Frequency Selective Surfaces (FSS) are surface structures usually comprising periodic patterns of thin conductor on a dielectric interface. (See FSS review). In recent times, any FSS with a unit cell small compared to a free space wavelength is often referred to as a metasurface. Dr Mackay was heavily involved in the development of this topic, beginning in the 1990s, before the term metasurface was invented.
For example, one such structure was patented, together with Dr Mike Wood, in 1991 (public release 1999) for narrow-band absorption applications (see here) based on the hexagonal interlocking element design:
These sorts of FSS are interesting because they give rise to a resonance at a wavelength much larger than a unit cell described by a surface impedance which is insensitive to the angle of incidence and polarisation of an incident plane wave.
A Floquet modal analysis of such structures is useful and provides a basis for numerical methods for the analysis of single and multi-layer FSS (see fss analysis method). This approach to multi-layer FSS can quantify the excitation of evanescent waves between interfaces and is numerically efficient when relatively few evanescent coupling modes are required.
Another interesting class of FSS are those comprising two surfaces on the top and bottom of a thin layer of lossless dielectric substrate composed of periodic arrays of offset slots. Intuitively, one might suppose no transmission through such a structure on a very thin substrate. However, this turns out not to be the case and transmission occurs at a narrow resonant frequency. This was demonstrated numerically and experimentally in 1990-1991 at RSRE leading to an award of the John Benjamine memorial prize.
There are many areas of FSS research which remain hot topics. One, for which Dr Mackay has conducted recent research, is in the properties of non-periodic FSS for which the geometry is hyperuniform stealthy.