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Organic photonic bandgap microcavities doped with semiconductor
nanocrystals for room-temperature on-demand single-photon sources
Svetlana G. Lukishova a; Luke J. Bissell a; Vinod M. Menon b; Nikesh Valappil b; Megan A. Hahn c; Chris M.
Evans c; Brandon Zimmerman a; Todd D. Krauss c; C. R. Stroud Jr a; Robert W. Boyd a
a The Institute of Optics, University of Rochester, Rochester, NY, USA b Department of Physics, Queens
College-CUNY, Flushing, NY, USA c Department of Chemistry, University of Rochester, Rochester, NY, USA

First Published on: 23 January 2009

To cite this Article Lukishova, Svetlana G., Bissell, Luke J., Menon, Vinod M., Valappil, Nikesh, Hahn, Megan A., Evans, Chris M.,
Zimmerman, Brandon, Krauss, Todd D., Stroud Jr, C. R. and Boyd, Robert W.(2009)'Organic photonic bandgap microcavities doped
with semiconductor nanocrystals for room-temperature on-demand single-photon sources',Journal of Modern Optics,

To link to this Article: DOI: 10.1080/09500340802410106

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Journal of Modern Optics
2009, 1–8, iFirst

Organic photonic bandgap microcavities doped with semiconductor nanocrystals for

room-temperature on-demand single-photon sources

Svetlana G. Lukishova
a*, Luke J. Bissell

a
, Vinod M. Menon

b
, Nikesh Valappil

b
, Megan A. Hahn

c
,

Chris M. Evans
c
, Brandon Zimmerman

a
, Todd D. Krauss

c
, C.R. Stroud Jr

a
and Robert W. Boyd

a

a
The Institute of Optics, University of Rochester, Rochester, NY USA;

b
Department of Physics, Queens College – CUNY,

Flushing, NY USA;
c
Department of Chemistry, University of Rochester, Rochester, NY USA

(Received 7 February 2008; final version received 14 August 2008)

We report the first experimental observation of fluorescence from single semiconductor nanocrystals (colloidal
quantum dots) in microcavities. In these room-temperature experiments we observed photon antibunching from
single CdSe nanocrystals doped into a chiral one-dimensional photonic bandgap liquid-crystal microcavity.
The chirality resulted in high-purity, circular polarization of definite handedness of the emitted single photons.
We also report the fabrication of chiral microcavities for telecom wavelengths doped with PbSe nanocrystals as
well as a solution-processed-polymer microcavity with a defect layer doped with CdSe nanocrystals between two
distributed Bragg reflectors. These systems with their low host fluorescence background are attractive for
on-demand single-photon sources for quantum information and communication.

Keywords: single-photon sources; colloidal quantum dots; microcavities

1. Introduction

The development of on-demand single-photon sources

(SPSs) with photons exhibiting antibunching has

recently been of significant interest for their applica-

tions in quantum cryptography [1–4]. A desirable
feature for a SPS is photon polarization, since in the

case of single photons with definite polarization, the

quantum cryptography system’s efficiency will be twice

that of an unpolarized SPS.
Room-temperature SPSs based on colloidal semi-

conductor quantum-dot (QD) fluorescence [5,6] are

very promising because of higher QD photostability at

room temperature than that of conventional dyes, and

relatively high quantum yield (up to�100%). Recently,
electrically driven light emission from a single colloidal

QD at room temperature was obtained [7], opening

up the possibility for electrical pumping of an SPS on

demand, based on colloidal QDs. At the same time,

colloidal QDs can be dissolved/dispersed in various
hosts, e.g. photonic crystal microcavities, from solution.

Placed into a microcavity environment, the QD

spontaneous emission rate can be enhanced via the

Purcell effect [4]. The cavity geometry can also influence
QD polarization.

In spite of the well-developed microcavity fabrica-

tion techniques [8], and wide use of these techniques

for heterostructured QDs operating at cryogenic

temperatures, e.g. [4,9], there are only few reports of

doping colloidal QDs into microcavities. For instance,

Poitras et al. [10] demonstrated spontaneous emission

enhancement by a factor of 2.7 from CdSe QDs

embedded in a half-wavelength one dimensional cavity

sandwiched between two distributed Bragg reflectors

(DBR). The DBRs were prepared using sputter

deposition of TiO2-SiO2 quarter-wavelength thick

layers. Kahl et al. [11] started with the same approach,

but also used a focused ion beam to etch micropillar

microcavities of both round and elliptical cross-section

from a planar cavity. Martiradonna et al. [12]

combined the DBR technique prepared by e-beam

evaporation of TiO2-SiO2 layers with imprint litho-

graphy of a defect layer. Lodahl et al. [13] embedded

CdSe QDs into a titania inverse opal photonic crystal

and observed both inhibition and enhancement of

decay rates. Fushman et al. [14] mapped cavity

resonances of PbS QDs in an AlGaAs membrane

using QD fluorescence. Wu et al. [15] achieved efficient

coupling of 1.5-mm emission from PbSe QDs to
a Si-based photonic-crystal-membrane microcavity

with a Purcell factor of 35. In [16], Bose et al. reported

weak coupling of PbS QDs with the same Si-based

microcavity. In addition, Hoogland et al. [17] reported

PbS QD 1.5-mm lasing of whispering gallery modes
in a microcapillary resonator, and Barrelet et al. [18]

studied CdS QDs in nanowire one-dimensional photo-

nic crystal structures.

*Corresponding author. Email: sluk@lle.rochester.edu

ISSN 0950–0340 print/ISSN 1362–3044 online

� 2009 Taylor & Francis
DOI: 10.1080/09500340802410106

http://www.informaworld.com

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It should be noted that no data exist in the
literature on the fluorescence of room-temperature,
single colloidal QDs embedded in the microcavity
environment, nor on preserving fluorescence anti-
bunching of single colloidal QDs in the cavity
materials. All results on SPSs based on QDs in
microcavities, see, e.g. [4,9], are reported for hetero-
structured QDs at cryogenic temperatures.

We describe here the results of doping single
colloidal semiconductor QDs in two types of organic
1-D photonic bandgap microcavities which permit us
to use single colloidal QDs directly from a solution:
(1) chiral microcavity made of monomeric and/or
oligomeric planar-aligned cholesteric liquid crystals
(CLCs) [19–21] and (2) polymer microcavity with
a defect layer between two DBRs [22–23]. These two
types of microcavity are robust and simple in
fabrication.

We report the first observation of single-emitter
circularly polarized fluorescence of definite handed-
ness. The polarization selectivity is produced by
a chiral microcavity. We also observed fluorescence
antibunching from a colloidal CdSe QD in this
microcavity. Earlier, we achieved both fluorescence
antibunching [19–21] and definite linear polarization of
single dye molecule fluorescence at room temperature
in planar aligned nematic liquid crystal (LC) hosts.
This method was applied for anisotropic dye molecules
which can be aligned by rod-like nematic LC molecules
[21,24]. A chiral microcavity provides circular polar-
ized fluorescence for emitters even without dipole
moments. Such a microcavity can be prepared from
CLCs for any optical wavelength. We also present here
chiral microcavities for PbSe QDs with a fluorescence
maximum at 1.5 mm prepared both from monomeric

(fluid-like) and oligomeric glassy CLCs. By imaging
the fluorescence of QDs in a 1.5-mm microcavity as
well as in a polymer DBR microcavity with a defect
layer we also demonstrate their potential for SPS
device applications.

Section 2 of this paper describes the experimental
setup for fluorescence imaging, antibunching and
polarization measurements. Section 3 is devoted to
chiral microcavities made of CLCs doped with single
colloidal CdSe, CdSeTe or PbSe QDs. It contains
details on sample preparation, photonic-bandgap
transmission curves matched with QD fluorescence at
580 nm, 700 nm and 1.5 mm, circular polarized fluores-
cence, and antibunching measurements. Section 4
describes the preparation of a polymer microcavity
with a defect layer doped with single CdSe QDs
between the DBRs and single-QD fluorescence ima-
ging in this structure.

2. Experimental setup

The experimental setup consists of a home-built
confocal fluorescence microscope based on a Nikon
TE2000-U inverted microscope with several output
ports. Figure 1 shows the abbreviated schematics of
our experiment for fluorescence imaging and anti-
bunching measurements (a) and polarization and
spectral measurement (b).

We excite our samples with 76 MHz repetition-rate,
6 ps pulse duration, 532-nm light from a Lynx mode-
locked laser (Time-Bandwidth Products Inc.). To
obtain a diffraction-limited spot on the sample, the
excitation beam is expanded and collimated by
a telescopic system with a spatial filter. The samples
are placed in the focal plane of a 1.3-numerical

A
P

D
 1

APD 2

50/50 BS

Dichroic
mirror

Objective
lens

Single emitters 
in host

(a) (b)

ND filters 

Filters

Microscope
cover slips

Pump

Variable
 delay

TimeHarp 200

PC Dichroic
mirror

Objective
lens

Single emitters 
in host

ND filters 

Filters

Microscope
cover slips

Pump

λ/4

Spectrometer

PC

Coupling lens

Fiber

Glan 
Thompson
Polarizer

Figure 1. Schematics of experimental setup for fluorescence imaging and antibunching measurements (a); spectral and
polarization measurements. (b) We used the following abbreviations: neutral density (ND); single-photon counting avalanche
photodiode module (APD); beamsplitter (BS). (The color version of this figure is included in the online version of the journal.)

2 S.G. Lukishova et al.

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aperture, oil-immersion microscope objective used in
confocal reflection mode. In focus, the intensities used
are of the order of several kW cm

�2
. Residual

transmitted excitation light is removed by a dichroic
mirror and a combination of two interference filters
yielding a combined rejection of nine orders of
magnitude at 532 nm. The sample’s holder is attached
to a piezoelectric, XY translation stage providing
a raster scan of the sample through an area up to
50 mm�50 mm.

The following diagnostics are placed in the separate
output ports:

(1) A Hanbury Brown Twiss interferometer con-
sisting of a 50/50 beamsplitter cube and two
cooled, Si single photon counting avalanche
photodiode modules (APDs) SPCM AQR-14
(Perkin Elmer). The time interval between two
consecutively detected photons in separate
arms is measured by a TimeHarp 200 time
correlated single-photon counting card using
a conventional start-stop protocol.

(2) Electron multiplying, cooled CCD-camera
iXon DV 887 ECS-BV (Andor Technologies).

(3) Fiber-optical spectrometer (Ocean Optics). The
method of defining the dissymmetry of circular
polarization is described in [25]. For circular-
polarization measurements both an achromatic
quarter waveplate and a Glan Thompson linear
polarizer on rotating mounts are placed in the
spectrometer port in front of the spectrometer.
An area with several single QDs is selected by
an EM-CCD camera and/or APD-detectors
with a raster-scan of the sample. It is imaged to
the spectrometer input fiber. The spectra are
recorded with a 6-s accumulation time with
background subtraction.

CdSe/ZnS core/shell QDs were obtained commercially
from Evident Technologies (maximum fluorescence
wavelength �o¼620 nm) or were synthesized according
to published methods (�o¼580 nm) [26,27]. CdSeTe
QDs (�o¼700 nm) were obtained commercially from
Invitrogen. PbSe/C18H34O2 QDs (�o¼1.5 mm) contain-
ing PbSe QDs of smaller-size (�o�900 nm) were
synthesized according to variations of literature
methods [28,29].

3. Chiral microcavity made of LCs doped with single

colloidal QDs

3.1. CLC sample preparation

In a planar-aligned CLC, the rod-shaped anisotropic
molecules with small chiral ‘tails’ form a periodic
helical structure with pitch p [30]. For sufficiently thick

CLC layers, the reflectance of normally incident,
circularly polarized light, with the same handedness
as the CLC structure, is nearly 100% within a band
centered at �c¼navp. The bandwidth is approximately
��¼�c�n/nav, where nav is the average of the ordinary
no and extraordinary ne refractive indices of the
medium: nav¼ (noþne)/2, and �n¼ne�no. This per-
iodic structure can also be viewed as a 1-D photonic
crystal, with a bandgap within which propagation of
light is forbidden. For emitters located within this
structure, the spontaneous emission rate is suppressed
within the spectral stopband and enhanced near the
band edge [31,32]. Lasing experiments in dye-doped
CLC structures with high dopant concentration [32]
confirmed that the best condition for coupling is when
the dopant fluorescence maximum is at a band edge
of the CLC selective transmission curve.

For sample preparation we use two types of LCs:
(1) monomeric mixtures of low-molecular-weight E7
nematic-LC blend with a chiral additive CB15 and
(2) oligomer CLC (OCLC) powders [19–20]. E7 and
CB15 are fluids at room temperature. Both materials
were supplied by EM Industries. We filtered the E7
and CB15 to remove fluorescent contaminants. Powder
OCLCs were supplied by Wacker GmbH.

For development of CLC hosts which form a chiral
photonic bandgap tuned to the QD fluorescence band,
two main aspects are important: (1) properly choosing
the concentration (or ratio) of different LC compo-
nents and (2) providing planar alignment of the CLC.
For the monomeric mixtures, the stopband position �c
of the photonic bandgap is defined roughly by C¼nav/
(�c�HTP), where C is the weight concentration of
CB15 in the CB15/E7 mixture, nav�1.6 for this
mixture, and HTP�7.3 mm�1 is the helical twisting
power of the chiral additive in nematic liquid crystal.
The actual stopband position relative to the fluores-
cence maximum of the QD was further defined
empirically by obtaining selective transmission curves
of different samples using a spectrophotometer. After
monomeric CLC preparation, a QD solution of �1nM
concentration was mixed with monomeric CLC and
solvent was evaporated.

For the OCLC powders there is not such a simple
relation between the concentration and �c. We found
the right ratio R of components only empirically by
mixing the different ratios of two OCLC with different
�c (1.17 mm and 2.15 mm) by dissolving them in
a solvent. By evaporating the solvent using
a procedure of heating this solution in a vacuum
inside a rotating retort, we obtained a new powder
oligomer with an intermediate �c.

After that, monomeric CLC doped with QDs is
placed between two cover glass slips and planar aligned
through uni-directional mechanical motion between

Journal of Modern Optics 3

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the two slides. For planar alignment of OCLC, a cover

slip with Wacker powder is placed on a hot plate and

melted at �120�C. The second slip is used to shear the

melted oligomer at this temperature. After the align-

ment the sample is slowly cooled into the glassy state,

preserving CLC order and planar alignment [20]. For

single-molecule fluorescence experiments Wacker pow-

ders need to be purified. For further details of CLC

doping and sample preparation from both monomeric

and oligomeric LCs with different photonic bandgaps,

see [19,20].

3.2. 1-D-photonic bandgap transmission curves

By properly choosing the concentration of different

LC monomers and/or Wacker glassy oligomers and

providing planar alignment of the LCs, we developed

CLC chiral 1-D photonic bandgap structures with

different stopband positions doped with single QDs

(either CdSe, CdSeTe or PbSe). In both cases of

monomeric and oligomeric liquid crystals, increasing

the concentration (or ratio) of a component with

higher HTP changes the position of the stopband in the

direction of shorter wavelengths. The error in defining

weight concentrations can smear this effect for

mixtures with concentrations close to one another.

The stopband positions are tuned to the QD fluores-

cence bands for the visible (Figure 2) and �1.5 mm
(Figure 3).

Figure 2 shows selective transmission of two

monomeric 1-D chiral photonic bandgap structures

with 36.6% (a) and 36.0% (b) weight concentrations of

chiral additive CB15 in E7/CB15 mixtures. It also

shows the fluorescence spectra of the CdSe (a) and

CdSeTe (b) QDs with the centers of the fluorescence

peaks near 580 and 700 nm.

400 500 600 700 800
Wavelength (nm)

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450 550 650 750

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20

40

60

80

100(a) (b)1.0

0.5

Figure 2. Selective transmission of two monomeric chiral photonic bandgap CLC hosts for right-handed circular polarized light
and the fluorescence spectrum of the CdSe (a) and CdSeTe (b) QDs. (The color version of this figure is included in the online
version of the journal.)

40

50

60

70

80

90

100(a) (b)

1.0 1.2 1.4 1.6 1.8 2.0
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(a
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%
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60

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100

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2.0

Ra = 1:0

Rc = 1:1
Rb = 1:0.24

Ca = 16.3%

Cc = 14.4%
Cb = 16.0%

a

b
c

a

b

c

Figure 3. PbSe QD fluorescence spectrum and selective transmission of chiral photonic bandgap cholesteric microcavities for
telecom wavelength 1.5 mm for unpolarized light [for circularly polarized light the minimum transmission value will be smaller
than 5% (see Figure 2, where measurements were made with a linear polarizer and achromatic quarter waveplate)]. (a) For
monomeric liquid crystals of different concentrations, C, of chiral additive CB15 in CB15 and E7 mixture; (b) for Wacker chiral
oligomeric powder mixtures of different ratios, R, of �c¼1.17 mm powder to �c¼2.15 mm powder. (The color version of this
figure is included in the online version of the journal.)

4 S.G. Lukishova et al.

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Figure 3 shows spectral transmission curves for

several prepared photonic bandgap structures with the

band edge at 1.5 mm made of monomeric CLC (left)
and OCLC (right). The fluorescence spectrum of

a PbSe QD solution at high QD concentration is

depicted in both figures.

3.3. Circular polarized fluorescence and antibunching

in a chiral microcavity

Figure 4(a), shows several single-CdSe/ZnS QD

fluorescence images in a CLC host with the same QD

fluorescence spectrum as presented in Figure 2(a). The

spectral transmission of this CLC photonic bandgap

structure is similar to the spectral transmission curve

also shown in Figure 2(a). Note that the dark

horizontal stripes in the pattern of separate QDs are

the result of single QD blinking, which is

a characteristic property of single-QD fluorescence.

The raster scan area is 15 mm�15 mm. Figure 4(b),
shows single-PbSe QD fluorescence images in a

CLC photonic bandgap structure with the spectral

transmission curve (a) in Figure 3(a). We used the

solution containing both 1.5 mm emitting QDs and a
small concentration of 900 nm emitting QDs as a result

of synthesis. Using Si APDs we are only able to record

QD fluorescence from the smaller size QDs with

�o�900 nm.
Figure 5(b) shows emission spectra for CdSe/ZnS

QDs in a chiral CLC cavity for right-handed (black

line) and left-handed circular polarizations (red line).

The spectral transmission of the cavity is presented in

Figure 2(a). The degree of circular polarization is

measured by the dissymmetry factor ge [25,33]:

ge ¼
2ðIL � IRÞ

ðIL þ IRÞ
, ð1Þ

where IL and IR are the intensities of left-handed and

right-handed circular polarizations.
At 580 nm, ge¼�1.6. For unpolarized light ge¼0.

The fluorescence spectrum of the same CLC

microcavity without QDs is depicted in Figure 5(a).

The nature of the spectral peaks which were observed

in the CLC cavity without QDs is unclear. It is not

a microcavity effect, because we observed the same

features from unaligned CLC without a microcavity.

It can be attributed to some impurities which we did

not remove during the LC purification procedure.
We also illuminated a single CdSe QD in the CLC

host and measured the fluoresced photon statistics

under saturation conditions. Figure 6(a) presents the

g
(2)
(t) histogram at different interphoton times t. The

value of g
(2)
(0) is 0.76�0.04. One sees that the peak at

zero interphoton time is clearly smaller than any of the

other peaks, which shows an antibunching property.

This antibunching histogram can be improved by using

QDs which fluoresce outside the fluorescence spectrum

of the CLC shown in Figure 5(a). One easily can see

a host fluorescence peak at the fluorescence maximum

of the selected QD. At wavelengths larger than

�700 nm, no host background is observed.

cts/ms

5 µm

44(a) (b)

20

10

 2

30

40

5 µm

cts/ms

32

28

24

20

16

Figure 4. Typical confocal fluorescence images (1) of single
CdSe QDs in CLC host (a) (15 mm�15 mm scan), and single
PbSe QDs in CLC host (25 mm�25 mm scan) (b). (The color
version of this figure is included in the online version of the
journal.)

0
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60
70
80

500 550 600 650 700
Wavelength (nm)

LHPLHP

RHPRHP

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Figure 5. Fluorescence spectrum of the monomeric CLC host without QDs (a) and CdSe QDs in a similar CLC host (Figure
2(a)) for two different circular polarizations of single photons: (b) black line – right-handed, red line – left-handed. (The color
version of this figure is included in the online version of the journal.)

Journal of Modern Optics 5

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We doped CdSeTe QDs with �0¼700 nm in the
CLC host with the stopband shown in Figure 2(b), and
when illuminating a single QD, obtained antibunching
with g

(2)
(0)�0 (Figure 6(b)). Note that the QD

fluorescence maximum is outside of the CLC back-
ground spectrum. It shows that excluding the CLC
background helps to obtain better antibunching.
The fitted curve g

(2)
(t)¼1 – (1/N )e

(-t/�)
[34] gives

g
(2)
(0)¼0.001�0.034, with a fluorescence lifetime

��15 ns and with the number N of illuminated
emitters equal to 1. This quantum dot has � larger
than the time between two laser pulses, so we cannot
observe fluorescence excited by the separate laser
pulses as in Figure 6(a).

Estimation of the efficiency P of on-demand
polarized antibunched photon emission into the col-
lecting objective showed P�10% with the second-
order correlation function g

(2)
(0)¼0.8, measured from

the antibunching histogram of Figure 6 (a), using
5.2 mW excitation power. We define P from the
following equation:

½1�gð2Þð0Þ�Nout ¼ NincQD��QAPDGP, ð2Þ

where Nout¼2�10
5
counts s

�1
is the measured photon

count rate by the APDs and NincQD¼Ninc�abs¼
(I/h�)�abs¼7.17�10

6
photons s

�1
is the number of

photons incident on the quantum dot per second.
Here I is the measured incident intensity in the focal
area of the sample (I¼2.23 kW cm

�2
), h�¼3.73�

10
�19

J for 532-nm light, and �abs¼1.2�10
�15

cm
2
is

the absorption cross-section of the quantum dot at
532 nm taken from the measurements of Leatherdale
et al. [35].

The parameters of Equation (2) defining losses in
the microscope detection system (��QAPD) are as
follows: �¼0.49 is the measured transmission of all
interference and colored glass filters in front of the
APDs (preventing cross-talk between them), �¼0.48 is
the measured transmission of the objective, microscope

optics, imaging lenses and nonpolarizing beamsplitter,
and QAPD¼0.58 is the quantum efficiency of the APD
at 580 nm as quoted by the vendor.

The parameter G¼0.4 is the measured CdSe/ZnS
QD quantum yield. The value of P characterizes the
cavity (collection efficiency from the source into the
collecting objective), and the value of GP characterizes
both the cavity and the fluorescent emitter together. In
our measurements for the on-demand polarized anti-
bunched photon source, P�10%, and GP�4%. The
value of GP can be increased to 10% by using
a quantum dot with G�1 (see, e.g. experimental
paper [36]).

4. Polymer microcavity with defect layer doped with

single QDs between the DBRs

The second type of photonic bandgap structure doped
with single CdSe/ZnS core/shell QDs from Evident
Technologies is prepared using solution processing.
The fluorescence maximum of these QDs is �620 nm.
The details of structure preparation are reported in
[22–23]. The DBRs are fabricated by spin coating
alternating quarter wavelength thick polymer layers
with different refractive indices n [poly-vinylcarbazole
(PVK) with n¼1.683 and poly-acrylic acid (PAA) with
n¼1.428 at 600 nm] [22,23]. Solvents are chosen such
that the solvent for one polymer does not dissolve the
other polymer. PVK is soluble in non-polar solvents
such as toluene or chlorobenzene but not in polar
solvents such as water or alcohol, whereas PAA is
soluble in alcohol but not in chlorobenzene. Greater
than 90% reflectivity is obtained using 10 periods of
the DBR structure.

The 1-D microcavity is formed by sandwiching
a �/nPVK thickness PVK defect layer doped with single
QDs between two such DBRs. The top and bottom
DBRs both comprise 10 periods. QDs are mixed with
a defect layer PVK polymer solution and spin coated

0
0.2
0.4
0.6
0.8

1
1.2
1.4
1.6
1.8

−80 −60 −40 −20 0 20
Interphoton times (ns)

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1(a) (b)

−80 −60 −40 −20 0 20
Interphoton times (ns)

g
(2

) (
t)

g
(2

) (
t)

Figure 6. Histograms of coincidence counts of single-QD fluorescence in a CLC host under pulsed excitation. The dip at zero
interphoton time indicates antibunching. (a) For the CdSe QD of Figure 2(a) with �0 inside the liquid crystal background; (b) for
CdSeTe QD of Figure 2(b) with �0 outside the liquid crystal background. (The color version of this figure is included in the online
version of the journal.)

6 S.G. Lukishova et al.

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onto the bottom DBR structure (QD concentration in
solution is �7.7 nM).

Figure 7(a) shows the reflectivity of the whole
structure (DBRs with a defect layer between them)
with the cavity mode at �620 nm. The quality factor
(Q) of such a microcavity was found to be �40. The
theoretical estimate of Q for the polymer distributed
Bragg mirrors to form a microcavity structure is within
the same order of magnitude (�60). This discrepancy
is attributed to lower reflectivities of the actually
fabricated DBR mirrors than anticipated by the theory
using the ideal mirrors.

The inset of Figure 7(a), shows the normalized
reflectivity spectra of the DBR with 10 periods without
any defect layer. Fluorescence imaging of single QDs
in a DBR structure with a defect layer is shown in
Figure 7(b). This raster scan image shows blinking of
single QDs (bright horizontal stripes) and a low host
fluorescence background, making these structures
suitable for single photon source applications.

It was reported earlier for a high concentration of
CdSe/ZnS QDs [22,23] that the photoluminescence
decay time � of the QDs in a microcavity was �150 ps.
For QDs on a bare glass substrate or embedded in
PVK without a bandgap structure � was found to be
�1000 ps and �400 ps, respectively [22,23].

It should be noted that this type of microcavity
provides 1-D light confinement so that light can
propagate in a lateral direction. Preparation of
a micropillar microcavity from the current 1-D
microcavity using focused ion beam etching is in
progress. Etching with a similar 1-D microcavity was
carried out in [11]. An elliptical cross-section micro-
pillar will provide linear polarized emission of desired
direction from single colloidal QDs. In contrast to [10],
in which DBRs and a defect layer doped with colloidal
QDs between them are prepared by sputtering

alternating SiO2 and TiO2 layers, the process described

here is more simple and robust and compatible with

the standard solution processing technique of colloidal

QDs. The technique of sputtering and/or thermal

evaporation require multiple deposition systems –

one for colloidal quantum dots and another for

DBRs. In addition, these techniques have detrimental

effects on the optical properties of the QDs due to the

introduction of surface defects.

5. Conclusion

Single semiconductor nanocrystal (colloidal QD)

fluorescence in microcavities was studied for the first

time. We report the first observation of single-emitter

circularly polarized fluorescence of definite handedness

due to microcavity chirality. The chiral microcavities

were prepared by a simple method of planar alignment

of cholesteric liquid crystals. Antibunching experi-

ments show that the fluorescence background of the

medium is low, so that antibunching of QD fluores-

cence is preserved. 1-D chiral photonic bandgap

structures possess an advantage over conventional

1-D photonic bandgap technologies. Because the

refractive index n varies gradually rather than abruptly

in chiral structures, there are no losses into the

waveguide modes, which arise from total internal

reflection at the border between two consecutive

layers with different n. Therefore, it is not necessary

to make a micropost structure as in [11] to reject ‘leaky’

modes in a lateral direction. In addition, liquid crystal

microcavities are wavelength-tunable by changing the

temperature and applied electric field.
Single QD fluorescence in a polymer microcavity

with a defect layer doped with single QDs between

DBRs was studied as well. Such easily prepared and

400 500 600 700 800 900 1000

0.2

0.4

0.6

0.8

1.0

Wavelength (nm)

Cavity Mode

400 600 800 1000

0.2

0.4

0.6

0.8

1.0

N
o
rm

a
liz

e
d
 r

e
fl
e
c
ta

n
c
e

Wavelength (nm)
N

o
rm

a
liz

e
d
 r

e
fl
e
c
ta

n
c
e

2 µm

cts/ms

14

12

10

8

6

4

Figure 7. (a) Reflectivity spectrum of the polymeric microcavity embedded with CdSe QDs with �0¼620 nm. Shown in the inset
is the normalized reflectivity spectrum showing the stop band of the ten-period Bragg reflector without the defect layer. (b) Single
QD fluorescence imaging in a polymeric microcavity under 532-nm excitation (10 mm�10 mm scan). (The color version of this
figure is included in the online version of the journal.)

Journal of Modern Optics 7

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robust organic microcavities using low fluorescence

background materials can be incorporated in visible

and telecom SPSs.
Our next steps will be to increase the efficiency of

our on-demand polarized antibunched photon source.

We will do this by: (1) selecting QDs with G�100%,

�o outside the host fluorescence background, and �
shorter than several ns, and (2) providing strong

coupling between a single QD and a cavity by refining

the cavity preparation technique. We also plan to

demonstrate fluorescence antibunching at telecom

wavelength.

Acknowledgements

The University of Rochester authors acknowledge support
by the NSF Award ECS-0420888 and EHR-0633621. L.J.
Bissell thanks the Air Force for a SMART fellowship. The
authors thank A. Lieb and L. Novotny for advice and help,
Z. Shi and H. Shin for assistance and J. Dowling for
providing better understanding of emitter fluorescence
in CLC photonic bandgap structures. Queens College
authors’ work was supported partly by the Army Research
Office, Short Term Analytical Service Grant at Queens
College – CUNY.

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