Half-Wave Optical Thickness (HWOT)
Definition and meaning of Half-Wave Optical Thickness (HWOT) in chemistry.
Half-wave optical thickness (HWOT) is the thickness of a thin-film coating layer, defined as its refractive index multiplied by its physical thickness, that equals one half of the design wavelength of light (or a whole-number multiple of that half-wavelength).
In more detail
A layer with this thickness introduces a round-trip phase shift of a full wavelength (360 degrees) at the design wavelength, which is optically equivalent to no phase shift at all. As a result, an HWOT layer does not change the reflectance or transmittance of the coating stack it sits within, so it is called an "absentee layer." HWOT layers are often formed by placing two identical quarter-wave optical thickness (QWOT) layers of the same material next to each other, and are used as protective outer layers or spacer layers in dielectric mirrors and interference filters without disturbing their optical performance.
Key facts
| Field | Physical Chemistry |
|---|---|
| Relation | optical thickness n·d = mλ/2 (m = 1, 2, 3...) |
| Also known as | absentee layer |
| Related term | QWOT (quarter-wave optical thickness) |
A silicon dioxide (SiO2, refractive index n is about 1.46) capping layer on a laser mirror designed for 1064 nm light needs a physical thickness of about 364 nm, since n times d then equals 532 nm, exactly half the design wavelength, so the layer protects the coating without changing its reflectivity.
Frequently asked questions
Why is an HWOT layer called an absentee layer?
Because at the design wavelength it produces a round-trip phase shift of a full wavelength (2 pi radians), which has no net effect on the coating's reflectance or transmittance, as though the layer were not there optically.
How is HWOT related to QWOT?
Two adjacent quarter-wave optical thickness (QWOT) layers of the same material, each with optical thickness lambda/4, combine to form a single half-wave optical thickness (HWOT) layer of thickness lambda/2.