1. Field
Apparatuses consistent with exemplary embodiments relate to radio frequency (RF) technology, and more particularly, to filtering devices for terahertz electromagnetic radiation.
2. Description of the Related Art
Terahertz (THz) radiation is an area of research having numerous applications in short-range high speed communication, security, medicine, various industries, space research and the like.
Recently, progress has been demonstrated in producing compact THz sources and detectors, but still, there is a shortage of devices for controlling THz radiation (switchers, modulators, phase shifters, etc,). Basically, this is caused by ‘THz gap’ phenomena. Natural materials are transparent to THz radiation and do not exhibit a strong magnetic or electric response within the range of 1 to 3 THz. To enable proper interaction in the THz range, it is necessary to create artificial materials with specified properties.
Among novel materials, artificial electromagnetic materials—metamaterials (MTMs) play an important role. MTMs enable desired electromagnetic properties in any frequency range. MTMs solve the problem of ‘THz gap’ phenomena and stimulate terahertz research and development. Electromagnetic MTMs have exhibited exotic properties, such as negative or zero dielectric and magnetic permittivity, negative or zero refractive index, and effects of super resolution and cloaking. MTMs are expected to a niche for THz manipulating devices, which is not possible by using natural materials.
The design of various MTMs with specified properties has been widely discussed. Structures such as MTMs based on split-ring resonators, MTMs based on resonant dielectric inclusions, ferroelectrics- and ferromagnetics-based MTMs, MTMs based on phase change materials, layered metal-dielectric structures of MTMs, and MTM structures including liquid crystals are most often used for the realization of MTMs in a microwave field in a range of millimeter waves and in a THz frequency range. The above-listed structures are often used in creating tunable MTMs.
Such devices based on MTMs may perform the role of a zero order resonator for the mobile terminal of a wireless energy transmitter. Such a structure is described, for example, in US patent application No. 2010/0123530. The devices in this patent reference possess properties of an MTM (zero dielectric and magnetic permittivity). An example of a band-pass THz filter is described in U.S. Pat. No. 7,483,088, in which the device is in the form of a frequency selector with frequency-tuning properties, based on a phase delay that is provided by using liquid crystals. A Lyot-filter, described in U.S. Pat. No. 7,483,088, represents a dual refractive filter, widely applicable in visible and infra-red (IR) radiation bands. The filter functions are based on principles of interference of polarized light by a combination of the dual refractive index devices, whose optical axes are inclined with respect to each other. Lyot-filters can be used in active phase-slowing-down dual refractive index devices such as electro-optical crystals and nematic liquid crystals.
However, the above-described structures require a large magnetic field to control a resonant frequency and magnets of a large size. Also, frequency tuning of the filter requires mechanical deflection of the position of the magnets, which is far slower than optical or electronic control. Furthermore, the level of insertion loss of the filter is relatively high (8 dB). In addition, the structure of the device is extremely complex to manufacture.
U.S. Pat. No. 7,826,504 discloses a device consisting of a plurality of metal (gold) electrical resonance elements (MTMs), allocated on a semiconducting substrate. The MTM elements represent resonators in the form of conductive frames with isolated gaps, or have a dual structure in the form of non-conductive circuits with conductive gaps. Both such structures provide control of a transfer constant of a structure. Various shapes of resonance elements are disclosed and compared. Schottky diodes, in which a saturation range or depletion of a charge is formed in a gap area, are built in a set of resonance elements. Modulation of charging density provides 50% modulation of a level of transparency of a structure in a THz range that is 10 times better than that of many existing devices. The disclosed device allows the modulation of radiation in a THz range that can be used in, for example, a quantum cascade laser.
However, the device disclosed in U.S. Pat. No. 7,826,504 has low efficiency with respect to its capacity as a band-pass filter. Selectivity of the disclosed device (used as the band-pass filter) is extremely small. The slope of fronts of the transfer constant is small, and thus, the effective filtering of waves of a specified length is not provided. Additionally, tuning the device according to frequency has negligible effect. Most of these types of resonators used in such structures are anisotropic. Such structures can be effectively used only for a specific type of polarization of waves radiated in a THz range.