Room-temperature single photon sources with definite circular and linear polarizations
ABSTRACT We report experimental results of two room-temperature single photon sources with definite polarization based on emitters
embedded in either cholesteric or nematic liquid crystal hosts. In the first case, a cholesteric 1-D photonic bandgap microcavity
provides circular polarization of definite handedness of single photons from single colloidal semiconductor quantum dots (nanocrystals).
In these experiments, the spectral position of the quantum dot fluorescence maximum is at the bandedge of a photonic bandgap
structure. The host does not destroy fluorescence antibunching of single emitters. In the second case, photons with definite
linear polarization are obtained from single dye molecules doped in a planar-aligned nematic liquid crystal host. The combination
of sources with definite linear and circular polarization states of single photons can be used in a practical implementation
of the BB84 quantum key distribution protocol.
Conference Proceeding: Quantum cryptography in optical fiber networks[show abstract] [hide abstract]
ABSTRACT: Quantum cryptography represents a new paradigm for secure communications in which security is based on fundamental physical laws rather than computational complexity. Recent progress in the development of practical optical fiber-based systems will be describedOptical Fiber Communication Conference, 1999, and the International Conference on Integrated Optics and Optical Fiber Communication. OFC/IOOC '99. Technical Digest; 02/1999
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ABSTRACT: Simple far-field emission polarization microscopy reveals that the emission transition dipole of CdSe colloidal quantum dots (QDs) is twofold degenerate at room temperature. We measure, model, and compare polarization anisotropy statistics of CdSe QDs and DiI (a one-dimensional emitter). We find excellent agreement between experiment and theory if the transition dipole of CdSe QDs is assumed to be twofold degenerate. This implies that the three-dimensional orientation of the unique crystal axis in QDs can be determined at room temperature with polarization microscopy. We describe an optical setup to measure four polarization angles of multiple single QDs simultaneously and in real time (approximately equal to 16 Hz). We use this setup in a proof-of-concept experiment to demonstrate that the rotational motion of QDs can be monitored in various host matrices.Proceedings of the National Academy of Sciences 02/2003; 100(2):405-8. · 9.74 Impact Factor
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ABSTRACT: In order to probe the local dynamics of lipid bilayers in the gel phase, we measured the rotational time trajectories of a membrane probe, diI(3), in supported bilayers of DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) using single molecule fluorescence polarization imaging. diI(3) has two hydrocarbon tails that mimic phospholipid tails and has its transition dipole moment lying mostly on the plane of the membrane; hence it is an excellent probe for rotational dynamics in membranes. Above the transition temperature, the probes are laterally mobile and do not display polarized emission. In the gel phase below the transition temperature, lateral mobility is severely reduced and the emission becomes polarized with its polarization direction changing in the milliseconds time scale. Molecule by molecule analysis of the rotational time scales revealed significant heterogeneities among molecules, much larger than would be due to statistical noise. Control experiments using small unilamellar vesicles suggest that the heterogeneities are not caused by surface interactions and are intrinsic to the gel phase membrane. The rotational dynamics is strongly temperature dependent and the thermally activated state for the rotational motion has a large entropic barrier (> 30kB), indicating that relatively large local disorder is required for the rotational motion to occur. Rotational hopping between discrete angles has been observed at the lowest temperatures (approximately 10 degrees C). Our results suggest that the gel phase membrane is not uniform at the microscopic level but is highly dynamic with the rigidity of local environments constantly changing.The Journal of Chemical Physics 03/2004; 120(6):3030-9. · 3.16 Impact Factor
ОПТИКА И СПЕКТРОСКОПИЯ, 2010, том 108, № 3, с. 452–459
The development of on?demand efficient single?
photon sources (SPSs) with photons exhibiting anti?
bunching has recently been of significant interest for
their applications in quantum cryptography . Weak
attenuated light sources are not satisfactory because in
order to attenuate the source sufficiently so that two si?
multaneous photons are very unlikely the probability
of no photons at all becomes large. In the BB84 quan?
tum key distribution protocol Alice (transmitter) and
Bob (receiver) employ the linear and circular polariza?
tion states of single photons. The linear and circular
bases are used to provide two different quantum level
representations of zero and one [2, 3]. So a desirable
feature for a SPS is definite photon polarization, since
if the photon has unknown polarization, then filtering
it through a polarizer to produce the desired polariza?
tion for quantum key distribution will reduce twice the
In this paper, experimental results of room?tem?
perature, robust SPSs on demand with definite polar?
ization using single?emitter fluorescence in different
liquid crystal (LC) hosts are discussed [4–8]. A desir?
able polarization state (either circular with definite
handedness or linear with definite direction) of a fluo?
rescence of the emitter in a LC host can be produced
either by providing a chiral microcavity environment
of cholesteric LC (CLC) or by aligning emitters’ di?
pole moments in a definite direction in nematic LC.
SPSs based on single emitters in LCs are the room
temperature alternatives to cryogenic SPSs based on
semiconductor heterostructured quantum dots in mi?
crocavities prepared by molecular beam epitaxy
(MBE) (see reviews [9, 10]). Definite linear polariza?
tion from heterostructured quantum dots both in ellip?
tical pillar microcavities [11–13] and in 2?D photonic
crystal [14, 15] was reported for the resonance wave?
length at cryogenic temperatures. In difference to ex?
pensive MBE, well?developed LC display technology
of preparation of planar?aligned 1?D photonic band?
gap CLC layers or planar?aligned nematic LC layers is
easy and fast. Different types of single emitters can be
easily dissolved or dispersed both in monomeric (flu?
id?like) or oligomeric (solid) LCs. In addition to emit?
ter alignment and self?assembled structures with pho?
tonic bandgap properties, LC hosts with special treat?
ment (oxygen depletion) can protect the emitters from
bleaching. In paper , we reported on a significant
diminishing of dye bleaching by saturation of LC with
helium. In that work molecules did not bleach for pe?
riods of more than one hour under continuous wave
(cw) excitation . Another remarkable advantage of
LC, e.g., changing its properties with temperature or
by external field variation, can provide SPS tunability.
The structure of this paper is as follows. Section 2
describes the results on antibunching and circular po?
larized fluorescence with desired handedness of col?
loidal semiconductor quantum dots (QD) embedded
into planar?aligned CLC monomeric host. Section 3 is
devoted to linearly polarized fluorescence with defi?
nite polarization state from single dye molecules
aligned in a glassy nematic LC oligomer. Section 4
concludes the paper.
2. SINGLE?PHOTON SOURCE WITH
CIRCULARLY POLARIZED PHOTONS
The first SPS is based on the single colloidal CdSe
(fluorescence wavelength λ0 ~ 580 nm) and CdSeTe
(λ0 ~ 700 nm) QDs suspended in a CLC hosts self?as?
sembled in a chiral photonic bandgap structures. This
ИСТОЧНИКИ ОДИНОЧНЫХ ФОТОНОВ
ROOM?TEMPERATURE SINGLE PHOTON SOURCES WITH DEFINITE
CIRCULAR AND LINEAR POLARIZATIONS
© 2010 S. G. Lukishova, L. J. Bissell, C. R. Stroud, Jr., and R. W. Boyd
The Institute of Optics University of Rochester, Rochester NY, 14627 USA
Received August 3, 2009
Abstract—We report experimental results of two room?temperature single photon sources with definite po?
larization based on emitters embedded in either cholesteric or nematic liquid crystal hosts. In the first case,
a cholesteric 1?D photonic bandgap microcavity provides circular polarization of definite handedness of sin?
gle photons from single colloidal semiconductor quantum dots (nanocrystals). In these experiments, the
spectral position of the quantum dot fluorescence maximum is at the bandedge of a photonic bandgap struc?
ture. The host does not destroy fluorescence antibunching of single emitters. In the second case, photons with
definite linear polarization are obtained from single dye molecules doped in a planar?aligned nematic liquid
crystal host. The combination of sources with definite linear and circular polarization states of single photons
can be used in a practical implementation of the BB84 quantum key distribution protocol.
ОПТИКА И СПЕКТРОСКОПИЯ том 108 № 3 2010
ROOM?TEMPERATURE SINGLE PHOTON SOURCES WITH DEFINITE CIRCULAR453
structure can provide not only spontaneous emission
enhancement and a diminishing of the fluorescence
lifetime, but also circular polarization of definite
handedness even for emitters without a dipole mo?
ment. In addition, because the refractive index n varies
gradually in chiral structures rather than abruptly,
there are no losses into the waveguide modes, which
arise from total internal reflection at the border be?
tween two consecutive layers with different n. We re?
port here for the first time fluorescence antibunching
of a QD doped in a LC. Earlier we reported fluores?
cence antibunching of dye embedded in LC host .
Room?temperature SPSs based on colloidal QD fluo?
rescence are very promising because of higher QD
photostability at room temperature than that of con?
ventional dyes, and relatively high quantum yield (up
to ~100%) . Electrically driven light emission from
a single colloidal QD at room temperature was ob?
tained, opening up the possibility for electrical pump?
ing of a SPS on demand, based on colloidal QDs .
Nonblinking, long?lasting colloidal QDs were report?
ed recently .
A. Experimental setup for antibunching
and circular polarization measurements
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 (left) and polarization and
spectral measurement (right).
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 tele?
scopic system with a spatial filter. The samples are
placed in the focal plane of a 1.3?numerical aperture,
oil?immersion microscope objective used in confocal
reflection mode. In focus, the intensities used are of
the order of several kW/cm2. Residual transmitted ex?
citation light is removed by a dichroic mirror and a
combination of two interference filters yielding a com?
bined 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 × 50 μm.
The following diagnostics are placed in the separate
(1) A Hanbury Brown Twiss arrangement consist?
ing of a 50/50 beamsplitter and two cooled, Si single
photon counting avalanche photodiode modules (AP?
Ds) SPCM AQR?14 (Perkin Elmer). The time interval
between two consecutively detected photons in sepa?
rate arms is measured by a TimeHarp 200 time corre?
lated single?photon counting card using a convention?
al 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 . For circular?polar?
ization 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 in a wide?
field mode and/or by APD?detectors with a raster?
scan of the sample in a confocal mode. It is imaged to
the spectrometer input fiber. The spectra are recorded
with a 6?s accumulation time with background sub?
CdSe/ZnS core/shell QDs were synthesized by
T. Krauss’ group (University of Rochester) according
Fig. 1. Schematics of experimental setup for fluorescence imaging and antibunching measurements (left); spectral and polariza?
tion measurements (right). We used the following abbreviations: neutral density (ND), single?photon counting avalanche photo?
diode module (APD), beamsplitter (BS).
ОПТИКА И СПЕКТРОСКОПИЯ том 108 № 3 2010
LUKISHOVA et al.
to published methods [20, 21]. CdSeTe QDs were ob?
tained commercially from Invitrogen.
B. Chiral microcavities made of cholesteric liquid
crystals doped with single colloidal quantum dots:
preparation and characterization
In a planar?aligned CLC, the rod?shaped anisotro?
pic molecules with small chiral “tails” form a periodic
helical structure with pitch p . For sufficiently
thick CLC layers, the reflectance of normally inci?
dent, circularly polarized light, with the same handed?
ness as the CLC structure, is nearly 100% within a
band centered at λc = navp. The bandwidth is approxi?
mately Δλ = λcΔn/nav, where nav is the average of the
ordinary no and extraordinary ne refractive indices of
the medium: nav = (no + ne)/2, and Δn = ne – no. This
periodic structure can also be viewed as a 1?D photo?
nic 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 [23, 24]. Both the literature  and our
lasing experiments  in dye?doped CLC structures
with high dopant concentration confirmed that the
best condition for coupling is when the dopant fluo?
rescence maximum is at a band edge of the CLC selec?
tive transmission curve.
For sample preparation we use monomeric mix?
tures of low?molecular?weight E7 nematic?LC blend
with a chiral additive CB15. E7 and CB15 are fluids at
room temperature. Both materials were supplied by
EM Industries. We filtered E7 and CB15 to remove
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 of different LC components and (2)
providing planar alignment of the CLC. For the
monomeric mixtures, the stopband position λc of
the photonic bandgap is defined roughly by C =
nav/(λc× HTP), where C is the weight concentration
of CB15 in the CB15/E7 mixture, nav ~ 1.6 for this
mixture, and HTP ~ 7.3 μm–1 is the helical twisting
power of the chiral additive in nematic LC. The actual
stopband position relative to the fluorescence maxi?
mum of the QD is defined empirically by obtaining se?
lective transmission curves of different samples using a
spectrophotometer with a thin film linear polarizer
and an achromatic quarter waveplate.
After monomeric CLC preparation, a QD solution
of ~ nM concentration is mixed with monomeric CLC
and solvent is evaporated. After that, monomeric CLC
doped with QDs is placed between two cover glass slips
and planar aligned through uni?directional mechani?
cal motion between the two slides. For further details
of CLC doping and sample preparation see .
By properly choosing the concentration of differ?
ent LC monomers and providing planar alignment of
the LCs, we developed CLC chiral 1?D photonic
bandgap structures with different stopband positions
doped with single QDs (CdSe and/or CdSeTe). In?
creasing the concentration of a component with high?
er HTP changes the position of the stopband in the di?
rection of shorter wavelengths. The error in defining
weight concentrations (~±5%) can smear this effect
for mixtures with similar concentrations. The stop?
band positions are tuned to the QD fluorescence bands
Figure 2 shows selective transmission curves of two
monomeric 1?D chiral photonic bandgap structures
with 36.6% (left) and 36% (right) weight concentra?
tions of chiral additive CB15 in E7/CB15 mixtures. It
also shows the fluorescence spectra of the CdSe (left)
and CdSeTe (right) QDs with the centers of the fluo?
rescence peaks near 580 and 700 nm.
C. Circular polarized fluorescence and antibunching
in chiral cholesteric liquid crystal microcavity
Figure 3 (left) shows emission spectra for
CdSe/ZnS QDs in a chiral CLC microcavity for right?
handed (black line) and left?handed circular polariza?
of CdSe/ZnS QDs, a.u.
of CdSeTe/ZnS QDs, a.u.
Fig. 2. Selective transmission of two monomeric chiral photonic bandgap CLC hosts for right?handed circular polarized light and
the fluorescence spectrum of the CdSe (left) and CdSeTe (right) quantum dots.
ОПТИКА И СПЕКТРОСКОПИЯ том 108 № 3 2010
ROOM?TEMPERATURE SINGLE PHOTON SOURCES WITH DEFINITE CIRCULAR455
tions (gray line). The degree of circular polarization is
measured by the dissymmetry factor ge :
where IL and IR are the intensities of left?handed and
right?handed circular polarizations. At 580 nm, ge=
= –1.6. For unpolarized light ge = 0. Using another
type of CLC, e.g., Wacker CLC oligomers , left?
handed circular polarization of single photons can be
The fluorescence spectrum of the same CLC mi?
crocavity without QDs is depicted in Fig. 3, right. The
nature of the spectral peaks which were observed in the
CLC cavity without QDs is unclear. It is not a micro?
cavity 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 un?
der saturation conditions. Figure 4, left presents the
g(2)(t) histogram at different interphoton times t.
One sees that the peak at zero interphoton time is
clearly smaller than any of the other peaks, which
shows an antibunching property [g(2)(0) = 0.76 ± 0.04].
This antibunching histogram can be improved by using
QDs which fluoresce outside the fluorescence spec?
trum of the CLC shown in Fig. 2, right. At wavelengths
larger than ~700 nm, no host background is observed.
For a CdSeTe colloidal QD with a 700 nm fluores?
cence maximum (outside of the background spectrum
of CLC with the stopband shown in Fig. 2, right), we
obtained antibunching under saturation conditions
with g(2)(0) = 0.11 ± 0.06 (Fig. 4, right). It shows that
excluding the CLC background helps to obtain better
antibunching. This QD has a fluorescence lifetime
larger than the pulse repetition period of 13.1 ns, so we
can not observe fluorescence excited by the separate
Estimation of the efficiency P of polarized single?
photon emission into the collecting objective showed
P ~ 15% with the second?order correlation function
g(2)(0) = 0.11 ± 0.06, measured from the antibunching
Fluorescence intensity, rel.units
Fluorescence intensity, rel.units
Fig. 3. Fluorescence spectrum of: CdSe QDs in the monomeric CLC host (selective transmission shown in Fig. 2, left) for two
different circular polarizations of single photons (RHP – right handed, LHP – left handed) – left. The same monomeric CLC
host without QDs – right.
Fig. 4. Histograms of coincidence counts of single?QD fluorescence in a CLC host under pulsed excitation. The dip at zero in?
terphoton time indicates antibunching. Left: for CdSe QD of Fig. 2, left with λ0 within the liquid crystal background; right: for
the CdSeTe QD of Fig. 2, right with λ0 outside the liquid crystal background.
Interphoton times, ns
Interphoton times, ns
ОПТИКА И СПЕКТРОСКОПИЯ том 108 № 3 2010
LUKISHOVA et al.
histogram of Fig. 4, right using 257 nW excitation
power. We defined P from the following equation:
where Nout = 2.86 × 104 counts/s is the measured pho?
ton count rate by the APDs and NincQD = NincσabsQD =
= (I/hν)σabsQD = 2.35 × 106 photons/s 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 (I = 110 W/cm2), hν = 3.73 × 10–19 J
for 532?nm light, and σabsQD = 8.0 × 10–15 cm2 is the
absorption cross?section of the quantum dot at 532 nm
taken from the measurements of Leatherdale et al.
 as well as calculated from vendor’s measurements
of extinction coefficient .
Other parameters of equation (2) are as follows:
α = 0.62 is the measured transmission of all interfer?
ence filters in front of the APDs, β = 0.29 is the mea?
sured transmission and collection of fluorescent light
by the objective, microscope optics, and imaging lens?
es, G = 0.59 is the measured CdSeTe/ZnS QD quan?
tum yield , and QAPD = 0.66 is the quantum effi?
ciency of the APD at 700 nm provided by the vendor.
The value of P characterizes the cavity (collection
efficiency from the source into the collecting objec?
tive), and the value of PG characterizes both the cavity
and the fluorescent emitter together. In our measure?
ments for on?demand polarized single?photon source,
P ~ 15%, and PG ~ 9%.
3. SINGLE PHOTON SOURCE WITH
LINEARLY POLARIZED PHOTONS
The second SPS is based on a single dye molecule
fluorescence suspended in LCs [4–7]. The results on
dye fluorescence antibunching in LC host are reported
in our papers [4–6]. We will describe here the results
on definite linear polarization of fluorescence of single
DiIC18(3) dye molecules in a planar?aligned glassy
nematic LC host which is solid at room temperature.
A. Experimental setup for linear polarization
measurements of single dye molecules
Single?molecule fluorescence microscopy for this
SPS is carried out on a Witec alpha?SNOM micro?
scope in confocal mode. See details of the setup in ref?
erence . The cw, spatially filtered, 532 nm, diode?
pumped Nd:YAG laser output excites single mole?
cules. To obtain a diffraction limited spot on the sam?
ple, the excitation beam is expanded and collimated
by the optical system of Witec microscope . In?fo?
cus, the intensities used were of the order of several
kW/cm2. The dye?doped nematic LC sample is placed
in the focal plane of a 1.4?numerical aperture, oil?im?
mersion microscope objective used in confocal trans?
mission mode. Light emitted by the sample is collect?
ed by a confocal setup using a second oil?immersion
2 ( )0 ( )
objective with a 1.25?numerical aperture, an imaging
lens with a focal length 12.5 cm and an aperture in a
form of optical fiber. For polarized fluorescence mea?
surements, we place inside the Witec microscope both
a 50/50 polarizing beamsplitter cube and the second
arm of the confocal detection. The confocal micro?
scope apertures were 100 μm core optical fibers placed
in each arm of the beamsplitter’s output. Residual
transmitted excitation light was removed by filters
yielding a combined rejection of better than seven or?
ders of magnitude at 532 nm. Photons in the two arms
were detected by SPCM AQR?14 APD modules.
B. Sample preparation from planar?aligned glassy
nematic LC oligomer doped with single dye molecules
For these experiments we use DiIC18(3) dye (DiI
dye) from Molecular Probes in planar?aligned, glassy
nematic liquid crystal host which is solid at room tem?
perature. The dye molecular structure is presented in
Fig. 5a. An oligomer host material was synthesized by
S.H. Chen’s group (University of Rochester) . The
nematic LC state of this material, which exists at ele?
vated temperatures, is preserved at room temperature
by slowly cooling to room temperature the LC to the
solid glassy state with frozen nematic order . An
oligomer solution with a 1% concentration by weight
in chloroform with 10 nM concentration of the dye in
chloroform was prepared. We prepared ~100 nm thick
film of this planar?aligned glassy, nematic LC guest?
host system by several procedures including spin?coat?
ing on a cover glass slip with a photoaligned polymer
layer, heating and slowly cooling the sample .
C. Linearly polarized fluorescence from single
Figure 5b shows images of single?molecule fluores?
cence for components perpendicular (left) and paral?
lel (right) to the alignment direction, under 532?nm,
cw?excitation. These two polarization components in
the plane of the sample have been separated with a po?
larizing beamsplitter cube . Figure 5b clearly shows
that for this sample, the polarization direction of the
fluorescence of single molecules is predominantly in
the direction perpendicular to the alignment of LC
molecules. It is important that the background level
of left and right images of Fig. 5b is the same
(~10 counts/pixel or ~640 counts/s). The single?mol?
ecule?fluorescence signal exceeds this background by
up to 15 times.
The polarization anisotropy for a DiI dye is defined
where Ipar and Iperp are fluorescence intensities for po?
larization components parallel and perpendicular to
the alignment direction . Processing the images of
Fig. 5b)with background subtraction shows that from