Background introduction
Background
The optical microscope is easy to use, the image interpretation is simple and clear, and it does not damage the sample. It can observe the natural state of the substance. It can study its chemical composition and so on, so it has a wide range of applications. But its resolution is limited by the diffraction of the half-wavelength of the light wave, about 0.2-0.5μm. The line width of modern semiconductor integrated circuits has been as small as a few tenths of a micrometer, and higher resolution is also required in fields such as cell biology.
Limits brought by the diffraction resolution limit
The effective magnification of conventional optical microscopes is limited by the diffraction limit of the optical system. According to the Rayleigh criterion space The resolution limit is an overview of the principle and application of the photon scanning tunneling microscope, wavelength, refractive index, and aperture angle of the optical system.
From the above formula, it can be seen that the resolution cannot reach the nanometer level in the visible light band.
The evanescent field existing on the surface of the sample
The field distribution outside the surface of the object can be divided into two regions: one is only a few points away from the surface of the object The wavelength area is called the near-field area; the other part is called the far-field area from outside the near-field area to infinity. Conventional observation tools such as microscopes, telescopes and various optical lenses are all in the far-field range. It is the structure of the object that is larger than the wavelength range, while the near field shows the fine structure of the object much smaller than the wavelength. The existence of the near field is the evanescent field.
The evanescent field is an electromagnetic field that leaves the surface of the object or the light source rapidly attenuating in space. The evanescent field is different from the propagation field, and its disturbance to the light source and the object itself can no longer be ignored. It only exists on the outermost surface of the object and cannot spread far away. It is known that the information of the fine structure in the object cannot be transmitted to the far field, but is restricted to the area close to the surface of the object. Therefore, the evanescent wave is a non-radiative field.
Introduction to PSTM
In 1928, EHSynge proposed a method to overcome the (far-field) diffraction limit: use a small aperture diaphragm with a diameter much smaller than the wavelength as the light source and If the distance from the sample is also less than one wavelength, the imaging resolution will be limited by the size of the small hole. Due to technical difficulties, this near-field optical microscope was not possible until the scanning tunneling microscope STM was invented in 1982. In 1989, R.C. Reddick and others made a photon scanning tunneling microscope PSTM. Its mechanism is similar to STM, and the resolution is better than the half-wavelength value of light. In addition, the mature imaging mechanisms and methods of optical microscopes can be used to study and observe transparent objects under atmospheric conditions, which are difficult to solve by general electron microscopes and scanning tunneling microscopes, which has attracted the attention of the world.
The photon scanning tunneling microscope is a special optical microscope that uses the evanescent field of total internal reflection to break the diffraction limit of traditional optical microscopes and achieve nanometer-level resolution. It can not only observe the surface morphology of the sample, but also measure the refractive index distribution of the sample.
In fact, the electron scanning tunneling microscope (ESTM) and the photon scanning tunneling microscope (PSTM) are very similar in terms of mechanism and structure. In contrast, the photon scanning tunneling microscope can be said to be a new member of the microscopy instrument family, whose theoretical basis comes from near-field optics.
Structure of photon scanning tunneling microscope
PSTM system consists of three parts: PSTM working head, corresponding image processing system and anti-vibration workbench. The working head includes an adjustable slope mirror, optical fiber probe, and a mechanical operating frame; the image acquisition and processing system includes a computer, a color printer and a high-precision monitor. This system can collect a variety of sample images And processing, see Figure 1 for details.
The mechanism of the photon scanning tunneling microscope
The basic idea of the photon scanning tunneling microscope is that the laser is totally reflected at the interface between the sample and the prism, and it will be lost on the surface of the sample. Field, as long as the probe is used to explore the Cain’s missing field, nanometer-level high resolution can be achieved.
The mechanism of PSTM is shown in Figure 2. The lower part of Figure 2 shows that the parallel light in the prism undergoes total internal reflection at the interface, the middle part is the sample, the dotted line represents the resulting evanescent field, the upper fiber probe detects the evanescent field signal, the evanescent field is near-field optics It is an extremely important concept in, it is a non-uniform field, which propagates along the boundary of the medium on the incident surface. The surface amplitude and field strength decay exponentially with the increase of the distance from the interface. The shape of the sample surface is different. The evanescent field of the optical fiber is used to detect the field, and then the detection signal is processed to obtain a three-dimensional image of the sample surface. When the optical probe enters within the wavelength range from the sample, that is, when the operating point is reached, It can be understood as having entered the tunnel. At this time, a tunnel current similar to the electron scanning tunnel microscope can be generated, and then the electronic tracking feedback system is used to keep the probe scanning at the same field intensity, and the signal is sent to the computing image processing system. You can get a realistic three-dimensional image of the surface on the monitor.
Features and scope of application of photon scanning tunneling microscope
Features
Compared with scanning electron microscope, photon scanning tunneling microscope also has the following advantages:
It can be used for the observation of non-conductive samples
To perform surface three-dimensional imaging
More Suitable for applications such as spectroscopy
The image is composed of digital data streams, which is convenient for remote observation, storage and processing. When using an electron microscope for observation, special processing must be performed on the sample , So many human factors will be introduced;
Because there is no need for vacuum conditions, the use and maintenance costs are very low, and its application range is relatively wide, especially in the fields of bioengineering , Is more advantageous, because the vacuum working environment of the electron microscope has a direct destructive effect on the living cells, while the PSTM can see the three-dimensional images of living cells.
Scope of application
PSTM has already developed the following specific applications in the world:
Surface shape observation, surface profile and roughness observation on biological samples, polymers, optical materials and other samples
Research on thin film
Research on optical waveguides
Observe the surface image of diffraction gratings to infer and evaluate the blaze angle and vertex angle of the grating, which will help Yu Guangshan’s standardization, non-destructive inspection of semiconductor wafers, etc.
Applied in many fields such as nanostructure analysis, chemical composition analysis and spectral analysis.
Prospect
In summary, the new technology of PSTM is expected to be used in medicine, bioengineering, optical waveguide and optoelectronic integrated circuit testing, materials science and There are broad application prospects in some high-tech disciplines and industries such as surface science. In addition, this emerging technology can continue to infiltrate other technical fields in many aspects, gradually open up and establish new application fields, and give full play to its inherent potential, which will have a series of important impacts on the development of science and technology and the construction of the national economy.