moving to shorter wavelengths fig2 a 2b and 2c show the electric field distributions in the z and x directions of the lowest resonant mode of the resonator in the x direction only the exponential envelop is shown in fig2 c while the high frequency structure related to the bragg gratings is omitted the fwhm of this exponentially decaying field is about 10 u03bcm for a pair of bragg structures that extend about 70 u03bcm the evanescent tail in the z direction probes the region immediately beneath of the cavity as seen in fig2 b the separation of probe 10 and sample 24 h is about 100 to 250 nm as seen in fig2 a the resonant frequency of the cavity 15 changes as the dielectric perturbation 25 is translated along the u03be direction this change in the resonant frequency results in a corresponding change in the transmission of monochromatic light through the waveguide resonator fig3 is a schematic drawing indicating one embodiment of mode of operation of the probe 10 shown in the figure is a scheme using second order grating couplers 30 31 couplers with 30 40 coupling efficiencies are relatively easy to fabricate however other methods such as prism coupling can also.