My Research

 

Postdoctoral Research in Caltech 2001-

     Development of surface plasmon enhanced light emitting diodes

     Characterization of photonic crystals by near field optical microscopy

     Near field transient lens microscopy of InGaN/GaN quantum wells

     Photothermal molecular sensing by using metal thin-film nano grating

 

Impact of the projects

 

Postdoctoral Research in Kyoto University 1998-2001

     Development of the unique laser spectroscopy and apply to several optical materials

     Temporal and Spatial-resolved Nonlinear Spectroscopy of InGaN/GaN

     Spectroscopy of Biological Cell

 

Graduate Research in Kyoto University 1992-1998

     Molecular dynamics of chemical reaction intermediate spaces probed by the nonlinear spectroscopy

  

 

 

Postdoctoral Research in Caltech (2001- )

     Development of surface plasmon enhanced light emitting diodes

Since 1993, InGaN light emitting diodes (LEDs) have been improved and commercialized as light sources in the ultraviolet and visible spectral regions, but their original promise as solid-state replacements for light bulbs has so far been delayed as their light emission efficiencies have been limited. The most important requirement for a competitive LED for solid-state lighting is the development of new methods to increase its quantum efficiency of emission.

Quite recently, we invented a novel method to enhance this light emission efficiency by using the coupling between surface plasmons (SPs) and quantum wells (QWs). SPs offer the unique ability to localize, extract and enhance electromagnetic fields. So far, the actual enhancement of light emission by SP-QW coupling had not been observed directly in visible spectral region. We found for the first time a significant SP enhancement of light emission from InGaN/GaN with metal layers deposited 10nm above the QWs. As the photoluminescence (PL) peak of the uncoated wafer at 470 nm was normalized to 1, it is clear that dramatic enhancements in the PL intensities from Ag and Al coated samples can be obtained. No such enhancements were obtained from samples coated with gold, as its well-known plasmon resonance occurs only at longer wavelengths. We also showed that SP-QW coupling actually increase the internal quantum efficiency (āint) of emission by measuring the temperature dependence of PL intensities. These āint values increased 6.8 times (to 41%) with Ag and 3 times (to 18%) with Al, explainable by spontaneous recombination rate enhancements. These surprisingly increasing of emission efficiencies suggests that SP enhancement of light emission from InGaN QWs is a very promising method for developing the super bright LED devices, which emission efficiency should be also over 6-hold larger than that of presents.

                   

(Left) Surface plasmon enhanced PL of InGaN/GaN quantum QWs. (Right) Temperature dependence of PL

 

We propose to fabricate the super bright LED structures by using the SP-QW coupling. In order to design and fabricate even more efficient optical devices for wider spectral range, we have to understand and optimize the mechanism and dynamics of SP-QW coupling and light extraction. We will test several nanostructures of metal layers and the dependence of various metals includes mixture metals, thickness and grain shapes/sizes of metal layers. Until now, a lot of efforts to increase the emission efficiency have been investigated based on the development of the crystal growth techniques, but, there are limited. Our SP-QW coupling method is one solution to increase dramatically the efficiencies of InGaN/GaN LEDs. The following figure shows the historical development of solid-state light emitters. We also plotted the expected efficacy of our super bright InGaN/GaN LEDs. We estimate that the increasing of the internal quantum efficiencies of emission is at least 2-fold and increasing of the light extraction efficiency is also at least 2-fold from those of the present best InGaN LEDs. Therefore, our proposed LEDs are expected to achieve extraordinary high efficiency (120lm/W) beyond that of the fluorescent tube (75lm/W). Our super bright LEDs can provide the evolutional rapid development of solid-state light sources taken place the present light sources (fluorescent tube or light bulb).

 

 

Historical development of the luminescence efficacies of LEDs and expected values of our super bright LEDs.

 

     Characterization of photonic crystals by near field optical microscopy

Recently, near-field scanning optical microscopy (NSOM) has been used as a powerful alternative method to analyze local electromagnetic field distributions in fabricated nanophotonic structures. Several groups have reported NSOM studies of planar photonic crystal (PPC), however, high spatially resolved near-field images of the field distribution inside the PPC cavity have not so far been reported. We succeeded for the first time to observe the optical mode images obtained by NSOM on very small PPC cavities based on fractional edge dislocations. A cw light from a He-Ne laser (633nm) and a 20ns pulsed diode laser (780nm) were used as the excitation laser source for illumination mode (I-mode) and the collection mode (C-mode) measurements, respectively. Photoluminescence (PL) signals were distinguished from the excitation laser by using the colored glass filter with a cut-off wavelength of 850nm, and detected with a high-sensitivity (fW) InGaAs photo-detector. The metal-coated fiber tip, with small aperture size, enables us 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 our system is as small as 50nm. By using the shear-force detection, we could also obtain a topographic image of the sample, in addition to the near-field optical image. We have also performed micro photoluminescence measurements on our structures in order to confirm the existence of the localized cavity modes. The photonic crystal nanocavities fabricated in active InGaAsP material used in this work are very similar to those used to realize low-threshold lasers described in our previous publication. We have observed localized defect modes of the compact photonic crystal nano-cavities by using I-mode. The size of the detected mode was roughly four by three lattice spacings. To the best of our knowledge, the optical modes and field distribution that we have observed are the smallest reported so far. High-resolution near-field optical images were obtained by using a metal-coated optical fiber tip with a 150nm aperture size. In addition to localized cavity modes, we have 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 by using C-mode. The confined cavity modes observed by near-field microscopy were confirmed by micro photoluminescence experiments, and very good agreement between the two analysis methods was found. We conclude that near-field scanning optical microscopy is a powerful tool for investigation of local profiles of confined modes in nanocavities.

 

NSOM Image of photonic crystal laser

 

     Near field transient lens microscopy of InGaN/GaN quantum wells

We developed two-beams NSOM measurements based on the transient lens effect, which provides a third order nonlinear optic signature for carrier properties in semiconductors. By using this meted, we succeeded to observe the non-radiative processes, for example, carrier diffusion/ recombination, phonon generation, or heat conduction in InGaN/GaN. Such processes are also important effects dominating the optical properties of light emitters, but have proven to be very difficult to observe experimentally by traditional optical measurements. We obtained the near-field PL image and TL image of InGaN/GaN at room temperature with time-modulated (400Hz) InGaN laser (405nm) and a AlGaAs laser (830nm) as pump and probe beams, respectively. We obtained the topography, reflection, PL and TL images of the InGaN/GaN at the same time. We found a sub-micron scale spatial inhomogeneity in the near-field image of both PL and TL signals. Spatial inhomogeneities of PL intensities and peak wavelengths in InGaN/GaN QWs were previously reported and interpreted to be a result of fluctuations of the indium composition, the QW thickness, and the piezoelectric field in InGaN active layers. These fluctuations are very important and may act as the carrier localization centers, and suppress the pathways for nonradiative recombination of carriers. Such carrier localization has been known as one of the reasons for the strong emission properties in InGaN/GaN materials. In this study, we found that the NSOM-TL image was also distributed inhomogeneously on a sub-micron scale. This suggests that the nonradiative carrier processes (diffusion, localization, or thermalization) should also be spatially distributed at submicron scales. We also found a strong correlation between the PL and TL images. Under this condition, the carrier density signal represents the main component in the NSOM-TL signal. Therefore, the TL images can be used to represent the carrier density in active layers and the bright region should correspond to the carrier localization areas. By comparing the PL and TL images, it was found that the many bright regions in the TL images correspond to the bright regions in the PL images. In these regions, carrier localized areas are well act as the radiative recombination (emission) centers. On the other hand, some bright TL regions correspond to the dark region in the PL image. We believe that these regions should be represented to the nonradiative recombination (thermalization) centers of carriers in the materials. To the best of our knowledge, this is the first report of the direct mapping of the distribution of the carrier density and the nonradiative recombination in InGaN/GaN-based optical devices. Such results are important to understand the carrier dynamics and the emission mechanism in InGaN but could not be obtained with previous spectroscopic techniques. In conclusion, the Near field transient lens microscopy is powerful method to characterize LEDs or other optical devices.

 

 

     Photothermal molecular sensing by using metal thin-film nano grating

Recently, microfluidic technologies have been developed and applied widely in biology and analytical chemistry. The fluorescent latex beads, DNA with fluorescent dyes, or colored solutions have been used as the host objects in such fluidic channels. However, many molecules and chemical compounds have neither color nor fluorescence/phosphorescence. High sensitive molecular sensing technique without human visibility is necessary for chemical and biomedical applications of microfluidic devices. In this article, we propose the highly sensitive molecular sensing technique based on the third order nonlinear spectroscopy with nano and micro-metal gratings fabricated in our laboratory in order to apply to the monitoring of the microfluidic devices. Time-modulated InGaN laser or a frequency-tripled Nd:YAG laser (355nm) was pump beam. A cw-He-Ne laser (633 nm, 0.05 mW) was used as probe beam. Both beams were focused by a lens at the fabricated metal grating in sample solution. Diffracted probe beam was isolated with a pinhole and a glass filter and detected with a InGaAs photodetector. Nano and micrometer scaled metal gratings were fabricated by chromium evaporation on quartz substrates, patterned by electron beam lithography, and chemical etching. Sample solution which we used was nitrobenzene in 2-propanol (5 volume %). Nitrobenzene is not colored and has neither fluorescence nor phosphorescence, so molecular detection of nitrobenzene has been very difficult. We obtained the diffracted signals which have two components; the large background offset component (Ir) and the time modulated signal component (Is). According to the nonlinear optics theory, proportional relationship is obtained between Is(t) and the refractive index change [ƒÂn(t)] induced by the temperature grating and density grating at this frequency scale (100 Hz). Then, Is is proportional to the molecular concentration. This suggests that the solute molecular concentration can be detected by this technique. In order to observe the actual time dependence of the relaxation of transient grating, we investigated the time-resolved measurement. Obtained signal decay times were reasonable to those values calculated by the thermal diffusivity in solvent. This excellent agreement shows the validity of the signal analysis method. In conclusion, we developed the new technique of the third order nonlinear optical spectroscopy by using the fabricated nano and micro gratings. This method has many advantages compared with traditional techniques; (1) simple setting and easy alignment, (2) high sensitivity and high S/N ratio (heterodyne detection), (3) high speed (high frequency) detection, and (4) easy analyzing (linear relationship between signal intensity and molecular concentration). This technique is expected to be a powerful tool to monitor the molecules in nano and microfluidic devices.

 

Molecular sensor by using nano-grating film in microfluidic devices

 

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Impact of the projects

 

These works provide a foundation for the rapid development of highly efficient and high-speed solid-state light emitters. First, our proposed super bright LEDs would bring the gillumination revolutionh to fruition, i.e., finally replace Edisonian light bulbs and fluorescent tubes with solid state light emitters as the dominant white light sources. Second, by using our developed optical characterizing technique, the detailed mechanisms and dynamics of plasmon optics and nanophotonics could be clearly understood. This should result in much more efficient and advanced optical devices. We also propose some new plans to use geometry to control the behavior of photons on the microscopic scale. In short, our project will result in very important and fundamental work for developing the underlying physics for enabling future photonic technologies.

       The development of highly efficient and high-speed solid-state optical devices would enable highly desirable new technologies in near future. For example, now the applicant holds US-patent (US-6,629,762) and a corresponding Japanese patent for the idea and theory for a 3D holographic movie player/recorder using nano-structured electric field sensors. This idea represents very interesting and important future technologies but has been so far very difficult to realize. Therefore, 3D holographic movies have not so far been realized and exists only in science fiction. As this new technique needs a super bright light emitters, high-speed optical modulators, and nano-sized structures of liquid crystal cells. We believe that it could be realizable by gathering the nanofabrication techniques and the thus the results of this project could impact even Hollywood.

 

Postdoctoral Research in Kyoto University (1998-2001)

     Development of the unique laser spectroscopy and apply to several optical materials

Many kinds of photonics materials have been developed and used for wider industrial and medical fields. Even more highly efficient and functional photonics devices are expected for future technology and society. In order to develop the photonics devices, detail elucidation and understanding of the optical properties and dynamics are necessary, but it is very difficult by using only traditional spectroscopic techniques. Therefore, we developed the unique and novel laser spectroscopic techniques, 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 confocal laser microscopy, etc. We apply these technique for several photonics materials, for example, semiconductors (GaN, ZnSe, ZnO-based quantum well, quantum dot, etc.), organic thin films (polysilane films, Alq3-based EL , etc.), nano-metal-particles (Pt, Au, etc.), molecular systems (solution, micelle, LB-layer, liquid crystal, etc.), and biological living cells (plant cell, nerve or liver cell of mouse, dopamine), etc.

 

 

Time-resolved micro-photoluminescence Spectroscopy

 

 

 

Scanning Near-field Optical Microscopy

 

     Temporal and Spatial-resolved Nonlinear Spectroscopy of InGaN/GaN

Currently, InGaN/GaN based quantum well (QW) optical devices (light emitting diodes; LED and laser diode; LD) have been studied and developed for a wide range of applications. Recently, we have reported on the temporal and spatial resolved observation of the radiative processes in InGaN/GaN-based QW probed by time-resolved micro-photoluminescence (TRMPL) and scanning near-field optical microscopy (NSOM). The device performances and optical emission properties of InGaN/GaN LEDs are determined by both radiative and nonradiative processes of carriers in InGaN active layers. However, only few investigations have so far been conducted to experimentally elucidate the nonradiative processes (thermalization, heat conduction and carrier diffusion), primarily due to the difficult nature of such measurements. We investigated the temporal and spatial-resolved nonlinear spectroscopy for the direct observation of the nonradiative processes of InGaN/GaN. We developed transient grating (TG) and transient lens (TL) spectroscopy which are one of the third order nonlinear spectroscopy. For TG, bright-dark stripe patterned modulations of the optical properties were created by the crossing of two pump beams and detected by the diffraction of the probe beam. On the other hand, for TL, Gaussian shaped spatial distribution of optical properties was created and detected by focus/defocus of the probe beam. Such modulations of optical properties are attributed to some nonradiative processes in materials (carrier dynamics, thermal dynamics, etc.). We obtained the time profiles of the TG and TL signals with picosecond and nanosecond time-scales. The signals have two components; a spike-like positive component of the refractive index change (ƒ¢n>0) and a slowly-decaying negative component (ƒ¢n<0). From the relationship of (Ýn/ÝN)>0 and (Ýn/ÝT)<0 in GaN, the fast and slow components were assigned to the carrier density change (ƒ¢N) and the temperature change (ƒ¢T), respectively. By fitting of the TG and TL time profiles, the carrier density and recombination rate and diffusivity were obtained by the signal intensity and decay of the fast component. On the other hand, the thermal energy released by the nonradiative recombination and the heat conductivity were obtained by the slow signal component. In order to know the special-resolved properties, for example the correlation between the optical property and threading dislocation density, time-resolved micro photoluminescence (TRMPL) spectroscopy was also developed and used. We found that both the intensities and the time profiles of the signal were different at high TD (109 cm-2) seed region and the low TD (106 cm-2) wing region of the air-bridged lateral epitaxial growth GaN and InGaN/GaN.

 

 Optical configuration of the third order nonliner spectroscopy (Transient Grating Method)

 

 

 

Transient Grating Method with femtosecond pulse laser syatem

 

 

     Spectroscopy of Biological Cell

The native fluorescence properties of dopamine were examined aiming the direct visualization of dopamine dynamics in the living cell. Dopamine hydrochloric acid salt in aqueous solution was excited at 266 nm femtosecond laser and the sufficient fluorescence emission peaking at 330 nm was detected with a streak camera. The fluorescence decay curve was fitted by single-exponential functions, with the lifetime of approximately 0.80 ns. The influence of deep-UV laser excitation on cells is also discussed for the direct observation of dopamine in the living cells. In addition, it is needed to detect the dopamine fluorescence in the living cell sensitively and separately from emission of other fluorescent species. When instrumental arrangement and time-resolved spectral analysis can make it possible to solve such problems, direct visualization of the secretion process of individual cells will be achieved by the laser-induced native fluorescence imaging microscopy, without using any additional fluorescent probes. This quantitative imaging technique will provide a useful noninvasive approach for the study of dynamic cellular changes and the understanding of the molecular mechanisms of information transporting. This direct imaging technique can be developed to apply a quantitative analysis, and can provide a useful noninvasive approach for the study of dynamic cellular changes and the understanding of the molecular mechanisms of secretory or information transportation processes. At the same time we are investigation the diffusion process of dopamine in solution by using Taylor dispersion technique and transient grating technique The diffusion properties in solution from these measurements will also serves as a basic data to understanding the dopamine behavior in the living cellh.

Time-resolved fluorescent measurement of dopamine chloride acid salt in water. 

 

 

Graduate Research in Kyoto University (1992-1998)

     Molecular dynamics of chemical reaction intermediate spaces probed by the nonlinear spectroscopy

Molecular dynamics (translation, rotation, and vibration) in solution is fundamental and important factor in chemistry and physics and attracted many investigations for a long time. Especially, molecular translation diffusion processes is very important processes because chemical reactions in solutions are usually limited by the mutual molecular diffusion. Therefore, the diffusion coefficients (Ds) have been measured by many methods. However, only few attempts had so far been made to elucidate the Ds of chemical reaction intermediate radicals mainly because of the experimental difficulties although it is very important to understand the mechanism and dynamics of chemical reactions in a fluid.

In my doctoral thesis, we succeeded in the first ever to measure Ds values of the short-lived radicals accurately by using the laser induced transient grating (TG) method which is one of the third order nonlinear spectroscopy. We found that Ds of the radicals created by photoinduced hydrogen abstracted reactions of ketons, quinines, and azoaromatic compounds from organic solvent are 2-3 times smaller than those of the parent molecules, even though the radicals and parent molecules possess nearly the same sizes and the same shapes. This surprising discovery of the anomalously slow diffusion of the radicals had a great impact on physics, chemistry, and other related research fields. By the extended investigation, such as the solvent dependence, the solute size dependence, the temperature dependence, it was found that the activation energies for diffusion of the radicals are larger than those of the parent molecules. This fact suggests the existence of an unknown strong intermolecular interaction between the radicals and the surrounding molecules. According to this fact, theory and analysis of the chemical reactions in solutions based on the simple hydrodynamic theory should be modified. We proposed that Ds of radicals could not be estimated by Ds of the stable molecules of similar size and shape.

 

Time profile of the TG signal taken for benzophenone in tetradecane solution.

 

On the other hand, Ds of the benzyl radical created by the photo-dissociation from dibenzyl ketone do not diffuse anomalously slowly but similar to the stable molecules. We compared the properties of benzyl radical with those of the other radicals produced by hydrogen abstraction in order to find a possible origin of the anomalous diffusion. The reasonable origin of the anomalous diffusion of the radicals were proposed by the theoretical chemistry group in Kyoto University by using the ab initio molecular orbital (MO) calculation of the intramolecular charge polarization indused by the external electrostatic field on each atoms in molecule. They found that the partial charge polarization of pyrazinyl radical and benzophenon ketyl radial were remarkably enhanced compare to the parent molecules pyrazine and benzophenon. They proposed that this remarkably enhancement of the partial charge polarization bring the strong radival-solvent interaction, which is the origin of the anomalous slow diffusion phenomenon. They also showed that the D of pyrazinyl radical was slower than that of pyrazine by using the molecular dynamics (MD) simulation with the partial change polarization effect. They described that the mechanism of the enhancement of partial charge polarization of radicals is due to the ƒÐ-ƒÎ mixture orbital reformed from the ƒÎ-conjugated orbital in solute molecules. Thus, our results were supported by theoretically and, at the same time, contributed to the development of the field of the theoretical chemistry.

   Other kinds of interesting molecular dynamics of radicals, ions, and radical ions were also founded in the aqueous, mixed, and micellar solutions. Accordingly, our results brought the foundation of the new research field of gRadical Chemistryh.

 

Special solute-solvent intermolecular interaction based on the intra-molecular partial charge polarization

 

 

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