Universal photonic tunneling time.

We consider photonic tunneling through evanescent regions and obtain general analytic expressions for the transit (phase) time tau (in the opaque barrier limit) in order to study the recently proposed "universality" property according to which tau is given by the reciprocal of the photon frequency. We consider different physical phenomena (corresponding to performed experiments) and show that such a property is only an approximation. In particular, we find that the "correction" factor is a constant term for total internal reflection and quarter-wave photonic band gap, while it is frequency dependent in the case of an undersized waveguide and distributed Bragg reflector. The comparison of our predictions with the experimental results shows quite good agreement with observations and reveals the range of applicability of the approximate "universality" property.