J-Net: Improved U-Net for Terahertz Image Super-Resolution.

Terahertz (THz) waves are electromagnetic waves in the 0.1 to 10 THz frequency range, and THz imaging is utilized in a range of applications, including security inspections, biomedical fields, and the non-destructive examination of materials. However, THz images have a low resolution due to the long wavelength of THz waves. Therefore, improving the resolution of THz images is a current hot research topic. We propose a novel network architecture called J-Net, which is an improved version of U-Net, to achieve THz image super-resolution. It employs simple baseline blocks which can extract low-resolution (LR) image features and learn the mapping of LR images to high-resolution (HR) images efficiently. All training was conducted using the DIV2K+Flickr2K dataset, and we employed the peak signal-to-noise ratio (PSNR) for quantitative comparison. In our comparisons with other THz image super-resolution methods, J-Net achieved a PSNR of 32.52 dB, surpassing other techniques by more than 1 dB. J-Net also demonstrates superior performance on real THz images compared to other methods. Experiments show that the proposed J-Net achieves a better PSNR and visual improvement compared with other THz image super-resolution methods.

Exploring the impedance-matching effect in terahertz reflection imaging of skin tissue.

Terahertz (THz) electromagnetic waves, known for their unique response to water, offer promising opportunities for next-generation biomedical diagnostics and novel cancer therapy technologies. This study investigated the impedance-matching effect, which enhances the efficiency of THz wave delivery into tissues and compensates for the signal distortion induced by the refractive index mismatch between the target and the sample substrate. Three candidate biocompatible materials, water, glycerol, and petroleum jelly were applied to a skin phantom and compared using THz two-dimensional imaging and time-of-flight imaging methods. Finally, we successfully demonstrated impedance-matching effect on mouse skin tissues.

Terahertz Spectral Properties of PEO-Based Anti-Adhesion Films Cross-Linked by Electron Beam Irradiation.

We investigated the spectral property changes in anti-adhesion films, which were cross-linked and surface-modified through electron beam irradiation, using terahertz time-domain spectroscopy (THz-TDS). Polyethylene oxide (PEO), which is a biocompatible and biodegradable polymer, was the main component of these anti-adhesion films being manufactured for testing. The terahertz characteristics of the films were affected by the porosity generated during the freeze-drying and compression processes of sample preparation, and this was confirmed using optical coherence tomography (OCT) imaging. An anti-adhesion polymer film made without porosity was measured by using the THz-TDS method, and it was confirmed that the refractive index and absorption coefficient were dependent on the crosslinking state. To our knowledge, this is the first experiment on the feasibility of monitoring cross-linking states using terahertz waves. The THz-TDS method has potential as a useful nondestructive technique for polymer inspection and analysis.

Improving signal-to-noise ratio of a terahertz signal using a WaveNet-based neural network.

When acquiring a terahertz signal from a time-domain spectroscopy system, the signal is degraded by measurement noise and the information embedded in the signal is distorted. For high-performing terahertz applications, this study proposes a method for enhancing such a noise-degraded terahertz signal using machine learning that is applied to the raw signal after acquisition. The proposed method learns a function that maps the degraded signal to the clean signal using a WaveNet-based neural network that performs multiple layers of dilated convolutions. It also includes learnable pre- and post-processing modules that automatically transform the time domain where the enhancement process operates. When training the neural network, a data augmentation scheme is adopted to tackle the issue of insufficient training data. The comparative evaluation confirms that the proposed method outperforms other baseline neural networks in terms of signal-to-noise ratio. The proposed method also performs significantly better than the averaging of multiple signals, thereby facilitating the procurement of an enhanced signal without increasing the measurement time.

MAX-Phase Films Overcome Scaling Limitations to the Resistivity of Metal Thin Films.

Metal thin films have been widely used as conductors in semiconductor devices for several decades. However, the resistivity of metal thin films such as Cu and TiN increases substantially (>1000%) as they become thinner (<10 nm) when using high-density integration to improve device performance. In this study, the resistivities of MAX-phase V2AlC films grown on sapphire substrates exhibited a significantly weaker dependence on the film thickness than conventional metal films that resulted in a resistivity increase of only 30%, as the V2AlC film thickness decreased from approximately 45 to 5 nm. The resistivity was almost identical for film thicknesses of 10-50 nm. The small change in the resistivity of V2AlC films with decreasing film thickness originated from the highly ordered crystalline quality and a small electron mean free path (11-13.6 nm). Thus, MAX-phase thin films have great potential for advanced metal technology applications to overcome the current scaling limitations of semiconductor devices.

Terahertz Wave Absorption Property of all Mixed Organic-Inorganic Hybrid Perovskite Thin Film MA(Sn, Pb)(Br, I)3 Fabricated by Sequential Vacuum Evaporation Method.

All mixed hybrid perovskite (MA(Sn, Pb)(Br,I)3) thin film was fabricated by sequential vacuum evaporation method. To optimize the first layer with PbBr2 and SnI2, we performed different annealing treatments. Further, MA(Sn, Pb)(Br, I)3 thin film was synthesized on the optimized first layer by evaporating MAI and post-annealing. The formed hybrid perovskite thin film exhibited absorptions at 1.0 and 1.7 THz with small absorbance (<10%). Moreover, no chemical and structural defect-incorporated absorption was found. In this study, the possibility of changing terahertz absorption frequency through the mixture of metal cations (Sn+ and Pb+) and halogen anions (Br- and I-) was verified.

Enhanced Spin-to-Charge Conversion Efficiency in Ultrathin Bi2Se3 Observed by Spintronic Terahertz Spectroscopy.

Owing to their remarkable spin-charge conversion (SCC) efficiency, topological insulators (TIs) are the most attractive candidates for spin-orbit torque generators. The simple method of enhancing SCC efficiency is to reduce the thickness of TI films to minimize the trivial bulk contribution. However, when the thickness reaches the ultrathin regime, the SCC efficiency decreases owing to intersurface hybridization. To overcome these contrary effects, we induced dehybridization of the ultrathin TI film by breaking the inversion symmetry between surfaces. For the TI film grown on an oxygen-deficient transition-metal oxide, the unbonded transition-metal d-orbitals affected only the bottom surface, resulting in asymmetric surface band structures. Spintronic terahertz emission spectroscopy, an emerging tool for investigating the SCC characteristics, revealed that the resulting SCC efficiency in symmetry-broken ultrathin Bi2Se3 was enhanced by up to ∼2.4 times.

Strong Linear Correlation between CH3NH2 Molecular Defect and THz-Wave Absorption in CH3NH3PbI3 Hybrid Perovskite Thin Film.

To control the density of a CH3NH2 molecular defect, which strongly contributed to a significant THz-wave absorption property in the CH3NH3PbI3 hybrid perovskite thin film formed by the sequential vacuum evaporation method, we performed post-annealing processes with various temperatures and times. In the thin film after post-annealing at 110 °C for 45 min, the density of the CH3NH2 molecular defect was minimized, and CH3NH3I and PbI2 disappeared in the thin film after the post-annealing process at 150 °C for 30 min. However, the density of the CH3NH2 molecular defect increased. Moreover, the THz-wave absorption property for each thin film was obtained using a THz time-domain spectroscopy to understand the correlation between the density of a molecular defect and the THz-wave oscillation strength at 1.6 THz, which originated in the molecular defect-incorporated hybrid perovskite structure. There is a strong linear correlation between the oscillator strength of a significant THz-wave absorption at 1.6 THz and the CH3NH2 molecular defect density.

Clean interface without any intermixed state between ultra-thin P3 polymer and CH3NH3PbI3 hybrid perovskite thin film.

Hole transport layers (HTL) are crucial materials to improve the power conversion efficiency in organohalide hybrid perovskite-based solar-cell applications. Two important physical properties are required in HTL materials: good hole mobility and air-protection. After HTL solution-based deposition, an intermixed chemical state at the interface between HTL and hybrid perovskite is key to confirming the physical property of HTL. We performed high-resolution x-ray photoelectron spectroscopy to investigate the chemical states at the interface between an ultra-thin P3 polymer and CH3NH3PbI3 hybrid perovskite thin film. At the interface, we found no apparent intermixed chemical state. Furthermore, we confirmed that the P3 HTL with the ultra-thin layer (7 nm) protected the hybrid perovskite material against air-exposure for 2 weeks.

Significant THz absorption in CH3NH2 molecular defect-incorporated organic-inorganic hybrid perovskite thin film.

The valid strong THz absorption at 1.58 THz was probed in the organic-inorganic hybrid perovskite thin film, CH3NH3PbI3, fabricated by sequential vacuum evaporation method. In usual solution-based methods such as 2-step solution and antisolvent, we observed the relatively weak two main absorption peaks at 0.95 and 1.87 THz. The measured absorption spectrum is analyzed by density-functional theory calculations. The modes at 0.95 and 1.87 THz are assigned to the Pb-I vibrations of the inorganic components in the tetragonal phase. By contrast, the origin of the 1.58 THz absorption is due to the structural deformation of Pb-I bonding at the grain boundary incorporated with a CH3NH2 molecular defect.

Strong polarization-dependent terahertz modulation of aligned Ag nanowires on Si substrate.

Optically tunable, strong polarization-dependent transmission of terahertz pulses through aligned Ag nanowires on a Si substrate is demonstrated. Terahertz pulses primarily pass through the Ag nanowires and the transmittance is weakly dependent on the angle between the direction of polarization of the terahertz pulse and the direction of nanowire alignment. However, the transmission of a terahertz pulse through optically excited materials strongly depends on the polarization direction. The extinction ratio increases as the power of the pumping laser increases. The enhanced polarization dependency is explained by the redistribution of photocarriers, which accelerates the sintering effect along the direction of alignment of the Ag nanowires. The photocarrier redistribution effect is examined by the enhancement of terahertz emission from the sample. Oblique metal nanowires on Si could be utilized for designing optically tunable terahertz polarization modulators.

Feasibility of using terahertz spectroscopy to detect seven different pesticides in wheat flour.

This study investigated the feasibility of detecting pesticides using terahertz (THz) spectroscopy in high-density polyethylene and/or wheat flour mixtures. The absorption spectra of seven pesticides (dicofol, chlorpyrifos, chlorpyrifos-methyl, daminozide, imidacloprid, diethyldithiocarbamate, and dimethyldithiocarbamate) were measured in the frequency range 0.1 to 3 THz at room temperature. Five of the seven pesticides exhibited specific absorption peaks in the low-energy THz range. The two remaining pesticides had no specific absorption peaks in this frequency range, but they exhibited different frequency-dependent refractive indices. The absorption coefficients of imidacloprid increased with its increasing weight ratio in high-density polyethylene, and the fitted power absorptions and refractive indices using a Maxwell-Garnett effective medium model were comparable to the measured data. Imidacloprid was also identified from its characteristic absorption peaks in wheat flour mixtures, and a linear relationship between the absorption coefficient and the weight ratio was observed. Our results show the potential of detection of selected pesticides in foods, such as wheat flour, using THz spectroscopy.

Ultrafast zero balance of the oscillator-strength sum rule in graphene.

Oscillator-strength sum rule in light-induced transitions is one general form of quantum-mechanical identities. Although this sum rule is well established in equilibrium photo-physics, an experimental corroboration for the validation of the sum rule in a nonequilibrium regime has been a long-standing unexplored question. The simple band structure of graphene is an ideal system for investigating this question due to the linear Dirac-like energy dispersion. Here, we employed both ultrafast terahertz and optical spectroscopy to directly monitor the transient oscillator-strength balancing between quasi-free low-energy oscillators and high-energy Fermi-edge ones. Upon photo-excitation of hot Dirac fermions, we observed that the ultrafast depletion of high-energy oscillators precisely complements the increased terahertz absorption oscillators. Our results may provide an experimental priori to understand, for example, the intrinsic free-carrier dynamics to the high-energy photo-excitation, responsible for optoelectronic operation such as graphene-based phototransistor or solar-energy harvesting devices.

Gate-controlled nonlinear conductivity of Dirac fermion in graphene field-effect transistors measured by terahertz time-domain spectroscopy.

We present terahertz spectroscopic measurements of Dirac fermion dynamics from a large-scale graphene that was grown by chemical vapor deposition and on which carrier density was modulated by electrostatic and chemical doping. The measured frequency-dependent optical sheet conductivity of graphene shows electron-density-dependence characteristics, which can be understood by a simple Drude model. In a low carrier density regime, the optical sheet conductivity of graphene is constant regardless of the applied gate voltage, but in a high carrier density regime, it has nonlinear behavior with respect to the applied gate voltage. Chemical doping using viologen was found to be efficient in controlling the equilibrium Fermi level without sacrificing the unique carrier dynamics of graphene.

Molecular imaging with terahertz waves.

We demonstrate a highly sensitive THz molecular imaging (TMI) technique involving differential modulation of surface plasmons induced on nanoparticles and obtain target specific in vivo images of cancers. This technique can detect quantities of gold nanoparticles as small as 15 µM in vivo. A comparison of TMI images with near infrared absorption images shows the superior sensitivity of TMI. Furthermore, the quantification property of TMI is excellent, being linearly proportional to the concentration of nanoparticles. The target specificity issue is also addressed at the ex vivo and cell levels. The high thermal sensitivity of TMI can help extend photonic-based photothermal molecular imaging researches from the in vitro level to the in vivo level. The TMI technique can be used for monitoring drug delivery processes and for early cancer diagnosis.

Nanoparticle-enabled terahertz imaging for cancer diagnosis.

This paper demonstrates the principle of the nanoparticle-contrast-agent-enabled terahertz imaging (CATHI) technique, which yields a dramatic sensitivity of the differential signal from cancer cells with nanoparticles. The terahertz (THz) reflection signal increased beam by 20% in the cancer cells with nanoparticles of gold nano-rods (GNRs) upon their irradiation with a infrared (IR) laser, due to the temperature rise of water in cancer cells by surface plasma ploritons. In the differential mode, the THz signal from the cancer cells with GNRs was 30 times higher than that from the cancer cells without GNRs. As the high sensitivity is achieved by the surface plasmon resonance through IR laser irradiation, the resolution of the CATHI technique can be as good as a few microns and THz endoscopy becomes more feasible.