Near-field characterization of photonic crystal waveguides
In the past few years, the development of photonic crystal waveguides and structures has progressed immensely revealing many interesting optical phenomena and demonstrating numerous potential applications. We show how a scanning near-field optical microscope (SNOM) in the collection mode can be used to investigate these powerful new optical structures, in particular photonic crystal waveguides (PhCWs). Unlike conventional far-field techniques, a SNOM can detect truly guided light propagating inside the structure without relying on scattering, and SNOM imaging is thereby not restricted by the diffraction limit of half of the wavelength used. In contrast to the regular input-output measurements, the SNOM collects local optical information which is not integrated or averaged because the light has propagated through the entire photonic structure, including possible in- and out-coupling waveguides. We show how intensity detection can be used to map mode profiles, determine losses locally, gain insight into the Bloch nature of the propagating light, and investigate more complex photonic crystal-based structures. Phase-sensitive detection allows the Bloch nature to be investigated in more detail as all the Bloch harmonics belonging to a single Bloch mode can be unravelled. Time-resolved SNOM measurements (pulse tracking measurements) allow pulse propagation to be visualized and phase and group velocities to be determined independently with the need for modelling. For certain optical frequencies ultraslow light can be observed. The possibilities and limitations of SNOM imaging for the characterization of PhCWs are discussed.