Millimeter-Wave Band Electro-Optical Imaging System Using Polarization CMOS Image Sensor and Amplified Optical Local Oscillator Source.

In this study, we developed and demonstrated a millimeter-wave electric field imaging system using an electro-optic crystal and a highly sensitive polarization measurement technique using a polarization image sensor, which was fabricated using a 0.35-µm standard CMOS process. The polarization image sensor was equipped with differential amplifiers that amplified the difference between the 0° and 90° pixels. With the amplifier, the signal-to-noise ratio at low incident light levels was improved. Also, an optical modulator and a semiconductor optical amplifier were used to generate an optical local oscillator (LO) signal with a high modulation accuracy and sufficient optical intensity. By combining the amplified LO signal and a highly sensitive polarization imaging system, we successfully performed millimeter-wave electric field imaging with a spatial resolution of 30×60 µm at a rate of 1 FPS, corresponding to 2400 pixels/s.

Exposure Time Control Method for Higher Intermediate Frequency in Optical Heterodyne Imaging and Its Application to Electric-Field Imaging Based on Electro-Optic Effect.

We propose and demonstrate a method for equivalent time sampling using image sensors to selectively detect only the target frequency. Shortening the exposure time of the image sensor and using equivalent time sampling allows for the detection of frequency components that are higher than the frame rate. However, the imaging system in our previous work was also sensitive to the frequency component at 1/4 of the frame rate. In this study, we control the phase relationship between the exposure time and observed signal by inserting an additional interval once every four frames to detect the target frequency selectively. With this technique, we conducted electric field imaging based on the electro-optic effect under high noise conditions in the low-frequency band to which the conventional method is sensitive. The results demonstrated that the proposed method improved the signal-to-noise ratio.

Far-infrared spectroscopy of salt penetration into a collagen fiber scaffold.

We employed far-infrared spectroscopy to observe the amount of salt that penetrates into collagen fiber masses. The absorption properties of collagen sheets prepared from tilapia skin, bovine skin, rat tail, and sea cucumber dermis were measured using a transmission Fourier transform spectrometer in a band from approximately 100 to 700 cm(-1). We confirmed that the absorbance spectra of the four types of dried collagen sheet show good agreement, even though the amino acid compositions differed. The absorbance peaks observed in the band corresponded to collective vibrations of plural functional groups such as methylene and imino groups in collagen. When salt solution was added to the collagen sheets and then dried, the spectral shapes of the sheets at approximately 166 cm(-1) were clearly different from those of the plain collagen sheets. The differential absorbance between wavenumbers 166 cm(-1) and 250 cm(-1) sensitively reflected the difference between higher-order structures, and the salt diffusion (crystallization) depended on the collagen fiber condition. From these results, we consider that spectral changes can be used for the numerical evaluation of salt penetration into a collagen fiber scaffold.

Infrared and Terahertz Spectroscopic Investigation of Imidazolium, Pyridinium, and Tetraalkylammonium Tetrafluoroborate Ionic Liquids.

We performed terahertz time-domain spectroscopy and infrared spectroscopy of imidazolium-based, pyridinium-based, and tetraalkylammonium-based tetrafluoroborate ionic liquids to study their characteristic intermolecular and intramolecular vibrational modes to clarify interactions between various cations and the tetrafluoroborate anion. It was found that the central frequency of the intermolecular vibrational band for these ionic liquids has a relatively high frequency, ranging from 90 to 100 cm-1. In the 900-1150 cm-1 range, the intramolecular vibrational absorption band of the 3-fold degenerate mode of tetrafluoroborate anions in the ionic liquids was observed. Although the tetrafluoroborate anion is attributable to one of the weakly coordinated anions, the spectroscopic splitting behavior of the 3-fold degenerate mode differs depending on the cation species. It was revealed that the degenerate mode is very sensitive to local interactions between the tetrafluoroborate anion and each cation.

Infrared Spectroscopy in the Middle Frequency Range for Various Imidazolium Ionic Liquids-Common Spectroscopic Characteristics of Vibrational Modes with In-Plane +C(2)-H and +C(4,5)-H Bending Motions and Peak Splitting Behavior Due to Local Symmetry Breaking of Vibrational Modes of the Tetrafluoroborate Anion.

Various alkyl-methylimidazolium ionic liquids (ILs) were inspected using infrared spectroscopy in the middle frequency range. In the 1050-1200 cm-1 range, there is a skeletal vibrational mode accompanied with a large in-plane +C(2)-H bending motion and +C(4)-H and +C(5)-H motions, and in the 1500-1650 cm-1 range, there are two skeletal vibrational modes with in-plane +C(4,5)-H bending motions. Interestingly, in both ranges, we found that skeletal vibrational modes with a large in-plane +C(2)-H bending motion and in-plane +C(4,5)-H bending motions are insensitive to increases in the basicity of anions or the strengthening of hydrogen bond-type interactions, and the behaviors are completely different from those in the +C-H stretching vibrational modes in the 3000-3200 cm-1 range and the skeletal vibrational modes with large out-of-plane +C-H motions in the 700-950 cm-1 range. Furthermore, in alkyl-methylimidazolium tetrafluoroborate [C n mim+][BF4 -] ILs, we found that absorption due to the (threefold) degenerate vibrational mode of [BF4 -] was observed as a broad absorption band with three splitting peaks in the 900-1150 cm-1 range as a result of local symmetry breaking due to the cation-anion interactions.

Dielectric property measurements of corneal tissues for computational dosimetry of the eye in terahertz band in vivo and in vitro.

The dielectric constant of the normal corneal tissue of a rabbit eye was obtained in vitro in the range from approximately 0.1 to 1 THz, and the drying process on the eye surface exposed to high-power terahertz waves was investigated by in vivo reflectance measurement using terahertz time-domain spectroscopy. When the rabbit eye was exposed to terahertz waves at 162 GHz for 6 min with an irradiation power of 360 or 480 mW/cm2, the reflectance temporally increased and then decreased with a temperature increase. Based on multiple-reflection calculation using the dielectric constant and anterior segment optical coherence tomography images, those changes in reflectance were attributed to drying of the tear and epithelium of the cornea, respectively. Furthermore, the drying progressed over a temperature increase of around 5°C under our exposure conditions. These findings suggest that the possibility of eye damage increases with the progress of drying and that the setting of the eye surface conditions can be a cause of disagreement between computational and experimental data of absorbed energy under high-level irradiation because reflectance is related to terahertz wave penetration in the eye tissue. The time-domain spectroscopic measurements were useful for the acquisition of the dielectric constant as well as for the real-time monitoring of the eye conditions during exposure measurement.

Characteristic Spectroscopic Features because of Cation-Anion Interactions Observed in the 700-950 cm-1 Range of Infrared Spectroscopy for Various Imidazolium-Based Ionic Liquids.

The 700-950 cm-1 range in infrared spectroscopy was investigated for various imidazolium-based ionic liquids (ILs). Two main vibrational modes of the methylimidazolium cation exist in this region. At 700-800 cm-1, there is an out-of-plane +C(2)-H and +C(4,5)-H bending mode with a larger motion of +C(4,5)-H in the imidazolium ring, whereas at 800-950 cm-1, there is an out-of-plane +C(2)-H and +C(4,5)-H bending mode with a larger motion of +C(2)-H in the imidazolium ring. The molar-concentration-normalized absorbance spectrum of the former band at 700-800 cm-1, which is related to the bending mode with a large +C(4,5)-H motion in the imidazolium ring, is not particularly sensitive to the interactions with anions. The molar-concentration-normalized absorbance spectrum of the latter band at 800-950 cm-1 was nearly identical for ILs that have methylimidazolium cations with carbon chains of different lengths and the same anions. The band shape of the latter band, which is related to the bending mode with a large out-of-plane +C(2)-H bending motion, was highly sensitive to the interactions with anions and, interestingly, both blue- and red-shifted tendencies in the spectrum for each system were observed.

Analysis of dermal composite conditions using collagen absorption characteristics in the THz range.

The absorption characteristics of the dermis were reviewed in the terahertz range from 0.2 to 2 THz. The absorption magnitude of the dermis was higher than that of the epidermis model owing to the inclusion of collagen fibers. The heat denaturation of the collagen and the decrease of water content in the dermis caused a decrease in the absorption magnitude of the dermis. We verified that the absorption magnitude of collagen sheets at 1 THz similarly decreased by nearly 43% upon the heat treatment at approximately 70° C. When the heat-treated sheet was used as a scaffold for cell culture, the average growth rate of the fibroblast increased. These findings suggest that variation in the ability of cell growth in the dermis can be predicted using the absorption of the collagen or the difference between the absorptions of dermis and water.

Monte Carlo simulations of skin exposure to electromagnetic field from 10 GHz to 1 THz.

In this study, we present an assessment of human-body exposure to an electromagnetic field at frequencies ranging from 10 GHz to 1 THz. The energy absorption and temperature elevation were assessed by solving boundary value problems of the one-dimensional Maxwell equations and a bioheat equation for a multilayer plane model. Dielectric properties were measured [Formula: see text] at frequencies of up to 1 THz at body temperature. A Monte Carlo simulation was conducted to assess variations of the transmittance into a skin surface and temperature elevation inside a body by considering the variation of the tissue thickness due to individual differences among human bodies. Furthermore, the impact of the dielectric properties of adipose tissue on temperature elevation, for which large discrepancies between our present measurement results and those in past works were observed, was also examined. We found that the dielectric properties of adipose tissue do not impact on temperature elevation at frequencies over 30 GHz. The potential risk of skin burn was discussed on the basis of the temperature elevation in millimeter-wave and terahertz-wave exposure. Furthermore, the consistency of the basic restrictions in the international guidelines set by ICNIRP was discussed.

Infrared Spectroscopy of Ionic Liquids Consisting of Imidazolium Cations with Different Alkyl Chain Lengths and Various Halogen or Molecular Anions with and without a Small Amount of Water.

Infrared spectroscopy was performed on ionic liquids (ILs) that had imidazolium cations with different alkyl chain lengths and various halogen or molecular anions with and without a small amount of water. The molar concentration normalized absorbance due to +C-H vibrational modes in the range of 3000 to 3200 cm-1 was nearly identical for ILs that had imidazolium cations with different alkyl chain lengths and the same anions. A close correlation was found between the red-shifted +C-H vibrational modes, the chemical shift of +C(2)-H proton, and the energy stabilization of the hydrogen-bonding interaction. The vibrational modes of the water molecules interacting with anions in the range between 3300 and 3800 cm-1 was examined. The correlation between the vibrational frequencies of water, the frequencies of +C-H vibrational modes, and the center frequency of intermolecular vibrational modes due to ion pairs was discussed.

Terahertz and Infrared Spectroscopy of Room-Temperature Imidazolium-Based Ionic Liquids.

The terahertz- and infrared-frequency vibrational modes of various room-temperature imidazolium-based ionic liquids with molecular anions were examined extensively. We found that the molar-concentration-normalized absorption coefficient spectra in the low-wavenumber range for imidazolium cations with different alkyl-chain lengths were nearly identical for the same anion. Regarding the overall view of a wide range of imidazolium-based ionic liquids, we found that the reduced mass of the combination of an imidazolium-ring cation and the anion and the force constant play significant roles in determining the central frequency of the broad absorption band. In addition to these findings, we also discuss the correlation between the (+)C-H stretching vibrational modes in the 3000-3300 cm(-1) range of the infrared spectra and the intermolecular vibrational band in the low-wavenumber range. Finally, we describe some interesting characteristics of the intermolecular vibrational band observed in a wide range of imidazolium-based ionic liquids.

Vibration Modes at Terahertz and Infrared Frequencies of Ionic Liquids Consisting of an Imidazolium Cation and a Halogen Anion.

The terahertz and infrared frequency vibration modes of room-temperature ionic liquids with imidazolium cations and halogen anions were extensively investigated. There is an intermolecular vibrational mode between the imidazolium ring of an imidazolium cation, a halogen atomic anion with a large absorption coefficient and a broad bandwidth in the low THz frequency region (13-130 cm-1), the intramolecular vibrational modes of the alkyl-chain part of an imidazolium cation with a relatively small absorption coefficient in the mid THz frequency region (130-500 cm-1), the intramolecular skeletal vibrational modes of an imidazolium ring affected by the interaction between the imidazolium ring, and a halogen anion with a relatively large absorption coefficient in a high THz frequency region (500-670 cm-1). Interesting spectroscopic features on the interaction between imidazolium cations and halogen anions was also obtained from spectroscopic studies at IR frequencies (550-3300 cm-1). As far as the frequency of the intermolecular vibrational mode is concerned, we found the significance of the reduced mass in determining the intermolecular vibration frequency.

Analysis of tissue condition based on interaction between inorganic and organic matter in cuttlefish bone.

The absorption properties of an inner layer of cuttlefish bone were measured using a transmission terahertz time-domain spectrometer in a band from approximately 0.1 to 4 THz. For oriented samples, an absorption peak related to the behavior of calcium carbonate appeared at approximately 2 THz. The peak magnitude and frequency depended on the direction of the incident terahertz electric field, indicating that calcium carbonate crystals constituting the inner layer were oriented in a certain direction. The absorbance of a sample heated to 350 °C for 0 to 2 h to remove organic matter tended to decrease with heating time in the oriented direction, while the peak frequency shifted to higher frequencies. Furthermore, we showed that the peak frequency depended on the interaction area within the unheated sample and we thus obtained a two-dimensional image reflecting crystal regularity inside the cuttlefish bone from the spectral data at each position.