Here, xa is the resonant frequency of the device, c0 is the intrinsic
loss rate of the device, and c1 is the coupling rate of the device
with the TM wave. The equation shows that at resonant frequency
(i.e., x = xa), complete absorption is possible (i.e., A = 100%) if
c0 = c1 is fulfilled, and this is the so called critical coupling condition.
It is worth noting that the metal back reflector in Fig. 1 is critical
for achieving complete absorption. If there is no such reflector,
the form of Eq. (1) should be modified and the maximum absorption
rate is no longer 100% but 50% [15].