Thin Films And Coatings

Question 1

Describe the phase changes that occur upon reflection?

Answer 1

From low to high index (n2 > n1) reflected wave is shifted in phase by 180° (1/2 cycle)

From high to low index (n2 > n1) Reflected wave continues with no change in phase

Question 2

Describe the phase change for thin film.

Answer 2

Phase change upon reflection at on interface (phase changes are not common)

Question 3

Describe the phase changes for coatings.

Answer 3

Phase change upon reflection at both interfaces (phase changes are common)

Question 4

What 3 main factors play a role in determining if there will be constructive and/or destructive interference?

Answer 4

Thickness
Wavelength
Indices of refraction

Question 5

Can we have interfence between reflections between two interfaces for thin films and coatings?

Answer 5

Yes

Question 6

How can index affect thin films and coatings?

Answer 6

There are differences between reflections from low to high vs. high to low index

Question 7

What are some examples of thin film?

Answer 7

Soap film
Oil slicks

Question 8

How do we use thickness to consider interference of thin films?

Answer 8

Consider the possible interference if the thickness of the film is 1 λ, ½ λ, and ¼ λ thick.

Assume that the light that strikes the front surface is at 0λ.

Question 9

What is total destructive/constructive interference of thin films?

Answer 9

Light reflected off the front surface and light reflected off the back surface of a thin film combine

Question 10

When do we have total constructive vs destructive interference?

Answer 10

If the thin film is a multiple of 1/2λ thick, there will be total destructive interference.

If a thin film is an odd multiple of 1/4λ thick, there will be total constructive interference.

Question 11

What is the optical thickness of the medium?

Answer 11

T/n

Question 12

What is the density of coating compared to surrounding materials? What is the material commonly used for optical coatings?

Answer 12

With coatings, the coating material is denser than the air and less dense than the optical material that is being coated.

The most common coating material is for anti-reflection and the material used is magnesium fluoride.

Question 13

What are the phase changes at the first and second surface of coatings?

Answer 13

With optical coatings there is a change in phase of 180° at the first surface with reflection.

With optical coatings there is also a change in phase of 180° at the second surface with reflection.

Question 14

Describe the law for optical coatings.

Answer 14

If light strickes a thin film (common phase changes at both surfaces):
total constructive interference, coating material must be multiple of 1/2 wavelength of the incident light

Total destructive interference, coating material must be odd multiple of 1/4 wavelength of the incident light

Question 15

Describe the basics for optical coatings.

Answer 15

The coating needs to be an odd multiple of 1/4λ of the incident light in thickness – usually vacuumed onto the surface of a lens

The ideal refractive index of the coating material for anti- reflection should be equal to the square root of the lens material that is to be coated.

The material used for A/R coating is magnesium fluoride which has a refractive index of 1.38

Question 16

How should we approach problems with optical coatings for A/R?

Answer 16

Only one wavelength can be placed on a lens with the proper thickness for complete destructive interference

The magnesium fluoride refractive index (n=1.38) does not match the square root of any of the known spectacle lens materials (still used for other practical reasons including good adhesion and relatively good stability over time).

Question 17

Why can ARCs appear purplish?

Answer 17

Green reflection is eliminated

Some reddish and bluish reflectance remains
(see graph)

Mixture of red and blue has purplish hue

Reflected color will change with angle since effective thickness of coating changes

Question 18

When are neighboring wavelengths affected by a coating that is at a thickness?

Answer 18

Creates complete total destructive interference for a particular wavelength

Multi-layer coatings with differing thickness are used o obtain the desired results

Question 19

Describe interference in microscope slides.

Answer 19

Fringes of equal thickness

When t = 1/4 you get constructive interference, when t = 1/2 you get destructive interference. In this case, the film is air so the wavelength need not be adjusted.

Question 20

Describe the application of newton rings.

Answer 20

Same as microscope slide interference except that the change in thickness is is not linear and follows a very predictable shape.