What is the difference between near-field optical microscopy and far-field microscopy

What is the difference between near-field optical microscopy and far-field microscopy

What is a near-field optical microscope?
Since the 1980s, with the advancement of science and technology towards small-scale and low dimensional space, and the development of scanning probe microscopy technology, a new interdisciplinary field - near-field optics has emerged in the field of optics. Near field optics has revolutionized the traditional optical resolution limit. The emergence of a new type of near-field scanning optical microscope (NSOM), also known as SNOM, has expanded people's field of view from half the wavelength of incident light to several tens of wavelengths, that is, nanoscale. In near-field optical microscopy, the lens in traditional optical instruments is replaced by a small optical probe, whose tip aperture is much smaller than the wavelength of light.

As early as 1928, Synge proposed that ultra-high resolution could be achieved by shining incident light through a small hole with an aperture of 10nm onto a sample with a distance of 10nm, scanning and collecting light signals in the micro region at a step size of 10nm. In this intuitive description, Synge has clearly predicted the main features of modern near-field optical microscopes.

In 1970, Ash and Nicholls applied the concept of near-field to achieve two-dimensional imaging with a resolution of K/60 in the microwave band (K=3cm). In 1983, the BM Zurich Research Center successfully prepared nanoscale optical pores at the tips of metal coated quartz crystals. Using tunneling current as feedback between the probe and sample, obtain an ultra-high optical resolution image of K/20. The driving force for near-field optics to attract more widespread attention comes from AT&T Bell Lab. In 1991, Betzig et al. made high-throughput conical optical holes using optical fibers, deposited metal thin films on the side, and used a unique shear force probe sample spacing control method. This not only increased the photon flux by several orders of magnitude, but also provided a stable and reliable control method, triggering a series of studies on high-resolution optical observation in different fields such as biology, chemistry, magneto-optical domains, high-density information storage devices, and quantum devices using near-field optical microscopy. The so-called near-field optics is relative to far-field optics. Traditional optical theories, such as geometric optics and physical optics, typically only study the distribution of light fields far away from light sources or objects, commonly referred to as far-field optics. Far field optics has a far-field diffraction limit in principle, which limits the minimum resolution size and minimum marking size when using far-field optics principles for microscopy and other optical applications. Near field optics studies the distribution of the light field within a wavelength range from a light source or object. In the field of near-field optics research, the far-field diffraction limit is broken, and the resolution limit is no longer limited in principle and can be infinitely small. Therefore, based on the principles of near-field optics, the optical resolution of microscopic imaging and other optical applications can be improved.

The optical resolution based on near-field optical technology can reach the nanometer level, breaking through the diffraction limit of traditional optics. This will provide powerful operation, measurement methods, and instrument systems for many fields of scientific research, especially the development of nanotechnology. At present, near-field scanning optical microscopes and near-field spectrometers based on hidden field detection have been applied in fields such as physics, biology, chemistry, and materials science, and their application scope is constantly expanding; Other applications based on near-field optics, such as nanolithography and ultra-high density near-field optical storage, nanooptical components, and the capture and manipulation of nanoscale particles, have also attracted the attention of many scientific workers.

Besides being called microscopes, there aren't many similarities.

Firstly, and also the biggest difference, the resolution is different. Far field microscopy, also known as traditional optical microscopy, is limited by the diffraction limit, making it difficult to image clearly in areas smaller than the wavelength of light; And near-field microscopy can achieve clear imaging.

Secondly, the principle is different. Far field microscopy utilizes the reflection and refraction of light, and can be achieved by combining lenses; In the near-field, probes are needed to achieve the acquisition of optical signals through the coupling and conversion of evanescent and transmission fields.

Also, the complexity and cost of the instruments, etc,