Single Gaussian beam interaction with a Kerr microsphere: characteristics of the radiation force (original) (raw)
Related papers
Radiation force on a nonlinear microsphere by a tightly focused Gaussian beam
Applied optics, 2002
We determine the characteristics of the radiation force that is exerted on a nonresonant nonlinear ͑Kerr-effect͒ rigid microsphere by a strongly focused Gaussian beam when diffraction and interference effects are significant ͑sphere radius a Յ illumination wavelength ͒. The average force is calculated from the surface integral of the energy-momentum tensor consisting of incident, scattered, and internal electromagnetic field vectors, which are expressed as multipole spherical-wave expansions. The refractive index of a Kerr microsphere is proportional to the internal field intensity, which is computed iteratively by the Rytov approximation ͑residual error of solution, 10 Ϫ30 ͒. The expansion coefficients for the field vectors are calculated from the approximated index value. Compared with that obtained in a dielectric ͑linear͒ microsphere in the same illumination conditions, we find that the force magnitude on the Kerr microsphere is larger and increases more rapidly with both a and the numerical aperture of the focusing objective. It also increases nonlinearly with the beam power unlike that of a linear sphere. The Kerr nonlinearity also leads to possible reversals of the force direction. The proposed technique is applicable to other types of weak optical nonlinearity.
Optics communications, 2006
We calculate the radiation force that is exerted by a focused continuous-wave Gaussian beam of wavelength λ on a non-absorbing nonlinear particle of radius a ≪ 50λ/π. The refractive index of the mechanically-rigid particle is proportional to the incident intensity according to the electro-optic Kerr effect. The force consists of two components representing the contributions of the electromagnetic field gradient and the light scattered by the Kerr particle. The focused intensity distribution is determined using expressions for the six electromagnetic components that are corrected to the fifth order in the numerical aperture (NA) of the focusing objective lens. We found that for particles with a < λ/21.28, the trapping force is dominated by the gradient force and the axial trapping force is symmetric about the geometrical focus. The two contributions are comparable with larger particles and the axial trapping force becomes asymmetric with its zero location displaced away from the focus and towards the beam propagation direction. We study the trapping force behavior versus incident beam power, NA, λ, and relative refractive index between the surrounding liquid and the particle. We also examine the confinement of a Kerr particle that exhibits Brownian motion in a focused beam. Numerical results show that the Kerr effect increases the trapping force strength and significantly improves the confinement of Brownian particles.
Radiation forces on a micrometer-sized sphere in an evanescent field
Journal of the Optical Society of America B, 1995
Electromagnetic wave theory is used to predict the radiation forces exerted upon a micrometer-sized spherical particle illuminated by evanescent waves penetrating across a dielectric interface. These forces are quantified for two incident beam polarizations ( p and s polarization) and for different refractive-index media. The electromagnetic formalism that we use is based on theoretical studies of Barton et al. [J. Appl. Phys. 64, 1632; 66, 4594 (1989)]. The novelty of the present research is to apply this general formalism to the calculation of forces when the evanescent field is identified with the incident field. Our theoretical results for the horizontal and vertical force components are shown graphically in nondimensional form as functions of the size parameter of the sphere. Moreover, our results are found to be in reasonable agreement with recent experimental findings of Kawata and Sugiura [Opt. Lett. 17, 772 (1992)].
DYNA, 2019
From the invention of the Optical Tweezer (OT) in 1986, these devices have been considered as high-level tools for research in the areas such as biology and microbiology. A theoretical study obtaining equations for gradient and scattering forces that exert an OT when the illumination beam is a doughnut-shaped mode TEM∗01 linearly polarized is realized. This work focuses on the behavior of radiation forces on a dielectric sphere in the Rayleigh regime. In order to facilitate the phenomenological analysis of the behavior of the radiation forces a graphical user interface is created.
Force measurement on microspheres in an optical standing wave
Journal of the Optical Society of America B, 2008
We have measured the optical forces on isolated particles trapped in an optical lattice generated by the interference of two coherent laser beams. Two independent methods are employed here that are based on the equipartition theorem and hydrodynamic drag. The optical force on a particle in an optical lattice depends strongly on the ratio of the particle diameter to the period of the lattice. Based on the observed size dependence, we developed an approach that allows tunable, size-dependent force selection of a subset of particles from an ensemble containing mixed particles.
2005 European Quantum Electronics Conference, EQEC '05, 2005
Up to now optical spectroscopies have analyzed the scattered light or the heat generated by absorption as a function of the wavelength to get information about the samples. Among the light matter interaction phenomena one that has almost never been used for spectroscopy is the direct photon momenta transfer. Probably because the forces involved are very small, varying from hundreds of femto to tens of pico Newtons. However, the nowadays very popular Optical Tweezers can easily accomplish the task to measure the photon momenta transfer and may be the basis for the Optical Force Spectroscopy. We demonstrate its potential as such a tool by observing more than eight Mie resonance peaks of a single polystyrene microsphere, and showed the capability to selective couple the light to either the TE, TM or both microsphere modes depending of the beam size, the light polarization and the beam positioning. The Mie resonances can change the optical force values by 30-50 %. Our results also clearly show how the beam polarization breaks the usually assumed azimuthal symmetry by Optical Tweezers theories. We also obtained the spectrum from the two photon excited luminescence using the Optical Tweezers to hold a single bead suspended and a femtosecond Ti:sapphire laser for the non-linear excitation. This spectrum shows the pair of peaks due to both TE and TM spherical cavity modes. We have been able to observe more than 14 Mie resonance peaks in the TPE luminescence. Our results are in good agreement with optical force calculations using Maxwell stress tensor and partial wave decomposition of the incident beam approximated to a 3 th order gaussian beam.
Optical forces on microparticles in an evanescent laser field
Optics Letters, 1999
We present a theory based on an exact calculation of the radiation forces on a microsized particle illuminated by evanescent waves created under total internal ref lection in a f lat substrate. The inf luence of the proximity of this interface to the particle is analyzed by a numerical simulation that addresses multiple scattering of light between the particle and the dielectric f lat surface. We thus give an interpretation of the experimental results of Kawata and Sugiura [Opt. Lett. 17, 772 (1992)] and put forward a method that is capable of predicting new effects.
Optical Gradient Forces of Strongly Localized Fields
PHYSICAL REVIEW LETTERS, 1998
We present a new approach for determining optical gradient forces applied by strongly focused laser beams on dielectric particles. We show that when the electromagnetic field is focused to a diffraction limited spot a dipole approximation is valid for any particle size. We derive intuitive predictions for force-displacement curves, maximal trapping forces, and force constants. The theory fits well with recent measurements of particles trapped by laser tweezers. We also discuss effects of radiation pressure and gravity. [S0031-9007(98)06883-5] 05.40. + j, 42.25.Fx The technique of optical tweezers has opened new experimental horizons in the biophysical and colloidal realm. Since the pioneering work of Ashkin [1-3], who first introduced the use of optical gradient forces, researchers have found diverse applications of trapping and manipulating single particles such as dielectric spheres and cellular organelles. Recent work with laser tweezers has allowed quantitative measurements of piconewton forces and nanometer displacements such as those produced by the action of single molecular motors. In addition, laser tweezers are widely used to measure mechanical elastic properties of cellular components such as DNA strands, molecular filaments, and membranes [4-6]. These experimental advances require an understanding and determination of optical gradient forces in a predictive and tractable manner. Theoretical calculations to date that predict the force acting on a general particle are strictly applicable to either small particles (electromagnetic theory) or very large particles (ray optics calculations) . In the intermediate regime (typically 1 10 mm), which is often the most interesting one, both theories are incompatible with experimental results .
Single-beam trapping of micro-beads in polarized light: Numerical simulations
Optics express, 2006
Using numerical solutions of Maxwell's equations in conjunction with the Lorentz law of force, we compute the electromagnetic force distribution in and around a dielectric micro-sphere trapped by a focused laser beam. Dependence of the optical trap's stiffness on the polarization state of the incident beam is analyzed for particles suspended in air or immersed in water, under conditions similar to those realized in practical optical tweezers. A comparison of the simulation results with available experimental data reveals the merit of one physical model relative to two competing models; the three models arise from different interpretations of the same physical picture.