15 Super-Resolution Imaging and Microscopy by Dielectric …
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interlinked and coupled, and complete model is still missing to date. Most existing theoretical studies are based one or two processes above, leading to incomplete
conclusions and debates [19, 20, 23, 31, 33–42]. At the beginning, Photon nanojet (PNJ) was considered as the main super-resolution mechanism [19]. However,
it was soon realized that the resolution of PNJ (typically between λ/2 and λ/3 for
n = 1.5 microsphere) is insufficient to explain the experimentally observed resolution of ~λ/7. Consequently, other mechanisms were discussed and investigated.
Duan and co-workers reported that classical Whispery Gallery Modes (WGM) in
microsphere can help boost imaging resolution up to ~λ/4. Besides PNJ and classical WGM, a new physical mechanism, super-resonance effect (SRE), was recently
discovered by Wang et al. and Hoang et al. [43, 44]. The SRE is caused by internal
particle partial wave modes (not by scattering waves as in PNJ) and surface wave
modes; it can generate super-resolution focusing with lateral resolution ~λ/5–λ/6.
This new effect helped bridge the resolution gap between PNJ/WGM theories (~λ/4)
and experiments (~λ/7). On the other hand, experimental resolution can be enhanced
by substrate effect [33, 45], partial and inclined illumination [27], microsphere partial immersion [46] and coherent illumination effect [20] as demonstrated by various
authors. In the following, we briefly review the key properties of PNJ, SRE, broadband lighting source effect, substrate and partial and inclined illumination effect on
super-resolution imaging. For more information on the basics of related theory (e.g.
Mie theory) and PNJ (e.g. how PNJ evolves with respect to size and refractive index,
medium effect, PNJ by other shaped particles, etc.), please refer to previous reviews
written by us [33, 36].
15.2.1.1 Photonic Nanojet
PNJ is a narrow, high-intensity electromagnetic beam that propagates into the background medium from the shadow-side surface of a plane-wave illuminated lossless
dielectric microcylinder or microsphere of diameter greater than the illuminating
wavelength, λ [19]. The PNJ is attained via light scattering by micro-sized particles
(spheres, cylinders, cubes, prisms and others, typical diameter between 1 and 50 μm)
[47, 48, 49–57]. This is an important effect which is now widely used for subwavelength imaging, sensing, patterning, nanoparticle trapping and manipulation, and
more. The earliest study relating to PNJ can be dated back to year 2000 by one of the
present author: Luk’yanchuk and co-worker first demonstrated that enhanced optical
near-fields of a 500-nm silica sphere can be used as super-resolution lens (superlens)
for subwavelength structuring of silicon surface [57]. Since then, the technique grows
rapidly [58]. Without knowing the initial work by Luk’yanchuk et al., in 2004 Chen
et al. coined the terminology PNJ—‘photonic nanojet’ [47], which is now widely
used [47, 59–70].
Figure 15.2 shows an example of PNJ subwavelength focusing, where a 1 μmdiameter Polystyne particle (n = 1.6) is illuminated in air by a plane wave laser
beam from the top (λ = 248 nm); these parameters equivalents to size parameter
q = 2 π a/λ = 12.67, where a is particle radius and λ the incident wavelength.
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interlinked and coupled, and complete model is still missing to date. Most existing theoretical studies are based one or two processes above, leading to incomplete
conclusions and debates [19, 20, 23, 31, 33–42]. At the beginning, Photon nanojet (PNJ) was considered as the main super-resolution mechanism [19]. However,
it was soon realized that the resolution of PNJ (typically between λ/2 and λ/3 for
n = 1.5 microsphere) is insufficient to explain the experimentally observed resolution of ~λ/7. Consequently, other mechanisms were discussed and investigated.
Duan and co-workers reported that classical Whispery Gallery Modes (WGM) in
microsphere can help boost imaging resolution up to ~λ/4. Besides PNJ and classical WGM, a new physical mechanism, super-resonance effect (SRE), was recently
discovered by Wang et al. and Hoang et al. [43, 44]. The SRE is caused by internal
particle partial wave modes (not by scattering waves as in PNJ) and surface wave
modes; it can generate super-resolution focusing with lateral resolution ~λ/5–λ/6.
This new effect helped bridge the resolution gap between PNJ/WGM theories (~λ/4)
and experiments (~λ/7). On the other hand, experimental resolution can be enhanced
by substrate effect [33, 45], partial and inclined illumination [27], microsphere partial immersion [46] and coherent illumination effect [20] as demonstrated by various
authors. In the following, we briefly review the key properties of PNJ, SRE, broadband lighting source effect, substrate and partial and inclined illumination effect on
super-resolution imaging. For more information on the basics of related theory (e.g.
Mie theory) and PNJ (e.g. how PNJ evolves with respect to size and refractive index,
medium effect, PNJ by other shaped particles, etc.), please refer to previous reviews
written by us [33, 36].
15.2.1.1 Photonic Nanojet
PNJ is a narrow, high-intensity electromagnetic beam that propagates into the background medium from the shadow-side surface of a plane-wave illuminated lossless
dielectric microcylinder or microsphere of diameter greater than the illuminating
wavelength, λ [19]. The PNJ is attained via light scattering by micro-sized particles
(spheres, cylinders, cubes, prisms and others, typical diameter between 1 and 50 μm)
[47, 48, 49–57]. This is an important effect which is now widely used for subwavelength imaging, sensing, patterning, nanoparticle trapping and manipulation, and
more. The earliest study relating to PNJ can be dated back to year 2000 by one of the
present author: Luk’yanchuk and co-worker first demonstrated that enhanced optical
near-fields of a 500-nm silica sphere can be used as super-resolution lens (superlens)
for subwavelength structuring of silicon surface [57]. Since then, the technique grows
rapidly [58]. Without knowing the initial work by Luk’yanchuk et al., in 2004 Chen
et al. coined the terminology PNJ—‘photonic nanojet’ [47], which is now widely
used [47, 59–70].
Figure 15.2 shows an example of PNJ subwavelength focusing, where a 1 μmdiameter Polystyne particle (n = 1.6) is illuminated in air by a plane wave laser
beam from the top (λ = 248 nm); these parameters equivalents to size parameter
q = 2 π a/λ = 12.67, where a is particle radius and λ the incident wavelength.