Design and Construction of Scanning Near-Field Optical Heterodyne Interferometer

Nanophotonics, photonic crystal and surface plasmons are hot topics in optics research in recently years. Optical phenomena in subwavelength structure and near- field region had been applied to many applications in this nano-technology trend. Conventional optical microscopy had encountered bottleneck in such small scale because of the far-field image resolution. Near-field scanning optical microscopy (NSOM) is the instrument which can overcome the diffraction limit by getting high resolution image in near-field region. By the development of related techniques and the knowledge of near-field optics, NSOM becomes the essential measurement tool for nanophotonics.
Photodetection generally transfer light intensity to electric signal, so it cannot analyze the complete characteristics of light field. Phase detection is more sensitive than intensity detection. In this thesis, interferometry is added into NSOM to measure the optical phase. The combination of NSOM system and heterodyne fiber interferometer has shown high precision and high sensitivity ability. Because of the exponential decay of high spatial frequency near-field optics and much smaller aperture of high resolution fiber tip, the optical signal is very weak. Heterodyne interferometer can enhance the contrast of signal and decrease the noise of amplifying low level signal. Besides improving the NSOM signal, heterodyne interferometer can help to get phase information which leads to better understanding of the electromagnetic wave propagation characteristics of light field.
This system contain near-field scanning optical microscopy, fiber interferometer, heterodyne technique, photodetecion, signal processing, noise reduction and environment control method. Experiments of focus spot, diffraction grating and subwavelength surface structure confirm the low-level signal measurement ability of the near-field heterodyne interferometer and further verify the higher sensitivity achieved by using the phase detection to examine the electromagnetic wave propagation behaviors.