Equation Icon  ( 2 ) ( 2 2 ) --r2t1exp-(− i2kD-)- r1 −-r2(r1-+-t1)exp(−-i2kD-)- a4 = a0 r1 − 1 − r1r2exp (− i2kD ) = a0 1 − r1r2exp (− i2kD ) , (7 )