COLD DIPOLAR EXCITONS ON A CHIP – FROM FUNDAMENTAL MANY-BODY PHYSICS TO MULTI-FUNCTIONAL CIRCUITRY...

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COLD DIPOLAR EXCITONS ON A CHIP – FROM FUNDAMENTAL MANY- BODY PHYSICS TO MULTI- FUNCTIONAL CIRCUITRY Ronen Rapaport The Racah Institute of Physics and the School of Engineering, The Hebrew University of Jerusalem λ S AW SAW 1 SAW 2

Transcript of COLD DIPOLAR EXCITONS ON A CHIP – FROM FUNDAMENTAL MANY-BODY PHYSICS TO MULTI-FUNCTIONAL CIRCUITRY...

COLD DIPOLAR EXCITONS ON A CHIP – FROM FUNDAMENTAL MANY-BODY

PHYSICS TO MULTI-FUNCTIONAL CIRCUITRY

Ronen RapaportThe Racah Institute of Physics and the School of

Engineering,The Hebrew University of Jerusalem

λ SAW

SAW1

SAW

2

The nanophotonics group

Yehiel Shilo

Kobi Cohen

Ronen Rapaport

Boris Laikhtman

Loren Pfeiffer

Ken West

Paulo Santos

Snezana Lazic

Adriano Violante

Rudolph Hey

The nanophotonics group

Outline

Fundamental aspects: I - experiments on trapped dipolar excitons – evidence for strong particle correlations, dark excitons condensate

Dipolar exciton functional devices:II - Demonstration of an exciton acoustic multiplexer circuit

III (not presented) - Remote dipolar interactions

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Dipolar excitons in semiconductor bilayers

+ + +

- - -

z

d

dEnergy

z

-

+

CB

VB

AlGaAs

GaAs-+

-+

-+

-+

z

AlGaAs

GaAs-+

-+

-+

-+

Energy

z

CB

VB

-

+

-

+

CB

VB

∆V

e∆V

e∆V

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dipolar excitons

+ +

- -

r

+ + +

- - -

z

d∆V

2D dipolar fluid – aligned dipoles – repulsive interaction

Boson quasi-particles (integer spin) – Bose fluid at low T (<4K)

Spin degeneracy of 4: 2-bright excitons (S=±1),

2-dark excitons (S=±2)

Long tunable lifetime (nanoseconds to microseconds)

Easy to observe and measure – emit photons!We can “see” excitons…

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Weakly interacting quantum fluids

Cold atoms Exciton-polaritons in semiconductor microcavities

Common feature:weakly interacting particles →

Local (contact) interactions

Point particles – weak spatial correlations – mean field

description (generally speaking)

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Cold dipolar fluids in two dimensionsComposed of particles with a permanent dipole moment

Longer range interactions

Non-trivial particle correlations in both quantum and classical regimes

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Cold dipolar fluids in two dimensions (2D)

BEYOND MEAN FIELD

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Cold dipolar fluids in two dimensionsnew correlation regimes and phases are expected, e.g.:

•Classical and quantum particle correlations•Gas – liquid transitions (both quantum and classical)•beyond Bogoliubov excitation spectrum – rotons•Superfluidity and crystalization.

BL, RR, PRB 2009

Measuring particle correlations is essential to understand the many-body classical and quantum physics of dipolar fluids

Schindler, Zimmerman, PRB, (2008)Astrakharchik et al. Phys. Rev. Lett. (2007) .

Buchler et al. Phys. Rev. Lett. (2007).Boning et al. Phys. Rev. B (2011).Berman et al.Phys. Rev. B (2012).

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Observation of spontaneous coherence of a cold dipolar exciton fluid

A. A. High et al. Nano Letters 12, 2605-2609 (2012).

A. A.High. et al. Nature 483, 584–588 (2012).

I – Dipolar exciton correlation measurements

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dipolar excitons

+ +

- -

r

+ + +

- - -

z

d∆V

Excitons emit photons an optical probe of the system:

Single exciton energy interaction energy with other dipoles

Direct measurement of d-d interaction!→Direct window to particle correlations, fluid phases

Energy of emitted Photon:

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Can we see evidence for particle correlations?

Technique: time resolved spectroscopy of trapped dipolar excitons -

Advantages:

•Homogeneous fluid in thermal equilibrium with no particle source

•Allows density calibration (at least relative) by “photon counting” and knowledge of the thermal distribution •Allows to see fast dynamics

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Trapped dipolar exciton fluid

Exciton electrostatic traps – dipoles are trapped under a semitransparent gate via electrostatic forces

Posi

tion

(mic

rons

)

Wavelength (nm))

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Trapped dipolar exciton fluid

Exciton electrostatic traps – dipoles are trapped under a semitransparent gate via electrostatic forces

Posi

tion

(mic

rons

)

Wavelength (nm))

Note:- Spatial confinement- Flat density distribution- Reduction of interaction energy as density decays

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Mapping from trapped fluid dynamics

Single exciton energy

Dipolar interaction energy

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Mapping from trapped fluid dynamics

Mean field prediction:No temperature dependence!

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• beyond mean field prediction- dipolar correlations!• Two correlation regimes

T>2.5K

E int

Mean field prediction

T dependent regim

e

T independent

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r0

Balance between quantum motion and repulsion

Lower T: r0 < T - Quantum correlations

3

22

2

2

~r

de

rM X

High T: r0 > T - Classical correlationsBalance between thermal motion and repulsion

Temperature dependence

No temperature dependence

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Deviation from thermal distribution below ~2.5K Missing particles!

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less bright excitons missing particles

Dark exciton (S=±2) accumulation (condensation)?(S=±2)

(S=±1)<0.1meV?

Mapping

larger ΔE larger density more particles

T< 2.5K

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(S=±2)

(S=±1)

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Mapping from trapped fluid dynamics

II – Multi-functional exciton circuit

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•Vision: Future coherent exciton circuitry

•More control and manipulation tools more access to investigate interesting physical phenomena

Why?

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Dipolar exciton devices: How to control exciton motion?

•Surface acoustic waves (SAW) introduce a traveling strain field.

•Causes bandgap modulation .

•Allows for exciton transport insidepotential minima .

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Transport by surface acoustic waves

•SAW is generated using RF transducers.

•Propagation distance of milimeters!

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A transistor with surface acoustic waves •Transport using SAW.

•Electrical switching between ON/OFF states.

Based on:High et al. Opt. Lett. (2007).High, et al. Science (2008).

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Switching using surface acoustic waves

•Channel switching by interfering SAWs

Simulation based on nonlinear exciton diffusion model:RR, GC, SS, APL (2006)

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A demonstration of a multi-functional device

III – Remote dipolar interactions(not presented in the talk)

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Remote dipolar interactions

•Dipolar interaction is relatively long range.•Can it have an effect over a macroscopic

distance?

Fluid A Fluid B

Intra-fluid

Inter-fluid

+ +

- -

r

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Remote interaction for density calibration

KC, PS, and RR, PRL 2011

Interaction energy of a homogeneous trapped fluid

But, for a remote dipole

Local correlations not important – only geometry

Model independent relation between density and density

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Using remote interactions to manipulate exciton flow

KC, PS, and RR, PRL 2011

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Measuring remote interactions

•Measure the interaction of one fluid on another

•Pump-probe experiment•Time and space resolved spectroscopy

Probe laser (CW)

Pump laser (pulsed)

TimeTime

Pumpdensity

Probeenergy

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Can remote dipolar interactions be measured?

+

-

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Energy profile

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Time resolved pump-probe experiments

Probe indirect exciton

Wavelength )nm(

Pos

ition

) m

(

810 815 820

0

50

100

150

200

t=1800ns after pulse

Wavelength )nm(

Pos

ition

) m

(

810 815 820

0

50

100

150

200

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Inte

nsity

Observing remote interactions

∆E

Better long time electrostatic stability is still required for a reliable density calibration

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Thank you!