Measurement of beam profiles by terahertz sensor card with cholesteric liquid crystals.

We demonstrate a sensor card with cholesteric liquid crystals (CLCs) for terahertz (THz) waves generated from a nonlinear crystal pumped by a table-top laser. A beam profile of the THz waves is successfully visualized as color change by the sensor card without additional electronic devices, power supplies, and connecting cables. Above the power density of 4.3 mW/cm2, the approximate beam diameter of the THz waves is measured using the hue image that is digitalized from the picture of the sensor card. The sensor card is low in cost, portable, and suitable for various situations such as THz imaging and alignment of THz systems.

Trapping waves with terahertz metamaterial absorber based on isotropic Mie resonators.

Quasi-monodisperse dielectric particles organized in a periodic hexagonal network on an aluminum surface are exploited numerically and experimentally as a single-layered near-perfect absorber in the terahertz regime. Of particular interest are titanium dioxide (TiO(2)) microspheres because of their large dielectric permittivity and isotropic shape leading to Mie resonances with insensitive polarization. Absorption higher than 80% at normal incidence covering two distinct ranges of frequencies is demonstrated experimentally. Furthermore, the performance of the metamaterial absorber is kept over a wide range of incident angles.

Crossover from capacitive to inductive electromagnetic responses in near self-complementary metallic checkerboard patterns.

Transmission spectra of near-self-complementary metallic checkerboard patterns (MCPs) exhibit a drastic change when the metal squares are brought into contact with each other from a noncontact state. Transmission spectra of near-self-complementary samples, which are fabricated by printing technology, show rather gradual systematic change with changing the nominal metal square size while keeping the period because of randomness naturally introduced by the limited accuracy of the printer. The spectra have transmission-invariant frequencies, which means that the spectra are a superposition of two types of spectra, the ratio of which depends on the nominal square size. The correlation seen in the real and imaginary parts of the complex amplitude spectra can be interpreted based on the Kramers-Kronig relation. As an application of the sensitiveness of the transmission spectrum of the MCPs to connectivity of the metal squares, the revealing of an optically hidden pattern by a terahertz beam is demonstrated.

Spirulina-templated metal microcoils with controlled helical structures for THz electromagnetic responses.

Microstructures in nature are ultrafine and ordered in biological roles, which have attracted material scientists. Spirulina forms three-dimensional helical microstructure, one of remarkable features in nature beyond our current processing technology such as lithography in terms of mass-productivity and structural multiplicity. Spirulina varies its diameter, helical pitch, and/or length against growing environment. This unique helix is suggestive of a tiny electromagnetic coil, if composed of electro-conductive metal, which brought us main concept of this work. Here, we describe the biotemplating process onto Spirulina surface to fabricate metal microcoils. Structural parameters of the microcoil can be controlled by the cultivation conditions of Spirulina template and also purely one-handed microcoil can be fabricated. A microcoil dispersion sheet exhibited optically active response attributed to structural resonance in terahertz-wave region.

Mechanism of optical terahertz-transmission modulation in an organic/inorganic semiconductor interface and its application to active metamaterials.

Terahertz (THz) transmission modulation through copper phthalocyanine (CuPc)-coated Si under various laser light irradiation conditions was investigated using THz time-domain spectroscopy. The charge carrier transfer from Si to CuPc is crucial for photo-induced metallization, and the thickness of the CuPc layer is a critical parameter for achieving high charge carrier density for metallization. Transmission through a split-ring resonator array metamaterial, fabricated on CuPc-coated Si, can be efficiently modulated by laser light irradiation. Our findings may open the way for various types of metamaterials using organic conjugated materials that are suitable for easy device fabrication using printing technologies.

Wavefront control by stacked metal-dielectric hole array with variable hole shapes.

A stacked metal-dielectric hole array (SHA) containing rectangular holes whose shape gradually varies in-plane is proposed as a means of achieving wavefront control. The dependence of the transmitted phase on the frequency can be tuned by the hole shape, in particular the length of the sides that are normal to the incident polarization. The combination of periodic holes along the polarization direction and the gradual change in hole shape normal to the polarization direction produce an inclined wavefront for 1-dimensional beam steering. An in-plane phase difference of 0.6π using an SHA with a thickness of one-sixth of the wavelength has been experimentally demonstrated.

Transmission phase control by stacked metal-dielectric hole array with two-dimensional geometric design.

Transmission phase control is experimentally demonstrated using stacked metal-dielectric hole arrays with a two-dimensional geometric design. The transmission phase varies drastically with small frequency shifts due to structural resonances. Laterally propagating surface plasmon polaritons excited by the periodic hole array roughly determine the resonance frequency, whereas localized resonances in each hole determine the dispersion. The transmission phase at various frequencies is directly evaluated using interferometric microscopy, and the formation of an inclined wavefront is demonstrated using a beam steering element in which the hole shapes gradually change in-plane from square to circular.

Wire-grid polarizer sheet in the terahertz region fabricated by nanoimprint technology.

Wire-grid polarizer sheets in the terahertz region have been fabricated on flexible substrates by nanoimprint technology. They show an ideal polarization property in the terahertz frequency region, whereas the cost is very low. Since the wire pitch is far smaller than the wavelength, the effective medium theory agrees well with experimental results. The effective medium theory shows the possibility of further improvement of polarization properties by selecting appropriate materials for wire grids.

Measurement of the dielectric constant of thin films by terahertz time-domain spectroscopic ellipsometry.

We present a reflection-type terahertz time-domain spectroscopic ellipsometry (THz-TDSE) technique for measuring the complex dielectric constants of thin-film materials without replacement of the sample. THz-TDSE provides complex dielectric constants from the ratio of the complex amplitude reflection coefficients between p- and s-polarized THz waves. The measured dielectric constants of doped GaAs thin films show good agreement with predictions of the Drude model, even though the film thickness is of the order of a tenth of the penetration depth of the THz waves. In addition, we demonstrate the measurements of soft-phonon dispersion in SrTiO(3) thin films deposited on a Pt layer. The obtained dielectric constants agree well with the predictions of a generalized harmonic oscillator model.

Measurement of electron paramagnetic resonance using terahertz time-domain spectroscopy.

We present a frequency-domain electron spin resonance (ESR) measurement system using terahertz time-domain spectroscopy. A crossed polarizer technique is utilized to increase the sensitivity in detecting weak ESR signals of paramagnets caused by magnetic dipole transitions between magnetic sublevels. We demonstrate the measurements of ESR signal of paramagnetic copper(II) sulfate pentahydrate with uniaxial anisotropy of the g-factor under magnetic fields up to 10 T. The lineshape of the obtained ESR signals agrees well with the theoretical predictions for a powder sample with the uniaxial anisotropy.

Characteristics and generation process of surface waves excited on a perfect conductor surface.

We investigate characteristics and generation process of surface waves excited on a structured perfect conductor surface in order to clarify the mechanism of resonant transmission in metal hole arrays made of the perfect conductor. By using a metal hole array of kagome lattice, we can separate two modes excited on the perfect conductor surface; a surface wave and an edge mode. Our calculation based on finite-difference time-domain method provides a generation process of the edge mode and the surface wave that is responsible for the resonant transmission in metal hole arrays.

Optical transmission anomalies in a double-layered metallic slit array.

We theoretically predicted the existence of an anomalous optical transmittance dip, which must be observed for a metamaterial structure with two metallic slabs with cut-through slit arrays of a constant period d under a normal incident condition. By changing the relative lateral displacement l between the two slabs, the dip frequency varies across that of a so-called Rayleigh-Wood's (RW) anomaly frequency. The mechanism of this anomaly is quite different from that of the RW anomaly and interpreted in terms of the interference between the propagating and evanescent waves. For the present double-layered system, furthermore, it is suggested that the RW anomaly vanishes for l = 0 and d/2. In experiments in the terahertz region, we observe that the fundamental features agree with these theoretical predictions.

Terahertz absorption spectra of original and generic ceftazidime.

The absorption spectra of ceftazidime and its generic versions (Modacin, Mosyl, and Mobenzocin) have been measured by terahertz time domain spectroscopy (THz-TDS). Differences in the THz absorption were observed between the original and generic versions. The results show small, but significant differences in the states of ceftazidime hydrate between the original and generic versions. THz-TDS can be used to evaluate the stability of medicines as well as to control their quality.

Enhancement and suppression of terahertz emission by a Fabry-Perot cavity structure with a nonlinear optical crystal.

We have fabricated Fabry-Perot (FP) cavities in the THz region with a ZnTe crystal as a cavity layer by a simple stacking method. We observed more than a three times enhancement of the THz emission intensity in the FP cavities compared with the bare ZnTe crystal at the frequencies of the resonant modes and stopband edges. On the other hand, suppression of the THz emission occurs at frequencies in the stopband. The enhancement and suppression of the THz emission are caused by the modification of the optical density of state in the FP cavities compared to the vacuum.

High-repetition-rate optical delay line using a micromirror array and galvanometer mirror for a terahertz system.

We developed a high-repetition-rate optical delay line based on a micromirror array and galvanometer mirror for terahertz time-domain spectroscopy. The micromirror array is fabricated by using the x-ray lithographic technology. The measurement of terahertz time-domain waveforms with the new optical delay line is demonstrated successfully up to 25 Hz.

Terahertz pulse radiation from argon clusters.

The characteristics of THz pulse radiation from argon clusters irradiated with intense femtosecond laser pulses have been investigated. The average power of the THz pulses radiated from argon clusters is approximately 40 times greater than that from argon gas. The enhancement of the THz radiation originates from large laser absorption by the clusters. The polarization state and the angular distribution of the power of the THz pulses show no dependence on laser polarization, which suggests that the radiation originates from laser-induced charge separation along the laser propagation direction.

Terahertz wire-grid polarizers with micrometer-pitch Al gratings.

We fabricated a terahertz wire-grid polarizer consisting of a micrometer-pitch Al grating on a Si substrate by photolithography and wet etching. The ratio of TM and TE transmittances (extinction ratio) was over 35 dB at 0.5 THz. At the Brewster angle of the Si substrate, the polarization transmittance of a TM wave through the fabricated polarizer exceeded 95% and the extinction ratio was over 45 dB at approximately 1 THz. The fabricated polarizer has a higher extinction ratio than conventional free-standing terahertz wire-grid polarizers.

Application of partial least square on quantitative analysis of L-, D-, and DL-tartaric acid by terahertz absorption spectra.

Absorption spectra of polycrystalline L-, D-, and DL-tartaric acid have been measured by terahertz time domain spectroscopy (THz-TDS). Different absorption bands are observed for DL-tartaric acid and its enantiomers (L- and D-tartaric acid). This result shows that the THz-TDS can be used for distinguishing between DL-tartaric acid and enantiomers (L- and D-tartaric acid). Moreover, partial least square (PLS) can be found to improve the quantitation of L-tartaric acid in L- and DL-tartaric acid mixture by THz-TDS.

Terahertz time-domain spectroscopy of imidazolium ionic liquids.

We have measured the terahertz (THz) complex dielectric spectra of imidazolium ionic liquids by THz time-domain spectroscopy (THz-TDS) in the frequency range from 5 (0.15 THz) to 140 cm(-1) (4.2 THz). The ionic liquids investigated are 1-ethyl-3-methylimidazolium (EMIm+)/trifluoromethanesulfonate (TfO-), EMIm+/tetrafluoroborate (BF(4)-), 1-butyl-3-methylimidazolium (BMIm+)/TfO-, and BMIm+/BF(4)-. The dielectric values of the ionic liquids in the THz region are similar to those of short-chain alcohols. The THz dielectric values are related to subpicosecond-to-picosecond dynamics. The same trend has been observed in the empirical polarity ET(30) although it is related to the static characteristics of polarity and hydrogen bonding ability. A difference between the two types of liquids is observed in the THz dielectric spectral shapes: the ionic liquids show structured lineshapes but short-chain alcohols show much less structured ones. The structured lineshapes of the ionic liquids reflect the low-frequency motions of interion and/or intramolecular vibrations. When the ionic liquids composed of the different imidazolium cations contain the same anions as counterions, their density-normalized THz dielectric spectra above 20 cm(-1) bear strong resemblance to each other in shape and magnitude. It shows clearly that the THz spectra do not originate from the intramolecular vibrations of the imodazolium cations. All of the intramolecular vibrations of the anions are located above 140 cm(-1) except the CF3-SO3 torsion of TfO-, the band of which alone cannot explain the broad THz dielectric spectra of the ionic liquids. Therefore, we conclude that the interion vibrations rather than the intramolecular vibrations dominantly contribute to the THz dielectric spectra. The results strongly indicate that even in the liquid phase the ionic liquids have local structures similar to their solid-phase structures.

Polarization state measurement of terahertz electromagnetic radiation by three-contact photoconductive antenna.

We have fabricated a three-contact photoconductive antenna for the polarization-sensitive detection of terahertz electromagnetic radiation. Taking into account all three photoconductive signal current components, this three-contact photoconductive antenna can measure the polarization state of pulsed THz radiation at an accuracy comparable to that achieved using the conventional method which employs a set of wire-grid polarizers. The three-contact photoconductive receiver may be useful for polarization-sensitive spectroscopy such as vibrational circular dichroism spectroscopy and ellipsometry in the THz frequency region.