15 Super-Resolution Imaging and Microscopy by Dielectric …
379
• Resolution: in most cases the lateral resolution of the jet is slightly better than
diffraction limit in air (0.5λ). However, in some cases the resolution could reach
the solid immersion limit λ/2n [72], where n is the refractive index of particle.
• PNJ usually appears for diameter d of the dielectric microsphere or microcylinder
from 2λ to about 40λ with the refractive index contrast relative to the background
medium less than 2:1.
• PNJ could maintain a subwavelength focusing jet along the path that can extend
more than 2λ beyond the dielectric cylinder or sphere. This has led to a widespread but misleading claim that PNJ is a non-evanescent propagating beam within
which evanescent wave doesn’t contribute [73]. In fact, evanescent waves could
play strong role in near-field PNJ as demonstrated in Fig. 15.2.
Besides, PNJ engineering has also been extended to other shapes and configurations of dielectric particles (spheroids [74, 75], disks [76], cones [77], cubes [78],
multilayer [79] and others [80–82]), incident beam modulation[83], phase masks
[84–86], photonic hook [87, 88], reflective PNJ [89–91] and applications in second
harmonic generation. For more information on these please refer to our previous
review article [36].
15.2.1.2 Super-Resonance Effect (SRE)
Besides PNJ, strong resonant modes can be excited in micro spherical cavity. For
example, at n = 1.5, q = 26.94163, a super-resonance mode (i.e. a new type WGM
mode) can be excited. We define ‘super-resonance’ as a giant field enhancement
mode caused by microsphere’s internal partial waves. In contrast, PNJ are induced
by scattering waves outside the particle. Unlike WGM which can be excited in
both microsphere and microcylinder, SRE only exist in microspheres. Figure 15.3
demonstrates a typical SRE mode field distribution and its giant field enhancement
amplitude.
In SRE mode, two near-symmetrical hotspots were generated near the top and
bottom apex points (points T, B) of the particle (see Fig. 15.3a). The enhancement
factor can be extremely large at these points, reaching the order of 10
4 (peak value:
43,774), two orders higher than those in PNJ (typically on order of several tens or
hundreds, Fig. 15.3c vs. Fig. 15.1). Importantly, the SRE hotspot spot has a super
resolution of 0.22λ which exceeds resolution limit of PNJ and WGM. The SRE is
caused by the excitation of special order of internal partial waves in Mie theory, e.g.
L = 43 for q = 26.94163. It is very sensitive to the size parameter. For example,
when q changes by 1e-4 from 26.94163 to 26.94153, the peak drops by more than
20,000 (see Fig. 15.3c). The top four strongest SRE modes for n = 1.5 particle are
seen at q = 38.6203 (with L = 51 electric partial wave mode excited, |E|
2
= 939286,
|H|
2
= 1793371), 66.5555 (L = 80 electric mode, |E|
2
= 773375, |H|
2
= 351073),
32.5623 (L = 43 magnetic mode, |E|
2
= 621863, |H|
2
= 55113), and 31.8619 (L
= 42 magnetic mode, |E|
2
= 263386, |H|
2
= 24880), respectively. Details on SRE
are outside the scope of this review and will be presented in a separate publication
379
• Resolution: in most cases the lateral resolution of the jet is slightly better than
diffraction limit in air (0.5λ). However, in some cases the resolution could reach
the solid immersion limit λ/2n [72], where n is the refractive index of particle.
• PNJ usually appears for diameter d of the dielectric microsphere or microcylinder
from 2λ to about 40λ with the refractive index contrast relative to the background
medium less than 2:1.
• PNJ could maintain a subwavelength focusing jet along the path that can extend
more than 2λ beyond the dielectric cylinder or sphere. This has led to a widespread but misleading claim that PNJ is a non-evanescent propagating beam within
which evanescent wave doesn’t contribute [73]. In fact, evanescent waves could
play strong role in near-field PNJ as demonstrated in Fig. 15.2.
Besides, PNJ engineering has also been extended to other shapes and configurations of dielectric particles (spheroids [74, 75], disks [76], cones [77], cubes [78],
multilayer [79] and others [80–82]), incident beam modulation[83], phase masks
[84–86], photonic hook [87, 88], reflective PNJ [89–91] and applications in second
harmonic generation. For more information on these please refer to our previous
review article [36].
15.2.1.2 Super-Resonance Effect (SRE)
Besides PNJ, strong resonant modes can be excited in micro spherical cavity. For
example, at n = 1.5, q = 26.94163, a super-resonance mode (i.e. a new type WGM
mode) can be excited. We define ‘super-resonance’ as a giant field enhancement
mode caused by microsphere’s internal partial waves. In contrast, PNJ are induced
by scattering waves outside the particle. Unlike WGM which can be excited in
both microsphere and microcylinder, SRE only exist in microspheres. Figure 15.3
demonstrates a typical SRE mode field distribution and its giant field enhancement
amplitude.
In SRE mode, two near-symmetrical hotspots were generated near the top and
bottom apex points (points T, B) of the particle (see Fig. 15.3a). The enhancement
factor can be extremely large at these points, reaching the order of 10
4 (peak value:
43,774), two orders higher than those in PNJ (typically on order of several tens or
hundreds, Fig. 15.3c vs. Fig. 15.1). Importantly, the SRE hotspot spot has a super
resolution of 0.22λ which exceeds resolution limit of PNJ and WGM. The SRE is
caused by the excitation of special order of internal partial waves in Mie theory, e.g.
L = 43 for q = 26.94163. It is very sensitive to the size parameter. For example,
when q changes by 1e-4 from 26.94163 to 26.94153, the peak drops by more than
20,000 (see Fig. 15.3c). The top four strongest SRE modes for n = 1.5 particle are
seen at q = 38.6203 (with L = 51 electric partial wave mode excited, |E|
2
= 939286,
|H|
2
= 1793371), 66.5555 (L = 80 electric mode, |E|
2
= 773375, |H|
2
= 351073),
32.5623 (L = 43 magnetic mode, |E|
2
= 621863, |H|
2
= 55113), and 31.8619 (L
= 42 magnetic mode, |E|
2
= 263386, |H|
2
= 24880), respectively. Details on SRE
are outside the scope of this review and will be presented in a separate publication