Dear Bokhari

Electromagnetic meta-materials, artificially engineered by periodically-arranged meta-elements which are smaller than the operating wavelength, exhibits special responses to the incident EM wave.Generally speaking, meta-surfaces could include some traditional concepts in the microwave spectrum such as frequency selective surfaces (FSS), impedance sheets and even Ohmic sheets. In the microwave regime, the thickness of these meta-surfaces can be much smaller than the wavelength of operation (for example, 1/1000 of the wavelength), since the skin depth could be extremely small for highly conductive metals, and also the substrates with this range in lamda (1/1000 of the lamda) are called ultrathin substrate. therefore your substrate with lamda/23 (0.0434 lamda) is not a ultrathin substate.

You can see the following links for your answer:

Article Flexible ultrathin metamaterial absorber for wide frequency ...

https://www.tandfonline.com/doi/abs/10.1080/17455030.2017.1418099?journalCode=twrm20

Best

Mousa

Thanks for the answer Mousa. Ill give the paper a read.

Simple thin All-metal FSS of slots (or slits) has the pair of eigen-oscillations and corresponding pair of transmission resonances. First ones are well-known and has PMW at the symmetry plane.For slits they are known as half-lambda resonances. The second ones have PEW, high-Q factor, placed very close to upper boundary of single-mode band. To made them visible in calculations or in experiment the metasurfaces has to be enough thick.

So if you do not recognize such resonances immediately before end of single mode band then you can consider your object as ultra-thin.

Regards. AO

@Kirilenko Afanasievich Thankyou Thats a great answer. Could you refer any paper or literature from which I can further clarify my concepts. especially these transmission resonances and eigen oscillations in metasurfaces.

@Denis Jaisson. I have seen research papers claiming metasurfaces to be ultra thin. I am interested to know the criteria for which one can claim that metasurface is ultra thin or not.

See? for example, A.A.Kirilenko and A.O.Perov, “On the

common nature of the enhanced and resonance transmission through the periodical set of holes,” IEEE Trans. Antennas Propag., vol. 56, no. 10, pp. 3210–3216, Oct. 2008. + references