The properties of spin fluctuations in antiferromagnets are largely unexplored, in particular at ultrafast timescales. Here, we employ femtosecond noise correlation spectroscopy to experimentally study magnetization fluctuations in the canted antiferromagnet Sm0.7Er0.3FeO3 across its spin-reorientation transition and under external magnetic fields. By comparing our measurements to atomistic spin noise and Monte Carlo simulations, we find that the amplitude of the spin noise is governed by the free energy, with stronger fluctuations in regions where the potential landscape softens. We furthermore demonstrate that external magnetic fields suppress spin fluctuations and enhance the quasiferromagnetic magnon frequency by effectively stiffening the potential. These results highlight an effective route for tuning ultrafast magnetization fluctuations via external parameters.

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.

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.

Efficiency of optical beats in a chaotically oscillating laser is confirmed comparing that of free running CW laser using a highly efficient plasmonic photomixer. Ultra-stability of optical beats with laser chaos continues down to the laser threshold level. The output power is also estimated using conventional bowtie antenna. The great potential of chaotically oscillating lasers is verified for THz systems.

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.

Nanocrystalline Ti 4 O 7 with controlled crystallite sizes was synthesized to investigate the effect of crystallite size on the metal–semiconductor transition.

Tritium in water is difficult to monitor because the low-energy beta particles emitted by tritium are strongly selfabsorbed. Although liquid scintillation counting provides high sensitivity, it requires sample preparation and generates radioactive organic liquid waste, limiting its suitability for on-site continuous monitoring. In this study, we investigated the inorganic single-crystal scintillator (La,Gd)2Si2O7:Ce (Ce:La-GPS) for tritiated water measurements. The as-grown sample showed long-term afterglow that increased the count rate in the region of interest for tritium detection, but air annealing at 1200 degrees C suppressed the afterglow to the background level. The annealed sample exhibited a light output of approximately 35,000 photons/MeV and an energy resolution of 5.5% at 662 keV under 137Cs gamma-ray excitation. Immersion measurements in tritiated water showed a linear counting response, with a minimum detectable activity of 3.85 MBq/L for a 600 s measurement. These results indicate that Ce:La-GPS is a promising solid scintillator for high-activity tritiated water monitoring, while application to low-level monitoring will require increasing the effective scintillator surface area.

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.

Efficiency of optical beats in a chaotically oscillating laser is confirmed comparing that of free running CW laser using a highly efficient plasmonic photomixer. Ultra-stability of optical beats with laser chaos continues down to the laser threshold level. The great potential of chaotically oscillating lasers is verified for THz systems.

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.

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).

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.

Low cost, compact, highly efficient, and highly stable THz wave using chaotically oscillating laser diode is investigated. Compared it to conventional continuous wave multi-mode semiconductor laser excitation system, about ten times output radio frequency power is increased because of chaotic supremacy.

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.