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Tunable liquid microlens arrays in electrode-less configuration and their accurate characterization by interference microscopy

L. Miccio, A. Finizio, S. Grilli, V. Vespini, M. Paturzo, S. De Nicola, and Pietro Ferraro

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Abstract

A special class of tunable liquid microlenses is presented here. The microlenses are generated by an electrowetting effect under an electrode-less configuration and they exhibit two different regimes that are named here as separated lens regime (SLR) and wave-like lens regime (WLR). The lens effect is induced by the pyroelectricity of polar dielectric crystals, as was proved in principle in a previous work by the same authors (S. Grilli et al., Opt. Express 16, 8084, 2008). Compared to that work, the improvements to the experimental set-up and procedure allow to reveal the two lens regimes which exhibit different optical properties. A digital holography technique is used to reconstruct the transmitted wavefront during focusing and a focal length variation in the millimetre range is observed. The tunability of such microlenses could be of great interest to the field of micro-optics thanks to the possibility to achieve focus tuning without moving parts and thus favouring the miniaturization of the optical systems.

©2009 Optical Society of America

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Supplementary Material (5)

Media 1: MOV (642 KB)     
Media 2: MOV (536 KB)     
Media 3: MOV (3977 KB)     
Media 4: MOV (2419 KB)     
Media 5: MOV (1599 KB)     

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Figures (10)
Fig. 1.
Fig. 1. (a) Optical microscope image of a typical PPLN with a square array of hexagonal reversed domains; (b), (c) schematic views of the sample cross section corresponding to the wave-like lenses regime and to the separated lenses regime of the microlens array, respectively. In both cases the temperature of the substrate is decreasing. The difference between the two regimes stands in the liquid thickness. The black arrows indicate the orientation of the spontaneous polarization.

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Fig. 2.
Fig. 2. Schematic view of the interferometric configuration. PBS polarizing beam splitter; MO microscope objective; PH pin-hole; M mirror; BS beam splitter.

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Fig. 3.
Fig. 3. Movies showing the evolution of the wrapped mod 2π phase map during (a) the heating (Media 1) and (b) the cooling process (Media 2).

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Fig. 4.
Fig. 4. Experimental and fitted 1D profiles of the unwrapped phase distribution corresponding to (a) the heating and (b) cooling process; (c) (d) focal length variation as a function of temperature in case of the heating and the cooling process, respectively.

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Fig. 5.
Fig. 5. Two-dimensional representation of (a) the wrapped and (b) the unwrapped phase map corresponding to 4×4 microlens array.

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Fig. 6.
Fig. 6. (a) Measured phase distribution and (b) corresponding fitted surface; surface distribution of (c) the focus term, (d) the third order spherical aberration, (e) the astigmatism at 45° term, (f) the astigmatism at 90°, (g) the triangular astigmatism on x base, (h) the triangular astigmatism on y base.

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Fig. 7.
Fig. 7. Movies of the wrapped phase distribution evaluated for a portion of the lens array in case of separated lenses SRL (a) heating (Media 3) and (b) cooling (Media 4).

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Fig. 8.
Fig. 8. Profile of the phase distribution during (a) heating and (b) cooling; (c),(d) temperature dependence of the focal length for the heating and the cooling process, respectively, in case of separated lenses.

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Fig. 9.
Fig. 9. (a) Measured and fitted phase profile; (b) list of the coefficient values of the linear expansion resulting from the fitting process.

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Fig. 10.
Fig. 10. Optical microscope movie (Media 5) of a single liquid lens of an array of separated lenses after formation and successive thermal tuning.

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Tables (1)

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Table 1. List of coefficient values.

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Equations (2)

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Φ ( x ) = π λ ( x 2 ) f
W ( x ) = a ( 1 ) x 4 + a ( 2 ) x 2 + a ( 2 ) x + a ( 4 )
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