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 SAW SAW 1 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 d Energy z - + CB VB AlGaAs GaAs z AlGaAs GaAs Energy z CB VB CB VB ∆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 dimensions Composed 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 dimensions (2D) → BEYOND MEAN FIELD

The nanophotonics group Cold dipolar fluids in two dimensions new 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, (2012). A. A.High. et al. Nature 483, 584–588 (2012).

I – Dipolar exciton correlation measurements

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 Position (microns) Wavelength (nm))

The nanophotonics group Trapped dipolar exciton fluid Exciton electrostatic traps – dipoles are trapped under a semitransparent gate via electrostatic forces Position (microns) 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 regime T independent

The nanophotonics group r0r0 Balance between quantum motion and repulsion Lower T: r 0 < T - Quantum correlations High T: r 0 > T - Classical correlations Balance between thermal motion and repulsion Temperature dependence No temperature dependence

The nanophotonics group Deviation from thermal distribution below ~2.5K  Missing particles!

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 (S=±2) (S=±1)

The nanophotonics group Mapping from trapped fluid dynamics

II – Multi-functional exciton circuit

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 inside potential 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 A demonstration of a multi-functional device

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

The nanophotonics group Remote dipolar interactions Dipolar interaction is relatively long range. Can it have an effect over a macroscopic distance? Fluid AFluid 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) Time Pump density Probe energy

The nanophotonics group Can remote dipolar interactions be measured? + -

The nanophotonics group Energy profile di r e ct ind irec t (di pol ar)

The nanophotonics group di r e ct ind irec t (di pol ar) Time resolved pump-probe experiments

The nanophotonics group Intensity Observing remote interactions ∆E di r e ct ind irec t (di pol ar) Better long time electrostatic stability is still required for a reliable density calibration

The nanophotonics group Thank you!