This book is focused on structural phase transformation mechanism at the interface of Gallium and Silica. The nonlinear response of the film has been studied by solving a NLSE at the interface by the method of phase plane analysis. Also, the Maxwell-Garnett Effective Medium Theory has been used for calculating dielectric constants for various phases of gallium. Moreover, the results obtained have been implemented for investigating the nonlinear response of carbon nanotubes. The light induced conversion of ?-gallium to a new, more metallic, more reflective phase give rise to a big nonlinearity which can be applied to perform passive Q-switching of fiber-lasers. The, film of SWNT deposited on a dielectric substrate give rise to the nonlinear response at the interface which can be applied to fast switching devices that demand for large optical nonlinearity.