Teraherz Optical Science
|Hirori, Hideki||Associate Professor|
|Wolpert, Christian||Research Associate|
|Tani, Shuntaro||Research Associate|
|Chen, Wen||Research Support Staff|
Terahertz (THz) waves, electromagnetic radiation in the frequency region from 0.1 to 10 THz, is the next frontier in optical science and technology*. THz waves have been used to characterize the electronic, vibrational, and compositional properties of solid, liquid, and gas phase materials. In particular, biological sensing and imaging are the most highly anticipated applications of THz waves. Important features of THz waves for biological applications are summarized as follows:
- Fingerprints: Many biological molecules have their rotational and vibrational modes in the THz frequency range.
- Water-sensitivity: THz radiation is quite sensitive to water and its dynamic behaviors depending on temperatures and interaction with various kinds of solutes.
- Safety: THz radiation has low phonon energies (4 meV @ 1 THz) and, therefore, does not ionize biological tissue.
However, compared to well-developed visible light optical technologies and electronics in the microwave region, basic research, new approaches, and advanced technology development in the THz band have been only limited, as THz wave emitters and receivers are not as well developed compared to microwave and optical equipment.
We are developing high-power THz wave generation techniques and their application to the biological sciences. Our method of high power THz wave generation is based on the Cherenkov-type rectification process in LiNbO3 crystals, or the four-wave-mixing process in laser induced gas-plasma with amplified femtosecond lasers (4mJ/pulse). This has allowed us to generate an intense THz wave over 200 kV/cm in the electric field in the repetition rate of 1 KHz. Recently, our group has been exploring non-linear optical responses of semiconductors and biological materials and we have found various novel phenomena that have never before been observed. Simultaneously we are developing a near-field THz microscope working at video rate. These technologies will open the doors to new THz sensing and imaging applications in the near future.
At the iCeMS, we have initiated new multidisciplinary research projects using high-power THz waves and related THz science and technologies including:
- Biological applications of THz near-field microscopy. We have developed a special sensing crystal that enables us to convert the THz near-field image to a visible image using a non-linear optical process inside the sample mount. The current target for special resolution is below 5 micrometers. Thanks to our high power THz-waves, the microscope will work at video rates. Biological applications are now possible and will be conducted in collaboration with the Harada and Kusumi Labs.
- Development of novel techniques to control materials with intense THz waves. Intense THz waves have the potential to modify or control optical and electrical properties in various functional materials. For example, non-linear properties in the THz frequency region are important in semiconductors for high-speed switching devices and future hopes in biological materials for new sensing and imaging technologies. Serious photo-blinking and darkening problems in fluorescent semiconductor quantum-dots may be overcome in part using resonant excitation of intense THz waves ranging from hidden dark levels to luminescent levels.
- Water-material interaction in meso-space is important to understand biological activities in living cells. We are developing a special THz spectrometer with attenuated total reflection (ATR) devices to measure accurately the response function in the THz frequency region including optical permittivity and conductivity. We intend to elucidate the dynamic properties of liquids, especially hydration effects in small molecules, proteins, and lipid layers.
- Ultrafast dynamics in meso-space. We have developed a time-resolved optical measurement system with femtosecond time-resolution to monitor light-induced chemical reactions. Using this technique, we are preparing to elucidate how molecules in meso-space behave under light irradiation. Along these same lines, we are studying porous materials developed by the Kitagawa Lab.
* In the different units, 1 THz = 1 ps = 300 µm = 33 cm-1 = 4.1 meV = 47.6 K.
- Tani, S., Blanchard, F., and Tanaka, K., "Ultrafast Carrier Dynamics Under High Electric Field In Graphene", Phys. Rev. Lett. 109, 166603 (2012).
- Blanchard, F., Ooi, K., Tanaka, T., Doi, A., and Tanaka, K., "Terahertz spectroscopy of the reactive and radiative near-field zones of split ring resonator", Optics Express, 20, pp. 19395-19403 (2012).
- Hishida, M., and Tanaka, K., "Long-range hydration effect of lipid membrane studied by terahertz time-domain spectroscopy", Phys. Rev. Lett. 106, 158102 (2011).
- Hirori, H., Shinokita, K., Shirai, M., Tani, S., Kadoya, Y., and Tanaka, K., "Extraordinary Carrier Multiplication Gated by a Picosecond Electric Field Pulse", Nature Communications 2:594 doi:10.1038/ncomms1598 (2011).
- Tanaka, K., Hirori, H., and Nagai, M., "THz Nonlinear Spectroscopy of Solids", IEEE Transactions on Terahertz Science and Technology 1, 301-312 (2011).