Antireflection coatings (AR) are used to reduce the Fresnel reflectance of optical components. In imaging systems, AR coatings improve the efficiency because they provide less light lost. In complex optical systems, the reduction in reflections also improves the contrast of the image by elimination of stray light.

OptiLayer allows designing all types of AR coatings:

• Narrow-band and broadband AR coatings;
• Single-band, dual-band, multiple-band AR coatings; 
• AR coatings at normal and oblique incidence;
• AR operating in all spectral ranges;
• AR coatings formed from two, three and more materials;
• AR coatings using absorbing layers;
• AR coatings with specified color properties;
• AR coatings in stacks

It follows from the theory that, at normal angle of incidence and at oblique incidence case (s-polarization), two-component AR designs with the highest and lowest available refractive indices form an optimal class of AR designs.

Residual reflectance of broadband AR coatings is decreasing with increase of optical thickness of the coating and with growing the number of layers. At the same time, there is a lower limit of the residual reflectance. 

We present a video example of the designing broadband AR coating at YouTube

It has been shown that there exists the minimum achievable residual reflectance of broadband AR coatings.

This limit depends on:

• Relative width of the antireflection spectral range;
• Ratio between high and low refractive indices;
• Ratio between low index material and refractive index of the incidence medium;
• Ratio between substrate refractive index and refractive index of the incidence medium.

You can estimate residual reflectance for your design problem here. This formula gives accurate estimations for broadband AR design problems when relative width of AR range is more than 2.

Example: two-octave AR coating operating in the spectral range from 450nm to 1800 nm. Layer materials are Ta2O5 and SiO2, refractive indices are specified by Cauchy formulas. Substrate is Suprasil. Minimum achievable reflectance can be calculated with the help of this formula with: 

• n_H=2.1 (Average index)
• n_L=1.46 (Average index)
• n_s=1.46 (Average index)
• λ_l=450 nm
• λ_u=1800 nm

The formula estimates minimum achievable reflectance as 1.1%

The simplest AR coating contains 10 layers and achieves reflectance equal to 1.6%.

This coating was produced at Helios deposition plant. See comparison of the theoretical and experimental data at this picture. The substrate is covered from one side.

See the details in our publications on AR coatings:

• T. Amotchkina, M.Trubetskov, Y. Pervak, L. Veisz, V. Pervak, Stress compensation with antireflection coatings for ultrafast laser applications: from theory to practice, Vol. 22, pp. 30387-30393 (2014)
• T. Amotchkina, M. Trubetskov, V. Pervak, and A. Tikhonravov. "Design, production and reverse engineering of two-octave antireflection coatings," Appl. Opt. 50, 6468-6475 (2011).
• T. V. Amotchkina, "Empirical expression for the minimum residual reflectance of normal- and oblique-incidence antireflection coatings", Appl. Optics, Vol. 47, Issue 17, pp. 3109-3113 (2008)

• A. V. Tikhonravov, M. K. Trubetskov, T. V. Amotchkina, and J. A. Dobrowolski, "Estimation of the average residual reflectance of broadband antireflection coatings," Appl. Opt. 47, C124-C130 (2008)
• T. Amotchkina, A. Tikhonravov, M. Trubetskov, "Estimation for the number of layers of broad band anti-reflection coatings", Proc. SPIE. 7101, Advances in Optical Thin Films III 710104 (2008)