What is the coupling strength of the DQWs?

Since the coupling strength U eV is given by f f: nutation frequency , U are estimated as 4.0, 9.9, and 14.2 eV for DQWs with respective separations of 10, 6, and 3 nm.

How was the near-field PL spectra obtained?

The near-field photoluminescence NFPL spectra were obtained using a He–Cd laser=325 nm , collected with a fiber probe with an aperture diameter of 30 nm, and detected using a cooled chargecoupled device through a monochromator.

What is the corresponding NFPL for the two QWs?

No emission was observed from the exciton ground state of QW1 EA1 or the excited state of QW2 EB2 at a photon energy of 3.435 eV, indicating that the excited energy in QW1 was transferred to the excited state of QW2.

What is the average concentration of Mg in ZnO?

The authors believe that these peaks originated from the respective ZnO QWs because their energies are comparable to the predicted ZnO well-layer thicknesses of 1.7 IS , 3.4 I1D , and 2.2 I3D nm, respectively, calculated using the finite square-well potential of the quantum confinement effect in ZnO SQWs.10

What is the difference between the two states?

For room-temperature operation, since the spectral width reaches thermal energy 26 meV , a higher Mg concentration in the barrier layers and narrower Lw are required so that the spectral peaks of the first excited state E2 and ground state E1 do not overlap.

(Open Access) Nanophotonic switch using ZnO nanorod double-quantum-well structures (2007) | Takashi Yatsui

Q: What is the emission peak of QW2?

Since the excited state of QW2 is a dipole-forbidden level, the observed 3.435 eV emission indicates that the energy transfer from the ground state of QW1 to the excited state of QW2 was blocked by the excitation of the ground state of QW2.

Nanophotonic switch using ZnO nanorod double-quantum-well structures

Takashi Yatsui, Suguru Sangu, Tadashi Kawazoe, Motoichi Ohtsu, Sung Jin An, Jinkyoung Yoo, and Gyu-Chul

Citation: Applied Physics Letters 90, 223110 (2007); doi: 10.1063/1.2743949

View online: http://dx.doi.org/10.1063/1.2743949

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/90/22?ver=pdfcov

Published by the AIP Publishing

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Nanophotonic switch using ZnO nanorod double-quantum-well structures

Takashi Yatsui

a兲

SORST, Japan Science and Technology Agency, Bunkyo-ku, Tokyo 113-8656, Japan

Suguru Sangu

Advanced Technology R&D Center, Ricoh Co. Ltd., Yokohama, Kanagawa 224-0035, Japan

Tadashi Kawazoe and Motoichi Ohtsu

b兲

School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan

Sung Jin An, Jinkyoung Yoo, and Gyu-Chul Yi

National CRI Center for Semiconductor Nanorods, Pohang University of Science and Technology

(POSTECH), San 31 Hyoja-dong, Pohang, Gyeongbuk 790-784, Korea and Department of Materials Science

and Engineering, Pohang University of Science and Technology (POSTECH), San 31 Hyoja-dong,

Pohang, Gyeongbuk 790-784, Korea

共Received 12 March 2007; accepted 5 May 2007; published online 30 May 2007兲

The authors report on time-resolved near-ﬁeld spectroscopy of ZnO/ZnMgO nanorod

double-quantum-well structures 共DQWs兲 for a nanometer-scale photonic device. They observed

nutation of the population between the resonantly coupled exciton states of DQWs. Furthermore,

they demonstrated switching dynamics by controlling the exciton excitation in the dipole-inactive

state via an optical near ﬁeld. The results of time-resolved near-ﬁeld spectroscopy of isolated DQWs

described here are a promising step toward designing a nanometer-scale photonic switch and related

Systems of optically coupled quantum dots 共QDs兲

should be applicable to quantum information processing.

1,2

Additional functional devices 共i.e., nanophotonic devices

3–6

兲

can be realized by controlling the exciton excitation in QDs.

ZnO is a promising material for room-temperature operation,

owing to its large exciton binding energy

7–9

and recent

achievements in the fabrication of nanorod hetero-

structures.

10,11

This study used time-resolved near-ﬁeld spec-

troscopy to demonstrate the switching dynamics that result

from controlling the optical near-ﬁeld energy transfer in ZnO

nanorod double-quantum-well structures 共DQWs兲. We ob-

served nutation of the population between the resonantly

coupled exciton states of DQWs, where the coupling strength

of the near-ﬁeld interaction decreased exponentially as the

separation increased.

To evaluate the energy transfer, three samples were pre-

pared 关Fig. 1共a兲兴: 共1兲 single-quantum-well structures 共SQWs兲

with a well-layer thickness of L

=2.0 nm 共SQWs兲, 共2兲

DQWs with L

=3.5 nm with 6 nm separation 共1-DQWs兲,

and 共3兲 three pairs of DQWs with L

=2.0 nm with different

separations 共3, 6, and 10 nm兲, where each DQW was sepa-

rated by 30 nm 共3-DQWs兲. These thicknesses were deter-

mined by the transmission electron microscopy 共TEM兲 mea-

surement. ZnO / ZnMgO quantum-well structures 共QWs兲

were fabricated on the ends of ZnO nanorods with a mean

diameter of 80 nm using catalyst-free metal organic vapor

phase epitaxy.

The average concentration of Mg in the Zn-

MgO layers used in this study was determined to be

20 at. %.

The far-ﬁeld photoluminescence 共PL兲 spectra were ob-

tained using a He–Cd laser 共␭ =325 nm兲 before detection

using near-ﬁeld spectroscopy. The near-ﬁeld photolumines-

cence 共NFPL兲 spectra were obtained using a He–Cd laser

共␭ =325 nm兲, collected with a ﬁber probe with an aperture

diameter of 30 nm, and detected using a cooled charge-

coupled device through a monochromator. Blueshifted PL

peaks were observed at 3.499共I

兲, 3.429共I

兲, and

3.467 共I

兲 eV in the far- and near-ﬁeld PL spectra

关Fig. 2共a兲兴. We believe that these peaks originated from the

respective ZnO QWs because their energies are comparable

to the predicted ZnO well-layer thicknesses of 1.7共I

兲,

3.4共I

兲, and 2.2 共I

兲 nm, respectively, calculated using the

ﬁnite square-well potential of the quantum conﬁnement ef-

fect in ZnO SQWs.

To conﬁrm the near-ﬁeld energy trans-

fer between QWs, we compared the time-resolved near-ﬁeld

PL 共TR

NFPL

兲 signals at the I

, I

, and I

peaks. For the

time-resolved near-ﬁeld spectroscopy, the signal was col-

lected with a ﬁber probe with an aperture diameter of 30 nm

and detected using a microchannel plate through a bandpass

ﬁlter with 1 nm spectral width. Figure 2共b兲 shows the typical

NFPL

of SQWs 共TR

兲, 1-DQWs 共TR

兲, and 3-DQWs

共TR

兲, respectively, using the 4.025 eV 共␭= 308 nm兲 light

with a pulse of 10 ps duration to excite the barrier layers of

ZnO QWs.

We calculate the exciton dynamics using quantum me-

chanical density-matrix formalism,

12,13

␳

=−

ប

关H,

␳

兴 +

兺

␥

共2A

␳

†

− A

†

␳

−

␳

†

兲共1兲

共

␳

: density operator, H: Hamiltonian in the considered sys-

tem, A

†

and A

: creation and annihilation operators for an

exciton energy level labeled n, and

␥

: photon or phonon

relaxation constant兲. The exciton population is calculated us-

ing matrix elements for all exciton states in the system con-

sidered. First, we apply the calculation to a three-level sys-

tem of SQWs 关inset of Fig. 2共c兲兴, where the continuum state

ប⍀

is initially excited using a 10 ps laser pulse. Then, the

initial exciton population in ZnO QWs is created in ប⍀

where an incoherent Gaussian excitation term with a tempo-

a兲

Electronic mail: yatsui@ohtsu.jst.go.jp

b兲

Also at SORST, Japan Science and Technology Agency, 2-11-16 Yayoi,

Bunkyo-ku, Tokyo 113-8656, Japan.

APPLIED PHYSICS LETTERS 90, 223110 共2007兲

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141.223.173.110 On: Tue, 21 Apr 2015 11:29:56

ral width of 2

␴

is added in Eq. 共1兲 because nonradiative

relaxation paths via exciton-phonon coupling make a

dephased input signal, statistically. Finally, an exciton carrier

relaxes due to the electron-hole recombination with relax-

ation constant

␥

. Figure 2共c兲 shows a numerical result and

experimental data. Here, we used 2

␴

=100 ps, and

␥

was

evaluated as 460 ps.

A similar calculation was applied for DQWs. We used

two three-level systems, coupled via an optical near ﬁeld

with a coupling strength of U

关inset of Fig. 2共d兲兴. Figure

2共d兲 shows the numerical results for the exciton population

in QWs

and the experimental data. Here 2

␴

and 2

␴

were set at 200 ps, which is twice the value for SQWs, be-

cause the relaxation paths extend the barrier energy state in

the two quantum well 共QW兲.

␥

and

␥

are evaluated as

200 ps. We believe that the faster relaxation for DQWs com-

pared with SQWs reﬂects the lifetime of the coupled states

mediated by the optical near-ﬁeld. Furthermore, the charac-

teristic behavior that results from near-ﬁeld coupling appears

as the oscillatory decay in Fig. 2共d兲. This indicates that the

time scale of the near-ﬁeld coupling is shorter than the de-

coherence time, and that coherent coupled states, such as

symmetric and antisymmetric states,

determine the system

dynamics. Furthermore, nutation never appears unless unbal-

anced initial exciton populations are prepared for ប⍀

and

ប⍀

. In the far-ﬁeld excitation, only the symmetric state is

excited because the antisymmetric state is dipole inactive. By

contrast, in the near-ﬁeld excitation, both the symmetric and

antisymmetric states are excited due to the presence of a

near-ﬁeld probe. Since the symmetric and antisymmetric

states have different eigenenergies, the interference of these

states generates a detectable beat signal. The unbalanced ex-

citation rate is given by A

=10 here. From the period of

nutation, the strength of the near-ﬁeld coupling is estimated

to be U

=7.7 ns

−1

共=4.9

␮

eV兲.

We evaluated nutation frequencies using Fourier analy-

sis. In Fig. 3共a兲, the power spectral density of SQWs 共PS

兲

does not exhibit any peaks, indicating a monotonic decrease.

By contrast, the power spectral density of 1-DQWs 共PS

兲

had a strong peak at a frequency of 2.6 ns

−1

. Furthermore,

that of 3-DQWs 共PS

兲 had three peaks at 1.9, 4.7, and

7.1 ns

−1

. Since, the degree of the coupling strength, which is

proportional to the frequency of the nutation, increases as the

separation decreases, the three peaks correspond to the sig-

nals from DQWs with separations of 10, 6, and 3 nm, re-

spectively. Since the coupling strength បU 共eV兲 is given by

ប

␲

f 共f: nutation frequency兲, បU are estimated as 4.0, 9.9,

and 14.2

␮

eV for DQWs with respective separations of 10,

6, and 3 nm. Furthermore, the peak intensity for the DQWs

with 3 nm separation is much lower than for those with

10 nm separation, which might be caused by decoherence of

the exciton state due to penetration of the electronic carrier.

Considering the carrier penetration depth, the strong peak of

DQWs with 10 nm separation originates from the near-ﬁeld

coupling alone. The solid line in Fig. 3共b兲 shows the separa-

tion dependence of the peak frequency. The exponentially

FIG. 1. ZnO / ZnMgO nanorod quantum-well structures. c: c axis of the ZnO

stem. 共a兲 Schematics of ZnO/ ZnMgO SQWs, DQWs 共1-DQWs兲, and triple

pairs of DQWs 共3-DQWs兲. 共b兲 Z-contrast TEM image of 3-DQWs clearly

shows the compositional variation, with the bright layers representing the

ZnO well layers. Scale bar: 50 nm.

FIG. 2. 共Color online兲 Near-ﬁeld time-resolved spectroscopy of ZnO nano-

rod DQWs at 15 K. 共a兲 NF

,NF

, and NF

: near-ﬁeld PL spectra. FF

, and FF

: far-ﬁeld PL spectra of ZnO SQWs

共Lw= 2.0 nm兲, 1-DQWs 共Lw= 3.5 nm, 6 nm separation兲, and 3-DQWs

共Lw= 2.0 nm and 3, 6, and 10 nm separation兲. 共b兲 TR

,TR

, and TR

show TR

NFPL

signal obtained at I

, and I

. Theoretical results on the

transient exciton population dynamics 共solid curves兲 and experimental PL

data 共ﬁlled squares兲 of 共c兲 SQWs 关same as curve TR

in 共b兲兴 and 共d兲

1-DQWs 关same as curve TR

in 共b兲兴. The insets schematically depict the

respective system conﬁgurations. ប⍀

: barrier energy state with a central

energy.

FIG. 3. 共Color online兲 Evaluation of the nutation frequencies between the

QWs. 共a兲 PS

,PS

, and PS

show the power spectra of TR

,TR

, and

, respectively. 共b兲 Separation D dependence of frequency of the

nutation.

223110-2 Yatsui et al. Appl. Phys. Lett. 90, 223110 共2007兲

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141.223.173.110 On: Tue, 21 Apr 2015 11:29:56

decaying dependence represented by this line supports the

origin of the peaks in the power spectra from the localized

near-ﬁeld interaction between the QWs.

Next, we performed the switching operation. Figures

4共a兲 and 4共b兲 explain the “off” and “on” states of the pro-

posed nanophotonic switch, consisting of two coupled QWs.

and QW

are used as the input/output and control ports

of the switch, respectively. Assuming L

=3.2 and 3.8 nm,

the ground exciton state in QW

and the ﬁrst excited state in

resonate. In the off operation 关Fig. 4共a兲兴, all the exciton

energy in QW

is transferred to the excited state in the neigh-

boring QW

and relaxes rapidly to the ground state. Conse-

quently, no output signals are generated from QW

. In the on

operation 关Fig. 4共b兲兴, the escape route to QW

is blocked by

the excitation of QW

, owing to state ﬁlling in QW

on ap-

plying the control signal; therefore, an output signal is gen-

erated from QW

Figure 4共c兲 shows the NFPL for the three pairs of DQWs

with L

=3.2 and 3.8 nm with different separations 共3, 6, and

10 nm兲. Curve NF

off

was obtained with continuous input

light illumination from a He–Cd laser 共3.814 eV兲. No emis-

sion was observed from the exciton ground state of QW

共EA

兲 or the excited state of QW

共EB

兲 at a photon energy

of 3.435 eV, indicating that the excited energy in QW

was

transferred to the excited state of QW

. Furthermore, the

excited state of QW

is a dipole-forbidden level. Curve

control

shows the NFPL signal obtained with control light

excitation of 3.425 eV with a 10 ps pulse. Emission from the

ground state of QW

at a photon energy of 3.425 eV was

observed. Both input and control light excitations resulted in

an output signal with an emission peak at 3.435 eV, in addi-

tion to the emission peak at 3.425 eV 共curve NF

兲, which

corresponds to the ground state of QW

. Since the excited

state of QW

is a dipole-forbidden level, the observed

3.435 eV emission indicates that the energy transfer from the

ground state of QW

to the excited state of QW

was blocked

by the excitation of the ground state of QW

Finally, the dynamic properties of the nanophotonic

switching were evaluated by using the time correlation single

photon counting method. We observed TR

NFPL

signals using

a ﬁber probe with an aperture diameter of 30 nm at 3.435 eV

with both input and control laser excitations 关see Fig. 4共d兲兴.

The decay time constant was found to be 483 ps. The output

signal increased synchronously, within 100 ps, with the con-

trol pulse. Since the rise time is considered equal to one-

quarter of the nutation period

␶

the value agrees with those

obtained for DQWs with the same well width in the range

from

␶

/4=36 ps 共3 nm separation兲 to

␶

/4 =125 ps 共10 nm

separation兲.

We observed the nutation between DQWs and demon-

strated the switching dynamics by controlling the exciton

excitation in the QWs. For room-temperature operation,

since the spectral width reaches thermal energy 共26 meV兲,a

higher Mg concentration in the barrier layers and narrower

are required so that the spectral peaks of the ﬁrst excited

state 共E

兲 and ground state 共E

兲 do not overlap. This can be

achieved by using two QWs with L

=1.5 nm 共QW

兲 and

2nm 共QW

兲 with a Mg concentration of 50%, where the

energy difference between E

and E

in QW

is 50 meV.

The work at POSTECH was supported by the National

Creative Research Initiative Project, Korea and AOARD

04-49 共Quotation No. FA5209-040T0254兲.

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FIG. 4. 共Color online兲 Switching operation by controlling the exciton exci-

tation. Schematic of the nanophotonic switch of 共a兲 “off” state and 共b兲 “on”

state. 共c兲 NF

,NF

control

, and NF

off

show NFPL signal obtained with the

illumination of input laser along, control laser alone, and input and control

laser, respectively. 共d兲 Near-ﬁeld time-resolved PL signal with on state.

223110-3 Yatsui et al. Appl. Phys. Lett. 90, 223110 共2007兲

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141.223.173.110 On: Tue, 21 Apr 2015 11:29:56

Nanophotonic switch using ZnO nanorod double-quantum-well structures

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Frequently Asked Questions (15)

Q1. What is the name of the paper?

Q2. What is the coupling strength of the DQWs?

Q3. What is the effect of the interference of the symmetric and antisymmetric states?

Q4. What is the degree of the coupling strength?

Q5. How was the near-field PL spectra obtained?

Q6. How did the authors control the exciton excitation in the dipole-inactive state?

Q7. What is the emission peak of QW2?

Q8. What is the emission peak of the ground state of QW2?

Q9. What is the effect of the symmetric state on the spectral density of the DQ?

Q10. What is the corresponding NFPL for the two QWs?

Q11. What is the time scale of the near field coupling?

Q12. What is the average concentration of Mg in ZnO?

Q13. How many ps are used for the relaxation of exciton populations?

Q14. How did the authors observe the emission from the ground state of QW2?

Q15. What is the difference between the two states?