I have seen unit cell designs that only perform at normal incidence while there are some designs that perform for a wide range of incidence angles. what is the reason or explanation behind this. what makes them angularly stable.
H. A Bokhari ,
In a word, resonance.
Phase on the surface may be related to the phase of an incident plane wave via phase-matching principles and the constitutive relations between wave vector components. So then, observing a large range of incident angles is equivalent to observing a large range of wave vector (k) values on the surface.
If the phase incurred across the metasurface element has a strong frequency dependence, the metasurface will tend to only operate over a small range of angles, which correspond to the frequency/frequencies of operation of the surface mode. Alternatively, if the phase across the metasurface has a weak frequency dependence, it will tend to operate over a wide range of incident angles.
We can visualize these modes on a dispersion (k-Beta or w-k) diagram. Modes which have a strong frequency dependence vs angle will exist over a large portion of these diagrams with some k-dependent slope. Resonant modes, however, will exist as pretty much straight lines -- indicating that over all possible k values, the frequency band stays pretty much constant.
See Yang and Rahmat-Samii's book, "Electromagnetic Bandgap Structures in Antenna Engineering" (ISBN: 9780521889919), Chapter 2, for a more detailed description.
H. A Bokhari ,
In a word, resonance.
Phase on the surface may be related to the phase of an incident plane wave via phase-matching principles and the constitutive relations between wave vector components. So then, observing a large range of incident angles is equivalent to observing a large range of wave vector (k) values on the surface.
If the phase incurred across the metasurface element has a strong frequency dependence, the metasurface will tend to only operate over a small range of angles, which correspond to the frequency/frequencies of operation of the surface mode. Alternatively, if the phase across the metasurface has a weak frequency dependence, it will tend to operate over a wide range of incident angles.
We can visualize these modes on a dispersion (k-Beta or w-k) diagram. Modes which have a strong frequency dependence vs angle will exist over a large portion of these diagrams with some k-dependent slope. Resonant modes, however, will exist as pretty much straight lines -- indicating that over all possible k values, the frequency band stays pretty much constant.
See Yang and Rahmat-Samii's book, "Electromagnetic Bandgap Structures in Antenna Engineering" (ISBN: 9780521889919), Chapter 2, for a more detailed description.
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