defects (air holes and triangles) were prepared carefully in this work. To experimentally evaluate the performance of the contributing to the structural imaging over soft samples. Two‐dimensional topographies of each sample were reconstructed successfully, and the obtained cross root‐mean‐square error (cross‐RMSE), cross peak signal‐to‐noise ratio (cross‐PSNR), and cross structural similarity (cross‐SSIM) were 151.6451, 26.3225, and 0.9616 for Sample 1 with a high dose of 180 projections respectively, as well as 30.3242, 33.3129, and 0.9711 for Sample 2 with a low dose of 36 projections, respectively. The obtained imaging indicators of this work showed a superiority of imaging quality over those of recent works. Furthermore, the investigation of the bearing capacity has shown promise in enhancing image quality even in low‐dose conditions. The presented results suggest that the unwrapped phase combining with Gaussian deconvolution in low‐frequency band of THz imaging would be useful to improve the reconstructed image quality, potential to highly feasible non‐destructive testing of polymer foam via large wavelength.

Anisotropic quasi-static permittivity of rare-earth scandate single crystals measured by terahertz spectroscopy

We report the real-valued static and complex-valued quasi-static anisotropic permittivity parameters of rare-earth scandate orthorhombic single crystal GdScO3 (GSO), TbScO3 (TSO), and DyScO3 (DSO). Employing continuous-wave terahertz spectroscopy (0.2–1 THz), the complex permittivity was extracted using an anisotropic ambient-film-ambient model. Data obtained from multiple samples of the same oxides and different surface cuts were analyzed simultaneously. The zero-frequency limit of the modeled data indicates that at room temperature the real part of the dielectric tensor components for GSO are ɛa = 22.7, ɛb = 19.3, and ɛc = 28.1; for DSO, ɛa = 20.3, ɛb = 17.4, and ɛc = 31.1; and for TSO, ɛa = 21.6, ɛb = 18.1, and ɛc = 30.3, with a, b, and c crystallographic axes constituting the principal directions for the permittivity tensor. These results are in excellent agreement with expectations from theoretical computations and with scarcely available data from previous experimental studies. Furthermore, our results evidence a noticeable attenuation, which increases with frequency, and are very significant especially at the higher frequency end of the measurement and along the c-direction in all samples. We suggest the attenuation is most likely caused by the onset of absorption due to long-wavelength active optical phonon modes. These results are important for electronic and potential sub-terahertz applications (e.g., quarter-wave plate) benefiting from the large index contrast along different directions in these materials.

Terahertz reflective imaging systems with improved angle compatibility realized by the designed quasi-scattering screen

Active imaging can provide significantly larger signal margins in the terahertz (THz) spectral region than passive imaging. However, the imaging effect of THz active imaging is severely constrained by the orientation requirement of target reflection, which limits the application scenarios of this technique. In view of the shortcomings of existing THz reflective imaging systems, this paper proposes a new system optimization scheme from the aspect of hardware - a compact quasi-scattering screen (QSS) is proposed and designed as a beam-shaping element to eliminate the need for the best direction of specular reflection to the greatest extent. The corresponding relationship between the divergent Gaussian beam and target light field is directly established by using the equal-energy mapping method in non-imaging optics, which rejects the collimation process of the THz source. By customizing the optimization function in ZEMAX® to automatically optimize the surface of the QSS, the direction of incident light propagation can be adjusted. A non-coaxial reflective THz imaging system using a 0.1 THz continuous wave source demonstrates the ability of the QSS to improve the angle compatibility of the traditional THz reflective imaging system. The results show that the imaging angle compatibility range of the system can be increased to more than 5 times without changing the original imaging optical path and its resolution. In addition, it can also improve the imaging quality of the original imaging optical path for the object with a smooth surface or certain structure.

Parasitic mixing in photomixers as continuous wave terahertz sources

We present observations of parasitic frequency components in the emission spectrum of typical photomixer sources for continuous wave (CW) terahertz generation. Broadband tunable photomixer systems are often used in combination with direct power detectors, e.g., for source and/or detector characterization. Here, spectral components besides the intended terahertz emission at the difference frequency of the two excitation lasers can significantly distort the measurement results. In this work, the appearance of parasitic mixing signals is observed in broadband measurements with a broadband antenna-coupled field-effect transistor as terahertz detector (TeraFET). The measurements reveal weaker spectral absorption features than expected and also a signal plateau towards higher frequencies, both strongly indicating a background in the detection signals. The photomixer emission is investigated in detail with a terahertz Fourier-transform infrared spectrometer (FTIR). We relate the observed parasitic frequency components with good quantitative agreement with the mode spectra of the semiconductor lasers. We also present one possible approach to overcome some of the issues, and we emphasize the importance of our findings to avoid distorted measurement results. To our knowledge, the essential aspect of parasitic mixing has so far been largely ignored in the literature where terahertz CW photomixer emitters are widely used for spectrally resolved measurements.

Continuous wave terahertz detection using 1550nm pumped nonlinear photoconductive GaAs metasurfaces.

Terahertz (THz) continuous wave (CW) spectroscopy systems can offer extremely high spectral resolution over the THz band by photo-mixing high-performance telecommunications-band (1530-1565 nm) lasers. However, typical THz CW detectors in these systems use narrow band-gap photoconductors, which require elaborate material growth and generate relatively large detector noise. Here we demonstrate that two-step photon absorption in a nano-structured low-temperature grown GaAs (LT-GaAs) metasurface which enables switching of photoconductivity within approximately one picosecond. We show that LT-GaAs can be used as an ultrafast photoconductor in CW THz detectors despite having a bandgap twice as large as the telecommunications laser photon energy. The metasurface design harnesses Mie modes in LT GaAs resonators, whereas metallic electrodes of THz detectors can be designed to support an additional photonic mode, which further increases photoconductivity at a desired wavelength.

Coherent Terahertz Detection via Ultrafast Dynamics of Hot Dirac Fermions in Graphene.

Graphene has recently been shown to exhibit ultrafast conductivity modulation due to periodic carrier heating by either terahertz (THz) waves, leading to self-induced harmonic generation, or the intensity beat note of two-color optical radiation. We exploit the latter to realize an optoelectronic photomixer for coherent, continuous-wave THz detection, based on a photoconductive antenna with multilayer CVD-grown graphene in the gap. While for biased THz emitters the dark current would pose a serious detriment for performance, we show that this is not the case for bias-free THz detection and demonstrate detection up to frequencies of at least 700 GHz at room temperature, even without optimized tuning of the doping. We account for the photocurrent and photomixing response using detailed simulations of the time-dependent carrier distribution, which also indicate significant potential for enhancement of the sensitivity, to become competitive with well-established semiconductor photomixers.

Pigments, minerals, and copper-corrosion products: Terahertz continuous wave (THz-CW) spectroscopic characterization of antlerite and atacamite

In this work, the optical properties of atacamite, antlerite and azurite are characterized for the first time by exploiting a high-resolution coherent THz-CW spectrometer in the spectral range 0.1–2.8 THz adopting transmission configuration; the spectral response of atacamite and antlerite is reported for the first time in literature in this frequency range. The absorption spectra of the two copper-based pigments showed the presence of distinct absorption peaks defining their fingerprints. In particular, atacamite presents a peak centered at 2.45 THz and antlerite presents two spectral fingerprints centered at 1.77 and 2.52 THz; the retrieved average refractive index is 1.19 and 1.29, respectively. The absorption spectrum of azurite is in good agreement with previous works in the far infrared region and presents two sharp peaks centered at 1.83 and 2.23 THz with an average value of the refractive index of 1.65. Furthermore, by means of the same technique, it is possible to discriminate and quantify pure components in binary mixtures. This result is of great interest for the identification of pigments and metallic corrosion products, and it demonstrates that THz-CW spectroscopy can open the way to an innovative approach for the Cultural Heritage field.

Miniaturized Continuous-Wave Terahertz Spectrometer With 3.6 THz Bandwidth Enabled by Photonic Integration and Microelectronics

Miniaturized Continuous-Wave Terahertz Spectrometer With 3.6 THz Bandwidth Enabled by Photonic Integration and Microelectronics

Terahertz Imaging Using Nanorectifier-Based Detectors and Broadband Thermal Sources

Several terahertz imaging experiments have been conducted at room temperature using a self-switching diode (SSD) rectenna as a detector, and a broadband thermal source (at 610 °C) as a continuous-wave terahertz generator. Since the terahertz emission produced by the source is non-coherent with random polarizations and has a wide-ranging spectrum, the SSD-based rectenna employed in this work utilizes a planar spiral micro- antenna which has a circular polarization that able to effectively capture all incident radiation regardless of the angles. The antenna has been designed for a broadband frequency response in the range of 0.1-10 THz. This is to ensure the terahertz images produced are ascribed to the terahertz radiation collected by the antenna, but without eliminating the possibility of thermal effects at frequencies higher than the terahertz region. In order to further validate the results obtained, an Airy pattern experiment has been conducted. Based on this experiment, the effective frequency response of the SSD rectenna is estimated at 2.29 THz. The utilization of thermal source and micro-size rectenna in this work may pave the way to explore many opportunities in developing flexible, compact, and low-cost terahertz imaging systems without the use of expensive components (e.g., typically lasers are used as terahertz sources).

Continuous-wave terahertz in-line holographic diffraction tomography with the scattering fields reconstructed by a physics-enhanced deep neural network

Diffraction tomography is a promising, quantitative, and nondestructive three-dimensional (3D) imaging method that enables us to obtain the complex refractive index distribution of a sample. The acquisition of the scattered fields under the different illumination angles is a key issue, where the complex scattered fields need to be retrieved. Presently, in order to develop terahertz (THz) diffraction tomography, the advanced acquisition of the scattered fields is desired. In this paper, a THz in-line digital holographic diffraction tomography (THz-IDHDT) is proposed with an extremely compact optical configuration and implemented for the first time, to the best of our knowledge. A learning-based phase retrieval algorithm by combining the physical model and the convolution neural networks, named the physics-enhanced deep neural network (PhysenNet), is applied to reconstruct the THz in-line digital hologram, and obtain the complex amplitude distribution of the sample with high fidelity. The advantages of the PhysenNet are that there is no need for pretraining by using a large set of labeled data, and it can also work for thick samples. Experimentally with a continuous-wave THz laser, the PhysenNet is first demonstrated by using the thin samples and exhibits superiority in terms of imaging quality. More importantly, with regard to the thick samples, PhysenNet still works well, and can offer 2D complex scattered fields for diffraction tomography. Furthermore, the 3D refractive index maps of two types of foam sphere samples are successfully reconstructed by the proposed method. For a single foam sphere, the relative error of the average refractive index value is only 0.17%, compared to the commercial THz time-domain spectroscopy system. This demonstrates the feasibility and high accuracy of the THz-IDHDT, and the idea can be applied to other wavebands as well.

Nondestructive Evaluation of Cryofoam with Uneven Surface by Continuous Wave Terahertz Imaging Using Dynamic Depth Focusing Technique

Nondestructive Evaluation of Cryofoam with Uneven Surface by Continuous Wave Terahertz Imaging Using Dynamic Depth Focusing Technique

Narrow linewidth and wideband tunable continuous-wave terahertz generator based on difference frequency generation with DAST crystal.

A narrow linewidth and wideband tunable continuous-wave terahertz generator with DAST crystal has been demonstrated in this paper. Two narrow-linewidth CW fiber lasers were used as the pump sources for difference frequency generation. The terahertz wave can be continuously tunable in the range of 1.1-3 THz. The maximum output power of 2.79nW was obtained at 2.568 THz. The linewidth of the output THz wave was estimated to be 56.5 MHz by fitting transmission spectrum of CO gas at 450 Pa pressure around 80.52 cm-1 with the Vogit gas model. Furthermore, the output spectra at room temperature and pressure was in good agreement with the air absorption lines in Hitran database. Moreover, the narrower absorption characteristic spectrum of 2-Deoxy-D-Glucose sample has been obtained through the spectrum measurements. Therefore, it could promote the practical prospect of tunable CW-THz source, which will have good potential in THz high-precision spectroscopic detection and multispectral imaging.