Katsuhiro Ajito

THz Chemical Imaging for Biological Applications

THz spectroscopy is advantageous in analytical chemistry because it can detect and identify intermolecular interactions in chemical compounds, such as hydrogen bonds and hydrations, and molecular networks. Recent advances in THz components, such as ultrashort pulsed lasers and photoconductive antennas, have improved the sensitivity of THz time-domain (TDS) spectroscopy and have made the THz chemical imaging possible. THz chemical imaging can reveal hydrogen bond distributions and will be a very powerful tool in biology, pharmacology, and life sciences. THz-TDS is also promising for the quantitative chemical analysis and detection of molecules and clusters in nanospace and ice.

Fully Integrated ASK Receiver MMIC for Terahertz Communications at 300 GHz

An ASK receiver MMIC operating at 300 GHz for future terahertz communications is presented. In the receiver IC, we fully integrated all necessary components-a receiving dipole antenna, high gain RF amplifier, envelop detector for demodulating ASK signal and output differential data amplifier-in a 1000×2500 μm2 area. A silicon lens was used to compensate for the small gain of the on-chip antenna. To ensure reliable and stable operation, we designed the MMIC with a thin-film microstrip line, which is expected to suppress crosstalk between the on-chip antenna and the RF amplifier through the substrate and silicon lens. The packaged receiver module with the silicon lens is expected to provide approximately 24-dBi beam directivity. Measured RF and baseband bandwidths are around 30 and 15 GHz, respectively, when a single bias of 3.3 V and total current of around 86 mA are applied. With the receiver module, simple wireless data transmission was conducted for up to 24 Gbps in the 300-GHz band. At 12.5 Gbps, error-free data transmission (bit error rate <; 10-9) over 0.3 m was achieved with the transmission power of - 16-dBm and a 25-dBi transmitting antenna. With -10-dBm transmission power, measured Q-factors of the received eye patterns were larger than 6 for up to 20 Gbps, which implies that the bit error rate will be less than 10-9.

Terahertz-wave generation from quasi-phase-matched GaP for 1.55μm pumping

We have realized a terahertz (THz)-wave source employing difference frequency generation (DFG) from a quasi-phase-matched GaP stack pumped at 1.55μm. We observed THz waves with enhanced power by quasi-phase matching (QPM) in the ⟨110⟩ direction of GaP with a ⟨111⟩ polarization direction for the incidence of two pump lights with the same propagation and polarization directions. We obtained THz-wave power proportional to the product of two pump-light powers due to DFG. We also confirmed that power peaks appeared at around 1 and 2.6THz reflecting the first- and the third-order QPM, respectively.

50-Gb/s Direct Conversion QPSK Modulator and Demodulator MMICs for Terahertz Communications at 300 GHz

We demonstrate direct quadrature modulator and demodulator monolithic microwave integrated circuits for future terahertz communications at 300 GHz based on the quadrature phase-shift keying (QPSK) modulation format. For the modulating and demodulating signal, we employed half-Gilbert cell mixers, which provide balanced signaling and moderate performance in conversion efficiency with a simple circuit configuration. In order to maintain the balance performance of the modulator and demodulator, passive baluns and couplers are implemented with thin-film microstrip lines, which exhibit less insertion loss than inverted microstrip lines (IMSLs), while the active mixers are based on IMSLs for short interconnections. The half-Gilbert-cell mixers have a wide enough operation bandwidth for high-throughput communications of more than 10% at 300 GHz. According to the static constellation of the modulator, imbalance is expected to be less than approximately ±0.6 dB ∠4°. A nonchip back-to-back experiment was conducted at up to 60 Gb/s, and 50-Gb/s operation was verified with a low bit error rate on the order of 10-8 or less. The results demonstrate that the QPSK modulation scheme can be applied to double the data rate at terahertz frequencies.

Uni-Travelling-Carrier Photodiode Module Generating 300 GHz Power Greater Than 1 mW

In this letter, we demonstrate over 1 mW power generation at 300 GHz with a uni-travelling-carrier photodiode (UTC-PD) packaged in a WR-3 waveguide module. To increase the maximum power, two identical UTC-PDs were monolithically integrated along with a T-junction to combine the power from the two PDs. The UTC-PD module exhibited peak saturated output power of approximately 1.2 mW at 300 GHz with photocurrent of around 20 mA per PD and bias voltage of -3.9 V. In addition, 3 and 10 dB bandwidths were measured to be around 70 GHz or 23% and over 150 GHz or 50%, respectively.

Characterization of dye-doped TiO2 films prepared by spray-pyrolysis

Thin films of TiO2 were prepared by spray-pyrolysis of titanium(IV)-oxy-acetylacetonate onto indium-tin-oxide glass electrodes. Depending upon the substrate temperature, morphology of the deposited TiO2 films changed from irregular aggregates at 200°C to homogeneous particles with a diameter of 50–100 nm above 400°C. Dye-doping was achieved at the substrate temperature below 260°C by dissolving phthalocyanine or rhodamine molecules into the oxy-acetylacetonate solution. Photoelectrochemical measurements indicated that anodic photocurrents at the wavelength range below 400 nm due to direct excitation of TiO2 increased by formation of homogeneous fine particles at higher temperature. The electrode doped with phthalocyanine exhibited photocurrents in its visible absorption band at 600–700 nm whereas that doped with rhodamine yielded no sensitized photocurrent.

Chemical Mapping of Pharmaceutical Cocrystals Using Terahertz Spectroscopic Imaging

Terahertz (THz) spectroscopic imaging is a promising technique for distinguishing pharmaceuticals of similar molecular composition but differing crystal structures. Physicochemical properties, for instance bioavailability, are manipulated by altering a drugs crystal structure through methods such as cocrystallization. Cocrystals are molecular complexes having crystal structures different from those of their pure components. A technique for identifying the two-dimensional distribution of these alternate forms is required. Here we present the first demonstration of THz spectroscopic imaging of cocrystals. THz spectra of caffeine-oxalic acid cocrystal measured at low temperature exhibit sharp peaks, enabling us to visualize the cocrystal distribution in nonuniform tablets. The cocrystal distribution was clearly identified using THz spectroscopic data, and the cocrystal concentration was calculated with 0.3-1.3% w/w error from the known total concentration. From this result, THz spectroscopy allows quantitative chemical mapping of cocrystals and offers researchers and drug developers a new analytical tool.

Single Nanoparticle Trapping Using a Raman Tweezers Microscope

We have obtained a Raman spectrum of single nanometer-sized particles (nano-particles) trapped by laser tweezers for the first time. The microscope used in this study is a new version of our Raman tweezers microscope (RTM); it contains an oil-immersed objective lens with high numerical aperture to increase the force of the optical radiation pressure of the near-infrared laser beam to trap single organic and biological nanoparticles and provides sufficient sensitivity for the Raman measurement of the trapped nano-particles. The confocal arrangement in the system completely eliminates the Raman signal of the oil under the objective lens. The RTM provides a Raman spectrum of a single polystyrene latex bead about 40 nm in diameter with an exposure time of 3 s and allows us to determine its molecular species. Furthermore, the RTM enables us to count the number of nanoparticles in the focal spot of the laser beam from the Raman spectra of trapped nanoparticles.

Combined Near-Infrared Raman Microprobe and Laser Trapping System: Application to the Analysis of a Single Organic Microdroplet in Water

A combined Raman microprobe and laser trapping system using near-infrared (NIR) laser light was developed for the investigation of single organic microdroplets. The NIR laser light is noninvasive and reduces fluorescence interference in the Raman spectrum for organic molecules. The focused laser beam used for the laser trapping of a microdroplet serves simultaneously as the laser microprobe for Raman measurement. With this system, the focused laser spot is about 1 μm in diameter, which is small enough for the laser trapping of a single toluene microdroplet in water. The system also makes it possible to visualize a focused laser spot together with a laser-trapped microdroplet by using holographic notch filters. The Raman spectrum for a single laser-trapped toluene microdroplet can be obtained from below 100 cm−1 to above 3000 cm−1 with a charge-coupled device (CCD) detector. Fluorescence interference in the Raman spectrum is completely removed by using NIR laser light. The signal-to-noise ratio (SNR), defined as the ratio of the peak height to the standard deviation of the baseline noise in the spectrum, exceeded 250 for the 1003 cm−1 band of a toluene microdroplet at 1 s, which is sufficient to allow identification of the molecular species of a microdroplet.

Near-infrared Raman spectroscopy of single particles

Raman spectroscopy using non-invasive near-infrared (NIR) laser light has become a powerful tool for the microscopic analysis of organic and biological materials. A Raman tweezers microscope (RTM) was developed by combining NIR Raman spectroscopy with the laser trapping technique, which enables us to expand the scope of single particle studies. Recent results obtained using the RTM for single droplets and polymer spheres in a micrometer range are reported.