High temperature cavity polaritons in epitaxial Er2O3 on silicon

On-chip optical interconnects with wavelength division multiplexing are being pursued as a low-power low-latency high-bandwidth alternative to metal interconnects. Significant research has focused on integrating optical gain material into the Si platform, with III-V wafer bonding to Si being the most successful candidate to date. In addition there is a long history of efforts to incorporate Er3+ into Si material systems for light emission and optical gain. Erbium-doped fibers are the dominant amplifiers for telecommunications because of their high quantum efficiency and because the shielded Er3+ 4f transition wavelengths are largely insensitive to temperature and the host matrix. However, the small gain coefficient of Er3+-doped materials is insufficient for dense microphotonic applications. To compensate for erbium’s small emission cross section, stoichiometric erbium compounds are potential alternatives that achieve Er3+ densities 100 times greater than erbium’s solubility limit in doped materials. The high density and weak inhomogeneous broadening of stoichiometric erbium crystals can intrinsically produce large dispersive resonances in the refractive index and vacuum-Rabi splitting of optical cavity modes. Rabi splitting and the associated cavity-polariton modes can be described as a consequence of linear dispersion or as nonperturbative coupling between the dipole s and an optical cavity, while perturbative coupling to a single ion produces Purcell-enhanced emission. For rare-earth emitters, cavity polaritons have been observed around a single Er3+ transition in an oxidized polycrystalline erbium layer, but the effect was quenched at T 40 K. We have previously shown that atomic layer epitaxy produces high quality single-crystal Er2O3 films on silicon and offers the prospect of electrical injection through precisely controlled heterostructures. In this letter, we describe the spectroscopy of small mode-volume Er2O3 microdisk resonators formed from this material, and we analyze the properties of high temperature T 361 K cavity polaritons formed between the cavity’s whispering-gallery modes WGMs and two Er3+ transitions in the 1500 nm band. Analyzing the polariton response involves continuously tuning a cavity mode across the Er3+ transitions. As a mode is shifted through an optical transition, the resonances anticross i.e., the vacuum-Rabi splitting and produce symmetric hybrid modes i.e., the cavity polaritons , which appear in both the cavity transmission and photoluminescence PL . In terms of cavity quantum electrodynamics cQED , the fundamental quantity for cavity polaritons is the Rabi frequency that describes the coherent coupling between the two-level system s and a near-resonant optical mode. For a uniform distribution of emitters, the many-dipole Rabi frequency ḡN is given by