Many types of optical tweezers arrays have been proposed and developed for use in conjunction with microfluidics for
bio-chemical essays. Trap arrays rely on different methods allowing various degrees of flexibility and relative trapping
efficiencies. Among the different techniques currently employed, it is not simple to distinguish which ones produce
adequate performances for a given task in bio-chemistry. Experimental results for trapping efficiently diverse biological
specimens allow distinguishing between the properties of optical trap arrays based on techniques as different as
interferometry, holography, Fresnel or Fraunhoffer diffraction of diffractive structures, generalized phase contrast,
microlens assemblies, micro-mirrors matrices, and also clusters of individual tweezers. The bulkiness of those systems is
another important factor in the design of labs-on-a-chip; in particular the use of cumbersome microscope objectives can
be detrimental to chip optimization. Arrangements of tweezers produced with different concepts should be compared in
terms of efficiency, ease of use, and number of traps simultaneously exploitable
Micro-optical components offer several possibilities for creating
large matrices of optical traps, either when working on inverted
microscopes, or by directly integrating miniaturized optical
components at the level of a micro-fluidic chip. In this article we
focus on two particular configurations, both allowing to generate
large arrays of 3D optical traps. The first configuration takes
advantage of an array of refractive microlenses to generate multiple
optical tweezers within the focal plane of a high-NA microscope
objective. The second configuration relies on an array of focusing
high-NA micromirrors which are directly integrated at the level of a
micro-fluidic chip. We also present measurements of the maximal
optical trapping forces that can be reached with several types of
cells commonly employed in biology and biotechnology, and
demonstrate that these forces are essentially related to the bulk
refractive index of the cells.
We present a multiple laser tweezers system based on refractive optics. The system produces an array of 100
optical traps thanks to a refractive microlens array, whose focal plane is imaged into the focal plane of a high-NA
microscope objective. This refractive multi-tweezers system is combined to micro-fluidics, aiming at performing
simultaneous biochemical reactions on ensembles of free floating objects. Micro-fluidics allows both transporting
the particles to the trapping area, and conveying biochemical reagents to the trapped particles. Parallel trapping
in micro-fluidics is achieved with polystyrene beads as well as with native vesicles produced from mammalian
cells. The traps can hold objects against fluid flows exceeding 100 micrometers per second. Parallel fluorescence
excitation and detection on the ensemble of trapped particles is also demonstrated. Additionally, the system is
capable of selectively and individually releasing particles from the tweezers array using a complementary steerable
laser beam. Strategies for high-yield particle capture and individual particle release in a micro-fluidic environment
are discussed. A comparison with diffractive optical tweezers enhances the pros and cons of refractive systems.
In biological investigations, many protocols using optical trapping call for the possibility to trap a large number
of particles simultaneously. Interference fringes provide a solution for massively parallel micro-manipulation of
mesoscopic objects. Concurrently, the strength of traps can be improved by raising the slope of fringe profiles,
such as to create intensity gradients much higher than the ones formed by sinusoidal fringes (Young's fringes). We
use a multiple-beam interference system, derived from the classical Fizeau configuration, with semitransparent
interfaces to generate walls of light with a very high intensity gradient (Tolansky fringes). These fringes are
formed into a trapping set-up to produce new types of trapping templates. The possibility to build multiple trap
arrays of various geometries is examined; a high number of particles can be trapped in those potential wells. The
period of the fringes can easily be changed in order to fit traps sizes to the dimensions of the confined objects.
This is achieved by modifying several parameters of the interferometer, such as the angle and/or the distance
between the beam-splitter and the mirror. It is well known that optical trapping presents a great potential when
used in conjunction with microfluidics for lab-on-a-chip applications. We present an original solution for multiple
trapping integrated in a microfluidic device. This solution does not require high numerical aperture objectives.
Shaping optical fields is the key issue in the control of optical forces that pilot the manipulation of mesoscopic polarizable dielectric particles. The latter can be positioned according to endless configurations. The scope of this paper is to review and discuss several unusual designs which produce what we think are among some of the most interesting arrangements. The simplest schemes result from interference between two or several coherent light beams, leading to periodic as well as pseudo-periodic arrays of optical traps. Complex assemblages of traps can be created with holographic-type set-ups; this case is widely used by the trapping community. Clusters of traps can also be configured through interferometric-type set-ups or by generating external standing waves by diffractive elements. The particularly remarkable possibilities of the Talbot effect to generate three-dimensional optical lattices and several schemes of self-organization represent further very interesting means for trapping. They will also be described and discussed. in this paper. The mechanisms involved in those trapping schemes do not require the use of high numerical aperture optics; by avoiding the need for bulky microscope objectives, they allow for more physical space around the trapping area to perform experiments. Moreover, very large regular arrays of traps can be manufactured, opening numerous possibilities for new applications.
Very high frequency oscillations of intense light fields interact with micron-size dielectric objects to exert dc optical forces that allow polarizable particles to levitate, to be trapped and to be bound. Such optical forces are also suitable to arrange cold atoms in optical lattices. Various assemblages of optical traps, including periodic arrays, can be constructed either with independent lasers, or with a single laser beam split into different parts later recombined by interference, as well as through the use of diffractive elements. These optical-well arrays serve as templates for writing and erasing dynamic two-dimensional and three-dimensional "optical crystals", composed of mono-dispersed polystyrene spheres in water. Subsequently, the crystals become diffractive structures themselves.
The association of micro-fluidics and optical trapping allows for the formation of optical traps into micro-channels. This leads to perform microchemistry experiments, such as fluorescence detection, on individual bodies attached to trapped particles. Self-trapping due to the optical binding force relates to the interaction between different dielectric objects located in an electromagnetic field; each one reacts not only to the field of the incident beam, but also to the induced fields radiated coherently by all other particles. Optical binding strongly influences the equilibrium state and the behavior of optical crystals. It must have the potential for creating collective effects.