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Superconducting Tunnel Junctions (STJ) are promising devices as a next generation of nuclear and optical detectors. Non-equilibrium experiments can provide useful information on the behaviour of a perturbed superconductor. Among possible nonequilibrium sources, we used quasi-particle injection in three-terminal double tunnel junction devices and a laser pumping source. We investigated the effect of perturbed conditions on sub-harmonic gap structures, which are present in I-V characteristics of high quality Nb-based STJs. Experimental results concerning the change of the energy gap measured at V=2/spl Delta//2e show features which can not be completely explained in terms of simple heating effects.