Noncausal time response in frustrated total internal reflection?

Tunneling of photons in frustrated total internal reflection has been studi ed in the time domain with single-cycle femtosecond pulses. It is seen that both the phase and energy of the pulse travel faster than the speed of lig ht in vacuum. Theoretical analysis of the experiments shows that the time-r esponse function for electromagnetic waves propagating in the air gap is no ncausal. However, it is found that superluminal signal propagation is not p ossible in this case because Of the inevitable diffractive spreading of the signal beam.