Angular momentum of light | EPFL Graph Search

The angular momentum of light is a vector quantity that expresses the amount of dynamical rotation present in the electromagnetic field of the light. While traveling approximately in a straight line, a beam of light can also be rotating (or "spinning, or "twisting) around its own axis. This rotation, while not visible to the naked eye, can be revealed by the interaction of the light beam with matter. There are two distinct forms of rotation of a light beam, one involving its polarization and the other its wavefront shape. These two forms of rotation are therefore associated with two distinct forms of angular momentum, respectively named light spin angular momentum (SAM) and light orbital angular momentum (OAM). The total angular momentum of light (or, more generally, of the electromagnetic field and the other force fields) and matter is conserved in time. Introduction Light, or more generally an electromagnetic wave, carries not only energy but also momentum, which is

Unconventional Fermi Surface Instabilities in the Kagome Hubbard Model

Ronny Thomale

We investigate the competing Fermi surface instabilities in the kagome tight-binding model. Specifically, we consider on-site and short-range Hubbard interactions in the vicinity of van Hove filling of the dispersive kagome bands where the fermiology promotes the joint effect of enlarged density of states and nesting. The sublattice interference mechanism devised by Kiesel and Thomale [Phys. Rev. B 86, 121105 (2012)] allows us to explain the intricate interplay between ferromagnetic fluctuations and other ordering tendencies. On the basis of the functional renormalization group used to obtain an adequate low-energy theory description, we discover finite angular momentum spin and charge density wave order, a twofold degenerate d-wave Pomeranchuk instability, and f-wave superconductivity away from van Hove filling. Together, this makes the kagome Hubbard model the prototypical scenario for several unconventional Fermi surface instabilities. DOI: 10.1103/PhysRevLett.110.126405

Amer Physical Soc2013

Generation of acoustic helical wavefronts using metasurfaces

Hervé Lissek, Juan Ramon Mosig, Seyyed Hussein Seyyed Esfahlani

It has been shown that acoustic waves with helical wavefronts can carry angular momentum, which can be transmitted towards a propagating medium. Such a wave field can be achieved by using a planar array of electroacoustic transducers, forming a given spatial distribution of phased sound sources which produce the desired helical wavefronts. Here, we introduce a technique to generate acoustic vortices, based on the passive acoustic metasurface concept. The proposed metasurface is composed of space-coiled cylindrical unit cells transmitting sound pressure with a controllable phase shift, which are arranged in a discretized circular configuration, and thus passively transforming an incident plane wavefront into the desired helical wavefront. This method presents the advantage of overcoming the restrictions on using many acoustic sources, and it is implemented with a transmitting metasurface which can be easily three-dimensionally printed. The proposed straightforward design principle can be adopted for easy production of acoustic angular momentum with minimum complexity and using a single source.

2017

Micro-optics for multiple laser trapping in microfluidics

Fabrice Merenda

Optical trapping allows manipulating very small objects – varying from Angstöms to micron size particles – without physical contact by taking advantage of laser light. This technique, relying on light momentum transfer, allows manipulating biological matter (such as cells, organelles, vesicles or chemically functionalized artificial particles, etc.) and offers promising potentialities for research in biotechnology and biochemistry. Individually confining a large number of microscopic objects opens new ways for the downscaling of analysis tools for drug screening, particles sorting and the assessment of statistical data. In particular, the combination of optical trapping with microfluidics greatly enlarges the possibilities offered by both techniques. This PhD work takes place in a research aiming at developing novel bio-analytical instrumentation relying on large arrays of optical traps compatible with microfluidic systems. The dissertation focuses on the use of micro-optics for generating extended matrices of three-dimensional optical traps capable of capturing a large number of particles within microfluidic environments. The stability of optical traps is first studied with special emphasis on the conditions present in microfluidic flows, both from a theoretical and an experimental point of view, and trapping forces achievable on biological cells are investigated. A multiple optical trapping system relying on microlens arrays, adapted to work on commercial microscopes, is shown to be capable of generating arrays of more than 500 three-dimensional traps. Simultaneously trapping in multiple planes is also evidenced using matrices of microlenses, thanks to a self-imaging effect of the array of traps. Furthermore, possibilities offered by multiple optical trapping within microfluidic environments are explored. Polystyrene spheres as well as biological particles, such as native vesicles, can be trapped in arrays, manipulated inside microfluidic devices and analyzed in parallel through fluorescence microscopy while extremely small quantities of chemical reagents are flown past the array. Finally, for the sake of system miniaturization and further extension of the number of traps, multiple three-dimensional optical trapping based on arrays of miniaturized high numerical aperture parabolic mirrors is proposed, allowing the optics necessary for optical trapping, fluorescence excitation and collection to be integrated at the level of a microfluidic chip. Such miniaturized mirror matrices did not even exist before this thesis; their fabrication and characterization are detailed in this dissertation.

EPFL2008

Micro-optics for multiple laser trapping in microfluidics

Fabrice Merenda

EPFL2008