Optical tweezers use the forces exerted by a strongly focused beam of light to trap and move objects ranging in size from tens of nanometres to tens of micrometres. Since their introduction in 1986, the optical tweezer has become an important tool for research in the fields of biology, physical chemistry and soft condensed matter physics. Recent advances promise to take optical tweezers out of the laboratory and into the mainstream of manufacturing and diagnostics; they may even become consumer products. The next generation of single-beam optical traps offers revolutionary new opportunities for fundamental and applied research.

http://physics.nyu.edu/grierlab/nreview2c/nreview2c.pdf

A revolution in optical manipulation

Photonic crystal fibers guide light by corralling it within a periodic array of microscopic air holes that run along the entire fiber length Largely through their ability to overcome the limitations of conventional fiber optics—for example, by permitting low-loss guidance of light in a hollow core—these fibers are proving to have a multitude of important technological and scientific applications spanning many disciplines The result has been a renaissance of interest in optical fibers and their uses

https://science.sciencemag.org/content/sci/299/5605/358.full.pdf

Photonic crystal fibers

Bose-Einstein condensation (BEC) has been observed in a dilute gas of sodium atoms. A Bose-Einstein condensate consists of a macroscopic population of the ground state of the system, and is a coherent state of matter. In an ideal gas, this phase transition is purely quantum-statistical. The study of BEC in weakly interacting systems which can be controlled and observed with precision holds the promise of revealing new macroscopic quantum phenomena that can be understood from first principles.

Bose-Einstein condensation in a gas of sodium atoms

As they travel through space, some light beams rotate. Such light beams have angular momentum. There are two particularly important ways in which a light beam can rotate: if every polarization vector rotates, the light has spin; if the phase structure rotates, the light has orbital angular momentum (OAM), which can be many times greater than the spin. Only in the past 20 years has it been realized that beams carrying OAM, which have an optical vortex along the axis, can be easily made in the laboratory. These light beams are able to spin microscopic objects, give rise to rotational frequency shifts, create new forms of imaging systems, and behave within nonlinear material to give new insights into quantum optics.

/pdf/orbital-angular-momentum-origins-behavior-and-applications-1sv8qfuhrs.pdf

Orbital angular momentum: origins, behavior and applications

Internet data traffic capacity is rapidly reaching limits imposed by optical fiber nonlinear effects Having almost exhausted available degrees of freedom to orthogonally multiplex data, the possibility is now being explored of using spatial modes of fibers to enhance data capacity We demonstrate the viability of using the orbital angular momentum (OAM) of light to create orthogonal, spatially distinct streams of data-transmitting channels that are multiplexed in a single fiber Over 11 kilometers of a specially designed optical fiber that minimizes mode coupling, we achieved 400-gigabits-per-second data transmission using four angular momentum modes at a single wavelength, and 16 terabits per second using two OAM modes over 10 wavelengths These demonstrations suggest that OAM could provide an additional degree of freedom for data multiplexing in future fiber networks

http://science.sciencemag.org/content/sci/340/6140/1545.full.pdf

Terabit-Scale Orbital Angular Momentum Mode Division Multiplexing in Fibers

Electromagnetically induced transparency in N-level cascade schemes

We examine coherent effects in a four-level Vee scheme. We show that the system can be made to display a number of different phenomena, including the inhibition of two-photon absorption and the production of multiple dark states. We show that this latter effect is due to the independent control of the Autler–Townes splitting of the upper and lower probe field by independent driving fields. We suggest possible experimental systems in rubidium for the observation of such effects.

https://iopscience.iop.org/article/10.1088/0953-4075/36/13/315/pdf

Coherent effects in a driven Vee scheme

The heating produced by a focused laser has been shown to provide a range of manipulation tools on droplets in microfluidic situations, through the generation of thermocapillary flows whose net result is to produce a force on the drop In particular, droplets of water in oil that are produced in microchannels can be blocked in a special test section Here, the manipulation of the flow within the droplet is explored through spatial and temporal modulation of the laser pattern used to block the drop When a stationary pattern of two laser spots is used, the flow preserves the mirror symmetry inside the drop, as happens in the case of two alternating spots if the frequency of the switching is higher than the response rate of the fluid Lower frequency switching produces a time periodic flow that breaks the mirror symmetry and which leads to efficient mixing inside the droplet The mixing that is produced by this alternating flow is studied both experimentally and using numerical simulations of particle trajectories from measured velocity fields This mixing can be optimized for certain parameter ranges, namely by varying the distance between the spots and the forcing frequency

https://iopscience.iop.org/article/10.1088/1367-2630/11/7/075033/pdf

Mixing via thermocapillary generation of flow patterns inside a microfluidic drop

Arrays of optically tweezed aerosol droplets, each of sub-picolitre volume, are manipulated by holographic optical tweezers and characterised by cavity enhanced Raman spectroscopy. A spatial light modulator is employed to generate arrays of optical traps from a single laser beam and to control the array dimensions and relative trap positions. Comparative hygroscopicity measurements are performed concurrently on five trapped droplets by monitoring the evolving size of each droplet. This is extended to the controlled coalescence of an array of droplets accompanied by spectroscopic measurements. These data represent the first ever simultaneous measurements of the evolving composition and size of an array of aerosol droplets. We consider the possibility of using aerosol arrays as a platform for studying chemical reactions in sub-picolitre volumes, exploiting the versatility of aerosol arrays for performing optical digital microfluidic operations accompanied by micro-total analysis.

David McGloin

Papers

Electromagnetically induced transparency in N-level cascade schemes

Coherent effects in a driven Vee scheme

Mixing via thermocapillary generation of flow patterns inside a microfluidic drop

Spectroscopic characterisation and manipulation of arrays of sub-picolitre aerosol droplets

Guiding a cold atomic beam along a co-propagating and oblique hollow light guide