[Paper Review] Collective excitations of exciton-polariton condensates in a synthetic gauge field

Dąbrówka Biegańska, Maciej Pieczarka+8 more|arXiv (Cornell University)|Nov 26, 2020

Strong Light-Matter Interactions59 references21 citations

TL;DR

This study experimentally measures collective excitations in a high-density exciton-polariton condensate under a synthetic gauge field induced by microcavity anisotropy. Using polarization-resolved photoluminescence tomography, the authors observe strongly anisotropic dispersion relations and extract spin-dependent interaction strengths, confirming theoretical predictions of Dirac cone-like crossings and non-Abelian gauge field effects in the nonlinear regime of spinor polaritons.

ABSTRACT

Collective (elementary) excitations of quantum bosonic condensates, including condensates of exciton polaritons in semiconductor microcavities, are a sensitive probe of interparticle interactions. In anisotropic microcavities with momentum-dependent TE-TM splitting of the optical modes, the excitations dispersions are predicted to be strongly anisotropic, which is a consequence of the synthetic magnetic gauge field of the cavity, as well as the interplay between different interaction strengths for polaritons in the singlet and triplet spin configurations. Here, by directly measuring the dispersion of the collective excitations in a high-density optically trapped exciton-polariton condensate, we observe excellent agreement with the theoretical predictions for spinor polariton excitations. We extract the inter- and intra-spin polariton interaction constants and map out the characteristic spin textures in an interacting spinor condensate of exciton polaritons.

Motivation & Objective

• To probe the nonlinear regime of spinor polariton condensates where synthetic gauge fields and spin-dependent interactions coexist.
• To measure collective excitation dispersions in momentum space under controlled anisotropy.
• To extract the triplet (α₁) and singlet (α₂) interaction strengths from experimental data.
• To map the spin texture and gauge field distribution in the interacting condensate.

Proposed method

• Employed a GaAs-based microcavity cooled to 4 K with a narrow photon linewidth (>100 ps) to resolve anisotropic polariton dispersion.
• Used off-resonant continuous-wave laser excitation shaped into a ring to create a 2D optical trap for polaritons.
• Performed polarization-resolved photoluminescence tomography using H/V and D/A polarization bases to access pseudospin states.
• Measured momentum-space dispersion by translating the imaging lens relative to the monochromator slit.
• Fitted experimental data to a spinor Hamiltonian (Eq. 1) including TE-TM splitting and cavity anisotropy to extract β, Ω, and ϕ.
• Calculated Stokes parameters S₁ and S₂ from polarized intensities to reconstruct the pseudospin texture in momentum space.

Experimental results

Research questions

• RQ1 How do collective excitations in a high-density exciton-polariton condensate behave under a synthetic gauge field?
• RQ2 What is the role of spin-dependent interactions (α₁, α₂) in shaping the excitation dispersion and spin texture?
• RQ3 Can the predicted anisotropic Dirac cone-like crossings and non-Abelian gauge field effects be experimentally observed in the nonlinear regime?
• RQ4 How do the measured dispersion relations compare to theoretical predictions for spinor polariton excitations?

Key findings

• The measured dispersion of collective excitations exhibits strong anisotropy, with Dirac cone-like crossings along the direction parallel to the anisotropy axis (k∥), consistent with theoretical predictions.
• The triplet interaction strength was extracted as α₁ = (14.89 ± 0.92) µeV µm⁻², and the singlet interaction strength as α₂ = (28.1 ± 2.2) µeV, confirming |α₂| ≪ α₁.
• The crossing point in momentum space was observed at k*∥ = 0.971 µm⁻¹, corresponding to the cancellation of TE-TM splitting and optical anisotropy effects.
• The polarization-resolved tomography revealed a characteristic monopolar pseudospin texture around the diabolical point, confirming the presence of an effective non-Abelian gauge field.
• The anisotropy axis was determined to be ϕ = −15° from the +x-axis, aligning with the sample orientation and consistent with the model.
• The experimental data show excellent agreement with the theoretical model of spinor polariton excitations in a synthetic gauge field, validating predictions in the nonlinear regime.