32
M. I. Stockman
emitted electrons over their energies). Additionally, PEEM requires clean surfaces
in high vacuum.
A fundamentally different non-perturbing approach to studying nanoplasmonic
hot spots has been pioneered in Refs. [181, 182]. It is based on the so-called photonlocalization super-resolution far-field microscopy. This method of far-field superresolution has originally been developed in application to biological imaging [183].
This method’s fundamentals can be very briefly described as the following.
Assume that there is a single radiating chromophore (say, fluorescing molecule)
in the view field of an optical microscope. Alternatively, there may be a number of
such chromophores but their concentration should be low enough so they are resolved
separately by the microscope (i.e., the distance between these molecules are greater
than the microscope’s resolution). The center of the emission of such a single (or
separately resolved) emitter can be found with any precision that is only limited by
statistical fluctuations of the number of the recorded photons but not by the resolution
of the microscope provided that this microscope or the system under study does not
change in the course of the observation.
After the position and brightness of a given single molecule are recorded, this
molecule is naturally bleached. Then another molecule comes into the hot spot and
its position and brightness are recorded until it is bleached. The process is repeated
until the distribution of the brightness of emitters is built with a sufficient statistical
precision.
It is assumed that the emission brightness of a single chromophore is proportional
to the local field intensity of the hot spot at its position and that this chromophore
exerts a negligibly weak perturbation on the local field of the hot spot. Thus this
photon-localization nanoscopy is a non-perturbative method allowing one to find the
intensity distribution at the hot spot on the nanoscale limited only by the statistical
fluctuations (inversely proportional to the accumulation time) and the size of the
chromophore itself, which is negligible in realistic situations.
The results of the hot spot local intensity-distribution measurements for an aluminum surface are shown in Fig. 1.13a. This distribution is a narrow peak with the
width of ≈20 nm. The observed fine structure of this distribution is attributed to
statistical fluctuations [181]. The cross section through this distribution displayed in
Fig. 1.13b suggests an exponential decay of this distribution function in space with
the FWHM = 20 nm.
Very similar results are obtained for the silver colloid clusters as shown in
Figs. 1.13c, d. Note that the aluminum surface studied is nominally smooth and
contains only random roughness while the silver colloid clusters are fractals whose
density fundamentally possesses large and correlated fluctuations.
M. I. Stockman
emitted electrons over their energies). Additionally, PEEM requires clean surfaces
in high vacuum.
A fundamentally different non-perturbing approach to studying nanoplasmonic
hot spots has been pioneered in Refs. [181, 182]. It is based on the so-called photonlocalization super-resolution far-field microscopy. This method of far-field superresolution has originally been developed in application to biological imaging [183].
This method’s fundamentals can be very briefly described as the following.
Assume that there is a single radiating chromophore (say, fluorescing molecule)
in the view field of an optical microscope. Alternatively, there may be a number of
such chromophores but their concentration should be low enough so they are resolved
separately by the microscope (i.e., the distance between these molecules are greater
than the microscope’s resolution). The center of the emission of such a single (or
separately resolved) emitter can be found with any precision that is only limited by
statistical fluctuations of the number of the recorded photons but not by the resolution
of the microscope provided that this microscope or the system under study does not
change in the course of the observation.
After the position and brightness of a given single molecule are recorded, this
molecule is naturally bleached. Then another molecule comes into the hot spot and
its position and brightness are recorded until it is bleached. The process is repeated
until the distribution of the brightness of emitters is built with a sufficient statistical
precision.
It is assumed that the emission brightness of a single chromophore is proportional
to the local field intensity of the hot spot at its position and that this chromophore
exerts a negligibly weak perturbation on the local field of the hot spot. Thus this
photon-localization nanoscopy is a non-perturbative method allowing one to find the
intensity distribution at the hot spot on the nanoscale limited only by the statistical
fluctuations (inversely proportional to the accumulation time) and the size of the
chromophore itself, which is negligible in realistic situations.
The results of the hot spot local intensity-distribution measurements for an aluminum surface are shown in Fig. 1.13a. This distribution is a narrow peak with the
width of ≈20 nm. The observed fine structure of this distribution is attributed to
statistical fluctuations [181]. The cross section through this distribution displayed in
Fig. 1.13b suggests an exponential decay of this distribution function in space with
the FWHM = 20 nm.
Very similar results are obtained for the silver colloid clusters as shown in
Figs. 1.13c, d. Note that the aluminum surface studied is nominally smooth and
contains only random roughness while the silver colloid clusters are fractals whose
density fundamentally possesses large and correlated fluctuations.