THz-TDS System Using a Mechanically Shared Dual-Comb Fiber Laser

Quantitative detection of substances without feature absorption peak by terahertz Rydberg atom sensor.

We demonstrate a 4-f terahertz time-domain spectroscopy (THz-TDS) system using an organic crystal DSTMS as the THz emitter and a low temperature grown (LTG) InGaAs/InAlAs photoconductive antenna as the receiver. The system covers a frequency range from 0.2 up to 8 THz. The influences of the pump laser power, the probe laser power and the azimuthal angle of the DSTMS crystal on the time-domain THz amplitude are experimentally analyzed. The frequency accuracy of the system is verified by measuring two metamaterial samples and a lactose film in this THz-TDS system. The proposed combination of DSTMS emission and PC antenna detection realizes a compact and low-cost THz-TDS scheme with an ultra-broad bandwidth, which may promote the development and the applications of THz-TDS techniques.

In this paper, we employ an ultrafast THz time-domain spectroscopy (TDS) system for computationally efficient imaging using a Kirchhoff migration algorithm. The imaging method makes use of uncollimated, unfocused terahertz beams, and eliminates the need for any opto-mechanical elements. Moreover, this method compensates for the low signal-to-noise ratio obtained with an uncollimated THz beam. We use a THz TDS system based on electronically controlled optical sampling (ECOPS), which achieves a measurement rate of 1600 traces per second. We validate the performance of the method with three different objects.

Summary form only given. We develop a new terahertz (THz) imaging approach with 90 μm image resolution at 750 GHz using an inverse-microscope quasi-optical setup with an extended hemispherical high-resistivity silicon lens and a vector network analyzer frequency extender with horn-antenna output port. Owing to the large refractive index of silicon lens (n~3.5), the achieved resolution is less than λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> /3, where λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> is the free-space wavelength. The test sample is placed on the focal plane of the silicon lens, making direct contact with the flat focal surface of the back side of the extended hemispherical lens and a raster-scan image is obtained using computer-controlled x-y translation stages. Reflected THz signal is digitized and recorded on a vector network analyzer, providing wideband reflection spectra of the sample as a function of position. Using the proposed imaging system, 2-dimensional THz images of planarized samples, including integrated circuits, chemical substances and biological tissue samples can be readily collected. This cost-effective quasi-optical approach can also easily be adapted to other signal acquisition systems, such as THz time-domain spectroscopy (THz-TDS) and/or continuous wave (THz-CW) systems to improve image resolution and expand their imaging capabilities. However, unlike existing THz-TDS and THz-CW systems where the source and receiver ports are disparate, the proposed VNA-based system uses the same port for the reflection (S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> ) measurements. As such, it allows a direct measurement of normal reflection from the sample, resulting in more straightforward analysis of the sample properties. We will present the details of the setup and its performance using several samples that exhibit distinct morphological and spectroscopic behavior in the THz band.

Terahertz filters based on the frequency selective surfaces (FSS) structure have been modeled and fabricated on the sapphire substrate. The terahertz (THz) transmission properties are investigated using a THz time-domain spectroscopy (THz-TDS) system. The designed two FSS-based THz filters exhibit the THz electromagnetic wave transmission and reflection characteristics, respectively, showing about 70% change of THz transmission at the frequency of 1.3 THz. It is suggested that the THz filtering effect results from the resonance behavior between THz plane-wave and resonant elements of designed metallic FSS structures. The agreement between experiments and simulations indicate that the designed THz filter show a great potential application for high-speed THz modulator.

Terahertz (THz) spectroscopy can potentially be used to probe and characterize inhomogeneous materials, however spectroscopic identification of such materials from spectral features of diffuse returns is a relatively underdeveloped area of study. In this thesis, diffuse THz scattering from granular media is modeled by applying radiative transfer (RT) theory for the first time in THz sensing. Both classical RT theory and dense media radiative transfer (DMRT) theory based on the quasi-crystalline approximation (QCA) are used to calculate diffuse scattered intensity. The numerical solutions of the vector radiative transfer equations (VRTE) were coded and calculated in MATLAB. The diffuse scattered field from compressed Polyethylene (PE) pellets containing steel spheres was measured in both transmission and reflection using a THz time domain spectroscopy (THz-TDS) system. Measurement results showed energy redistribution by granular media due to volume scattering as well as angle dependent spectral features due to Mie scattering. The RT model was validated by successfully reproducing qualitative features observed in experimental results. Diffuse intensity from granular media containing Teflon, lactose sugar, and C4 explosive was then calculated using the RT models. Simulation results showed the amplitude of diffuse intensity is affected by factors such as grain size, fractional volume of grains, thickness of scattering layer, and scattering angles. Spectral features were also observed in the diffuse intensity spectra from media containing grains with THz spectral signatures. The simulation results suggest the possibility of identifying materials from diffuse intensity spectra.

Terahertz (THz) is an electromagnetic spectrum with a frequency range from 0.1 to 10 THz, which is located in between the microwave and infrared regions. The unique features of THz waves make them eligible for use in various medical applications. The THz imaging is one of them, which is mainly based on the analysis and processing of the transmission and reflection spectrum information of the sample. This chapter mainly presents the research status and prospects of several THz medical imaging systems and their applications for medical imaging in biological tissues. As the demand for technology grows, high-performance THz imaging systems are becoming indispensable. The ability of the THz time-domain spectroscopy (THz-TDS) system to extract spectra from amplitude and phase information opens virtually unlimited possibilities for imaging applications. The performance achievements of THz-TDS-based imaging with potential research on its fast-imaging components for solving the existing limitations of imaging speed are also presented. Furthermore, the latest developments of several THz-TDS-based imaging methods, including tomography imaging and near-field imaging, are highlighted with their performance improvement. Additionally, the rapid development of THz-TDS as a highly versatile analytical tool for the characterisation of pharmaceutical materials has drawn considerable attention. One of the greatest biomedical potentials of THz imaging is the use of molecular spectroscopy for diagnostics, which is exponentially advanced and moving closer to progress. It is now evident that different types of biomolecules leave distinctive spectral fingerprints in the THz region, which considerably widens the coverage of its technology application including in-vitro and in-vivo measurements of small molecules of clinical importance in point of care and diagnostic systems. In-vivo molecular imaging is considered the next frontier in medical diagnostics, which would be ideally performed non-invasively. Recent achievements in the field of medical imaging have dramatically enhanced the early detection and treatment of many pathological conditions. THz imaging systems can help in detecting early cancer before it is visible or sensitive to any other identification resources. The THz images can distinguish between healthy tissue and basal cell carcinoma and therefore help in mapping the exact margins of early-stage tumours. By obtaining both frequency and time-domain information, the THz imaging can ensure enhanced detection of cancer and provide sharper imaging and molecular fingerprinting. THz biomedical imaging has become a modality of interest due to its ability to simultaneously acquire both image and spectral information. Advanced digital image processing algorithms are greatly needed to assist in screening, diagnosis and treatment. Finally, we summarise the obstacles in the way of the application of THz biomedical imaging application technology in clinical detection, which need to be investigated and overcome in the future.

We give a brief review of the developments in terahertz time-domain spectroscopy (THz-TDS) systems and micro-cavity components for probing samples in the University of Shanghai for Science and Technology. The broadband terahertz (THz) radiation sources based on GaAs m-i-n diodes have been investigated by applying high electric fields. Then, the free space THz-TDS and fiber-coupled THz-TDS systems produced in our lab and their applications in drug/cancer detection are introduced in detail. To further improve the signal-to-noise ratio (SNR) and enhance sensitivity, we introduce three general micro-cavity approaches to achieve tiny-volume sample detection, summarizing our previous results about their characteristics, performance, and potential applications.

Electron beam (e-beam) irradiations are often involved for characterizing graphene-based terahertz (THz) devices or realizing graphene surface plasmons. Here, based on THz time-domain spectroscopy (TDS) and two-dimensional scanning system, the time dependence and the in situ mapping images of changed THz conductivity of graphene induced by e-beam irradiations are studied. The change in THz signals with irradiation indicates a decrease in the THz conductivity as a result of electron doping in graphene through irradiation. And the spatial imaging maps of the decreased THz conductivity reveal diverse electron doping speeds in graphene during different current e-beam irradiations. Additionally, different theoretical doping models are given for explanations of the imaging maps and the calculated results by doping models are in good accordance with the experimental results. Our findings are of significance for understanding the change in THz conductivity and carrier transport of graphene under e-beam irradiations.

We describe the design, build and characterization of a novel two-output port configuration for a THz-Time Domain Spectroscopy (TDS) system. By introducing a tilted THz ultra-broadband polarizer, we split the THz beam in two orthogonal polarization detector branches. The probe laser is similarly split (with an optical polarizer) replicating the detection chain to obtain two independent orthogonal polarization detection units. We describe the system’s performance highlighting some of the advantages of this system in one of its two modes of operation: optimized polarimetry for Jones matrix measurements. A bi-refringent sapphire standard was measured to confirm its capabilities and assess the performance of the system showing good agreement with existing literature data.

Sugar substitutes, which generally refer to a class of food additives, mostly have vibration frequencies within the terahertz (THz) band. Therefore, THz technology can be used to analyze their molecular properties. To understand the characteristics of sugar substitutes, this study selected mannitol and erythritol as representatives. Firstly, PXRD and Raman techniques were used to determine the crystal structure and purity of mannitol and erythritol. Then, the THz time-domain spectroscopy (THz-TDS) system was employed to measure the spectral properties of the two sugar substitutes. Additionally, density functional theory (DFT) was utilized to simulate the crystal configurations of mannitol and erythritol. The experimental results showed good agreement with the simulation results. Finally, microfluidic chip technology was used to measure the THz spectroscopic properties of the two sugar substitutes in solution. A comparison was made between their solid state and aqueous solution state, revealing a strong correlation between the THz spectra of the two sugar substitutes in both states. Additionally, it was found that the THz spectrum of a substance in solution is related to its concentration. This study provides a reference for the analysis of sugar substitutes.

This study presents a self-supervised deep learning methodology for efficient denoising of terahertz (THz) images acquired through THz time-domain spectroscopy (THz-TDS) systems. Unlike traditional approaches that require paired clean and noisy images, the proposed method leverages the Noise2Noise framework to enable effective model training using only noisy data. Quantitative evaluations on 1.20 THz images demonstrate a significant noise level reduction from 16.87 to 0.61, corresponding to a 96% improvement across diverse signal-to-noise ratio (SNR) conditions. Furthermore, the model exhibits strong generalization capabilities across various frequency bands and structural characteristics, consistently achieving noise reductions of 74% to 96% throughout the 1.00–2.00 THz spectrum.

Polarization measurement is important in the Terahertz (THz) band as certain samples may have polarization‐related fingerprints. Due to some limitations, obtaining complete polarization measurements is still quite difficult. In this article, we propose several simple methods to measure partial polarization parameters in the THz band by using a THz time‐domain spectroscopy (THz‐TDS) system. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 2005–2007, 2010; Published online in Wiley InterScience (www.interscience. wiley.com). DOI 10.1002/mop.25407

In this work, a second-order Minkowski fractal broadband absorber based on a conventional sandwich structure is proposed. The design was developed from the conventional square-ring microwave absorber. The improvement is divided into two major parts: the evolution of structural parameters as well as the evolution of patterns. The first part is realized based on the analysis of the model structure and the equivalent circuit, and the second part is realized by introducing the internal coupling of the pattern using a second-order Minkowski structure. This proposed absorber exhibits an absorption greater than 90% between 0.283 terahertz (THz) and 0.689 THz. The absorber is polarization-insensitive and possesses good incidence angle stability. In order to verify the performance of the absorber through experimental research, ultraviolet lithography technology is used to manufacture an absorber sample. A terahertz time-domain spectroscopy system is used to measure the sample, and the measurement results are consistent with the simulation results, verifying the correctness of the absorber design.

We present a new generation of photoconductive (PC) terahertz (THz) near-field probes based on freestanding Beryllium-doped low-temperature-grown InGaAs/InAlAs cantilevers. The PC probes are compatible to optical gating with a femtosecond laser having a center wavelength of 1550 nm. Therefore, they are well suited for a cost-efficient direct integration with fiber-coupled THz time-domain spectroscopy (TDS) systems. The photoconductor material features electron lifetimes of 500 fs allowing for broadband detection up to 2 THz, which is comparable to existing LT-GaAs-based near-field detectors, which require 800-nm wavelength excitation. We demonstrate the detector operation in a state-of-the-art fiber-based TDS system with fast and coherent data acquisition and show obtained sheet-resistance mappings of conductive thin-films featuring subwavelength resolution.