Mohammad Memarian

Quad-mode and dual-mode dielectric resonator filters

This paper introduces for the first time a quad-mode dielectric resonator filter, using a simple cylinder resonator. A four-pole single cavity filter is designed, simulated, and fabricated based on this quadruple mode resonator. Additionally, a new type of dual-mode dielectric resonator filters is introduced, using the same cylindrical resonator cut in half along its axis. Center frequency control, intra/inter/input-coupling mechanisms, tuning, and spurious improvement methods are discussed, showing versatility of the proposed structures to realize different filtering functions and specifications. Measured results are presented for practical filters, employing the proposed quad-mode and dual-mode resonators. The dielectric resonator filters presented in this paper offer a significant size and mass reduction in comparison with conventional dielectric resonator filters. They promise to be useful for both wireless and satellite applications.

Dirac leaky-wave antennas for continuous beam scanning from photonic crystals

Leaky-Wave Antennas (LWAs) enable directive and scannable radiation patterns, which are highly desirable attributes at terahertz, infrared and optical frequencies. However, a LWA is generally incapable of continuous beam scanning through broadside, due to an open stopband in its dispersion characteristic. This issue is yet to be addressed at frequencies beyond microwaves, mainly as existing microwave solutions (for example, transmission line metamaterials) are unavailable at these higher frequencies. Here we report leaky-wave radiation from the interface of a photonic crystal (PC) with a Dirac-type dispersion and air. The resulting Dirac LWA (DLWA) can radiate at broadside, chiefly owing to the closed Γ-point bandgap of the Dirac PC. Thus, the DLWA can continuously scan a directive beam over a wide range of angles by varying the frequency. These DLWAs can be designed at microwave as well as terahertz to optical frequencies, with feasible dimensions and low losses.

Dual-mode half-cut dielectric resonator filters

This paper shows that it is feasible to realize a quadruple-mode dielectric resonator. A half-cut of this resonator is then shown to support two modes. A new type of dual-mode dielectric resonator filters is introduced, using a cylindrical resonator cut in half along its axis. Center frequency control, intra/inter/input-coupling mechanisms, tuning, and spurious improvement methods are discussed, showing versatility of the proposed structure. Practical filter measurement results are presented which verify the proposed concept. The filter provides improvements in terms of size and mass reduction, in comparison with existing dual-mode dielectric resonator filters, while maintaining better spurious performance, with a small degradation in quality factor. It is a low-cost compact solution, suitable for both wireless and satellite applications.

Evanescent-to-propagating wave conversion in sub-wavelength metal-strip gratings

Transmitting sub-wavelength information to the far-zone is of great interest for various electromagnetics and optics applications, e.g., for overcoming the diffraction limit in imaging or enabling multiple-input/multiple-output operation in closely spaced antennas. The common phenomenon in these applications is the conversion of evanescent to propagating waves. In this paper, we rigorously analyze and study evanescent-to-propagating wave conversion based on sub-wavelength metallic strip gratings. A theory is provided, which fully captures all diffracted fields and clearly highlights this conversion phenomenon. The Greens function of the strip gratings is constructed by introducing the spectral impulse response. This method solves the problem of the aperiodic excitation of the periodic grating and along the way provides insight and clear explanation of the evanescent-to-propagating wave conversion that takes place. All proposed results are validated against full-wave electromagnetic simulations. The theory is used to highlight and explain “extraordinary” transmission through a sub-wavelength metal strip grating when excited by a current source. Finally, an interesting application is presented where a nonradiating arrangement of sources is made to radiate by converting and diffracting its reactive near-field spectrum into the propagating regime using a simple metal strip grating.

Metasurface quantum-cascade laser with electrically switchable polarization

Dynamic control of a laser’s output polarization state is desirable for applications in polarization sensitive imaging, spectroscopy, and ellipsometry. Using external elements to control the polarization state is a common approach. Less common and more challenging is directly switching the polarization state of a laser, which, however, has the potential to provide high switching speeds, compactness, and power efficiency. Here, we demonstrate a new approach to achieve direct and electrically controlled polarization switching of a semiconductor laser. This is enabled by integrating a polarization-sensitive metasurface with a semiconductor gain medium to selectively amplify a cavity mode with the designed polarization state, therefore leading to an output in the designed polarization. Here, the demonstration is for a terahertz quantum-cascade laser, which exhibits electrically controlled switching between two linear polarizations separated by 80°, while maintaining an excellent beam with a narrow divergence of ∼3°×3° and a single-mode operation fixed at ∼3.4  THz, combined with a peak power as high as 93 mW at a temperature of 77 K. The polarization-sensitive metasurface is composed of two interleaved arrays of surface-emitting antennas, all of which are loaded with quantum-cascade gain materials. Each array is designed to resonantly interact with one specific polarization; when electrical bias is selectively applied to the gain material in one array, selective amplification of one polarization occurs. The amplifying metasurface is used along with an output coupler reflector to build a vertical-external-cavity surface-emitting laser whose output polarization state can be switched solely electrically. This work demonstrates the potential of exploiting amplifying polarization-sensitive metasurfaces to create lasers with desirable polarization states—a concept which is applicable beyond the terahertz and can potentially be applied to shorter wavelengths.

DIPOLE RADIATION NEAR ANISOTROPIC LOW- PERMITTIVITY MEDIA

We investigate radiation of a dipole at or below the interface of (an)isotropic Epsilon Near Zero (ENZ) media, akin to the classic problem of a dipole above a dielectric half-space. To this end, the radiation patterns of dipoles at the interface of air and a general anisotropic medium (or immersed inside the medium) are derived using the Lorentz reciprocity method. By using an ENZ half-space, air takes on the role of the denser medium. Thus we obtain shaped radiation patterns in air which were only previously attainable inside the dielectric half-space. We then follow the early work of Collin on anisotropic artiflcial dielectrics which readily enables the implementation of practical anisotropic ENZs by simply stacking sub-wavelength periodic bi-layers of metal and dielectric at optical frequencies. We show that when such a realistic anisotropic ENZ has a low longitudinal permittivity, the desired shaped radiation patterns are achieved in air. In such cases the radiation is also much stronger in air than in the ENZ media, as air is the denser medium. Moreover, we investigate the subtle difierences of the dipolar patterns when the anisotropic ENZ dispersion is either elliptic or hyperbolic.

Wide-band/angle Blazed Surfaces using Multiple Coupled Blazing Resonances.

Blazed gratings can reflect an oblique incident wave back in the path of incidence, unlike mirrors and metal plates that only reflect specular waves. Perfect blazing (and zero specular scattering) is a type of Wood’s anomaly that has been observed when a resonance condition occurs in the unit-cell of the blazed grating. Such elusive anomalies have been studied thus far as individual perfect blazing points. In this work, we present reflective blazed surfaces that, by design, have multiple coupled blazing resonances per cell. This enables an unprecedented way of tailoring the blazing operation, for widening and/or controlling of blazing bandwidth and incident angle range of operation. The surface can thus achieve blazing at multiple wavelengths, each corresponding to different incident wavenumbers. The multiple blazing resonances are combined similar to the case of coupled resonator filters, forming a blazing passband between the incident wave and the first grating order. Blazed gratings with single and multi-pole blazing passbands are fabricated and measured showing increase in the bandwidth of blazing/specular-reflection-rejection, demonstrated here at X-band for convenience. If translated to appropriate frequencies, such technique can impact various applications such as Littrow cavities and lasers, spectroscopy, radar, and frequency scanned antenna reflectors.

Blazed metasurface grating: The planar equivalent of a sawtooth grating

The blazing behavior of non-planar sawtooth gratings are mimicked with a planar metasurface. We show that with an appropriate design, an equivalent planar “blazed metasurface” can reflect most of the incident power back in the path of incidence, and with reduced power reflected in the specular direction. We then utilize such metasurface to create high rejection stopband in a waveguide, by lining the sidewall of the guide with this metasurface. The results are justified by theory, as well as simulations and measurements. The blazed metasurface may find various applications in microwaves up to optical frequencies, e.g. for stopband filters, Fabry-Perot resonator mirrors, or replacing corner reflectors.

Modal analysis and closure of the bandgap in 2D transmission-line grids

Two-dimensional transmission line (TL) grids have been shown to demonstrate interesting behaviors, such as effective negative refractive index and growing of evanescent waves. The analytical treatment of periodic TL-grid structures in the literature predicts a closed bandgap dispersion relation and 2 eigenmodes for a symmetric unperturbed TL-grid unit cell. In this paper, it is shown that unloaded 2D TL-grids inherently exhibit an open bandgap in the dispersion relation and the TL-grid has to be perturbed for successful closure of the bandgap. Furthermore, it is shown that there is a flat (deaf) band in the dispersion relation which is regarded as an additional 3rd eigenmode. Finally, based on this analysis, a 2D single-point fed TL-grid Dirac leaky-wave antenna (DLWA) design is fabricated. Experimental results show true broadside radiation, which is a proper indication of the successful closure of the bandgap in the dispersion relation.

All-dielectric steerable leaky-wave THz antenna

Epsilon Near Zero (ENZ) and Zero Index Metamaterials (ZIM) offer remarkable behaviors such as tailoring the radiation phase pattern, tunneling of waves through channels and bends, and directive emission of sources (Enoch et al., Phys. Rev. Lett., 2002), demonstrated both in theory and experiments. The directive emission effect is due to a unique and simple refracting feature that ZIMs offer. An oblique incident wave going from the ZIM to air refracts with a smaller angle to the normal at the interface. This is due to the transverse wave-number matching and Snells law. With air having the higher refractive index than the ZIM, the angle of refraction in air is smaller than in the ZIM. If a radiator is embedded inside the ZIM, the waves exiting the ZIM are collimated, causing a highly directive beam.