Scanning near-field optical microscope based on double-resonant fiber probe montage and its operation in liquids

The operation of the scanning near-field optical microscope based on the double-resonant montage of a fiber probe onto the tuning fork (working frequency of the latter, that is 32 kHz, coincides with the second resonance frequency of the bending oscillations of the free standing part of a fiber beam) in liquid is reported. It is shown that due to the peculiarities of the probe montage (initially large, around 3,000 - 5,500 quality factor of the dithering and long projection of the fiber beam beyond the tuning fork body) and microscope electronics, this SNOM is very fit to work in liquids. Quality factor of the sensor drops down to the values around 300 - 600 when the probe tip is submerged on the depth of 0.2 - 0.3 mm, thus remaining large enough to enable high quality imaging with rather small acting force value laying in the subnanoNewton region. We also discuss the joint liquid recipient construction which connects the liquid cell containing a sample with the large water reservoir via a flexible tube. This reservoir is placed onto separate Z-stage and hence the water level in the cell can be regulated independently from the sample position which facilitates the SNOM operation a lot.