Thermal Effects on NSOM Probes
Thermal heating of the NSOM probe occurs in the taper region due to the absorption of light by the metallic coating engulfing the exterior of the probe. In illumination mode, light is coupled into the core of the fiber, and upon reaching the tapered region of the probe, begins to propagate through the cladding to the metal coating. Because the metal has a non-unity reflection coefficient, some photon absorption occurs in the metal layer, and heat is generated. The optical signal transmitted through the probe is limited by the ability of the tapered region to physically tolerate and to dissipate this heat.
Upon initialization, the tutorial window displays a graph of probe temperature as a function of distance to the tip, and the Temperature/Distance radio button is selected. Adjusting the Light Power slider from its default left-hand position causes the optical power coupled into the tip to increase. The selected value is indicated above the slider, ranging from 0.1 to 9.4 milliwatts. As the power is increased, the red curve reflects the temperature response in the probe. Note that the temperature rise is much more extreme near the tip, with a relatively minor change occurring at distances of more than about 600 micrometers. The image window to the right of the graph illustrates the nature of the probe tip failure that occurs with rising temperature. With low power settings, light is emitted only from the aperture region at the probe tip. As the power into the probe is increased, light leakage through the metal coating increases along the probe length. Eventually large gaps in the metal coating occur as the metal and glass separate, and probe tip failure results.
Selecting the Temperature/Light Power radio button changes the graph display to a plot showing the linear correspondence of light power to tip temperature, measured at a fixed distance (80 micrometers) from the probe aperture. Adjusting the Light Power slider moves the red dot on the graph to the corresponding position on the curve. The small window labeled Probe Tip Profile illustrates the location at which the temperature was measured for the graph data.
The thermal destruction of an aluminum-coated NSOM probe tip has been experimentally shown to occur at approximately 470 degrees Celsius (considerably lower than the melting temperature of aluminum, 660 degrees Celsius), corresponding to an input coupled power of 9.4 milliwatts. The tip damage most likely results from the different thermal expansion coefficients of the aluminum and the optical fiber, which causes the aluminum coating to separate from the glass.
In addition to tip damage, other experimental problems arise from the localized tip heating. These temperature dependant factors include fluorescence lifetime, quantum yield, and the photostability of the chromophores. Among the strategies for reducing probe temperature are increasing the thickness of the deposited metal layer, and increasing the cone angle of the tapered tip. Interestingly, in certain NSOM applications, controlled tip heating could be considered a beneficial side effect of the probe's light transmission. For example, precise localized heating of a data storage disk above the Curie temperature could be employed to facilitate magneto-optical recording.