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HISTORY OF TECHNOLOGY, TECHNOLOGY, OBJECTS AROUND US
Microscope on surface plasmons. History of invention and production
Directory / The history of technology, technology, objects around us It is usually impossible to observe objects of angstrom thickness using visible light. However, there is a microscope that allows you to do this. The limit of the resolving power of a microscope sets the phenomenon of light diffraction. Diffraction is the bending of waves around obstacles. In a broader sense, any deviation in the propagation of waves from the laws of geometric optics. In the case of a microscope, diffraction determines the minimum distance between two luminous points at which we would see them in a microscope as two, and not one. After a little calculation, it turns out that the minimum distance at which two luminous points can be located will be on the order of half the wavelength of light at which they emit. Thus, for radiation at a wavelength of 630 nm, one can count on the resolution of objects no larger than 315 nm in size. But the phenomenon of diffraction can be looked at from another angle. It is known that light is a stream of photons, quantum particles. It is quantum mechanics that will help us figure out how to get resolution far beyond the diffraction limit. The fact is that the uncertainty relation connects two vectors, the momentum of a particle and its radius vector. As S.I. Valyansky in the "Soros Educational Journal": "Now if we ask ourselves the uncertainty in the definition of momentum, then we have set that uncertainty in the definition of the coordinate of a quantum object, which we can no longer reduce. This gives us some volume in the coordinate space. Let it be some a cube of known volume, but no one forbids us to deform it without changing its volume and thereby violating the general uncertainty relation, and we deform this cube into some thin pancake with a large area, but a small thickness. If the quantum moves in a direction parallel to the plane of this pancake, then due to the large uncertainty of its localization in the plane of the pancake, it is possible to obtain a sufficiently large certainty in the projection of the momentum onto this plane. At the same time, we obtain a sufficiently high localization of the quantum in the direction perpendicular to this plane, but a huge uncertainty in the projection of the momentum onto this direction. Thus, the accuracy of determining the direction of quantum motion in a plane parallel to the plane of the pancake is directly related to the thickness of this pancake. In other words, the thinner we roll our volume into a pancake, the more accurately we will be able to measure the direction of quantum motion in the plane of the pancake. So, it turns out that we can accurately determine one of the projections of the radius vector and one of the projections of the momentum. Only these projections are mutually perpendicular. But how can theory be put into practice? After all, in order to work with large fluxes of quanta localized in a thin layer, it is necessary that they propagate quite well in this thin layer, since we want to make the region of their localization in the direction perpendicular to their movement, of nanometer dimensions.
This is where plasmons come to the rescue. Plasmons are quasi-particles (quanta) resulting from vibrations of conduction electrons relative to ions. For solids, such as metals, these are vibrations of conduction electrons relative to the ionic core of the crystal. They are called quasi-particles in order to distinguish them from real quantum particles - electrons, protons, neutrons, etc. Their difference lies in the fact that if you heat the metal so that it turns into a gas of its original atoms, then there will be no plasmons . They exist only when there is metal as a whole.
In what follows, we will be interested in electromagnetic field quanta associated with oscillations of surface charges in the absence of an exciting field. By analogy with ordinary plasmons, quasiparticles are introduced - surface plasmons (SP). The area of their localization is located near the interface, where surface charges are localized. In 1902, the American optician Robert Wood discovered a change in the intensity of a beam of light diffracted by a grating. This was the first experimental observation of surface plasmons in the optical range. But this was understood only in 1941, when the Italian theoretical physicist Hugo Fano managed to explain Wood's anomalies. And only in the late 1960s, Andreas Otto applied the ideas developed in the works of the German physicist to electromagnetic waves in the optical range. He formulated conditions under which it is possible to excite PP waves on smooth surfaces and indicated a method for their excitation in the optical wavelength range. Thus, the way was opened for the experimental study of surface plasmons in the optical range. In 1971, three years after the publication of Otto's work, Erwin Kretschmann proposed another scheme for excitation of surface plasmons in the optical range. In the Kretschmann geometry, a thin conducting film, on the surface of which surface plasmons are excited, is deposited directly on the prism with which they are excited. In 1988, Wolfgang Knohl and Benno Rothenhäusler proposed the use of surface plasmons for microscopy. They demonstrated a working model of a microscope, in which surface plasmons were excited according to the Kretschmann scheme, to study a specially made grid with known parameters. The results were so impressive that soon this new device began to be used in physics, chemistry, biology and technology. Many researchers have turned to this instrument because of its simple design and high resolution.
The design of the surface plasmon microscope is based on the scheme of excitation of surface plasmons by the Kretschmann method. S.I. Valyansky: "A thin metal film is deposited on the hypotenuse face of a rectangular triangular prism. It is illuminated from the side of the prism with monochromatic linearly polarized light with a divergence an order of magnitude less than the half-width of the resonance curve for this film. Moreover, the polarization vector lies in the plane of incidence of light - the so-called P- polarized light. The light reflected from the film hits the photomatrix, the signal from which is processed by the computer.We remember that the resolution in the plane of the film we have a few microns.Therefore, a telescope is placed between the prism and the photomatrix in the light path, expanding the beam so that the light coming from micron area of the film, captured several elements of the photomatrix. This is one of the simple schemes of a surface plasmon microscope, but far from being the only one. There are a large number of their modifications, convenient for solving specific problems. How does a surface plasmon microscope work? The conditions for resonant excitation of surface plasmons depend not only on the properties of the metal film on whose surface they are excited, but also on the dielectric properties of the medium with which this film borders. Any thin film on a metal surface can be represented as a local change in the dielectric properties of the environment. And this immediately affects the condition of resonant excitation in this place of surface plasmons. In other words, the resonance curve is shifted in this place relative to the curve for a pure film to the region of large angles. This means that if we adjust our microscope to an angle corresponding to the optimal excitation of surface plasmons for a pure metal film, then in those places where the measured object will be, the intensity of the reflected light will be greater, and the greater the thicker this fragment." The microscope does not respond to thickness, but to changes in a parameter that depends on the permittivity and thickness of the object being measured. The main element of the entire device is a thin metal film. The resolution of the entire device depends on the correct choice of its thickness and quality. The excitation of surface plasmons occurs not at a certain angle of incidence, but at a set of angles. If we remember that the set of angles corresponds to the set of photon momenta, then everything becomes clear. The reason for this is the finite lifetime of surface plasmons. The resolution of the microscope will be the better, the longer the PP will be able to propagate. If its propagation speed is fixed, then in a shorter lifetime it will spread over a shorter distance. And it is clear that due to absorption and scattering by the roughness of the metal film, the path length can only decrease. However, not only the film surface is responsible for the lifetime of surface plasmons, but also its bulk properties. The dielectric constant of a metal has both a real and an imaginary part. Due to the presence of the latter, electromagnetic energy is absorbed and, accordingly, the lifetime of surface plasmons decreases. Therefore, to increase the resolution of the microscope, it is necessary to take a metal with a minimum value of the imaginary permittivity. Silver is such a metal. An unfavorable aspect, however, is that the silver film degrades rapidly, oxidizing in about a week. But this difficulty was overcome by developing a method for protecting the surface of the silver film. If the metal film is thin, then the close boundary of the prism will lead to the fact that it will be more profitable for surface plasmons to decay and transform into bulk radiation than to remain surface excitation, that is, its lifetime will be short. For the same reason, the fraction of energy that goes into the generation of surface plasmons will be small. Obviously, if the thickness of the metal film is too large, then practically all the energy of the exciting electromagnetic wave will be absorbed in the volume of the film, not reaching its surface. And the film will work like a mirror. Naturally, there is an optimal thickness, which must be determined. This effect is widely used as a method for studying various transition layers and thin films. This is its main area of application. The microscope was originally designed to observe the organization of monomolecular oriented films at the moment of their formation on the surface of a liquid and during their transfer to solid substrates. Another area of application is biology, direct observation of biological objects. In this case, it is important not so much the high resolution of the microscope in terms of thickness as the high resolution of objects whose internal structure is determined by elements with small changes in the permittivity. Usually, biologists inject contrast fluid to observe their objects, after which they can be observed. A plasma microscope allows you to observe them without these tricks. Using such a microscope, one can, for example, distinguish the boundary between the cytoplasm and the cell wall in an aqueous medium. A microscope - a sensor based on PP resonance - can be used to record the kinetics of chemical and biochemical reactions, to control the size of complexes formed on the surface. Author: Musskiy S.A.
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