Mariko Yamaguchi

Noninvasive Inspection of C-4 Explosive in Mails by Terahertz Time-Domain Spectroscopy

We have measured the refractive index, extinction coefficient, and absorption coefficient of the explosive C-4 by THz time-domain spectroscopy and examined its applicability to detecting C-4 in mails. The explosive C-4, which is used in almost all mail-bomb terrorist activities, shows six THz bands in the frequency region from 5 to 90 cm-1 (150 GHz–2.7 THz). The 26.9 cm-1 band is very strong and can be used as a fingerprint of C-4. We confirmed that the THz absorption spectrum of C-4 hidden inside an envelope coincides essentially with that of C-4 itself. We also examined the applicability of the THz reflection spectroscopy to C-4 detection by calculating the reflectance spectra. Our results demonstrate that the THz techniques are very promising for counterterrorism against C-4 mail bombs.

Terahertz absorption spectra of L-, D-, and DL-alanine and their application to determination of enantiometric composition

Absorption spectra of polycrystalline L-, D-, and DL-alanine have been measured by the terahertz time domain spectroscopy (THz-TDS) in the frequency range from 10to90cm−1 at room temperature. We observed several absorption bands, which have the large difference between enantiomers (L- and D-alanine) and the racemic compound (DL-alanine) in their peak frequencies. This obvious difference shows that the THz absorption bands are strikingly sensitive to the crystal structures. This result indicates that the THz-TDS can be used for distinguishing between the enantiomers and the racemic compound. We propose and demonstrate a method to determine the enantiometric composition of amino acids from the THz absorption spectra.

Investigation of inflammable liquids by terahertz spectroscopy

We have examined a nondestructive and contactless screening method for water and inflammable liquids stored in common beverage plastic bottles by the terahertz time-domain spectroscopy. No THz transmission has been observed between 10 and 60cm−1 for 45-mm-thick water in plastic bottles. Part of the THz wave, on the other hand, has been transmitted through the commercially available inflammable liquids in the same bottles. The differences in the absorption intensities and the refractive indices in the THz region, which reflect the ingredients of organic compounds, allow us to distinguish these inflammable liquids. We confirm that the screening method based on the THz transmission can be applied to classifying the commercially available inflammable liquids stored in plastic bottles and to distinguishing these inflammable liquids from water as well.

Degradation diagnosis of ultrahigh-molecular weight polyethylene with terahertz-time-domain spectroscopy

We investigated ultrahigh-molecular-weight-polyethylene (UHMWPE) samples prepared by various conditions with terahertz-time-domain spectroscopy (THz-TDS). Degradation of the virgin UHMWPE samples by γ irradiation induced a drastic increase of the absorption ranging continuously over the THz region. The increase of the absorption continuum is interpreted to originate in the oxidation of the amorphous region within the sample. Only slight THz spectral changes induced by the γ irradiation were, however, observed for the UHMWPE samples doped with 0.1 and 0.3 wt % vitamin E. This result agrees with the earlier indication that vitamin E has an antidegradation effect on UHMWPE. The present result shows that the THz-TDS can be used for the quality control of UHMWPE by monitoring the absorption continuum in the THz region.

Time-Domain Coherent Anti-Stokes Raman Scattering Signal Detection for Terahertz Vibrational Spectroscopy Using Chirped Femtosecond Pulses

A new scheme for low-frequency coherent anti-Stokes Raman scattering spectroscopy (CARS) using chirped femtosecond pulses is proposed and demonstrated. Two chirped broadband optical pulses created terahertz (THz) polarization in the sample and generated CARS signal. The chirped CARS signal was then compressed by a pulse compressor and detected by a time gating technique. A resonant Raman band of GaSe at around 0.6 THz was successfully observed with the time-domain CARS technique. With the present system, CARS spectra from 0.2 to 5 THz are obtainable.

Impedance spectroscopy of organic magnetoresistance devices—Effect of interface disorder

Organic magnetoresistance (OMAR) can be caused by either single carrier (bipolaron) or double carriers (electron-hole)-based mechanisms. In order to consider applications for OMAR, it is important to control the mechanism present in the device. In this paper, we report the effect of traps on OMAR resulting of disorder at the interface between the organic active layer with the hole injection layer [poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate): PEDOT:PSS]. It has been found that while the single carriers OMAR is enhanced by the presence of traps, the double carriers OMAR is totally removed in a sample with a high interface trap density. The reasons for these results are discussed based on the impedance spectroscopy measurements. First, the mechanism (single or double carriers) responsible of the OMAR was determined with the support of the capacitance measurement. Then, the influence of traps was discussed with the Nyquist diagrams and phase angle-frequency plots of the samples. The results sugge...

Low-Frequency Coherent Raman Spectroscopy Using Spectral-Focusing of Chirped Laser Pulses

Vibrational spectroscopy is generally implemented using two schemes; that is, absorption spectroscopy and Raman spectroscopy. Conventionally, low frequency absorption spectroscopy is carried out using Fourier transform spectrometers equipped with a far infrared radiation source and a thermal detector. On the other hand, low frequency Raman spectroscopy is carried out by way of double or triple monochromaters and high-quality notch filters, whose performance determines the low frequency limit of the Raman spectrometer. In addition, in recent years, terahertz time-domain spectroscopy (THz-TDS) (Hangyo et al., 2005), utilizing femtosecond lasers as the excitation source, has been developed. THz-TDS enabled us to obtain absorption and dispersion spectra with a high signal-to-noise ratio in a frequency region less than 3 THz (100 cm-1) and can be applied for absorption spectroscopy of various substances (Kawase et al., 2009; Korter et al., 2006; Taday et al., 2003;Tani et al., 2004; Tani et al., 2010; Walther et al., 2003; Yamamoto et al., 2005; Yamaguchi et al., 2005) and imaging measurements (Kawase, 2004). There is a keen interest in low frequency vibrational spectroscopy for biomolecules since large amplitude, low frequency modes in macro biomolecules are believed to be associated with their respective function, as in the case of proteins (Chou, 1985, 1988). In order to fully understand the dynamics and function mechanisms of biomolecules, it is necessary to study their large amplitude and anharmonic low-frequency vibrational motions as these govern their thermal and physiochemical properties. A normal mode analysis of protein molecules revealed that large amplitude vibrational modes which are delocalized in the whole molecule lie within the THz region (< 120 cm-1) (Brooks & Karplus, 1985; Go et al., 1983). In addition, the calculations suggested that the entropy of the whole molecule, specifically its thermodynamic characteristics, is governed by the large amplitude vibrational modes in the sub-THz region (<30 cm-1). Consequently, important information related to the functions and dynamics of proteins can be derived by investigating THz vibration spectra.

Terahertz optical constants of ytterbium-doped yttrium aluminum garnet crystals

Terahertz time-domain spectroscopy has been employed to measure the optical constants of ytterbium-doped yttrium aluminum garnet (YbxY1−x)3Al5O12 (Yb:YAG) crystals for nominal x values of 0.0, 0.1, 0.2, 0.5, 0.8, and 1.0 in the frequency range from 0.2to1.8THz. The real refractive indices for Yb:YAG crystals increase linearly with the concentration of Yb3+. The experimental results can be fitted by the Sellmeier equation. The results imply that the phonon modes of the Yb:YAG crystals shift to lower frequencies with substitution of Yb for Y. The extinction coefficients of the Yb:YAG crystals are smaller than 0.05.

Determination of the mechanism behind the organic magnetoresistance (OMAR) effect by using impedance spectroscopy

We investigated the organic magnetoresistance (OMAR) mechanism in poly(3–hexylthiophene–2,5–diyl) (P3HT)–based diodes, by using impedance spectroscopy. We have prepared layered structures consisting of indium tin oxide, poly(3,4–ethylenedioxythiophene):poly(styrenesulphonate) (PEDOT:PSS), P3HT and aluminium and measured the bias voltage dependence of the OMAR and of the impedance spectra. The capacitance deduced from the impedance data indicated that the OMAR was explained by the single–carrier (bipolaron) model at lower bias voltages as well as double–carrier models at higher bias voltages. The impedance response also suggested that the OMAR from the single–carrier model was governed by the PEDOT:PSS/P3HT interface whereas the double–carrier OMAR was mainly related to the P3HT layer.

Systematic alanine insertion reveals the essential regions that encode structure formation and activity of dihydrofolate reductase

Decoding sequence information is equivalent to elucidating the design principles of proteins. For this purpose, we conducted systematic alanine insertion analysis to reveal the regions in the primary structure where the sequence continuity cannot be disrupted. We applied this method to dihydrofolate reductase (DHFR), and examined the effects of alanine insertion on structure and the enzymatic activity by solubility assay and trimethoprim resistance, respectively. We revealed that DHFR is composed of “Structure Elements”, “Function Elements” and linkers connecting these elements. The “Elements” are defined as regions where the alanine insertion caused DHFR to become unstructured or inactive. Some “Structure Elements” overlap with “Function Elements”, indicating that loss of structure leads to loss of function. However, other “Structure Elements” are not “Function Elements”, in that alanine insertion mutants of these regions exhibit substrate- or inhibitor-induced folding. There are also some “Function Elements” which are not “Structure Elements”; alanine insertion into these elements deforms the catalytic site topology without the loss of tertiary structure. We hypothesize that these elements are involved essential interactions for structure formation and functional expression. The “Elements” are closely related to the module structure of DHFR. An “Element” belongs to a single module, and a single module is composed of some number of “Elements.” We propose that properties of a module are determined by the “Elements” it contains. Systematic alanine insertion analysis is an effective and unique method for deriving the regions of a sequence that are essential for structure formation and functional expression.