Mode structure of the radiation emitted through high-gain parametric down-conversion
Quantumness and brightness of light are not usually found together in today’s optical quantum information, communication or measurement technologies. Yet, the last two decades saw an increased interest in the study of bright sources of nonclassical light where lossless control of their spatio-temporal spectrum promises to improve the capabilities of existing applications or to create novel ones. One of the most popular light sources for quantum optics is parametric down-conversion (PDC), a nonlinear process that occurs inside certain dielectrics where non-classical radiation originates from the parametric amplification of vacuum fluctuations. Even though the PDC process is known for almost six decades, the quantum aspects of PDC have mostly been studied at low pump power regimes. In this case, much less than one photon per mode is spontaneously produced. At strong pump power regimes, where several photons per mode are produced, PDC becomes a source of a multiphoton non-classical state of light known as bright squeezed vacuum (BSV). BSV is recognized as a good candidate for efficient light-light and light-matter interactions, multichannel quantum communications, parallel information processing, high-resolution metrology and imaging, among other applications. This thesis addresses one out of several unexplored questions regarding BSV, namely, what is its spatial modal structure? To answer this question, orthonormal modes dictated by the spatial coherence of the radiation (or so-called Schmidt modes) which also account for photon number correlations, have been considered. The Schmidt-mode formalism has been successfully used in the description of the radiation generated by low-gain PDC but their usage in the BSV case was challenging. This thesis presents several experiments that validated the results of an analytical theory for BSV based on Schmidt modes. According to the theory, the modes describing BSV, to a good approximation, are invariant to the pump power used in the process, while the photon population of each mode is not. The observed changes in the spatial intensity spectrum, spatial photon number correlations and the effects of the spatial anisotropy on the spectrum shape corroborated this prediction. Additionally, different methods for engineering the spatial spectrum of BSV were proposed and implemented by using unseeded and strongly pumped traveling-wave optical parametric amplifiers (OPAs) made of bulk nonlinear crystals, where the radiation produced is highly multimode in several degrees of freedom. For instance, spatial walk-off was exploited to obtain tunable, bright, narrowband and diffraction-limited twin beams through the amplification of the radiation in the direction of the pump Poynting vector. Alternatively, tailoring of the BSV state was achieved through the amplification of the radiation produced in one unseeded traveling-wave OPA by the presence of a second OPA, up to the generation of a single spatial mode. The modal content of the output light was further studied in terms of radial and orbital angular momentum modes. Finally, as an alternative to generation, lossless projective filtering of a single spatial BSV mode by means of a single-mode fiber was performed. The examples given in this thesis on the lossless control of the BSV spatial spectrum are extensible to the temporal domain, as proved in subsequent works. Since the elucidation of the mode structure of BSV radiation is a requirement for further development of reliable light tailoring strategies that preserve BSV nonclassical properties, this thesis is a direct contribution to the know-how that will allow full involvement of BSV in quantum technologies in the near future.