Periodic structures are the building blocks of laser mirrors, bandpass filters, rejection filters and edge filters. The concept of such a structure furnishes insight into the functioning of such multilayers. An example is a stack of 20 layers airH1L1H2L2H3L3H3L4H5L5 â¦ H10L10substrate where each layer has a different thickness and possibly a different refractive index.
Although the digital computer can readily compute its reflectance, it is difficult to gain insight into how that multilayer functions because of the overwhelming number of variables. If, however, the coating contains only two types of layers H, LairH L H L H L H L H L H L H L H L H L H Lsubstrate, which may be written air (H L)10substrate, then a good idea of the performance of the 20 layer stack is obtained by analyzing the properties of the H L basic period iterated 10 times. An additional advantage of treating this as a periodic structure is that efficient algorithms are used to compute its reflectance with improved computational efficiency (see Â§5.8.1).
Figure 5-1 shows the results of adding HL pairs to a stack. Each layer has a quarterwave optical thickness. Starting with a single H layer, the maximum reflectance is 31%. The addition of an HL pair to form HLH increases the maximum reflectance to 63%. Augmenting the stack with still another HL pair to produce HLHLH boosts the maximum reflectance to 85%.