The interaction between terahertz (THz) electromagnetic fields and collective lattice excitations offers a fundamental route to access nonlinear and nonequilibrium light-matter dynamics in solids. However, how THz perturbations induce coherent oscillations in a bulk nonlinear optical medium remains unclear. Here we show that THz irradiation induces a counter-propagating parametric oscillation in a periodically poled lithium niobate crystal through phonon-polariton coupling. The resulting dynamics reveal that the collective vibrational mode bridges a widely separated THz-optical frequency range. This process establishes an unprecedented interaction regime free from the conventional requirements of resonant cavities or strict phase matching. As a proof-of-principle demonstration, we achieved omnidirectional THz detection enabled by this interaction, including counter injection from the phase-matching and excitation along the crystal optical axis. These results establish a platform for flexible THz-optical photon bridging across disparate frequency regimes.

We present a terahertz time-domain spectroscopy (THz-TDS) approach integrated with single-pixel imaging (SPI) using a metallic random mask, enabling simultaneous acquisition of spatial and spectral information with high quantitative accuracy. While computational ghost imaging (CGI) provides excellent noise robustness, it typically yields relative correlation values, losing the quantitative scale proportional to the THz electric field. Conversely, ordinary least squares (OLS) reconstruction maintains quantitative scaling but is highly susceptible to noise due to the ill-conditioned nature of random masks. To overcome this trade-off, we introduce a statistical scale-adaptive computational ghost imaging (SSA-CGI) method that hybridizes the correlation-based stability of CGI with the quantitative scaling of OLS. This approach statistically transfers the quantitative voltage scale from OLS to the high-SNR CGI reconstruction, enabling accurate amplitude and phase mapping. The proposed technique was experimentally validated using a metallic aperture and patterned SU-8 structures, demonstrating spatially resolved spectroscopic imaging with high scaling fidelity and yielding group refractive indices consistent with reported values. Owing to its simple and robust configuration, the SSA-CGI method offers a practical, low-cost route toward quantitative THz spectroscopic imaging for applications in material characterization, nondestructive testing, and biomedical diagnostics.

We propose and experimentally demonstrate an angle sensing technique using a mid-infrared ultrashort pulse laser. The system exploits the wavelength-angular dependence of diffracted light from a fixed diffraction grating, enabling high-resolution angle measurements by monitoring the spectral centroid of the reflected beam. A femtosecond laser with a center wavelength of 2.8 mu m and a pulse width of 300 fs, generated through nonlinear polarization rotation, serves as the broadband light source. The angle of a measurement target is encoded in the center wavelength of the diffracted spectrum, which is acquired by a spectrometer via a single-mode fiber. To mitigate the impact of atmospheric water vapor absorption, which is particularly significant in the mid-infrared region, we implemented a reference light measurement scheme. By normalizing the measurement spectrum with the reference spectrum, we successfully compensated for spectral distortions, allowing for accurate centroid extraction even under absorption-influenced conditions. Initial evaluations using a mirror as the target yielded a sensitivity of 0.0320 nm/arcsecond and demonstrated excellent linearity over an angular range of 223 arc-seconds. The noise level, determined from repeated measurements, led to an estimated angular precision of approximately 0.313 arcseconds. Most notably, we investigated the sensor's performance when measuring a rough-surfaced target fabricated by end milling with a surface roughness Rq of 0.4 mu m. Despite a notable decrease in reflected signal intensity, the system maintained a comparable sensitivity of 0.0319 nm/arcsecond. This result confirms the robustness of our method against surface roughness, as traditional angle sensors relying on specular reflection suffer from significant sensitivity degradation under such conditions. The proposed system, by relying on spectral encoding of angle information rather than spatial beam properties, offers a powerful alternative to conventional autocollimation-based sensors. It enables accurate angle measurements even for rough or non-specular surfaces, which are often encountered in practical applications. This work paves the way for robust and compact infrared angle sensors capable of operating in complex real-world environments.

We demonstrate magnetic permeability time-varying metamaterials at GHz frequencies using ferromagnetic permalloy (Ni80Fe20; Py). We observe frequency up and down conversion of 4 GHz microwaves through the metamaterials, which is caused by the temporal modulation of permeability in the Py layer. Moreover, the efficiency of the up-conversion to a higher frequency is much larger than that of the down conversion to a lower frequency. These experimental results are reproduced well via numerical calculation, verifying that the significant up-conversion efficiency is traced back to nonlinear magnetization dynamics in the metamaterials. The present study opens a door to microwave sources toward the 6th-generation mobile communication system, four-dimensional metamaterials with spatio-temporal modulation, and nonlinear spintronics.

Terahertz circular dichroism (THz-CD) spectroscopy provides unique access to low-energy chiral interactions between circularly polarized THz light and multiscale chirality in materials. Yet it has remained unclear whether the THz band can function as a practical, high-fidelity probe that produces visualization of spatial distribution of such interactions. Here, we establish that it can by introducing a frequency-tunable THz-CD spectroscopic imaging platform operating across 3-6 THz with narrow spectral bandwidth, circularly polarized excitation, and two-dimensional readout. Using twisted-layer moire metasurfaces (TMMSs) as a top-down, reconfigurable model system that allows deterministic design and control of chirality-in contrast to bottom-up chiral media such as biomolecules, whose handedness is constrained by biochemical stability-we spatially resolve chiral responses that elude spatially averaged spectra: the dissymmetry (g-factor) inverts sign between topological singularities within a single planar architecture and reverses with the twist handedness, whereas scale moire counterparts remain achiral. We formulate complementary indicators for local displacement asymmetry and macroscopic rotational geometry and show that only their combined action reproduces the measured 2D images, thereby providing experimental validation of a multiscale description in the THz regime. Beyond this platform, our results position spatial THz-CD imaging (3-6 THz) as a practical tool for site-specific chiral metrology in metasurfaces, soft-phonon materials, and biomolecular assemblies.

We present a robust phase-sensitive imaging approach for the terahertz (THz) region based on spiral-phase filtering combined with single-pixel imaging (SPI). By introducing a spiral phase plate at the Fourier plane of a 4f optical system, spatial phase variations are converted into measurable intensity distributions through the shadow effect. Unlike conventional THz techniques that rely on interferometric or time-domain detection, the proposed approach operates in a monochromatic, common-path configuration, enabling compact and alignment-insensitive implementation. The method is intrinsically sensitive to spatial phase gradients and discontinuities, as uniform phase regions are suppressed while phase variations are converted into measurable intensity patterns through spiral-phase filtering. Experiments demonstrate characteristic phase-dependent responses, including orbital angular momentum (OAM)-dependent intensity inversion, initial-phase-controlled rotation of intensity patterns, multi-vortex generation, and phase inversion in structured objects. Furthermore, spatial phase information can be retrieved from intensity-only measurements using a phase-modulation-based reconstruction approach. These results establish spiral-phase filtering as a practical platform for phase-gradient-sensitive imaging in the THz region and highlight its potential for compact and versatile phase-sensitive THz imaging using monochromatic radiation.

We propose a simple method to generate a frequency-multiplexed terahertz multiple vortex beam using a spiral phase plate and a metallic mask. Using a broadband terahertz source, we experimentally demonstrate the conversion of a terahertz Gaussian beam into a frequency-multiplexed single or multiple vortex beam with topological charges ranging from 1 to 3, which is supported by simulations. This multifunctional device opens new possibilities for high-speed THz communication, information processing, and high-efficiency terahertz wavefront manipulation devices.

We demonstrated frequency stabilized terahertz wave parametric generation by two infrared beams, frequency fixed pumping and tunable seeding (stabilized by spectral drill cavity) beams using a nonlinear MgO:LiNbO3 crystal.

Terahertz (THz) circular dichroism (CD) spectroscopic imaging is used to determine the anisotropy and structural chirality of chiral metasurfaces fabricated by printed electronics techniques. This THz-CD spectroscopic imaging system utilizes THz frequency tunable difference frequency generation with a nonlinear 4-N,N-dimethylamino-4’-N’-methyl stilbazolium tosylate (DAST) crystal.

In a vector beam (VB), the polarization rotates spatially within the electromagnetic-field distribution. In this paper, we propose a simple method for using a circularly polarized pump pulse that is tightly focused onto a (100)-cut ZnTe crystal to generate a VB with anti-vortex polarization (i.e., with topological charge ℓ = −1) in the terahertz (THz) regime. Relative to normal incidence, we expect the THz output to be enhanced for the oblique incidence of a focused pump pulse. We formed a ring-shaped pump beam using a pair of axicon lenses and focused it onto a (100)-cut ZnTe crystal to achieve oblique-dominant excitation. We then performed two-dimensional polarization imaging based on THz time-domain spectroscopy, and we evaluated the results using cylindrically polarized Laguerre–Gaussian (CPLG) mode decomposition optimized by using the information entropy. We confirmed the successful generation of THz VBs with ℓ = −1, with the output efficiency increasing with focal angle from 6.4° to 17.3°. These results indicate feasibility of using THz VBs for various applications.

We have focused on terahertz (THz) radiation by longitudinal components of nonlinear polarization in the second order nonlinear processes. Previously, we reported experimental observation of THz radiation by the longitudinal components of nonlinear polarization. This time we report its application to beam steering by manipulating the shape of the pump beam on the nonlinear crystal.

We demonstrated phase measurement of terahertz wave using bi-directional parametric wavelength conversion in nonlinear MgO:LiNbO3 crystals. Optical heterodyne detection of terahertz wave allowed their frequency, energy, and phase to be observed.

We experimentally observe nonlinear spin torque in metallic bilayers of platinum and permalloy by means of spin-torque ferromagnetic-resonance (ST-FMR) under massive dc current injection. The observed nonlinear spin torque exerted to permalloy magnetization is attributed primarily to nonlinear spin polarization. Additional origin of the nonlinear spin torque is magnon generation (annihilation) followed by shrinkage (expansion) of effective magnetization, which is reveled by STFMR and unidirectional spin Hall magnetoresistance measurements. The present study paves a way to spin-Hall effect based nonlinear spintronic devices as well as time-varying nonlinear magnetic metamaterials with tailor-made permeability.

Optical metamaterial elements that break mirror symmetry, such as swastika-shaped lattice structures, have been shown to exhibit chirality in the terahertz (THz) region, which is due to the spiral character of their hierarchical three-dimensional structure. However, in ordinary THz imaging with a linearly polarized beam, it has been difficult to quantify chiral optical characteristics, limiting the ideal design of metamaterials.

We demonstrated terahertz wave parametric wavelength conversion between frequency-controlled infrared (stabilized pumping and tunable seeding) beams and terahertz wave in a nonlinear crystal. The frequency stabilized pumping beam is amplified by an injection-seeded PPLN-OPG and KTAOPA pumped by a Nd:YAG MOPA system. The seeding beam is stabilized 1.5$\mu$ m beam as traceable to the national standard. The master laser of the MOPA system is a SLM Nd:YAG laser with duration of about 1 ns. The frequency of tunable seeding beam is monitored by using a “spectral drill” cavity. The cavity provides a continuous one-way sweep of the axis modes in a Fabry-Pérot cavity without sweeping the cavity length. When a geometric phase shifter composed of fixed and rotating phase plates is arranged within the cavity, the frequencies of the axis modes are controlled by the angle of the phase plate. The frequency of seeding beam is observed as intensity error signals. We used a nonlinear MgO:LiNbO 3 crystal with a Si-prism as an efficient output coupler for the terahertz wave. The output terahertz wave was measured using a pyroelectric detector. We speculate that the frequency-controlled terahertz wave could be powerful tools not only for solving real world problems but also fundamental physics. We expect that these methods will open new fields.

We have constructed a two-dimensional imaging system based on terahertz time-domain spectroscopy and successfully generated a terahertz vector beam with topological charge l = 1 using a (100) cut ZnTe crystal excited by circularly polarized pulse. (C) 2024 The Authors

We demonstrate a terahertz circular dichroism imaging system. This system allows the identification of left- and right-handed polarization singularities, associated from local left- and right-handed chiral structures, of a twisted-layered moire metasurface. (C) 2024 The Authors

A spectrum analyser using a virtually imaged phase array etalon is constructed to observe spectral drill laser (SDL) modes. The SDL is expected to realise the continuous mode tunning by rotating a constituent phase plate. (c) 2024 The Authors

We demonstrated parametric wavelength conversion between frequency-controlled two infrared (frequency fixed pumping and tunable (controlled by spectral drill cavity) seeding) beams and sub terahertz wave using a nonlinear MgO:LiNbO3 crystal. (c) 2024 The Authors

We demonstrate spoof surface plasmon coupling of a terahertz wave propagating in free space into a planar silicon waveguide through a bull's eye structure with a subwavelength aperture. Spoof surface plasmon polaritons induced by the bull's eye structure on the backside of the substrate propagate to the frontside through the aperture and couple into the waveguide. Electromagnetic field simulations revealed that the spoof surface plasmon polaritons propagating to the frontside show directivity along the incident polarization direction, and that the phase can be controlled by placing a Y-branched silicon waveguide beside the aperture. A prototype device was fabricated by bonding a copper-plated substrate with a bull's eye aperture and a waveguide fabricated by silicon micromachining. A monochromatic wave of 0.42-0.49 THz from a backward terahertz-wave parametric oscillator was injected into the bull's eye structure, and the intensity of the emitted wave from the end of the waveguide was measured. Directional coupling into the waveguide was confirmed from the intensity change depending on the incident polarization direction when using a straight waveguide. In addition, the phase difference between the two ends of the Y-branched waveguide was confirmed by the intensity change showing constructive or destructive interference depending on the polarization direction. These results indicate that it is possible to couple an incident wave into a planar waveguide perpendicular to it by controlling the phase via spoof surface plasmon coupling, suggesting its applicability to new experimental and practical systems in the terahertz band, such as beyond 5G/6G communications.