Purcell effect

One way of seeing why the Purcell effect arises is by using cavity quantum electrodynamics.[3] Fermi's golden rule dictates that the transition rate for the atom-vacuum (or atom-cavity) system is proportional to the density of final states. In a cavity at resonance, the density of final states is enhanced (though the number of final states may not be). The Purcell factor is then just the ratio of the cavity

${\displaystyle \rho _{\rm {c}}={\frac {1}{V\Delta \nu }}}$

to that of the free space density of states[4]

${\displaystyle \rho _{\rm {f}}={\frac {8\pi n^{3}\nu ^{2}}{c^{3}}}}$

Using

${\displaystyle Q={\frac {\nu }{\Delta \nu }}}$

we get that

${\displaystyle {\frac {\rho _{\rm {c}}}{\rho _{\rm {f}}}}={\frac {c^{3}}{8\pi n^{3}\nu ^{2}}}{\frac {Q}{\nu V}}={\frac {1}{8\pi }}\left({\frac {\lambda _{\rm {free}}}{n}}\right)^{3}\left({\frac {Q}{V}}\right)}$

which is correct up to a constant.

It has been predicted theoretically [5][6] that a 'photonic' material environment can control the rate of radiative recombination of an embedded light source. A main research goal is the achievement of a material with a complete photonic bandgap: a range of frequencies in which no electromagnetic modes exist and all propagation directions are forbidden. At the frequencies of the photonic bandgap, spontaneous emission of light is completely inhibited. Fabrication of a material with a complete photonic bandgap is a huge scientific challenge. For this reason photonic materials are being extensively studied. Many different kinds of systems in which the rate of spontaneous emission is modified by the environment are reported, including cavities, two,[7] [8] and three-dimensional[9] photonic bandgap materials.

The Purcell effect can also be useful for modeling single-photon sources for quantum cryptography.[10] Controlling the rate of spontaneous emission and thus raising the photon generation efficiency is a key requirement for quantum dot based single-photon sources.[11]