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T. Umakoshi and P. Verma
been proposed [46, 68, 69], it has still been challenging to perform TERS measurements in liquid condition. Once TERS becomes a useful tool to observe living biological samples under their physiological conditions, it would show a strong impact to
the related fields due to its label-free nature with a nanoscale spatial resolution to
observe natural behaviors of bio-molecules.
In an effort to add the benefits of nonlinearity in TERS and to observe non-living
bio-molecules, an adenine crystal was investigated by combining the coherent antistokes Raman scattering (CARS) with TERS [70]. Also, recently stimulated Raman
scattering (SRS) was demonstrated with TERS tip, where anomalous enhancement
of SRS signals was reported [71]. The combination with coherent Raman techniques
is another promising future of TERS.
4.2 Polarization Control in TERS
One of the concerns in TERS has been to control the polarization of near-field light.
It is easy to control and understand the polarization in far-field optics by simply using
polarizers or related optical components. However, it becomes complicated in the
near-field regime because the polarization of near-field light in TERS does not have
a direct and simple relation with the polarization of the incident light as it is also
strongly affected by the size, shape and the orientation of the plasmonic structure
around the apex of the TERS tip. Even if the plasmonic structure at the tip apex is
illuminated with a linearly polarized light, the resulting polarization of the near-field
light may not be oriented to the same direction as that of the incident light. Even a
slight difference in the shape, size or orientation of the plasmonic structure modifies
the polarization of near-field light. In fact, this is one of the reasons why the near-field
polarization has often been ignored from the considerations in TERS experiments,
or at most has been considered based on some assumptions. However, since Raman
intensity strongly depends on the polarization of excitation light, quantitative analysis
is not possible without understanding the polarization of near-field light in TERS.
Here, we introduce some techniques to study the polarization of near-field light
that have been recently developed for TERS [72]. One can apply the defocused
imaging technique for the evaluation of the polarization of near-field light generated
at the tip apex. This technique is used to investigate the direction of the dipole moment
of the plasmonic structure at the tip apex. It was originally developed to investigate
the dipole moment of a single fluorescent molecule [73]. If the observation image
is perfectly focused at the sample plane, a single fluorescent molecule would look
like a single dot. However, by slightly defocusing the observation image, a unique
defocused pattern appears, as shown in Fig. 9a. This defocused pattern shows a
non-symmetric pattern with a dark spot, as indicated by the dotted enclosure. The
location and the shape of the dark spot has a direct relation with the direction of the
dipole moment. Therefore, one can understand the dipole moment by analyzing the
defocused pattern.
T. Umakoshi and P. Verma
been proposed [46, 68, 69], it has still been challenging to perform TERS measurements in liquid condition. Once TERS becomes a useful tool to observe living biological samples under their physiological conditions, it would show a strong impact to
the related fields due to its label-free nature with a nanoscale spatial resolution to
observe natural behaviors of bio-molecules.
In an effort to add the benefits of nonlinearity in TERS and to observe non-living
bio-molecules, an adenine crystal was investigated by combining the coherent antistokes Raman scattering (CARS) with TERS [70]. Also, recently stimulated Raman
scattering (SRS) was demonstrated with TERS tip, where anomalous enhancement
of SRS signals was reported [71]. The combination with coherent Raman techniques
is another promising future of TERS.
4.2 Polarization Control in TERS
One of the concerns in TERS has been to control the polarization of near-field light.
It is easy to control and understand the polarization in far-field optics by simply using
polarizers or related optical components. However, it becomes complicated in the
near-field regime because the polarization of near-field light in TERS does not have
a direct and simple relation with the polarization of the incident light as it is also
strongly affected by the size, shape and the orientation of the plasmonic structure
around the apex of the TERS tip. Even if the plasmonic structure at the tip apex is
illuminated with a linearly polarized light, the resulting polarization of the near-field
light may not be oriented to the same direction as that of the incident light. Even a
slight difference in the shape, size or orientation of the plasmonic structure modifies
the polarization of near-field light. In fact, this is one of the reasons why the near-field
polarization has often been ignored from the considerations in TERS experiments,
or at most has been considered based on some assumptions. However, since Raman
intensity strongly depends on the polarization of excitation light, quantitative analysis
is not possible without understanding the polarization of near-field light in TERS.
Here, we introduce some techniques to study the polarization of near-field light
that have been recently developed for TERS [72]. One can apply the defocused
imaging technique for the evaluation of the polarization of near-field light generated
at the tip apex. This technique is used to investigate the direction of the dipole moment
of the plasmonic structure at the tip apex. It was originally developed to investigate
the dipole moment of a single fluorescent molecule [73]. If the observation image
is perfectly focused at the sample plane, a single fluorescent molecule would look
like a single dot. However, by slightly defocusing the observation image, a unique
defocused pattern appears, as shown in Fig. 9a. This defocused pattern shows a
non-symmetric pattern with a dark spot, as indicated by the dotted enclosure. The
location and the shape of the dark spot has a direct relation with the direction of the
dipole moment. Therefore, one can understand the dipole moment by analyzing the
defocused pattern.