Whispering gallery modes in novel silicon nanophotonic resonators

The emerging research field of silicon (Si) nanophotonics is pointing out promising ways towards revolutionary new lighting and modulator devices that rely on the nonlinear interaction of light with Si nanostructures. We... [ view full abstract ]

The emerging research field of silicon (Si) nanophotonics is pointing out promising ways towards revolutionary new lighting and modulator devices that rely on the nonlinear interaction of light with Si nanostructures. We provide a detailed understanding of photonic modes hosted in monolithic silicon nanophotonic resonators of a novel geometry (inverse half ellipsoid). Cathodoluminescence (CL) spectra were acquired along the height of the resonators. The intensity distributions were found to precisely match the numerically predicted distributions of hosted whispering gallery modes (WGMs). This enabled the derivation of an analytical design rule that describes the mode alignment in the resonators. We also show that a precise mode modulation can be performed via the application of a thin dielectric oxide cladding (Al₂O₃ or HfO₂).

Samples were prepared using cryogenic reactive ion etching and CL measurements were performed in a scanning electron microscope (SEM). For the numerical mode analysis, finite difference time domain (FDTD) simulations were used and for each emission peak the position and shape of the corresponding WGM was determined. Al₂O₃ or HfO₂ coatings were deposited by thermal atomic layer deposition (ALD) at 200°C. The wavelength shifts of the photonic modes were detected by micro-photoluminescence (PL) spectrometry.

Fig. 1a shows an SEM image in 90° tilt view of a typical Si nano-resonator under study. CL measurements permit a selective excitation of the photonic modes as indicated by the spectra shown in Fig. 1b. The spectra, collected at two positions along the nanostructure (see Fig. 1a), clearly exhibit spectrally coinciding peaks with a different distribution of intensities and permit to predict the position of the modes in the structures. X-Y cross sectional energy density (E²) maps obtained by FDTD simulations for two specific modes are given as insets. Fig. 2b shows a cross section image of a nanostructure conformally coated with alumina. As can be seen from the graph in Fig. 2a, the ALD coating induces peak shifts. The shifts are shown to be precisely tunable by the thickness and the refractive index of the applied optical cladding.