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T. Umakoshi and P. Verma
This creates an undesired far-field background in TERS measurements. A pure nearfield Raman signal is therefore obtained by subtracting Raman signal without tip
(far-field signal only) from the Raman signal with tip (combination of near-field and
far-field signals).
One of the crucial processes in TERS measurements is to position the tip apex
exactly at the center of the focus spot of the incident laser with nanoscale precision.
Therefore, it is highly recommended to install piezo actuators to control relative
position of tip and sample in both x- and y- directions, in addition to the z direction,
which is for AFM feedback. By using x- and y- piezo actuators, one can rasterscan the tip within the focus spot while detecting the Rayleigh scattered signal by a
photodetector so that one can obtain an image plotted by the tip-scattered intensity.
It usually shows a concentric pattern formed by the incident laser, as shown in the
inset of Fig. 3. This image represents the laser intensity distribution within the focus
spot and helps one recognize the exact location where the intensity is the highest. By
locating the tip apex exactly at the center of the concentric pattern using the piezo
actuators, it is possible to properly excite near-field light at the tip apex with the
maximum efficiency, and it is ready for the TERS measurements. Since the relative
position of the tip and the focus spot is important for efficient excitation of the nearfield light, the positions of the tip and the incident focus spot are kept fixed, and the
sample is moved in the x- and y- directions to search a region of interest or to facilitate
the raster scanning for TERS imaging. Therefore, at least x- and y- piezo actuators
are necessary for the sample stage. TERS imaging is performed by scanning the
sample rather than scanning the tip. The sample stage scanner is synchronized with
the CCD camera to obtain Raman spectrum at each pixel of the image.
There are a few more important points about the experimental setup that should
also be noted here. In the incident path in Fig. 3, a z-polarizer is inserted to convert
linier polarization to radial polarization. As shown in Fig. 4a, the z-polarizer is
composed of four or eight segmented half-wave plates, which rotates the linear
polarization of incident light in each segment in such a way that the resultant polarization is converted into the radial polarization after the light passes through the
z-polarizer [28]. When this radially polarized light bends into the focus spot as it
gets focused through the objective lens, the lateral components of polarization cancel
out while the z-components add up to result in the generation of a pure z-polarization
at the focus spot, the direction of which is parallel to the tip axis. To excite the nearfield light underneath the tip, it is important that the plasmons oscillate parallel to
the tip axis, i.e. in the z-direction. Therefore, a z-polarizer can efficiently induce
near-field light at the very end of the tip. In contrast, a linear polarization in the
absence of a z-polarizer generates the polarization parallel to the sample plane after
the objective, because the z-components get cancelled in this case, as shown in
Fig. 4b. It cannot thus efficiently excite near-field light at the tip apex. In addition to
the z-polarizer, a spatial mask is also inserted in the incident path. It is basically an
opaque disk with a diameter slightly smaller than the diameter of laser beam, which
blocks the central part of the laser, and allows only the external part in the form of
a ring to pass through. If the objective lens has sufficiently large numerical aperture
(NA), the incident light from this external ring is focused at the sample plane at
T. Umakoshi and P. Verma
This creates an undesired far-field background in TERS measurements. A pure nearfield Raman signal is therefore obtained by subtracting Raman signal without tip
(far-field signal only) from the Raman signal with tip (combination of near-field and
far-field signals).
One of the crucial processes in TERS measurements is to position the tip apex
exactly at the center of the focus spot of the incident laser with nanoscale precision.
Therefore, it is highly recommended to install piezo actuators to control relative
position of tip and sample in both x- and y- directions, in addition to the z direction,
which is for AFM feedback. By using x- and y- piezo actuators, one can rasterscan the tip within the focus spot while detecting the Rayleigh scattered signal by a
photodetector so that one can obtain an image plotted by the tip-scattered intensity.
It usually shows a concentric pattern formed by the incident laser, as shown in the
inset of Fig. 3. This image represents the laser intensity distribution within the focus
spot and helps one recognize the exact location where the intensity is the highest. By
locating the tip apex exactly at the center of the concentric pattern using the piezo
actuators, it is possible to properly excite near-field light at the tip apex with the
maximum efficiency, and it is ready for the TERS measurements. Since the relative
position of the tip and the focus spot is important for efficient excitation of the nearfield light, the positions of the tip and the incident focus spot are kept fixed, and the
sample is moved in the x- and y- directions to search a region of interest or to facilitate
the raster scanning for TERS imaging. Therefore, at least x- and y- piezo actuators
are necessary for the sample stage. TERS imaging is performed by scanning the
sample rather than scanning the tip. The sample stage scanner is synchronized with
the CCD camera to obtain Raman spectrum at each pixel of the image.
There are a few more important points about the experimental setup that should
also be noted here. In the incident path in Fig. 3, a z-polarizer is inserted to convert
linier polarization to radial polarization. As shown in Fig. 4a, the z-polarizer is
composed of four or eight segmented half-wave plates, which rotates the linear
polarization of incident light in each segment in such a way that the resultant polarization is converted into the radial polarization after the light passes through the
z-polarizer [28]. When this radially polarized light bends into the focus spot as it
gets focused through the objective lens, the lateral components of polarization cancel
out while the z-components add up to result in the generation of a pure z-polarization
at the focus spot, the direction of which is parallel to the tip axis. To excite the nearfield light underneath the tip, it is important that the plasmons oscillate parallel to
the tip axis, i.e. in the z-direction. Therefore, a z-polarizer can efficiently induce
near-field light at the very end of the tip. In contrast, a linear polarization in the
absence of a z-polarizer generates the polarization parallel to the sample plane after
the objective, because the z-components get cancelled in this case, as shown in
Fig. 4b. It cannot thus efficiently excite near-field light at the tip apex. In addition to
the z-polarizer, a spatial mask is also inserted in the incident path. It is basically an
opaque disk with a diameter slightly smaller than the diameter of laser beam, which
blocks the central part of the laser, and allows only the external part in the form of
a ring to pass through. If the objective lens has sufficiently large numerical aperture
(NA), the incident light from this external ring is focused at the sample plane at