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
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 (ηint) of emission by
measuring the temperature dependence of PL intensities. These ηint
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 [δn(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.
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 “illumination revolution” 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
Postdoctoral
Research in Kyoto University
• 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 (Δn>0) and a
slowly-decaying negative component (Δn<0). From the relationship of (∂n/∂N)>0 and (∂n/∂T)<0 in GaN, the fast and slow
components were assigned to the carrier density change (ΔN) and the temperature change (ΔT),
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
Time-resolved fluorescent measurement of dopamine chloride acid salt in
water.
Graduate Research in Kyoto University
• 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 σ-π mixture orbital reformed from the
π-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 “Radical Chemistry”.
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