Polaritons in Waveguides

In this work, we directly observe the strong coupling between inorganic quantum well excitons and the guided mode of a planar film waveguide. Compared to using distributed Bragg reflectors (DBRs), optical confinement by total internal reflection (TIR) in principle provides for smaller losses and also larger Rabi splitting through better spatial overlap of optical field and quantum well(s). Waveguides naturally operate at large in-plane wavevectors and group velocities so that polaritons should propagate large distances within their lifetime. To couple light in and out of the structure, we employ a grating coupler, which consists of a periodic modulation of the guidecladding layer interface.

Where the dispersion of the electromagnetic mode crosses that of the excitons and the coupling strength is large enough, there is an avoided crossing of the modes, which is the signature of the strong coupling regime. Figure below shows the angle-resolved PL spectrum for excitation and detection at the same region within a grating patterned area, which clearly show the anti-crossing. Observed Rabi splitting of 5–6 meV comes from only a single quantum well. The polaritons propagate with a group velocity of 26 µm/ps on resonance and with a lifetime of 11.4 ps.

Polariton dispersion in a waveguide.

Polariton dispersion in a waveguide.

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