Tunable terahertz and optical materials

Electromagnetic metamaterials are artificially structured materials consisting of subwavelength structural units with novel electromagnetic properties not found in natural materials. According to the spectrum range of electromagnetic metamaterials, they can be simply divided into microwave metamaterials, THz metamaterials and optical metamaterials.

Electromagnetic metamaterials were first realized in the microwave frequency band and then quickly evolved to cover almost the entire electromagnetic spectrum from terahertz to infrared and visible light. The study of metamaterials has gone far beyond the initial category of "negatively refractive materials". On the basis of metamaterials, new concepts such as super-surfaces and super-devices have been proposed.

Metamaterials in light polarization, phase, and amplitude control have many traditional optical materials and devices that can not be compared with the unique advantages, in the development of new guangxi belly-hungry components that have great potential. The use of metamaterials for ultra-long control of light can realize some new components, such as broadband circular polarizers, new perfect absorbers, and planar lenses with the ability to image without phase difference. Optical elements based on metamaterials, especially planar metamaterials, can greatly reduce the size and weight of traditional optical elements, thus more conducive to miniaturization and integration. At the same time, by changing the dimensions of the metamaterial structural unit, it can be made to work in different wavelength bands. This is of particular value in optical frequency bands (e.g., mid- and far-infrared or terahertz bands), which are lacking in conventional optical elements.

Tunable Terahertz & Optical Metamaterials

-Graphene Tunable Metamaterials

One of the most important properties of graphene is that it has tunable electronic and optical properties. It can be regarded as a band-free semiconductor, and the carrier concentration and Fermi energy levels of graphene can be effectively altered by means of chemical or electrostatic doping, for example. Graphene has equally important applications in tunable terahertz and optical metamaterials research.

Another compelling optical property of graphene is that it can support surface plasmon excitations in the mid- and far-infrared and terahertz bands. Surface plasmon excitations in graphene can greatly enhance the interaction between light and graphene at wavelengths much smaller than the wavelength of electromagnetic waves of the same frequency in free space. Therefore, like metals such as gold and silver, graphene itself can be processed into infrared and terahertz band metamaterials by fabricating some subwavelength structures.

-Development trends and application scenarios

Despite the many achievements in the research of tunable terahertz and optical metamaterials, many aspects of their optical and modulatable properties still need to be further improved, such as the modulation amplitude and modulation rate, the reproducibility of the modulation, the power consumption required for the modulation, and the stability and durability of the metamaterial structure. In the future development, the following aspects will be worthy of attention:

1.  First, the functions of tunable metamaterials will be further expanded, and the modulation freedom and flexibility will be further improved. For example, in radar and communication technology, phased array antenna is an extremely important device. If the individual units of metamaterials can be controlled as in the case of phased array antennas, the functionality of metamaterial devices will undoubtedly be greatly improved. The current tunable optical metamaterials are basically modulated on the entire array of structural units together. The realization of programmable optical metamaterials with unit modulation control will be a challenging goal, and similar terahertz and optical devices have broad application prospects in the fields of beam control, wavefront modulation, and so on.

2. Second, research on tunable metamaterials will move further from proof-of-principle to functional device development and application. From the application point of view, planar structure metamaterials compared to three-dimensional structure metamaterials are easy to process, transmission loss is relatively small, easy to integrate and in the amplitude of the light wave, polarization and phase control can achieve a lot of functions, so based on the planar structure or multi-layer planar structure of the tunable optical metamaterials is still the focus of the research. Tunable optical metasurfaces will be a very meaningful research direction. Hybrid structure tunable optical metamaterials have an important position in tunable metamaterials. Semiconductor materials, liquid crystals and sulfur-based glasses have been widely used in industry and life.

Tunable terahertz and optical metamaterials research has a strong application background. From the earliest active terahertz modulators to the latest tunable terahertz perfect absorbers and megahertz bandwidth near-infrared electro-optical modulators, tunable metamaterials show great potential in the development of photonic functional devices such as electro-optical modulators, tunable filters, multi-color-spectrum infrared focal-plane detectors, tunable planar lenses, and nonlinear and adaptive optics elements. The new planar optical elements based on tunable optical metamaterials can not only realize some functions that are difficult to be realized by traditional optical elements, but also have the advantages of ultra-thin thickness, light weight and small volume, which are conducive to the miniaturization and integration of photonic devices.

Tunable optical metamaterials may be the first to be used in the terahertz and infrared bands. In the mid- and far-infrared bands, the choice of conventional optical materials is limited.

The two main factors currently limiting the application of optical metamaterials are processing and loss. In the mid- and far-infrared and terahertz wavelength bands, the loss of metal materials is relatively small, and semiconductor materials or graphene can also be used to replace metal materials, etc. Moreover, metamaterials in this wavelength band can be processed using mature photolithography, which offers the possibility of metamaterial device applications. Mid- and far-infrared and terahertz devices based on tunable optical metamaterials can play an important role in communication, medical detection, national defense, homeland security, aerospace and other fields.

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