Tip-Enhanced Raman Spectroscopy
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the objective lens up and down with nanoscale precision. However, one first needs
to sense the amount of drift to set off the feedback to the objective lens positioner.
There could be several methods to do this. One of them is, for example, by utilizing
a parallel-plate capacitor with variable capacitance. When the distance between the
two metallic plates of a capacitor changes, the capacitance also changes with high
sensitivity. If a capacitor is attached to the alignment system in such a way that the
distance between the two metallic plates is mechanically changed with the change in
separation between the sample and the objective lens, then one can precisely sense
the change in this separation, which would correspond to the drift in the z-direction,
by monitoring the change in the capacitance. Once the drift in z-direction is sensed
by the capacitor in terms of an electric signal, it can send a corresponding feedback
to the objective lens positioner [31]. Alternatively, an optical method could also be a
good candidate to detect the drift in z-direction. A few major optics companies have
already manufactured some optical microscopy systems that are integrated with the
auto-focus system using optical drift detection. However, since they are not designed
for nano-photonics applications, the movement of the objective lens is operated by
a stepping-motor that moves in steps with jerk. It makes vibrations that affect SPM
operation, and does not have enough accuracy. Therefore, a piezo-actuated system
is highly recommended for TERS imaging.
By implementing these compensations in three dimensions, one can keep tip
within the focus spot for long time, which makes TERS more reliable and practical
technique.
3.3 Laser Illumination Configurations
Thus far we have discussed some practical suggestions for stable TERS measurements based on the TERS experimental system shown in Fig. 3. However, this is just
one of several possible experimental configurations. One of the important bases to
categorize the experimental setup is the configuration of laser illumination. In this
sense, the TERS experimental setup can be characterized as the bottom-illumination,
side-illumination, and top-illumination (Fig. 6).
In the setup shown in Fig. 3, the bottom-illumination is used. The incident laser
literally illuminates the tip from the bottom (Fig. 6a). In this configuration, the incident laser transmits through the substrate to illuminate the tip apex. Therefore, the
substrate needs to be transparent. However, as one can bring the objective lens quite
close to the substrate in this configuration, it is possible to use an objective lens
with high NA that usually has short working distance (WD), which allows for an
evanescent illumination and also provides high signal-collection efficiency [32–34].
In contrast, for the side- [35, 36] and top-illumination modes [37, 38], because one
does not have to guide light from the bottom, it is possible to use opaque substrates and
samples so that these configurations impose less restrictions on the sample choice.
However, the disadvantage of these configurations is that the laser is illuminated
from the same side of the substrate where the tip is located. This makes it technically
219
the objective lens up and down with nanoscale precision. However, one first needs
to sense the amount of drift to set off the feedback to the objective lens positioner.
There could be several methods to do this. One of them is, for example, by utilizing
a parallel-plate capacitor with variable capacitance. When the distance between the
two metallic plates of a capacitor changes, the capacitance also changes with high
sensitivity. If a capacitor is attached to the alignment system in such a way that the
distance between the two metallic plates is mechanically changed with the change in
separation between the sample and the objective lens, then one can precisely sense
the change in this separation, which would correspond to the drift in the z-direction,
by monitoring the change in the capacitance. Once the drift in z-direction is sensed
by the capacitor in terms of an electric signal, it can send a corresponding feedback
to the objective lens positioner [31]. Alternatively, an optical method could also be a
good candidate to detect the drift in z-direction. A few major optics companies have
already manufactured some optical microscopy systems that are integrated with the
auto-focus system using optical drift detection. However, since they are not designed
for nano-photonics applications, the movement of the objective lens is operated by
a stepping-motor that moves in steps with jerk. It makes vibrations that affect SPM
operation, and does not have enough accuracy. Therefore, a piezo-actuated system
is highly recommended for TERS imaging.
By implementing these compensations in three dimensions, one can keep tip
within the focus spot for long time, which makes TERS more reliable and practical
technique.
3.3 Laser Illumination Configurations
Thus far we have discussed some practical suggestions for stable TERS measurements based on the TERS experimental system shown in Fig. 3. However, this is just
one of several possible experimental configurations. One of the important bases to
categorize the experimental setup is the configuration of laser illumination. In this
sense, the TERS experimental setup can be characterized as the bottom-illumination,
side-illumination, and top-illumination (Fig. 6).
In the setup shown in Fig. 3, the bottom-illumination is used. The incident laser
literally illuminates the tip from the bottom (Fig. 6a). In this configuration, the incident laser transmits through the substrate to illuminate the tip apex. Therefore, the
substrate needs to be transparent. However, as one can bring the objective lens quite
close to the substrate in this configuration, it is possible to use an objective lens
with high NA that usually has short working distance (WD), which allows for an
evanescent illumination and also provides high signal-collection efficiency [32–34].
In contrast, for the side- [35, 36] and top-illumination modes [37, 38], because one
does not have to guide light from the bottom, it is possible to use opaque substrates and
samples so that these configurations impose less restrictions on the sample choice.
However, the disadvantage of these configurations is that the laser is illuminated
from the same side of the substrate where the tip is located. This makes it technically