Paul Fons
Optoelectronics Materials Molecular Biology Nano-Optics Research Team Techniques



  Paul Fons

Senior Staff Researcher

Team Leader

Nano-Optics Research Team

Center for Applied Near-Field Optics

National Institutes of Advanced Industrial Science & Technology

I am a senior research scientist here at AIST (National Institutes for Advanced Industrial Science & Technology). I am a staff member of the Center for Applied Near-Field Optics Research (CAN-FOR). I am also an affiliated staff member at SPring-8, the world's largest synchrotron. As of 2007, I became group leader of the ‹ßÚêŠî‘bŒ¤‹†ƒ`[ƒ€BI would roughly translate this as the Near-Field Fundamental Research Team, but I guess Nano-Optics is shorter.

I graduated from the University of Illinois where I took my Ph.D. There a took a sort of hybrid path involving the growth of metastable III-IV-V semiconductor alloys, experimental characterization of them (XPS and XRD), and modeling of them (kinetic Monte-Carlo growth models and Lancos recursion methods for electronic structure simulation). After a two year stint in the Institute of Applied Physics Department at the University of Tsukuba, I moved to the Electrotechnical Laboratory (now part of AIST) and became a permanent staff member there in 1993. After a lot of molecular beam epitaxy experience, I spent a lot of time learning analytical technqiues such as transmission electron microscopy, photoluminescence, and high-resolution x-ray diffraction to name a few. Following up on the desire to be able to characterize point defects, I started using x-ray absorption spectroscopy (XAFS).

XAFS allows probing of local order about a selected atom type up to a characteristic distance of <0.8 nm. As such it provides a method for measuring atomic ordering on a length scale inaccessible to diffraction (which typically probes lengths from tens of nanometers up to the coherence length of the probe beam). XAFS is actually quantum mechanical interference of a photoelectron with itself. As short range order exists even in amorphous structures, XAFS is an idea tool for investigating transformations from the amorphous to the crystalline state.

I am also a heavy user of the Photon Factory located about 20 minutes from AIST. The Photon Factory is a 2.5 GeV electron storage ring with a storage current of 450 mA and an emittance of 27 nm*rad. The beam lifetime is also over 50 hours resulting in people often referring to it as a-second-and-a-half generation synchrotron. In addition to the wavelength tunability (from the IR to the hard x-ray), a very large advantage from my point of view is that it is about 20 minutes by car from the laboratory. In addition to the very large increase in brilliance, the tunability of the synchrotron allows for elementally selective determination of local structure about atoms in a material via X-ray Absorption Spectroscopy (XAFS). At CAN-FOR, I am now a heavy user of SPring-8, the world's highest energy (largest) light source. It is only about an eight hour trip from my house by Shinkansen (so much for close!). The higher energy and larger source brightness allow for use of higher energy excitation (heavier elements) as well as smaller focus spot size.



 !   Research Highlights

Why is doping ZnO p-type so difficult: Direct determination of N location in ZnO:N
ZnO is a wide band gap semiconductor that at first glance would seem to have all of the advantages and none of the drawbacks of GaN. It has a large excitonic energy (low power optical devices), brilliant emission even when chock full of defects, and ... there are even bulk substrates! What is the catch? Doping it p-type is a challenge. We have used XANES at the N K edge (~409 eV) to directly probe the location of N in molecular beam epitaxial grown Nitrogen doped ZnO.


Direct Measurements of the Location of Cd atoms in CBD-Cd treated CuInSe2
We have used the Cd L3 absorption edge (3538 eV) to investigate the location of in-diffused Cd atoms during Cd chemical bath deposition of CuInSe2.

Understanding the phase-change mechanism of optical media
Most readers have probably have probably used re-writable DVD or CDs before. These devices are based upon the small reflectivity changes between the amorphous and metastable (crystalline) phases of chalcogenide materials such as Ge2Sb2Te5 or Ag-In-Sb-Te compounds. Interestingly enough until recently very little was understood of the structure of these materials and almost nothing was known about the amorphous phase.
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Measurements of Structural Changes in the Amorphous to Crystalline Phase transition in Near-Field Recorded DVD Media
Modern optical media encode information via small changes in reflectivity. As such the ultimate resolution is determined in the Fraunhofer(far field) diffraction limit (wavelength/d) in effect limited by the wavelength of the laser used. By using light in the Fresnel (near-field) diffraction limit, spots as small as 80 nm have been fabricated. The near-field light is generated by a thin film PtOx layer SuperRENS structure.

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