In-situ tunable, room-temperature polariton condensation in individual states of a 1D topological lattice

Abstract

Microcavity exciton-polaritons are bosonic, light-matter quasiparticles resulting from strong coupling between a cavity's resonance mode and the excitonic transition of an active material placed inside the cavity. Due to their bosonic nature, at sufficient densities, polaritons can undergo Bose Einstein condensation, making them a promising platform for a semiconductor based analog quantum simulator. In this work, we use a wavelength-tunable cavity with an organic active layer to demonstrate room temperature, selective condensation of polaritons in individual states of a one dimensional (1D) topological lattice. The investigated lattice is comprised of adjacent sites with alternating weak and strong bonds, a so called Su-Schrieffer–Heeger (SSH) chain, which was originally used to model polyacetylene. First, to probe the topological signatures of the studied structure, we measured the angle-resolved photoluminescence by locally exciting the structure with a 400 nm continuous-wave laser. By exciting the center of the structure we observe a well resolved bandstructure with clearly formed S-like and P-like energy bands. Repeating the measurement at the edge of the structure leads to the observation of a slightly different bandstructure. Inside the first energy gap a discrete energy state has appeared, indicating the formation of a topological edge state in the studied lattice's bandstructure. Furthermore, we studied the system above polariton condensation threshold, using a frequency-doubled, amplified Ti:sapphire laser at 400 nm, with a 1 kHz repetition rate and approximately 150 fs pulse duration. Using the tunability of our cavity and a vibron assisted relaxation mechanism, unique to organic materials, we are able to selectively condense polaritons to individual states of the 1D topological lattice (Fig. 1). Using a Michelson interferometer we investigated the coherence of the condensates, which exhibited long range spatial coherence spanning through almost the whole structure. Finally, we measured and compared three structures with different ratio between the weak and the strong bond. We showcased engineering of the energy gap and of the topological edge state localization by tuning the coupling of the weak bond. These results display the high level of tunability and engineerability of our platform and showcase its potential for the study of topological effects and the simulation of complex Hamiltonians at room temperature.