Polymeric photonic molecule super-mode lasers on silicon

Abstract

Optically coupled microcavities have emerged as photonic structures with promising properties for investigation of fundamental science as well as for applications. We report on the fabrication and spatially resolved spectroscopy of on-chip photonic molecule (PM) lasers consisting of two coupled, dye-doped polymeric microdisks on a silicon substrate. We investigate the fundamental lasing properties with focus on the spatial distribution of modes, the coupling dependent suppression of lasing modes, and in particular the application-oriented operation of these devices in aqueous environments. By depositing an additional polymer layer onto the lithographically structured cavities made of dye-doped poly(methyl methacrylate), coupling-gap widths below 150 nm with aspect ratios of the micro-/nanostructure exceeding 9∶1 are achieved. This enables strong optical coupling at visible wavelengths despite relatively small resonator radii of 25 µm. The lasing properties of dye-doped PMs are investigated using spatially resolved micro-photoluminescence (μ-PL) spectroscopy. This technique allows for the direct imaging of whispering-gallery modes (WGMs) in the photonics molecules. For subwavelength coupling gaps, we observe lasing from delocalized eigenstates of the PMs (termed in the following as super-modes). Using size-mismatched cavities, the lasing mode suppression for different coupling-gap widths is investigated. We further demonstrate single-mode lasing operation in aqueous environments with PMs, which are realized on a low-cost, polymer-on-silicon platform. Scientists in Germany have constructed miniature single-mode lasers comprised of two coupled polymer microdisk cavities on silicon. Tobias Grossmann and co-workers from the Karlsruhe Institute of Technology say that their ‘photonic molecule’ lasers may prove useful for experiments in quantum optics or optofluidic sensing. The lasers are made by spin-coating a 1.2-μm-thick layer of the polymer PMMA onto a silicon substrate and using the laser dye Pyrromethene 597 as the gain material. Electron-beam lithography can then be used to create a pair of closely spaced microdisks with radii of 20–25 μm, subsequent deposition of PPX can be applied to accurately reduce the coupling gaps to 0–200 nm. When optically pumped with short green laser pulses, the photonic molecule lasers exhibited single-mode lasing in the yellow/orange spectral range.