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Hexagonal porous optical fiber without a central defect is proposed and numerically analyzed with the finite element method (FEM) for transmitting terahertz (THz) electromagnetic wave pulse. In experiments, the transmission characteristics of polytetrafluoroethylene (PTFE) hexagonal porous optical fibers were measured using a THz time-domain spectroscopy (THz-TDS) system. To precisely estimate the effective material loss (EML), we measured the refractive index and absorption coefficient of PTFE in the THz range to use them in FEM analyses, and the EML of the porous fiber was estimated to be lower than that of a bulk rod as large as by a factor of 2 in the frequency range from 0.1 to 0.33 THz. In experiments, we measured the transmission characteristics of both the porous fibers and the bulk rod, to confirm a significant improvement in THz wave transmission nearly by an order of magnitude in the 6G telecommunication window, showing a better performance than theoretical estimations.

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.

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 V

The terahertz (THz)-wave absorption properties in organic-inorganic hybrid perovskite (OHP) materials are investigated with the in-depth development of OHP-based THz applications. In the THz range from 0.5 to 3 THz, OHPs typically show several interesting phonon modes such as transverse, longitudinal, and halogen self-vibrations. To modulate these frequencies, the density changes in defect-incorporated structures and element mixtures were tested and confirmed. In the literature, the origin of phonon modes in OHP materials have been mostly explained. However, we found new phonon vibration modes in formamidinium (FA)-based hybrid perovskite structures. FAPbI3 single crystals, organic–inorganic hybrid perovskites, of the δ-, δ/α-mixed-, and α-phases were prepared. We intriguingly found that the δ/α-mixed-phase exhibited significant THz-wave absorption peaks at 2.0 and 2.2 THz that were not related to any phonon modes from either the δ- or α-phases, although the δ/α-mixed-phase sample was confirmed to be formed by a physical combination of the δ- and α-phases without the creation of any new chemical states. Our theoretical study performed with ab initio calculations provides an explanation for these unusual THz-wave absorption behaviors; they originate from the novel vibration modes excited at the seamless interfaces in the mixed phase of FAPbI3. We found new phonon vibration modes in formamidinium (FA)-based hybrid perovskite structures. We intriguingly found that the δ/α-mixed-phase exhibited significant THz-wave absorption peaks at 2.0 and 2.2 THz that were not related to any phonon modes from either the δ- or α-phases, although the δ/α-mixed-phase sample was confirmed to be formed by a physical combination of the δ- and α-phases without the creation of any new chemical states. From the theoretical study, they originate from the novel vibration modes excited at the seamless interfaces in the mixed phase of FAPbI3. New vibrational modes generated by low-cost organic–inorganic crystals could be of value for terahertz devices used for non-invasive cell and tissue imaging. Vibrationally excited molecules can often be identified from terahertz energy signals, but these signals are hard to detect with conventional electronics. Researchers led by Young-Kyun Kwon from Kyung Hee University in Seoul, South Korea, and Min-Cherl Jung at the University of Tsukuba in Ibaraki, Japan, report insights into how optically active materials known as hybrid perovskites can be engineered to improve terahertz detection. The team’s analysis of formamidinium-lead-iodide perovskites showed that new terahertz-sensitive modes could be seen when the material solidified into a crystal containing two different structural arrangements known as phases. Theoretical simulations revealed the modes appear because of vibrations generated at the phase interfaces.

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 Bi

We investigated THz emission from Ar-ion-implanted Cu(In,Ga)Se2 (CIGS) films. THz radiation from the CIGS films increases as the density of implanted Ar ions increases. This is because Ar ions contribute to an increase in the surface surge current density. The effect of Ar-ion implantation on the carrier dynamics of CIGS films was also investigated using optical pump THz probe spectroscopy. The fitted results imply that implanted Ar ions increase the charge transition of intra-and carrier–carrier scattering lifetimes and decrease the bandgap transition lifetime.

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 &deg, 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 &deg, 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.

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.

Understanding the controllability of the THz-wave absorption property of organic–inorganic hybrid perovskite (OHP) thin films is necessary to advance the THz-based application field. Here, we found the correlation of THz-wave absorption property by different halogen elements in FAPb(Br, I)-based hybrid perovskite thin films fabricated by sequential vacuum evaporation. We confirm that the selection of halogen elements and their concentration ratio can be changed to systematically control the THz conductivity property.

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.

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.

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.

In this study, a native oxide layer was transformed from TeO2 into Bi2O3 on Bi2Te3 by changing the annealing temperature, and the change in the effects of the interface between the oxide layers and Bi2Te3 nanowire (NW) was studied. Caused by the change in the surface oxide layer, the change in the interface between the Bi2Te3 NW and oxide layer depending on the annealing temperature alters the band-alignment and charge states of defects, which significantly modulates the carrier dynamics (optical excitation and recombination processes) at the interface. Because of the trap/de-trap processes at a defect of the Bi2Te3/TeO2 interface, the photoresponse of the Bi2Te3/TeO2 system shows the defect-induced photogating effect, whereas the photoresponse of the Bi2Te3/Bi2O3 system shows a capacitive response without trap/de-trap processes. The systematic analysis of the effects of the surface oxidation and related defects on the photoresponse of TIs gives insight into TI-related photodetector devices and their properties under various conditions. Finally, a simple method for controlling the oxidation of the surface and accompanied interface properties such as the valance band offset (VBO), conduction band offset (CBO), and trap/de-trap processes, which can directly affect the operation of TI-based photodetector devices, is proposed.

A CH3NH3PbBr3 (MAPbBr3) single crystal was subjected to 2 and 10 V biasing for 10 min under ultra-high vacuum to investigate the bias effect on the surface of MAPbBr3. Under these controlled conditions, we performed high-resolution photoelectron spectroscopy on the MAPbBr3 single crystals at photon energies of 460 and 100 eV under synchrotron radiation to obtain core-level and valence structure spectra. The CH3NH2 molecular defect relative intensity area ratio decreased from 22% to 10%. It was assumed that the bias effect can induce a low density of defect states to suppress large defect-pinned energy states at the interfaces.

The exploration of new physical properties for various THz-based applications, such as THz-wave sensing, modulation, and imaging devices, is a key challenge in the research on organic–inorganic hybrid perovskite materials. These THz-based applications require satisfactory, sensitive, and stable absorption properties with values between 0.5 and 3 THz. To achieve these properties, candidate materials should possess a purified structure that induces regular and fixed phonon modes without any defects or impurities. CH3NH3PbBr3, an organic–inorganic hybrid perovskite thin film produced by a sequential vacuum evaporation method on a flexible PET substrate, was investigated in this study. Although the thin film contains only molecular defects related to CH3NH2 incorporated into the perovskite structure, our THz-wave absorption measurement and first-principles simulation confirmed that these molecular defects do not influence the three phonon modes originating from the transverse vibration (0.8 THz), the longitudinal optical vibrations (1.4 THz) of the Pb–Br–Pb bonds, and the optical Br vibration (2.0 THz). After spin-casting an ultrathin PTAA polymer protective layer (5 nm) on the hybrid perovskite thin film, it was additionally observed that there was no significant effect on the phonon modes. Thus, this novel flexible organic–inorganic hybrid perovskite material is a potential candidate for THz-based applications. A material useful for security imaging has been identified by an international team of scientists. Radiation between the infrared and microwave parts of the electromagnetic spectrum can pass through clothing. This so-called terahertz radiation is valuable for security screening technology as it can identify a wider range of materials than metal detectors and is much quicker than a manual pat-down. A team led by Young-Kyun Kwon from Kyung Hee University, Seoul, South Korea, and Min-Cherl Jung from the University of Tsukuba, Ibaraki, Japan, demonstrated that a material already used with great success in optoelectronic and solar cell applications, known as an organic–inorganic hybrid perovskite, also has useful terahertz properties. The team created thin films of this material on a flexible substrate and showed that it absorbed radiation at fixed terahertz frequencies. Phonon modes of CH3NH3PbBr3 hybrid perovskite thin film.

Understanding the defect structure in organic-inorganic hybrid perovskite material (OHP) is a crucial role to explain several physical properties such as material stability, energy band, carrier mobility, and so on. In the solar-cell applications using OHP, finding, understanding, and controlling defects is essential to making a more advanced device with high efficiency and stability. Naturally, we need to find, understand, and control the possible defects in OHP. However, the defect research field in OHP material is just beginning now. In this short review, we will explore the kinds of defects and their effects on OHP.

We observed the enhancement of terahertz radiation emitted from GaAs microstructures under wavelength tuned optical excitation. GaAs microstructure thin films were prepared by molecular beam epitaxy method. The peak amplitude of terahertz radiation from GaAs microstructure is more than eight times that from semi-insulating GaAs.

In mechanical engineering, truss structures have attracted much attention because of their mechanical strength and light weight. However, electromagnetic properties of truss structures have been rarely reported. We experimentally and numerically investigated a terahertz transmission through a truss structure composed of metallic meshes. In the experiments, a resonant transmission was observed through the metallic mesh truss structure. The frequencies and spatial distributions of the resonant modes were investigated in the numerical simulations. The frequencies of the resonant modes obtained from the numerical simulations agreed well with the observed frequencies. The field distributions of the resonant modes resemble those of the resonant modes of a Fabry–Perot resonator. Because truss structures are practically empty, they could be applied in realizing sensors to detect the chemical reactions of gases or molecular biomaterials in response to their changing refractive index.

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.

The optical excitation effect on the transmission of a terahertz pulse through aligned silver nanowires on a silicon substrate was studied. Terahertz pulses almost pass through the silver nanowires without optical excitation. However, the optically excited sample significantly reduces the transmittance, comparing with the excited silicon substrate only.

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.

CH3NH3PbI3 thin films were formed with various deposition rates of PbI2 first layer using a sequential vacuum evaporation method (SVE) to understand the formation behavior. Under low PbI2 deposition rates with 1 and 3 A s−1, the thin films did not form the hybrid perovskite structure. Over the 6 A s−1 deposition rate, the perovskite structure is observed with the remaining PbI2 and an intermediate phase together. In the 10 A s−1, the intermediate phase is increased, and the CH3NH2 molecular defect is observed. The hybrid perovskite thin film formed by SVE is confirmed with tiny grains and CH3NH2-incorporated intermediate state.

We employed terahertz (THz) spectroscopy to study the optical/electrical properties of various nanocrystal (NC)−carbon nanotube (i.e., single-, double-, and multiwalled) and −graphene oxide (GO) hy...

High-efficiency optical modulation of a terahertz pulse transmitted through aligned silver nanowires on a silicon substrate is demonstrated. Without optical excitation, the terahertz pulses mostly pass through the silver nanowires. However, an optically excited sample significantly modulates the transmittance compared with an excited silicon substrate. The enhanced modulation efficiency is explained by the redistribution effect of photo-carriers due to the nanowires. The simple structure of metal nanowires on a semiconductor substrate could be useful in implementing optically tunable terahertz wave modulators.

The accurate terahertz (THz) electrical and optical properties of biomaterials are required as the biological applications of THz electromagnetic waves were recently increased. The frequency-dependent characteristics of enamel and dentin of human tooth were measured in this study through THz time-domain spectroscopy. The power absorptions of enamel and dentin were found to be similar while the index of refraction of enamel was found to be bigger than that of dentin. The power absorption results of enamel and dentin were also compared with those of the wet samples.

We report strong emission of terahertz radiation from germanium wafers with nanostructured surfaces. The power of the terahertz radiation from a Ge wafer with an array of nano-bullets is comparable to that from n-GaAs wafers, which have been widely used as a terahertz source. We find that the THz radiation from Ge wafers is even more powerful than that from n-GaAs for frequencies below 0.6 THz. Our results suggest that introducing properly designed nanostructures on indirect band gap semiconductor wafers is a simple and cheap method to improve the terahertz emission efficiency of the wafers significantly.

The conducting polymer polyaniline (PANI) has a wide range of optoelectronic applications due to its unique electronic and optical characteristics. Although extensive works have been performed to understand the equilibrium properties, the nature of the charge type that governs its non-equilibrium optical response has been barely understood; a number of studies have debated the nature of photo-generated charge type in PANI, specifically whether it is polaron or exciton based. Here, we report experimental evidence that the charge relaxation dynamics of PANI are dominated by excitons. Utilizing ultrafast spin-resolved pump-probe spectroscopy, we observed that PANI charge dynamics are strongly spin-polarized, exhibiting a spin Pauli-blocking effect. Investigations including both spin-independent and spindependent dynamics reveal that there is no spin-flip process involved in charge relaxation. This provides compelling evidence of an exciton-dominated photo-response in PANI.

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.

We investigate the relativistic zero balance of the oscillator strength sum rule in a single-layer graphene. The full energy-dependent characteristics are studied by ultrafast THz spectroscopy to probe the low-energy quasi-free absorption oscillator in conjunction with ultrafast optical spectroscopy to investigate the high-energy oscillator near Fermi level.

We investigate the complex conductivity of silver nanowire thin films using terahertz time-domain spectroscopy. The measured conductivity shows a characteristic non-Drude response unlike bulk metals. The interesting behavior is explained by the Gans approximation and the Drude-Smith model.

Low energy region correspond to the Boltzmann thermal energy \( \left( {k_{B} T} \right) \) show condensed matter properties and electric phenomena. Terahertz (THz) spectroscopy technique enables us to access this low energy level excitation including the conductivity of itinerant charges, plasmons and polarons. THz spectroscopy measures material’s ground state at finite temperature and its dynamics as a result of electromagnetic or electronic perturbation.

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.