Waves fail to propagate in random media. First predicted for quantum particles in the presence of a disordered potential, Anderson localization has been observed also in classical acoustics, electromagnetism and optics. Here, for the first time, we report the observation of Anderson localization of pairs of entangled photons in a two-particle discrete quantum walk affected by position dependent disorder. A quantum walk on a disordered lattice is realized by an integrated array of interferometers fabricated in glass by femtosecond laser writing. A novel technique is used to introduce a controlled phase shift into each unit mesh of the network. Polarization entanglement is exploited to simulate the different symmetries of the two-walker system. We are thus able to experimentally investigate the genuine effect of (bosonic and fermionic) statistics in the absence of interaction between the particles. We will show how different types of randomness and the symmetry of the wave-function affect the localization of the entangled walkers.In 1958 P.W. Anderson [1] predicted that the wavefunction of a quantum particle can be localized in the presence of a static disordered potential. As a consequence of this mechanism it is expected that particle and energy transport through a disordered medium should be strongly suppressed and that an initially localized wave packet should not spread out with time. After more than fifty years from its discovery Anderson localization is still widely studied and it has pervaded many different areas of physics ranging from condensed matter and cold atomic physics to wave dynamics and quantum chaos [2]. This phenomenon emerges quite generically in the behavior of waves in complex media, and it has been experimentally observed in a variety of different systems: BoseEinsten condensates [3,4] Anderson localization is a single-particle process which arises from the destructive interference among different scattering paths. Nevertheless, even in the absence of a direct interaction between particles, pure quantum correlations [13] are expected to influence in a non-trivial way the underlying localization dynamics [14][15][16][17]. By taking advantage of the perfect phase stability provided by miniaturized integrated waveguide circuits [18], we experimentally simulate a quantum walk of a two-photon polarization-entangled state in a disordered medium. We are thus able, through a mapping derived in Ref. [14], to test the localization of a pair of non interacting particles obeying bosonic/fermionic statistics [19].A quantum walk (QW) [20] is an extension of the classical random walk, where the walker goes back and forth along a line and the direction at each step depends on the result of a fair coin flip. At the quantum level, interference and superposition phenomena lead to a non-classical behavior of the walker giving rise to new interesting effects, which can be harnessed to exponentially speed up search algorithms [21] and to realize universal quantum computation [22]. Besides, QWs have also been pr...
Deterministic fractal antennas are employed to realize multimodal plasmonic devices. Such structures show strongly enhanced localized electromagnetic fields typically in the infrared range with a hierarchical spatial distribution. Realization of engineered fractal antennas operating in the optical regime would enable nanoplasmonic platforms for applications, such as energy harvesting, light sensing, and bio/chemical detection. Here, we introduce a novel plasmonic multiband metamaterial based on the Sierpinski carpet (SC) space-filling fractal, having a tunable and polarizationindependent optical response, which exhibits multiple resonances from the visible to midinfrared range. We investigate gold SCs fabricated by electron-beam lithography on CaF 2 and Si/SiO 2 substrates.Furthermore, we demonstrate that such resonances originate from diffraction-mediated localized surface plasmons, which can be tailored in deterministic fashion by tuning the shape, size, and position of the fractal elements. Moreover, our findings illustrate that SCs with high order of complexity present a strong and hierarchically distributed electromagnetic near-field of the plasmonic modes. Therefore, engineered plasmonic SCs provide an efficient strategy for the realization of compact active devices with a strong and broadband spectral response in the visible/mid-infrared range. We take advantage of such a technology by carrying out surface enhanced Raman spectroscopy (SERS) on Brilliant Cresyl Blue molecules deposited onto plasmonic SCs. We achieve a broadband SERS enhancement factor up to 10 4 , thereby providing a proof-of-concept application for chemical diagnostics.
A hierarchical structure is an assembly with a multi-scale morphology and with a large and accessible surface area. Recent advances in nanomaterial science have made increasingly possible the design of hierarchical surfaces with specific and tunable properties. Here, we report the fractal analysis of hierarchical single-walled carbon nanotube (SWCNT) films realized by a simple, rapid, reproducible, and inexpensive filtration process from an aqueous dispersion, then deposited by drytransfer printing method on several substrates, at room temperature. Furthermore, by varying the thickness of carbon nanotube random networks, it is possible tailoring their wettability due to capillary phenomena in the porous films. Moreover, in order to describe the wetting properties of such surfaces, we introduce a two-dimensional extension of the Wenzel-Cassie-Baxter theory. The hierarchical surface roughness of SWCNT coatings coupled with their exceptional and tunable optical and electrical properties provide an ideal hydrophobic composite surface for a new class of optoelectronic and nanofluidic devices.
Multi-walled carbon nanotube (MWCNT) films form efficient heterojunction solar cells with n-type crystalline silicon (n-Si), due to their superior optical and electrical properties. Here, we report air-stable photovoltaic devices with record photoconversion efficiency of 10%. We realized thin films consisting of MWCNTs arranged in semitransparent random networks deposited on n-Si substrates by a simple, rapid, reproducible, and inexpensive vacuum filtration process at room temperature. Such heterojunctions favor high and broadband carrier photogeneration, extending the Si spectral response from near infrared to near ultraviolet range; charge dissociation of ultrafast hot carriers [1]; transport of electrons through n-Si and high-mobility [2] holes through the MWCNT percolative network. Furthermore, by varying the MWCNT film thickness, it is possible tailoring its optical and electrical properties, therefore the overall device optoelectronic features.These results not only pave the way for low-cost, efficient, and broadband photovoltaics, but also are promising for the development of MWCNT-based optoelectronic applications. (Francesco De Nicola) URL: 0039 0672594532 (Francesco De Nicola) ultrafast exciton/charge transfer (1-10 ps) [1, 6-11]; extraordinary high mobility (10 5 cm 2 /Vs) [2]; simple, rapid, and inexpensive solution-processability [12]. Furthermore, the unique one-dimensional structure of CNTs and the inherent properties of the graphitic sp 2 carbon lattice allow the fabrication of photovoltaic cells that are highly thermally conductive, mechanical, chemical, and radiation resistant [13]. Moreover, high aspect ratio and flexibility enable CNTs to be weaved into two-dimensional networks with tunable electrical and optical properties, which can be easily deposited on the surface of several materials [12]. In general, heterojunctions [14] offer more choice of materials with appropriate band gaps covering a wider range of the solar spectrum, and also potential for the development of a variety of nanomaterial/semiconductor solar cells. So far, a number of researchers have investigated the possibility of utilizing CNT films in devices based on crystalline semiconductors such as Si for heterojunction solar cells (we suggest the review in Ref. [15]). In this framework, on one hand Si photogenerates electron-hole pairs and transports electrons to leads. On the other hand, the CNT film acts both as a photogenerating layer and as holetransporting layer, eventually avoiding metal wiring from the device surface, which shade a portion of incident light, thus replacing the costly indium tin oxide (ITO). In this way, charge generation, separation, transport, and collection can be realized partly by the semi-transparent and conductive CNT thin film itself. Currently, researcher main efforts have been focused on single-wall carbon nanotube/silicon (SWCNT/Si) heterojunctions, achieving a photoconversion efficiency (PCE) up to ≈ 12% [16] without any post-process treatments, up to ≈ 11% after chemical doping [17], up to ≈...
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