COLD DIPOLAR EXCITONS ON A CHIP – FROM FUNDAMENTAL MANY-BODY PHYSICS TO MULTI-FUNCTIONAL CIRCUITRY...
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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
The nanophotonics group
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
The nanophotonics group
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…
The nanophotonics group
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)
The nanophotonics group
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
The nanophotonics group
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).
The nanophotonics group
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).
The nanophotonics group
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:
The nanophotonics group
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
The nanophotonics group
Trapped dipolar exciton fluid
Exciton electrostatic traps – dipoles are trapped under a semitransparent gate via electrostatic forces
Posi
tion
(mic
rons
)
Wavelength (nm))
The nanophotonics group
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
The nanophotonics group
Mapping from trapped fluid dynamics
Single exciton energy
Dipolar interaction energy
The nanophotonics group
Mapping from trapped fluid dynamics
Mean field prediction:No temperature dependence!
The nanophotonics group
• 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
The nanophotonics group
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
The nanophotonics group
•Vision: Future coherent exciton circuitry
•More control and manipulation tools more access to investigate interesting physical phenomena
Why?
The nanophotonics group
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 .
The nanophotonics group
Transport by surface acoustic waves
•SAW is generated using RF transducers.
•Propagation distance of milimeters!
The nanophotonics group
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).
The nanophotonics group
Switching using surface acoustic waves
•Channel switching by interfering SAWs
Simulation based on nonlinear exciton diffusion model:RR, GC, SS, APL (2006)
The nanophotonics group
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
The nanophotonics group
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
The nanophotonics group
Using remote interactions to manipulate exciton flow
KC, PS, and RR, PRL 2011
The nanophotonics group
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
The nanophotonics group
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
The nanophotonics group
Inte
nsity
Observing remote interactions
∆E
Better long time electrostatic stability is still required for a reliable density calibration