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The operation of photovoltaic (PV) tweezers, using the evanescent light-induced PV fields to trap and pattern nano- and micro-meter particles on a LiNbO(3) crystal surface, is discussed. The case of a periodic light pattern is addressed in detail, including the role of particle shape and the modulation index of the light pattern. The use of a single Gaussian light beam is also considered. Illustrative experiments for the two situations are presented. The performance of such PV tweezers in comparison to the best established case of optical tweezers, using optical forces, is considered. Differential features between the two trapping approaches are remarked.

The application of evanescent photovoltaic (PV) fields, generated by visible illumination of Fe:LiNbO 3 substrates, for parallel massive trapping and manipulation of micro-and nano-objects is critically reviewed. The technique has been often referred to as photovoltaic or photorefractive tweezers. The main advantage of the new method is that the involved electrophoretic and/or dielectrophoretic forces do not require any electrodes and large scale manipulation of nano-objects can be easily achieved using the patterning capabilities of light. The paper describes the experimental techniques for particle trapping and the main reported experimental results obtained with a variety of micro-and nano-particles (dielectric and conductive) and different illumination configurations (single beam, holographic geometry, and spatial light modulator projection). The report also pays attention to the physical basis of the method, namely, the coupling of the evanescent photorefractive fields to the dielectric response of the nano-particles. The role of a number of physical parameters such as the contrast and spatial periodicities of the illumination pattern or the particle deposition method is discussed. Moreover, the main properties of the obtained particle patterns in relation to potential applications are summarized, and first demonstrations reviewed. Finally, the PV method is discussed in comparison to other patterning strategies, such as those based on the pyroelectric response and the electric fields associated to domain poling of ferroelectric materials.

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Optical and optoelectronic techniques for micro-and nano-object manipulation are becoming essential tools in nano-and bio-technology. A remarkable optoelectronic technique that has experimented a strong development in the last few years is the so called photovoltaic optoelectronic tweezers. It is based on the light-induced electric fields generated by the bulk photovoltaic effect in certain ferroelectrics such as LiNbO3. The technique is simple and versatile, enabling a successful manipulation of a large variety of micro-and nano-objects with only optical control, without the need of electrodes or power supplies. However, it is still a challenge for this tool, to handle objects immersed in aqueous solution due to the electric screening effects of polar liquids. This has hindered their application in biotechnology and biomedicine where most processes develop in aqueous solution. In this work, a new efficient route to overcome this problem has been proposed and demonstrated. It uses photovoltaic optoelectronic tweezers to manipulate aqueous droplets, immersed in a non-polar oil liquid, but hanging at the interface air-oil. In this singular configuration, the high electric fields generated in the photovoltaic substrate allow a simple and flexible manipulation of aqueous droplets controlled by the light. Droplet guiding, trapping, merging and splitting have been achieved and efficient operation with water and a variety of biodroplets (DNA, sperm, and PBS solutions) have been demonstrated. The reported results overcome a main limitation of these tweezers to handle bio-materials and promises a high potential for biotechnological and biochemistry applications including their implementation in optofluidic devices.