  We study spectral singularities of an infinite planar slab of homogeneous
optically active material in focus of a thin lens under illumination of a
Gaussian beam. We describe the field distribution of the Gaussian beam under
this configuration as a plane wave propagated near the optical axis which its
phase and amplitude vary with distance from center of the beam. Based on this
approximation, we carry out the transfer matrix for the slab. We explore the
consequences of this configuration on determining the threshold gain of the
active medium and tuning the resonance frequencies related to spectral
singularities. We show that the spectral singularities and the threshold gain
besides that vary with distance from center of the Gaussian beam, also they
change with relative aperture of the focusing lens. As a result, using a thin
lens with higher relative aperture, the spectral singularities corresponding
resonances shift to the higher frequencies (lower wavelengths). Numerical
results confirm the theoretical findings
