14 Label-Free Super-Resolution Imaging with Hyperbolic Materials
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it is exponentially sensitive to any noise in the system, and as a result so far only
produced a limited success.
A number of super-resolution methods rely on the nonlinear response of the dyes
that are used to label biological samples in fluorescence imaging. Stimulated emission
depletion (STED) microscopy [5] accomplishes so by selectively deactivating part
of the illumination region by an additional light and localizing the final excitation
spot for the fluorophores to a very small region. However, this optical technique is
inherently nonlinear, and suffers from additional stray excitation in the dyes leading
to photobleaching [17].
Another super-resolution method in fluorescence imaging is stochastic optical
reconstruction microscopy (STORM), [18] which surpasses the diffraction limit by
allowing the dyes to fluoresce not all at once, but sequentially, so that very few
active molecules are emitting light at any given time. The problem of resolving two
closely spaced objects are therefore eliminated, and a complete image of the sample
can be formed by superimposing multiple time-resolved recordings of the individual
emitters. Apart from having the same drawbacks as those of fluorescence imaging
like phototoxicity, photobleaching etc., this method suffers in addition in that it is
not a deterministic method and can not capture a biological process in real time [19].
While fluorescence-based methods offer a remarkable improvement of the resolution, as compared to the conventional diffraction-limited optical imaging, fluorescent
dyes can induce an unwanted chemical reaction in the sample, or they can only be
attached to a certain part of the specimen. The resulting quest for a label-free superresolution imaging, led to the emergence of an alternative approach that is based on
the novel concept of electromagnetic metamaterials.
14.3 Metamaterials-Based Super-Resolution
Metamaterials [20] are artificial composites having properties that are usually not
available in nature. The constituents are arrayed on a scale much smaller than the
wavelength, so that the incoming radiation cannot fully distinguish between the
grainy variation, and the medium as a whole shows a response that can be substantially different from the properties of the individual material components. These new
emergent properties can be controlled by engineering the geometric structure of the
metamaterial unit cell.
One of the earliest examples of these new emergent properties of metamaterials is
the negative index which leads to negative refraction, reversed Cherenkov radiation
and the possibility of realizing a flat lens [21]. When an object is placed close to a slab
of such a medium, the evanescent waves scattered by the object excite surface states
at the metamaterial interfaces. For the surface state at the “right” metamaterial–air
interface (see Fig. 14.1a), its decay into the negative index slab manifests itself as the
effective growth of the field in the metamaterial when viewed from the position of
the source at the “left” of the slab. This effective “amplification” of the evanescent
field that carries the subwavelength information on the structure of object, is what
347
it is exponentially sensitive to any noise in the system, and as a result so far only
produced a limited success.
A number of super-resolution methods rely on the nonlinear response of the dyes
that are used to label biological samples in fluorescence imaging. Stimulated emission
depletion (STED) microscopy [5] accomplishes so by selectively deactivating part
of the illumination region by an additional light and localizing the final excitation
spot for the fluorophores to a very small region. However, this optical technique is
inherently nonlinear, and suffers from additional stray excitation in the dyes leading
to photobleaching [17].
Another super-resolution method in fluorescence imaging is stochastic optical
reconstruction microscopy (STORM), [18] which surpasses the diffraction limit by
allowing the dyes to fluoresce not all at once, but sequentially, so that very few
active molecules are emitting light at any given time. The problem of resolving two
closely spaced objects are therefore eliminated, and a complete image of the sample
can be formed by superimposing multiple time-resolved recordings of the individual
emitters. Apart from having the same drawbacks as those of fluorescence imaging
like phototoxicity, photobleaching etc., this method suffers in addition in that it is
not a deterministic method and can not capture a biological process in real time [19].
While fluorescence-based methods offer a remarkable improvement of the resolution, as compared to the conventional diffraction-limited optical imaging, fluorescent
dyes can induce an unwanted chemical reaction in the sample, or they can only be
attached to a certain part of the specimen. The resulting quest for a label-free superresolution imaging, led to the emergence of an alternative approach that is based on
the novel concept of electromagnetic metamaterials.
14.3 Metamaterials-Based Super-Resolution
Metamaterials [20] are artificial composites having properties that are usually not
available in nature. The constituents are arrayed on a scale much smaller than the
wavelength, so that the incoming radiation cannot fully distinguish between the
grainy variation, and the medium as a whole shows a response that can be substantially different from the properties of the individual material components. These new
emergent properties can be controlled by engineering the geometric structure of the
metamaterial unit cell.
One of the earliest examples of these new emergent properties of metamaterials is
the negative index which leads to negative refraction, reversed Cherenkov radiation
and the possibility of realizing a flat lens [21]. When an object is placed close to a slab
of such a medium, the evanescent waves scattered by the object excite surface states
at the metamaterial interfaces. For the surface state at the “right” metamaterial–air
interface (see Fig. 14.1a), its decay into the negative index slab manifests itself as the
effective growth of the field in the metamaterial when viewed from the position of
the source at the “left” of the slab. This effective “amplification” of the evanescent
field that carries the subwavelength information on the structure of object, is what