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2024
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Will Transformation Optics Be the Next Optical Revolution?
Metamaterials have changed the rules of optics
Transforming optics (Transformation optics) and metamaterials (metamaterial, also known as artificial structural materials) can create sci-fi cloaks of invisibility in the laboratory, promising to achieve many previously unseen optical feats-the big challenge now is to turn new concepts in the ivory tower into reality.
Metamaterials have changed the rules of optics. Nanostructured materials can control light in the sub-wavelength range, with effects ranging from negative refraction to invisibility cloaks that have long been considered unfeasible. Over the past 12 years, previously overreacting metamaterials have developed into one of the more exciting fields of photonics.
However, this bright future for metamaterials and transformation optics stems from the fact that they have slowly expanded into practical applications. Invisibility cloaks can be demonstrated in the laboratory, but usually only small objects hidden under monochromatic light can be seen from a specific angle. The main effect depends on resonance, so they cannot work under broadband illumination. Nanostructures that are much smaller than the wavelength of light are difficult to manufacture with precision, and even more difficult to manufacture in batches. It is difficult to find structural materials that can achieve ideal interaction with light waves. The challenge is to overcome these limitations and develop practical applications.
Development of Metamaterials
Metamaterials were first demonstrated at microwave frequencies, developed from earlier studies of man-made media. The basic idea is to assemble many arrays of sub-wavelength elements (including conductors and dielectrics) into a bulk structure, otherwise the metamaterial properties, especially the refractive index, cannot be obtained.
In traditional optics, the refractive index n is usually defined as the ratio of the speed of light traveling in a vacuum to the speed in the material. However, the potential physical significance depends on two more basic data-the dielectric constant ε and the magnetic permeability μ,n is actually equal
N=±em
In a vacuum, both ε and μ are defined as 1, so n = 1 in a vacuum. In the medium (such as glass materials), these two data are positive; but in the visible light band conductor has a negative dielectric constant, positive permeability, so their complex refractive index has a large negative component, so the metal has a large attenuation.
The refractive index of a natural material with uniform composition is uniform because the light waves only "see" the bulk material, not the atoms. Similarly, for light waves, metamaterials have a uniform refractive index because many of the same, evenly spaced elements are much smaller than the wavelength. However, the permeability and permittivity values of such constructed metamaterials can be designed to achieve refractive indices that are not possible with conventional optics, such as n =-1, which can bend light backwards when it enters the metamaterial.
The effectiveness of the permittivity and permeability of metamaterials depends on how the light waves interact with the internal components. A regular array of metal lines can produce an effective dielectric constant that can vary from positive to negative depending on the size, spacing, and arrangement. Also, adjusting the design of the split ring unit can produce a wide range of permeability. The optical effect is similar to that of a radio wave equipped with an array of subwavelength antennas.
transform optics
Metamaterials composed of uniform arrays of sub-wavelength units are essentially custom materials designed for unique properties. However, when the design is expanded to include non-uniform arrays of sub-wavelength units, more options will be created, opening the door to a new field of transformation optics that will go beyond geometric optics and manipulate electromagnetic fields in metamaterials.
"Our intuitive understanding of light is to approximate it as a ray, which is closely related to our vision. To our eyes, light behaves like a stream of particles." Metamaterials pioneer, Imperial's John Pendry, writes in Science journal. The standard ray approximation holds that light passes through an object in a straight line, resulting in a uniform refractive index. However, in the sub-wavelength range, the light image fails, and the structural design can change the propagation of electric and magnetic field lines in any way, which is impossible in traditional block optics. This is the domain of transformed optics.
Traditional optics uses Fermat's principle to describe how changes in refractive index affect the propagation path of light. "The emerging field of change optics allows us to solve the opposite problem, that is, how to realize a specific optical path by designing the properties of various materials." Liu Yongming and Zhang Xiang wrote. So, to hide an object using an invisibility cloak, they can specify the light path they want to guide the light, and then use transformation optics to design the metamaterials needed for the light to follow that path.
Designing a two-dimensional invisibility cloak requires bypassing the optical path of the hidden object; transformation optics is used to design such a metamaterial cloak. Transformation optics can also be used to design and manufacture various types of lenses, beam rotators, beam shifters, and metamaterial structural units used in optical illusion.
Metamaterial Building Blocks
The transformed optics building block is essentially the same as the sub-wavelength units used in other metamaterials. In addition to metals, there are surface plasmons generated by dielectrics and metal-dielectric structure interfaces. The size and shape of these cells are varied by the desired deformation of the metamaterial. In the visible band, individual units must be smaller than 400?700nm, or some more than 1000 atom wide in solids.
Researchers are exploring more directions. Strong magnetic reactions of metals can be used to produce unique interactions, but with large losses in the near-infrared and visible wavelengths. Metals react differently in the optical and infrared bands. The resonance effect of the metal structure can produce strong interaction, but the peak value of the effect is beyond the limit range. Dielectrics can have more broadband response and lower loss.
Among the possibilities being explored, dielectric nanocavity resonances have a high refractive index, a property that provides lower losses than metals and smaller scales than vacuum wavelengths. For example, Lei Shi and colleagues at the Valencia Institute of Technology (Madrid, Spain) studied the near-infrared resonance of silica gel as small as 250 nanometers. They are bullish on metamaterials that use liquids such as air and water.
planar metamaterial surface
The fabrication of optical metamaterial bodies has already encountered challenges, and some researchers are working on planar structures. "Optically artificially structured surfaces have the fascinating possibility of controlling light with surface-localized, planar components. "Vladimir Shalaev and colleagues at Purdue University (BloomingTon, IN) more recently wrote." The metamaterial surface promotes a distinct difference in the aspects of the new physics and phenomena corresponding to the three-dimensional structure. They are also compatible with on-chip nanopotonics.
These metamaterial surfaces deposit thin metal and dielectric patterns on a substrate. The response is inconsistent with the reflection and refraction of the bulk material and depends on the optical dispersion of the subwavelength layer. So far, demonstration experiments have been done on the negative refractive surface antenna array including infrared band and the planar chiral effect of optical band. The metamaterial surface also produces 3D effects without using bulk material, and Purdue's team hopes they can also control the phase, polarization and frequency of light.
Purdue's team writes that phase change materials can be integrated into planar semiconductor metamaterial surfaces for light opening, beam control, pulse shaping or modulation. They also propose the possibility of fabricating hyperbolic metamaterial surfaces, similar to the mass production of hyperbolic metamaterials through the interaction of metal and dielectric elements to produce strong anisotropy and hyperbolic dispersion. The bulk material version can change the behavior of light very significantly, but there is loss and it is difficult to manufacture. However, the Purdue team believes that the quasi-two-dimensional hyperbolic metamaterial has lower surface loss and can be used in standard semiconductor technologies.
Transformation optics has become a powerful paradigm for the potential applications of luminescence metamaterials. "The key benefit of this approach for electromagnetism is the physical connotation," Pendry writes, predicting that "transformation optics will be the design tool of choice in electromagnetic theory."
The challenges may be as great as the potential development prospects. At this moment, the metamaterial revolution is underway in many research directions in laboratories around the world. New breakthroughs are needed in the design and fabrication of nanostructures. There is no consensus on better shapes and raw materials for making metamaterials. The field of application is also uncertain. Like lasers half a century ago, today's metamaterials seem to be a solution, looking for problems, but we know how to make it happen.
Optics,Rules,Nano
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