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ABSTRACT: We present terahertz spectroscopic measurements of Dirac fermion dynamics from a large-scale graphene that was grown by chemical vapor deposition and on which carrier density was modulated by electrostatic and chemical doping. The measured frequency-dependent optical sheet conductivity of graphene shows electron-density-dependence characteristics, which can be understood by a simple Drude model. In a low carrier density regime, the optical sheet conductivity of graphene is constant regardless of the applied gate voltage, but in a high carrier density regime, it has nonlinear behavior with respect to the applied gate voltage. Chemical doping using viologen was found to be efficient in controlling the equilibrium Fermi level without sacrificing the unique carrier dynamics of graphene.
Nano Letters 01/2012; 12(2):551-5. · 13.20 Impact Factor
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ABSTRACT: Terahertz (THz) spectroscopic imaging technique was applied to classify the tumor region in the rabbit liver with VX2 hepatocellular carcinoma. Within the measurement range of 0.1–2 THz, the average reflectance values for all tumor samples were more than 4% higher than those for healthy cells, and the terahertz measurements correlated well with histological analysis results. This study on paraffin-embedded tissues showed the alteration of cell density and protein content in tumors, excluding the effect of water.
Journal of Applied Physics 04/2011; · 2.17 Impact Factor
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ABSTRACT: We demonstrate a highly sensitive THz molecular imaging (TMI) technique involving differential modulation of surface plasmons induced on nanoparticles and obtain target specific in vivo images of cancers. This technique can detect quantities of gold nanoparticles as small as 15 µM in vivo. A comparison of TMI images with near infrared absorption images shows the superior sensitivity of TMI. Furthermore, the quantification property of TMI is excellent, being linearly proportional to the concentration of nanoparticles. The target specificity issue is also addressed at the ex vivo and cell levels. The high thermal sensitivity of TMI can help extend photonic-based photothermal molecular imaging researches from the in vitro level to the in vivo level. The TMI technique can be used for monitoring drug delivery processes and for early cancer diagnosis.
Optics Express 02/2011; 19(5):4009-16. · 3.59 Impact Factor
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ABSTRACT: In this study, we have designed, fabricated, and characterized a miniaturized optical fiber-coupled terahertz (THz) endoscope system. The endoscopic system utilized a photoconductive generator and detector driven by a mode-locked Ti:sapphire laser. In reflection mode, the endoscope showed a high signal-to-noise ratio and a wide frequency spectrum similar to the conventional THz time-domain spectroscopic system. The cross section of the endoscope including generator and detector head is (2 x 4 mm) x 6 mm, which is small enough to be inserted into a human body. For a feasibility test, the endoscopic system was used to measure reflective THz signals from the side wall of the mouth, tongue, and palm skin as well as from water for comparison. The absorption and refractive index of the side wall of the mouth and tongue were similar to those of water but those of the palm skin were less than water.
Optics Express 09/2009; 17(19):17082-7. · 3.59 Impact Factor
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ABSTRACT: In this study, we measured the conformational-state-dependent power absorptions, refractive indices and dielectric constants of beta-glucans in laminarin, and the two states, SSH and TSH, of beta-glucans were compared.
Lasers and Electro-Optics 2009 and the European Quantum Electronics Conference. CLEO Europe - EQEC 2009. European Conference on; 07/2009
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Advanced Materials 06/2009; 21(43):4339 - 4342. · 13.88 Impact Factor
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ABSTRACT: D{sub 2}O used in nuclear power generation is very slightly different from H{sub 2}O. Deuterium (D) is the isotope of hydrogen (H) and has one more neutron than H. D{sub 2}O is heaver than H{sub 2}O by about 11% and its melting and boiling points are 3.81 deg. C and 101.42 deg. C, respectively. Moreover, D{sub 2}O is toxic. It is harmful for human body if it replaces more than approximately 25-50% of a human's body water. So, knowledge of the properties of D{sub 2}O included in H{sub 2}O is necessary. The confinement energy and hydrogen bonding energy of large molecules are in the terahertz frequency region (2{approx}10 THz, 3{approx}66 cm{sup -1}). Particularly, the polar molecules such as H{sub 2}O and D{sub 2}O react to this frequency region well because the time of hydrogen relaxation motion is subpicosecond. Terahertz time-domain spectroscopy (THz-TDS) using terahertz pulses has been used in the study of ultrafast dynamics of hydrogen bonds in biomolecules and carriers in semiconductors. We obtained the refractive index and power absorption of H{sub 2}O and D{sub 2}O at room temperature. The results show that the refractive index and power absorption of H{sub 2}O are larger than for D{sub 2}O.
AIP Conference Proceedings 04/2009; 1119(1).
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ABSTRACT: The penetration property of the terahertz electromagnetic (THz) wave is relevant to its use. We used the THz wave spectroscopy system which easily penetrates some materials that do not contain water, e.g., plastic and ceramics. The system has been developed for several purposes, including measuring the properties of semiconductors and bio-materials, and detecting plastic bombs and ceramic knives at airports. It is also used for medical imaging systems, such as magnetic resonance imaging (MRI), at some research institutes. It can show not only the difference in amplitude, but also the difference of the phase of each point of sample. MRI technology usually uses contrast agents to enhance the quality of the image. Gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA), made with a heavy metal ion, is commonly used as a clinical MRI contrast agent. Gadolinium oxide (Gd{sub 2}O{sub 3}) nanoparticle is a new contrast agent. It serves to equip the core of each particle with antibodies or ligands. It can freely circulate in blood vessels without amassing in the liver or lungs. This study shows the characteristics of gadolinium oxide nanoparticles to further advance terahertz medical imaging.
AIP Conference Proceedings 04/2009; 1119(1).
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ABSTRACT: This paper demonstrates the principle of the nanoparticle-contrast-agent-enabled terahertz imaging (CATHI) technique, which yields a dramatic sensitivity of the differential signal from cancer cells with nanoparticles. The terahertz (THz) reflection signal increased beam by 20% in the cancer cells with nanoparticles of gold nano-rods (GNRs) upon their irradiation with a infrared (IR) laser, due to the temperature rise of water in cancer cells by surface plasma ploritons. In the differential mode, the THz signal from the cancer cells with GNRs was 30 times higher than that from the cancer cells without GNRs. As the high sensitivity is achieved by the surface plasmon resonance through IR laser irradiation, the resolution of the CATHI technique can be as good as a few microns and THz endoscopy becomes more feasible.
Optics Express 04/2009; 17(5):3469-75. · 3.59 Impact Factor
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ABSTRACT: We measured the binding-state-dependent power absorptions, refractive indices, and dielectric constants of triple-stranded helices (TSHs) and single-stranded helices (SSHs) β-glucans in laminarin using terahertz time-domain spectroscopy (TDS). The SSH β-glucan was obtained from a TSH β-glucan laminarin by a chemical treatment with NaOH solution. The power absorption of TSH β-glucan increased more rapidly than that of the SSH β-glucan with the frequency increment. The refractive index and dielectric constants of TSH β-glucan were also larger than those of the SSH β-glucan. This result implies that terahertz-TDS is a very effective method in classifying the conformational state of β-glucans.
Applied Physics Letters 03/2009; 94(11):111911-111911-3. · 3.84 Impact Factor
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ABSTRACT: We measured the optical constants of aqueous biomaterial mixtures with various electrolyte concentrations using terahertz time-domain spectroscopy. The mixtures were divided into water and other electrolyte parts in mass fractions for analysis. The optical constants of the electrolyte, excluding water, were obtained by applying the ideal mixture equation, and the power absorption of the electrolyte was observed to be larger than that of water above 1 THz. Data from the measurement were fitted with the modified double Debye model, and the reorientation and hydrogen-bond formation decomposition times were found to decrease as the electrolyte concentration increased.
Journal of Applied Physics 10/2007; 102(7):074702-074702-5. · 2.17 Impact Factor
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ABSTRACT: We have characterized the optical properties of fluorine-doped thin-multiwalled carbon nanotubes (f-doped t-CNTs) compared with thin multiwalled carbon nanotubes (t-CNTs) using terahertz time-domain spectroscopy (THz-TDS). The THz power absorption and the index of refraction of f-doped t-CNTs were smaller than those of the raw t-CNTs. In addition, the transport parameters were acquired using the Bruggemann model fitting.
Infrared and Millimeter Waves, 2007 and the 2007 15th International Conference on Terahertz Electronics. IRMMW-THz. Joint 32nd International Conference on; 10/2007
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ABSTRACT: The influence of Cd molar fraction on the terahertz generation from Zn<sub>1-x</sub>Cd<sub>x</sub>Te has been studied. The optimal Cd compositions have been obtained as x=0.10 at lambda=820 nm and x=0.65 at lambda=850 nm.
Lasers and Electro-Optics, 2007. CLEO 2007. Conference on; 06/2007
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ABSTRACT: The authors investigated the carrier dynamics of n-type modulation-doped InAs/GaAs quantum dots (QDs) using terahertz time-domain spectroscopy to estimate the total number of electrons captured by the QDs. The terahertz power absorption of the sample with QDs was less than that of the sample without QDs. This is attributed to the fact that the carriers are confined in the QDs. The experiment results were fitted into the Drude model and the number of electrons captured by QDs was determined through the difference in the numbers of free electrons of the samples with and without QDs.
Applied Physics Letters 03/2007; 90(13):131906-131906-3. · 3.84 Impact Factor
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ABSTRACT: The electrical and optical properties of double-walled carbon nanotubes (DWNTs) have been characterized and compared with those of single-walled carbon nanotubes (SWNTs) utilizing terahertz time-domain spectroscopy. The power absorption and the complex refractive indices of DWNTs are smaller than those of SWNTs. The conductivity of DWNTs was also observed to be smaller. The experimental results have been fitted with the Bruggman effective medium approximations, which has yielded the transport parameters of DWNTs such as plasma frequency, damping rate, etc.
Applied Physics Letters 01/2007; 90(5):051914-051914-3. · 3.84 Impact Factor
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ABSTRACT: We propose a method for a diameter-selective removal of metallic single-walled carbon nanotubes (m-SWCNTs) from semiconducting
(s-) ones. Our separation technique is capable of 100% separation of semiconducting and metallic nanotubes for small diameter
nanotubes. We dispersed SWCNT powder by sonication in a mixed solution of tetramethylene sulfone and chloroform, where nitronium
ions were well disolved. Positively charged nitronium ions were intercalated into nanotube bundles, where the intercalation
was promoted also by the counter ions. Nitronium ions selectively attacked the sidewall of m-SWCNTs due to the abundant presence
of electron density at the Fermi level, thus yielding stronger binding energy compared to the counterpart s-SWCNTs. The s-SWCNTs
were left on the filter after filtration, whereas m-SWCNTs were perfectly destroyed by nitronium ions and drained away as
amorphous carbons. This preferable adsorption became obscured for nanotubes with diameters greater than 1.1 nm. The effectiveness
of removing m-SWCNTs was confirmed by the transmission electron microscope observations, x-ray photoemission spectra, resonant
Raman spectra, and absorption spectra.
Journal of Electronic Materials 01/2006; 35(2):235-242. · 1.47 Impact Factor
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ABSTRACT: The frequency-dependent optical constants and electrical conductivities of hydrogen-functionalized single-walled carbon nanotubes have been measured from the 0.2- to 1.5-THz region using a terahertz time domain spectroscopy. The indices of refraction and electrical conductivities of the sample after hydrogen functionalization were smaller than those of the sample before hydrogen functionalization. The experimental results were fitted using the Maxwell-Garnett model, and a reduction of plasma frequency was observed. This can be attributed to the fact that the hydrogen functionalization has reduced the number of free carriers with the bonding change from sp<sup>2</sup>to sp<sup>3</sup>.
Microwave Photonics, 2005. MWP 2005. International Topical Meeting on; 11/2005
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ABSTRACT: We have found a method for a diameter-selective removal of metallic single-walled carbon nanotubes (m-SWCNTs) from semiconducting (s-) ones by stirring or sonicating SWCNT powder in tetramethylene sulfone (TMS)/chloroform solution with nitronium hexafluoroantimonate (NO2SbF6: NHFA) and nitronium tetrafluoroborate (NO2BF4: NTFB). Positively charged nitronium ions (NO2+) were intercalated into nanotube bundles, where the intercalation was promoted also by the counterions. Nitronium ions selectively attacked the sidewall of the m-SWCNTs due to the abundant presence of electron density at the Fermi level, thus yielding stronger binding energy as compared to the counterpart s-SWCNTs. The s-SWCNTs were left on the filter after filtration, whereas the m-SWCNTs were disintegrated and drained away as amorphous carbons. The effectiveness of removing m-SWCNTs was confirmed by the resonant Raman spectra and absorption spectra.
Journal of the American Chemical Society 05/2005; 127(14):5196-203. · 9.91 Impact Factor
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ABSTRACT: Cordyceps, an entomopathogenic fungus, contains many health-promoting ingredients. Recent reports indicate that the consumption of cordyceps helps reduce blood-sugar content in diabetics. However, the mechanism underlying this reduction in circulatory sugar content is not fully understood. Methanolic extracts were prepared from the fruiting bodies of Paecilomyces tenuipes, and 4-beta acetoxyscirpendiol (4-ASD) was eventually isolated and purified. Na(+)/Glucose transporter-1 (SGLT-1) was expressed in Xenopus oocytes, and the effect of 4-ASD on SGLT-1 was analyzed utilizing a voltage clamp and by performing 2-deoxy-D-glucose (2-DOG) uptake studies. 4-ASD was shown to significantly inhibit SGLT-1 activity compared to the non-treated control in a dose-dependent manner. In the presence of the derivatives of 4-ASD (diacetoxyscirpenol or 15-acetoxyscirpendiol), SGLT-1 activity was greatly inhibited in an 4-ASD-like manner. Of these derivatives, 15-acetoxyscirepenol inhibited SGLT-1 as well as 4-ASD, whereas diacetoxyscirpenol was slightly less effective. Taken together, these results strongly indicate that 4-ASD in P. tenuipes may lower blood sugar levels in the circulatory system. We conclude that 4-ASD and its derivatives are effective SGLT-1 inhibitors.
Journal of biochemistry and molecular biology 04/2005; 38(2):211-7. · 2.02 Impact Factor
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ABSTRACT: The absorption and dispersion of aligned single-walled carbon-nanotube films were measured from 0.2 to 2.0 THz using a source of freely propagating subpicosecond pulses of THz electromagnetic radiation. The real conductivity increased rapidly with increasing frequency up to 0.45 THz and decreased at a high-frequency range. The Maxwell–Garnett model, where the nanotubes were embedded in a dielectric host, fit the results of this study with the Drude–Lorentz model for nanotube network. We have observed the transverse phonon mode of 2.4 THz propagating along the c direction. This suggested that the carbon nanotube network is composed of metallic and semiconducting nanotubes embedded in an air dielectric host. © 2004 American Institute of Physics.
Journal of Applied Physics 05/2004; 95(10):5736-5740. · 2.17 Impact Factor
