Navid P. Gandji

Superluminal media formed by photonic crystals for transformation optics-based invisibility cloaks

We have developed an approach to building superluminal medium for transformation optics‐based devices, including invisibility cloaks, from photonic crystals. Analysis of dispersion diagrams of 2D arrays composed from dielectric rods has shown that at frequencies corresponding to the second bands formed due to bandgap opening at increase of rod permittivity, the medium formed by arrays exhibits refractive indices providing for superluminal phase velocities of propagating waves. It is further demonstrated that rod arrays with various lattice constants could be used for realizing a range of superluminal index values prescribed by transformation optics for cylindrical cloaks at arbitrary chosen operating frequency. The performed studies allowed for solving a row of problems with employment rod arrays in the cloak medium: in particular, formulating transformation optics‐based prescriptions for refractive index dispersion in the cloaking shell, defining the dimensions of array fragments capable of responding similar to infinite arrays, finding optimal distribution of linear arrays sets at their coiling to form concentric material layers in the cloaking shell, and employing interaction between neighboring array sets with various lattice constants to assist the realization of prescribed index dispersion. The performance of the superluminal medium formed by rod array sets was demonstrated on an example of a cloaking shell developed for microwave frequency range. In contrast to metamaterial-based cloak media, the developed media requires neither material homogenization, nor obtaining the effective parameters with peculiar values and Lorentzs type resonances in rods. Combination of these advantages and low losses makes photonic crystals perspective materials for invisibility cloaks operating in THz and optical ranges.

Unconventional designs of RF probes for high-field MRI to enhance magnetic field uniformity

We present approaches to enhancing magnetic field uniformity produced by RF probes in high-field Magnetic Resonance Imaging (MRI) scanners by introducing compensating non-uniformities in the probe designs. Unconventional designs of RF probes for operating at 600 MHz in 14 T MRI scanners are demonstrated, including a solenoid coil with non-uniform wrapping and dielectric separators, a system of microstrip patch antennas with substrates engineered by using high permittivity inserts, and a system of specifically shaped patch antennas. The advantages of the proposed probe designs are discussed.

Unconventional RF Probes Comprised of Microstrip Patch Antennas for High-Field MRI Scanners

We have developed a system of microstrip patch antennas for replacing conventional RF coils in high-field Magnetic Resonance Imaging (MRI) scanners. The substrates of antennas were engineered by using high permittivity inserts to provide required resonance responses of miniaturized antennas at 600 MHz for 14T MRI scanners and to produce highly uniform magnetic field distribution in the sampling volume. A tuning circuit for the antenna system has been worked out.

Employing self-collimation phenomena in photonic crystals for the invisibility cloak development

We demonstrate that self-collimation effects in the invisibility cloak medium formed from 2D dielectric Photonic Crystal (PhC) can support wave propagation along curvilinear paths around an object to be hidden, with superluminal phase velocities controlled by deformation of PhC lattice cells. These effects can be used for the development of uni-directional cloaks with wave phase preservation, which can be scaled from microwave to optical frequencies. The obtained results show that employment of self-collimation effects can make PhCs advantageous with respect to metamaterials at the formation of the media requested by coordinate transformation techniques for the development of new electromagnetic devices.

Frequency selective transmission through waveguides with ENZ sections

Multi-sectional waveguide structures providing frequency selective transmission controlled by transverse insertion of metal rods in the ENZ sections, are proposed and investigated by using full-wave simulations. It is demonstrated, that when the diameter of the inserted rod is adequately chosen, the position of the full transmission peak in the waveguide spectrum can be furnished at any frequency within the range of 0.75 GHz above the cut-off frequency of the ENZ section.