THz Imaging System Research Articles

Investigating the Impact of Antenna Dispersion on Time Reversal Wideband THz Imaging Systems

Investigating the Impact of Antenna Dispersion on Time Reversal Wideband THz Imaging Systems

Antireflection Coatings Based on PEDOT:PSS Conductive Polymer Using d-Sorbitol Additives for Terahertz Spectroscopy and Imaging.

Optical antireflection has been employed for a variety of applications in terahertz spectroscopy and detectors. However, current methods encounter challenges in terms of cost, bandwidth, structural complexity, and performance. In this study, a low-cost, broadband, and easily processed THz antireflection coating scheme based on the model of impedance-matching effect is proposed, using a 6 wt % d-sorbitol-doped poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (s-PEDOT:PSS) film. By adjusting the thickness of the s-PEDOT:PSS film, these biocompatible conductive polymers enable a significant reduction of Fresnel reflection and operate over a broad bandwidth between 0.2 and 2.2 THz. Applying the antireflective coating to the surface of the sample substrate and electro-optic probe crystal in THz spectroscopy and near-field imaging shows that the spectral resolution is significantly improved, and the devices exhibit more excellent intended performance. The findings of this study could aid in improving the measurement capability of various THz time-domain spectroscopy and imaging system.

Performance Analysis of an Aperture-Coupled THz Antenna for Diagnosing Breast Cancer.

The most important technique for exposing early-stage breast cancer is terahertz imaging. It aids in lowering the number of breast cancer-related fatalities and enhancing the quality of life. An essential component of developing the THz imaging system for high-quality photos is choosing the right sensor. In this article, a wideband antenna for microwave imaging of breast tissue with an operating frequency of 30 GHz (107 GHz to 137 GHz) is constructed and analyzed. An aperture-coupled antenna with an optimized ground aperture is proposed and analyzed, which made it possible to obtain better and consistent impedance matching in the wideband spectrum. The variation of backscattered signal energy in body tissue is assessed with healthy breast tissue and in the presence of malignant cells. A significant difference in energy scattering is observed for both situations. The suggested antenna's linear and stable time domain characteristics make it an appropriate component for THz imaging technology.

Performance of an Active THz Imaging System for Recognition of Concealed Faces

Performance of an Active THz Imaging System for Recognition of Concealed Faces

Defect Detection in Bidirectional Glass Fabric Reinforced Thermoplastics Based on 3-D-THz Imaging

Non-homogeneous, bi-directional glass fabric reinforced thermoplastic (GFRT) composites are used in the automobile industry to produce economic and environmentally friendly vehicles. Despite the fact that GFRT materials have superior mechanical characteristics and lower manufacturing cost compared with traditional materials, they are prone to significant deformation and defect emergence during the forming/manufacturing of parts from prefabricated laminates. Some defects, especially hidden ones like delamination and consolidation defects, can significantly reduce the mechanical properties of the final parts. It is essential to use 3D non-destructive testing (NDT) technology to detect and identify such hidden defects. This research investigates the ability of 3D THz imaging to detect hidden defects (i.e., delamination and consolidation) in GFRT composite materials. Frequency modulated continuous wave (FMCW) technique is used to generate 3D-THz cross-sectional images. Non-defective and defective samples with a thickness of 1.5mm have been measured with two 3D THz imaging systems operating in the frequency range from 238 to 316GHz and from 499 to 733 GHz. In addition, a simulation of the reflected FMCW spectrum is presented to analyze the measurements and intercompare the performance of the two imaging systems. A systematic analysis in a large sample set demonstrates that a delamination defect with a thickness of 125μm as well as a consolidation defect with a difference in thickness less than 0.4mm can be successfully detected by the 700-GHz imaging system.

Skeletonization and 3D rendering with real time terahertz tomography

Terahertz technology (spanning between 0.1 and 10 THz) is now a well-established tool to achieve contactless sensing and non-destructive testing (NDT). Among the advanced approaches, THz computed tomography (THz CT) is an emerging technique for 3D reconstruction and has been extensively investigated over the last decade. This work focuses on those capabilities for 3D volumetric reconstructions of complex objects through the use of a real-time THz imaging system operating at 2.5 THz. Further work demonstrates that the resulting data are compatible with automated processing for (i) an ad-hoc segmentation, extracting the sample from the background and reconstruction surrounding noise, (ii) a component labelling, and (iii) a skeletonization, providing crucial additional metadata about the sample morphology.

Performance comparison of different deconvolution methods on 2D terahertz imaging

AbstractTerahertz (THz) images are always to be degraded because of the diffraction phenomenon. In this letter, the wave nature is considered in the resolution enhancement of terahertz imaging. The phase operation is extended to three classical deconvolution algorithms, which are used in THz complex‐value deconvolution field. The resolution improvement can be seen, and it is impressive that the resolution can reach 0.5 times of the wavelength via the complex‐value total variation deconvolution. We believe that the current resolution of THz imaging system can be significantly upgraded by taking advantage of such approaches.

Terahertz techniques for better hazelnut quality

In recent years, technological innovation has acquired a fundamental role in the agri-food sector and in particular in food quality control. The development of technology allowed to improve the quality of the food before it is placed on the market. Recently, non-invasive techniques such as those operating in the THz spectral band were applied to the field of food quality control.
 In the laboratories of the ENEA centre in Frascati, close to Rome-Italy, has been developed a THz imaging system operating in reflection mode and an experimental setup able to measure both reflection and transmission of the samples in the frequency range between 18-40 GHz. With these two setups will distinguish rotten and healthy hazelnuts acquiring in real time both images of the fruit inside the shell by using the imaging system and the transmission data exploiting the 18-40 GHz system.

Terahertz Imaging Simulation Using Silicon-based Microstrip Antenna and Horn Antenna for Breast Cancer Detection

Breast cancer is one of the most common cancers in the world that cause a lot of mortality. Early cancer detection is crucial to decrease morbidity and mortality rates worldwide. Effective treatment or early intervention is crucial before the disease becomes more incurable. This research contributes to proposing a THz imaging system for early cancer detection, especially breast cancer, by using the benefit of THz radiation. Some approaches are made differently from the previous research, such as the imaging method, the antenna type, and the material for the antenna with the expectation of producing an efficient system and better imaging results. The system consists of one microstrip antenna as a transmitter, 25 horn antenna as a receiver and a breast tissue model. All antenna is designed to meet the requirement specification. The receiver antenna will receive power from the transmitter which will vary due to the absorption of the breast model. The received power will be visualized into a 2D color image. The simulation was able to visualize an image of the breast tissue model. Received power varies from -16.280 dB to -55.241 dB which leads to different color levels to represent the model. Antenna radiation patterns also take a role to cause the phenomenon occurred that leads to differentiation of the breast tissue type. Based on the results, this research has able to simulate a THz imaging system for breast cancer. Further modification to the system can be done to improve the imaging results.

Single optical element to generate a meter-scale THz diffraction-free beam.

Diffraction-free electromagnetic beam propagates in free space without change in its two-dimensional transverse profile. Elongating diffraction-free length can benefit the practical application of this beam. Here, we demonstrate that a THz diffraction-free beam with meter-scale length can be achieved by using only one optical element. By circumscribing the line-shape of spherical harmonic function on a traditional axicon, such optical element is designed, and then can be fabricated by 3D-printing technique. Simulated, experimental, and theoretical results all show that the diffraction-free length of generated beam is over 1000 mm. Further analysis based on Fourier optics theory indicates that the spatial frequency of this beam has a comb distribution, which plays a key role during the beam generation process. Moreover, such distribution also demonstrates the beam generated by our invented optical element is not the Bessel beam, but a new diffraction-free beam. It is believed that this meter-scale THz diffraction-free beam can be useful in a non-contact and non-destructive THz imaging system for large objects.

Clinical Diagnosis of Gastric Cancer by High-Sensitivity THz Fiber-Based Fast-Scanning Near-Field Imaging.

Simple SummaryIn order to realize rapid and complete pre-screening in the pathological examination of gastric cancer and assist in quickly determining the boundary between cancerous and healthy cells, we developed a fast-scanning near-field terahertz (THz) imaging system. This imaging system is compact, low-cost, and easy to operate. THz examination does not need any processing after sectioning, and the diagnostic results are directly displayed in images within one minute. Compared with the H&E staining method, THz imaging diagnosis uses a quantitative absorption coefficient to distinguish cancer tissue and healthy tissue, which makes the automation of the tissue sampling pre-screening procedure possible and will save valuable time to help quickly define cancer tissue. At the same time, the spatial resolution of our near-field imaging system reaches λ/17. Using THz imaging to accurately define the margins of cancer can not only conserve healthy tissues but also minimize the number of second surgical procedures, which would save a lot of additional hospital resources.The distinguishable absorption contrast among healthy gastric tissues, carcinoma in situ and cancer tissues in the THz frequency range is one of the keys to realizing gastric cancer diagnosis by THz imaging. Based on microwave devices and a sub-wavelength fiber, we developed a fast-scanning THz imaging system combined with the principle of surface plasmon resonance enhancement. This imaging system has a near-field λ/17 spatial resolution and imaging S/N ratio as high as 108:1, and the image results are directly displayed within 1 min. We also successfully demonstrated the image diagnostic capability on sliced tissues from eight patients with gastric cancer. The results indicate that THz absorption images can not only clearly distinguish cancer tissue from healthy tissues but also accurately define the margins of cancer. Through a medical statistical study of 40 sliced tissues from 40 patients, we prove that THz imaging can be used as a standalone method to diagnose gastric cancer tissues with a diagnostic specificity and sensitivity of 100%. Compared with the H&E staining method, THz imaging diagnosis makes the automation of tissue-sampling pre-screening procedure possible and assists in quickly determining the boundary between cancerous and healthy tissues.

Fast subpixel displacement measurement, part 1: one-dimensional analysis, simulation, and experiment

Many image processing applications require a quantitative estimation of the underlying correspondence (of the relative position in space) between two or more images. Here, we propose a unique, less numerically intensive method, which we refer to as the subtraction method (SM), to measure in-plane object motion with subpixel accuracy. This method can be usefully employed in conjunction with the correlation method (CM) to provide fine motion information very quickly (∼100 times faster). We first review the CM, then, demonstrate, explain, and examine the proposed SM using analytic functions, standard digital images, and using both simulated and experimental data. Specifically, subpixel motion results for Gaussian functions, four different standard images, and speckle images are examined. Performance is compared to that of the CM. Experimentally, images are captured using both optical and THz imaging systems. A high-resolution megapixel camera is used in the optical system and a low-resolution camera with 16 × 16 pixels is used to perform the THz measurements. One-dimensional subpixel motion in both x and y is examined and analyzed. The results demonstrate that the method can be flexibly used in different areas.

Plasmonic Metasurfaces for Controlling the Spectrum of Terahertz Radiation Sources

We report on the development and fabrication of a plasmonic metasurfaces for PCA based on InAlAs/InGaAs superlattice heterostructures. As a result of THz time-domain measurements, it was found that for PCA emitters with a 150 nm thick plasmonic metasurface, the maximum recorded overall THz power was 5.3 (the optical-THz conversion efficiency was 0.19 %) under laser pumping with an average power of 2.8 mW and a bias voltage of 8.6 V. The increase in the signal-to-noise ratio due to the excitation of plasmon modes exceeds ~80 dB. The experiments have shown that the increase of metallization thickness provides the increase of the emitted power. The results could open a pathway towards the development of the Russian THz spec-troscopic and imaging systems.

Magnetron sputtering deposited large-scale Weyl semimetal THz detector

THz detectors are considered as a building block for a wide range of applications including THz imaging system as well as THz communication system. However, beset by low sensitivity, small effective detection area, high preparation cost and high noise equivalent power, further research still needs especially in high-performance and large-scale THz detectors. In order to fulfill this requirement, Weyl semimetal MoTe2 was chosen as the active layer due to its lower dark current and Weyl-point enhancement. Moreover, high-quality and large-scale THz MoTe2-based detectors were fabricated with an effective detection area of 1 mm × 6.3 mm by means of magnetron sputtering deposition method. With an external voltage applied, our device exhibits excellent surface plasmon polaritons effect for THz detection and demonstrates a photoresponse of 4 A/W and a noise equivalent power of 9.74 × 10-12 W Hz−1/2 under the illumination of 0.1 THz wave at room temperature. Our research paves the way to the fabrication of large-scale THz detectors, also proves that the Weyl semimetal can be an excellent candidate of high-performance material in long-wave detection.

Highly sensitive CMOS plasmon detector with a low‐gain buffer amplifier for terahertz imaging systems

AbstractA CMOS plasmon detector with a low‐gain buffer amplifier, which enhances the characteristics of the detector core, is proposed for improving the performance of THz imaging systems. A high voltage gain (AV) of the main amplifier integrated with the detector improves the voltage responsivity (RV) but not the performances of the detector core itself. The increase in the ratio of the RV to the noise‐equivalent power (NEP) in the proposed detector with a low‐gain buffer can improve the signal‐to‐noise ratio (SNR) of the THz images. The proposed detector with a − 1.5 dB AV buffer amplifier is fabricated by TSMC 0.25 μm CMOS technology and measured to have the RV of 357.1 kV/W and NEP of 57.3 pW/√Hz at the gate bias of 0.17 V for 0.2 THz input signals. The measurement results indicate that the proposed detector has an RV lower than that of a detector with an AV of 11.2 dB exhibiting a similar NEP. The obtained 0.2 THz raster scanning images show that the image SNR can be improved by 14.8 dB using in the proposed detector.

Low-voltage driven graphene-loaded metal wire grating device in total internal reflection geometry for broadband THz modulation

Terahertz modulators with capability of both intensity and phase are essential for THz imaging and communication systems. The low-voltage driven THz modulation technique is crucial for integrating the modulators with electronics components. There is still a lack of broadband devices able to achieve both amplitude and phase modulation with low voltage, due to the underlying physics behind existing approaches. Here, we demonstrate a graphene-loaded metal wire grating THz modulator in the total internal reflection geometry to achieve intensity modulation of 80% and phase modulation of 70 degree within 3 volts gate voltage. Quite different from using the strategy of metamaterials based on the electromagnetic resonance effects, our design has performed a broadband modulation for over 1 THz bandwidth.

A review on applications of integrated terahertz systems

A review on Terahertz end-to-end systems with an emphasis on integrated approaches is presented. Four major catalogs of THz integrated systems, including THz communication systems, THz imaging systems, THz radars, and THz spectroscopy systems, are reviewed in this article. The performance of integrated systems is compared with non-integrated solutions, followed by a discussion on the trend in future research avenues and applications.

Photoconductive Semiconductor-Based Optoelectronic Devices for Generation of Terahertz Radiation

We report on the development and fabrication of a superlattice-based InAlAs/InGaAs heterostructure featuring an ultrashort photocarrier lifetime which has been used as a material base for a photoconductive antenna (PCA)-emitter with a frequency bandwidth of 10 GHz — 3.0 THz and dynamic range above 60 dB. We show an efficiency of plasmonic electrodes embedded into the 10 μm PCA’s gap demonstrating the enhancement of the emitted THz power up to 10 μW (at bias voltage 20—30 V and average pump power 10 mW). The results could open a pathway towards the development of the Russian THz spectroscopic and imaging systems.

Low cost and long-focal-depth metallic axicon for terahertz frequencies based on parallel-plate-waveguides

In this work we demonstrate a triangular surface lens (axicon) operating at frequencies between 350 and 450 GHz using parallel-plate-waveguide technology. The proposed axicon offers longer focal depth characteristics compared to conventional plastic lenses, surpassing common TPX lenses by one order of magnitude. Additionally, due to the triangular surface of the axicon, this device is able to focus THz radiation onto smaller areas than TPX lenses, enhancing the resolution characteristics of THz imaging systems. The frequency range of operation of the proposed axicon can be easily tuned by changing the space between plates, making this approach a very attractive candidate for low-cost, robust and easy to assemble solutions for the next generation of active THz devices.

340-GHz Heterogeneously-Integrated THz Imager With 4°-Beamwidth 16×16 IPD Antenna Array for Lensless Terahertz Imaging Applications

A low-cost and planar heterogeneously-integrated 340-GHz THz imager is proposed for lensless THz imaging applications. The proposed THz imager is composed of a 16×16 antenna array realized in an integrated-passive-device (IPD) technology, an IPD-to-CMOS THz interconnect, and a power detector implemented in a 0.18- μm CMOS technology. The 16×16 IPD antenna array can provide simulated antenna gain of 23.9 dBi, antenna directivity of 30 dB, and half-power beamwidth (HPBW) of 5.6° at 340 GHz. The proposed THz interconnect utilizes a transmission line coupling technique to provide low-loss and broadband signal transition from a CMOS chip to an IPD one while occupying a small chip area. The simulated insertion loss of the THz interconnect is only 1.8 dB at 340 GHz while providing 3-dB bandwidth from 230 to 446 GHz. The power detector exploits the transistor's inherent even-order nonlinearity to rectify the input signal for power detection. The power detector can give simulated voltage responsivity R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</sub> and noise equivalent power (NEP) of 190.2 kV/W and 1.9 nW/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sup> at 340 GHz, respectively, as the chopping frequency f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mod</sub> is 1 kHz. A nonlinear curve fit technique is proposed to tackle the undesired fluctuation of the measured output voltage due to a standing-wave effect. Experimental results show that the proposed heterogeneously-integrated THz imager can provide measured effective R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</sub> and NEP of 0.967 MV/W and 0.18 nW/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sup> at 328 GHz, respectively, as f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mod</sub> is 1 kHz. The measured antenna directivity and HPBW can be 30 dB and 4° at 340 GHz, respectively. Such a THz imager with advantages of high antenna directivity and narrow HPBW can be employed to realize a simple, low-cost, and lensless THz imaging system. To the best of the authors' knowledge, the proposed THz imager integrates the highest number of antennas and exhibits the highest antenna directivity and the narrowest HPBW at THz frequencies reported thus far.