There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

In order to enable optical systems to operate with a high degree of compactness and reliability it is necessary to combine large number of optical functions in small monolithic structures. A development, somewhat reminiscent of that that took place in Integrated Electronics, is now beginning to take place in optics. The initial challenge in this emerging field, known appropriately as "Integrated Optics", is to demonstrate the possibility of performing basic optical functions such as light generation, coupling, modulation, and guiding in Integrated Optical configurations. The talk will review the main theoretical and experimental developments to date in Integrated Optics. Specific topics to be discussed include: Material considerations, guiding mechanisms, modulation, coupling and mode losses. The fabrication and applications of periodic thin film structures will be discussed.

Integrated optics

A novel design of a transmission window within the absorption band of a circuit analog absorber, named as frequency-selective rasorber (FSR), is presented. Based on an equivalent circuit model, the conditions are formulated to produce a passband with small insertion loss and to reduce the reflection at frequencies below and above the passband in the meanwhile. Simple design guidelines of our proposed FSR are then developed. With loaded lumped elements, the arrays of square-loop and cross-dipole are combined to realize its implementation. It is shown through measurements that an insertion loss of 0.68 dB can be obtained at 4.42 GHz and the fractional bandwidth for at least 10 dB reflection reduction within the lower and upper frequency bands is 92.3% under the normal incidence. A good agreement between simulated and measured results validates our design.

Frequency-Selective Rasorber Based on Square-Loop and Cross-Dipole Arrays

Geometric metasurfaces primarily follow the physical mechanism of Pancharatnam-Berry (PB) phases, empowering wavefront control of cross-polarized reflective/transmissive light components. However, inherently accompanying the cross-polarized components, the copolarized output components have not been attempted in parallel in existing works. Here, a general method is proposed to construct phase-modulated metasurfaces for implementing functionalities separately in co- and cross-polarized output fields under circularly polarized (CP) incidence, which is impossible to achieve with solely a geometric phase. By introducing a propagation phase as an additional degree of freedom, the electromagnetic (EM) energy carried by co- and cross-polarized transmitted fields can be fully phase-modulated with independent wavefronts. Under one CP incidence, a metasurface for separate functionalities with controllable energy repartition is verified by simulations and proof-of-principle microwave experiments. A variety of applications can be readily expected in spin-selective optics, spin-Hall metasurfaces, and multitasked metasurfaces operating in both reflective and transmissive modes.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/advs.202001437

A Fully Phase-Modulated Metasurface as An Energy-Controllable Circular Polarization Router.

This paper introduces the concept, theory, and design of 3-D frequency selective rasorbers (FSRs), which have a transmission window transparent to the incident electromagnetic wave with two absorption bands located at both sides of the window. The proposed rasorber consists of a 2-D periodic array of parallel waveguides. The transmission characteristics with high selectivity are produced by lossless resonators implemented using a parallel waveguide with a metallic post in the center. On the other hand, the absorption bands are obtained by lossy resonators constructed by loading of lumped resistors at the entry port of short-circuited waveguides. Physical mechanism of the proposed FSRs is explained with the aid of an equivalent circuit model, and relevant design equations are formulated. Two prototypes of the designed FSRs are fabricated and measured as a proof of concept. The experimental results show that a bandwidth of 50% for the insertion loss less than 3 dB and two absorption bands with a high absorptance of around 90% can be achieved. Moreover, the simulated results also show that the proposed structure exhibits stable performance against the variation of the incident angle of an incoming plane wave.

3-D Frequency Selective Rasorber: Concept, Analysis, and Design

Two printed wide-slot antennas with E-shaped patches and slots, for broadband applications, are proposed They are fed by a coplanar waveguide (CPW) and a microstrip line with almost the same performances Detailed simulation and experimental investigations are conducted to understand their behaviors and optimize for broadband operation Good agreement between the measurement and simulation has been achieved The impedance bandwidths, determined by 10-dB reflection coefficient, of the proposed slot antennas fed by microstrip line and CPW, from both measurement and simulation, are about 136% (285 to 1512 GHz) and 146% (283 to 182 GHz), respectively This large operating bandwidth is obtained by choosing suitable combinations of feed and slot shapes In order to achieve wider operation bandwidth both of the designed antennas have round corners on the wide slot and patch Meanwhile, the proposed antennas exhibit almost omnidirectional radiation patterns, relatively high gain, and low cross polarization A comprehensive numerical sensitivity analysis has been done to understand the effects of various dimensional parameters and to optimize the performance of the designed antennas Results for reflection coefficient, far-field E and H-plane radiation patterns, and gain of the designed antennas are presented and discussed

Bandwidth Enhancement of Printed E-Shaped Slot Antennas Fed by CPW and Microstrip Line

A procedure for the optimal design of thin wideband radar absorbers is presented. The resulting absorbers are implemented by printing a frequency selective surface on a lossy perforated substrate. A binary hill climbing optimization scheme with random restart is used to find optimal solutions. The method of moments in conjunction with the transmission line method is used to calculate the fitness of structures designed by the optimizer. The optimization procedure is done in two steps in order to find a structure with best performance in terms of both operation bandwidth and angular stability. Three types of lossy substrate are considered: 1) unmodified radar absorbing substrate, 2) substrate with one circular hole per unit cell, and 3) substrate with several circular holes per unit cell. It is shown that by drilling only one hole - with optimized radius - in each unit cell, a %100 improvement in terms of operation bandwidth may be obtained. For the case with several holes per unit cell the observed performance was not as good, which might be due to a too rough optimization.

Thin Wideband Radar Absorbers

Transformation of space coordinates can be used as a convenient tool for producing a controlled electromagnetic field pattern. In this paper, using a conformal transformation, the design procedure of a lens antenna with high gain and low sidelobes is presented which can be realized by isotropic graded refractive index (GRIN) materials. Applying proper simplifying techniques, the designed lens can be made by non-resonant metamaterials or non-magnetic dielectrics having wide frequency band and low loss. A graded photonic crystal (GPC) operating in metamaterial regime is used for this purpose. By placing the lens in a horn antenna, a highly directive beam is achieved. It is also shown that by lateral displacement of the lens medium, the beam direction can be controlled to some extent. A multibeam antenna in which each beam can be controlled independently is also presented. Simulation results are used to justify the design approach.

Design of High-Gain Lens Antenna by Gradient-Index Metamaterials Using Transformation Optics

A cosecant squared pattern reflector antenna fed by a pyramidal double-ridged horn for 2-18 GHz is presented. Invasive weed optimization (IWO) algorithm is used for synthesizing the point source doubly curved reflector antenna. IWO method makes antenna synthesis flexible to achieve extra desired features such as low side lobe level (SLL) and low ripples in the shaped beam region. The simulation results via FEKO software package further prove the validity and versatility of this technique for solving reflector synthesis problems. In addition, experimental investigations are conducted to understand the complete reflector antenna system behaviors. Good agreement between the simulation and measurement has been achieved. The ripple in the cosecant squared region and the SLL is less than 1.5 dB and 25 dB, respectively. Based on the obtained results, the proposed reflector antenna can be used in broadband surveillance-search radar systems.

Design of a Broadband Cosecant Squared Pattern Reflector Antenna Using IWO Algorithm

Dielectric resonator antennas with fractal cross sectional areas have been investigated. Two main configurations of these novel types of dielectric resonator antennas have been examined. Analyses of these proposed dielectric resonator antennas are performed numerically using the finite element method and verified by the finite integration technique. Agreement between the methods is excellent. The effects of antenna parameters, such as fractal iteration level and tapering rate of dielectric resonator, are investigated. © 2007 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2007.

Habibollah Abiri

Papers

Bandwidth Enhancement of Printed E-Shaped Slot Antennas Fed by CPW and Microstrip Line

Thin Wideband Radar Absorbers

Design of High-Gain Lens Antenna by Gradient-Index Metamaterials Using Transformation Optics

Design of a Broadband Cosecant Squared Pattern Reflector Antenna Using IWO Algorithm

Parametric study of novel types of dielectric resonator antennas based on fractal geometry