Spin selective filtering of polariton condensate flow

Abstract

Spin-selective spatial filtering of propagating polariton condensates, using a controllable spin-dependent gating barrier, in a one-dimensional semiconductor microcavity ridge waveguide is reported. A nonresonant laser beam provides the source of propagating polaritons, while a second circularly polarized weak beam imprints a spin dependent potential barrier, which gates the polariton flow and generates polariton spin currents. A complete spin-based control over the blocked and transmitted polaritons is obtained by varying the gate polarization. The generation of spin currents from the passage of electrons through ferromagnets is an important building-block for spintronic devices. For example, magnetic random access memory has developed from the spin valve concept, in which the current through a pair of ferromagnets is strongly attenuated when the magnets have opposite orientations. 1 This principle has a striking analogy in optics, where the attenuation of light passing through crossed polarizes also leads to information processing devices such as spatial light modulators based on liquid crystals. 2 Indeed analogies between spintronics and optics led to the emerging field of spinoptronics, 3 aiming to exploit a hybridization of the different fields.This hybridization is well illustrated by semiconductor microcavities in the strong coupling regime, which have become a promising basis for all-optical devices and circuits. [4][5][6][7][8] The strong coupling regime gives rise to excitonpolaritons (for simplicity, polaritons), whose light mass and integer spin facilitates condensation at elevated temperatures. 9,10 The excitonic fraction leads to strong carrier-carrier interactions 11 and sensitivity to gating electromagnetic fields. 12 At the same time, the photonic fraction allows the spin state of the polariton to be directly imprinted onto the polarization of the emitted light. An advantage over conventional spintronic devices is that, being neutral particles, polaritons do not experience strong dephasing due to Coulomb scattering and the coherent propagation of spin currents can be achieved over hundreds of microns. 13 Optically controlled carrier-carrier interactions in strongly coupled semiconductor microcavities have been shown to be a highly effective tool in the engineering of polaritons' energy landscapes, 14 for both the study of polariton condensate phenomena [15][16][17][18] and the realization of nascent polariton devices. [19][20][21][22][23][24] The interaction strength is spin dependent; 25-27 excitons with parallel spins experience a repulsive force, while the interaction between excitons with anti-parallel spin is weaker and can be attractive in nature. 28 In this work, we show that these anisotropic interactions allow the construction of a photonic analogue of current spin polarization as in a ferromagnet. Namely, we demonstrate spin polarization control of a polariton condensate signal using an optical gate in a high-finesse microcavity ridge. A schematic of the device is shown in Fig. 1(a)

All-optical quantum fluid spin beam splitter

“…Moreover, polaritonic systems have been shown to feature a wide range of resonant spin switching and multi-stability regimes [10,11], while latest advances have also demonstrated spin switching and bistability in the non-resonant optical pumping configuration [12][13][14]. These advantages have lead to extensive theoretical suggestions of polariton based spin circuits [15,16] as well as the realization of polariton optical spin filters [17]. However, a configuration that would allow for the directional and spatial separation of the spin components of a polariton condensate is yet to be demonstrated Coherent polariton circuits can be engineered by modifying the potential landscape of the microcavity through deep etching of the structure.…”

“…Although MC structures in the resonant excitation and OPO regime have been proposed as the basis of spin switching devices [10,11,26], the fine control of the excitation energy required for the switching operation hinders efficient applicability of these techniques. Nevertheless, optical control of the circular polarization of polariton emission has also been demonstrated under nonresonant optical injection in planar MCs [12,13,17,27,28], where intricate spin patterns have been observed due to the TE-TM splitting of the cavity mode [3,29,30].…”

Polariton spin whirls

“…Due to the OSHE, the long range coherence 24 and fast spin dynamics 25 , polaritons have been proposed as a potential candidate for the realization of a new generation of spinoptronic devices 26 . In this regard, the contribution of a spin dependent exciton reservoir has not been considered thoroughly, although in non-resonant experiments and in the proximity of the excitation spot, exciton interactions dominate over other types of interactions 27 and can directly affect the spin dynamics of polaritons 28 . In this Letter, we report on the experimental observation of spin whirls in the radial expansion of a polariton condensate formed under non-resonant optical excitation in a GaAs quantum well (QW) microcavity.…”

Realization of Hofstadter's butterfly and a one-way edge mode in a polaritonic system

“…We note that this spin dependent hopping corresponds to a breaking of time reversal symmetry, which will later be shown to give rise to an artificial gauge field. This kind of potential landscape can be created by non-resonant excitation with a localized optical pump which induces a spin-dependent potential that can selectively gate different spin polarizations [50]. Recent work also shows that alternating patterns of spin polarization may form spontaneously under near resonant excitation, offering a further alternative for engineering spin-dependent coupling for our lattice [51].…”