Optical binding in nanoparticle assembly: Potential energy landscapes

Optical binding is an optomechanical effect exhibited by systems of micro- and nanoparticles, suitably irradiated with off-resonance laser light. Physically distinct from standing-wave and other forms of holographic optical traps, the phenomenon arises as a result of an interparticle coupling with individual radiation modes, leading to optically induced modifications to Casmir-Polder interactions. To better understand how this mechanism leads to the observed assemblies and formation of patterns in nanoparticles, we develop a theory in terms of optically induced energy landscapes exhibiting the three-dimensional form of the potential energy field. It is shown in detail that the positioning and magnitude of local energy maxima and minima depend on the configuration of each particle pair, with regards to the polarization and wave vector of the laser light. The analysis reveals how the positioning of local minima determines the energetically most favorable locations for the addition of a third particle to each equilibrium pair. It is also demonstrated how the result of such an addition subtly modifies the energy landscape that will, in turn, determine the optimum location for further particle additions. As such, this development represents a rigorous and general formulation of the theory, paving the way toward full comprehension of nanoparticle assembly based on optical binding

RURAL-URBAN LINKAGES: PROBLEMS AND OPPORTUNITIES

This article examines recent evidence from a wide range of countries in Latin America, Asia and Africa, about the intensity of ties linking urban and rural areas. An understanding of how rural-urban linkages operate in different contexts and how they are shaped by factors such as economic policies, administrative measures and planning regulations is important, as this has an effect on the livelihoods particularly of the poorer and more vulnerable households and enterprises. With the help of illustrations, the article highlights the problems and opportunities that arise from processes of change resulting from, among others, urban growth, migration, and flows of services and goods (including solid and liquid waste).Esse artigo examina a recente evidencia observadas em vários países da América Latina, Ásia e África sobre a intensidade das relações entre as áreas urbanas e rurais. Uma compreensão de como as relações operam em diferentes contextos e como são moldadas pelos fatores como as políticas econômicas, medidas administrativas e planejamento regulador são importantes, como esses têm um efeito sobre a subsistência empresarial e privada dos mais pobres e mais vulneráveis. Com o emprego de ilustrações, o artigo elucida os problemas e oportunidades que vêem a partir dos processos de mudanças, entre outros, crescimento urbano, migração e fluxo de serviços e (incluindo dejetos sólidos e líquidos)

Urban transport and land-use in Latin America: Historical legacies and ways forward

Insofar as passenger transport offers accessibility, it has been a central organising factor in modern cities as well as a powerful driver of economic growth and, potentially, also a contributor to human wellbeing. But it can also be a cause of major, and often growing, inter-personal inequalities, mainly in societies that are already markedly unequal. Accessibility is a core determinant of land and housing prices and is a main factor in shaping spatial and social segregation in Latin American cities. The Modernist planning legacy of giving priority to motorised transport while separating land uses and classifying some de facto residential areas as illegal or informal still exerts a powerful influence over density and urban form and, by extension, over proximity to opportunities. Transport systems (whether formal or informal) are, in turn, shaped by these factors. This 45-minutes keynote address explores these claims, while sharing examples of interventions seeking to counterbalance this legacy

A retarded coupling approach to intermolecular interactions

A wide range of physical phenomena such as optical binding and resonance energy transfer involve electronic coupling between adjacent molecules. A quantum electrodynamical description of these intermolecular interactions reveals the presence of retardation effects. The clarity of the procedure associated with the construction of the quantum amplitudes and the precision of the ensuing results for observable energies and rates are widely acknowledged. However, the length and complexity of the derivations involved in such quantum electrodynamical descriptions increase rapidly with the order of the process under study. Whether through the use of time-ordering approaches, or the more expedient state-sequence method, time-consuming calculations cannot usually be bypassed. A simple and succinct method is now presented, which provides for a direct and still entirely rigorous determination of the quantum electrodynamical amplitudes for processes of arbitrarily high order. Using the approach, new results for optical binding in two- and three-particle systems are secured and discussed

Quasi-chemical approximation for polyatomic mixtures

The statistical thermodynamics of binary mixtures of polyatomic species was developed on a generalization in the spirit of the lattice-gas model and the quasi-chemical approximation (QCA). The new theoretical framework is obtained by combining: (i) the exact analytical expression for the partition function of non-interacting mixtures of linear $k$-mers and $l$-mers (species occupying $k$ sites and $l$ sites, respectively) adsorbed in one dimension, and its extension to higher dimensions; and (ii) a generalization of the classical QCA for multicomponent adsorbates and multisite-occupancy adsorption. The process is analyzed through the partial adsorption isotherms corresponding to both species of the mixture. Comparisons with analytical data from Bragg-Williams approximation (BWA) and Monte Carlo simulations are performed in order to test the validity of the theoretical model. Even though a good fitting is obtained from BWA, it is found that QCA provides a more accurate description of the phenomenon of adsorption of interacting polyatomic mixtures.Comment: 27 pages, 8 figure

Circular dichroism of cholesteric polymers and the orbital angular momentum of light

We explore experimentally if the light's orbital angular momentum (OAM) interacts with chiral nematic polymer films. Specifically, we measure the circular dichroism of such a material using light beams with different OAM. We investigate the case of strongly focussed, non-paraxial light beams, where the spatial and polarization degrees of freedom are coupled. Within the experimental accuracy, we cannot find any influence of the OAM on the circular dichroism of the cholesteric polymer.Comment: 3 pages, 4 figure

Transport, Urban Development and the Peripheral Poor in Colombia: Placing Splintering Urbanism in the Context of Transport Networks

In rapidly growing cities the evolution of utility and communication infrastructures has enabled the creation of ‘premium networked spaces’ exclusively for wealthier groups thus deepening already large social inequalities. By the same token, in a context of spatially concentrated income-earning opportunities and other urban functions, as well as limited purchasing power, accessibility to adequate means of connectivity with the rest of the urban fabric can be a determining factor in overcoming conditions of poverty for residents in physically marginal areas. Using the case study of Soacha, a municipality adjacent to Bogotá, Colombia’s capital city, we explore the evolution of vulnerable populations in peripheral informal settlements in parallel with the development of transport networks in the decade 2000-2010. Drawing on the splintering urbanism thesis we examine the apparent mismatch between the evolution of informal settlements and transport networks with the aim of identifying the effects of an uneven provision of material infrastructures and services for mobility on marginalization. We observe central elements in the structure of the networks of connectivity between Bogotá and Soacha, identifying the main gaps that lead to a fragmented set of connections. We develop a set of criteria for planners and policy makers in search of a better informed analysis of transport supply and policy development praxis for poor peripheral populations in similar regions and contexts

Raman scattering mediated by neighboring molecules

Raman scattering is most commonly associated with a change in vibrational state within individual molecules, the corresponding frequency shift in the scattered light affording a key way of identifying material structures. In theories where both matter and light are treated quantum mechanically, the fundamental scattering process is represented as the concurrent annihilation of a photon from one radiation mode and creation of another in a different mode. Developing this quantum electrodynamical formulation, the focus of the present work is on the spectroscopic consequences of electrodynamic coupling between neighboring molecules or other kinds of optical center. To encompass these nanoscale interactions, through which the molecular states evolve under the dual influence of the input light and local fields, this work identifies and determines two major mechanisms for each of which different selection rules apply. The constituent optical centers are considered to be chemically different and held in a fixed orientation with respect to each other, either as two components of a larger molecule or a molecular assembly that can undergo free rotation in a fluid medium or as parts of a larger, solid material. The two centers are considered to be separated beyond wavefunction overlap but close enough together to fall within an optical near-field limit, which leads to high inverse power dependences on their local separation. In this investigation, individual centers undergo a Stokes transition, whilst each neighbor of a different species remains in its original electronic and vibrational state. Analogous principles are applicable for the anti-Stokes case. The analysis concludes by considering the experimental consequences of applying this spectroscopic interpretation to fluid media; explicitly, the selection rules and the impact of pressure on the radiant intensity of this process