THz Imaging Applications Research Articles

Highly Sensitive Quad-Ray shaped Polarization Insensitive THz Meta-Biosensor

Abstract Metasurface-based sensors are now becoming crucial for label-free and rapid-detection technologies in biomedical applications, leading to a growing demand for new highly sensitive meta-biosensors. This paper demonstrates a perfectly symmetrical quad-ray X-shaped THz meta-absorber (QRXMA), enabling narrowband and polarization insensitivity performance. The theoretical modelling and performance investigation focuses on its potential for improved refractive index (RI) sensing for multiple biological samples. The simulation outcomes reveal that the proposed optimal QRXMA achieves an absorption rate of 99.98% at 1.3 THz for both TE and TM polarized incidences. Furthermore, we highlight the tunability of absorptive characteristics of the proposed device by altering the different geometric parameters. These findings underscore the QRXMA potential in RI sensing applications, achieving a sensitivity of approximately 120 GHz RIU-1 and a Figure of Merit (FoM) of about 2 with an 8 μm thick non-destructive analyte layer. This research paves the way for the development of highly efficient meta-biosensors with promising applications in THz detection, sensing, and imaging.

A tunable chiral metasurface useable in terahertz imaging and wavefront shaping

We proposed a tunable chiral metasurface comprising a reflective bottom layer of gold, a dielectric layer of polyimide, and a structural top layer of gold-graphene. Its main properties were studied via numerical simulations conducted using CST Studio Suite. The results indicate that, based on the chiral metasurface, we achieved dual-band circular dichroism of −0.5 and 0.77 at 0.9 THz and 1.06 THz, respectively, and complementary near-field imaging applications were realized by tuning the Fermi level (E f ) of graphene. Subsequently, exploiting the exceptional selective characteristics of circularly polarized waves using a chiral metasurface, eight chiral phase-gradient metasurfaces were constructed by rotating the chiral structure. Moreover, based on the Pancharatnam-Berry phase principle, tunable wavefront shaping applications were further realized, including anomalous reflection, vortex beams, and focusing. In anomalous reflection, the reflection angles for left-circularly polarized (LCP) and right-circularly polarized (RCP) incidences are opposite when adjusting the E f of graphene. For example, when the graphene E f is 0 eV and the LCP wave is incident at 0°, the reflection angle is −18°. Conversely, when the graphene E f is 1 eV and the RCP wave is incident at 0°, the reflection angle is 18°. In the application of vortex beams, by adjusting the E f of graphene, we achieved vortex beams with opposite topological charges under different circularly polarized incidences. In the focusing application, the incident LCP and RCP can achieve focusing and defocusing, respectively. And the graphene E f can dynamically control the focusing efficiency at the incident LCP, increasing it from 13.63% to 44.84%.

An ultra broadband metamaterial absorber based on metal-dielectric-metal technology for THz spectrum

This paper introduces a novel ultra-broadband Metamaterial Absorber (UBMA) demonstrating significant absorption capabilities across a wide terahertz frequency range from 2.42 THz to 6.11 THz. The 3.7 THz bandwidth represents 87% of the central frequency. The proposed UBMA comprises three layers: a star-shaped metal patch on top, a dielectric substrate in the middle, and a metallic ground plane below. Simulations using CST Microwave Studio software reveal that the design achieves high absorption at five distinct frequencies: 2.47, 3.45, 4.89, 6.01, and 6.87 THz, with absorption rates of 99% for the first four peaks and 90% for the fifth peak. The study of electric field and surface current distribution provides insights into the absorption mechanism. While the UBMA exhibits polarization-independent performance, its angular response shows some sensitivity to the incident angle, especially beyond 30° Despite this, the absorber maintains over 70% absorptivity up to a 45° incidence angle for both TE and TM polarizations within specific frequency ranges. The simple structure combined with high absorption efficiency makes the UBMA suitable for THz imaging, detection, and stealth applications, although its angular sensitivity must be considered for certain applications.

Microstructured large-area photoconductive terahertz emitters driven at high average power

Emitters based on photoconductive materials excited by ultrafast lasers are well-established and popular devices for THz generation. However, so far, these emitters – both photoconductive antennas and large area emitters - were mostly explored using driving lasers with moderate average powers (either fiber lasers with up to hundreds of milliwatts or Ti:Sapphire systems up to few watts). In this paper, we explore the use of high-power, MHz repetition rate Ytterbium (Yb) based oscillator for THz emission using a microstructured large-area photoconductive emitter, consist of semi-insulating GaAs with a 10 × 10 mm2 active area. As a driving source, we use a frequency-doubled home-built high average power ultrafast Yb-oscillator, delivering 22 W of average power, 115 fs pulses with 91 MHz repetition rate at a central wavelength of 516 nm. When applying 9 W of average power (after an optical chopper with a duty cycle of 50%) on the structure without optimized heatsinking, we obtain 65 µW THz average power, 4 THz bandwidth; furthermore, we safely apply up to 18 W of power on the structure without observing damage. We investigate the impact of excitation power, bias voltage, optical fluence, and their interplay on the emitter performance and explore in detail the sources of thermal load originating from electrical and optical power. Optical power is found to have a more critical impact on large area photoconductive emitter saturation than electrical power, thus optimized heatsinking will allow us to improve the conversion efficiency in the near future towards much higher emitter power. This work paves the way towards achieving hundreds of MHz or even GHz repetition rates, high-power THz sources based on photoconductive emitters, that are of great interest for example for future THz imaging applications.

Tunable and switchable multifunctional terahertz meta-mirror based on graphene and vanadium dioxide.

We design a multifunctional THz polarization modulation meta-mirror integrated with polarization conversion and dichroism functions switched by temperature and voltage. The meta-mirror is composed of two-layered graphene metasurfaces and a layer of vanadium dioxide (VO2) on a gold film substrate. Linear-to-linear polarization conversion and linear dichroism (LD) can be switched by temperature control in the VO2 film and Fermi level adjustments in the graphene metasurfaces, where the polarization conversion ratio (PCR) is higher than 0.9 in the range of 2.89 THz to 4.02 THz, LD value reached a maximum of 0.6 at 3.84 THz, and linear-to-circular polarization conversion and circular dichroism (CD) can also be tuned with ellipticity higher than 0.9 in the range of 2.32 THz to 2.69 THz and CD value as high as 0.71 at 2.45 THz. The proposed meta-mirror is the first THz metamaterial device integrating four switchable functions, including linear-to-linear polarization conversion, linear-to-circular polarization conversion, linear dichroism and circular dichroism. The meta-mirror is a promising design for compact system integration in THz imaging, sensing and biological detection applications.

A robust adaptive-thresholding primal-dual sparse recovery for THz imaging

Terahertz time-domain spectroscopy (THz-TDS) system is a powerful tool in material spectral analysis and non-destructive testing (NDT). However, the data acquisition is time-consuming due to the point-by-point scanning process, which leads to limitations in THz imaging applications. In this paper, a robust adaptive-thresholding primal-dual sparse recovery (APSR) method is proposed to reconstruct high-resolution THz images and reduce the acquisition time. A sparsity averaging dictionary, which is a concatenate of multiple bases, is applied to improve the robustness and generalization. A reweighted scheme is also adopted to enhance the sparse solution. As a crucial consideration in the sparse recovery method, we propose a robust adaptive threshold estimator based on the median absolute deviation in the sense that the final threshold determines the reconstruction quality and the convergence rate. Meanwhile, together with the Nesterov acceleration technique, a large threshold can be applied at the beginning of the iteration to help accelerate the convergence. Numerical experiments demonstrate that the APSR method can reconstruct THz images with robustness even with low SNR (of 30 dB) and low sampling rate (of 30%) compared to conventional methods. In the real THz-TDS system, our proposed sparse imaging method can successfully recover a composite object with much fewer THz spectra and reduces the acquisition time to 30% at the cost of a relative error of 2.5%, demonstrating the efficiency of our proposed method and the penetrating ability of THz technology in the NDT applications.

Terahertz cancer imaging and sensing: open research challenges and opportunities

There has been a rapid development of THz technology—sources, detectors and various THz imaging and sensing techniques. The THz technology demonstrates great potential as a modality for early, label free, non-ionizing and non-invasive detection of cancer. Some progressive technological development milestones have been achieved in this regard, however, to become clinically competitive and to provide the sought after real operational convenience, there is need for further research and development to overcome the existing challenges. This paper provides recent trends and perspectives through identification of existing challenges for the development of THz imaging and sensing systems that can evolve into actual medical modalities. We provide an overview of various aspects of THz technology, including techniques for imaging and sensing, mechanisms for THz image contrast and models for tissue dielectric responses to THz waves. The THz imaging application for detection of various cancers is briefed. The advantages of THz cancer imaging and sensing as well as the existing challenges are identified, with recommendations provided in contribution to future research. Further, some recent THz imaging and sensing developments such as the near-field methods to break the diffraction limit including waveguides, resonance and plasmonic metasurfaces are discussed. We emphasize the contribution of analytical algorithms that are based on machine learning, in particular, deep learning for the development of THz technology.Graphical abstract

Ion‐Bolometric Effect in Grain Boundaries Enabled High Photovoltage Response for NIR to Terahertz Photodetection

AbstractThe excellent performance of bolometers in the infrared and terahertz regions has attracted great attention. Understanding the transport process of charged particles is an efficient approach to determine detector performance. However, the lack of studies on the fine‐scale spatial motion of microscopic particles in bolometers has prevented a full understanding of the physical process. Herein, using micro‐nano‐scale optoelectronic performance correlation measurements, it is described how prevalent defect states at the grain boundaries (GBs) decrease current responses. Ions at the GBs of the polycrystalline perovskite bolometer contribute to the photocurrent via thermal excitons. In addition, the built‐in electric field established by ion migration fluctuates periodically with the strength of the light‐heating process due to the interaction between the bolometric effect and the Coulomb force. Additionally, the first ion‐bolometric detector is demonstrated with a significant photovoltage response to infrared and THz waves (75.3 kV W−1 at 1064 nm and 2.3 kV W−1 at 0.22 THz). An examination of the THz images shows the potential for large‐area THz imaging applications. The ion‐bolometric effect combines the broad spectral characteristics of the bolometer effect with the temperature sensitivity due to ion migration and provides a unique perspective on detector technology.

Terahertz phase modulator based on a metal-VO2 reconfigurable metasurface.

Actively controlling the phase of a terahertz (THz) wave is of great significance for beaming, tunable focusing, and holography. We present a THz phase modulator based on an electrically triggered vanadium dioxide (V O 2) reconfigurable metasurface. The unit cell of the device consists of two split-ring resonators embedded with a V O 2 ribbon. By electrically triggering the insulator-to-metal transition of V O 2, the resonance mode and resonance intensity of the unit cell can be dynamically controlled. The simulation results show that the structure can achieve a phase shift of about 360° in the range of 1.03-1.13THz, and the reflection amplitude can reach 80%. The device has potential applications in THz imaging, radar, broadband wireless communications, and array phase control.

THz beam shaping based on diffractive transformation for forming patterned simulation lightfields and wavefronts

In this paper, the traditional Gerchberg-Saxton (GS) algorithm is used to achieve a key phase recovery in the THz band via common Fourier transformation to realize desired beam manipulation. The main technological process includes a single mask UV-photolithography and a dual-step KOH wet etching, thus enabling a kind of THz diffractive optical element (THz-DOE) with the needed phase-step arrangement. The experimental results about THz beam shaping for generating patterns and wavefronts based on the THz-DOEs are given to demonstrate the feasibility of realizing complex lightfield generation and modulation for THz imaging applications. The developed THz-DOEs exhibit a significant THz amplitude modulation for constructing some particular patterns selected. The non-ideal surface roughness and phase-step configuration of the silicon-based DOEs for both the visible and infrared wavelength ranges is perfectly acceptable in the THz band due to its long wavelength nature. A better imaging exhibition based on the beam modulation of the developed THz-DOEs can be expected since the phase configuration accuracy can be further improved according to the technological and micro-structural parameter optimization.

Novel polarization independent multiband THz perfect metamaterial

AbstractThe design and characterization of a multiband THz metamaterial are reported in this article. The proposed metamaterial is constructed over the polyimide substrate embedded with a novel ring‐shaped frequency‐sensitive copper trace on the top providing the desired frequency response. The proposed metamaterial is developed from a unit cell with a base cross shape. The arms of the cross shape are deployed with three folded circular resonators providing the desired frequency‐selective property. The proposed metamaterial offers band‐stop characteristics at 0.83, 2.15, 2.91, 4.1, and 4.86 THz. The proposed unit cell metamaterial has the smallest foot‐print of 0.089λo × 0.089λo where λo is the free‐space wavelength. The electric field distribution and surface current contours are used to describe the operation of the proposed metamaterial. The proposed metamaterial has a symmetric configuration resulting in a polarization‐independent operation. The angular stability of the proposed metamaterial is investigated by subjecting the metamaterial to the various oblique incidence of THz radiation. The proposed THz metamaterial unit cell is suitable for THz imaging and spectroscopy applications.

Tunable six-band miniaturised metamaterial absorber for IoT and THz imaging applications

Tunable six-band miniaturised metamaterial absorber for IoT and THz imaging applications

Design and Comparative Analysis of Ultra-wideband and High Directive Antennas for THz Applications

This work presents a comprehensive detailed comparative study of the three ultra-wideband and high directive antennas for the THz imaging, spectroscopy, and communication applications. Three different types of photoconductive antennas (log-spiral, Vivaldi, and bowtie antennas) are designed and simulated in the frequency range of 1 to 6 THz in the CST microwave studio (MWS). The enhanced directivity of the designed PCAs is achieved with the integration of the hemispherical silicon-based lens with the PCA gold electrode and quartz substrate of the proposed antennas. The performance of the designed PCAs is compared in terms of impedance and axial ratio bandwidths, directivity, and radiation efficiency of the proposed antennas. The reported log spiral, Vivaldi PCAs with added silicon lens exhibit the -10 dB impedance bandwidth of 6 THz, 3dB AR bandwidth of 5 THz, 6 THz, and 6 THz and peak total radiation efficiencies of 45%, 65%, and 95% respectively.

Tailoring the band-pass properties of FSS-based THz filters

High-performance THz band-pass filters are highly demanded for THz spectroscopy and imaging applications. In this work, we have analyzed the spectral features of a THz band-pass filter and divided the spectrum into three spectral regions, including a pure waveguide region, a subwavelength region and a cut-off region. Based on the understanding of the spectrum, we have proposed and fabricated two kinds of THz band-pass filters, which were characterized by time-domain THz spectroscopy and show narrow bandwidths and high stopband suppression.

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.

Design and Characterization of a 10 × 10 Pixel Array THz Camera in 350 nm CMOS Technology

In this paper, a $10\times 10$ pixel array camera for room-temperature THz imaging application in 350 nm CMOS technology is presented. The camera consists of a $10\times 10$ focal-plane array (FPA) of THz detectors, on-chip column amplifiers, digital-to-analog converter (DAC) and on-chip memory for per-pixel calibration. High performance THz detectors are realized by integrating on-chip antennas and sub-threshold Si MOSFETs. The detectors are biased with a current source for increased responsivity and improved NEP. Our designed detector improves responsivity and NEP by a factor of 900 and 7, respectively, compared to conventional photo-voltaic (cold) FET-based detector in the same technology node and detection settings. Additionally, we proposed a successive-approximation (SA) calibration scheme to adjust the operating point of the pixels to maximize the THz response and improve uniformity, in the presence of pixel-to-pixel process variation. At 200 GHz illunimation frequency, the average responsivity and NEP of the 2D imager are measured as 16.4 kV/W and $216~\frac {nW}{\sqrt {Hz}}$ , respectively, with an image acquisition rate of 2 Hz. The responsivity and NEP of a representative pixel are extrapolated to 10 KHz (where 1/f noise has substantially decreased) for comparison with literature. The extrapolated values are 19 kV/W and $535~\frac {pW}{\sqrt {Hz}}$ , respectively. By placing the THz camera in an optical setup, we were able to acquire images of concealed metal objects in a cardboard box at 200 GHz illumination frequency.

Terahertz time domain spectroscopy and imaging application for analysis of sandwich panel with embedded fibre Bragg grating sensors and piezoelectric transducers

Sandwich structures are commonly used in many objects, e.g. deck components in bridge deck panel. Safety requirements result in development of structural health monitoring (SHM) systems that are implemented into sandwich structures. Such systems based on different types of sensors that can be embedded into the structure. The paper presents THz time domain spectroscopy and imaging application for internal structure analyses of a sandwich panel with glass fibre reinforced polymer skin and polyurethane foam fulfilment. In the adhesive bond between skin and fulfilment, an array of fibre Bragg grating (FBG) sensors and piezoelectric (PZT) transducers was embedded. The analyses were focused on detection and localisation of embedded sensors and transducers and their influence on THz wave propagation. The analysis contains differences in THz wave parameters, like refractive index and absorption coefficient and image analyses. Methods that allow to detect and localise of transducers and fibre optics with FBG sensors as well as damage (debonding between GFRP skin and foam fulfilment in sandwich structure) were proposed.

Fabrication of large-scale freestanding THz wire grid polarizers by femtosecond laser micromachining

High-performance THz polarizers, as a quasi-optical device to manipulate THz waves, are highly demanded for THz spectroscopy and imaging applications. In this work, we have employed the femtosecond laser micro-machining technique to fabricate freestanding THz wire grid polarizers with two kinds of reinforced line structures which weaken the effects of gravity sagging and entanglement of long free wires, thus enabling large-scale processing. The effect of the reinforced lines on the polarizers were simulated by the Finite-Difference Time-Domain (FDTD) method, and the performances of the polarizer were characterized by THz time-domain spectroscopy and compared with that of a commercial wire gird polarizer. The calculated transmittance is in good agreement with the FDTD results. The degree of polarization and the extinction ratio of the polarizer with discontinuous reinforced lines are better than those of a commercial wire grid polarizer.

A flexible, multifunctional, active terahertz modulator with an ultra-low triggering threshold

Flexible, multifunctional, active THz modulators with ultra-low triggering threshold were developed by aligned carbon nanotube thin films coated with VO2. These active THz modulators find applications in THz communication and THz imaging.

Ultrathin polarization-insensitive tri-band THz perfect metamaterial absorber

In this paper, an ultrathin and polarization-insensitive THz perfect metamaterial absorber (PMA) was proposed using the traditional sandwiched structure with circular patch resonators on the top layer. The simulated spectrum shows that the proposed PMA has three distinctive absorption peaks at f1 = 0.8 THz, f2 = 2.28 THz and f3 = 3.62 THz, with absorbance of 96.7%, 97.9% and 99.8%, respectively. The electric field distributions of the PMA reveal that the absorption mainly originates from the standing wave resonances between the top and bottom layers. The proposed PMA is polarization insensitive due to its axisymmetric unit cell structure. By adjusting the structure parameters, the resonance frequency, intensity and Q-factor of absorption peak can be tuned effectively. Our design may find potential applications in THz imaging, sensing and signal detection.