Designing Illumination source for Near-field Scanning Optical Microscope

Abstract

Near-field microscopy was invented to overcome the diffraction limit of the conventional optical microscopy. With the advent in nanostructures functional at optical wavelengths, such a device has become inevitable for studies in fundamental optics and photonics. Near-field microscopy captures the information contained in the evanescent waves to create high (subwavelength) resolution images. A major challenge in imaging subwavelength structures is the incoupling of excitation light. Free-space coupling is procedurally simple, but comes with the demerit of considerable unwanted scatter. The motivation for the project is to produce a guide for evanescent waves using a single-mode optical fiber. The evanescence sustained by the fiber can excite a sample in the Near-Field Scanning Optical Microscope(NSOM). This can circumvent substantial stray light in the imaging process. To serve this purpose, we obtained evanescent waves through micrometer thin optical fiber thread loop and coupled it to the samples. Characterisation and comparison of the incoupled light was done through the thin fiber loop and free space method. It was concluded that the incoupling of light using thin fiber loop offered high contrast and high signal to noise ratio optical signals in both near-field and far-field regimes.