Silvia Gentilini

Biomimetic antimicrobial cloak by graphene-oxide agar hydrogel

Antibacterial surfaces have an enormous economic and social impact on the worldwide technological fight against diseases. However, bacteria develop resistance and coatings are often not uniform and not stable in time. The challenge is finding an antibacterial coating that is biocompatible, cost-effective, not toxic, and spreadable over large and irregular surfaces. Here we demonstrate an antibacterial cloak by laser printing of graphene oxide hydrogels mimicking the Cancer Pagurus carapace. We observe up to 90% reduction of bacteria cells. This cloak exploits natural surface patterns evolved to resist to microorganisms infection, and the antimicrobial efficacy of graphene oxide. Cell integrity analysis by scanning electron microscopy and nucleic acids release show bacteriostatic and bactericidal effect. Nucleic acids release demonstrates microorganism cutting, and microscopy reveals cells wrapped by the laser treated gel. A theoretical active matter model confirms our findings. The employment of biomimetic graphene oxide gels opens unique possibilities to decrease infections in biomedical applications and chirurgical equipment; our antibiotic-free approach, based on the geometric reduction of microbial adhesion and the mechanical action of Graphene Oxide sheets, is potentially not affected by bacterial resistance.

Ultrashort pulse propagation and the Anderson localization.

We investigate the dynamics of a 10 fs light pulse propagating in a random medium by the direct solution of the three-dimensional Maxwell equations. Our approach employs molecular dynamics to generate a distribution of spherical scatterers and a parallel finite-difference time-domain code for the vectorial wave propagation. We calculate the disorder-averaged energy velocity and the decay time of the transmitted pulse versus the localization length for an increasing refractive index.

Nonlinear Gamow vectors, shock waves, and irreversibility in optically nonlocal media

Dispersive shock waves dominate wave-breaking phenomena in Hamiltonian systems. In the absence of loss, these highly irregular and disordered waves are potentially reversible. However, no experimental evidence has been given about the possibility of inverting the dynamics of a dispersive shock wave and turn it into a regular wave-front. Nevertheless, the opposite scenario, i.e., a smooth wave generating turbulent dynamics is well studied and observed in experiments. Here we introduce a new theoretical formulation for the dynamics in a highly nonlocal and defocusing medium described by the nonlinear Schroedinger equation. Our theory unveils a mechanism that enhances the degree of irreversibility. This mechanism explains why a dispersive shock cannot be reversed in evolution even for an arbitrarirly small amount of loss. Our theory is based on the concept of nonlinear Gamow vectors, i.e., power dependent generalizations of the counter-intuitive and hereto elusive exponentially decaying states in Hamiltonian systems. We theoretically show that nonlinear Gamow vectors play a fundamental role in nonlinear Schroedinger models: they may be used as a generalized basis for describing the dynamics of the shock waves, and affect the degree of irreversibility of wave-breaking phenomena. Gamow vectors allow to analytically calculate the amount of breaking of time-reversal with a quantitative agreement with numerical solutions. We also show that a nonlocal nonlinear optical medium may act as a simulator for the experimental investigation of quantum irreversible models, as the reversed harmonic oscillator.

Physical realization of the Glauber quantum oscillator

More than thirty years ago Glauber suggested that the link between the reversible microscopic and the irreversible macroscopic world can be formulated in physical terms through an inverted harmonic oscillator describing quantum amplifiers. Further theoretical studies have shown that the paradigm for irreversibility is indeed the reversed harmonic oscillator. As outlined by Glauber, providing experimental evidence of these idealized physical systems could open the way to a variety of fundamental studies, for example to simulate irreversible quantum dynamics and explain the arrow of time. However, supporting experimental evidence of reversed quantized oscillators is lacking. We report the direct observation of exploding n = 0 and n = 2 discrete states and Γ0 and Γ2 quantized decay rates of a reversed harmonic oscillator generated by an optical photothermal nonlinearity. Our results give experimental validation to the main prediction of irreversible quantum mechanics, that is, the existence of states with quantized decay rates. Our results also provide a novel perspective to optical shock-waves, potentially useful for applications as lasers, optical amplifiers, white-light and X-ray generation.

Gamow vectors explain the shock profile

The description of shock waves beyond the shock point is a challenge in nonlinear physics and optics. Finding solutions to the global dynamics of dispersive shock waves is not always possible due to the lack of integrability. Here we propose a new method based on the eigenstates (Gamow vectors) of a reversed harmonic oscillator in a rigged Hilbert space. These vectors allow analytical formulation for the development of undular bores of shock waves in a nonlinear nonlocal medium. Experiments by a photothermal induced nonlinearity confirm theoretical predictions: the undulation period as a function of power and the characteristic quantized decays of Gamow vectors. Our results demonstrate that Gamow vectors are a novel and effective paradigm for describing extreme nonlinear phenomena.

Erratum: Biomimetic antimicrobial cloak by graphene-oxide agar hydrogel

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Optothermal nonlinearity of silica aerogel

We report on the characterization of silica aerogel thermal optical nonlinearity, obtained by z-scan technique. The results show that typical silica aerogels have nonlinear optical coefficient similar to that of glass (≃10−12 m2/W), with negligible optical nonlinear absorption. The nonlinear coefficient can be increased to values in the range of 10−10 m2/W by embedding an absorbing dye in the aerogel. This value is one order of magnitude higher than that observed in the pure dye and in typical highly nonlinear materials like liquid crystals.

Optomechanics of random media

Using light to control the movement of nanostructured objects is a great challenge. This challenge involves fields like optical tweezing, Casimir forces, integrated optics, biophysics, and many others. However, when the complexity of the light-activated devices increases, disorder unavoidably occurs and induces a number of effects, such as multiple-scattering, diffusion, and the localization of light. We show that these effects radically enhance the mechanical effect of light. We determine theoretically the link between optical pressure and the light diffusion coefficient and unveil that optical forces and their statistical fluctuations reach a maximum at the onset of the photon localization. Disorder may thus be exploited for increasing the mechanical action of light on complex objects.

Nonlinear optics, optomechanics, and antibacterial coating by graphene oxide

Antibacterial items are one of the major queries from the medical community in the fight against medical infections. Indeed, bacteria are resistant and their multiplication and biofilm formation on devises are one of the major causes of infections. Finding antibacterial surfaces, which are biocompatible, cost-effective, not toxic, and spreadable over large and irregular surfaces, is not easy. However, we created an antibacterial cloak by laser printing of Graphene Oxide (GO) hydrogels by mimicking the Cancer Pagurus carapace. This surface provides up to 90% reduction of bacteria cells through a bacteriostatic and bactericidal effect. Indeed, Laser treating allows GO sheets gel to cut and wrap microorganisms. Our findings are confirmed by a theoretical active matter model. This new technology based on antibiotic-free biomimetic Graphene Oxide gels opens untrodden roads to the fight against infections in biomedical applications and chirurgical equipment.

Optical supercavitation in graphene-oxide hydrogel for antimicrobial cloaks

By nonlinear optical supercavitation we realize an antibacterial cloak on graphene-oxide (GO) based hydrogel. By combining the GO antimicrobial effects with a multiscale bio-inspired pattern, we observe up to 90% reduction of bacteria cells.