Super resolution mapping of the near optical field and the gradient optical force

Abstract

We have developed a NSOM technique that can map both the near optical field and the optical force using an atomic force microscope. This technique could be very useful for characterizing MEMs/NEMs devices, plasmonic nanoantennas, nano-photonic devices and biologically active substrates. Unlike conventional NSOM techniques that rely on an aperture fabricated on the end of an AFM tip to collect the optical signal this apertureless technique uses a lockin amplifier locked to the AFM tip vibrational frequency, to correlate the amplitude modulation of the back reflected optical signal to the strength of the optical field. And since we are not limited by the fabrication of an aperture the spatial resolution of the map is limited only by the size of a sharp AFM tip which for metallic coated tips can have a radius of curvature of 10 to 20 nm. For optical force mapping the incident laser is modulated and the lock-in amplifier is used to correlate the amplitude modulation of the vibrating AFM tip to strength of the optical gradient force. And in this way one can get a very accurate mapping of both the optical force and the optical field for any substrate of interest as long as it can be back illuminated. Lastly with an electrically monolithic substrate it is possible to correlate the amplitude modulation of the tunneling current to the optical field and obtain a spatial mapping that has a resolution of an STM, about 1 nm or maybe less.