Towards routine near-field optical characterization of silicon-based photonic structures: An optical mode analysis in integrated waveguides by transmission AFM-based SNOM

We report on the characterization of modal propagation in integrated waveguides using a transmission AFM-based scanning near-field optical microscope. This technique probes the field inside an integrated waveguiding structure by measuring the change in transmission due to the local interaction with the AFM tip. In this paper, we clearly show that it is sensitive to the full spectrum of modes, which includes propagating and counterpropagating guided and radiation modes. For a rather simple waveguiding sample (ion exchange monomode straight waveguide), the technique allows for retrieving the effective indexes of the propagating modes. This is achieved by measuring the standing wave pattern due to interference between the propagating and counterpropagating modes and by performing a Fourier analysis. In the case of porous silicon channel waveguide, although very contrasted, the image was found to be too complex to be analyzed. Phase sensitive measurements are in that case needed to discriminate in k space the different modes involved. Potentiality and limitation of the technique are discussed focusing on the modal analysis.