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Plasmonics and Nanophotonics Studies by Koichi Okamoto
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Review of Career

Dr. Koichi Okamoto received his Bachelor’s degree from the Department of Chemical Engineering, Doshisha University, Japan, in 1991. He then completed his M.S. and Ph.D. degree from the Department of Chemistry, Graduate School of Science, Kyoto University, Japan, in 1994 and 1998, respectively. In his doctoral thesis, he succeeded in the first direct observation of molecule dynamics of a chemical reaction within a solution. This work has had a great impact on physics, chemistry, and other related research fields. After graduation, he worked on the field of Electronic Science and Engineering because he intended to apply these powerful spectroscopic tools not only for the observation of chemical reaction dynamics but also for research within solid state problems (in physics, electronics, materials, and biology, etc.). From 1998 until 2000, he was Researcher in the Venture Business Laboratory (VBL), associated with Kyoto University. From 2000 until 2002, he was a Research Fellow of the Japan Society for the Promotion of Science (JSPS), and worked in the Department of Electronic Science and Engineering, Kyoto University. Since 2001, Dr. Okamoto joined Professor Scherer’s research group as a postdoctoral scholar at the Department of Electrical Engineering, California Institute of Technology. He is currently engaged in research within the field of Quantum-optics, Nonlinear-optics, Opt-electronics, Solid-states-physics, and Photo-chemistry. His current research interests include the design, fabrication, and characterization of novel solid-state light emitters based on plasmon optics and nano-photonics. In particular, he is interested in the development of both time- and spatially resolved spectroscopy at the limits of present measurement techniques. In his most recent devices, he has shown dramatic (15-fold) increases in the light emission intensity from quantum wells when these are placed within the high electromagnetic fields of surface plasmons formed at the interface between metals and dielectric materials. He verified the mechanism for this luminescence enhancement through careful modelling and developed new designs for even more efficient surface plasmon enhanced LEDs. He has developed vast experience in the field of optical characterization and obtained an intuitive feeling for the measurement techniques required to understand the physics of light emission in optical cavities and at high fields.

Review of Scientific Accomplishments

As an experimentalist, Dr. Koichi Okamoto is unusually versatile. He is talented both in the development of new optical characterization techniques [for example, ultra-fast time-resolved laser spectroscopy, pump & probe spectroscopy, nonlinear spectroscopy, Raman spectroscopy, near-field scanning optical microscopy, confocal scanning laser microscopy, photoluminescence, electroluminescence, cathodeluminescence, photothermal spectroscopy, time-space resolved spectroscopy, etc.] as well as in applying new nanofabrication techniques [for example, electron beam lithography, UV photolithography, laser beam writhing, reactive ion etching (RIE), chemically assisted ion etching (CAIBE), sputter deposition, metal evaporation, ion milling, chemical etching, etc.].

For his Ph.D. dissertation in the Department of Chemistry at Kyoto University, he succeeded in the first accurate observation of the molecular dynamics of short-lived radicals by using the laser induced transient grating (TG) method, a third order nonlinear spectroscopy technique. He found that the translational diffusion coefficients of radicals created by photoinduced hydrogen abstracted reactions of ketones, quinines, and azoaromatic compounds from organic solvent are 2-3 times smaller than those of their parent molecules, even though the radicals and parent molecules possess nearly the same sizes and shapes. This surprising discovery of the anomalously slow diffusion of the radicals had a great impact on physics, chemistry, and other related research fields. Dr. Okamoto’s discovery suggests the existence of an unknown strong intermolecular interaction between the radicals and the surrounding molecules in such systems. The origin of this anomalous diffusion of the radicals was later proposed by the theoretical chemistry group in Kyoto University in the term of the enhanced intramolecular charge polarization induced by the external electrostatic field on each atom within the molecule. Accordingly, Dr. Okamoto’s discovery provided the foundation of the new research field of “Radical Chemistry”.

After graduation, he worked at the Department of Electronic Science and Engineering and Venture Business Laboratory (VBL) at Kyoto University as a researcher. Dr. Okamoto’s aim was to develop optical methods not only for chemistry but also for the wider research fields within physics, electronics, materials, and biology. He helped developed new photonic materials that were useful in industrial and medical applications. For example, highly efficient and functional photonic light emitters are expected to have a profound impact for future technology and society. In order to develop such photonics devices, detailed understanding of the optical properties and dynamics of light emission are necessary. However, this is a very difficult endeavor if only traditional spectroscopic techniques are used. Therefore, Dr. Okamoto developed new and unique laser spectroscopic techniques, such as for example, time resolved micro photoluminescence spectroscopy (TR-m-PL), scanning near-field optical microscopy (NSOM) with illumination-collection mode, third order nonlinear optical spectroscopy, photo-thermal microscopic spectroscopy, Raman spectroscopy with scanning con-focal laser microscopy, etc. He applied these technique to the study of promising photonic materials, such as GaN, ZnSe, ZnO-based quantum wells and quantum dots, organic thin films (polysilane films, Alq3-based EL) , nano-metallic-particles (Pt, Au), molecular systems (solution, micell, LB-layer, liquid crystal), and biological living cells (plant cell, nerve or liver cell of mouse, dopamine).

In particular, Dr, Okamoto characterized InGaN/GaN based quantum wells (QW) to understand the emission dynamics and brightness of these materials that are rapidly emerging as the light emitters of choice for a wide range of applications. The device performances and optical emission properties of InGaN/GaN light emitting diodes (LEDs) are determined by both radiative and nonradiative processes of carriers within the InGaN active layers. However, only few investigators have so far conducted experiments to elucidate the nonradiative processes (thermalization, heat conduction and carrier diffusion). This is primarily due to the difficult nature of such optical measurements. Nevertheless, Dr. Okamoto investigated the temporal and spatial-resolved nonlinear spectroscopy for the direct observation of the nonradiative processes of InGaN/GaN, and developed transient grating (TG) and transient lens (TL) spectroscopy which are one of the third order nonlinear spectroscopy. Analyzing the obtained TG and TL signals, the carrier density and recombination rate and diffusivity could be obtained by the signal intensity and decay of the fast component. Moreover, the thermal energy released by the nonradiative recombination and the heat conductivity were obtained by the slow signal component.

Since Dr. Okamoto joined Professor Scherer’s group, he has held a key role in the development and application of nanophotonic surface plasmon devices. He invented a novel method to enhance the light emission efficiency from quantum wells by using the coupling between surface plasmons (SPs) and these QWs. Before Dr. Okamoto’s experiments, the enhancement of light emission by SP-QW coupling had not been experimentally observed in visible spectral region. For the first time, Dr. Okamoto demonstrated a significant SP enhancement of light emission from InGaN/GaN with metal layers deposited 10nm above the QWs – measuring a 14 fold enhancement in the peak PL intensity at 470 nm after silver coating a quantum well emitter. No such enhancements were obtained from samples coated with gold, as the plasmon resonance occurs only at longer wavelengths for that metal. The coupling lifetimes between QWs and SPs are expected to be very short due to the high electromagnetic fields introduced by the large density of states from the surface plasmon dispersion diagram. As spontaneous emission rates are increased, so are the internal quantum efficiencies (hint). These hint values increased 6.8 times (to 41%) with Ag and 3 times (to 18%) with Al, a result of the spontaneous recombination rate enhancements. These surprising increasing of emission efficiencies suggest that SP enhancement of light emission from InGaN QWs represents a very promising method for developing the super bright LEDs, with emission efficiencies over 6-hold larger than those of present LEDs. SP coupling thus is one of the most interesting methods for developing efficient light emitters as the metal can be used both as an electrical contact and for exciting plasmons, and Dr. Okamoto’s work provides a foundation for the rapid development of highly efficient and high-speed solid-state light emitters, not only limited to III-V materials.

Dr. Okamoto also succeeded for the first time to observe optical mode images on very small photonic crystal nanocavities by near field scanning microscopy (NSOM). Photoluminescence (PL) signals were distinguished from the excitation laser by using the colored glass filter and detected with a high-sensitivity (fW) InGaAs photo-detector. The metal-coated fiber tip, with small aperture size, enabled Dr. Okamoto to distinguish between localized cavity modes and propagating far-field modes, and to obtain more precise mode profiles when the tip probes into holes of Photonic crystals. The best resolution in the system assembled by Dr. Okamoto is ~50nm. By using the shear-force detection, He could also obtain topographic images from the photonic crystals, in addition to the near-field optical image. He has also performed micro PL measurements on photonic crystal nanocavity lasers in order to confirm the existence of the localized cavity modes. The size of the detected mode was roughly four by three lattice spacings, and the optical modes and field distribution observed by Dr. Okamoto represent the smallest reported so far. In addition to localized cavity modes, he has observed dielectric band modes in bulk photonic crystals surrounding the nanocavity by geometrically altering the bands in emission range and eliminating localized modes out of the emission range. Altogether, these careful NSOM measurements have provided Professor Scherer’s group with new insight into the physics of photonic crystals.

Finally, Dr. Okamoto also succeeded to fabricate nanopatterned GaN substrates and demonstrated interesting optical properties by confocal scanning photoluminescence microscopy of such InGaN pillar devices grown on such nanopatterned GaN substrates. By using this technique, Dr. Okamoto has been able to design and control the sizes and geometries of quantum dot or quantum wire structures through lithography. These optimized nanophotonic devices are applicable for use in new high-speed optical modulators, super bright LEDs, as well as low threshold laser. He has also reported a highly sensitive molecular detection technique based on the third order nonlinear optical effects obtained using nano-metallic-grating, and built the pump-probe measurement system and the microfluidic devices that had to be integrated into the experiment for this purpose. Dr. Okamoto has transferred the optical contrast from the nano-pattern of the pump beam through a metallic grating to the materials to be analyzed. This method is another powerful tool of highly sensitive molecular detection, similar to that of surface plasmon enhanced Raman scattering (SERS), and many materials or molecules are detectable using this new method. Dr. Okamoto proposes to apply this molecular sensing approach to the interrogation of solutions in integrated micro and nano-fluidic devices presently developed as a collaboration between Professors Scherer and Quake.

Contributions to Intellectual Life at Caltech

As descried in previous section, Dr. Okamoto developed novel light emitters based on the surface plasmon optics and nanophotonics. The work provides a foundation for the rapid development of highly efficient and high-speed solid-state light emitters. First, our proposed super bright LEDs have the potential of bringing the “illumination revolution” to fruition, i.e., finally to replace Edisonian light bulbs and fluorescent tubes with solid state light emitters as the dominant white light sources. Second, by using our proposed optical characterizing techniques, the detailed mechanisms and dynamics of plasmon optics and nanophotonics can now be clearly understood. This should result in much more efficient and advanced optical devices and a rapid prototyping opportunity for new device designs. Dr Okamoto also intends to pursue new plans to use geometry to control the behavior of photons on the microscopic scale. In short, his project will result in fundamental work for developing the underlying physics for enabling future photonic technologies. Dr. Okamoto has significant experience and talent in optical spectroscopy, and has developed many of the techniques described above. This will ensure rapid progress and completion of this project in the laboratory. The necessary fabrication and measurement equipment, as well as the technology and background knowledge is already available to support this project.
*This document was prepared by Prof. Scherer for my promotion to a faculty member of Caltech in 2005.