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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: 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: 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: 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: The accurate terahertz (THz) electrical and optical properties of biomaterials are required as the biological applications of THz electromagnetic waves were recently increased. The frequency-dependent characteristics of enamel and dentin of human tooth were measured in this study through THz time-domain spectroscopy. The power absorptions of enamel and dentin were found to be similar while the index of refraction of enamel was found to be bigger than that of dentin. The power absorption results of enamel and dentin were also compared with those of the wet samples.
Current Applied Physics.
