UC Irvine

ABSTRACT OF THE THESIS

Numerical Simulation of Photoinduced Force Microscopy

By

Mohammad Almajhadi

Master of Science in Electrical and Computer Engineering

University of California, Irvine, 2016

Professor H. Kumar Wickramasinghe, Chair

In this thesis we introduce a method of imaging: photoinduced force microscopy (PiFM). In this method, the images are accomplished by detecting tip-sample force at the near field which is optically excited. The origin of this force is related to the dipole momentum of the tip and its image in the sample. The dipole momentum strength is function of the polarizability of the sample under test; the polarizability is a wavelength dependent. Therefore, driving sample at resonance will give higher force than off resonance. This polarizability contains the spectroscopic information about the sample. By tuning the excited laser over broad range of frequency spectra, sample can by identified by comparing the resulting force measurement to a well-known absorption curve. PiFM Spatial resolution is well below 10nm and it is limited only by the tip geometry. This is because the field is spatially confined and the gradient of tip-sample force is proportional to z-4 where z is the distance away from the tip. Finite Element Method (FEM) is utilized to calculate the force gradient, and the tip-sample force.

The probe is attached to the end of a cantilever vibrating at one or two mechanical frequencies. Simultaneously, topographic and optical imaging process can be done by detecting them at different mechanical frequencies. In fact, the force will shift the mechanical frequencies, and this shift will be detected through quad-photodetector and the extracted signal will be translated into an image.