Terahertz (THz) imaging is a nondestructive, label-free, rapid imaging technique which gives the possibility of a real-time tracing of drugs within the skin. We evaluated the feasibility of THz dynamic imaging for visualizing serial changes in the distribution and penetration of a topical agent, dimethyl sulfoxide (DMSO) containing ketoprofen, using excised mouse skins. THz imaging was performed for 6 h after drug application to the skin and was compared with the results obtained using the Franz cell diffusion test, a standard in vitro skin absorption test. THz dynamic reflection imaging showed that the reflection signals decreased rapidly during the early time period, and remained constant through the late time period. The area of drug permeation within the skin layer on THz imaging increased with time. The dynamic pattern of THz reflection signal decrease was similar to that of DMSO absorption analyzed by the Franz cell diffusion test, which indicates that THz imaging mainly reflects the DMSO component. This study demonstrates that THz imaging technique can be used for imaging the spatial distribution and penetration of drug-applied sites.

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For the application of gadolinium oxide (Gd 2 O 3 ) nanoparticles as terahertz contrast agents, their optical properties in a solvent were studied using terahertz timedomain spectroscopy. The power absorption and refractive index of the samples were measured with various concentrations of nanoparticles. The power absorption was extremely large, as much as three orders of magnitude higher than that of water, so that a few ppms of Gd 2 O 3 nanoparticles were distinguished in terms of their power absorption capacity. The results show that the interaction between the terahertz electromagnetic waves and the Gd 2 O 3 nanoparticles is strong enough to allow their exploitation as contrast agents for terahertz medical imaging.

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High-power terahertz radiation was observed to be emitted from a gas jet irradiated by 100-terawatt-class laser pulses in the laser-wakefield acceleration of electrons. The emitted terahertz radiation was characterized in terms of its spectrum, polarization, and energy dependence on the accompanying electron bunch energy and charge under various gas target conditions. With a nitrogen target, more than 4 mJ of energy was produced at <10 THz with a laser-to-terahertz conversion efficiency of ~0.15%. Such strong terahertz radiation is hypothesized to be produced from plasma electrons accelerated by the ponderomotive force of the laser and the plasma wakefields on the time scale of the laser pulse duration and plasma period. This model is examined with analytic calculations and particle-in-cell simulations to better understand the generation mechanism of high-energy terahertz radiation in laser-wakefield acceleration.

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High-power terahertz radiation was observed to be emitted from a gas jet irradiated by 100-terawatt-class laser pulses in laser-wakefield acceleration of electrons. The emitted terahertz radiation was characterized in terms of its spectrum, polarization, and energy dependence on the accompanying electron bunch energy and charge under various gas target conditions. With a nitrogen target, more than 4 millijoule of energy was produced at < 10 terahertz with a laser-to-terahertz conversion efficiency of ~ 0.15%. Such strong terahertz radiation is hypothesized to be produced from plasma electrons accelerated by the ponderomotive force of the laser and plasma wakefields on the time scale of the laser pulse duration. This model is examined with analytic calculations and particle-in-cell simulations to better understand the generation mechanism of high-energy terahertz radiation in laser-wakefield acceleration.