Particles undergoing a stochastic motion within a disordered medium is a ubiquitous physical and biological phenomenon. Examples can be given from organelles as molecular machines of cells performing physical tasks in a populated cytoplasm to human mobility in patchy environment at larger scales. Our recent results showed that it is possible to use the disordered landscape generated by speckle light fields to perform advanced manipulation tasks at the microscale. Here, we use speckle light fields to study the anomalous diffusion of micron size silica particles (5 μm) in the presence of active microswimmers. The microswimmers we used in the experiments are motile bacteria, Escherichia coli (E.coli). They constitute an active background constantly agitating passive silica particles within complex optical potentials. The speckle fields are generated by mode mixing inside a multimode optical fiber where a small amount of incident laser power (maximum power = 12 μW/μm2) is needed to obtain an effective random landscape pattern for the purpose of optical manipulation. We experimentally show how complex potentials contribute to the anomalous diffusion of silica particles undergoing collisions with swimming bacteria. We observed an enhanced diffusion of particles interacting with the active bath of E.coli inside speckle light fields: this effect can be tuned and controlled by varying the intensity and the statistical properties of the speckle pattern. Potentially, these results could be of interest for many technological applications, such as the manipulation of microparticles inside optically disordered media of biological interests.
Optical forces can affect the motion of a Brownian particle. For example, optical tweezers use optical forces to trap a particle at a desirable position. Unlike passive Brownian particles, active Brownian particles, also known as microswimmers, propel themselves with directed motion and thus drive themselves out of equilibrium. Understanding their motion in a confined potential can provide insight into out-of-equilibrium phenomena associated with biological examples such as bacteria, as well as with artificial microswimmers. We discuss how to mathematically model their motion in an optical potential using a set of stochastic differential equations and how to numerically simulate it using the corresponding set of finite difference equations.
Optical tweezers have been widely used in physics, chemistry and biology to manipulate and trap microscopic and nanoscopic objects. Current optical trapping techniques rely on carefully engineered setups to manipulate nanoscopic and microscopic objects at the focus of a laser beam. Since the quality of the trapping is strongly dependent on the focus quality, these systems have to be very carefully aligned and optimized, thus limiting their practical applicability in complex environments. One major challenge for current optical manipulation techniques is the light scattering occurring in optically complex media, such as biological tissues, turbid liquids and rough surfaces, which give rise to apparently random light fields known as speckles. Here, we discuss an experimental implementation to perform optical manipulation based on speckles. In particular, we show how to take advantage of the statistical properties of speckle patterns in order to realize a setup based on a multimode optical fiber to perform basic optical manipulation tasks such as trapping, guiding and sorting. We anticipate that the simplicity of these “speckle optical tweezers” will greatly broaden the perspectives of optical manipulation for real-life applications.
A project to introduce secondary school students to statistical physics and biophotonics by means of an optical tweezers is presented. Interestingly, the project is completely experimental and no advanced calculus or physics knowledge is necessary. The project starts from the construction of the optical tweezers itself and therefore is also useful to introduce basic concepts of optics.
Some randomness is present in most phenomena, ranging from biomolecules and nanodevices to financial markets and human organizations. However, it is not easy to gain an intuitive understanding of such stochastic phenomena, because their modeling requires advanced mathematical tools, such as sigma algebras, the Itô formula and martingales. Here, we discuss a simple finite difference algorithm that can be used to gain understanding of such complex physical phenomena. In particular, we simulate the motion of an optically trapped particle that is typically used as a model system in statistical physics and has a wide range of applications in physics and biophysics, for example, to measure nanoscopic forces and torques.
The study of diffusion in a crowded and complex environment, such as inside a cell or within a porous medium, is of fundamental importance for science and technology. Combining blinking holographic optical tweezers and sub-pixel video microscopy permits one to study Brownian motion in confined geometries. In this work, in particular, we have studied the Brownian motion of two colloidal particles interacting hydrodynamically with each other. The proximity between the two microspheres induces a space-dependence in the particles diffusion coefficient and, therefore, a spurious drift. We measure this drift and evaluate the magnitude of the spurious force associated with it. We present the optoelectronic tools employed in the experiment and we discuss the experimental results.
We propose a novel approach for trapping micron-sized particles and living cells based on optical feedback. This approach can be implemented at low numerical aperture (NA=0.5, 20X) and long working distance. In this configuration, an optical tweezers is constructed inside a ring cavity fiber laser and the optical feedback in the ring cavity is controlled by the light scattered from a trapped particle. In particular, once the particle is trapped, the laser operation, optical feedback and intracavity power are affected by the particle motion. We demonstrate that using this configuration is possible to stably hold micron-sized particles and single living cells in the focal spot of the laser beam. The calibration of the optical forces is achieved by tracking the Brownian motion of a trapped particle or cell and analysing its position distribution.
Optical forces can affect the motion of a Brownian particle. For example, optical tweezers use optical forces to trap a
particle at a desirable position. Using more complex force fields it is possible to generate more complex configurations.
For example, by using two optical traps placed next to each other, it is possible to obtain a bistable potential where a
particle can jump between the two potentials with a characteristic time scale. In this proceeding, we discuss a simple
finite difference algorithm that can be used to simulate the motion of a Brownian particle in a one-dimensional field of optical forces.
The possibility of measuring microscopic forces down to the femtonewton range has opened new possibilities in fields
such as biophysics and nanophotonics. I will review some of the techniques most often employed, namely the photonic
force microscope (PFM) and the total internal reflection microscope (TIRM), which are able to measure tiny forces
acting on optically trapped particles. I will then discuss several applications of such nanoscopic forces: from plasmonic
optical manipulation, to self-propelled microswimmers, to self-organization in large ensembles of particles.
Optically trapped Brownian particles move under the effect of both the random thermal motion and the deterministic
optical forces. They can, therefore, be a very powerful tool to study statistical physics phenomena, relying both on the
presence of a natural noisy background and on a finely controllable deterministic force field. Here we will take a closer
look to a few of these phenomena and to the insights that optical manipulations techniques have permitted us to gain.
Abstract : In many countries the potential of optics as an exciting part of science is not fully exploited in high-school education. In addition, optics is often not taught in relation with daily experiences. With the motivation to expose the potential of doing otherwise to motivate students, we developed and implemented a two hour-long hands-on introduction to optics for high school students. We termed the program: The Day of Light. By attending the program, students learn basic concepts such as polarization, wavelength, color, stereoscopic vision, reflection and refraction in connection to everyday experiences based on applications of optics. The demonstration was fully organized and carried out by the ICFO Ph.D students who were members of the ICFO Optical Society of America (OSA) Student Chapter.
One of the most promising ways to study the biochemistry of single floating cells is to combine the techniques of
optical tweezers and Raman spectroscopy (OTRS). This can reveal the information that is lost when ensemble
averages are made over cell populations, like in biochemical assays. However, the interpretation of the acquired
data is often ambiguous. Indeed, the trapped living cell continues to move and rotate in the optical trap not only
because of the Brownian motion, but also because of its inherent biological motility and the variation of its shape
and size. This affects both Rayleigh and Raman light scattering. We propose the use of Rayleigh scattering to
monitor the growth of a single optically trapped yeast cell, while OTRS measurements are being performed. For
this purpose, we added a quadrant photodiode to our OTRS setup. The cell orientation in the optical trap is
shown to vary as the cell growth proceeds, especially when it becomes asymmetrical (budding) or it changes its
size or shape considerably (living and growing cell). Control experiments, performed using heat-treated cells and
polystyrene beads, confirm that this behavior is a consequence of the cell growth. These measurements have to
be taken into account in the interpretation of Raman spectra so as not to incorrectly attribute variations in the
spectra to change in the biochemical constituents of the cell if they are in fact due to a change of the orientation
of the cell in the trap.
Living cells show a variety of morphological traits upon which numerous identification techniques have already
been developed. However most of them involve lengthy biochemical procedures and can compromise the viability
of the cell. We demonstrate a method to differentiate cells only on the basis of its trapping dynamics while it is
being drawn into an optical trap (Optical Trapping Dynamics). Since it relies only on the inherent properties of
the optical trap, without requiring external markers or biochemically sensitive spectroscopic techniques, it can be
readily combined with existing optical tweezers setups. We applied it to the study of the yeast cell-cycle stages,
showing, in particular, how it can be amenable for the measurement of the budding index of a cell population.
The combination of Raman spectroscopy and Optical Tweezers has been used to trap living cells and collect information
about their biochemical state. Cells can continue living in such traps for periods of hours, allowing acquisition of time
resolved Raman spectra. However no spatial information can be acquired as the cells continue to rotate and move in the
single beam trap.
Here we describe the development of Holographic Optical Tweezers (HOT) for the controlled movement of floating cells
in order to construct their Raman images. Instead of a single trap, rapidly programmable multiple trapping points can be
produced around the periphery of the cells to impede the rotational motion of the cell. By trapping and scanning the cell
using HOT relative to a fixed Raman exciting laser, a point by point image of the cell can be constructed. We use an
interactive program that permits us to position the trapping points relative to the live image feed we see from the
microscope, using point and click. To demonstrate the possibilities of this technique images are shown of floating Jurkat
We report quantitative measurements of the radiation forces exerted on a micrometer dielectric sphere by a Surface Plasmon Polariton (SPP) excited at a gold/water interface. We separate the contributions of the two constituents of the plasmon wave - the electromagnetic field and the charge-density oscillations - to the total radiation force. Measurements performed with a Photonic Force Microscope (PFM) show an enhanced attraction to the surface compared to a conventional evanescent wave at the dielectric interface (102 enhancement factor).
Living cells initiate a stress response in order to survive environmentally stressful conditions. We monitored changes in the Raman spectra of an optically trapped Saccharomyces cerevisiae yeast cell under normal and hyperosmotic stress conditions. When the yeast cells were challenged with a high concentration of glucose so as to exert hyperosmotic stress, it was shown that two chemical substances - glycerol and ethanol - could be monitored in real time in a single cell. The volume of the detection area of our confocal microspectrometer is approximately 1 fL. The average quantities of detected glycerol and ethanol are about 300 attomol and 700 attomol respectively. This amounts to the detection of approximately 108 glycerol molecules and 4 X 108 ethanol molecules after 36 min of hyper osmotic stress. Besides this, we also optically trapped a single yeast cell for up to three hours under normal conditions and monitored the changes in the Raman spectra during the lag phase of its growth and the G1 phase of its cell cycle. During the lag phase the cell synthesises new proteins and the observed behavior of the peaks corresponding to these proteins as well as those of RNA served as a sensitive indicator of the adaptation of the cell to its changed environment. The changes observed in the Raman spectra of a trapped yeast cell in the late G1 phase or the beginning of S phase corresponded to the growth of a bud.
Gradient radiation forces exerted by strongly focused cylindrical
vector beams of radial and azimuthal polarizations on dielectric
spheres of different radii and refractive indices were calculated.
The effect of longitudinal and transversal components of the
focused electrical field on trapping properties was studied.
Experiments on optical trapping were performed using low-mode
optical fiber excited with Laguerre-Gaussian beam as a source of
the trapping beams.