Artificial Dielectric Flat Lenses

Abstract

Flat lens antennas are convenient solutions to realize highly directive antennas for millimeter wave and terahertz frequencies. Unlike the traditional three-dimensional bulky lens antennas, flat lenses are compact, low profile, and planar structures that can be manufactured with standard multi-layer technology, e.g. printed circuit board (PCB) or low temperature cofired ceramic (LTCC).

A common tradeoff in the design of flat lenses is between bandwidth and thickness. Electrically thin lenses are characterized by narrow frequency bandwidth, resulting from the phase wrapping adopted in the design. On the other hand, a wide bandwidth can be achieved by avoiding phase wrapping and using true-time-delay phase delay, but this is achieved at the cost of increased electrical thickness.

In this thesis, artificial dielectric layers (ADLs), consisting of periodic metal patches within a dielectric substrate, are proposed to realize flat lenses with large effective refractive index, which is a key property for reducing the thickness of wideband true-time-delay flat lenses. As such, ADLs are promising solution to achieve a good compromise between bandwidth and thickness.

Different aspects of ADL flat lenses are investigated in this thesis, going from the analysis to the design and experimental validation. For the analysis, a general procedure to find the permittivity profile of a gradient index (GRIN) lens is introduced. The method allows to design GRIN lenses that manipulate the phase front in different ways, by using a Geometrical Optics (GO) approach. Different cases are studied, including collimating lenses with on-axis and off-axis feeds; lenses that transform spherical wavefronts across different media; lenses changing the focal number of a quasi-optical system and Fresnel zone lenses. The design equations are validated by ray-tracing simulations in non-homogeneous media, implemented by numerical solution of the Eikonal equation.

Once the permittivity profile is defined, the continuous variation of refractive index is discretized into unit cells. Each unit cell is then implemented as an ADL stack, using ADL synthesis models developed earlier in the THz sensing group.

To validate the design procedure, an ADL flat lens with an operation band from 30 to 60 GHz is designed and fabricated using an eight-layer print circuit board (PCB) stackup. The measurement results reach good agreements with the simulation results and validate the design. The achieved performance demonstrate wideband operation, with a high taper efficiency (> 90%) and a maximum directivity of 25.5 dB. The lens is thinner than 1 wavelength within the band of operation.

The lens performance is demonstrated with a simple open-end waveguide as feed, which has high spillover losses. Diverse feeding antennas that can achieve higher aperture efficiency are also analysed by means of simulations.

Additionally, other types of GRIN lens, that manipulates the wavefront in distinct ways for different applications are investigate, to highlight the flexibility of the concept.