MacLeod Coatings: Typical examples

This article shows typical examples of a range of thin-film components manufactured by MacLeod. As the manufacturer says, they should not be thought of as the best that can be produced.
For these coatings, the incident medium is air unless specifically defined otherwise.
Angus MacLeod, Sandrine Auriol, Thomas Pickering
08/24/2017
OpticStudio
Polarization and Thin Film Coatings

Antireflection coatings

For the antireflection coatings, the substrate is assumed to be a crown similar to N-BK7 glass unless specifically defined otherwise.
The only coating that uses magnesium fluoride is the single-layer antireflection coating “AR_400-700_1L”. In the other antireflection coatings, the low-index material is SiO2. Although MgF2 has a slightly lower index and, therefore, gives improved performance, its environmental resistance is inferior to SiO2. An additional consideration is that SiO2 is readily sputtered whereas MgF2 presents great problems in sputtering.
 
OpticStudio File: You will find an example of these antireflection coatings in the file called “AR_Conic interconnect.zar” in the attachments at the top of this article.
That file shows the coupling of two single mode fibers with a conic lens. The front and back surfaces of the lens have an antireflective coating. Each configuration is a different coating.
The merit function gives a comparison of the coupling between the different configurations/coatings.

Antireflection coatings plot
 
  • AR_400-700_1L:

This is the well-known single-layer antireflection coating consisting of a quarter wave of magnesium fluoride. The coating is very simple to apply by thermal evaporation and is very tolerant to errors. Although one of the oldest antireflection coatings it is still produced in huge volumes. It usually has a characteristic magenta color in reflection.

 
  • AR_400-700_3L:

This famous and much used 3-layer coating was first conceived by the Schott company in Jena during WWII and was later described in L.B. Lockhart and P. King, "Three-layered reflection-reducing coatings," Journal of the Optical Society of America 37(9), 689-694, 1947.

 
  • AR_400-700_4L:
AR_400-700_3L uses three materials. The layer closest to the substrate is of intermediate index. Manufacturers prefer to use just two materials wherever possible. In this coating, the intermediate index quarter wave layer is replaced by a two-layer equivalent using the same materials as the other two layers. Slight gentle refinement then completes the design that also has slightly broader performance than the three-material coating.
 
  • AR_400-900_12L:
AR_400-900_12L is a 12-layer antireflection coating for the visible and near infrared.
 

Cold mirror coatings

OpticStudio File: You will find an example of these cold mirror coatings in the file called “Cold_mirror.zar” in the attachments at the top of this article.
The file contains a collimated source ellipse (black body spectrum 6000K 0.4-1.0µm) and a window that is coated. The window is coated on the front face and there are different configurations (coating and angle of the window).
There are two “flux vs wavelength” analyses to compare the reflected and transmitted sides. 
 
  • COLD_MIRROR_0DEG
A cold mirror reflects the visible region and transmits the infrared. This particular cold mirror is designed for angles of incidence at and near normal. The design has 31 layers.


 
  • COLD_MIRROR_45DEG_A
This cold mirror is designed for use at 45° incidence in air. It has 29 layers. The materials used have no significant absorption in the visible region.


 
  • COLD_MIRROR_45DEG_B
A cold mirror for 45° incidence with slightly higher reflectance than COLD_MIRROR_45DEG_A. It uses titania as high index material rather than niobia because titania has slightly higher index. However, titania suffers from absorption at the blue end of the spectrum hence the ripple for p-polarization seen there. The design has 47 layers.
 

 

Beamsplitter coatings

OpticStudio File: You will find an example of these beamsplitter coatings in the file called “Cube_BS.zar” in the attachments at the top of this article.
The file contains a collimated source ellipse (system wavelengths) and a beamsplitter that is coated. The merit function calculates the reflected and transmitted flux.
 
 
  • CUBE_BS_500-600_99L
This 50/50 dielectric (i.e. lossless) beamsplitter has an incident medium and substrate of N-BK7 and is designed for 45° incidence in the incident medium. The component is in the form of a cube with the coating on the diagonal. The problem with such components is the very low contrast in p-polarized layer properties (45° incident is not far from the Brewster angle between the two-layer materials) and with only 99 layers it is difficult to achieve the required reflectance and transmittance and at the same time to eliminate the ripple completely. The range 500nm to 600nm is near the limit of what is reasonably possible.
 

 
  • CUBE_BS_500-600_148L
This is another 50/50 dielectric beamsplitter with N-BK7 as incident medium and substrate. The specification is similar to CUBE_BS_500-600_99L but the greater number of layers, 148 rather than 99, allows better ripple elimination at the expense of more demanding manufacture.
 

 
  • CUBE_BS_460-640_316L
Another 50/50 beam splitter with N-BK7 as incident medium and substrate and 45° angle of incidence, thus a cube beam splitter with coating on the diagonal. The range has been increased to 460nm to 640nm but even with 316 layers there is some residual ripple. Further ripple reduction implies more layers. The manufacture of such a coating would be a major challenge and it is difficult to say if it might succeed.
 

 
  • CUBE_POL_45DEG_21L
A simple 21-layer cube polarizer for the visible region. The design is a quarter wave stack where the interface between the high and low index layers is arranged to be at the Brewster angle so that the p-polarized reflectance is very low.
 

 

Protected and Enhanced Reflectivity coatings

 
OpticStudio File: You will find an example of these protected and enhanced reflectivity coatings in the file called “Mirror_enhanced-reflectivity.zar” in the attachments at the top of this article.
The file contains a collimated source ellipse (blackbody spectrum 6000K) and a mirror that is coated. There are 6 configurations for the different coatings. There is a detector viewer that displays the true color of the reflection.
 
  • PROTECTED_AL
A simple aluminized reflector with a protecting layer of silica designed to give maximum luminous reflectance in a simple coating. The reflectance falls on either side of the visible region.
 
  • ENHANCED_AL

This simple four quarter wave reflectance-enhancing overcoat gives high luminous reflectance at near normal incidence. With more complex designs a wider range of high reflectance is possible.
 

  • PROTECTED_SILVER

In the visible and infrared, silver has the highest reflectance of all metals but offers poor environmental resistance. Here protection is afforded by a two-layer system. The thin Al2O3 layer next to the silver gives good adhesion while the outer SiO2 layer offers good protection. The thicknesses were taken from G. Hass, J.B. Heaney, H. Herzig, J.F. Osantowski, and J.J. Triolo, "Reflectance and durability of Ag mirrors coated with thin layers of Al2O3 plus reactively deposited silicon oxide," Applied Optics 14(11), 2639-2644, 1975, but they can be varied to modify the shape of the reflectance curve.
 


 
  • HR_400-700_81L
The quarter wave stack reflector does not cover the whole of the visible. Here is an 81-layer extended zone coating to give high reflectance over the whole of the visible region.
 
  • HR_405-840_101L
To extend the zone of high reflectance over a region greater than the visible demands still more layers than the 81 of the previous coating. Here we use 101 layers.
 
  • HR_633_21L
This is a simple quarter wave stack of 21 layers to give high reflectance at 632.8nm.
 

 

Near-Infrared Blocker coating

OpticStudio File: You will find an example of this near infrared blocker coating in the file called “IR_blocker.zar” in the attachments at the top of this article.
The file contains a source (black body spectrum 6000K) and a lens. The front face is coated with the Near Infrared blocker coating.
 
  • IR_BLOCKER_45L
There are many applications for a coating that transmits the visible region but blocks the near infrared. This 45-layer filter has better infrared blocking than would be required for a simple heat-reflecting filter.

References

http://thinfilmcenter.com/