Spontaneous emission in photonic crystals

[fisico30]

Hello forum,
I have a question about photonic crystals.

From Wikipedia: "photons (behaving as waves) propagate through this structure - or not - depending on their wavelength. Wavelengths of light that are allowed to travel are known as modes, and groups of allowed modes form bands. Disallowed bands of wavelengths are called photonic band gaps. This gives rise to distinct optical phenomena such as inhibition of spontaneous emission..."

I am not clear on the last line, "This gives rise to distinct optical phenomena such as inhibition of spontaneous emission..."

Why is spontaneous emission inhibited? Is it really? Where, on those dielectric regions where light does not propagate? But why? Isn't the absence of propagation pure due to destructive interference? Atoms still absorb and emit...

thanks
fisico30

 

[Andy Resnick]

Photonic bandgap materials are often described analogously to direct-gap semiconductors: similar dispersion relations, etc. The Wiki item may refer to the suppression of allowed modes within the bandgap- light of frequencies (energies) less than the bandgap can't propagate through the material. It's important to realize that (AFAIK) the majority of real photonic bandgap materials don't have a bandgap in all directions- the only one I know of was made to work in the millimeter-wave region.

 

[chrisbaird]

Spontaneous emission is typically suppressed, but not totally inhibited at frequencies in the band gap. Yes, the photonic band-gap effect can be thought of as destructive interference, or better yet as a resonant cavity effect. A resonant cavity can only support certain wavelengths. The spontaneous emission of light by atoms is also a resonance effect, so the PBG resonance effect interferes with the atom's resonance effects and it literally suppresses certain atomic oscillations. Remember that in the quantum field theory world, photon emission is not a classical electron ball throwing off a classical photon ball, but rather an interaction of an electric charge with the quantum electromagnetic field.