Edge trapping of exciton-polariton condensates in etched pillars

Abstract

In this letter, we present a study of the condensation of exciton-polaritons in large etched pillar structures that exhibit shallow edge trapping. The ≈ 100 µm ×100 µm pillars were fabricated using photolithography and a BCl 3 /Cl 2 reactive ion etch. A low energy region emerged along the etched edge, with the minima ≈ 7 µm from the outer edge. The depth of the trap was 0.5 − 1.5 meV relative to the level central region, with the deepest trapping at the corners. We were able to produce a Bose-Einstein condensate in the trap near the edges and corners by pumping non-resonantly in the middle of the pillar. This condensate began as a set of disconnected condensates at various points along the edges, but then became a single mono-energetic condensate as the polariton density was increased. Similar edge traps could be used to produce shallow 1D traps along edges or other more complex traps using various etch geometries and scales.In the past two decades, many experiments have used polaritons resulting from strong coupling between trapped microcavity photons and quantum well (QW) excitons. These bosonic particles have a very light mass (∼ 10 −4 m e ) due to being partially photonic, but also strong particle-particle interactions from being partially excitonic.1 This combination of a light mass and strong interactions leads to the formation of Bose-Einstein condensates (BECs) at relatively high temperatures (∼ 10 K).2-4 Polaritons provide a promising system for studying bosonic particles at even higher temperatures, and polariton lasing has been observed at room temperature in both GaN 5 and organic 6 systems. Many methods of confinement have been used to study polariton dynamics in a variety of geometries. Applying stress to a thin (≈ 100 µm) GaAs sample can be used to shift the exciton energy, resulting in a harmonic trap. 4,7Pumping such a stress trap non-resonantly in the center forms a repulsive barrier and can be used to form a ring geometry.8 Complex pumping geometries can also be used to confine polaritons, including the use of two or more pump spots in various arrangements or using a ring-shaped pump spot.9-13 More permanent methods of confinement include producing a spacer in certain regions of the cavity during the growth process, 14-16 using sub-wavelength gratings as the top mirror, 17,18 depositing metal strips onto the top mirror, 19 and etching the sample after growth to form 1D wires, 2D pillars, and 2D arrays of coupled pillars. 20-25While optically induced trapping potentials have the advantage of being easily reconfigured, etched trapping allows the confinement to be somewhat independent of the pump laser. Post-growth etching also produces much higher potential barriers at the etched edges than the a) Electronic mail: dmm154@pitt.edu b) J. K. Wuenschell is now at Physical Sciences Laboratory, The Aerospace Corporation, El Segundo, CA 90245, USA deposition of metal strips, and it is compatible with our existing sample materials and growth methods, unlike sub-wavelength gratings or modulating the cavi...

Superlinear increase of photocurrent due to stimulated scattering into a polariton condensate

“…As reported in Ref. [6] and shown here, in our structures the strain at the edges of the pillar leads to energy minima for the polaritons along the sides which trap the polariton condensate. Figure 1(c)…”

“…Theoretical modeling shows that this is due to a stimulated exciton-exciton scattering process in which one exciton relaxes into the condensate, while another one dissociates into an electron-hole pair.Polariton condensation is now a well established effect [1][2][3]. The polaritons, which can be viewed as dressed photons with effective mass and repulsive interactions, undergo Bose-Einstein condensation and show many of the effects of bosonic stimulation and superfluidity [4,5], including rapidly flowing to the ground state of whatever potential profile they occupy [6]. Polariton condensates can range from strongly nonequilibrium all the way to equilibrium [7,8].…”

Polariton-enhanced exciton transport

“…The sample used in this experiment is the same as those used in previous work 8,10,22,24,32,33 The sample was mounted in a cold-finger cryostat, which could be rotated by angle θ. The pump laser was reflected off a beam splitter (BS) through the imaging microscope objective (MO).…”

Zitterbewegung of exciton-polaritons

“…at this point that recently substantially longer polariton lifetimes γ −1 C ∼ 200 ps and much longer travel distances (hundreds of microns to millimeters) have been achieved [20,21]. In order to achieve a strong zitterbewegung effect at long distances, we propose to excite polaritons resonantly by a cw probe beam E p± (r) with ω p being the frequency of the resonant pump relative to the minimum of the lower polariton branch.…”