Photosensitization of nanostructured TiO2 electrodes with CdSequntum dots: effects of microstructure in substrates

Q. Shen1,2) and T. Toyoda1,2)

Department of Applied Physics and Chemistry1), and

Course of Coherent Optical Science2)

The University of Electro-Communications

Chofu, Tokyo 182-8585, Japan

(b) dye or semiconductor quantum dot sensitization toextend the photoresponse of TiO2 to the visible region.

Nanocrystalline TiO2 has received much attention in numerous fields of applications (e.g., photocatalysis, photoelectrochemical solar cell and gas sensor).

Dye-Sensitized Solar Cell (DSSC) with high energy conversion efficiency exceeding 10� has been developed by(a) preparation of highly porous, nanostructured TiO2

electrodes to increase the surface area;

BACKGROUNDSBACKGROUNDS

For higher efficiency, further research on the nanostructured TiO2 electrodes (morphology, optical absorption, charge injection, transport and recombination) is essential.

RESEARCH OBJECTIVESRESEARCH OBJECTIVES

TiO2 electrodes�Anatase-type TiO2 nanoparticles made from TiCl4 hydrolysis and oxidation processes.

�Composition of different size anatase-type TiO2 nanoparticles.

�Composition with rutile-type content on anatase-type TiO2nanoparticles.

�TiO2 nanotubes and nanowires.

�TiO2 photonic crystals.

SensitizerCdSe quantum dots.

Advantages of semiconductor Advantages of semiconductor quantum dots quantum dots

Quantum confinement allows for energygap tunable across the solar spectrum.

Large extinction coefficient resulting from quantum confinement.

Robust inorganic nature.

Large intrinsic dipole moment which maylead to rapid charge separation.

Impact ionization occurs with the possibility of high efficiency.

MOTIVATIONSMOTIVATIONSIn order to investigate morphological dependence,

two types of nanostructured TiO2 electrodes were prepared and CdSe quantum dots with various sizes were adsorbed on TiO2 as sensitizer.

TiO2 electrodes were characterized using SEM, XRD, photoacoustic (PA) and photoelectrochemical current (PEC) methods.

PA method is powerful for the investigation of optical absorption of optically opaque or scattering solid materials.

TiOTiO22 electrode preparationelectrode preparation

Nanocrystalline TiO2 powder (30 wt�)Pure waterAcetylacetone �10 wt%) Polyethylene glycol�PEG) (MW:500000)

(40 w�� vs TiO2)

A ( 15 nm)B ( 27 nm)

TiO2 paste

Stirring for one hour

Depositing TiO2 paste on FTO using squeegee method.

Heating in air at 450� for 30 min.Two types of TiO2

electrodes

Chemical solution deposition (CD) method of Chemical solution deposition (CD) method of CdSeCdSequantum dots on the TiOquantum dots on the TiO22 electrodeelectrode

S. Gorer, G. Hodes, J. Phys. Chem. 98 (1994) 5338.

� 80mM CdSO4

� 120mM N(CH2COONa)3 (NTA)� 80 mM Na2SeSO3

mixed

(1:1:1)Chemical Deposition

(CD) Solution

Solution

TiO2

For some Time

Sample

CD Solution

1h 3h 5h 9h 23h 49h

SEM micrographs of TiO2 electrodesSEM micrographs of TiOSEM micrographs of TiO22 electrodeselectrodes

Size of TiO2 in the paste: 15 nm

500 nm

Electrode A

500 nm

Electrode B

Size of TiO2 in the paste: 27 nm

SEM micrographs of SEM micrographs of CdSeCdSe--quantum dots quantum dots adsorbed on TiOadsorbed on TiO22 electrodeselectrodes

�µm

CdSe 5h CdSe

�µm

CdSe 60h�µm

CdSe 13h CdSe

CdSe 28h

�µm

X-Ray diffraction patternsXX--Ray diffraction patternsRay diffraction patterns

25 30 35 40 45 50

C dSeC dSe

C dSe

FTOFTO

TiO 2

TiO 2TiO 2

C dSe / TiO 2/ FTO TiO 2/ FTO

(311)(220)

(111)

2θ (De gre e s )

Int

ensi

ty (

Arb

. U

nits

)

φβλ

cos9.0

=D

Scherrer equation:

D: CdSe quantum dot sizeβ: half-width of the

diffraction peakΦ: diffraction angleλ: 0.154 nm

40 41 42 43 44 45

CdSe/ TiO2/ FTO

(220)

2θ (Degrees)

Int

ensi

ty (

Arb

. U

nits

)

D�5-6 nm (CdSe120h)

Photoacoustic Spectroscopy ( PAS )

Light source : 300W xenon arc lampNormalization : carbon black sheet Wavelength range�250nm�800nmModulation frequency : 33Hz

Xe-Lamp0 5 0 0

Monochromator

Optical Lens

Microphone

Pre-Amplifier

Lock-in Amplifier

DATA

Computer

PA Cellsample

Chopper

1.5 2.0 2.5 3.0 3.5 4.0

0.0

0.5

1.0

1.5A (TiO2: 15nm)

Wavelength (nm)

bulk 120h 23h 9h 5h 0h

PA I

nten

sity

(A

rb.

Uni

ts)

Photon Energy (eV)

900 750 600 450 300

EgE1

PA spectra of TiO2 electrodes A (TiO2:15nm) deposited with CdSe quantum dots for various times.

)2( 8

:ionapproximat mass Effective

2

2

1 aDa

hEEE g ==−=∆µ

0 20 40 60 80 100 1200

2

4

6

8

10

Deposition Time (Hours)

Dia

met

er (

nm)

Dependence of CdSe quantum dot size on deposition time.

1.5 2.0 2.5 3.0 3.5 4.00.0

0.5

1.0

1.5 B (TiO2: 27nm)

Wavelength (nm)

120h 49h 23h 9h 5h 3h 0h

PA I

nten

sity

(A

rb.

Uni

ts)

Photon Energy (eV)

900 750 600 450 300

1.5 2.0 2.5 3.0 3.5 4.00.0

0.5

1.0

1.5 B (TiO2: 27nm)

Wavelength (nm)

120h 49h 23h 9h 5h 3h 0h

PA I

nten

sity

(A

rb.

Uni

ts)

Photon Energy (eV)

900 750 600 450 300

1.5 2.0 2.5 3.0 3.5 4.00.0

0.5

1.0

1.5 B (TiO2: 27nm)

Wavelength (nm)

120h 49h 23h 9h 5h 3h 0h

PA I

nten

sity

(A

rb.

Uni

ts)

Photon Energy (eV)

900 750 600 450 300

1.5 2.0 2.5 3.0 3.5 4.00.0

0.5

1.0

1.5 B (TiO2: 27nm)

Wavelength (nm)

120h 49h 23h 9h 5h 3h 0h

PA I

nten

sity

(A

rb.

Uni

ts)

Photon Energy (eV)

900 750 600 450 300

1.5 2.0 2.5 3.0 3.5 4.00.0

0.5

1.0

1.5 B (TiO2: 27nm)

Wavelength (nm)

120h 49h 23h 9h 5h 3h 0h

PA I

nten

sity

(A

rb.

Uni

ts)

Photon Energy (eV)

900 750 600 450 300

1.5 2.0 2.5 3.0 3.5 4.00.0

0.5

1.0

1.5 B (TiO2: 27nm)

Wavelength (nm)

120h 49h 23h 9h 5h 3h 0h

PA I

nten

sity

(A

rb.

Uni

ts)

Photon Energy (eV)

900 750 600 450 300

1.5 2.0 2.5 3.0 3.5 4.00.0

0.5

1.0

1.5 B (TiO2: 27nm)

Wavelength (nm)

120h 49h 23h 9h 5h 3h 0h

PA I

nten

sity

(A

rb.

Uni

ts)

Photon Energy (eV)

900 750 600 450 300

1.5 2.0 2.5 3.0 3.5 4.00.0

0.5

1.0

1.5 B (TiO2: 27nm)

Wavelength (nm)

120h 49h 23h 9h 5h 3h 0h

PA I

nten

sity

(A

rb.

Uni

ts)

Photon Energy (eV)

900 750 600 450 300

PA spectra of TiO2 electrodes B (TiO2:27nm) deposited with CdSe quantum dots for various times.

Photoelectrochemical Current (PEC)Spectroscopy

Light source : 300W xenon arc lampNormalization : carbon black sheetMaterial of the PEC cell � quartzWavelength range�250nm�800nmElectrolyte : 1M KCl + 0.1M Na2S

AmmeterLock-in Amplifier����

Computer

Monochromator

Xe-Lamp1 2 3 4

PEC Cell

Pt

Sample

Chopper

Optical Lens

Electrolyte

1.5 2.0 2.5 3.0 3.5 4.0 4.5

0

100

200

300

400A (TiO2: 15nm) 120h

49h 23h 9h 5h 3h 0h

PEC

Int

ensi

ty (

Arb

. U

nits

)

Photon Energy (eV)

750 600 450 300

1.5 2.0 2.5 3.0 3.5 4.0 4.5

0

100

200

300

400A (TiO2: 15nm) 120h

49h 23h 9h 5h 3h 0h

PEC

Int

ensi

ty (

Arb

. U

nits

)

Photon Energy (eV)

750 600 450 300

1.5 2.0 2.5 3.0 3.5 4.0 4.5

0

100

200

300

400A (TiO2: 15nm) 120h

49h 23h 9h 5h 3h 0h

PEC

Int

ensi

ty (

Arb

. U

nits

)

Photon Energy (eV)

750 600 450 300

1.5 2.0 2.5 3.0 3.5 4.0 4.5

0

100

200

300

400A (TiO2: 15nm) 120h

49h 23h 9h 5h 3h 0h

PEC

Int

ensi

ty (

Arb

. U

nits

)

Photon Energy (eV)

750 600 450 300

1.5 2.0 2.5 3.0 3.5 4.0 4.5

0

100

200

300

400A (TiO2: 15nm) 120h

49h 23h 9h 5h 3h 0h

PEC

Int

ensi

ty (

Arb

. U

nits

)

Photon Energy (eV)

750 600 450 300

1.5 2.0 2.5 3.0 3.5 4.0 4.5

0

100

200

300

400A (TiO2: 15nm) 120h

49h 23h 9h 5h 3h 0h

PEC

Int

ensi

ty (

Arb

. U

nits

)

Photon Energy (eV)

750 600 450 300

1.5 2.0 2.5 3.0 3.5 4.0 4.5

0

100

200

300

400A (TiO2: 15nm) 120h

49h 23h 9h 5h 3h 0h

PEC

Int

ensi

ty (

Arb

. U

nits

)

Photon Energy (eV)

750 600 450 300

1.5 2.0 2.5 3.0 3.5 4.0 4.5

0

100

200

300

400A (TiO2: 15nm) 120h

49h 23h 9h 5h 3h 0h

PEC

Int

ensi

ty (

Arb

. U

nits

)

Photon Energy (eV)

750 600 450 300

PEC spectra of TiO2 electrodes A (TiO2:15nm)deposited with CdSe quantum dots for various times.

1.5 2.0 2.5 3.0 3.5 4.0 4.5

0

200

400

600

800B (TiO2: 27nm)

Wavelength (nm)

120h 49h 23h 9h 5h 3h 0h

PEC

Int

ensi

ty (

Arb

. U

nits

)

Photon Energy (eV)

750 600 450 300

1.5 2.0 2.5 3.0 3.5 4.0 4.5

0

200

400

600

800B (TiO2: 27nm)

Wavelength (nm)

120h 49h 23h 9h 5h 3h 0h

PEC

Int

ensi

ty (

Arb

. U

nits

)

Photon Energy (eV)

750 600 450 300

1.5 2.0 2.5 3.0 3.5 4.0 4.5

0

200

400

600

800B (TiO2: 27nm)

Wavelength (nm)

120h 49h 23h 9h 5h 3h 0h

PEC

Int

ensi

ty (

Arb

. U

nits

)

Photon Energy (eV)

750 600 450 300

1.5 2.0 2.5 3.0 3.5 4.0 4.5

0

200

400

600

800B (TiO2: 27nm)

Wavelength (nm)

120h 49h 23h 9h 5h 3h 0h

PEC

Int

ensi

ty (

Arb

. U

nits

)

Photon Energy (eV)

750 600 450 300

1.5 2.0 2.5 3.0 3.5 4.0 4.5

0

200

400

600

800B (TiO2: 27nm)

Wavelength (nm)

120h 49h 23h 9h 5h 3h 0h

PEC

Int

ensi

ty (

Arb

. U

nits

)

Photon Energy (eV)

750 600 450 300

1.5 2.0 2.5 3.0 3.5 4.0 4.5

0

200

400

600

800B (TiO2: 27nm)

Wavelength (nm)

120h 49h 23h 9h 5h 3h 0h

PEC

Int

ensi

ty (

Arb

. U

nits

)

Photon Energy (eV)

750 600 450 300

1.5 2.0 2.5 3.0 3.5 4.0 4.5

0

200

400

600

800B (TiO2: 27nm)

Wavelength (nm)

120h 49h 23h 9h 5h 3h 0h

PEC

Int

ensi

ty (

Arb

. U

nits

)

Photon Energy (eV)

750 600 450 300

1.5 2.0 2.5 3.0 3.5 4.0 4.5

0

200

400

600

800B (TiO2: 27nm)

Wavelength (nm)

120h 49h 23h 9h 5h 3h 0h

PEC

Int

ensi

ty (

Arb

. U

nits

)

Photon Energy (eV)

750 600 450 300

PEC spectra of TiO2 electrodes B (TiO2:27nm)deposited with CdSe quantum dots for various times.

IPCE: incident photon-to-current conversion efficiency for monochromatic radiation

)cmW(photoflux (nm) wavelength)cmA(density nt photocurre nm)(eV 1240

numbersphoton incident numberselectron injectedIPCE(%)

2-

2-

⋅×⋅×⋅

=

=

µµ

1.5 2.0 2.5 3.0 3.5

0

4

8

12

16

20

24

28A (TiO2: 15nm)

Wavelength (nm)

CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h

IPC

E (

%)

Photon Energy (eV)

800 600 400

1.5 2.0 2.5 3.0 3.5

0

4

8

12

16

20

24

28A (TiO2: 15nm)

Wavelength (nm)

CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h

IPC

E (

%)

Photon Energy (eV)

800 600 400

1.5 2.0 2.5 3.0 3.5

0

4

8

12

16

20

24

28A (TiO2: 15nm)

Wavelength (nm)

CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h

IPC

E (

%)

Photon Energy (eV)

800 600 400

1.5 2.0 2.5 3.0 3.5

0

4

8

12

16

20

24

28A (TiO2: 15nm)

Wavelength (nm)

CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h

IPC

E (

%)

Photon Energy (eV)

800 600 400

1.5 2.0 2.5 3.0 3.5

0

4

8

12

16

20

24

28A (TiO2: 15nm)

Wavelength (nm)

CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h

IPC

E (

%)

Photon Energy (eV)

800 600 400

1.5 2.0 2.5 3.0 3.5

0

4

8

12

16

20

24

28A (TiO2: 15nm)

Wavelength (nm)

CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h

IPC

E (

%)

Photon Energy (eV)

800 600 400

1.5 2.0 2.5 3.0 3.5

0

4

8

12

16

20

24

28A (TiO2: 15nm)

Wavelength (nm)

CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h

IPC

E (

%)

Photon Energy (eV)

800 600 400

1.5 2.0 2.5 3.0 3.5

0

4

8

12

16

20

24

28A (TiO2: 15nm)

Wavelength (nm)

CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h

IPC

E (

%)

Photon Energy (eV)

800 600 400

IPCE spectra of TiO2 electrodes A (TiO2:15nm)deposited with CdSe quantum dots for various times.

1.5 2.0 2.5 3.0 3.5

0

4

8

12

16

20

24

28

Wavelength (nm)

B (TiO2: 27nm) CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h

IPC

E (

%)

Photon Energy (eV)

800 600 400

1.5 2.0 2.5 3.0 3.5

0

4

8

12

16

20

24

28

Wavelength (nm)

B (TiO2: 27nm) CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h

IPC

E (

%)

Photon Energy (eV)

800 600 400

1.5 2.0 2.5 3.0 3.5

0

4

8

12

16

20

24

28

Wavelength (nm)

B (TiO2: 27nm) CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h

IPC

E (

%)

Photon Energy (eV)

800 600 400

1.5 2.0 2.5 3.0 3.5

0

4

8

12

16

20

24

28

Wavelength (nm)

B (TiO2: 27nm) CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h

IPC

E (

%)

Photon Energy (eV)

800 600 400

1.5 2.0 2.5 3.0 3.5

0

4

8

12

16

20

24

28

Wavelength (nm)

B (TiO2: 27nm) CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h

IPC

E (

%)

Photon Energy (eV)

800 600 400

1.5 2.0 2.5 3.0 3.5

0

4

8

12

16

20

24

28

Wavelength (nm)

B (TiO2: 27nm) CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h

IPC

E (

%)

Photon Energy (eV)

800 600 400

1.5 2.0 2.5 3.0 3.5

0

4

8

12

16

20

24

28

Wavelength (nm)

B (TiO2: 27nm) CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h

IPC

E (

%)

Photon Energy (eV)

800 600 400

1.5 2.0 2.5 3.0 3.5

0

4

8

12

16

20

24

28

Wavelength (nm)

B (TiO2: 27nm) CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h

IPC

E (

%)

Photon Energy (eV)

800 600 400

IPCE spectra of TiO2 electrodes B (TiO2:27nm)deposited with CdSe quantum dots for various times.

SummaryTwo types of nanostructured TiO2 electrodes A (15nm) and B (27nm) deposited with CdSe quantum dots by chemical deposition, have been characterized using PA, PEC spectra and IPCE measurements.

Photosensitization by CdSe quantum dots was demonstrated and red shift of the spectra with increasing CdSe sizes can be clearly observed.

PEC and IPCE spectra in the visible region are quite different for the two types of TiO2 electrodes, even for the same deposition time of CdSe quantum dots.

The electron diffusion coefficient of the TiO2 electrode A was found to be two times larger than that of electrode B using tran-sient photocurrent responses.

PEC and IPCE in the visible region in the CdSe-sensitized TiO2 nanostructured electrodes largely depend on both the microstructure and electron transport property in the TiO2 electrodes as well as the size and quantity of CdSe nanoparticles .

Future studies:TiO2/CdSe interfacial property;Energy conversion efficiency; Ultrafast relaxation dynamics etc.

UltrafastUltrafast Carrier Dynamics of Carrier Dynamics of CdSeCdSe Quantum Quantum Dots Adsorbed on Dots Adsorbed on NanostructuredNanostructured TiOTiO2 2 FilmsFilms

The ultrafast carrier dynamics were investigated by using lens-free heterodyne detection transient grating (LF-HD-TG) technique.

One kind of optical pump and probe technique:

Measurements of the decay of excited carrier densities after pulsed injection.

LFLF--HDHD--TGTG methodmethod

By approaching a sample to the grating, 1: Grating excitation2: Heterodyne detection

Excitation of sample

Heterodyne detection

Reference

Signal

Detector

Probe

Detection of signal

→ Easy TG method without lenses (No daily setup)

Sample

Pump

Transmission grating

K. Katayama et al., APL 82, 2775 (2003).

Lens-free heterodyne detection transient grating technique ( LF-HD-TG )

Grating

Titanium/sapphire laserWavelength: 800 nm; Pulse width: 150 fs; Intensity: 5 µJ/pulse

Lock-in Amp.

Probe beam

Sample

Optical Delay Line

SHG

Chopper Advantages1) Easy and compact.2) High sensitivity. → Measurements under

low pump intensity.3) Suitable for rough surfaces

or optically scattering samples.

0 10 20 30 40 50 60 70

0.0

0.2

0.4

0.6

0.8

1.0

Delay Time (ps)

LF

-HD

-TG

Sig

nal (

arb.

uni

ts)

LF-HD-TG response of nanostructured TiO2electrode without CdSe quantum dots.

0 10 20 30 40 50 60 70

0.0

0.2

0.4

0.6

0.8

1.0 5h 20h 44h 125h

LF-H

D-T

G S

igna

l (ar

b. u

nits

)

Delay Time (ps)

LF-HD-TG responses of nanostructured TiO2 electrodes B (TiO2: 27 nm) with CdSe quantum dots for different deposition times.

0 20 40 60 80 100

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Pump intensitySi

gnal

inte

nsit

y (a

rb.

unit

s)

Time (ps)

Pump intensity dependence of the LF-HD-TG response of nanostructured TiO2 electrode B (TiO2: 27 nm) deposited with CdSe quantum dots for 49h. The pump intensity is ranging from 2.5 to 16.5 µJ/pulse.

0 10 20 30 40 50 60 700.0

0.5

1.0

1.5

2.0

2.5

3.0

Sign

al in

tens

ity

(arb

. un

its)

Relative pump intensity (arb. units)

The pump intensity dependence of the maximum signal intensity for nanostructured TiO2 electrode B (TiO2: 27 nm) deposited with CdSe quantum dots for 49 h.

Analyses of LF-HD-TG response

21 /2

/10

ττ tt eAeAyy −− ++=

0 10 20 30 40 50 60 700.0

0.5

1.0

y0 0.11043 �}0.00715A1 1.00703 �}0.03463τ1 2.55526 �}0.12042A2 0.39276 �}0.00834τ2 27.48046 �}1.86301

cdse20h fit t ing

Tra

nsie

nt G

rati

ng S

igna

l

Delay Time (ps)

0 10 20 30 40 50 60 700.0

0.5

1.0 cdse20h fit t ing

Tra

nsie

nt G

rati

ng S

igna

l

Delay Time (ps)

0 10 20 30 40 50 60 700.0

0.5

1.0

Tra

nsie

nt G

rati

ng S

igna

l

Delay Time (ps)

Results of the Fitting Parameters

CdSedeposition time (h)

CdSesize (nm)

τ1(ps)

τ2(ps) A1 A2 y0

5 4.8 1.8 16 0.74 0.34 0.15

9 5.1 2.1 19 0.68 0.37 0.16

44 6.5 2.2 29 0.63 0.28 0.18

20 5.8 2.1 20 0.65 0.31 0.20

125 6.7 1.9 30 0.60 0.31 0.27

tr: trapped et: electron transferEg: energy gap

: excitation energynonradiativeradiative

υh

Eg υh

e-

h+

trtr

et

excitation

Energy

(1) Fast decay process (τ1 ): decrease of hole carrier numbers by trapping at the CdSe QD surface states;

(2) Slow decay process (τ2 ): photoexcited electron relaxation process, i.e. recombination and/or transfer to the TiO2 electrode;

(3) Base line (y0): thermal diffusion signal owing to thermal grating.

Summary

Three decay processes can be shown with decay time of 2 ps (τ1), 16-30 ps (τ2) and larger than a few hundreds ps, respectively.

Nanostructured TiO2 electrodes adsorbed with CdSe quantum dots by chemical deposition, have been characterized using LF-HD-TG measurements for the first time.

It is found that τ1 is independent of quantum dot size, but τ2 depends on quantum dot size.

AcknowledgementsD. Arae (now at Anritsu)M. Hayashi (now at CANON)I. Tsuboya (now at SONY)Y. KumagaiK. Katayama (University of Tokyo, now at MIT)T. Sawada(University of Tokyo, now at Tokyo University of Agriculture and Technology)

Financial supports:�A Grant-in Aid for Scientific Research (No. 14750645 and No. 15510098) and one on on Priority Area 417 (No. 15033224) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government.�The 21st Century COE program on “Coherent Optical Science”.