Veronica Vespini

Liquid micro-lens array activated by selective electrowetting on lithium niobate substrates.

Lens effect was obtained in an open microfluidic system by using a thin layer of liquid on a polar electric crystal like LiNbO3. An array of liquid micro-lenses was generated by electrowetting effect in pyroelectric periodically poled crystals. Compared to conventional electrowetting devices, the pyroelectric effect allowed to have an electrode-less and circuit-less configuration. An interferometric technique was used to characterize the curvature of the micro-lenses and the corresponding results are presented and discussed. The preliminary results concerning the imaging capability of the micro-lens array are also reported.

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

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.

3D lithography by rapid curing of the liquid instabilities at nanoscale.

In liquids realm, surface tension and capillarity are the key forces driving the formation of the shapes pervading the nature. The steady dew drops appearing on plant leaves and spider webs result from the minimization of the overall surface energy [Zheng Y, et al. (2010) Nature 463:640–643]. Thanks to the surface tension, the interfaces of such spontaneous structures exhibit extremely good spherical shape and consequently worthy optical quality. Also nanofluidic instabilities generate a variety of fascinating liquid silhouettes, but they are however intrinsically short-lived. Here we show that such unsteady liquid structures, shaped in polymeric liquids by an electrohydrodynamic pressure, can be rapidly cured by appropriate thermal treatments. The fabrication of many solid microstructures exploitable in photonics is demonstrated, thus leading to a new concept in 3D lithography. The applicability of specific structures as optical tweezers and as novel remotely excitable quantum dots–embedded microresonators is presented.

Surface-Charge Lithography for Direct PDMS Micro-Patterning

Direct patterning of PDMS films is achieved by modulating the wettability of polar dielectric substrates. Periodic array structures of microbumps can be madeup by functionalizing periodically poled lithium niobate crystals. The modulation of surface wettability is obtained through the spatial distribution of the surface electric charges generated by the pyroelectric effect under electrode-less configuration. An appropriate thermal treatment of the substrates assures both the wettability patterning and the fast cross-linking of the PDMS film.

Wettability patterning of lithium niobate substrate by modulating pyroelectric effect to form microarray of sessile droplets

The wettability of lithium niobate crystals was patterned by inducing the pyroelectric effect onto domain engineered samples. Compared to conventional electrowetting experiments, this technique allows one to obtain wettability patterning by an electrodeless configuration. In fact, the surface charges inducing the effect were generated pyroelectrically and spatially modulated by the domain grating. A microarray of sessile droplets was obtained on a single chip. Results and physical interpretation of the phenomena are reported and discussed.

Terahertz tuning of whispering gallery modes in a PDMS stand-alone, stretchable microsphere

We report on tuning the optical whispering gallery modes (WGMs) in a poly dimethyl siloxane-based (PDMS) microsphere resonator by more than 1 THz. The PDMS microsphere system consists of a solid spherical resonator directly formed with double stems on either side. The stems act like tie-rods for simple mechanical stretching of the microresonator, resulting in tuning of the WGMs by one free spectral range. Further investigations demonstrate that the WGM shift has a higher sensitivity (0.13 nm/μN) to an applied force when the resonator is in its maximally stretched state compared to its relaxed state.

Characterization of Bessel beams generated by polymeric microaxicons

We present a quick, simple and accurate digital holographic characterization of the Bessel beams produced by polymeric microaxicons. This technique allows the numerical reconstruction of both intensity and phase of the beam at whichever point starting from a single acquired hologram. From these data, it is possible to go back to the axicon structure, and to gather information about their characteristics. In particular, the focal length and the depth of focus of the axicon lens are experimentally measured, and the full width at half maximum of the beam is obtained too. The depth of focus, very large for a Bessel beam with respect to a Gaussian one, is successfully exploited for optical trapping of micrometric objects.

Hemicylindrical and toroidal liquid microlens formed by pyro-electro-wetting

We found that by opportune functionalization of a polar dielectric substrate, a self-arrangement of hemicylindrical or toroidal-shaped liquid droplets can be obtained. The process takes place when a thermal stimulus is provided to a poled substrate whose surface is covered by an oily substance layer. Liquid droplet self-arrangement is due to the pyroelectric effect, and interferometric characterization of the droplets is also reported. We investigated this open microfluidic system for exploring the possibility to obtain liquid cylindrical microlens with variable focal length. Liquid microtoroidal structures arrays are also realized. They could find application as resonant liquid microcavities for whispering gallery modes.

Printing of polymer microlenses by a pyroelectrohydrodynamic dispensing approach.

The investigation of a method for fabricating microlenses by a nozzle-free inkjet printing approach is reported. The new method, based on a pyroelectrohydrodynamic mechanism, is also able to dispense viscous liquids and to draw liquid phase drops directly from the reservoir. Specifically, by dispensing optical grade polymer dissolved in different solvent mixtures, microlenses were printed with a pattern defined directly through this deposition method. The reliability of the microlenses and the tunability of their focal properties were demonstrated through profilometric and inteferometric analyses.

Pyroelectric Adaptive Nanodispenser (PYRANA) microrobot for liquid delivery on a target

Manipulation of micro-sized objects in lab-on-a-chip and microfluidic environments is essential for different experiments and procedures ranging from chemical to biological applications and for experimental biotechnologies. For example polymeric particles, useful as targets for encapsulating or for being covered by drug vaccines, can be manipulated and controlled with the aim of releasing them to specific sites. Here we show a novel ElectroHydroDynamic tool able to control and manipulate dielectric micro-targets by a touch-less approach. This approach allows one to manipulate liquids and nano-particles simultaneously for specific delivery applications (i.e. decoration and coating). Thus a sort of EHD micro-robot is proposed. This flexible tool provides a new and powerful way to operate various tasks as demonstrated by the experiments reported here.