CHIN. PHYS. LETT. Vol. 32, No. 10 (2015) 107804
Au Microdisk-Size Dependence of Quantum Dot Emission from the Hybrid
Metal-Distributed Bragg Reflector Structures Employed for Single Photon
Sources *
WANG Hai-Yan(王海艳)1 , SU Dan(苏丹)1 , YANG Shuang(杨爽)1 , DOU Xiu-Ming(窦秀明)1 ,
ZHU Hai-Jun(朱海军)1 , JIANG De-Sheng(江德生)1 , NI Hai-Qiao(倪海桥)1 , NIU Zhi-Chuan(牛智川)1 ,
ZHAO Cui-Lan(赵翠兰)2 , SUN Bao-Quan(孙宝权)1**
1
State Key Laboratory for Superlattices and Microstructure, Institute of Semiconductors,
Chinese Academy of Sciences, Beijing 100083
2
College of Physics and Electronic Information, Inner Mongolia University for Nationalities, Tongliao 028043
(Received 9 July 2015)
We investigate metallic microdisk-size dependence of quantum dot (QD) spontaneous emission rate and microantenna directional emission effect for the hybrid metal-distributed Bragg reflector structures based on a particular
single QD emission. It is found that the measured photoluminescence (PL) intensity is very sensitive to the size
of metallic disk, showing an enhancement factor of 11 when the optimal disk diameter is 2 𝜇m and the numerical
aperture of microscope objective NA=0.5. It is found that for large metal disks, the Purcell effect is dominant
for enhanced PL intensity, whereas for small size disks the main contribution comes from plasmon scattering at
the disk edge within the light cone collected by the microscope objective.
PACS: 78.67.Hc, 42.50.Pq, 73.20.Mf
DOI: 10.1088/0256-307X/32/10/107804
Single semiconductor quantum dots (QDs) have
been considered as promising solid-state single photon sources. To obtain bright quantum sources, the
key issue is to enhance extraction efficiency of the
QD emission, which is challenging since QDs normally emit isotropically in a high refractive index
material. Several approaches have been proposed to
extract single photons by using micro-pills,[1,2] photonic crystals,[3] and metal nanostructures.[4−8] Recently, confined Tamm plasmon (TP) modes were proposed to accelerate the spontaneous emission rate and
to achieve a high collection efficiency, in which the
Fabry–Perot cavity is formed by depositing a micronsized metallic disk on QD with planar distributed
Bragg reflector (DBR) structures.[9−14] The enhancement of QD spontaneous emission rate as well as
a high directionality of emission were reported due
to the resonant coupling of the QD emission to the
confined TP modes.[9,10,12] Their calculations demonstrated that the maximum Purcell factor corresponds
to the microdisk diameter between 2 and 3 µm.[9,12]
To achieve effective coupling between the QD emission
and the optical cavity mode, or the plasmonic mode, it
is necessary to address the QDs in cavity or metallic
nanostructures by using atomic force microscopy,[15]
cryogenic laser photolithography,[16] and fluorescence
positioning methods.[17,18]
In this Letter, we investigate the metallic
microdisk-size dependence of spontaneous emission
rate and extraction efficiency using a single QD coupled to a TP mode, based on the techniques of opti-
cal positioning and single QD emission detection. It
is found that the measured photoluminescence (PL)
intensity is very sensitive to the metallic size, showing that an optimal disk with a diameter of 2 µm
corresponds to an 11-fold enhancement of QD emission, whereas the lifetime is independent of the metallic size, and always equals to a half in comparison
with the same QD without metallic microdisk. This
demonstrates that the Purcell factor is 2 and the extraction efficiency increases up to 5.5 times due to
the existence of a cavity based on the hybrid metalDBR structure, which accelerates QD emission rate as
well as enhances QD directional emission from a metal
micro-antenna. A dipolar antenna model can be used
to simulate the experimental data satisfactorily.
The studied InAs QD samples were grown by using the molecular beam epitaxy on a (001) GaAs
substrate. The studied sample consists of, in sequence, a 200 nm GaAs buffer layer, a 20-period
GaAs/Al0.9 Ga0.1 As DBR, a layer of 130 nm GaAs, a
QD layer, and a 130 nm GaAs cap layer. During the
PL measurements the optical method is used to locate
the position of the QDs with various sizes of metallic
microdisk coated above the QDs. The detailed methods can be found elsewhere in our previous work.[18]
To study the metallic microdisk-size dependence of
single photon emission, a typical QD was first chosen
from the sample and then its emission with certain
disk size was measured following the procedures: (1) A
Au microdisk was deposited on the sample surface by
using the electron beam lithography and a lift-off tech-
* Supported by the National Key Basic Research Program of China under Grant No 2013CB922304, and the National Natural
Science Foundation of China under Grant Nos 11474275 and 11464034.
** Corresponding author. Email: [email protected]
© 2015 Chinese Physical Society and IOP Publishing Ltd
107804-1
CHIN. PHYS. LETT. Vol. 32, No. 10 (2015) 107804
curves are shown in Fig. 1(b). It is found that the QD
PL lifetimes are 1.1 and 0.61 ns for the QD without
and with Au disk, respectively. The decrease of QD
lifetime is attributed to the Purcell effect owing to the
formation of a cavity between the Au disk and DBR,
and the cavity mode is resonant with the QD emission
wavelength. This explanation is further examined by
calculating reflective spectra for both the hybrid Au
disk-DBR structure (red line) and DBR alone without
Au disk layer (black line) as a function of wavelength
as shown in Fig. 1(c). The result indicates that the
QD emission wavelength of 906.2 nm is very close to
the cavity mode of 907 nm in the former case.
(a)
1100
1000
900
800
700
600
500
2
3
4
Delay time (ns)
5
6
(b)
With Au disks
Without Au
0
5
10
15
Diameter (mm)
Normalized PL
intensity
PL intensity
(arb. units)
2 mm
5 mm
10 mm
20 mm
1
With 20 m Au
Without Au
With 20 m Au
Without Au
Reflectivity
(2)
( )
10-1
0
Without Au
With Au
Fig. 1. (Color online) (a) PL spectra of a single quantum dot without (black) and with (red) an Au disk. (b)
The corresponding temporal dependences. (c) Calculated
reflectance spectra for the hybrid Au disk-DBR structure
(red line) and DBR alone without the Au disk layer (black
line).
Without Au
With 1 mm Au
10-2
100
0.9
m
m
0.6
10-1
0.3
(a)
10-2 (b)
0.0
904 905 906 907 908
0 1 2 3 4 5 6
Wavelength (nm)
Delay times (ns)
1.2 (c)
1.0
0.8
0.6
0.4
0.2
907 nm
0.0
0.6 0.7 0.8 0.9 1.0 1.1 1.2
Wavelength (mm)
Correlation function
intensity
100
Lifetime (ps)
Normalized PL
nique, in which the QD position is 130 nm below the
center of a metal Au disk; (2) optical measurements
were carried out; (3) the Au microdisk was etched by
using corrosive liquid (KI:I2 :H2 O=4:1:100); (4) steps
(1)–(3) were repeated for different disk diameters of
𝜑 = 1, 2, 5, 10, and 20 µm with the same disk thickness of 36 nm. This technique enables us to monitor
the same QD emission under the Au microdisk with
different sizes.
The QD sample was mounted in a cryostat cooled
down to 5 K, excited by illumination of a 640 nm diode
laser (cw or pulsed with a repetition frequency of
80 MHz and a pulse width of 63 ps). The excitation
laser beam was focused to an approximately 2 µm
spot on the sample by using a microscope objective
(NA=0.5) which was mounted on the NanoCube 𝑋𝑌 𝑍
piezo nanopositioning stage with a scanning range of
100 × 100 × 100 µm3 and a positioning resolution of
2 nm. The PL emission was collected by using the
same objective and measured by using a 0.5 m focal
length monochromator equipped with a silicon chargecoupled device (CCD). The PL decay measurements
were performed by using silicon avalanche photodiode (APD) and a time-correlated single-photon counting (TCSPC) board. To assess the actual excitation
power that reaches the QDs and to compare the PL
intensities as a function of excitation power, the reflection power from the sample with and without Au disk
has been subtracted from the pumped laser power.
Figure 1(a) presents the PL spectra of a single QD
with and without the 𝜑 = 20 µm Au disk, showing an
increase of the emission intensity of about 2 times for
QD with the Au disk. The corresponding PL decay
20
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
(c)
Without Au
0.12
-40 -20
0
20
Delay time (ns)
(d)
40
With 5 mm Au
0.1
-40 -20
0
20
Delay time (ns)
40
Fig. 2. (Color online) (a) Time-resolved PL spectra obtained for the Au disk of different sizes (𝜑 = 1–20 µm). (b)
The obtained lifetime versus the Au disk size, indicating that the lifetime with a Au disk is distributed over a range
of 550–650 ps, whereas its lifetime is 1100 ps for the QD emission without a Au disk. (c) and (d) Second-order
correlation function 𝑔 2 (𝜏 ) measurements for the QD emission without and with the 5 µm Au disk, respectively.
To investigate the influence of Au disk-size dependence on the QD spontaneous emission rate, the disks
of diameters 𝜑 = 1, 2, 5, 10 and 20 µm and with a
fixed thickness of 36 nm are deposited on the surface
of the QD sample. Figure 2(a) presents their PL decay
curves. The obtained lifetime versus disk diameter is
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CHIN. PHYS. LETT. Vol. 32, No. 10 (2015) 107804
summarized in Fig. 2(b), displaying that the lifetime
remains to be approximately a half in comparison with
the QD without a Au disk. As a result, the Purcell factor (∼1.8) calculated by using a ratio Γ R /Γ0 between
the radiative decay rates of QD with and without a
Au disk, ΓR and Γ0 , respectively, is independent of
the metal disk size. This conclusion is not consistent
with those reported in Refs. [9,12], where they demonstrated an existence of maximum Purcell factor corresponding to a disk diameter between 2 and 3 µm.
Note that the Purcell factor is proportional to the ratio of 𝑄/𝑉 , where 𝑄 and 𝑉 are the quality factor of
resonant mode in a cavity and the actual volume occupied by a given mode. It was reported that 𝑄-factor
decreases with reducing the disk diameter due to the
optical field coupling to the leaky modes at the disk
edge.[9,12] For small disks, it also leads to the appearance of discrete TP modes corresponding to a small
spatial distribution volume. Therefore, an unchangeable Purcell factor versus the disk size as observed in
our experiment may be due to the small spatial distribution mode volume for small disks. The secondorder correlation function 𝑔 2 (𝜏 ) measurements reveal
that no matter whether there is a metal disk or not
the QD always has a good single-photon property as
shown in Figs. 2(c) and 2(d) with 𝑔 2 (0) ∼ 0.1.
7
(a)
s)
5
Counts (10
(b)
(a)
Microscope
objective
sin2(θ)
(b)
6
θ
Without Au disk
With Au disk
Fitting data
5
4
2d
θ
Dipole radiation
↼π↽-1D↼θ↽ ∝ sin2↼θ↽
1 mm
Au disk
φ
d=(1/2)Tdepth of focus
2 mm
3
5 mm
10 mm
2
20 mm
Without
Au disk
1
0
0
decrease. As shown in Fig. 3(b), the maximum PL
intensity at saturation excitation power versus disk
diameter is plotted. It is found that the maximum
photon emitting rate is approximately 5.88 × 104 and
1.01 × 105 cps for QD without and with the 20 µm
metal disk, respectively. The enhancement factor is
1.7, which is very close to the Purcell factor of 1.8.
For the case of 2 µm Au diameter, the maximum photon emitting rate is 6.41 × 105 cps corresponding to an
enhancement factor of 11. By deducting the contribution to the cavity-induced increase of emission rate
from the total enhancement factor, a 5.6-fold increase
of PL intensity is attributed to the directional emission
of the metal antenna. As a result, the total enhancement factor can be written as (ΓR /Γ0 )𝐷R (𝜃)/𝐷0 (𝜃),
where 𝐷R (𝜃) and 𝐷0 (𝜃) correspond to the emission
towards a given 𝜃 direction for QD with and without the Au antenna, respectively. Actually, for the
dipole radiation, 𝐷R (𝜃) is proportional to a function
of sin2 (𝜃).[20]
10
20
30
40
50
60 0
Excitation power (mW)
5
10
15
Diameter (mm)
20
Fig. 3. (Color online) (a) PL emission intensity as a function of the excitation power for the QD with different Au
disk sizes. (b) The maximum PL intensity at the saturation excitation power versus disk diameter, where the
red line is connected between the fitting points calculated
based on the dipole radiation model as a guide for the
eyes.
Note that the QD emits isotropically in a high refractive index material matrix, which limits the extraction efficiency of the QD emission. Metal nanostructures can be considered as a very efficient directional plasmonic nanopatch dipole antenna.[14,19] To
inspect the influence of the metal disk on the extraction efficiency of the QD emission, PL intensity was
measured as a function of the excitation power, as
shown in Fig. 3(a), which displays an increase of the
QD emission intensity with reducing the Au diameter from 20 to 2 µm. For the QD with the 𝜑 = 1 µm
Au disk, however, its emission intensity becomes to
Fig. 4. (Color online) (a) Schematic illustration of the
microscope objective lens and QD sample with a Au disk
used in PL measurements. Here 2𝑑 is the depth of focus of
objective and 𝜃 is a projection angle of the light cone with
respect to the sample surface. This 𝜃 angle corresponds
to the radiation direction (emission can be collected by
objective) of the dipole antenna defined by the edge of the
Au disk as shown in (b).
For small Au disks, QD emission will excite localized surface plasmons at the disk edge, and plasmons will scatter from the edge, like the dipole radiations from the nanoantennas.[19] Thus the dipole
number 𝑁 for a diameter 𝜑 (𝑅 = 𝜑/2) is proportional to the circumference of 𝜋𝜑. A summation of 𝑁
dipole radiations collected by microscope objective is
responsible for the measured PL intensity, as schematically shown in Fig. 4(a), where the yellow disk represents the Au disk and 𝑑 is half the depth of focus
of objective. For surface dipole antennas, a radiation pattern of 𝐷R (𝜃) shown in Fig. 4(b) is proportional to the function of sin2 (𝜃). As the geometry
relationship shown in Fig. 4(a), we can obtain that
sin2 (𝜃) = 𝑑2 /(𝑑2 + 𝑅2 ) for each dipole radiation antenna. Thus 𝑁 dipole antennas will produce radiation
intensity 𝐼𝑁 ∝ 𝑁 sin2 (𝜃) = (𝜋𝜑)(𝑑2 /(𝑑2 + 𝜑2 /4)). Us-
107804-3
CHIN. PHYS. LETT. Vol. 32, No. 10 (2015) 107804
ing an expression of (ΓR /Γ0 )(𝜋𝜑)(𝑑2 /(𝑑2 + 𝜑2 /4)), we
can fit the experimental data as a function of the diameter 𝜑, and the obtained result is indicated by the
red curve shown in Fig. 3(b). The obtained fitting parameter 𝑑 is 1 µm, implying that the depth of focus of
objective corresponds to 2 µm, which is well consistent
with the parameter of the used microscope objective
of NA=0.5.
In conclusion, we have obtained the experimental
evidence for the existence of cavity in hybrid Au diskDBR structures which can accelerate QD spontaneous
emission rate. The Au disk can play a role of microantenna for directional emission. It results in an enhancement factor of 11 for an optimal Au diameter of
2 µm. For large metal disks, the Purcell effect is dominant for the enhancement of PL intensity, whereas
for small disks the main contribution comes from the
plasmon scattering at the disk edge within the light
cone collected by a microscope objective. We anticipate that such experimental results will help to design
a cavity based on the metal disk-DBR structures for
bright single photon sources.
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