EQMResearch group
Level 4 · Light in layered structures

Distributed Bragg reflector (DBR)

(SiO₂ / SiN)ₙ

Many thin layers tuned so all their reflections add up. The result is a near-perfect mirror for one chosen color.

Build on:Thin-film interference,Transfer-matrix method (TMM)

A mirror you can design

A distributed Bragg reflector (DBR) is just many thin films stacked with alternating high and low refractive indices, each one exactly λ_c / 4 thick. At the design wavelength λ_c all the tiny reflections add up coherently. Outside that band they interfere away.

More pairs, sharper stopband

With one or two pairs you barely have a mirror. With ten pairs you have a near-perfect reflector. The simulator uses N = 8.

λc = 940 nmWavelength λ (nm)R

Each pair adds another little reflection at λc. Stack enough pairs and the in-band reflectance sails toward 100%, with characteristic side-lobes flanking the stopband. The simulator ships an N = 8 DBR centered at 940 nm.

Slide the number of pairs and watch the stopband (the high-R region around λc) grow taller and steeper.
Key takeaways
  • DBR = repeated quarter-wave high/low layers.
  • Reflects nearly 100 % in a narrow band around λc; transparent elsewhere.
  • Combined with a top metal mirror it forms the experiment's microcavity.
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