What is the Definition of Photonics?

A beam of white light enters a prism and emerges as a vibrant spectrum, illustrating the fundamental photonics principle of light dispersion.

Photonics is a field of physics and a sub-discipline of the broader branch of optics, both of which have ties within engineering.

There are debates regarding the definition of photonics, with some research broadly defining this as the interaction of photons with matter¹
and others describing it as about the electronic applications ². There is also the route where photonics either relates to the application of photons³ or the manipulation of photons⁴. Finally, there is another approach that considers this topic to be about photons transmitting information⁵.

Regardless of the definition there are some applications we can take away from these ideas surrounding photonics. If we consider the broadest of the definitions for photonics, as how photons interact with matter, we can find some categories that link to some specific wording of other definitions. Those being the detection, emission, manipulation and transmission of photons⁶.

Emission and detection

Detection in photonics is about recognising a single or multiple photons, whereas emission is the releasing of a single or multiple photons. Emission occurs in two different ways, spontaneous or stimulated emission⁷.

Spontaneous emission doesn’t need a trigger, it results from high energy atoms and when chance dictates photons will be released. From this action, the stability within the atom is regained.

Stimulated emission instead has a trigger; an atom needs to be activated before it can emit its photons.

Stimulated Emission: Laser and Phosphor Screens

An example of stimulated emission is from lasers. This is where its name is from, with the acronym “laser” standing for “Light Amplification by Stimulated Emission of Radiation”

Lasers work by having radiation, either light or electricity, “excite” atoms, this brings them to a higher energy level. From this they will try to return to the base level by emitting photons⁸.

Another example is demonstrated through phosphor screens. Microchannel plate detectors⁹ convert photons or other subatomic particles into electron pulses for the detection from a phosphor screen. The screen receives this pulse and emits light demonstrating spatial information about the pulse and consequently the initial photons/particles.

Manipulation of Photons

The term “manipulation of photons” is a broad category, which includes concepts such as how you alter the direction, speed and polarisation of light.

To break it down a little, we are going to discuss 3 phenomena for light – reflection, refraction and transmission.

Reflection and refraction:

Often within vacuum optics reflections are something we want to remove. This is highlighted by the popularity of anti-reflective (AR) coatings within Torr Scientific. This would be classed as an important aspect of photonics for us.

But how do you stop reflections?

AR coatings act as a stepping-stone between the refractive index of the air and the optic. Minimising the severity of change is part of the reason for decreased reflections, but the larger aspect is being able to create destructive inference, matching the wavelengths to cancel each other out¹⁰.

A diagram demonstrating the effect of the Brewster window

Brewster windows and Brewster angles

Brewster windows are a special type of viewport, where the goal is to reflect s‑polarised light. The transmitted light is slightly p‑polarised.

The s- and the p- correspond to the two orthogonal components of the random vibrations of the electric field vector of light, parallel and perpendicular, respectively, to the reflecting surface.

Transmission

This may seem cut and dried that light travels through a material as determined by the refractive index, but the structure of the material can impact how this works: with birefringence.

Diagrams demonstrating how linear polarisation considers rays of light, showing s‑polarised and p‑polarised light and their direction of oscillation.

Birefringence

As light collides with a birefringent material along a non-equivalent axis, the two components of light (p‑polarised and s‑polarised) will interact with the material differently. They will be impacted by different refractive indices, so they will obtain distinct velocities, becoming split and polarised.

One wave will be faster, closer to the speed and the angle at which it entered the material. The other will be slower and be more distinct in angle to the original incidence. This is because the structure of the material is not the same viewed from one direction as it is from another (hence “non-equivalent” axis).

One particular direction is called the optical axis. At this angle the beam experiences a single refractive index and creates a new combination. This means the beam doesn’t split or become polarised.

Diagram demonstrating how light behaves as it interacts with a birefringent material.

Materials like our natural z-cut quartz¹¹ are intrinsically birefringent. This gives the optic a range of new purposes. Quartz is transmissive from between 250nm and 3µm, as well as from 50µm to 1000µm+, these dual transmission capabilities mean it can be used with IR, UV and visible applications!

Conclusion

Photonics is a complex subject without a clear definition, but this doesn’t mean we can’t utilise applications from photonics and make them accessible for physics, engineering and science more generally.

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