Abstracts
The way in which reflection of the trapping beam from a dielectric interface influences the distance of the trapped sphere from the beam waist is studied theoretically and experimentally. The reflected wave interferes with the incident wave and they create a standing-wave component in the total axial intensity distribution. This component then modulates the trapping potential and creates several possible equilibrium positions for the trapped sphere. When the beam waist approaches the surface, the potential profile changes, which consequently causes jumps of the trapped probe from its current location to a deeper potential well. We suggested theoretically and proved experimentally that the magnitude of these unwanted jumps between the neighbouring equilibrium position s can be decreased by a suitable size of the sphere.
We used generalised Lorenz-Mie scattering theory (GLMT) to compare submicron-sized particle optical trapping in a single focused beam and a standing wave. We focus especially on the study of maximal axial trapping force, minimal laser power necessary for confinement, axial trap position, and axial trap stiffness in dependancy on trapped sphere radius, refractive index, and Gaussian beam waist size. In the single beam trap (SBT), the range of refractive indices, which enable stable trapping depends strongly on the beam waist size (it grows with decreasing waist). On the contrary to the SBT, there are certain sphere sizes (non-trapping radii) that disable sphere confinement in standing wave trap (SWT) for arbitrary value of refractive index. For other sphere radii we show that the SWT enables confinement of high refractive index particle in wider laser beams and provides axial trap stiffness and maximal axial trapping force at least by two orders and one order bigger than in SBT, respectively.
Keywords:single beam trap, optical trapping, optical tweezers, standing wave, Mie scattering, Gaussian laser beam
We use a ray-optics-based model to determine the magnitude of forces exerted by laser light as the functions of laser beam, object and surrounding medium parameters. We study the influence of these parameters on total force in order to find the optimal parameter combination for the most effective manipulation. We have employed these theoretical results in practice and succeeded in building up a 3-D laser trap which we use to manipulate divinylbenzen spherical particles (10 - 35 micm sized) and also irregularly shaped living protozoa cells in water medium.
Keywords: optical tweezers
intensity and acting (according to the relation between the refractive indices of the object - nint - and surrounding medium - next - in the direction of the light intensity gradient (nint > next ) or against the direction of this gradient (nint < next ). Then the total force can easily be determined as the vector sum of both components.
Tightly focused laser beams can achieve
steep light intensity gradients nearby the beam
focal point; these gradients are directed to
the focal point. This implies that if the microobject (for
which the condition nint > next is valid) is situated
beyond the beam focal point, the scattering and gradient components
of the total force compete. The total force is thus able to act
against the direction of the light propagation within a certain range
of distances from the beam focal point; this occurs until the balance
between force components is reached. Let's consider a laser beam
impinging on the surface of a dielectric spherical particle. If the
beam is focused tightly enough, stable equilibrium of
forces can be achieved - the particle is 3-D-trapped
within the potential well of the electromagnetic field
of the incident laser beam. The trap stability and stiffness
are strongly influenced by the parameters of the beam (spot
size, wavelength of incident light), particle and surrounding medium (especially
the ratio of nint and next ). We have studied
theoretically the influence of these parameters upon both axial (i.e.
those parallel to the incident beam axis) and radial
(perpendicular to the beam axis) forces acting on a dielectric
microsphere in order to find an optimal parameter
combination for the most
efficient object trapping. Our calculations
were based on ray optics formalism and we
used Gaussian TEM00 and TEM01*
modes to describe the intensity profile of the incident laser
beam and directions of rays impinging on the sphere surface.
We succeeded in designing of a 3-D laser trap using a
He--Ne laser, a high N.A. objective for beam focusing and additional lenses
to set the beam parameters. We are able to manipulate divinylbenzen
particles within the size range 10-35 micm in water medium
and also irregulary shaped living protozoa cells. These experiments
have led into a good agreement (considering experimental uncertainty
and theoretical approximations) with theoretical predictions in case the
particle
size suited well the ray-optics validity conditions.
The interference of the incident and reflected beams forms a standing wave which is especially useful for nanoobjects trapping. Nanoobjects of diameters equal to tens of nanometers are stably trapped near the antinodes (intensity maxima) of the standing wave due to the restoring dipole force which is thousands times greater than the scattering force in this case.
The paper presents the qualitative description of the trapped object behaviour, theoretical study and calculations of forces acting on the microobjects and nanoobjects. The influence of the standing wave on the observed microsphere behaviour is also discussed.
Keywords: Single beam trap, optical tweezers, Rayleigh scattering, Mie scattering, Gaussian laser beam.
Keywords: Optical trap, Optical tweezers, Gaussian standing wave, Rayleigh particles, Mie particles, Optical force, Two-photon fluorescence, Paramecium, Mitochondria, Sub-cellular organelles.
The force and trap stiffness are studied as the functions of the laser beam parameters (beam waist w0, distance of the beam waist from the mirror z0) and particle properties (radius a, relative refractive index nrel). The effect of the structural resonances on the force magnitude is also discussed.
Keywords: near-field probe, micropositioning, intensity profile measurement
Keywords: laser trapping, optical tweezers, laser scissors, cell fusion, optical rotation
Keywords: optical tweezers, optical trapping, standing wave, Rayleigh scattering, Mie scattering, Gaussian laser beam.
Keywords: trap stiffness, force measurement, standing wave, spectral analysis
Keywords: trapping, optical confinement and manipulation, electromagnetic theory, interference, Mie theory, scattering, particles
We propose an efficient method of cooling atoms in a strong Gaussian standing wave. The steep gradients of the atomic potential energy give rise to large dipole forces, which can be much stronger than the maximum radiation pressure force and can therefore stop atoms in a much shorter distance. We have simulated the cooling process using a semi-classical Monte Carlo method, which includes the radial motion, in addition to the motion along the beams. Both motions are calculated directly without separation of the dynamics into force and diffusion terms.
To cool a large range of atomic velocities
the frame in which the standing wave is at rest was swept by
changing the frequencies of the counterpropagating beams, in
a similar way to the well-known chirp cooling technique using
the radiation pressure force. The simulations show that it
is possible to stop caesium atoms in a distance of 8 cm starting
from a room temperature distribution and keep them focussed
near the centre of the beam using red detuning. For red detuning atoms
are attracted towards the regions of high intensity at the
centre of the beam but if we employ the curvature of
Gaussian beams far from
beam waist to prevent atoms spreading cooling can
be obtained even in blue detuned beams.
To cool a large range of atomic velocities the frame in which the standing wave is at rest was swept by changing the frequencies of the counter-propagating beams, in a similar way to the well-known chirp cooling technique using the radiation pressure force. If the curvature of Gaussian beams far from beam waist is employed the radial motion and velocities can be reduced even for the blue detuning comparing to the near waist case.
The simulations show that it is possible
to stop caesium atoms in a distance of several centimetres
(the exact value depends on the laser power, beam waist
radius and acceptable chirping force) starting from the most probable
velocity at room temperature.Narrower radial and wider
axial velocity distribution was obtained for red detuning comparing
with the blue one.
Keywords: laser cooling, laser trapping
is not completely cancelled at all radial positions across the beam. This creates an intensity dip in both the axial and radial directions that can be used as an atomic trap for blue detuning of the light. We simulated the behaviour of two level atoms in this trap using dressed state Monte-Carlo method and in this paper we show that it gives good trapping when the residual intensity at the bottom of traps is small.
Keywords: Atomic dipole trap, Gaussian laser beam.
Keywords: Atomic dipole trap, Gaussian laser beam.
Keywords: Optical force; Rayleigh particle; Colloidal particle; Interference optical trap; Optical lattice; Sorting; Refractive index; Optical chromatography
We demonstrate an optical conveyor belt that provides trapping and subsequent precise delivery of several submicron particles over a distance of hundreds of micrometers. This tool is based on a standing wave (SW) created from two counter-propagating nondiffracting beams where the phase of one of the beams can be changed. Therefore, the whole structure of SW nodes and antinodes moves delivering confined micro-objects to specific regions in space. Based on the theoretical calculations, we confirm experimentally that certain sizes of polystyrene particles jump more easily between neighboring axial traps and the influence of the SW is much weaker for certain sizes of trapped object. Moreover, the measured ratios of longitudinal and lateral optical trap stiffnesses are generally an order of magnitude higher compared to the classical single beam optical trap.
The influence of size of the trapped object on its position near the dielectric interface is studied experimentally. The trapping beam is reflected on a surface and creates weak standing wave component in resulting field distribution. This component causes unwanted jumps of the trapped particle, when the beam waist moves axially in the surface vicinity. Particles of di.erent sizes are more and less in.uenced by the standing wave, respectively. The position of the trapped particle is measured with quadrant photodiode and photomultiplier tube at the same time.
We present and analyse a method that uses an interference of counter-propagating Bessel beams for 3D confinement of high-index micro-particles in array of optical traps. Due to this interference a sort of standing wave (SW) is created with intensity maxima separated by more than half a wavelength of the trapping beam and arranged along propagation axis. Thanks to the non-diffracting nature of this beam the region of SW existence is much longer comparing to the Gaussian beam of the similar beam diameter. Steep axial optical intensity gradients cause axial optical force that enables 3D particle con.nement. Moreover, the self-healing property of Bessel beam suppresses the beam modification due to the presence of confined objects.
We describe a general way how to calculate optical forces acting on Rayleigh particles or colloids placed into general interference field. In this paper we focus on a configuration with 3 beams laying in one plane and we present an analysis of the particles behaviour. We found that this arrangement can be used for sorting of particles having refractive index higher or lower comparing to the surrounding immersion medium and even for sorting of particles according to their size.
We present two methods of surface profiles measurement using optically trapped probe in tightly probe in tightly focused laser beam (optical tweezers). The first method is basesd on a continuous contact of the probe with the surface (contact mode) and the second one employes the alternating contact (tapping mode). The probe defiations are detected by two-photon fluorescence excited by the trapping beam and emitted by the trapped dyed probe.
We present an experimental demonstration of multiple optical tweezers based on interference of two co-propagating beams that intersect at a given angle and form interference fringes (asymmetric optical traps) at the focal plane of a focusing lens. Since this arrangement provides only two-dimensional trapping when the objects are pushed against the coverglass, we added the third counter-propagating beam. This beam did not interfere with the previous two but compensated their radiation pressure. Therefore, stable three-dimensional confinement into multiple fringes is achieved. We quantified experimentally the maximal optical forces exerted on 1 lm polystyrene bead in both con.gurations and compared them with theoretical predictions. Reasonable good coincidence was found especially for two-dimensional trapping.
The effects of pulse laser irradiation on the cortex of Paramecium caudatum and Blepharisma undulans undulans were studied. The character and extent of cortical injury was video recorded and subsequently analyzed. Destruction of the cytoskeleton by laser irradiation was detected by immunofluorescence staining. A difference in the development and healing of the wound was observed between Paramecium and Blepharisma cells. A more immediate reaction was recorded in Blepharisma cells containing blepharismin, a red pigment, known to absorb light energy. The damage to the infusorian cortex due to laser irradiation was compared with that produced by mechanical devices.
We describe a general way how to calculate optical forces and torque acting on colloids placed into laser interference field. In this paper we focus on a polystyrene particle placed into three interfering beams laying in one plane and creating two dimensional optical lattice. Colloids behavior in this type of optical landscape differs according to the colloid size. Spatial distribution of the optical force and torque is studied and particle behavior is predicted. Total optical force and torque are presented for various optical lattice configurations.
Recently a non-contact organization of submicron colloidal particles on the surface attracted a great attention in connection with development of imaging techniques using total internal reflection. We focus here on the theoretical description of the forces acting on a submicron particle placed in an interference field created by two counter-propagating evanescent waves. Numerical results elucidate how these forces or trap depth depend on the particle size and angle of incidence of both beams. Experimental results proved these conclusions and several polystyrene particles of diameter 520 nm were confined in evanescent standing wave.
In this paper we present the standing wave created from two counter-propagating non-diffracting (Bessel) beams as a device for confinement and precise delivery of sub-micron sized particles. The particle position in direction of beam propagation is controled by changing the phase shift between these two beams. We succeeded in delivery of polystyrene particles of diameter 410 nm over a distance of 300 um. At the same time we experimentaly confirmed the theoretical prediction how the optical forces acting on particles in this kind of field depend on the size of the objects.
We focus on a numerical study using coupled dipoles to explore the interaction between two spheres of micron size. This interaction (sometimes called optical coupling or binding) is studied in counter-propagating Gaussian laser beams that do not interfere. We found that for a certain range of the refractive indices of the particle there exist several stable and unstable positions where the total forces acting on particles are zero. Moreover we observed an oscillations of the force acting between both objects coming from the interference of backscattered field.
Článek popisuje zařízení určené k bezkontaktní manipulaci s velkým spektrem mikroobjektů o průměrech od 0,5 um do 30 um. Zařízení je unikátní v tom, že jej lze použít s většinou komerčních mikroskopů a není nutné je jakkoli modifikovat. Klíčovými vlastnostmi tohoto zařízení jsou malé rozměry, univerzálnost a jednoduchá obsluha. Jsme přesvědčení, že zařízení nalezne četné aplikace v biologii, medicíně, fyzice či technických oborech, kde zejména vysoké pořizovací náklady komerčních optických pinzet a skalpelů bránily jejich masovějšímu nasazení.
In this article we present laser diode based tool for optical manipulation with microobjects. This tool is very suitable for micromanipulations with large spectrum of speciments in the diameter range 0.5 - 30 um. Adapter is directly mounted to the microscope without any additional improvements and fits to many commercially available microscopes. Key feature of this adapter is compactness, usability and simple handling. With this adapter user takes advantage of wide spectrum of commercially available laser diodes with different wavelengths. For this reason the tool can be used in many areas such as biology, medicine and measurements.
Laboratories of optical micromanipulation techniques (OMITEC – see http://www.isibrno.cz/omitec) were founded ten years ago as a part of Department of Quantum Light Generators of Institute of Scientific Instruments. In cooperation with Institute of Physical Engineering of Faculty of Mechanical Engineering of Technical University in Brno intensive research of interactions between laser radiation and solid objects has begun. This research deals with three basic categories – non-contact manipulations with micro- and nano-objects, microablation and using photopolymerization to create structures in microscopic scale.
The redistribution of light between micro- or nanoobjects placed in counter-propagating laser fields leads to their steady-state spatial configurations. Under appropriate conditions, the objects are spatially separated and form optically bound matter. This is a very exciting phenomenon that is still not fully understood. In this article we present a new theoretical model of how to study this phenomenon, which is based on a coupled dipole method particularly amenable to nanoparticle optical binding. Predictions of this model are compared with experimental data and other theoretical models with satisfactory results.
Recently a non-contact organization of sub-micron colloidal particles in the vicinity of liquid?solid interface attracted great attention in connection with the development of imaging techniques using total internal reflection. We focus here on the theoretical description of the optical forces acting on a sub-micron particle placed in an interference field created by two counter-propagating evanescent waves. We study the behavior of nanoparticles by means of Rayleigh approximation, and also the behaviour of sub-micron particles by Lorentz?Mie scattering theory. Numerical results show how these forces depend on the particle size and angle of incidence of both beams. The alternating dependence on the bead size was proven experimentally, and the sub-micron beads behavior was experimentally studied in the motional evanescent standing wave. Self-organization of the beads into linear chains was also observed.
We present the theoretical and experimental study of nondiffracting Bessel beams as a device for optical manipulation and confinement of nanoparticles. We express analytically the optical forces acting on a nanoparticle placed into a single and two counter-propagating non-paraxial nondiffracting beams created behind the axicon. Nanoparticle behavior in these configurations is predicted by computer simulations. Finally we demonstrate experimentally how standing waves created from two independent counter-propagating nondiffraction beams confines polystyrene beads of radii 100 nm, and organizes them into a one-dimensional chain 1 mm long. Phase shift in one beam causes the motion of the whole structure of the standing wave together with any confined objects over its extent.
We study the transfer of the cell nucleus and individual chromosomes from one living cell to the other one during their fusion. To achieve this, the nuclei of the two fused cells are stained with different fluorescent dyes which serve as identification markers. The fusion itself is done in an inverted optical microscope by combined system that uses optical tweezers to bring two living cells into contact and optical scalpel to punctuate their membranes at the contact point. This process initiates a fusion of both cells into one hybrid cell containing two nuclei. If the fusion product is viable, these nuclei tend to mix together. The dynamics of the fusion process is then visualized by exciting the fluorescently labeled fusion product with a suitable light source. The time evolution of the mutual position of the fused cell nuclei and their final orientation is traced from a video record of the experiment. The spatial distribution of the nuclear material in the resulting hybrid nucleus is studied by analysis of positions of FISH (fluorescent hybridization in situ) signals of specific genetic loci in automated fluorescence microscope (high resolution cytometer). The obtained results are compared to the signals distribution of FISH in the original cells.
In this article we describe a combined system that uses optical tweezers to bring two living cells into contact and optical scalpel to punctuate their membranes at the contact point. This process initiates a fusion of both cells into one hybrid cell containing two nuclei. If the fusion product is viable, these nuclei tend to mix together. The spatial distribution of the nuclear material in the resulting hybrid nucleus is studied by analysis of positions of FISH (fluorescent hybridization in situ) signals of specific genetic loci in automated fluorescence microscope (high resolution cytometer). The obtained results are compared to the signals distribution of FISH in the original cells.
We describe a general way how to calculate analytically optical forces acting on Rayleigh particles or colloids placed into interference field made by evanescent waves. In this paper we focus on a configuration with three interfering waves and we present a comprehensive analysis of optical trap positions, depths, and forces depending on the configuration and polarisation of the incident waves. Particle behaviour is predicted including optical sorting according to the particle refractive index.
Send comments to webmaster
Last modification: 30 May 2007
