Abnormal focal length increase of terajet generated in reflection mode

We report an interesting and abnormal electromagnetic phenomenon with regard to a terajet (TJ) that is generated in a reflection mode, which is realized by placing a dielectric scatterer onto a metal reflection plate. We show that the introduction of an air hollow into metal reflection plate beneath the scatterer does not induce an expected decrease but an abnormal increase of focal length of the TJ by as much as more than three times. This abnormal phenomenon takes place in case that the air hollow is shallow and there exists a critical hollow depth for a given lateral size of air hollow. Larger than the critical depth, the phenomenon no longer occurs. It is explained from viewpoints of both ray optics in terms of role of relative portion of central waves in TJ formation and electromagnetic field theory with regard to hollow-induced phase singularities.