Conventional optical microscope and near-field optical microscope
The near-field optical microscope is a revolution over conventional optical microscopes. It does not use optical lenses for imaging, but uses the probe tip to scan above the sample surface to obtain information about the sample surface. Analyzed the physical essence of imaging principles between traditional optical microscopes and near-field optical microscopes, as well as the similarities and differences in the structures of the two microscope systems. Introduced the manufacturing method of fiber optic probes. The focus was on the principles of near-field detection, optical tunneling effects, and the properties of non radiative fields.
Traditional optical microscopes are the oldest members of the microscope family, with a history of several hundred years. It used to be the only means of observing small structures. Traditional optical microscopes mainly use optical lenses to magnify or image objects. Generally speaking, a single lens can magnify an object several tens of times, and using a combination of lenses can almost magnify it up to nearly a thousand times. The diffraction effect of light limits the possibility of further improving the resolution of optical microscopes. This is the Rayleigh resolution limit.
Overview of Traditional Optical Microscopes
Traditional optical microscopes are composed of optical lenses. By utilizing the refractive index of the material and the curvature of the lens, the observed object is magnified to obtain its detailed information. However, the magnification of an optical microscope cannot be arbitrarily increased, as it is limited by the optical diffraction limit.
Where r is the distance between two points, λ is the wavelength of the beam, n is the refractive index of the medium, and θ is the half angular aperture of the lens that collects and focuses the beam onto the detector. It specifies the distance at which two points can be precisely distinguished, which is determined by the parameters of the imaging system. The above inequality indicates that in order to improve resolution (i.e. reduce distance r), there are only three ways: (1) choose shorter wavelengths (if UV electromagnetic radiation, X-rays, or electron beams are chosen, they will be more effective). (2) To improve n, work with materials with high refractive index. This is the principle of immersion microscopy, invented by Amici in the mid-19th century. (3) Increase the aperture angle of the microscope. Electron microscopes use electron beams instead of light beams, greatly improving resolution. It should be noted that the Rayleigh criterion is based on the assumption of propagating waves. If non radiative fields can be detected, it is expected to avoid the Rayleigh criterion and completely break through the limitations of diffraction barriers.
Principle of near-field optical microscope
We can understand the imaging process as follows: when a photon or electron emitted by a light source is projected onto a target object, it is reflected and captured or received by some detector (such as the observer's eyes or camera). Due to the fact that the trajectory and number of reflected particles are related to the properties of the object, particle beams carry information about the characteristics of the object. We call the projection on a target an 'image'. Physically, objects and images are extremely different: objects are generally three-dimensional; And it is usually a two-dimensional projection of physical quantities related to the structure of the object, because the recording medium is two-dimensional. This physical quantity is usually light intensity, as detectors are only sensitive to light intensity. If we replace the object itself with a light field related to the object, we may study the relationship between the object field and the image field, that is, the relationship between the intensity of the object field and its intensity on the image plane. However, the first question that needs to be answered is: What is the relationship between the structure of an object and its light field? In principle, Maxwell's equations provide a way to study this problem: the distribution changes of electron current or charge density inside an object under the action of an external electromagnetic field; The oscillating charges and currents can cause changes in the electromagnetic field, allowing it to propagate from the surface of an object to the external space. According to the principle of continuity, it seems logical to infer that the distribution of charges and currents on the surface of an object can be reconstructed from the spatial field distribution extremely close to the object. Due to the fact that the distribution of charges or currents only changes at extremely small distances (generally less than the wavelength), we also assume that the "space field extremely close to the object" only changes at such small distances.
