Optical nonlinear effects and photon pair production in modulated materials

Abstract

According to quantum field theory an oscillation of boundary conditions in the vacuum can generate real photon pairs, which pop into existence from the zero-point energy. This surprising effect is known as the dynamical Casimir effect (DCE). In this thesis, we focus our attention on using the DCE as a route to experimentally study the quantum vacuum. Initially we explore experimental approaches that rely on an oscillation of vacuum boundary conditions. These require a direct temporal modulation of the boundary condition, which we achieve by temporally modulating the refractive index through the second order nonlinearity of the medium. We show that the key condition for this approach is that the medium is subwavelength in thickness. This leads to a temporal modulation of the refractive index. We use a mechanically exfoliated MoS2 monolayer. We present a detailed study of the second order nonlinearity of 2D (mono-atomic layer) dichalcogenide MoS2, and test its potential for spontaneous parametric down-conversion (SPDC) i.e. amplification of vacuum fluctuations mediated by optical nonlinearity. We present a model of SPDC in MoS2 monolayers and show that our data are compatible with theoretical estimates. We show exciting indications that SPDC may be possible in this material by performing polarization and lifetime measurements. We also discuss and characterise a new photo-luminescence emission around 1500 nm which is enhanced at the edges of multi-layer MoS2. In chapter three we present experimental approaches using spatially modulated third order nonlinear media along the propagation axis. We show that under certain conditions the spatial modulation of the boundary conditions along the nonliner media can be considered as a temporal modulation. Modulated waveguides provide an intensity variation of the pulse along the waveguide. Two different materials are used as substrate material for the waveguides; gallium lanthanum sulphide (GLS), which has a high third order nonlinear susceptibility and fused silica. We present numerical simulation results for photon generation from the modulated waveguides in GLS sample. We also characterise the nonlinear response and effects of the waveguides. In both substrates, we show that the lack of nonlinearity and different sources of losses prevents the creation of photon pairs. Finally, we present an experimental method in which the group velocity dispersion (GVD) is modulated along a photonic crystal fiber. Measurements of the temporal correlations between the newly generated frequencies is presented with a coincident to accidental ratio (CAR) of 7.5: yielding proof of a quantum correlation between the generated photon pairs.