  As a part of my doctoral thesis, I studied the effect of phase fluctuations
in two dimensional (2D) and three dimensional (3D) superconducting films of
conventional s-wave superconductor, NbN, by measuring the electrodynamics
response through magnetic penetration depth at low frequency and the complex
conductivity at microwave frequencies. We show that the phase transition in 2D
or quasi-2D superconductors can be reconciled with the standard
Berezinskii-Kosterlitz-Thouless (BKT) transition when the small vortex core
energy is taken into consideration. In 3D NbN films, we show that in presence
of strong disorder, superconductivity can get destroyed by phase fluctuations
even if Cooper pairs continue to survive in the system. Our study provides a
paradigmatic picture of importance of phase fluctuations in 2D and strongly
disordered 3D superconductors.
