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