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Near-field optical microscope



Background introduction

Traditional optical microscope (i.e. far-field optical microscope) is the oldest member of the microscope family. It used to be the only way to observe tiny structures. Traditional optical microscopes are composed of optical lenses, which use refractive index changes and lens curvature changes to magnify the observed object to obtain detailed information. However, the diffraction limit of light limits the further improvement of optical microscope resolution. According to the Rayleigh resolution limit, the magnification of an optical microscope cannot be increased arbitrarily. The Rayleigh criterion is based on the assumption of propagating waves. If it can detect evanescent waves that carry detailed information about objects, the Rayleigh criterion can be circumvented and the limit of diffraction limit can be broken.

Near-field optics is not only an effective optical means to break the diffraction limit, it is a new interdisciplinary subject in the field of optics that has emerged with the advancement of science and technology to small-size and low-dimensional space. Its research object It is an optical phenomenon within one wavelength (several nanometers) from the surface of the object. Near-field optical microscopy is a new type of ultra-high-resolution microscopy imaging technology, which is a product of the combination of probe technology and optical microscopy technology, and is an important part of near-field optics.

Near-field optical imaging is different from classical optics. It involves optical theories and phenomena in a wavelength range. The so-called "near-field" area contains: (l) radiated field: field components that can be transmitted outward; (2) non-radiation field: field components that are confined to the sample surface and decay rapidly at a distance. Since the near-field wave embodies the transient changes in the discontinuity of spatial optical properties when light is propagating, the sub-wavelength structure and optical information of the sample can be detected by detecting the evanescent wave of the sample. In recent years, near-field optical microscopy has achieved breakthrough developments in theory and practice.

Because photons have some special properties, such as no mass, electrical neutrality, relatively long wavelength (compared with electrons), easy to change polarization characteristics, can propagate in air and many dielectric materials, etc. , Near-field optics plays a role that other scanning tunneling microscopes and atomic force microscopes cannot replace in nano-scale observations, which has triggered near-field optical microscopes in nano-scale optical imaging, nano-scale optical micro-processing and lithography, and ultra-high-density information storage. , And a series of studies such as in-situ and dynamic observation of biological samples. In this field, another new development is the combination of near-field optical technology with near-field spectroscopy and time resolution. People can not only distinguish a single molecule, but also obtain the fluorescence spectrum emitted by a single molecule and the mesoscopic system information combined with time resolution (10-15s). At the same time, new theoretical issues of resolution, contrast, polarization and light propagation characteristics under near-field conditions are also raised.

The principle of near-field optical microscope

Traditional optical microscopes are composed of optical lenses, which can magnify objects up to several thousand times to observe details. Due to the diffraction effect of light waves, the magnification can be increased infinitely It is impossible, because it will encounter the obstacle of the diffraction limit of light waves, and the resolution of traditional optical microscopes cannot exceed half of the wavelength of light. For example, using green light with a wavelength of λ=400nm as the light source, only two objects with a distance of 200nm can be resolved. In practical applications, λ>400nm, the resolution is lower. This is because the general optical observation is at a position far away from the object (>>λ).

Based on the principle of non-radiation field detection and imaging, near-field optical microscopes can break through the diffraction limit of ordinary optical microscopes, and can perform nano-scale optical imaging and nano-scale spectroscopy studies at ultra-high optical resolution. .

The near-field optical microscope consists of probes, signal transmission devices, scanning control, signal processing and signal feedback systems. The principle of near-field generation and detection: incident light illuminates objects with many tiny structures on the surface. Under the action of the incident light field, the reflected waves generated by these fine structures include evanescent waves confined to the surface of the object and travel far away. The spreading wave at the place. Evanescent waves come from fine structures in objects (objects smaller than the wavelength). The propagating wave comes from the rough structure of the object (the object larger than the wavelength), which does not contain any information about the fine structure of the object. If a very small scattering center is used as a nanodetector (such as a probe) and placed close enough to the surface of the object, the evanescent wave will be excited to make it glow again. The light generated by this excitation also contains undetectable evanescent waves and propagating waves that can be propagated to a distance for detection. This process completes the near-field detection. The conversion between the evanescent field and the propagation field is linear, and the propagation field accurately reflects the change of the evanescent field. If a scattering center is used to scan the surface of the object, a two-dimensional image can be obtained. According to the principle of reciprocity, the roles of the illumination light source and the nano detector are interchanged, and the nano light source (evanescent field) is used to illuminate the sample. Due to the scattering effect of the object's fine structure on the illumination field, the evanescent wave is converted into a remote The results of the detected propagating waves are exactly the same.

Near-field optical microscopy is digital imaging by scanning and recording point by point on the sample surface by a probe. Figure 1 is a diagram of the imaging principle of a near-field optical microscope. The x-y-z coarse approximation method in the figure can adjust the distance between the probe and the sample with an accuracy of tens of nanometers; while the x-y scanning and z control can control the probe scanning and the feedback follow-up in the z direction with a precision of 1 nm. The incident laser in the picture is introduced into the probe through an optical fiber, and the polarization state of the incident light can be changed according to requirements. When the incident laser irradiates the sample, the detector can separately collect the transmission signal and the reflection signal modulated by the sample, which are amplified by the photomultiplier tube, and then directly collected by the computer after the analog-to-digital conversion or enter the spectrometer through the spectroscopic system to obtain the spectrum information. System control, data acquisition, image display and data processing are all completed by the computer. It can be seen from the above imaging process that the near-field optical microscope can collect 3 types of information at the same time, namely the surface topography of the sample, the near-field optical signal and the spectral signal.

Components of a near-field optical microscope

Objective lens

The core component of a near-field optical microscope is a small hole device with an aperture smaller than the wavelength, such as a fiber probe, Its geometric aperture is similar to the numerical aperture of a microscope objective. When the distance between the fiber probe and the illuminated sample is certain, the size of the optical probe's transparent aperture plays a key role in the resolution of the near-field optical microscope. For near-field optical microscopes, in order to obtain higher resolution, on the one hand, the light beam passing through the optical probe must be restricted as much as possible in the lateral direction; on the other hand, the light flux passing through the restricted area must be as large as possible. To improve the signal-to-noise ratio.

Measurement and control of the distance between the probe and the sample

Near-field optical microscope uses nanometer-level highly localized near-field light to obtain the topography of the object, which requires the use of a grid Point-by-point scanning technology to obtain the topography of the sample. During the scanning process, a very critical issue is that the distance between the probe and the sample must be controlled within the near-field (a few nanometers to tens of nanometers) scale and maintain a certain constant value. Therefore, the precise measurement and control of the distance between the probe and the sample is a very important part of the near-field optical microscope. So far, several measurement and control technologies for controlling the distance between the probe and the sample have been developed, such as: shear force intensity measurement and control technology, contact measurement and control technology, tunneling current intensity measurement and control technology, and near-field light intensity measurement and control technology.

Light path

The light path is another main structural component of the near-field optical microscope, which mainly includes the light source and the illumination light path as well as the collection light path and the light detector.

Application of near-field optical microscope

Because near-field optical microscope can overcome the shortcomings of traditional optical microscope's low resolution and scanning electron microscope and scanning tunneling microscope's damage to biological samples, so It has been widely used, especially in the fields of biomedicine, nanomaterials and microelectronics, and has become an optical means to explore the mysteries of biological macromolecule activities, bringing powerful experimental weapons to biologists. Using near-field optical microscopy, we have started work in many fields involved in the research of biology, not only the observation and research of static image, such as cell mitosis, chromosome resolution and local fluorescence, in situ DNA, RNA Sequencing, gene recognition, etc., as well as research using the dynamic process of observing the appearance of the image over time.

Biological research applications

Because of the characteristics of photons, near-field optical microscopy has many advantages in biological research:

(1) Beyond the optical diffraction limit The resolution can even reach the sub-nanometer level;

(2) Optical microscopy technology is non-invasive and can be observed and studied in the natural environment of organisms;

( 3) Ability to observe absorption, reflection, fluorescence, polarization contrast, and see through the internal optical properties of biological samples;

(4) Spectroscopy analysis, with high resolution for chemical states;

( 5) Interaction between local (nano-level) light and the sample;

(6) Single-molecule level observation sensitivity, 1photon/sec;

(7) Nanometer spatial resolution, High time resolution (femtosecond);

(8) It can work at room temperature.

Application of microelectronic technology

The core of information technology is the high-density storage of information. Because the near-field optical microscope has low requirements for environmental conditions and the existing mature optical disk technology foundation, it has become a strong competitor of various near-field high-density information storage technologies. Improving information storage density is a major issue of great concern to scientific research and industry. The current optical and magneto-optical reading and writing methods use far-field technology. Due to the limitation of the diffraction limit, the size of the reading and writing spot is controlled at about 1mm, the storage density is about 55Mbit/cm2, and a shorter laser wavelength is used. The storage density is not improved much. The development of near-field optics provides a new principle. Since scanning near-field optical microscopes can break through the diffraction limit, the storage density is greatly improved.

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