SEDL / STP / STP689-EB / STP39126S

Simplified Description of Dielectric Reflectors

Sparks, M.
Xonics, Incorporated Santa Monica, California

Flannery, M.
Xonics, Incorporated Santa Monica, California

Pages: 10    Published: Jan 1979

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Abstract

The spectral reflectance, R(ω), of a quarter-wave reflector is quite asymmetric in general as a result of absorption. The maximum electric field E in an infinite stack is in the center of the high-index (low-index) layer at the low-frequency (high-frequency) end of the high-reflection band. Thus, the absorptance minimum of a stack having much greater absorption in the high-index material occurs at ω> ω^c (band center). Operation at ω> ω^c could increase the damage resistance if the high-index material is easily damaged. At ω^c, E decays rapidly. As ω departs from ω^c in an infinite reflector, E penetrates deeper into the coating, with no decay at the band edges. The peak-to-peak distance in an infinite dielectric stack is constant for all frequencies in the band. By using the continuity of E and dE/dz, E can be obtained from E= 0 at the metallic substrate or, in an infinite stack, from E=0 at the reflector surface for ω = ω^C or E=0 in the center of the high- or lowindex layer at the band edges. Simple closed-form approximations for the spectral absorptance and the phase of E are accurate and sufficient for present applications.