Professor, Applied Physics, Graduate School of Engineering, Osaka University
Chief Scientist, Nanophotonics Laboratory,
The Institute of Physical and Chemical Research (RIKEN)
Beyond the diffraction limit
— Photonics explores the nano world —
The world’s smallest bull, which will appear in next year’s Guinness Book of World Records, is 8µm in length and 5µm in height. The legs, horns, and the tail are so small as to be beyond the diffraction limit and can not be imaged by visible light. The bull’s body is made from photopolymerizable resin, a polymer that solidifies by absorption of light. Prof. Kawata has developed a nanophotonics method to manipulate nanoscale structures using an ultrafast femtosecond laser of several hundred nm wavelength.
When photons are strongly confined both temporally and spatially, two photons can simultaneously excite an electron in a target. The femtosecond laser has made this strong confinement possible. A 1kW peak power femtosecond laser compresses energy into a 100fs pulse so that strong confinement is achieved with an exposure of only 1mW near infrared light for 10ns. When this compressed pulse meets the resin target, the multiphoton process is confined to the region of high photon density and solidification occurs only at the focal point. Nanodevices can be developed from this new method, completely different from the conventional method of “cutting”. The micro-bull is proof indeed.
Femtosecond lasers have also proved to be very powerful as a tool for biology. Since a cell is transparent, a femtosecond laser beam can propagate through the interior of a cell without damaging the surface and manipulate only the subcellular organelles at the focal point. Photonics is becoming indispensable in the field of biology, as well as the more conventional semiconductor applications.
Another field of photonics research that Prof. Kawata is active in is the optical near-field. He has observed DNA and carbon nanotubes by near-field microscopy based on Raman scattering where the light spectra is shifted by molecular vibrations in the sample. The ability to simultaneously observe and analyse the sample is one primary aim of photonics. Prof. Kawata aims to develop optical microscopy that can observe the vibrations of single molecules. “I am still in the world of 10nm; I intend to go into the world smaller than 1nm to really see what is there. I intend to observe DNA in its natural state, without cutting or using chemical preparations. I have the ideas for how to get there and I think I will be able to reach it within a few years.”
Prof. Kawata has been the Director of the Handai Frontier Research Center (FRC) at Osaka University since October 2001. The FRC has established the e-learning nano-engineering program within FRe-University for people who are now working in related fields. “Those with science or engineering degrees have never taken nanotechnology class at university before, and even people with non-science or engineering degrees can now learn nanotechnology. Take DNA as an example; it cannot be categorized precisely as biology, chemistry or physics. Nanotechnology explores not just a single field but a combination of cutting-edge science.” Along with pursuing great advances in science, he also realizes the necessity of bringing nanotechnology to the wider society.
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| Fig. 1 |
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| Ultra-fine fabrication system using two-photon absorption process with femtosecond laser beams. |
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| Fig. 2 |
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| Microscopic sculptures fabricated using two-photon process, (a) bull, (b) chain and (c) gearwheel. |
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| Fig. 3 Large Image |
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| Stimulation of a cell by using femtosecond laser beams, (a) schematic diagram and (b) calcium ion transmission. |
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| Fig. 4 Large Image |
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| Electron microscopic images of a probe for a near-field optical microscopy (a) and (b), and Raman spectra of Rhodamine 6G dye observed by the near-field optical microscopy (c). |
