Overview of the Development of Near-Field Optical Microscopy

As the semiconductor industry, ultra-precision machining, and biomedical fields delve into the nanoscale, the demand for advanced measurement technologies has heightened. Optical methods, with their non-contact, non-destructive, and rapid properties, have been widely employed, yet the resolution of traditional optical microscopes is bounded by the diffraction limit. Scanning Near-Field Optical Microscopy (SNOM), operating on the principles of non-radiative field detection and imaging, overcomes this limit to facilitate nanoscale optical imaging with ultra-high resolution. This review introduces the imaging principles of near-field optical microscopy and analyzes its advantages and disadvantages across various operational modes. It then delves into two crucial aspects of SNOM's ongoing development: enhancing resolution and signal-to-noise ratio (SNR). In terms of improving resolution, approaches like adding nano-antennas to the tips of aperture-type SNOM probes have been explored to break through the probe aperture's resolution constraint. Techniques such as coating optical fibers with metal films or embedding metal nanowires to convert light into surface plasmon forms for high-resolution measurements are discussed. Additionally, the use of a minimal gap mode between the probe and sample to compress the light spot, thereby exceeding the resolution limitations of scattering-type SNOMs, is examined. The review also addresses strategies to augment the light field intensity at the probe tip and improve SNR. These include fabricating various nanostructures at the probe tip to excite surface plasmons, thereby mitigating background noise caused by direct far-field light, and enhancing the SNR. Innovations in fiber-optic probes, such as the inclusion of ring gratings to improve surface plasmon conversion efficiency, are explored to increase the focused light field's intensity and signal strength. The utilization of special structures like platforms to induce surface plasmon reflective resonance enhancement, thus increasing the focused light field's intensity, is also discussed. The article concludes by summarizing the state of near-field optical microscopy and projecting its future development trajectory.