Transport of indirect excitons in a potential energy gradient

Abstract

We realized a potential energy gradient -a ramp -for indirect excitons using a shaped electrode at constant voltage. We studied transport of indirect excitons along the ramp and observed that the exciton transport distance increases with increasing density and temperature.An indirect exciton in a coupled quantum well structure (CQW) is a bound state of an electron and a hole in separate wells (Fig. 1a). The spatial separation allows one to control the overlap of electron and hole wavefunctions and engineer structures with lifetimes of indirect excitons orders of magnitude longer than those of direct excitons. Long lifetimes of the indirect excitons allow them to travel over large distances before recombination [1][2][3][4][5][6][7][8][9][10] . Furthermore, indirect excitons have a built-in dipole moment ed, where d is close to the distance between the quantum well (QW) centers that allows their energy to be controlled by voltage: an electric field F z perpendicular to the QW plane results in the exciton energy shift E = edF z 11 . These properties allow studying transport of indirect excitons in electrostatically created in-plane potential landscapes E(x, y) = edF z (x, y).Exciton transport was studied in various electrostatic potential landscapes including circuit devices 12-14 , traps 15 , lattices 16,17 , moving lattices -conveyerscreated by a set of ac voltages 18 , and narrow channels 14,19,20 .Several exciton transport phenomena have been observed, including the inner ring in emission patterns 4,6,8,10,21,22 , transistor effect for excitons [12][13][14] , localization-delocalization transition in random potentials 4,6,8 and in lattices 16,17 , and dynamical localization-delocalization transition in conveyers 18 . Exciton transport was also studied in potential energy gradients created by voltage gradients in electrodes 1,7 .In this work, we study exciton transport in a potential energy gradient -a ramp -created by a shaped electrode at constant voltage. We utilize the ability to control exciton energy by electrode shape 23 and design the shape of a top electrode on the sample surface so that a voltage applied to it creates a constant potential energy gradient for indirect excitons in the CQW. The excitonic ramp realizes directed transport of excitons as a diode realizes directed transport of electrons.The advantages of this shaped-electrode-method include the suppression of heating by electric currents in electrodes (such currents may appear in the case when the ramp potential is created by a voltage gradient in the top electrode) and the opportunity to engineer the exciton energy profile along the ramp by designing the electrode shape. We also measure exciton transport in a narrow channel formed by a voltage applied to an electrode stripe of constant width -a flat-energy channel 14,20 .The CQW structure is grown by molecular beam epitaxy. An n + -GaAs layer with n Si = 10 18 cm 3 serves as a homogeneous bottom electrode. A semitransparent top electrode is fabricated by depositing a 100 nm indium tin oxide...

Pattern formation in the exciton inner ring

“…1(a) and 1(b)]. For samples with the same layer structure, designed patterns of top electrodes create the required in-plane potential landscapes for excitons, including traps, 20 lattices, 18,23,29 ramps, 30 and circuit devices. 14,15,19,21 In this work, we use a top electrode unstructured on a large area.…”

“…Transport of indirect excitons was studied in a variety of potential landscapes created by applied electric fields, including ramps, 4,11,12,30 traps, 20 lattices, 18,29 moving lattices-conveyers, 23 narrow channels, 16,19,25 and circuit devices, 14,15,19,21 as well as in optically induced traps. 10,13,24,27 A set of exciton transport phenomena was observed, including the transistor effect for excitons, 14,15,19,21 localizationdelocalization transition in random potentials 7,9,10,13,17,22 and in static and moving lattices, 18,23,29 and the inner ring in emission patterns.…”

Exciton Transport Phenomena in GaAs Coupled Quantum Wells

“…A potential energy gradient -a ramp -is created by a shaped electrode at constant voltage with no energy-dissipating voltage gradient [Leonard et al, 2012]. The design utilizes the ability to control exciton energy by electrode density [Kuznetsova et al, 2010a].…”

“…The development of excitonic devices that control exciton fluxes is mainly concentrated on indirect excitons in coupled quantum wells (CQW) [Hagn et al, 1995;Gärtner et al, 2006;High et al, 2007;1 Vögele et al, 2009;Kuznetsova et al, 2010b;Cohen et al, 2011;Winbow et al, 2011;Schinner et al, 2011;Leonard et al, 2012] and excitonpolaritons in microcavities [Amo et al, 2010;Gao et al, 2012;Ballarini et al, 2013;Nguyen et al, 2013;Sturm et al, 2014].…”

Indirect excitons in a potential energy landscape created by a perforated electrode

“…One instance of this method based on varying the electrode width-the shaped electrode methodhas been used to create confining potentials for IXs in traps 18 and ramps. 14,24 In this work, we present an excitonic device based on the electrode density modulation in which a potential energy gradient is created by a perforated electrode at a constant …”