Interference In Thin Films

Thin-film interference occurs when a lightwave is reflected off two surfaces separated by a distance equal to its wavelength. We witness distinct coloured patterns when light waves that reflect off the top and bottom surfaces collide. During this process, light strikes the border between the two media. With some of it being reflected and some being transmitted.

There are two reflections that occur close together when the second medium is a thin film. It occurs at the thin film’s top and bottom boundary surfaces. Thus, two waves emerge from a thin film, one reflected off the top surface and the other reflected off the bottom surface. For example, you’ve probably seen a rainbow-coloured reflection in a puddle of water by the side of the road or in a bubble. Thin-film interference is the name given to this observable phenomenon.

Thin-film interference explains the phenomenon of various colours being visible in light reflected from soap bubbles. Anti-reflection coatings on glasses and camera lenses operate in the same way. Continue reading to learn more about thin-film interference.

Interference Pattern and Light Sources

• Monochromatic source

When the incident light comes from a monochromatic source, interference patterns appear as bright and dark lines. Light lines depict constructive interference zones between reflected waves. And dark lines represent destructive interference zones. The thickness of the coating changes from one place to another then the interference also changes from constructive to destructive.

• Broadband source

When the incident light comes from a broadband source or white source, the interference patterns appear as coloured lines. An example of it is sunshine. There are many wavelengths of light that induce constructive interference at different film thicknesses. Many film parts appear in different colours, which are dependent on the local film thickness.

• Phase interaction

Assume that there are two incident light beams (X and Y). Each beam generates a reflected beam (dashed). Beam X’s reflection off the lower surface and beam Y’s reflection off the higher surface are the reflections of interest. These reflected beams combine to form a composite beam (Z). The resulting beam is relatively powerful if the reflected beams are in phase. If the reflected beams have the opposite phase, the resultant beam is reduced.

Condition for Constructive Interference in Thin Films 

The interference of light waves, both constructive and destructive, is also responsible for the colourful patterns seen in thin films such as soap bubbles. Thin-film interference occurs when light waves reflected from the top surface of a film interfere with light waves reflected from the bottom surface. The thickness of the film must be on the order of the wavelength of light to produce an excellent colourful pattern.

Consider a small layer of oil floating on water. Thin-film interference can occur if these two light waves constructively interact:

• The light is reflected from the top surface by the air.

• The light travels from the air through the oil, reflects off the bottom surface, and travels back through the oil into the air.

Light reflects off a surface with a greater index of refraction. It causes a 180° phase shift in the wave. This is crucial in evaluating whether these waves interact constructively or destructively.

The two reflected waves must be displaced by an integer of multiple wavelengths to produce constructive interference. This must account for any phase shift caused by a reflection off a higher-n material and the extra distance travelled by the wave as it travels down and back through the film. In the case of the oil film, constructive interference will occur if the film thickness is 1/4 or 3/4 or 5/4 wavelength, and so on. When the thickness of the oil film is 1/2 or 1 or 3/2 wavelength, and so on, destructive interference develops.

Application of Interference in Thin Film

Some of the applications of interference in the thin film are:

• Thin films are used in anti-reflection coatings and optical filters. They are used to control the amount of light reflected or transmitted at a certain wavelength at a surface.
• Thin-film interference is used to allow only specific wavelengths of light to pass through the device. Deposition processes are used to create these films. It includes the controlled addition of material to a substrate. Other techniques used are chemical vapour deposition and physical vapour deposition processes.

• Thin films can also be found naturally. In many species, the Tapetum lucidum is found. It is a layer of tissue behind the retina which helps in light collecting.

• The effects of thin-film interference are seen in oil slicks and soap bubbles. A thin film’s reflectance spectrum exhibits distinct oscillations, and the spectrum’s extrema may be used to calculate the thickness of the thin film.
• Ellipsometry is a technique for testing the properties of thin films. Polarised  light is reflected off a film surface and measured by a detector in an ellipsometry experiment. The complex reflectance ratio of the system is computed. It is then used to compute film layer thicknesses and refractive indices in a model study.

• Dual polarisation interferometry is a new technique. It calculates the refractive index and thickness of molecular scale thin films. And how they change when they are stimulated.

Conclusion 

By now, you would have understood what the interference of light in a thin film is and what is the condition for constructive interference in a thin film. Thin-film interference occurs when a lightwave is reflected off two surfaces separated by a distance equal to its wavelength. The application of interference in a thin film is an important topic from the point of view of several competitive exams. Make sure to have a good grasp of this topic.