Thin films of material are commonly used in an optical system to manipulate various wavelengths of light in a desired manner.
There are many different kinds of optical thin films and each type of film has an explicit purpose.
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Anti-reflection
Anti-reflection films are used to optimize transmittance, increase contrast and eliminate ghost images. As these coatings are highly durable, most transmissive optics have some type of anti-reflection thin film.
High-reflective
High-reflective films are essentially calibrated mirrors, meant to optimize reflectance in either single wavelengths or along a wide spectrum.
Beamsplitter
Beamsplitter films cleave incident light into multiple desired output beams. These films can be found in cameras, projectors and lasers.
Filter
Filter films can attenuate light at specific wavelengths. These thin films can reflect, absorb or transmit light.
Optical thin films are made for a particular incident angle and a particular polarization of light like S-polarization or P-polarization. If the incident angle is shifted in an optical system, the internal angles and optical path lengths will be shifted, impacting the phase change in the resultant beams coming from the film. When a non-normal incidence is applied, S-polarized and P-polarized light acts differently from one another at each interface, causing different optical results at the two polarizations. This phenomenon is used in the design of beamsplitter films.
Using a thin film at an angle of incidence other than what it is made for will typically lead to a substantial drop in performance, and significant deviations in the angle of incidence can cause a total loss of function. Likewise, using a different polarization than what was intended will produce unwanted outcomes.
Optical thin films are produced by the deposition of dielectric and metallic materials like aluminium oxide. To optimize or minimize interference, they are generally designed to be quarter-wave optical thickness (QWOT) or halfwave optical density (HWOT) of the light wavelength used in the application.
While the wavelength and angle of incidence are the predominant design considerations, the index of refraction and thickness can also be adjusted to maximize performance. Alterations in any of these factors will have an impact on the path length of light inside the film, which will affect phase values of the light as it travels.
Fabricating optical thin films
Optical thin films are typically fabricated through various vapor deposition techniques. During vapor deposition, a source material in a vacuum chamber is vaporized, through either heating or particle bombardment. The resulting vapor condenses on a focus on substrate to produce a thin film.
Optical thin films of specified thicknesses can be produced through the control of temperature, pressure, substrate location and substrate rotation.
The fairly delicate nature of vapor deposition produces thin films that are loosely structured. These films have problems with water absorption, as it changes the effective refractive index to cause a drop-off in performance. Thin films can be bolstered using Ion Beam Assisted Deposition, which involves an ion beam being directed at the exterior of the substrate. This technique increases adhesion energy to create denser, more robust films.
Next-generation optical thin films
Possibly used in the near future to make powerful optical circuits, metasurfaces are next-generation thin films that are currently being designed and developed.
The conventional method for creating metasurfaces is an arduous lithography process that takes several hours and must be done inside a clean room. However, a recently-published study reported that researchers at the Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland have now developed a simpler method that requires just a few minutes. The new process can be done outside of a clean room and at room temperature.
With their technique, the Swiss researchers were the first to generate dielectric glass metasurfaces, as opposed to metallic metasurfaces. The main benefit of dielectric metasurfaces is that they don’t absorb much light and have a large refractive index, allowing for the effective modulation the light as it propagates through these materials.
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