THz irradiation inhibits cell division by affecting actin dynamics.

Biological phenomena induced by terahertz (THz) irradiation are described in recent reports, but underlying mechanisms, structural and dynamical change of specific molecules are still unclear. In this paper, we performed time-lapse morphological analysis of human cells and found that THz irradiation halts cell division at cytokinesis. At the end of cytokinesis, the contractile ring, which consists of filamentous actin (F-actin), needs to disappear; however, it remained for 1 hour under THz irradiation. Induction of the functional structures of F-actin was also observed in interphase cells. Similar phenomena were also observed under chemical treatment (jasplakinolide), indicating that THz irradiation assists actin polymerization. We previously reported that THz irradiation enhances the polymerization of purified actin in vitro; our current work shows that it increases cytoplasmic F-actin in vivo. Thus, we identified one of the key biomechanisms affected by THz waves.

Terahertz spectroscopy technology as an innovative technique for food: Current state-of-the-Art research advances.

With the dramatic development of source and detector components, terahertz (THz) spectroscopy technology has recently shown a renaissance in various fields such as medical, material, biosensing and pharmaceutical industry. As a rapid and noninvasive technology, it has been extensively exploited to evaluate food quality and ensure food safety. In this review, the principles and processes of THz spectroscopy are first discussed. The current state-of-the-art applications of THz and imaging technologies focused on foodstuffs are then discussed. The advantages and challenges are also covered. This review offers detailed information for recent efforts dedicated to THz for monitoring the quality and safety of various food commodities and the feasibility of its widespread application. THz technology, as an emerging and unique method, is potentially applied for detecting food processing and maintaining quality and safety.

Propagation of THz irradiation energy through aqueous layers: Demolition of actin filaments in living cells.

The effect of terahertz (THz) radiation on deep tissues of human body has been considered negligible due to strong absorption by water molecules. However, we observed that the energy of THz pulses transmits a millimeter thick in the aqueous solution, possibly as a shockwave, and demolishes actin filaments. Collapse of actin filament induced by THz irradiation was also observed in the living cells under an aqueous medium. We also confirmed that the viability of the cell was not affected under the exposure of THz pulses. The potential of THz waves as an invasive method to alter protein structure in the living cells is demonstrated.

Improvement of the depth resolution of swept-source THz-OCT for non-destructive inspection.

We construct a terahertz swept source optical coherence tomography system using a continuous-wave diode multiplier source in the 600-GHz band for defect inspection in multilayer objects and evaluate its performance. Using this system, we image a multilayer plastic sample to demonstrate the effectiveness of nondestructive three-dimensional imaging. To enhance the depth resolution, we apply an annihilating filter to the analysis and confirm that two surfaces of a 1-mm-thick plastic plate can be resolved. In addition, the repeatability of measured thicknesses is 0.22 mm. These values are approximately one-half and one-tenth of the resolution achievable by conventional Fourier analysis, respectively.

The thermodynamic properties and molecular dynamics of [Li+@C60](PF6-) associated with structural phase transitions.

Calorimetric and terahertz-far-infrared (THz-FIR) spectroscopic and infrared (IR) spectroscopic measurements were conducted for [Li+@C60](PF6-) at temperatures between 1.8 and 395 K. [Li+@C60](PF6-) underwent a structural phase transition at around 360 K accompanied by the orientational order-disorder transition of Li+@C60 and PF6-. The transition occurred in a step-wise manner. The total transition entropy (ΔtrsS) of 40.1 ± 0.4 J K-1 mol-1 was smaller than that of the orientational order-disorder transition in a pristine C60 crystal (ΔtrsS = 45.4 ± 0.5 J K-1 mol-1). Thus, the orientational disorder of Li+@C60 in the high-temperature phase of [Li+@C60](PF6-) was much less excited than that of the pristine C60 owing to the Coulombic interactions, which stabilized the ionic crystal lattice of [Li+@C60](PF6-). At T < 100 K, upon cooling, Li+ ions were trapped in two pockets on the inner surface of C60, and no phase transition was observed. Finally, the Li+ ions achieved a complete order at 24 K through antiferroelectric transition. The ΔtrsS value of 4.6 ± 0.4 J K-1 mol-1 was slightly smaller than R ln 2 = 5.76 J K-1 mol-1 expected for the two-site order-disorder transition. The extent of the Li+ motion in the C60 cage was related to the selection rule in the THz-FIR and IR spectroscopy of the C60 internal vibrations, because a C60 cage should be polarized by the Li+ ion. It is shown that the local symmetry of the caged molecule can be modified by the rotational or hopping motion of the encaged ions.

Rotational dynamics of Li+ ions encapsulated in C60 cages at low temperatures.

Li+ ions encapsulated in fullerene C60 cages (Li+@C60) are expected to be suitable as molecular switches that respond to local electric fields. In this study, the rotational dynamics of Li+ ions in C60 cages at low temperatures are experimentally revealed for the first time using terahertz absorption spectroscopy. In crystalline [Li+@C60](PF6-), the Li+ ion rotates in the carbon cage even at 150 K. The rotational mode gradually changes into a librational mode below 120 K, which is associated with the localization of Li+ ions due to the electrostatic interactions with its screening image charge on the C60 cage as well as with the neighboring Li+@C60 and PF6- ions. A simple rotational/librational energy scheme for the Li+ ions successfully explains the spectroscopic results, and the potential of Li+@C60 as a molecular switch is discussed based on the energy scheme.

Polymer Morphological Change Induced by Terahertz Irradiation.

As terahertz (THz) frequencies correspond to those of the intermolecular vibrational modes in a polymer, intense THz wave irradiation affects the macromolecular polymorph, which determines the polymer properties and functions. THz photon energy is quite low compared to the covalent bond energy; therefore, conformational changes can be induced "softly," without damaging the chemical structures. Here, we irradiate a poly(3-hydroxybutylate) (PHB) / chloroform solution during solvent casting crystallization using a THz wave generated by a free electron laser (FEL). Morphological observation shows the formation of micrometer-sized crystals in response to the THz wave irradiation. Further, a 10-20% increase in crystallinity is observed through analysis of the infrared (IR) absorption spectra. The peak power density of the irradiating THz wave is 40 MW/cm(2), which is significantly lower than the typical laser intensities used for material manipulation. We demonstrate for the first time that the THz wave effectively induces the intermolecular rearrangement of polymer macromolecules.

Analytical solutions of electromagnetic waves in focusing and magnifying cylindrical hyperlenses: Green's function approach.

The analytical solutions of the electromagnetic waves in the inhomogeneous cylindrical hyperlens (CH) comprising concentric cylindrical layers (CCLs) with multiple point sources located either outside the structure in the focusing process or inside the core in the magnifying process are obtained by means of Green's function analysis. The solutions are consistent with FDTD simulation in both processes. The sub-wavelength focal spot λ/16.26 from two point sources with wavelength 465 nm is demonstrated in the CH made by alternating silver and silica CCLs. Our solutions are expected to be the efficient tools for designing the sub-wavelength focusing and imaging cylindrical hyperlens.

THz phase-contrast computed tomography based on Mach-Zehnder interferometer using continuous wave source: proof of the concept.

In this study, we propose a THz computed tomography (CT) method based on phase contrast, which retrieves the phase shift information at each data point through a phase modulation technique using a Mach-Zehnder interferometer with a continuous wave (CW) source. The THz CT is based on first-generation CT, which acquires a set of projections by translational and rotational scans using a thin beam. From the phase-shift projections, we reconstruct a spatial distribution of refractive indices in a cross section of interest. We constructed a preliminary system using a highly coherent CW THz source with a frequency of 0.54 THz to prove the concept and performed an imaging experiment using phantoms to investigate its imaging features such as artifact-immune imaging, quantitative measurement, and selective detection.

Terahertz beam steering using interference of femtosecond optical pulses.

A terahertz (THz) beam steering method is demonstrated by applying the characteristic of grating lobe (GL) radiation from a linear array antenna and the interference of femtosecond optical pulses. A photoconductive device is illuminated by two femtosecond laser beams combined at an angle of less than 0.5°. Considering the interference pattern as a THz point source array, THz GL radiation is generated through the superposition of radiation emitted from all point sources and steered by varying the interval of the interference pattern. The THz beam direction could be changed by 20° at 0.93THz by varying the relative incidence angle of the pump beams by 0.033°.

THz emission characteristics from p/n junctions with metal lines under non-bias conditions for LSI failure analysis.

We have investigated the characteristics of THz emissions from p/n junctions with metallic lines under non-bias conditions. The waveforms, spectra, and polarizations depend on the length and shape of the lines. This indicates that the transient photocurrents from p/n junctions flow into the metallic lines that emit THz waves and act as an antenna. We have successfully demonstrated the non-contact inspection of open defects of multi-layered interconnects in a large-scale integrated circuit using the laser THz emission microscope (LTEM). The p/n junctions connected to the defective interconnects can be identified by comparing the LTEM images of normal and defective circuits.

Polarization and temperature dependent spectra of poly(3-hydroxyalkanoate)s measured at terahertz frequencies.

Temperature-dependent terahertz (THz) absorption spectra of poly(3-hydroxyalkanoate)s (PHAs) were measured by using a Fourier transform far-infrared (FT-FIR) spectrometer and a THz time-domain spectrometer over a temperature range of 10 K to 465 K with a liquid helium cryostat and a heating cell. Clear differences were observed between the spectra of crystalline and amorphous polyhydroxybutyrate (PHB), indicating that the absorption peaks observed in the THz spectra originated in the higher-order conformation of PHB. The polarization spectra of a stretched PHB sample were measured, and the direction of the vibrational transition moment was determined. The temperature dependences of the spectra reveal frequency shifts and broadening of the absorption peaks with temperature, suggesting large anharmonicity of the vibrational potential. The temperature shift behaviour is quite different in each transition. Some of the transitions show a blue shift, which cannot be explained by a simple anharmonic potential model. Frequency shifts of the peaks were mainly observed below 10 THz, which suggests a large anharmonicity of the vibrational potential at lower frequencies.

Depth-resolving THz imaging with tomosynthesis.

We demonstrated a depth-resolved 3D imaging technique based on absorption contrast using tomosynthesis. Tomosynthesis is similar to computed tomography except that the number of projections is much smaller. We constructed a tomosynthesis imaging system, which detects a transmitted continuous THz wave. We applied a backprojection method that was suitable for the constructed detection configuration, to reconstruct an image. Using this system, we imaged a test sample made from paper and reproduced characters written by pencil.

Tunability enhancement of a terahertz-wave parametric generator pumped by a microchip Nd:YAG laser.

We have enhanced the tunability of a terahertz-wave parametric generator using MgO:LiNbO3 pumped by a subnanosecond, passively Q-switched, microchip Nd:YAG laser. This pump source allows high-intensity pumping without damage to the nonlinear crystal and generates a narrow linewidth and tunable terahertz wave with injection seeding by an external cavity diode laser for the idler wave. The high-intensity pumping causes a gain curve broadening of the terahertz-wave parametric generation, especially in the high-frequency region. We obtained an output terahertz wave with a tuning range of 0.9-3 THz (100-330 microm), a peak power of 100 mW, and a linewidth of less than 10 GHz. This room temperature operated, palm-sized source promises to be a widely tunable terahertz-wave source that is suited to a variety of applications.

Noninvasive mail inspection system with terahertz radiation.

Using the high penetrability of the terahertz waves and the characteristic absorption spectra in this frequency range, we have built a noninvasive mail inspection system targeting drugs and explosives. The system is composed of two stages; in the first stage, the scattering of a continuous terahertz wave is used for selecting mail that contains concealed powder; in the second stage, the absorption spectrum of the suspicious mail is measured and the material is identified using a terahertz spectrum database. We evaluated the performance and the limits of the inspection system.

Interference terahertz label-free imaging for protein detection on a membrane.

We demonstrate a highly sensitive imaging method combined a terahertz time-domain spectroscopy and an interference effect for label-free protein detection on a polyvinylidene difluoride membrane. The method is based on terahertz time-domain spectroscopy and uses an interference effect. Biotin is linked to the membrane using poly ethylene glycol or poly ethylene glycol methyl ether to prevent it from being washed off. Binding of the biotin with streptavidin is then observed by measuring the terahertz signal change due to the variation of the membrane refractive index. We demonstrate the detection of the binding streptavidin protein in gradually decreasing concentrations, down to 27 ng mm-2, using the image recorded at 1.5 THz.

Terahertz beam steering and frequency tuning by using the spatial dispersion of ultrafast laser pulses.

We demonstrate a terahertz (THz) beam steering method using difference frequency generation that is based on the principle of phased array antennas. A strip-line photoconductive antenna was illuminated by two spatially dispersed beams produced from an ultrafast laser. THz radiation with a bandwidth of 65 GHz was generated from the overlapping area of the two beams, between which the frequency difference was approximately constant. We confirmed that the THz beam can be steered by tilting one of the incident pump beams so as to change their relative phase relation. The steering range of the THz beam was 29 degrees when the angle between the incident pump beams was only varied within a range of 0.155 degrees, that is, 187 times less. In addition, by laterally shifting one of the pump beams, the frequency of the THz radiation could be tuned from 0.3 to 1.7 THz. This technique can be applied to high-speed terahertz imaging and spectroscopy systems.

Cherenkov phase-matched monochromatic THzwave generation using difference frequency generation with a lithium niobate crystal.

We demonstrated a Cherenkov phase-matching method for monochromatic THz-wave generation using the difference frequency generation process with a lithium niobate crystal, which resulted in high conversion efficiency and wide tunability. We successfully generated monochromatic THz waves across the range 0.2-3.0 THz. We obtained efficient energy conversion in the low frequency region below 0.5 THz, and achieved a flat tuning spectrum by varying the pumping wavelength during THz-wave tuning.

Output power enhancement of a palmtop terahertz-wave parametric generator.

Broadband terahertz (THz) waves were generated by optical parametric processes based on laser light scattering from the polariton mode of a nonlinear crystal. By using the parametric oscillation of a MgO-doped LiNbO3 crystal pumped by a nanosecond Q-switched Nd:YAG laser, we have realized a broadband, high-energy and compact THz-wave source. We report the development of a THz-wave parametric generator (TPG) using a small pump source with a short pulse width and a top-hat beam profile. These characteristics of the pump beam permit high-intensity pumping especially close to the output surface of the THz wave without thermal damage to the crystal surface. We also calculated the outcoupled THz wave for beams with two different intensity profiles: a top-hat beam (in this experiment) and a Gaussian beam (previously reported). The result shows the mechanism of the output energy and/or power enhancement.

Component spatial pattern analysis of chemicals by use of two-dimensional electro-optic terahertz imaging.

We demonstrate a method of chemical mapping by using the transillumination terahertz (THz) images obtained by two-dimensional electro-optic THz imaging. The images and spectral data were measured between 0.1 and 1.0 THz. An experimental sample consisting of three chemicals was prepared, with one in two concentrations. By introducing the component spatial pattern analysis based on the least-squares method, the chemical composition, spatial distribution, and difference in concentration were clearly determined.