Svetlana G. Lukishova, Luke J. Bissell, Ansgar W. Schmid1, Zhimin Shi, Heedeuk Shin, Russel Knox2, Patrick Freivald2, Simon K.H. Wei3 Robert W. Boyd, Carlos R. Stroud, Jr., Show-H. Chen3, Kenneth Marshall1
RoomRoom--Temperature Single Photon Sources withTemperature Single Photon Sources withFluorescence Emitters in Liquid Crystal HostsFluorescence Emitters in Liquid Crystal Hosts
The Institute of Optics, University of Rochester1
Laboratory for Laser Energetics, University of Rochester 2
Department of Physics, University of Rochester3 Department of Chemical Engineering, University of Rochester
The International Conference on Quantum Information, Rochester NY, June 14, 2007
Our goal: Device: efficient, polarized, stable, robust single photon
source on demand operating at room temperatureUS Patent (allowed in April 2007):S.G. Lukishova, R.W. Boyd, C.R. Stroud, “Efficient room-temperature source of
polarized single photons”
(Appl. # US 10/753,323, PCT International Patent Application No PCT/US04/00362).
Current state of the art (“best”
SPSs):(1)
Semiconductor heterostructured
quantum dots (optically [1] and electrically pumped [2]) –
cryogenic temperatures;(2) Single color centers in diamond -
definite wavelength only;(3) Atoms and ions in the cavity –
need robustness.
[1]. Y. Yamamoto et al., Prog. In Inform., No 1, 5 (2005) –
review.
[2]. A.J. Bennett et al., Phys. Stat. Sol. B, 243, is. 14, 3730 (2006) –
review.
To use liquid crystal hosts doped with single emitters (including liquid crystal oligomers/polymers) to align emitter dipoles along the direction preferable for excitation efficiency (along the light polarization).
We propose
Deterministic molecular alignment will provide single photons of definite linear polarization.
Ek
Chiral
liquid crystal hosts provide 1-D photonic crystal environment for the emitter (for efficient lasing in these hosts
see review 1];
Chiral
liquid crystal hosts provide circular polarization of definite handedness to emitted photons;
Liquid crystals can easily infiltrate photonic crystals [2,3] or
photonic crystal fibers [4] providing bandgap
tunability;
2-D and 3-D photonic crystals can be prepared from holographic polymer-dispersed liquid crystals [5];
Removing oxygen from liquid crystal reduces emitter bleaching
[1]. V. Kopp, Z. Zhang, A. Genack, Prog. Quant. Electron., 27, 369 (2003). [2]. K. Bush and S. John, Phys. Rev. Lett, 83, 967 (1999).[3]. E. Yablonovich, Nature, 401, 539 (1999).[4]. T.T. Larsen, A. Bjarklev, et al., Optics Express, 11, 2589 (2003).[5]. M.J. Escuti, J. Qi
and G.P. Crawford, Opt. Lett., 28, No 7, 522 (2003).
Outline
SPS with circularly polarized photons of definite handedness (CdSe/ZnS
colloidal quantum dots in aligned chiral
nematic
liquid crystal host);
SPS with linearly polarized photons (single dye molecules in aligned nematic
liquid crystal host);
Some other possibilities of organic microcavities
for SPS
The results of several experiments on SPS using liquid crystal host:
Po
Planar-alignedcholesteric
Transmitted LH light
Incident unpolarized light
Reflected RH light
Po
Planar-alignedcholesteric
Transmitted LH light
Incident unpolarized light
Reflected RH light
where pitch Po = 2a (a is a period of the structure);
λo = nav Po , Δλ = λo Δn/nav ,
nav
= (ne
+ no
)/2; Δn = ne
- no .
In addition, planar-aligned cholesterics
with 1-D chiral
photonic bandgap structure provides circularly polarized
fluorescence of definite handedness even for emitters without dipole moments
Cholesteric
(chiral
nematic) provides a photonic crystal environment into which the chromophore
will emit.
Chiral
liquid crystal hosts
The helical structure of the cholesteric phase, with the helical axis in the plane of the substrate.
Cholesteric (chiral nematics) liquid crystals: low molecular weight (fluid) and glassy oligomers (solid).
(For sample preparation details see Ref. 1).
Single emitters: CdSe/CdS
quantum dots and/or single dye molecules
1.
S.G. Lukishova, A.W. Schmid, C. M. Supranowitz, N. Lippa, A.J. McNamara, R.W. Boyd and C.R. Stroud, J. Mod. Optics, Special issue “Single Photon: Detect., Appl. and Measur. Methods”, 51, No 9-10, 1535 (2004).
Confocal fluorescence microscope and Hanbury
Brown and Twiss
setup
76 MHz repetition rate, ~6 ps
pulsed-laser excitation at 532 nm
Fluorescence antibunching
from CdSe quantum dotin 1-D photonic bandgap cholesteric host
15 μm x 15 μm scan
8
10
12
14
16
18
20
22
-80 -60 -40 -20 0 20 40interphoton time (ns)
coin
cide
nce
coun
ts
Selective transmission curves of prepared photonic bandgap structures and fluorescence spectrum
of CdSe/CdS
QD solution in hexane
0
10
20
3040
50
60
70
80
500 550 600 650 700
Wavelength, nm
Fluo
resc
ence
int
ensi
ty,
rel.
units
Circularly polarized fluorescence from several single quantum dots in photonic bandgap CLC host
The degree of circular polarization is measured by the dissymmetry factor ge
= 2(IL
– IR
)/(IL
+IR
) [1].
At 575 nm ge
= -1.6. For unpolarized
light ge
= 0.
[1] S. H. Chen et. al, Nature, 397, 506-508 (1999)
RHP
LHP
Host background can be removed using QDs
which fluoresce outside host’s fluorescence band
Host background fluorescence (cholesteric
mixture of E7 and CB15)
Estimation of an efficiency P of polarized single-photon emission into the collecting objective
P ~ 48 %
Noutput
=Nincmolec
αβγQP
α
= 0.4 is the measured transmission of all filters, β
= 0.45 is the measured transmission and collection of the objective and microscope optics, γ=0.4 is the QD quantum yield, and Q=0.58 is quantum efficiency of the APD at 580 nm.
Experimental setup for linearly polarized fluorescence measurements [1]
Witec
alpha-SNOM microscope was used in confocal
transmission mode
[1]. S.G. Lukishova
et al., J. Modern Optics, Special Issue on Single Photon: Sources, Detectors, Applications and Measurement Methods, 54, iss. 2 & 3, 417-429 (2007).
532 nm cw laser APD
APD
Singlemodefiber
Sample Filters
50/50 polarizing beamsplitter cube
Transmissionconfocalmicroscope
532 nm cw laser APDAPD
APD
APD
Singlemodefiber
Sample Filters
50/50 polarizing beamsplitter cube
Transmissionconfocalmicroscope
Rotationof 45o
Photoalignment with polarized UV-light
Glassy nematic liquid crystal layers doped with single DiI
dye molecules (layer thickness is ~ 100nm)
Polarizing microscope images of photoaligned
glassy nematic liquid crystal layers show good planar alignment
Deterministically linearly polarized fluorescence of single dye molecules [DiIC18
(3)] in glassy nematic liquid crystal host
Perpendicular Parallel
Figures clearly show that for this sample, the polarization direction of the fluorescence of single molecules is predominantly in the direction perpendicular to the alignment of liquid crystal molecules.It is important that the background levels of left and right figures
are the same.
Deterministically polarized fluorescence of single dye molecules
[DiIC18
(3)] in glassy nematic liquid crystal host (continued)
0
2
4
6
8
10
12
14
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
Polarization anisotropy
Num
ber o
f mol
ecul
es
0
2
4
6
8
10
12
14
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
Polarization anisotropy
Num
ber o
f mol
ecul
es ρ = ( I par
- I perp
) / ( I par
+ I perp
)Polarization anisotropy
Random orientation of DiIC18
(3) molecules (left -
theory; right -
experiment) –
from I. Chung, K.T. Shimizu, M.G. Bawendi, PNAS, 100, 405 (2003).
38 molecules
Processing the images with background subtraction shows that from a total of 38 molecules, 31 molecules have a negative r value, three molecules a zero value, and only four molecules have a positive value of ρ.
A highly asymmetrical histogram for ρ
is depicted (left figure), that greatly differs from both theoretical (center) and experimental (right) histograms of ρ
for the same DiIC18
(3) dye when randomly oriented.
For random orientation this histogram is symmetrical: the number
of molecules with positive ρ
is the same as that with negative ρ
in contrast with the histogram in left figure.
NN
N
CH
CH
CH
Molecular structure of DiIC18
(3) dye.
Absorbing and emitting dipoles are nearly parallel to the bridge (perpendicular to two alkyl chains) *)
.
*) B. Stevens and T. Ha, J. Chem. Phys., 120, 3030 (2004).
The two alkyl chains likely orient themselves parallel to the rod-
like liquid crystal molecules, but the emitting/absorbing dipoles that are nearly parallel to the bridge (perpendicular to alkyl chains) will be directed perpendicular to the liquid crystal alignment. DiIC18
(3) molecules orient in the same manner in cell membranes*).
This predominance of “perpendicular”
polarization can be explained by the DiIC18
(3)’s molecular structure.
05
101520253035
570 590 610 630 650 670 690
Wavelength, nm
Fluo
resc
ence
inte
nsity
, ar
bitr
ary
units
Spectrofluorimeter
measurements of a polarized fluorescence of DiIC18
(3) dye doped in planar-aligned glassy nematic
liquid crystal (~0.5% concentration by weight, 4.1 μm layer thickness)
Dye fluorescence shows polarization anisotropyρ
= (Ipar – Iperp
)/ (Ipar
+ Iperp
) = -
0.5
Perpendicular
Parallel
532-nm excitation
For the laser power incident on the sample (input power on the first objective) ~8 μW, a beam radius ~ 0.2 μm, and using value of σ
~ 1.3 ×
10-16
cm2
for DiIC18
(3) molecules, we arrive at photons/s per molecule Nincmol
= 2.21 ×
106
photons/(s mol).
We can evaluate the efficiency P from the following expression: Noutpol
= 0.96 Nincmol
αβPDQ. Here Noutpol
= 3.65 kc/s is the measured polarized photon count rate from a single molecule with ρ
= 1, α
= 0.525 is the measured transmission of rejection filters, β
= 0.45 is the measured total transmission and collection coefficient of two objectives and microscope optics after the second objective but before the beam splitter cube (without rejection filters), D = 0.2 is the measured coupling efficiency of the fiber optics used in this setup, Q = 0.58 is the photon detection efficiency of the avalanche photodiode at 580 nm, 0.96
is the coupling efficiency from the fiber to the APD-FC-connector. From these data we deduced P ≈
6.3% .
Estimation of an efficiency P of polarized single-photon emission into the collecting objective
Antibunching in the fluorescence of single terrylene molecule in a glassy liquid crystal host [1]
Coi
ncid
ence
eve
nts
n(τ )
0 0- 26 26Time interval τ, ns
-26 260
10
Coi
ncid
ence
eve
nts
n(τ )
0 0- 26 26Time interval τ, ns
-26 260
10
Left histogram exhibits a dip at τ
= 0 indicating photon antibunching in the fluorescence of the single dye molecule; no antibunching is observed in the right histogram in the fluorescence of the clusters of the molecules.
1. S. G. Lukishova
et al., IEEE J. Selected Topics in Quantum Electronics, Special Issue on Quantum Internet Technologies, 9, No. 6, 1512 (2003).
02468
101214
0 10 20 30 40 50 60Time, min
Fluo
resc
ence
inte
nsity
, kc
ount
s/s
Preventing terrylene dye bleaching in liquid crystal host [1]
Over the course of more than one hour, no dye bleaching was observed in the oxygen - depleted liquid crystal host (upper curve) in difference with the host without treatment (lower curve).
1. S. G. Lukishova
et al., IEEE J. Selected Topics in Quantum Electronics, Special Issue on Quantum Internet Technologies, 9, No. 6, 1512 (2003).
Overcoming the emitter bleaching problem (some other papers):
B. Lounis
and. W.E. Moerner, Nature, 407, 491 (2000). – several hours of pulsed excitation of single terrylene
dye molecules in p-terphenyl
molecular crystal host without bleaching;
L. Fleury, J.-M. Segura, G. Zumofen, B. Hecht, U.P. Wild, Phys. Rev. Lett., 84, 1148 (2000) –
reducing bleaching of single molecule fluorescence of terrylene
in p-terphenyl
host under cw-excitation;
T.-H. Lee, P. Kumar, A. Mehta, K. Xu, R.M. Dickson, M.D. Barnes, Appl. Phys. Lett., 85, 100 (2004). –
conducting polymer nanoparticles
prepared by microdroplet
technique (~2.5 hours cw
excitation without chromophore
bleaching).
Scanning electron micrographs of the cubic photonic crystal formed in holographic polymer
dispersed liquid crystals. (From Reference 1).
Micrograph of the guided modes in a photonic crystal fiber filled with chiral
nematic
liquid crystal at four different temperatures: green -
T =77oC; yellow -
T =89oC; nothing –
phase transition; blue -
T = 94oC. The bandgap
location sensitivity is ~ 1nm/oC and 3nm/oC in visible and infrared region respectively. (From Reference 2).
[1]. M.J. Escuti, J. Qi
and G.P. Crawford, Opt. Lett., 28, No 7, 522 (2003).[2]. T.T. Larsen, A. Bjarklev, D.S. Hermann, J. Broeng, Optics Express, 11,
No. 20, 2589 (2003).
3-D photonic crystal structures in liquid crystals (left) and photonic crystal fiber infiltrated with liquid crystals (right)
Polymer MicrocavityPolymer Microcavity
LLaaNNMMPP
Distributed Bragg Reflecting (DBR) mirrors comprising of alternating layers of λ/4n thickness. Alternating layers comprise of poly –
vinyl carbazole
(PVK) and poly-acrylic acid (PAA)Refractive index of the two layers: nPVK
=1.683, and nPAA
=1.47Cavity layer formed by λ/n thick PVK
layer with embedded CdSe/ZnS
core-
shell quantum dots. Glass substrate
DBR MirrorPVK/PAACavity with CdSe
Qdots
Glass Substrate
10 periods
5 periodsλ/n
Cavity
400 500 600 700 800 900 10000.0
0.2
0.4
0.6
0.8
1.0
Nor
mal
ized
Ref
lect
ance
Wavelength (nm)
Cavity Mode
400 600 800 1000
0.2
0.4
0.6
0.8
1.0
Nor
mal
ized
Ref
lect
ance
Wavelength (nm)
400 500 600 700 800 900 10000.0
0.2
0.4
0.6
0.8
1.0
Nor
mal
ized
Ref
lect
ance
Wavelength (nm)
Cavity Mode
400 600 800 1000
0.2
0.4
0.6
0.8
1.0
Nor
mal
ized
Ref
lect
ance
Wavelength (nm)
Results of V. Menon
group
LLaaNNMMPP
Time Time ––
Resolved PhotoluminescenceResolved Photoluminescence
500 1000 1500 2000 2500 3000
0.0
0.2
0.4
0.6
0.8
1.0 QDs in microcavity Bare QDs QDs in PVK
τ ∼ 150 ps
τ ∼ 400 ps
Nor
mal
ized
PL
inte
nsity
Time (ps)
τ ∼ 1000 ps
Results of V. Menon
group
N. V. Valappil, I. Zeylikovich, T. Gayen, B. B. Das, R R. Alfano, and V. M. Menon, Paper # M10.1, MRS Fall Meeting, Boston (2006).
70
20
30
40
50
60
200
0
25
50
75
100
125
150
175
2000 25 50 75 100 125 150 175
Single-CdSe/ZnS-QD fluorescence in DBR mirror structure with a defect layer (~7.7 nM
concentration of QDs
in solution)
Future plans: micropillar
microcavity
(in a progress)
SEM image is taken from Ref. 1
[1]. M. Kahl, T. Thomay, V. Kohnle, K. Beha, et al., QELS 2007, paper QFC5.
10μm x 10 μm raster scanCooled EM-CCD-camera image
Room-temperature, robust single photon sources were demonstrated, based on colloidal CdSe/CdS
QD and/or
single-dye molecule fluorescence in liquid crystal hosts (fluorescence antibunching);Circular polarized fluorescence of definite handedness from single CdSe/CdS
QDs
was observed for the first time
(owing to 1-D photonic bandgap
chiral
cavity);Deterministic linear polarization of single-dye-molecule fluorescence was observed;Dye-bleaching was significantly reduced using oxygen depleted liquid crystals.
Summary
Future PlansPbSe
QDs
in photonic bandgap
cavities will be used to
generate single photons at telecom wavelengths;Increasing SPS efficiency, narrowing spectral width and diminishing fluorescence lifetime of single photons;Electroluminescent deterministically polarized single-photon source with single colloidal QDs
in photonic bandgap
cavities
(recently electroluminescence of single colloidal CdSe
QD was reported [1]).Other types of microcavities/nanostructures in liquid crystals (including 2D and 3D);Liquid crystal infiltrated photonic crystal/photonic crystal microcavities
for tunable single photon source.
[1] H. Huan
et. al, App. Phys. Lett. 90, 023110 (2007)
Acknowledgments
The authors acknowledge the support by the U.S. Army Research Office under Award No. DAAD19-02-1-0285 and National Science Foundation Awards ECS-0420888 and EEC-
0243779. L.B. gratefully acknowledges the support of an Air
Force SMART fellowship.
Receipt of QDs
from M. Hann
and T. Krauss is gratefully acknowledged. The authors also thank A. Lieb
and L. Novotny for advice and help and J. Dowling for discussions.
Slide Number 1Slide Number 2Slide Number 3Slide Number 4Slide Number 5Slide Number 6Slide Number 7Confocal fluorescence microscope and Hanbury Brown and Twiss setupSlide Number 9Slide Number 10Slide Number 11Slide Number 12Slide Number 13Slide Number 14Slide Number 15Slide Number 16Slide Number 17Slide Number 18Slide Number 19Slide Number 20Slide Number 21Slide Number 22Slide Number 23Slide Number 24Polymer MicrocavitySlide Number 26Slide Number 27Slide Number 28Slide Number 29Slide Number 30
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