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Negative Refractive Index

· 4 min read
Kinjal Raykarmakar

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As studied in our physic classes, refractive index is generally a positive value. However, can this theory be turned upside down to view materials from another point of view? That’s the story of Metamaterials!

Refractive Index

Before diving in much, let’s review what do we mean by the refractive index of a material. Firstly, refraction is a phenomenon in which light bends when it passes from one material to another. Now, classically speaking: the refractive index is a measure to describe the speed of light in a material.

Mathematically,



Where, c = speed of light in vacuum = 299,792,458 m/s

Abnormalities

Refractive index for material not unique. It depends on the wavelength of the light. If different wavelengths have a different refractive index, means that different wavelengths have different speeds of light. (keep this in mind)

Simply, the phase velocity of a wave is the rate at which the wave propagates in some medium. In a negative phase velocity (NPV) medium, the phase velocity is directed opposite to the direction of energy flow. This phenomenon is totally beyond geometrical optics. This occurs in certain materials only. (We’re getting there!)

Negative Refractive Index

What could go wrong with our knowledge?

After some complicated math, we can infer that, the refractive index of electromagnetic radiation equals:

εr = electrical permittivity and µr = magnetic permittivity

Convention the positive square root is chosen for n. But in materials with ε and μ negative, we tend to keep the value of n is negative too, just to match its physical significance. (We’re playing with abnormalities!)

Metamaterials

A negative-index metamaterial or negative-index material (NIM) is a metamaterial whose refractive index for an electromagnetic wave has a negative value over some frequency range. These materials are not naturally occurring. So, there’s a need to synthesize these materials artificially.

Negative permittivity ε < 0 had already been discovered and realized in metals for frequencies up to the plasma frequency. There are two requirements to achieve a negative value for refraction. The first is to fabricate a material that can produce negative permeability μ < 0. Second, negative values for both permittivity and permeability must occur simultaneously over a common range of frequencies.

“This is the first time that both total refraction, that is zero reflection, and amphoteric refraction — both positive and negative refraction — have been seen in the same material,”

~ Zhang told PhysicsWeb

“Reflection is sometimes thought of as the penalty electromagnetic waves must pay when they are refracted. Our result shows that this does not have to be the case.”

Yong Zhang and co-workers have demonstrated total negative refraction in a ‘twinned’ alloy that contains yttrium, vanadium and oxygen. Furthermore, depending on the angle of incidence, the interface could also positively refract light.

Who cares about the negative refractive index?

The whole concept of negative refractive index materials can be used to change our conventional devices. Those devices broadcast, transmit, shape, or receive electromagnetic signals that travel over cables, wires, or air. Moreover, their properties can be upgraded.

Moreover, in the wireless domain, these metamaterial apparatuses continue to be researched. Other applications are also being researched. These are electromagnetic absorbers such as radar-microwave absorbers, electrically small resonators, waveguides that can go beyond the diffraction limit, phase compensators, advancements in focusing devices (e.g. microwave lens), and improved electrically small antennas.

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