Progettazione, fabbricazione e caratterizzazione di un interferometro non lineare integrato

Quantum description of light offers a lot of new possibilities in physics and engineering: it allows one to understand physical phenomena that can't be described through a classical theory and promises a revolution in communication, computation, simulation, and metrology. However, not all the outcomes of a “quantum” experiment depend on the non-classical nature of the radiation. Indeed, in some cases a classical analogue of a given quantum phenomenon could be found, and similar outcomes can be obtained also using classical light. In 2014, Lemos et al. proposed a “quantum” experiment aiming at measuring the interaction of light with a sample without detecting the beam directly involved in the process. The system is based on Spontaneous Parametric Down Conversion (SPDC), which is a nonlinear process that occurs in materials with second-order susceptibility. In this process, a pump photon interacts with the nonlinear material, and it splits spontaneously in two photons (signal and idler) such that the sum of their frequencies is equal to the pump frequency. In the work, two nonlinear sources (ppKTP crystals) are pumped; the optical configuration makes only the idler photons generated by the first crystal to interact with the object, while only signal photons from both sources are detected. Since the generations of signal beams are indistinguishable events (due to the specific design proposed by the authors), the probability of detecting a photon results from a quantum interference which depends on a phase-shift induced by the sample. The main advantage of this interferometric technique is the possibility to measure some properties of a sample by detecting photons that don't interact with it. This can be very useful for imaging or sensing applications, since with this approach one can use a low-frequency beam for the interaction and a high-frequency beam for the detection. Even if this experiment can be explained only in the framework of a quantum theory of light, a classical analogue was proposed by Shapiro in 2015. Basically, it can be shown that the same effect of sensing with undetected photons is achievable using a stimulated nonlinear process, which can be described classically. The aim of this thesis is to prove it experimentally in an integrated nonlinear interferometer. First, I designed the integrated optical circuit. The basic idea is to realise the same architecture proposed by Lemos et al. in an integrated photonic chip based on SOI technology (micrometric-sized Silicon waveguides in a Silica matrix). This platform is promising for several applications, since it is CMOS compatible, and Silicon is a material with a strong third-order nonlinearity. In addition, the small cross-section of SOI-based waveguides (220 nm x 500 nm) further enhances the nonlinearity. Our approach makes use of Four Wave Mixing (FWM), in which a signal is generated by the interaction of a pump beam and an idler beam. During the design process, each bulk optical element of the original experiment has been substituted with integrated structures: nonlinear crystals, dichroic mirrors, and beam splitters are replaced with spirals, microring resonators, and Multi-Mode Interference (MMI) couplers, respectively. Some custom software have been used to simulate and design the integrated optical circuit. In the second part of the project I performed the characterization of the device and realised the experiment. First, I used a tunable laser to test the spectral response of every optical components of the circuit. As in the work by Lemos et al., the experiment consists in the detection of a phase-shift by measuring the signals generated through FWM, and the perturbation is induced by heating a portion of the interferometric structure by a custom external heater which was built during the project.