Experimental and theoretical investigations of optical absorption and emission spectra of alkali-halides doped with Tl⁺-like impurities are reviewed. The Jahn—Teller effect plays an important role. An extensive interpretation of the complex phenomena is presented and critically assessed.

The possibility of observing superluminal behavior in the propagation of localized microwaves over distances of tens of wavelengths is experimentally demonstrated. These types of waves, better than the evanescent modes of tunneling, can contribute to answering the question on the luminal limit of the signal velocity.

Measures of pulse delay in microwave propagation, in open air and for short distances (not much greater than 1 m), were made by using launcher and receiver horns. When these are facing each other we observe a delay time corresponding to a speed equal to c while, if the receiver horn is shifted or tilted with respect to the launcher horn, the delay time decreases showing a superluminal behavior. In other words the modulation phase shift, interpreted as a propagation time, turns out to be surprisingly smaller than the one relative to the light speed. This effect, which disappears for longer distances, is here interpreted on the basis of the existence of a special kind of evanescent waves (leaky waves). Just the presence of evanescent waves allows one to make a comparison with the tunneling processes where superluminal transport properties have been theoretically predicted.

Delay-time measurements with a microwave setup, where a step narrowing in the waveguide simulates a quantum mechanical potential barrier, have been performed even beyond the cutoff. This allows experimental measurements in a very accessible temporal range of the order of nanoseconds. The results, interpreted as tunneling times, are compared with the existing quantum mechanical models, translated into the electromagnetic framework.

Delay times relative to a beat envelope are deduced from a path-integral solution of the telegrapher equation, analytically continued to imaginary time when the square of the effective velocity becomes negative. The results are approximately described by a semiclassical model translated in frequency from the nominal to an effective cutoff, thus improving the agreement with experimental results.