In order to collect the light in an efficient way one strategy is reducing the light reflection from surface of materials. In optic this can be done by using an Anti–Reflective Coating (ARC). The idea of anti–reflective coating has been found accidentally by Lord Rayleigh in the 19th century, however the first real anti–reflector has been made by Fraunhofer in 1817. Nowadays, anti-reflectors are widely used or have this potential to be used in photovoltaic devices and optical-electrical sensors To develop a ARC system one should consider the main conditions of anti–reflectivity. To do so, first we start by examining the case of single layer ARC and then we will generalize it for multilayer system. following figure shows the light propagation through a single layer film with refractive index of n on a glass substrate with refractive index ns.
In this case anti–reflection conditions can be written as:
- The phase difference of reflected waves must be nπ/2.
- The thickness of film must be kλ/4 of incident beam where k is a odd number.
This conditions developed by assuming that there is no other optical interaction such as scattering and absorption. Based on Fresnel’s equation the reflection coefficient of an interface between two media is:
Hence by this definition the reflectance R for such structure like can be written as:
Now in order to get a perfect anti-reflector the R must be equal to zero and as a result the anti–reflectivity condition for such system is a circumstance when :
In next post we will extend this model for a multilayer system