logIE logEI y lum Fig. 3. Computed distributions of the electric field intensity E(units: [/mr)in the xz-and yz-planes near the focus of a 0.9NA, /=5.0mm, diffraction- limited objective. The 70=532nm plane-wave illuminating the entrance pupil of the lens is linearly polarized along the x-axis. Sx1 Sx10 0.604-020.00.20.40.60.6-040.20.00.20.40.6 x Jum x um Fig 4 Distribution of the Poynting vector S(units: [/m))in and around a glass micro-sphere (n= 1.5, d=460nm). The focused beam, obtained by sending a linearly-polarized plane-wave (polarization along y)through a 0. 9NA objective, propagates along the negative --axis. Sphere center offset from the focal point:(250,0, 50)nm. The(S, S_)vector-field is superimposed on the color-coded S. plot on the right-hand side if the laser power level is adjusted to a few microwatts, then the radiation pressure will work against the force of gravity to hold the bead in a stable trap along the :-axis )The computed anisotropy of this trap in the lateral direction is s)=1-(K/K,)=-015, where Kr and K, are the trap stiffness coefficients along the x-axis given by dFx/ ax for x- and y-polarized beams, respectively, Ref. [12]. The aforementioned s/ was computed at the x-offset value of 50nm, where z-offset A Oum is chosen to yield the maximum of Fr in the vicinity of the center of the small rectangles depicted in Fig. 5, where Fx is fairly insensitive to small variations of =. The computed stiffness anisotropy is plotted in Fig. 6 versus the particle diameter d for spherical #67575-$15.00USD Received 15 February 2006, revised 7 April 2006, accepted 10 April 2006 (C)2006OSA 17 April 2006/Vol 14, No 8/OPTICS EXPRESS 3665Fig. 3. Computed distributions of the electric field intensity |E| 2 (units: [V2/m2]) in the xz- and yz-planes near the focus of a 0.9NA, f = 5.0mm, diffraction-limited objective. The λ0 = 532nm plane-wave illuminating the entrance pupil of the lens is linearly polarized along the x-axis. Fig. 4. Distribution of the Poynting vector S (units: [W/m2]) in and around a glass micro-sphere (n = 1.5, d = 460nm). The focused beam, obtained by sending a linearly-polarized plane-wave (polarization along y) through a 0.9NA objective, propagates along the negative z-axis. Sphere center offset from the focal point: (250,0,50)nm. The (Sx,Sz) vector-field is superimposed on the color-coded Sz plot on the right-hand side. if the laser power level is adjusted to a few microwatts, then the radiation pressure will work against the force of gravity to hold the bead in a stable trap along the z-axis.) The computed anisotropy of this trap in the lateral direction is sl = 1−(κx/κy) = −0.15, where κx and κy are the trap stiffness coefficients along the x-axis given by ∂Fx/∂x for x- and y-polarized beams, respectively, Ref. [12]. The aforementioned sl was computed at the x-offset value of 50nm, where z-offset ≈ 0μm is chosen to yield the maximum of Fx in the vicinity of the center of the small rectangles depicted in Fig. 5, where Fx is fairly insensitive to small variations of z. The computed stiffness anisotropy is plotted in Fig. 6 versus the particle diameter d for spherical #67575 - $15.00 USD Received 15 February 2006; revised 7 April 2006; accepted 10 April 2006 (C) 2006 OSA 17 April 2006 / Vol. 14, No. 8 / OPTICS EXPRESS 3665