Electromagnetic field, decay lengths

Here, m is the refractive-index sensitivity of the sensor Uadsorbate and Ubiank are the refractive indices of the adsorbate (i.e., analyte) and the bulk environment prior to the sensing event, respectively (iadsorbate is the effective thickness of the adsorbate layer and k is the characteristic electromagnetic field decay length associated with the sensor. 

Flat-surface SPR sensors have a large refi-active-index sensitivity ( 2 x 10 nm/RIU), which is the chief component of their overall sensitivity [54]. LSPR nanosensors have modest refractive-index sensitivity ( 2 x 10 nm/RIU) [55], in contrast. Nevertheless, both types of sensors have approximately equivalent sensitivity for a given adsorbate. In addition to the difference in refractive-index sensitivity, the electromagnetic field decay length Zdis also different for SPR and LSPR sensors. SPR sensors have a decay length on the order of 200 nm. For LSPR... 

A. The electromagnetic field decays evanescently into the adjacent media, with a decay length of several hundreds of nanometers. 

Figure 5. Decay lengths for the electromagnetic field of a surface electromagnetic wave on a Cu-vacuum interface. The right-hand scale is for the decay length into the Cu and the left-hand scale for the decay length into vacuum.

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