ThermoMicroscopes Aurora-2, User Manual

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T
HEORY OF
O
PERATION
1-3
Aurora-2 User’s Manual
The fiber-optic probe is attached to one prong of a piezoelectric tuning fork. The tip
extends slightly beyond the end of the tuning fork. The tuning fork is vibrated using a
dithering piezoelectric device, which produces a vibrational amplitude at the tip of approx-
imately one nanometer. As the piezoelectric material of the fork vibrates, it produces a
small current. The vibration amplitude of the fiber can then be measured by measuring the
piezoelectric signal from the tuning fork. The current is amplified and input into a lock-in
amplifier. The phase change of the tuning fork signal relative to the driving signal is
measured and used in the feedback loop.
Figure 1-3 Tuning Fork Mechanism
M
ODES OF
O
PERATION
The NSOM instrument is a unique combination of a scanning probe microscope and an
ultra-high resolution optical microscope. Generally both modes—topography and
optical—are used together. However, there may be some situations where topography is
used alone.
The NSOM, used as an optical microscope, can be operated either in tip collection or tip
illumination mode. In tip collection mode, the sample is the light source, and the tip acts
as a way to collect this light. This method is best for samples such as waveguides and laser
diodes.
Tip illumination mode is perhaps the most common NSOM mode. Tip illumination uses
the tip as a “light funnel” to illuminate the sample in a precise, controlled manner. This
mode can be further subdivided into reflection collection, transmission collection, and li-
thography modes. Reflection collection gathers light that has been reflected from the
sample. It is used for opaque samples, such as semiconductor materials. This method is
not as efficient for gathering light, since the physical position of the tip collector does not
allow it to collect much of the reflected light. Reflection mode might also suffer from
image artifacts created by tip shadowing on the sample surface. Transmission mode is
more commonly used and is more efficient. The collector is placed behind the sample and
collects a majority of the light as it passes through the sample. The drawback to this mode
is that it requires the use of thin, transparent samples.