Mapping the field distribution of a resonator in a photonic crystal slab, using transmission SNOM

Transmission scanning near-field optical microscopy (T-SNOM) is a recently introduced method for imaging optical intensity distributions in dielectric waveguiding structures, with a resolution below the diffraction limit. It is in particular suitable for mapping out standing-wave patterns in resonators. Application of the method to a Fabry-Perot-like cavity in a line-defect waveguide in a photonic crystal slab is demonstrated. Two different two-dimensional models have been used to verify that the T-SNOM image is a good representation of the optical intensity distribution in the resonator. Introduction Scanning near-field optical microscopy (SNOM) methods [1], can provide the resolution below the diffraction limit that is required for measuring the field distribution in high-refractive-index microcavities, e.g. in photonic crystals. The interpretation of images obtained by such methods may be challenging because the probe that is used for local light collection and/or illumination also perturbs the optical structure under investigation. Although probe tips can have diameters below 100 nm, their effect can certainly not be neglected when small high-Q resonators are to be characterised. Especially metal-clad tips, that provide the best spatial resolution, may strongly affect the optical field, thus making the interpretation of such measurements difficult. In particular, this can be a problem with high-Q resonators which can be considerably detuned by the presence of a nano-sized probe [2, 3, 4]. The perturbation of the field by the probe is essential for obtaining resolution below the diffraction limit by converting part of the evanescent field to radiation. Recently we [4] and others [5] have demonstrated a related method, now called transmission scanning near-field optical microscopy (T-SNOM), where the functions of light collection and illumination on the one hand are separated from the function of perturbation on the other hand. The effect of this perturbation of the field by a dielectric or metallic probe is measured through the light that is transmitted through (or back-reflected from) the structure. The method is especially useful for mapping out standing-wave patterns. Although other SNOM types, like photon scanning tunnelling microscopy (PSTM) have a broader field of applicability (not requiring transmitted or back-reflected light to be available for analysis), T-SNOM has several advantages compared to PSTM. The scanning tip can be significantly smaller (potentially increasing the resolution) without necessarily decreasing the sensitivity; there is a large freedom in choosing the tip material, allowing to select the strength of the mechano-optical interaction; the availability of a larger optical signal improves the signal-to-noise ratio, thus allowing less complicated detectors and signal processing electronics. In this paper we will explain the method, show some results for a Fabry-Perot-like cavity resonator in a photonic crystal slab, discuss the interpretation of the results, and show through simulations that the T-SNOM image indeed closely resembles the intensity distribution in the unperturbed resonator. T-SNOM operation The principle of the set-up is shown in Fig. 1. The tip of an atomic force microscope (AFM) probe is scanned over the surface of the photonic structure, while the light transmission through the structure is recorded for each tip position. The strongest interaction can be expected at those positions where the optical field has the largest intensity, so that the optical field distribution is effectively mapped out. The AFM probe can be scanned across the device either in contact mode or in tapping mode. In contact mode the highest sensitivity and resolution can be expected, but damage to the tip or the structure may easily occur. In tapping mode, damage is much less likely to occur, but resolution and sensitivity may be less because of the reduced optical intensity in the evanescent field compared to that of the diffraction limited radiation field. However, as was recently pointed out by Li et al. [6], this height-dependent differential sensitivity may be exploited for discriminating between the evanescent field and the radiation field by using the tapping mode with a suitable signal processing method.