2. Reflection from a two-pair Distributed Feedback Reflector (DBR) One way to increase the reflectance from a substrate is to add a Distributed Bragg Reflector (DBR) on top of the substrate. A DBR structure consists of a stack _ pairs of high/low refractive index layers as shown below. layer I 2 34 nig Infinitely air n1 n2 n nz Ro extending substrate nin 4 4 4 4 air n1= 3.5, n2 = 3.0, ns = 3.6 nin R 27 air n n2 ns ning R 37 air n n n 2 nin air The structure is designed to enhance reflectance at 1, = 900 nm in free space. (a) Find the thickness for layer 1, 2, 3, and 4.(b) Calculate the Reflectance R1, R2, R3, R4, and Rs using the refractive index transformation method ni Cos(BI) + jn,Sin(BI) nin (1, A) = no . (hint: calculate nin and then r for each structure) no Cos(BI) + jn, Sin(BI) (c) Repeat part (b) using Dynamic Matrix method