Towards absolute calibration of optical tweezers

Aiming at absolute force calibration of optical tweezers, following a critical review of proposed theoretical models, we present and test the results of MDSA (Mie-Debye-Spherical Aberration) theory, an extension of a previous (MD) model, taking account of spherical aberration at the glass/water interface. This first-principles theory is formulated entirely in terms of experimentally accessible parameters (none adjustable). Careful experimental tests of the MDSA theory, undertaken at two laboratories, with very different setups, are described. A detailed description is given of the procedures employed to measure laser beam waist, local beam power at the transparent microspheres trapped by the tweezers, microsphere radius and the trap transverse stiffness, as a function of radius and height in the (inverted microscope) sample chamber. We find generally very good agreement with MDSA theory predictions, for a wide size range, from the Rayleigh domain to large radii, including the values most often employed in practice, and at different chamber heights, both with objective overfilling and underfilling. The results asymptotically approach geometrical optics in the mean over size intervals, as they should, and this already happens for size parameters not much larger than unity. MDSA predictions for the trapping threshold, position of stiffness peak, stiffness variation with height, multiple equilibrium points and `hopping' effects among them are verified. Remaining discrepancies are ascribed to focus degradation, possibly arising from objective aberrations in the infrared, not yet included in MDSA theory.Comment: 15 pages, 20 figure

Determining Index Data from Refracted/Diffracted Rays

An optical fiber is a cylindrical waveguide of visible (or near visible) light composed of silica doped with germanium oxide (Ge02). The guiding is accomplished by varying the level of Ge02 in the fiber to create an index of refraction in the fiber that varies with the radius of the fiber. The fiber is manufactured by creating a large cane with a radius on the order of centimeters that goes through a sequence of heatings and extrusions until it reaches the finished size, which has a radius on the order of microns. To assess the quality of optical fibers during their manufacture, it is common to measure the index of refraction of a cane during an intermediate step of the process. The index of refraction varies with the radius of the cane, and is written n(r). The desired profile varies depending on the future use of the optical fiber, but a standard profile is a simple parabola. The actual profile in an optical fiber does not match the desired profile due to the way in which optical fibers are manufactured. A glass blank is spun on a lathe while a flame that is fed an appropriate level of silica and Ge02 moves rapidly back and forth along the cane. Soot from the flame is deposited on the spinning blank. Naturally the deposition will create spiral patterns of doping on the cane. This creates oscillations in the level of Ge02, and therefore in the desired refractive index. Because soot is being deposited at a constant volumetric rate, the wavelength of the oscillation decreases as the radius of the cane increases. The flame travels up and back along the cane in each layer, so the layer structure has two local maxima in each full oscillation. Because the oscillatory behavior of n(r) is unimportant in the final product, Corning asked to determine a way to remove the noise in the measurements of n(r) caused by the oscillations, and determine the background profile

Transmission properties in waveguides: An optical streamline analysis

A novel approach to study transmission through waveguides in terms of optical streamlines is presented. This theoretical framework combines the computational performance of beam propagation methods with the possibility to monitor the passage of light through the guiding medium by means of these sampler paths. In this way, not only the optical flow along the waveguide can be followed in detail, but also a fair estimate of the transmitted light (intensity) can be accounted for by counting streamline arrivals with starting points statistically distributed according to the input pulse. Furthermore, this approach allows to elucidate the mechanism leading to energy losses, namely a vortical dynamics, which can be advantageously exploited in optimal waveguide design.Comment: 8 pages, 4 figure

High frequency integrable regimes in nonlocal nonlinear optics

We consider an integrable model which describes light beams propagating in nonlocal nonlinear media of Cole-Cole type. The model is derived as high frequency limit of both Maxwell equations and the nonlocal nonlinear Schroedinger equation. We demonstrate that for a general form of nonlinearity there exist selfguided light beams. In high frequency limit nonlocal perturbations can be seen as a class of phase deformation along one direction. We study in detail nonlocal perturbations described by the dispersionless Veselov-Novikov (dVN) hierarchy. The dVN hierarchy is analyzed by the reduction method based on symmetry constraints and by the quasiclassical Dbar-dressing method. Quasiclassical Dbar-dressing method reveals a connection between nonlocal nonlinear geometric optics and the theory of quasiconformal mappings of the plane.Comment: 45 pages, 4 figure

Geometrodynamics of polarized light: Berry phase and spin Hall effect in a gradient-index medium

We review the geometrical-optics evolution of an electromagnetic wave propagating along a curved ray trajectory in a gradient-index dielectric medium. A Coriolis-type term appears in Maxwell equations under transition to the rotating coordinate system accompanying the ray. This term describes the spin-orbit coupling of light which consists of (i) the Berry phase responsible for a trajectory-dependent polarization variations and (ii) the spin Hall effect representing polarization-dependent trajectory perturbations. These mutual phenomena are described within universal geometrical structures underlying the problem and are explained by the dynamics of the intrinsic angular momentum carried by the wave. Such close geometro-dynamical interrelations illuminate a dual physical nature of the phenomena.Comment: 25 pages, 4 figures, review to appear in special issue of J. Opt. A: Pure Appl. Op

Brightness of Synchrotron radiation from Undulators and Bending Magnets

We consider the maximum of the Wigner distribution (WD) of synchrotron radiation (SR) fields as a possible definition of SR source brightness. Such figure of merit was originally introduced in the SR community by Kim. The brightness defined in this way is always positive and, in the geometrical optics limit, can be interpreted as maximum density of photon flux in phase space. For undulator and bending magnet radiation from a single electron, the WD function can be explicitly calculated. In the case of an electron beam with a finite emittance the brightness is given by the maximum of the convolution of a single electron WD function and the probability distribution of the electrons in phase space. In the particular case when both electron beam size and electron beam divergence dominate over the diffraction size and the diffraction angle, one can use a geometrical optics approach. However, there are intermediate regimes when only the electron beam size or the electron beam divergence dominate. In this asymptotic cases the geometrical optics approach is still applicable, and the brightness definition used here yields back once more the maximum photon flux density in phase space. In these intermediate regimes we find a significant numerical disagreement between exact calculations and the approximation for undulator brightness currently used in literature. We extend the WD formalism to a satisfactory theory for the brightness of a bending magnet. We find that in the intermediate regimes the usually accepted approximation for bending magnet brightness turns out to be inconsistent even parametrically.Comment: 72 pages plus cover, 4 figure

The wave energy flux of high frequency diffracting beams in complex geometrical optics

We consider the construction of asymptotic solutions of Maxwell's equations for a diffracting wave beam in the high frequency limit and address the description of the wave energy flux transported by the beam. With this aim, the complex eikonal method is applied. That is a generalization of the standard geometrical optics method in which the phase function is assumed to be complex valued, with the non-negative imaginary part accounting for the finite width of the beam cross section. In this framework, we propose an argument which simplifies significantly the analysis of the transport equation for the wave field amplitude and allows us to derive the wave energy flux. The theoretical analysis is illustrated numerically for the case of electron cyclotron beams in tokamak plasmas by using the GRAY code [D. Farina, Fusion Sci. Technol. 52, 154 (2007)], which is based upon the complex eikonal theory. The results are compared to those of the paraxial beam tracing code TORBEAM [E. Poli et al., Comput. Phys. Commun. 136, 90 (2001)], which provides an independent calculation of the energy flow