Cold Mirror

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A cold mirror reflects short wavelengths and transmits long wavelengths. The are many applications for this type of coating. In this example, we require that the coating consist of layers of SiO2 and TiO2 on a glass substrate and have the following performance at normal incidence:

> 90% reflectance for 380-750 nm

< 10% reflectance for 800-1000 nm

In TFCalc it is very easy to enter these requirements using continuous targets. We will use TFCalc's needle/tunneling optimization to design the coating from "scratch". That is, we will begin the design process with a single thin layer of TiO2 and allow TFCalc to increase the thickness and number of layers in the design. TFCalc finds a design in a few minutes. After we remove two thin layers and reoptimize the design, an acceptable 21-layer design is found:

Plot of cold mirror design

The prime difficulty with this type of coating is the design of the reflector. After an acceptable reflector has been designed, it is usually easy to extend the transmitting region. For instance, the plot below shows the 21-layer design extended to 2000 nm.

Plot of cold mirror design

Here are the designs, starting with the layer closest to the substrate, and with thicknesses given in nm:

TIO2      44.21     33.76
SIO2     102.57    124.04
TIO2      43.97     28.39
SIO2      73.13     66.36
TIO2      42.56     51.05
SIO2      82.52     86.41
TIO2      48.92     51.97
SIO2      87.18     88.77
TIO2      46.48     39.55
SIO2      66.56     70.41
TIO2      36.16     36.85
SIO2     162.54    162.37
TIO2      53.07     53.15
SIO2      97.19    102.08
TIO2      79.13     81.48
SIO2     102.19     99.64
TIO2      67.77     61.86
SIO2     128.87    137.38
TIO2      53.24     51.30
SIO2     158.61    191.13
TIO2      34.40     19.03

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