Super-resolution reconstruction algorithm for terahertz imaging below diffraction limit

Terahertz(THz)imaging has drawn significant attention because THz wave has a unique capability to transient,ultrawide spectrum and low photon energy.However,the low resolution has always been a problem due to its long wavelength,limiting their application of fields practical use.In this paper,we proposed a complex one-shot super-resolution(COSSR)framework based on a complex convolution neural network to restore superior THz images at 0.35 times wavelength by extracting features directly from a reference measured sample and groundtruth without the measured PSF.Compared with real convolution neural network-based approaches and complex zero-shot super-resolution(CZSSR),COSSR delivers at least 6.67,0.003,and 6.96%superior higher imaging efficacy in terms of peak signal to noise ratio(PSNR),mean square error(MSE),and structural similarity index measure(SSIM),respectively,for the analyzed data.Additionally,the proposed method is experimentally demonstrated to have a good generalization and to perform well on measured data.The COSSR provides a new pathway for THz imaging super-resolution(SR)reconstruction below the diffraction limit.

A promising diagnostic method:Terahertz pulsed imaging and spectroscopy

The terahertz band lies between the microwave and infrared regions of the electromagnetic spectrum.This radiation has very low photon energy and thus it does not pose any ionization hazard for biological tissues.It is strongly attenuated by water and very sensitive to water content.Unique absorption spectra due to intermolecular vibrations in this region have been found in different biological materials.These unique features make tera-hertz imaging very attractive for medical applications in order to provide complimentary information to existing imaging techniques.There has been an increasing interest in terahertz imaging and spectroscopy of biologically related applications within the last few years and more and more terahertz spectra are being reported.This paper introduces terahertz technology and provides a short review of recent advances in terahertz imaging and spectroscopy techniques,and a number of applications such as molecular spectroscopy,tissue characterization and skin imaging are discussed.

Feasibility of terahertz imaging for discrimination of human hepatocellular carcinoma

BACKGROUND Hepatocellular carcinoma(HCC) is one of the most common malignant tumors worldwide, and novel methods for early/rapid diagnosis of HCC are needed.Terahertz(THz) spectroscopy is considered to have the potential to distinguish between normal liver tissue and HCC tissue; however, there are few reports on it.We conduct this observational study to explore the feasibility of THz imaging for the diagnosis of HCC.AIM To evaluate the feasibility of THz for discriminating between HCC and normal liver tissues using fresh tissue specimens obtained from HCC patients who had undergone surgery.METHODS Normal liver tissue and HCC tissue were cryosectioned into 50 μm-thick slicesand placed on cover glass. Two adjacent tissue sections were separated subjected to histopathological examination by hematoxylin and eosin staining or THz transmission examination, and THz images were compared with pathologically mapped images. We determined the typical tumor and normal liver tissue regions by pathological examination; the corresponding areas of adjacent sections were examined by THz transmission.RESULTS The transmission rate of HCC tissue was 0.15-0.25, and the transmission rate of typical HCC tissue was about 0.2. THz transmittance in normal liver tissue is slightly higher than 0.4, but there were many influencing factors, including the degree of liver cirrhosis, fat components, ice crystals in frozen sections, and apoptosis.CONCLUSION In conclusion, this study shows that THz imaging can detect HCC tissue. Further research will yield more detailed data of the THz transmission rates of HCC tissue with different degrees of differentiation.

Terahertz composite imaging method

In order to improve the imaging quality of terahertz(THz) spectroscopy, Terahertz Composite Imaging Method(TCIM) is proposed. The traditional methods of improving THz spectroscopy image quality are mainly from the aspects of de-noising and image enhancement. TCIM breaks through this limitation. A set of images, reconstructed in a single data collection, can be utilized to construct two kinds of composite images. One algorithm, called Function Superposition Imaging Algorithm(FSIA), is to construct a new gray image utilizing multiple gray images through a certain function. The features of the Region Of Interest(ROI) are more obvious after operating, and it has capability of merging ROIs in multiple images. The other, called Multi-characteristics Pseudo-color Imaging Algorithm(Mc Pc IA), is to construct a pseudo-color image by combining multiple reconstructed gray images in a single data collection. The features of ROI are enhanced by color differences. Two algorithms can not only improve the contrast of ROIs, but also increase the amount of information resulting in analysis convenience. The experimental results show that TCIM is a simple and effective tool for THz spectroscopy image analysis.

Single‐pixel terahertz imaging:a review

This paper is devoted to reviewing the results achieved so far in the application of the single-pixel imaging technique to terahertz(THz)systems.The use of THz radiation for imaging purposes has been largely explored in the last twenty years,due to the unique capabilities of this kind of radiation in interrogating material properties.However,THz imaging systems are still limited by the long acquisition time required to reconstruct the object image and significant efforts have been recently directed to overcome this drawback.One of the most promising approaches in this sense is the so-called“single-pixel”imaging,which in general enables image reconstruction by patterning the beam probing the object and measuring the total transmission(or reflection)with a single-pixel detector(i.e.,with no spatial resolution).The main advantages of such technique are that i)no bulky moving parts are required to raster-scan the object and ii)compressed sensing(CS)algorithms,which allow an appropriate reconstruction of the image with an incomplete set of measurements,can be successfully implemented.Overall,this can result in a reduction of the acquisition time.In this review,we cover the experimental solutions proposed to implement such imaging technique at THz frequencies,as well as some practical uses for typical THz applications.

Terahertz endoscopic imaging for colorectal cancer detection:Current status and future perspectives

Terahertz(THz) imaging is progressing as a robust platform for myriad applications in the field of security,health,and material science.The THz regime,which comprises wavelengths spanning from microns to millimeters,is non-ionizing and has very low photon energy:Making it inherently safe for biological imaging.Colorectal cancer is one of the most common causes of death in the world,while the conventional screening and standard of care yet relies exclusively on the physician's experience.Researchers have been working on the development of a flexible THz endoscope,as a potential tool to aid in colorectal cancer screening.This involves building a single-channel THz endoscope,and profiling the THz response from colorectal tissue,and demonstrating endogenous contrast levels between normal and diseased tissue when imaging in reflection modality.The current level of contrast provided by the prototype THz endoscopic system represents a significant step towards clinical endoscopic application of THz technology for invivo colorectal cancer screening.The aim of this paper is to provide a short review of the recent advances in THz endoscopic technology and cancer imaging.In particular,the potential of single-channel THz endoscopic imaging for colonic cancer screening will be highlighted.

Exploration of high-speed 3.0 THz imaging with a 65 nm CMOS process

This paper describes a promising route for the exploration and development of 3.0 THz sensing and imaging with FET-based power detectors in a standard 65 nm CMOS process.Based on the plasma-wave theory proposed by Dyakonov and Shur,we designed high-responsivity and low-noise multiple detectors for monitoring a pulse-mode 3.0 THz quantum cascade laser(QCL).Furthermore,we present a fully integrated high-speed 32×32-pixel 3.0 THz CMOS image sensor(CIS).The full CIS measures 2.81×5.39 mm^(2) and achieves a 423 V/W responsivity(Rv)and a 5.3 nW integral noise equivalent power(NEP)at room temperature.In experiments,we demonstrate a testing speed reaching 319 fps under continuous-wave(CW)illumina-tion of a 3.0 THz QCL.The results indicate that our terahertz CIS has excellent potential in cost-effective and commercial THz imaging and material detection.

Near-field 3D imaging approach combining MJSR and FGG-NUFFT

A near-field three-dimensional(3 D)imaging method combining multichannel joint sparse recovery(MJSR)and fast Gaussian gridding nonuniform fast Fourier transform(FGGNUFFT)is proposed,based on a perfect combination of the compressed sensing(CS)theory and the matched filtering(MF)technique.The approach has the advantages of high precision and high efficiency:multichannel joint sparse constraint is adopted to improve the problem that the images recovered by the single channel imaging algorithms do not necessarily share the same positions of the scattering centers;the CS dictionary is constructed by combining MF and FGG-NUFFT,so as to improve the imaging efficiency and memory requirement.Firstly,a near-field 3 D imaging model of joint sparse recovery is constructed by combining the MF-based imaging method.Secondly,FGG-NUFFT and reverse FGG-NUFFT are used to replace the interpolation and Fourier transform in MF-based imaging methods,and a sensing matrix with high precision and high efficiency is constructed according to the traditional imaging process.Thirdly,a fast imaging recovery is performed by using the improved separable surrogate functionals(SSF)optimization algorithm,only with matrix and vector multiplication.Finally,a 3 D imagery of the near-field target is obtained by using both the horizontal and the pitching interferometric phase information.This paper contains two imaging models,the only difference is the sub-aperture method used in inverse synthetic aperture radar(ISAR)imaging.Compared to traditional CS-based imaging methods,the proposed method includes both forward transform and inverse transform in each iteration,which improves the quality of reconstruction.The experimental results show that,the proposed method improves the imaging accuracy by about O(10),accelerates the imaging speed by five times and reduces the memory usage by about O(10~2).

Terahertz Three-Dimensional Imaging Based on Computed Tomography with Photonics-Based Noise Source

Computed tomography has been proven to be useful for non-destructive inspection of structures and materials. We build a three-dimensional imaging system with the photonically generated incoherent noise source and the Schottky barrier diode detector in the terahertz frequency band （90–140GHz）. Based on the computed tomography technique, the three-dimensional image of a ceramic sample is reconstructed successfully by stacking the slices at different heights. The imaging results not only indicate the ability of terahertz wave in the non-invasive sensing and non-destructive inspection applications, but also prove the effectiveness and superiority of the uni-traveling-carrier photodiode as a terahertz source in the imaging applications.

Approach for ISAR imaging of near-field targets based on coordinate conversion and image interpolation

Inverse synthetic aperture radar(ISAR)imaging of near-field targets is potentially useful in some specific applications,which makes it very important to efficiently produce highquality image of the near-field target.In this paper,the simplified target model with uniform linear motion is applied to the near-field target imaging,which overcomes the complexity of the traditional near-field imaging algorithm.According to this signal model,the method based on coordinate conversion and image interpolation combined with the range-Doppler(R-D)algorithm is proposed to correct the near-field distortion problem.Compared with the back-projection(BP)algorithm,the proposed method produces better focused ISAR images of the near-field target,and decreases the computation complexity significantly.Experimental results of the simulated data have demonstrated the effectiveness and robustness of the proposed method.

A new horizon for neuroscience:terahertz biotechnology in brain research

Terahertz biotechnology has been increasingly applied in various biomedical fields and has especially shown great potential for application in brain sciences.In this article,we review the development of terahertz biotechnology and its applications in the field of neuropsychiatry.Available evidence indicates promising prospects for the use of terahertz spectroscopy and terahertz imaging techniques in the diagnosis of amyloid disease,cerebrovascular disease,glioma,psychiatric disease,traumatic brain injury,and myelin deficit.In vitro and animal experiments have also demonstrated the potential therapeutic value of terahertz technology in some neuropsychiatric diseases.Although the precise underlying mechanism of the interactions between terahertz electromagnetic waves and the biosystem is not yet fully understood,the research progress in this field shows great potential for biomedical noninvasive diagnostic and therapeutic applications.However,the biosafety of terahertz radiation requires further exploration regarding its two-sided efficacy in practical applications.This review demonstrates that terahertz biotechnology has the potential to be a promising method in the field of neuropsychiatry based on its unique advantages.

Spectroscopic measurements and terahertz imaging of the cornea using a rapid scanning terahertz time domain spectrometer

Spectroscopic measurements and terahertz imaging of the cornea are carried out by using a rapid scanning terahertz time domain spectroscopy（THz-TDS） system.A voice coil motor stage based optical delay line（VCM-ODL） is developed to provide a rather simple and robust structure with both the high scanning speed and the large delay length.The developed system is used for THz spectroscopic measurements and imaging of the corneal tissue with different amounts of water content,and the measurement results show the consistence with the reported results,in which the measurement time using VCM-ODL is a factor of 360 shorter than the traditional motorized optical delay line（MDL）.With reducing the water content a monotonic decrease of the complex permittivity of the cornea is observed.The two-term Debye relaxation model is employed to explain our experimental results,revealing that the fast relaxation time of a dehydrated cornea is much larger than that of a hydrated cornea and its dielectric behavior can be affected by the presence of the biological macromolecules.These results demonstrate that our THz spectrometer may be a promising candidate for tissue hydration sensing and practical application of THz technology.

Experimental research on spectrum and imaging of continuous-wave terahertz radiation based on interferometry

A system for measuring terahertz spectrum is proposed based on optical interferometer theory, and is experimentally demonstrated by using a backward-wave oscillator as the terahertz source. A high-resolution, high-precision interferometer is constructed by using a pyroelectric detector and a chopper. The results show that the spectral resolution is better than 1 GHz and the relative error of frequency is less than 3%. The terahertz energy density distribution is calculated by an inverse Fourier transform and tested to verify the feasibility of the interferometric approach. Two kinds of carbon-fiber composites are imaged. The results confirm that the interferometer is useful for transmission imaging of materials with different thickness values.

Terahertz two-pixel imaging based on complementary compressive sensing

A compact terahertz（THz） imaging system based on complementary compressive sensing has been proposed using two single-pixel detectors. By using a mechanical spatial light modulator, sampling in the transmission and reflection orientations was achieved simultaneously, which allows imaging with negative mask values. The improvement of THz image quality and anti-noise performance has been verified experimentally compared with the traditional reconstructed image, and is in good agreement with the numerical simulation. The demonstrated imaging system, with the advantages of high imaging quality and strong anti-noise property, opens up possibilities for new applications in the THz region.

Terahertz Transmission Imaging with 2.52 THz Continuous Wave

In this article, two terahertz transmission imaging systems are built with a 2.52 THz continuous wave laser and two types of sensors. One is array scanning system using a 124×124 pyro-electric array camera as the detector; the other is a point-wise scanning system utilizing a Golay cell as the detector. The imaging speed and quality is briefly analyzed. Terahertz （THz） imaging results demonstrate that the array scanning system has higher imaging speed with lower resolution. The point-wise scanning system has higher imaging quality with lower speed.

Pixel for Passive Terahertz Imaging System （PTIS） at the Base of Josephson Junction in Self-Pumping Regime

The use of Josephson junctions （JJ） in self-pumping mode as one of the heterodyne detection method can stay the new trend for design modern passive terahertz imaging system （PT1S）. Combination superconductive and semiconductor technologies on the one substrate in any case will born principal trend of the critical electronic technologies, including the realization superconductive JJ near the gate of a semiconductor transistor. Further development will allow to realize the PTIS system with electronic tuning of the receiving frequency. At the same time there exists yet additional new possibility to measure the frequency of the receiving signals （very briefly） on JJ.

Two-color laser PEEM imaging of horizontal and vertical components of femtosecond surface plasmon polaritons

Explicit visualization of different components of surface plasmon polaritons(SPPs) propagating at dielectric/metal interfaces is crucial in offering chances for the detailed design and control of the functionalities of plasmonic nanodevices in the future. Here, we reported independent imaging of the vertical and horizontal components of SPPs launched from a rectangular trench in the gold film by a 400-nm laser-assisted near-infrared(NIR) femtosecond laser time-resolved photoemission electron microscopy(TR-PEEM). The experiments demonstrate that distinct imaging of different components of SPPs field can be easily achieved by introducing the 400-nm laser. It can circumvent the risk of sample damage and information loss of excited SPPs field that is generally confronted in the usual NIR laser TR-PEEM scheme. The underlying mechanism for realizing distinct imaging of different components of the SPPs field with two-color PEEM is revealed via measuring the double logarithmic dependence of photoemission yield with the 800-nm and 400-nm pulse powers of different polarizations. Moreover, it is found that the PEEM image quality of the vertical and horizontal components of the SPPs field is nearly independent of the 400-nm pulse polarization. These results pave a way for SPPs-based applications and offer a possible solution for drawing a space-time field of SPPs in three dimensions.

Bridging the terahertz near-field and far-field observations of liquid crystal based metamaterial absorbers

Metamaterial-based absorbers play a significant role in applications ranging from energy harvesting and thermal emitters to sensors and imaging devices.The middle dielectric layer of conventional metamaterial absorbers has always been solid.Researchers could not detect the near field distribution in this layer or utilize it effectively.Here,we use anisotropic liquid crystal as the dielectric layer to realize electrically fast tunable terahertz metamaterial absorbers.We demonstrate strong,position-dependent terahertz near-field enhancement with sub-wavelength resolution inside the metamaterial absorber.We measure the terahertz far-field absorption as the driving voltage increases.By combining experimental results with liquid crystal simulations,we verify the near-field distribution in the middle layer indirectly and bridge the nearfield and far-field observations.Our work opens new opportunities for creating high-performance,fast,tunable,terahertz metamaterial devices that can be applied in biological imaging and sensing.

Mapping an on-chip terahertz antenna by a scanning near-field probe and a fixed field-effect transistor

In the terahertz（THz） regime,the active region for a solid-state detector usually needs to be implemented accurately in the near-field region of an on-chip antenna.Mapping of the near-field strength could allow for rapid verification and optimization of new antenna/detector designs.Here,we report a proof-of-concept experiment in which the field mapping is realized by a scanning metallic probe and a fixed AlGaN/GaN field-effect transistor.Experiment results agree well with the electromagnetic-wave simulations.The results also suggest a field-effect THz detector combined with a probe tip could serve as a high sensitivity THz near-field sensor.

Improvement of femtosecond SPPs imaging by two-color laser photoemission electron microscopy

Clear imaging of surface plasmon polaritons(SPPs)is a prerequisite for SPPs-based applications.In this work,we demonstrate an improvement of near-field imaging of SPPs via directly comparing the visibility of the photoemission electron microscopy(PEEM)image of SPPs under one-and two-color laser excitation(also known as one-or two-color laser PEEM).By measuring the photoelectron yield and the contrast of the interference fringes of SPPs,we demonstrate that in addition to enhancing the photoemission yield,two-color laser PEEM can significantly improve the contrast between bright and dark fringes(nearly 4 times higher than that of one-color laser case).By recording the nonlinear order of the photoelectrons ejected from the bright and dark fringes,respectively,the underlying mechanism for the improved visibility is revealed.In addition,the influences of the polarization direction of 400-nm laser on the PEEM images of the SPPs with different wave vector directions are shown.These results can provide technical support for the development of SPPs-based communication devices and catalysis.