can the imaginary part of complex permittivity be negative?

[mtxx]

The complex permittivity has a real and imaginary part.
Er = Er' + i Er"

the real part is usually referred as the dielectric constant, this must be a positive value,

but the imaginary part, is it always positive as well?

it should be right?
since Q, the quality factor should be a positive value as well.

 

[hshah8970]

For mediums that exhibit energy loss for the signal, the imaginary permittivity is positive.
For mediums that have energy gain, imaginary permittivity is negative.