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T. Umakoshi and P. Verma
well as the possible scattering noise from the tip shaft since no incident light can
transmit above the sample plane to hit the shaft or the upper part of the tip.
3.2 Some Advanced Instrumentations for TERS
As positioning the tip into the focus spot is important, keeping the tip within the focus
spot throughout the measurement time is also important. Since the near-field light at
the tip apex can drastically reduce or vanish even with a position shift of a few tens
of nanometers from the focus spot, it is crucial to suppress mechanical and thermal
drift of the experimental systems. Every component such as the optical microscope,
AFM, and cantilever tip can cause a mechanical or a thermal drift. One of the simple
and effective ways to suppress the drift is to put the system in an enclosure box to
block wind and sound from the external environment and to suppress the ambient
temperature change or fluctuation. However, it is still not possible to completely
eliminate the effect of the drift. It becomes more crucial, especially in the case of
TERS imaging, as it requires to detect weak Raman scattering from multiple points.
It usually takes close to 1 h or even more to obtain one TERS image of a reasonable
size. In such a case, suppression of the drift by employing an enclosure is not enough,
and a more active method such as compensation for the drift is required [29–31].
As the relative position between the tip and the laser focus can be shifted in any
directions in the three dimensional space, drift compensation methods are required
for all x-, y- and z- directions. In x- and y- directions, i.e. in-plane direction to the
sample plane, the drift compensation can be easily achieved. As discussed with Fig. 3,
one can obtain an optical image of the intensity pattern of the focus spot by scanning
the tip on the focus spot, which can be used to position the tip apex in the right location
within the focus spot. One can repeat the same process periodically at a certain time
interval before a non-negligible drift occurs. For example, one can obtain the image
of the focus spot every time after obtaining one line scan in a TERS image. If one
finds that there is a shift between the tip position and the focus spot, the tip can be
moved back to its original position before the next scan. In this way, if it takes, say,
one minute to scan one line of the TERS image, it would be possible to compensate
the drift every minute. For the compensation, it is of course better to make such an
algorithm to automatically obtain the focus spot image and compensate the drift. It
can be easily implemented by performing gaussian fitting to the focus spot image to
find out the focus spot position. Although the tip-scanning configuration is discussed
in Fig. 3, laser-scanning configuration with a galvano mirror system would be better
because one needs to take the focus spot images multiple times during the TERS
imaging. The galvano mirrors can in general scan so quick that the focus spot image
can be obtained almost instantly [30].
In terms of the drift in z-direction, it is mostly caused by the drift of the objective
lens. As the objective drifts in z-direction, the laser can be defocused at the sample
plane, which would deteriorate the measurement. To actively compensate this drift,
an objective lens positioner actuated by a piezo device is useful. It can precisely move
T. Umakoshi and P. Verma
well as the possible scattering noise from the tip shaft since no incident light can
transmit above the sample plane to hit the shaft or the upper part of the tip.
3.2 Some Advanced Instrumentations for TERS
As positioning the tip into the focus spot is important, keeping the tip within the focus
spot throughout the measurement time is also important. Since the near-field light at
the tip apex can drastically reduce or vanish even with a position shift of a few tens
of nanometers from the focus spot, it is crucial to suppress mechanical and thermal
drift of the experimental systems. Every component such as the optical microscope,
AFM, and cantilever tip can cause a mechanical or a thermal drift. One of the simple
and effective ways to suppress the drift is to put the system in an enclosure box to
block wind and sound from the external environment and to suppress the ambient
temperature change or fluctuation. However, it is still not possible to completely
eliminate the effect of the drift. It becomes more crucial, especially in the case of
TERS imaging, as it requires to detect weak Raman scattering from multiple points.
It usually takes close to 1 h or even more to obtain one TERS image of a reasonable
size. In such a case, suppression of the drift by employing an enclosure is not enough,
and a more active method such as compensation for the drift is required [29–31].
As the relative position between the tip and the laser focus can be shifted in any
directions in the three dimensional space, drift compensation methods are required
for all x-, y- and z- directions. In x- and y- directions, i.e. in-plane direction to the
sample plane, the drift compensation can be easily achieved. As discussed with Fig. 3,
one can obtain an optical image of the intensity pattern of the focus spot by scanning
the tip on the focus spot, which can be used to position the tip apex in the right location
within the focus spot. One can repeat the same process periodically at a certain time
interval before a non-negligible drift occurs. For example, one can obtain the image
of the focus spot every time after obtaining one line scan in a TERS image. If one
finds that there is a shift between the tip position and the focus spot, the tip can be
moved back to its original position before the next scan. In this way, if it takes, say,
one minute to scan one line of the TERS image, it would be possible to compensate
the drift every minute. For the compensation, it is of course better to make such an
algorithm to automatically obtain the focus spot image and compensate the drift. It
can be easily implemented by performing gaussian fitting to the focus spot image to
find out the focus spot position. Although the tip-scanning configuration is discussed
in Fig. 3, laser-scanning configuration with a galvano mirror system would be better
because one needs to take the focus spot images multiple times during the TERS
imaging. The galvano mirrors can in general scan so quick that the focus spot image
can be obtained almost instantly [30].
In terms of the drift in z-direction, it is mostly caused by the drift of the objective
lens. As the objective drifts in z-direction, the laser can be defocused at the sample
plane, which would deteriorate the measurement. To actively compensate this drift,
an objective lens positioner actuated by a piezo device is useful. It can precisely move