La translocation de biopolymères à travers un nanopore intervient dans de nombreux processus biologiques et technologiques, comme le transport nucléocytoplasmique dans le pore nucléaire des cellules eucaryotes, la sécrétion de protéines, le séquençage rapide de l’ADN ou l’électrophorèse capillaire.Nous proposons une technique optique en molécule unique originale pour l’étude de la translocation de biopolymères à travers un nanopore basée sur l’effet Zero-Mode Waveguide. Nous nous sommes intéressés au passage d’ADN double-brin de plusieurs tailles, d’ADN simple-brin et d’ARN, entraînés par un flux à travers une membrane nanoporeuse track-etched. Nous montrons qu’il existe un flux critique régissant le passage des biopolymères indépendant du rayon des pores ainsi que de la taille des biopolymères et de leur nature, conformément aux prédictions théoriques de Brochard et de Gennes.Le pore nucléaire est un nanopore biologique responsable du transport sélectif entre le noyau et lecytoplasme des cellules. Nous avons étudié l’influence de la concentration en importinBeta1 – une protéine nécessaire au transport nucléocytoplasmique – sur l’organisation du canal central du pore nucléaire deXenopus laevis en mesurant la diffusion de molécules de Dextran fluorescentes à travers celui-ci. Nous observons une ouverture du canal central à basse concentration suivi d’un rétrécissement de celui-ci à plus forte concentration. Cette évolution du rayon du canal central avec la concentration en importin Beta1est conforme aux modèles en champ moyen de Opferman et coll. et de Ando et coll. et aux observations expérimentales sur des systèmes reconstitués in vitro de Lim et coll. et Zahn et coll. / The translocation of biopolymers through a nanopore is a feature common to many biological andtechnological processes such as the nucleocytoplasmic transport through the nuclear pore complex(NPC), protein secretion, fast DNA sequencing or capillary electrophoresis.We have developed an original single molecule optical detection technique for the study of biopolymerstranslocation through a nanopore based on the Zero-Mode Waveguide effect. We studied thepassage of double stranded DNA of different sizes, of single stranded DNA and of double-stranded RNAdriven by a flux through track-etched nanoporous membranes. We demonstrate that translocation isgoverned by a critical flux independent of both biopolymer size and nature and of the pore radius inagreement with the theoretical predictions of Brochard and de Gennes.The NPC is a biological nanopore responsible for the selective transport between cytoplasm andnucleus in cells. We studied the influence of importinBeta1 concentration – a protein involved in the nucleocytoplasmictransport – on the structure of the central channel of the NPC of Xenopus laevis byassessing the diffusion of fluorescently labeled Dextran molecules through the NPC. We observe anopening of the central channel at low concentration followed by a shrinking at higher concentrationin importinBeta1 in agreement with mean-field models from Opferman et al. and Ando et al. and withexperiments on biomimetic in vitro systems from Lim et al. and Zahn et al.

Dominated by Heisenberg's uncertainty principle, vacuum is not quantum mechanically an empty void, i.e. virtual pairs of particles appear and disappear persistently. This nonlinearity subsequently provokes a number of phenomena which can only be practically observed by going to a high-intensity regime. Pair production beyond the so-called Sauter-Schwinger limit, which is roughly the field intensity threshold for pairs to show up copiously, is such a nonlinear vacuum phenomenon. From the viewpoint of Dirac's front form of Hamiltonian dynamics, however, vacuum turns out to be trivial. This triviality would suggest that Schwinger pair production is not possible. Of course, this is only up to zero modes. While the instant form of relativistic dynamics has already been at least theoretically well-played out, the way is still open for investigating the front form. The aim of this thesis is to explore the properties of such a contradictory aspect of quantum vacuum in two different forms of relativistic dynamics and hence to investigate the possibility of finding a way to resolve this ambiguity. This exercise is largely based on the application of field quantization to light-front dynamics. In this regard, some concepts within strong field theory and light-front quantization which are fundamental to our survey have been introduced, the order of magnitude of a few important quantum electrodynamical quantities have been fixed and the basic information on a small number of nonlinear vacuum phenomena has been identified. Light-front quantization of simple bosonic and fermionic systems, in particular, the light-front quantization of a fermion in a background electromagnetic field in (1+1) dimensions is given. The light-front vacuum appears to be trivial also in this particular case. Amongst all suggested methods to resolve the aforementioned ambiguity, the discrete light-cone quantization (DLCQ) method is applied to the Dirac equation in (1+1) dimensions. Furthermore, the Tomaras-Tsamis-Woodard (TTW) solution, which expresses a method to resolve the zero-mode issue, is also revisited. Finally, the path integral formulation of quantum mechanics is discussed and, as an alternative to TTW solution, it is proposed that the worldline approach in the light-front framework may shed light on different aspects of the TTW solution and give a clearer picture of the light-front vacuum and the pair production phenomenon on the light-front.