116
N. Rahbany et al.
in the vicinity of metallic nanoantennas [14]. Therefore, characterizing NSOM probes
is not a straightforward and easy task. Their radiation patterns strongly depend on the
type of probe as well as on the environment that they interact with. When such probes
are placed close to the surface of a sample, the scattered field is highly affected by
multiple reflections and interferences at the surface. This strong coupling between
the probe and the surface makes it even more difficult to predict their EM radiation
pattern in a given environment [15, 16].
In addition to its detection role, a NSOM tip can also be used to excite and launch
surface plasmon polaritons on the surface of metallic films [17]. It is important
to mention here that the excitation of surface plasmons can also be performed by
other techniques such as exciting surface defects [6, 17], point-like dipoles [18],
and tunneling electrons [19, 20], in addition to classical techniques such as prism or
grating couplers. The far-field radiation pattern of leaky surface plasmons launched
on metallic thin films by NSOM tips was previously studied using conventional backfocal plane imaging techniques [17, 21, 22]. In such techniques, the intensity of the
EM field is accurately measured in the Fourier plane of an optical system [23, 24].
However, no information about the phase is obtained. In order to fully characterize
the scattered EM field in three dimensions, both the intensity and the phase must be
calculated. For this reason, we developed a combined NSOM-holography technique
to accurately describe the radiation patterns of individual NSOM probes as a function
of the local environment. The main advantage of our combined system is that it can
directly deliver information about both the amplitude and the phase of the scattered
light through the NSOM probe from a single recorded hologram. Then, following
the procedure described in Sect. 5.2, we can reconstruct the full three-dimensional
scattered field coupled to the environment [25]. It is important to mention that Digital
Holographic Microscopy was previously combined with NSOM for the purpose of
achieving super-resolution imaging through a disordered scattering medium that is
illuminated by subwavelength tips that act as point-like sources [26].
5.2 Principles of Digital Holography
Holography is a technique that combines the processes of interference and diffraction
to record and reconstruct the amplitude and phase of an electromagnetic field in three
dimensions. It was discovered in 1948 by Denis Gabor [27, 28] who received a Nobel
Prize in Physics for his work later in 1971. It wasn’t until the development of the
lasers in 1960 that holography took its place in the optics domain [29].
The advantage of holography over conventional photography techniques is that it
contains information about the entire three-dimensional wavefield which is contained
in interference patterns. These patterns arise when the wave scattered by the object, or
object wave, is illuminated by a reference wave, creating a hologram. This hologram
is then illuminated with the reference wave again to obtain the three-dimensional
reconstructed image of the electromagnetic field scattered by the object [30–36].
N. Rahbany et al.
in the vicinity of metallic nanoantennas [14]. Therefore, characterizing NSOM probes
is not a straightforward and easy task. Their radiation patterns strongly depend on the
type of probe as well as on the environment that they interact with. When such probes
are placed close to the surface of a sample, the scattered field is highly affected by
multiple reflections and interferences at the surface. This strong coupling between
the probe and the surface makes it even more difficult to predict their EM radiation
pattern in a given environment [15, 16].
In addition to its detection role, a NSOM tip can also be used to excite and launch
surface plasmon polaritons on the surface of metallic films [17]. It is important
to mention here that the excitation of surface plasmons can also be performed by
other techniques such as exciting surface defects [6, 17], point-like dipoles [18],
and tunneling electrons [19, 20], in addition to classical techniques such as prism or
grating couplers. The far-field radiation pattern of leaky surface plasmons launched
on metallic thin films by NSOM tips was previously studied using conventional backfocal plane imaging techniques [17, 21, 22]. In such techniques, the intensity of the
EM field is accurately measured in the Fourier plane of an optical system [23, 24].
However, no information about the phase is obtained. In order to fully characterize
the scattered EM field in three dimensions, both the intensity and the phase must be
calculated. For this reason, we developed a combined NSOM-holography technique
to accurately describe the radiation patterns of individual NSOM probes as a function
of the local environment. The main advantage of our combined system is that it can
directly deliver information about both the amplitude and the phase of the scattered
light through the NSOM probe from a single recorded hologram. Then, following
the procedure described in Sect. 5.2, we can reconstruct the full three-dimensional
scattered field coupled to the environment [25]. It is important to mention that Digital
Holographic Microscopy was previously combined with NSOM for the purpose of
achieving super-resolution imaging through a disordered scattering medium that is
illuminated by subwavelength tips that act as point-like sources [26].
5.2 Principles of Digital Holography
Holography is a technique that combines the processes of interference and diffraction
to record and reconstruct the amplitude and phase of an electromagnetic field in three
dimensions. It was discovered in 1948 by Denis Gabor [27, 28] who received a Nobel
Prize in Physics for his work later in 1971. It wasn’t until the development of the
lasers in 1960 that holography took its place in the optics domain [29].
The advantage of holography over conventional photography techniques is that it
contains information about the entire three-dimensional wavefield which is contained
in interference patterns. These patterns arise when the wave scattered by the object, or
object wave, is illuminated by a reference wave, creating a hologram. This hologram
is then illuminated with the reference wave again to obtain the three-dimensional
reconstructed image of the electromagnetic field scattered by the object [30–36].