Introduce material with the loss or gain in Transient model comsol 4.2

Hi,
I am working at transient model for optical application(wavelength = 1.55um) with loss or gain material and also nonlinear material.
I know that I can put loss/gain into my material by introducing refractive index and put in Im(n) in comsol a minus number which works as loss.
1) but, my problem is that I put loss or gain, in both case the amplitude of light go to infinite value in my loss/gain marerial??? On the other hand, Comsol can not realize that Im(n) is loss (-) or gain(+)?

2)In addition, when I increase my loss value to bigger value, the comsol doesn't show any change in transition(output/input)?

3)another question is that about step time? why do I decrease my step time from a special value, my simulation will show warning? it is strange for me because I anticipate my result will be better?

Some useful information :
cross section of my structure : 50 um^2
3 type of material are used in structure
mesh size : extremely finer
time step: 1[fs]

Thanks in advance
Hadi