Propagation in a waveguide requires proper termination of signals to prevent reflections from interfering with the desired circuit functionality. Conventional termination designs for a two-dimensional planar waveguide topology provide maximum signal suppression in the direction perpendicular (so-called normal incidence in optics) to the structure, and can exhibit poor performance when the signal is incident at a modest angle of incident. The purpose of the innovation is a calculable metamaterial design for an absorber or termination that has broadband frequency response and low signal reflection over large incident angles.
An engineered metamaterial electromagnetic structure is described that absorbs radiation over a significantly wider spectral bandwidth and angular range than the present state-of-the-art. The innovation was conceived as an element of a micromachined planar circuit to control undesired reflections over a wide range of angles in a parallel-plate waveguide cavity. Structures with the suggested topology can be designed to provide the minimum return loss at any specific angle while producing an acceptable return loss at other incident angles. The one-dimensional unit cell described has broadband response (demonstrated up to 170% of fractional bandwidth while employing readily available manufacturing and material processes), and is convenient for use as a transmission line termination in planar integrated circuits such as in microwave and sub-millimeter wavebands. By replicating copies of this unit cell, high-performance, two-dimensional absorptive termination arrays can be realized. Moreover, by layering or stacking copies of these two-dimensional arrays (or their electromagnetic complement), similar absorption properties can be realized in three dimensions, thus enabling an improvement in reflection control relative to currently used free-space absorbers.
For a planar waveguide, the invention consists of two parallel conductive plates between a dielectric media. One side of the conductor has resistive elements patterned in an array configuration. The unit cell in the large array is made of circular resistors with their diameter gradually tapered from small to large toward the end. The unit cell array is placed in an infinite array at an inclined angle to provide maximum power absorption from that direction. For design simplicity and broadband incident angle coverage, the Cartesian or hexagonal tiling pattern can be used to maximally absorb plane wave at an incident angle of 45 and 60 degrees, respectively. Plane wave radiation is also absorbed efficiently at other angles, dependent on the length and width of the array. The high-frequency response is limited by the width of the unit cell (i.e., when the wavelength of light is shorter than twice the unit cell width, radiation will be scattered from the metamaterial structure), and the low-frequency response is set by the length of the array of resistors (i.e., as the wavelength of the radiation approaches the length of the array, the structure will not appear to be adiabatic, and reflections from the finite extent of the absorber structure can occur). The surface impedance of the thin film is chosen such that the absorption encountered in twice the length of the array is greater than the targeted reflection coefficient. Variations on this rule of thumb consistent with the desired performance and available device area can also be employed.
The invention can effectively absorb more than 99% of plane wave signal in the operating band. The incident angle can be varied dependent on each design. For general applications where design angle is at 45 degrees, the infinite array of the invention can suppress plane waves at an angle ranging from normal (0 degrees) to near parallel (180 degrees) incidence.
The novel feature of the proposed metamaterial is the realization of an absorptive structure with improved performance. This invention allows radiation to be absorbed over a large range of incident angles while using a similar amount of area when compared to the absorber with uniform tapered structure, which is only effective at near normal incidence. The invention can be designed to have maximum power absorption at any specified incident angle. It is also simple to fabricate, requiring an additional resistor film layer overlay on the existing conductor of a planar circuit. The design rules are simple and enable optimization for a variety of uses.
The invention can be used in a wide range of applications. The metamaterial absorber could find use as a microwave circuit termination, and for glint and reflection reduction for instrumentation and anechoic chamber settings.