I.Evanescent Wave

In the field of optics, evanescent waves, are oscillating electric and/or magnetic fields that may not propagate like electromagnetic waves but whose energy is spatially amassed near the source rather than moving waves of

#### **Table 2.** *An overview of confining optical interactions at nanoscale.*

gradually decaying field intensity in a specific spatial direction that are often encountered. They also do not contribute to energy transfer in that direction; despite the fact that the Poynting vector (averaged over one oscillation cycle) can have non-zero segments in various ways. Evanescent waves can also occur in various types of waves, such as sound waves and quantummechanical waves. There are likewise situations where a light field can be disintegrated into an evanescent and a propagating part. A light field may also be disintegrated into an evanescent and a propagating element under some circumstances. As a waveguide surface reaches a lower refractive index medium, an evanescent wave is formed, which decays exponentially in the axial direction (away from the waveguide) and this evanescent field has a magnitude of 50–100 nm that can be used to cause nanoscale optical interactions as well as evanescent wave excitation for high near-field fluorescence sensing [7]. The coupling of two waveguides by an evanescent wave is another example of nanoscale optical interaction, as seen schematically in **Figure 2(a)**, in which photons are launched from one waveguide to another, and these evanescent wave-coupled waveguides can be used as directional couplers in an optical communication network to relay signals. It has also been proposed that evanescent wave-coupled waveguides be used for sensor applications, where sensing induces a transition in photon tunneling from one waveguide channel to the next. There are optical sensors for specific material species where one can experiment with how a light field, which is essentially directed, such as in a glass structure, is connected to an evanescent field and atoms or molecules in that area will be able to interact with the light field; for example, after being excited by light, they could emit fluorescence [6].

Total internal reflection, which involves the scattering of light through a prism with a refractive index of n1 to an atmosphere with a lower refractive index of n2, is just another illustration of a geometry that produces an evanescent wave [7, 8]. As the incidence angle is small enough, light refracts and passes through the second medium to some degree, such that as the incidence angle reaches a critical angle, the light ray is reflected from the interface, as seen in **Figure 2(b)**.
