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Cooperative microlenses

L.E. Helseth and T.M. Fischer

Open Access Open Access

Abstract

We demonstrate that microspheres above a substrate act as microscopic lenses. Their Brownian motion causes the focal point to change abruptly, thereby creating characteristic intensity fluctuations which depend on the interaction between the spheres. To this end, superparamagnetic spheres in a magnetic field assemble into long pearl chains, where the intensity fluctuations depend on the stiffness of the chain. Upon assembling the superparamagnetic beads into a two-dimensional colloidal crystal, the fluctuations are restricted in two dimensions, and temporal network structures develop.

©2004 Optical Society of America

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Supplementary Material (1)

Media 1: AVI (788 KB)     

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Figures (9)
Fig. 1.
Fig. 1. A single bead focusing and collecting the light.

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Fig. 2.
Fig. 2. Intensity fluctuations of a single bead diffusing on top of a garnet surface.

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Fig. 3.
Fig. 3. (AVI 199 KB) Blinking chain above a glass slide in presence of a magnetic field of about 500 A/m (6 Oe).

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Fig. 4.
Fig. 4. Intensity fluctuations of three beads in a chain consisting of about 30 beads.

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Fig. 5.
Fig. 5. Chain above a glass slide in presence of a magnetic field of about 3000 A/m (38 Oe).

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Fig. 6.
Fig. 6. Schematic drawing of the beads moving toward the domain wall.

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Fig. 7.
Fig. 7. The intensity increases as the bead is getting closer to the domain wall.

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Fig. 8.
Fig. 8. (AVI 788 KB) Two dimensional colloidal crystal observed in reflection with crossed polarizers.

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Fig. 9.
Fig. 9. The real crystal structure observed in transmission mode (a) and its Fourier transform (b). The image also shows the inverted intensity distribution of the same structure seen in reflection mode a few seconds later (c) as well as its Fourier Transform (d).

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Equations (4)

Equations on this page are rendered with MathJax. Learn more.

h m = l D ( B l D G ) ,
f ≈ a 2 n w n b − n w .
Δ r ∼ λ f 2 n w a ,
Δ z ∼ λ 2 ( f n w a ) 2 ,
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