With 3-D finite difference time domain (FDTD) method, we investigate the effect of the geometry parameters and near
field distribution on sensitivity of the bowtie type nanoresonator biosensor in terms of refractive index. The calculation
results show that the geometry parameters including gap (G), radius of curvature (R) and tip angle (A) have different
effects on bulk sensitivity of bowtie metallic nanostructure. The sensitivity linear decreases as G increases while
exponential decreases as R increases. Moreover, with A taking 15° to 135°, the sensitivity descends initially and then rises. As for the influence of near field on sensitivity, we can conclude that sensitivity is proportional to maximal local field and
the scale factor of fitted curve for R is larger than the one for G.
Lengthways and transverse offset of the position of the virtual image relative to the object are analyzed theoretically in
different situations of bi-grating imaging, and an experiment on the verification of the offset is carried out. The result
shows that the position of the virtual image is just behind the object relative to the positive direction of Z axis, the
distance between virtual image and object increases with the increasing of the distance between object and grating G2;
Therefore, the position of virtual image deviates from objects in the X-axis if looking it through grating G2. Thus the
grating G2 and the virtual image are "located on" the same side of the Z-axis.
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