Quantum Optics with single Nano-Objects

Outline:

• Introduction : nonlinear optics with single molecule

• Single Photon sources

• Photon antibunching in single quantum dot fluorescence

• Conclusion

ADVANTAGES

• No ensemble averaging• Statistical correlations• Extreme sensitivity to immediate local

nanoenvironment• No synchronisation needed, time evolution• Single quantum system

Inhomogenous spectrum at 2 K

-400 -200 0 200 4000

20

40

60

80

Laser detuning (MHz)

Flu

ores

cenc

e

Saturation of a SM line

-10 -5 0 5 100

1

2

3

Laser detuning (GHz)

Flu

ores

cenc

e

hom= 20 MHzI ~ 250Is

= 620 nm

s

s

IIII

SS

1

- Light Shift- Hyper-Raman Resonance

0

150

-100 -50 0 50 100100

120

140

c

b

a

Pump-Probe Detuning (MHz)

Flu

ores

cen

ce (

a. u

.)

p s

p

p

0s

Pump-probe experiments:

Lig

h t s

h ift

(

)

1000

= 2.4 = 4.7

-4

-2

0

2

4

0.50-0.5Inverse pump detuning (-1)

Electro-Optical Effects

Linear Stark Shift: ~ 0.3 D

- 1st case: High RF field, low laser intensity

Modulation of the molecular transition frequency Side Bands

RFRFE

0

RF

RFnJ

Modulation of the laser frequency

Linear Coupling with the RF field

E0RF cos(RF t)

RF = 80 MHz

0

20

40

60

-500 0 5000

10

20

Fluo

resc

ence

(Cou

nts/m

s)

Laser Detuning (MHz)

0

20

40

60

2nd case: High laser intensity, low RF field

- Molecule dressed by the laser field- RF coupling between 1,n> and |2,n> states

L

0

|e,n>

|g,n+1>

|1,n>

|2,n>

|g,n+1>

|e,n>

L

|g,n+1>

|e,n> |1,n>

|2,n>

L22LLG

- Rabi Resonance: RF = G

fixedRF, tuned L

22LRFres

100 MHz

0 1 2 3 4 5 6 70

1

2

3

4

5

6

7

Side

band

Pos

ition

( )

Laser Rabi Frequency ( )

Shift of the Rabi resonance function of the laser field amplitude

L=1.5

L=3.5

Quantum Cryptography:

* Quantum mechanics provide unconditional security for communication

* Encoding information on the polarization of single photons

Quantum computing:

Quantum logic gates based on single photons have been demonstrated

Triggered Single Photon Sources

Practical need for Single Photons:

Present day sources of single photons

- Correlated photon pairs* Atomic cascade* Parametric down conversion

- Highly attenuated laser (or LED) pulses

Problems: - Random time generation- Average photon’s number <<1

Present day sources of single photons

Use of a single quantum system

Yamamoto’s group experiment:- Coulomb Blockade in a mesoscopic double barrier p-n junction- temperature 50 mK (dilution cryostat)- low detection efficiency ~10-4

- low e-h recombination rate

Controlled excitation of a single molecule

Deterministic generation of single photons

Photon antibunching in single molecule fluorescence

Excitation: Rapid adiabatic passage

Two conditions:

Adiabatic passageTpass > TRabi

Rapid passageTpass << f

L >>

|1,n-2>

|1,n>

|1,n-1>

|2,n>

|2,n-1>

|2,n-2>

|e,n>

|g,n+1>

|g,n>

|g,n-1>

|e,n-1>

|e,n-2>

|e,n>

|g,n+1>

|g,n>

|g,n-1>

|e,n-1>

|e,n-2>

+0-0 0

400 8000

1

2

Time (ns)

Flu

ores

cenc

eV

olta

ge (

V)

Det

unin

g (

)

+45

-45

0

+30

-30

0

RF = 3 MHz, L = 2.6

- T = 250 ns, = 3 , 0 = 80 - Short rise time- Relaxation time (-1 8 ns)- Oscillations

1

2

3

4

5

Flu

ores

cenc

e (u

.a.)

Temps (ns)-10 0 10 20 30 40

.0

.5

Exc

ited

sta

te p

opul

atio

n

Detailed shape of a fluorescence burst

How many emitted photons per sweep?

Measurement of the autocorrelation function g(2)()Comparaison with Q.M.C. simulations

Histogram of time delays:

RF = 3MHz, L = 3.2

400

800

1200

0

2

4

400

800

0

2

4

6

-200 0 200

400

600

-200 0 200

0246

Num

ber

of C

oinc

iden

ces

Co r

rela

tio n

Fu n

c ti o

n (u

. a.)

Delay (ns)

0 = 25

0 = 44

0 = 65

0

10

20

30

40

50

60

70

Poissonian SubPoissonian

p(0)p(1)p(2)p(n>2)

- = 6 MHz, 0 = 44

- nav = 1.12

- p(1) = 0.68

- p(n>1) ~ 0.21

- Mandel Parameter

Q sour.= - 0.65 Qdetc.= - 0.006

Comparaison with a Coherent source

Room Temperature Single Room Temperature Single photon sourcephoton source

Principe de l’expérience- Pulsed excitation to a vibrationnaly excite level- Rapid Relaxation (ps) to the fluorescent state- Emission of a single photon

Experimental setup - Inverted Microscope- Piezo-electric Scanner- Coincidence Setup- Detection effeciency 6%

T.A.C.&

P.H.A. Stop

Start

Flu

ores

cenc

e

Exc

itatio

n (5

32nm

)

System: Terrylene in p-terphenylFavorable photophysical parameters and high photostability

Confocal fluorescence image(10m*10m) of single Terrylene molecules

CW ExcitationCW Excitation : Photon : Photon Antibunching Antibunching

20 W

210 W

W

670 W

1600 W

Fluorescence autocorrelation function g(2)() proportional to the excited state population, >0:

Signature of a single molecule emission

fss

sII

IIII

1exp1

1

Pulsed excitationPulsed excitation : : Triggered single photon emissionTriggered single photon emission

- Single exponential decay- fluorescence lifetime:

f = 3.8 ns

- M.L. 532 nm laser: - Pulse width: 35ps, - Repetition rate: = 6.25 MHz

FFluorescence autocorrluorescence autocorreelation Flation Fuunctionnction

Laser spot positioned• on a single molecule

(Signal/Background~ 6)(b) Away from any molecule

(background coherent emission )

central peak area / lateral peak area= (B2+2BS) / (B+S)2 ~ 0.27

Saturation du taux d’émissionSaturation du taux d’émission

Fit: S=343 kHz, Is=1.2 MW/cm2 86% of the pulses

lead to a single photon emission

At the maximum power :Smax=310 kHz, pmax(1)=0.86

Short laser pulse p<<f

p(2) ~ 0p(1) =(=p)

Emission rate:S= p(1)= S

(p)

0

10

20

30

40

50

60

70

80

90

Poissonian SubPoissonian

p(0)p(1)p(2)p(n>2)

- nav = 0.86

- p(1) = 0.86

- = 6.25 MHz

- Mandel Parameter

Q sour.= - 0.86 Qdetc.= - 0.03

- P(n>1) < 8 10-4 !

Comparaison Comparaison with a coherent with a coherent sourcesource

PPhoton statistics hoton statistics of sof single ingle quantum dot fluorescencequantum dot fluorescence

CdSe

ZnS- Colloidal CdSe/ZnS quantum dots

- 2 nm Radius, 575 peak emission

- fluorescence quantum yield ~50%, ~ 105 M-1 cm-1

- QDs bridge the gap between single molecules and bulk solid state- Size-dependent optical properties - Tunable absorbers and emitters- Applications from labeling to nano-devices

0 200 400 600 8000

100

200

300

400

Cou

nt R

ate

(kH

z)

Time (ms)

Intermittence in single QD Fluorescence

- Blinking attributed to  Auger ionization 

- High S/B ratio- Low photobleaching rate blea < 10-8

Blinking:ton , Intensity dependencetoff , no I dependence, inversepower law

Time Delay (ns)

-60 -40 -20 0 20 40 60

Coi

ncid

ence

s

0

50

100

150

Photon antibunching in single QD fluorescence

- Start-Stop setup- Coincidence histogram C()(TAC time window tTAC of 200 ns, bin width tbin of 0.2 ns)

Dip at =0, signature of a strong photon antibunching

C(0) ~ 0 for a large range of intensities (0.1 – 100 kW/cm2)

High Auger ionization rates (~1/20 ps-1, Klimov et al.)

No multi-excitonic radiative recombination

Many QDs vs single QD

Time delay (ns)

-40 0 40

Coi

nci

den

ces

0

50

100

150

Fluorescence Lifetime (ns)12 14 16 18 20 22 24 26 28

Num

ber

of Q

Ds

0

2

4

6

8

10

12

14

Quantum dot lifetimes measurements

0 20 40 60 80 100 120 140 160

0,01

0,1

1

fluo

resc

ence

dec

ay (

a.u.

)

time (ns)

Bulk measurement with TCSPC(M. Dahan et al., 2000)

Multi-exponential decay

- Experimental accuracy ~ ns- Width of f histogram : heterogenety in the QDs structure !?

Single QDs measurements at low intensities

fGC exp12

Intensity (kW/cm2)

5 15 25 35 45

Ris

e R

ate

(ns-1

)

0,04

0,06

0,08

0,10

Delay (ns)

-80 -40 0 40 80

Co

inci

den

ces

0

20

40

Delay (ns)

-80 -40 0 40 80

Co

inci

den

ces

20

40

Saturation intensities:Isat ~ 10-80 kW/cm2

Cross-section :abs ~ 2 – 16 10-16 cm2

From the QD state filling equations fsatIIC 1exp1

Delay (ns)

0 2000 4000 6000 8000

Coi

ncid

ence

s

0

40

80

120

160

Intensity (kW/cm2)

0 20 40 60 80 100

Cou

nt R

ate

(kH

z)

0

100

200

300

400

500

Count rate saturation

- At high intensities, very short ton , average count rate Rav skewed- Use coincidence histogram for accurate value Rav in the On state- With large tTAC and high Rav ~ interphoton mean time (1/ Rav)

)exp()()( 2 avRGC

Good agreemeny For the measured Isat

Conclusion

- Demonstration of a single photon source based on controlled fluorescence from single molecule

- Room temperature operation- Improve the collection efficiency de collection (cavity…)- Other systems:

* NV centers (antibunching observed)* Quantum dots : at low T (<5K) antibunching in

spectrally selected fluorescence from InAs QDs

- Photon antibunching in colloidal CdSe QDs Efficient Auger ionization effect