Figure 1

(Color online) Experimental principle: Oil and salt-water droplets are trapped in two counterpropagating Gaussian beams (150 mW at 532 nm) and illuminated by a blue LED through a field lens (F.L.). Particles are imaged with a 0.5 NA microscope objective (M.O.). A set of dichroic filters (not detailed) allows us to balance the blue and the green light. A polarizer ($p$) and $\lambda /4$ set was also used to avoid feedback in the laser. The photo in (a) comes from the image formed when the focus of the microscope objective is tuned to the green bright dots (apparently corresponding to the median plane of the droplet) while that in (b) is obtained from the sharpest blue light focus of the droplet. In (b), the bright scattering dots are barely observable in this image since they are now defocused.Reuse & Permissions

Figure 2

(Color online) Experimental photos and a simulation for a $2.70\mu \text{m}$ radius oil-droplet of refraction index 1.5 trapped in a counterpropagating circularly polarized beam configuration. Picture (a) is taken of the scattered trapping green light only. The circle indicates the perimeter of the sphere as deduced from spectroscopic measurements. In picture (b), imaging blue light incident along the axis of the microscope objective has been added. In (c) we display results from Mie calculations of counterpropagating plane waves followed by Fourier optics simulations of the imaging system (the boundary of the sphere is indicated by a circle).Reuse & Permissions

Figure 3

(Color online) Optically bound salt-water droplets. On each photograph, the largest droplet is roughly $1.4\mu \text{m}$ in radius. The smaller droplet radii are approximately $\pm 25\text{nm}$ in (a) $0.35\mu \text{m}$, (b) $0.45\mu \text{m}$, and (c) $0.45\mu \text{m}$ (upper) and $0.35\mu \text{m}$ (lower). The droplets sizes were deduced from comparison of glare points with numerical simulations.Reuse & Permissions

Figure 4

(Color online) Experimental setup for a spectroscopic determination of droplet properties. The laser is protected with a polarizer set (polarizer $P$ and $\lambda /4$-plate). Blue light coming from a LED is superimposed collinearly thanks to a dichroic beam splitter (DBS) through a field lens $\left(L2\right)$. The light collected by the microscope objective (MO) is sent onto a blazed grating $\left(g\right)$ and imaged $\left(L3\right)$ on a camera. The resulting camera image gives the spectrum along the longitudinal coordinate and the image of the trapping beam axis on the vertical coordinate. The lens $\left(L3\right)$ is actually a set of two cylindrical lenses, allowing different magnifications along the two transverse directions.Reuse & Permissions

Figure 5

(Color online) Top figure illustrates the normalized spectra of a trapped oil droplet ($a=1.29\mu \text{m}$ radius, 1.48 index) illuminated by a polarized wide spectrum blue LED (spectrum centered on 455 nm, $ka=16.54–18.85$). The dotted curves are numerical simulations corresponding to a modeling of the experimental setup of Fig. 4. The two lower pictures correspond to spectral dispersed images for TE $\left(n=21,22,23\right)$ and TM $\left(n=20,21,22\right)$ incident polarizations, respectively. The vertical coordinate is the trapping beam axis and the spectrum is spread over the horizontal (wavelength) coordinate. The bright regions correspond to the “glare points.” The global intensity envelope is that of the illuminating LED source.Reuse & Permissions

Figure 6

(Color online) From left to right, near field, pupil plane, and image of a $\left(n,n\right)$ whispering gallery resonance. The far field fringe pattern collected by an imaging lens has the same angular frequency as the near field. In the image plane, the resonance gives two bright dots appearing outside the physical perimeter of the sphere.Reuse & Permissions

Figure 7

(Color online) Ratio between the mode numbers of the transverse magnetic resonances and the size parameter at which their sharpest resonance occurs (refractive index contrast of the spheres $n=1.5$). Resonances corresponding to mode numbers higher than 10 will give rise to source regions that will appear to lie outside the physical perimeter since $n$ in these cases is larger than the size parameter $ka$ for which they occur.Reuse & Permissions

Figure 8

(Color online) Spectrally dispersed upper glare point of a $3.66\mu \text{m}$ radius oil-droplet with TE-polarized illuminating light. The droplet’s center is adopted as the origin of the axial coordinates, $z=0$. The lower graph is a horizontal mirror of the top picture using a logarithmic scale in intensity. The physical position of the droplet perimeter is indicated by the (green) lines. The images of the sharp gallery resonances (more readily visible on the logarithmic scale) appear as if located outside of the physical droplet perimeter.Reuse & Permissions

Figure 9

(Color online) Far field scattering from counterpropagating plane waves impinging on a spherical dielectric sphere (index $1.5$, $ka=23$) as a function of $\theta$. Blue dotted curve: ${\mathbf{e}}_{\theta }$ polarized field; green dashed curve: ${\mathbf{e}}_{\phi }$ polarized field; red solid curve: total intensity. The energy scattered in the 90° direction is roughly one hundred times as small as in the forward direction.Reuse & Permissions