Cerium-doped yttrium aluminum garnet (Y3Al5O12:Ce, YAG:Ce) thin films were prepared by reactive co-sputtering from two separate elemental sources (Y3Al5 alloy and pure cerium) using a dual hollow cathode plasma-jet system in an Ar/O-2 atmosphere. This plasma-based approach enabled spatial control of Ce incorporation through the geometric configuration of the sputtering sources and tailored power input. Film composition and structure were analyzed using LIBS, XRD, and optical methods, revealing a gradient in Ce content across the substrate array. Post-deposition annealing at 1000 degrees C was essential for crystallization and luminescence activation, resulting in the formation of single-phase YAG at low Ce concentrations and a gradual transition toward CeO2-rich films at high Ce loading. Photoluminescence and cathodoluminescence studies showed Ce3+ emission at moderate doping levels, while higher Ce content led to phase segregation, Ce4+ formation, and luminescence quenching. These results demonstrate that dual hollow cathode reactive sputtering provides a flexible approach for controlling the Ce distribution and for exploring the structural and optical behavior of YAG:Ce films under extreme Ce loading conditions.

We demonstrate electrically detected spintronic terahertz energy detection based on ferromagnet/heavy-metal bilayers operated under continuous-wave terahertz excitation. Using zero dc bias current and ON-OFF modulation combined with lock-in detection, we observe a finite dc voltage exhibiting pronounced modulation-frequency, magnetic-field, and polarization-angle dependencies. To clarify the physical origin of the signal, we introduce a unified framework based on terahertz-driven spin torques and nonlinear magnetization dynamics governed by the Landau-Lifshitz-Gilbert equation.

Perfluorocubane is an unconventional electron acceptor whose lowest unoccupied molecular orbital is composed entirely of σ* orbitals. In contrast to conventional acceptors that stabilize excess charge through π*-orbital delocalization, perfluorocubane confines an electron with its compact cubic scaffold, giving rise to an almost point-charge-like electronic structure. Such an electronic configuration is expected to produce electron-transfer (ET) behavior beyond established donor-acceptor models. Here we investigate intramolecular photoinduced ET in a donor-acceptor molecule combining dimethylfluorene and perfluorocubane. Picosecond time-resolved transient absorption spectroscopy directly reveals charge separation and recombination in polar solvents. Analysis of the ET kinetics within the semiclassical Marcus framework yields an unusually large electronic coupling together with a substantial reorganization energy, an uncommon combination in π-conjugated donor-acceptor systems. These distinctive ET characteristics are attributed to the σ*-derived LUMO and the compact cubic architecture of perfluorocubane, establishing this molecule as a platform for accessing previously unexplored regimes of molecular electron-transfer chemistry.

For the sustainable development of beyond fifth-generation (B5G) or sixth-generation (6G) wireless communication technologies operating in the sub-terahertz and terahertz range, it is highly desirable to utilize environmentally friendly materials in B5G/6G devices. Here, we report a sub-terahertz absorbing sheet made of biodegradable cellulose-nanofiber (CNF), which can be useful for achieving electromagnetic compatibility (EMC) in B5G/6G devices in a green manner. The absorber consists of one pure CNF layer sandwiched between two layers of carbon-nanotube (CNT)-dispersed CNF layers. The CNT concentration is engineered to be low (0.15 wt.%) in the layer on the wave-incident side and high (6 wt.%) in the layer on the opposite side, emulating the resistive and reflective layers of a Salisbury screen absorber, respectively. The total thickness of our absorber is as thin as 120 mu m. Terahertz time-domain spectroscopy (THz-TDS) reveals its good shielding effectiveness of 28.5 dB, low reflectance of 2.5%, and high absorptance of 97.4% at 300 GHz. Additionally, an absorptance higher than 80% is achieved over a broadband frequency range from 250 to 480 GHz. Good agreement between the measured results and calculated results based on the ABCD matrix method confirms that the cancellation of reflected waves is realized as designed.

Abstract The growing demand for noncontact sensing of environmental pollutants and toxic gases has increased interest in low-cost, compact, and easy-to-operate optical gas-sensing systems. In this study, we propose a low-concentration ammonia (NH3) detection method based on terahertz time-domain spectroscopy (THz-TDS) using a chaotic multimode laser diode (MLD). Introducing porous glass into the gas cell enhanced interactions occurring between the NH3 molecules and the internal silanol surfaces, leading to a decrease in the effective refractive index and a temporal advance of the THz time-domain waveform. The system achieved NH3 detection down to 0.2 ppm. The response increased rapidly at low concentrations and tended to saturate above approximately 1 ppm, which could be described by a Langmuir-type adsorption model. These results demonstrate that chaotic MLD THz spectroscopy combined with porous glass provides a low-cost and compact approach for sub-ppm-level ammonia detection.

The photophysical properties of the para-connected MA-TA and meta-connected mMA-TA donor-acceptor thermally activated delayed fluoresence (TADF) structural isomers were investigated to clarify how substitution patterns affect excited-state relaxation dynamics in TADF molecules. Time-resolved photoluminescence measurements showed that MA-TA exhibits relatively simple emissive-state relaxation, whereas mMA-TA shows a time-dependent red shift of the emission band and biexponential photoluminescence decay. Using reference spectra obtained from steady-state absorption and fragment transient absorption measurements, femtosecond transient absorption spectroscopy revealed that MA-TA undergoes rapid formation of a charge-transfer-like excited state within several picoseconds, as indicated by the decay of a broad MA-like near-infrared excited-state absorption component monitored around 850 nm and the concomitant rise of a CT-like excited-state absorption around 600 nm. In contrast, mMA-TA exhibited a spectroscopic signature of an additional intermediate excited-state species, observed as a transient absorption band around 700 nm, that was not clearly observed for MA-TA. The delayed appearance and relatively fast decay of this signal indicate that mMA-TA forms an intermediate excited-state species during early relaxation. These results demonstrate that the substitution pattern strongly influences the early excited-state relaxation pathway in these TADF molecules. The formation of an additional intermediate excited-state species in mMA-TA may provide a plausible origin for its more complex photoluminescence dynamics and lower emission efficiency. This study highlights the importance of controlling substitution-dependent intermediate excited-state formation in the molecular design of efficient TADF emitters.

Controlling trap depth is crucial to improve photocatalytic activity, but designing such crystal structures has been challenging. In this study, we discovered that in 2D materials like BiOCl and Bi4NbO8Cl, composed of interleaved [Bi2O2]2+ and Cl- slabs, the trap depth can be controlled by manipulating the slab stacking structure. In BiOCl, oxygen vacancies (VO) create deep electron traps, while chlorine vacancies (VCl) produce shallow traps. The depth is determined by the coordination around anion vacancies: VO forms strong σ bonds with Bi-6p dangling bonds below the conduction band minimum (CBM), while those around Cl are parallel, forming weak π-bonding. The strong re-hybridization makes the trap depth deeper. In Bi4NbO8Cl, VCl also creates shallow traps, but VO does not produce deep traps although Bi-6p orbitals are also forming strong σ bonding. This difference is attributed to the difference of the energy level of CBM. In both cases, the CBM consists of Bi-6p orbitals extending into the Cl layers. However, these orbitals are isolated in BiOCl, but those in Bi4NbO8Cl are bonded with each other between neighboring [Bi2O2]2+ layers. This unique bonding-based CBM prevents the formation of deep electron traps, and significantly enhances H2 evolution activity by prolonging the lifetime of highly reactive free electrons.

Our research focuses on calculating and analyzing the dynamics of free electrons in GaAs semiconductors driven by infrared femtosecond vortex lasers. We found that under laser drive, electrons exhibit a trend of motion under gradient forces on average over time. At the same time, we also found that the motion of electrons exhibits a special spatial distribution under special high-intensity lasers.

In this study, we investigate the anisotropic properties of β-Ga₂O₃ epitaxial layer along the a-axis [100] and baxis [010] directions using terahertz time-domain spectroscopy (THz-TDS). The THz response spectra were measured over a frequency range of 0.2 - 3 THz and at temperature range of 90– 400 K. By employing a Drude-Lorentz model, we extracted the anisotropic transport parameters. Our findings provide fundamental insights into the anisotropic behavior of β-Ga₂O₃, offering theoretical support for its optimization in high-frequency electronic and optoelectronic applications.

Photocatalytic water splitting utilizing visible-light-active materials holds promise for a greener and more facile way to generate H2 and O2. Herein, we analyze the strong potential of BaTaO2N for oxygen generation via water splitting based on a fundamental understanding of the behavior of photoinduced charge carriers. We found that the photoexcited electrons in BaTaO2N undergo trapping at trap states 0.4 eV below the conduction band minimum. This electron trapping occurs following significant electron-hole recombination within 200 ps after excitation. We also identified the important role of oxygen evolution cocatalysts not only for effectively enhancing charge-carrier separation but also for reducing the trap-state energy level from 0.4 to 0.1 eV. Overall, the combination of sluggish charge-carrier recombination and effective and rapid hole transfer to CoO x resulted in an almost 100-fold increase in the population of accumulated long-lived electrons in defect-rich BaTaO2N. This observation is consistent with the high O2 evolution rate exhibited by this material. These findings open up new directions and strategies to capitalize on the properties of BaTaO2N and further allow the development of highly effective oxynitride-based materials for water splitting.

From a perspective of application to 6G communication devices, we developed a sub-terahertz (THz) wave absorber made of cellulose nanofiber (CNF). The absorber consists of two carbon-nanotube (CNT)-dispersed CNF layers and one pure CNF layer, and its total thickness is 120 μm. Using THz time-domain spectroscopy, its good shielding effectiveness, low reflectance and high absorptance at 300 GHz was confirmed.

We investigated the transport properties of polarization-induced carriers in AlN/GaN/AlN and GaN/AlN heterostructures using terahertz spectroscopy. These heterostructures are uniquely designed to host a two-dimensional electron gas (2DEG), a 2D hole gas (2DHG), or both. By comparing their terahertz spectral responses, we experimentally confirm the presence of a 2DHG channel that is theoretically predicted to coexist with the 2DEG in AlN/GaN/AlN quantum well structures.

Terahertz (THz) measurements are increasingly valued for nondestructive testing of materials in power devices and other applications. Hence, there is a growing demand for highly accurate characterization methods in the THz range. Here we demonstrate the application of THz time-domain ellipsometry (THz-TDE) to large-scale, quantitative mapping of semiconductor wafers. While THz-TDE is an established technique, its application in wafer-scale mapping, which is an important process in the semiconductor industry, has not yet been demonstrated. In this work, we highlight the effectiveness of THz-TDE by mapping the electrical properties of a widely used semiconductor, silicon carbide (SiC). Spatial distribution maps of conductivity, carrier density, and mobility of a commercial 4-in. SiC wafer are derived using the measured ellipsometric parameters. THz-TDE mapping offers a nondestructive, contactless testing method to evaluate semiconductor quality and electrical homogeneity and is notably suitable for doped semiconductors characterized by high THz absorption.

This study investigates the formation of two types of laser-induced periodic surface structures (LIPSS) on amorphous Ge2Sb2Te5 (GST) using terahertz free-electron laser (THz-FEL) irradiation. The THz pulses used in this study consist of a macro pulse containing 150 micro pulses, each with a duration of 2 ps. The central wavelength λ of the THz radiation is 75 μm. By exposing GST to THz-FEL pulses at fluences up to 35 J/cm2, two distinct periodic structures were observed: low spatial frequency LIPSS (LSFL) and high spatial frequency LIPSS (HSFL). The LSFL formed parallel to the laser's polarization direction with a period of approximately λ/4.5. The HSFL, with a period λ/18, formed perpendicular to the polarization near the ablation edges. The periods of the generated HSFL and LSFL are 4 μm and 16 μm, respectively, and can be observed with an optical microscope. The period of the LSFL is generally consistent with λ/n (where n is the material’s refractive index), which is the approximate value expected based on previous reports in the optical region. This work contributes to a deeper understanding of LIPSS formation mechanisms under long-wavelength terahertz irradiation and suggests pathways for fine-tuned surface structuring applications using THz-FEL.

Colloidal inorganic nanocrystals (NCs) consist of an inorganic core and organic ligands, which contribute to their physical properties and colloidal stability, respectively. Indium tin oxide (ITO) NCs exhibit a localized surface plasmon resonance (LSPR) band in the near-infrared (NIR) region, and the position of this band can be tuned by adjusting the ratio of indium to tin. Due to these properties, ITO NCs have attracted considerable attention for applications in photoenergy conversion materials [1]. The terahertz (THz) region is generally defined as the frequency range between approximately 0.1-10 THz. Although the terahertz region has lower photon energy compared to the visible range (1 THz = ~4.1 meV), it enables the measurement of physical phenomena—such as free electron behavior—that are not accessible with visible light. While the physical properties of bulk ITO have been investigated using terahertz spectroscopy [2], those of colloidal ITO NCs remain largely unexplored. In this study, we synthesized a series of ITO NCs with varying indium-to-tin ratios and different surface ligands, and investigated their physical properties using terahertz time-domain spectroscopy (THz-TDS).

Terahertz time-domain spectroscopy (THz-TDS) systems based on femtosecond (fs) pulsed lasers have shown great potential in nondestructive and non-contact inspection of semiconductor and biomedical materials [1]. Due to their time-domain measurement approach using mechanical delay scanning, THz-TDS systems can retrieve both amplitude and phase information, enabling their use in gas sensing applications [2]. However, the high cost and bulky nature of femtosecond lasers limit their suitability for practical, on-site, or commercial deployment. Therefore, a more cost-effective alternative is provided by continuous-wave multimode laser diode (cw-MLD) based TDS [3-4]. In this work, we demonstrate that a MLD-THz-TDS system can detect phase shifts caused by ammonia gas at parts-per-million (ppm) levels.

We conducted gas sensing experiments using a continuous wave multimode laser-based terahertz time domain spectroscopy system. Ammonia gas diluted with dry air was detected by monitoring the peak time shift of the terahertz waveform transmitting the gas cell containing a piece of porous glass that adsorbed the gas molecules. By controlling the gas flow rate, we observed a peak time advancement in the terahertz time-domain signal caused by ammonia gas at different concentrations. According to the results, we have successfully achieved ppm-level ammonia gas sensing.

We demonstrate the applicability of terahertz time-domain ellipsometry (THz-TDE) for characterizing liquid samples, using water and glycerol as test cases. Measurements were performed with a windowless setup employing a rotating analyzer to extract polarization-resolved waveforms. While glycerol, a nonvolatile liquid, exhibited stable waveforms, water showed time delays and amplitude reductions due to evaporation. By analyzing the differences between time-domain waveforms measured at equivalent analyzer angles, we developed a correction method that compensates for these effects. The corrected absorption spectra of water showed good agreement with conventional THz-TDS results. These findings indicate that THz-TDE, combined with multi-angle analysis, enables accurate, windowless characterization of both volatile and nonvolatile liquids.

In order to demonstrate the application of magneto-optical terahertz time-domain ellipsometry (MO-THz-TDE) in the characterization of semiconductors, doped and undoped InSb wafers were evaluated with a magnetic field of B = 0.4 T. The dielectric tensors of both InSb samples were obtained, and the influence of plasma resonance on the Kerr effect was observed. Furthermore, electron transport properties such as effective mass and carrier density were derived using the Drude model without any assumptions.

Gallium oxide (Ga 2 O 3 ), an emerging ultra-wide bandgap semiconductor (~4.9 eV), holds great promise for power electronics and deep-ultraviolet optoelectronic devices due to its high breakdown field and Baliga’s figure of merit. Among its polymorphs, β-Ga 2 O 3 is the most stable and exhibits strong anisotropy owing to its monoclinic structure. Terahertz time-domain spectroscopy (THz-TDS) offers a non-contact approach to probe anisotropic charge transport and phonon modes. While prior studies focused on room-temperature behavior [1-3], this work investigates anisotropic carrier transport in β-Ga 2 O 3 over a wide temperature range, providing insights critical for device optimization.