Slussarenko, Sergei (2010) Novel tools for manipulating the photon orbital angular momentum and their application to classical and quantum optics. [Tesi di dottorato] (Unpublished)

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In recent times, optical beams with spiraling wave fronts carrying definite value of Orbital Angular Momentum (OAM) have found a number of applications in several fields of classical and quantum optics. However, the inherent difficulty in manipulating the OAM of laser beams has prevented since now a wide spread of techniques based on photon OAM. A novel device, invented in 2006 by our group able to generate OAM-carrying beams of defined order, made handling the light OAM so easy as handling the light polarization, giving a renewed boost to the worldwide research on the light OAM. The device, called "q-plate" (QP) is essentially a birefringent liquid crystal cell whose optical axis is locally inhomogeneous in the transverse plane and follows a pattern, defined by the topological charge. When a light beams passes through the QP the topological charge is transferred into the optical phase endowing the beam with a definite value of OAM. The working mechanism of the QP is the conversion of the spin of each photon in the beam into OAM (Spint-to-Orbit Conversion, STOC process). The QP has a large number of advantages compared to other well-known OAM generation techniques, in terms of efficiency, production costs, versatility etc. In this thesis, I studied QP manufacturing procedures, implementing the rapidly-developing photoalignment technique to create QPs with integer and semi-integer values of the topological charge (which is restricted to unit charge in case of standard rubbing technique). I have demonstrated experimentally the possibility to control the QP by external electric field, reaching the SAM-OAM conversion efficiencies higher that 97%. Using the QP I was able to generate arbitrary Hermite-Gaussian as well as Laguerre-Gaussian modes by controlling the polarization of the input light. Such method, together with the available fast electro-optical switching allows a high-speed beam mode control, up to rates of MHz. I have also found a novel way to encode and read two bits of information in the 4D OAM space by using only one QP and I have introduced a novel interferometric setup, that by itself and together with the QP can perform a wide range of quantum logic operations in the spinorbit space of single photon. Finally, in the field of quantum optics using QP, a new type of SAM-OAM entanglement, called "hybrid" was demonstrated and tested by violation of Bell's inequalities and an adjustable quantum gate, based on QP technology, called Universal Unitary Gate, was proposed that can perform arbitrary operation in the photon spinorbit space.

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