Far-Field Transmittance

For a NFT that is an aperture, the simplest theoretical and experimental procedure for evaluating NFT efficiency is to calculate or measure the far-held transmittance. The total power transmitted through the aperture must be normalized in some manner. The incident beam in a theoretical calculation is frequently a plane wave however, the incident power in a plane wave is inhnite. Because only a hnite amount of power is transmitted through an aperture, the transmittance of the aperture as a ratio of transmitted power to incident power is exactly zero for a plane wave therefore, the transmittance of an aperture for a plane wave is not a useful FOM. However, there is a hnite amount of power in a plane wave in the cross-sectional area of the aperture. A popular FOM is the ratio of the transmitted power from a plane wave incident upon the NFT (or the absorbed power of the medium next to the NFT) to the power/area of the plane wave multiplied by the cross-sectional area of the aperture. For periodic arrays of NFTs, the FOM is the ratio of the transmitted power to the power/area of the plane wave multiplied by the area of a uiut cell. The power/area for a plane wave is 

Unfortunately, as discussed in the previous section, it is easily demonstrated that the far-field transmittance is not necessarily related to the near-field coupling efficiency for a NFT meant to be 

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B), lensing results in additional focusing of the beam and its increased size in the far field, and so the transmittance through the aperture is reduced. The focal plane... 

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