Characterization Techniques for Planar Optical Microresonators

Abstract

Abstract. Optical microresonators that are coupled to optical waveguides often behave quite similar to Fabry-Perot resonators. After summarising key properties of such resonators, three characterization methods will be discussed. The first involves the analysis of transmission and reflection spectra, from which important parameters like (waveguide) loss and coupling or reflection coefficients can be extracted. The second method is called transmission-based scanning near-field optical microscopy (T-SNOM) which allows to map out the intensity distribution inside a high-Q resonator with subwavelength resolution. For such resonators conventional SNOM suffers from inaccuracies introduced by the disturbing effect of the presence of the probe on the field distribution. T-SNOM avoids this problem by exploiting this disturbing effect. The third method is most applicable to large-size (many wavelengths across) resonators. It involves simultaneous analysis of scattered light and transmission/reflection spectra in order to relate spectral features to large-scale field distributions inside the resonator. Together, these techniques form a convenient toolbox for characterizing many different planar optical microresonators.Keywords: optical microresonators; optical measurements; SNOM; NSOM; AFM; scattered-light analysis. IntroductionOptical resonators have many applications, such as wavelength filtering, add-drop multiplexing of several wavelength channels, lasers, and field enhancement [1][2][3][4]. Recent developments in integrated optics towards high-refractive index contrast technology (e.g. silicon photonics [5]) and related fields such as photonic crystals have opened the road towards a strong miniaturisation of optical circuits. In particular microring resonators [e.g. 1, 2, 5, 6] and cavities in photonic crystals [e.g. 3-5, 7-10] have been intensively investigated. Integrated optical circuits with high functionality have been conceived [2,11], based on arrays of microresonators. Conventionally, optical resonators are characterized by performing spectral transmission and/or reflection measurements. In these methods, the input-output relationships are analysed, and through device models, some data on internal parameters can be inferred. These methods will provide essential information on the functionality and in:

Quantitative analysis and near-field observation of strong coupling between plasmonic nanogap and silicon waveguides

“…Similar configurations have been applied for the characterization of glass 15 and lithium niobate 16 waveguides as well as on Si-based structures 17,18 and planar microresonators. 19 This technique measures the change in transmission at the device output induced by the AFM probe. 20 Our motivation here is to demonstrate that this configuration is a simple yet powerful technique for the observation and measurement of local electric fields in integrated plasmonic devices.…”