An evanescent wave is an electromagnetic field that does not carry energy away from its source as a propagating wave. Its energy is concentrated near its origin. Unlike typical propagating waves, which transmit energy over long distances, an evanescent wave’s intensity quickly diminishes with distance from where it forms. The term “evanescent” means “tending to vanish,” accurately describing this behavior.
How Evanescent Waves Form
Evanescent waves form primarily through total internal reflection (TIR). This occurs when a wave, such as light, travels from a denser medium into a less dense medium and strikes the boundary at an angle greater than a specific “critical angle.” For example, if a laser beam in glass hits an interface with air at a sufficiently large angle, the light will be entirely reflected back into the glass.
Even though the main wave is reflected, an evanescent wave still appears on the side of the less dense medium. This is because electric and magnetic fields cannot be discontinuous at the boundary, a requirement for Maxwell’s equations. A disturbance must extend into the less dense medium to maintain field continuity, even without energy being transmitted across the interface. This disturbance is the evanescent wave.
The formation of an evanescent wave is related to the mathematical concept of an imaginary wave vector component perpendicular to the interface. While the wave propagates parallel to the interface, its amplitude decays exponentially in the direction perpendicular to it. This decay ensures no net energy flow away from the boundary into the less dense medium, upholding energy conservation during total internal reflection.
Defining Characteristics
Evanescent waves have distinct properties. They are non-propagating, meaning they do not transport net energy away from the interface in the direction of their decay. While they contain energy, this energy remains localized near the boundary where the wave is formed.
Another defining feature is their exponential decay. The intensity of an evanescent wave diminishes very rapidly with increasing distance from the interface, meaning its influence is felt only over extremely short distances. This penetration depth is typically on the order of the wavelength of the incident light, often tens to hundreds of nanometers.
The frequency of an evanescent wave matches that of the incident wave. Its “wavelength” perpendicular to the interface is considered imaginary, which mathematically describes its rapid decay rather than oscillatory behavior in that direction. The absence of net energy transfer across the interface in the direction of decay is a hallmark of an evanescent field, where the time-averaged Poynting vector (representing energy flow) in that direction is zero.
Where Evanescent Waves Appear
Evanescent waves manifest in various natural phenomena and engineered systems. A common example is in fiber optics, where light is guided through optical fibers using total internal reflection. At the interface between the core (denser medium) and the cladding (less dense medium), an evanescent wave extends slightly into the cladding, even as the light remains confined within the core. This evanescent field is crucial for the fiber’s ability to guide light efficiently over long distances.
Prisms and other light guides, often found in binoculars or periscopes, also rely on total internal reflection, creating evanescent fields at their reflective surfaces. These fields are typically not directly observed but are a consequence of the underlying physics. A related phenomenon is frustrated total internal reflection (FTIR). If another medium is brought very close to the interface where an evanescent wave exists, the evanescent wave can “couple” into this new medium, allowing energy to be transmitted across the gap, even though it would normally be totally reflected. This principle finds use in devices like some fingerprint scanners, where the ridges of a finger make contact with a surface, frustrating the reflection and allowing light to pass through.
Beyond optics, evanescent fields can appear in other wave phenomena, such as surface water waves or acoustic waves, where similar decaying fields can exist near boundaries. For instance, in acoustics, evanescent waves can form when sound waves encounter an interface at certain angles, decaying rapidly into the second medium without propagating.
Using Evanescent Waves in Technology
Evanescent waves are valuable in various technological applications. Evanescent wave sensors, particularly biosensors, are a significant application. These devices exploit the sensitivity of the evanescent field to changes in the refractive index of the medium immediately adjacent to the sensor surface. When target molecules bind to the surface, they alter the local refractive index, which in turn changes the properties of the evanescent wave, allowing for highly sensitive detection of biomolecules in medical diagnostics or environmental monitoring.
Total Internal Reflection Fluorescence (TIRF) microscopy is another application. In TIRF, an evanescent field is generated at the interface between a glass slide and a biological sample. This field selectively excites fluorescent molecules (fluorophores) located only within a shallow region, typically about 100 nanometers, immediately above the surface. This selective excitation provides high-resolution imaging of cellular processes that occur directly at cell membranes or other surfaces, minimizing background fluorescence from the bulk of the sample.
Evanescent waves are also fundamental to the operation of optical waveguides and integrated optics. In photonic circuits, the evanescent field allows for light to be confined and guided within tiny structures. This field can also facilitate coupling between adjacent waveguides or components without direct physical contact, enabling the transfer of light signals in compact optical devices like fiber-optic splitters and couplers.
The concept of evanescent waves also finds an analogy in quantum mechanics, specifically in quantum tunneling. In this quantum phenomenon, particles can “tunnel” through energy barriers even if they do not possess enough energy to classically overcome them. This behavior is described by a decaying probability wave (similar to an evanescent wave) that extends into the forbidden region, allowing a non-zero probability of the particle appearing on the other side of the barrier.