I would like to calibrate the torsional spring constant of rectangular cantilevers using the thermal noise method of Sader. The cantilevers have a torsional resonance frequency of ~ 300 kHz. How can I obtain the torsional PSD for Simple Harmonic Oscillator/Lorentzian fitting? Is it as easily accessible as the normal (z) PSD in the default thermal tune calibration? We have a Multimode 8 and Nanoscope V controller.

the vertical deflection is ((A+B) - (C+D))/SUM, and the lateral deflection is ((A+C)-(B+D))/SUM, where A, B, C and D are the signals from the quadrants on the photodiode and SUM is A+B+C+D.

Looking at the above equations, you can trick the microscope into thinking that a lateral deflection is a vertical one by swapping the B and C photodiode signals. The easiest way to do this is to make a patch cable that goes between the cable from the optical head and the socket on the microscope base. If you swap the photodiode signals for quadrants B and C, the microscope will interpret lateral deflections as vertical ones, so just doing a regular thermal tune with the patch cable in place will give you the torsional PSD.

Obviously, you have to remember to remove the cable before you do any regular AFM!

Bear in mind that using a home made patch cable will probably invalidate any warranty you have, and if you make the wrong connections you could do some serious damage to the head and the base.