Abstract:Light field microscopy is a new technique for high-speed volumetric imaging of weakly scattering or fluorescent specimens. It employs an array of microlenses to trade off spatial resolution against angular resolution, thereby allowing a 4-D light field to be captured using a single photographic exposure without the need for scanning. The recorded light field can then be used to computationally reconstruct a full volume. In this paper, we present an optical model for light field microscopy based on wave optics, instead of previously reported ray optics models. We also present a 3-D deconvolution method for light field microscopy that is able to reconstruct volumes at higher spatial resolution, and with better optical sectioning, than previously reported. To accomplish this, we take advantage of the dense spatio-angular sampling provided by a microlens array at axial positions away from the native object plane. This dense sampling permits us to decode aliasing present in the light field to reconstruct high-frequency information. We formulate our method as an inverse problem for reconstructing the 3-D volume, which we solve using a GPU-accelerated iterative algorithm. Theoretical limits on the depth-dependent lateral resolution of the reconstructed volumes are derived. We show that these limits are in good agreement with experimental results on a standard USAF 1951 resolution target. Finally, we present 3-D reconstructions of pollen grains that demonstrate the improvements in fidelity made possible by our method.

Light field microscopy has been proposed as a new high-speed volumetric computational imaging method that enables reconstruction of 3-D volumes from captured projections of the 4-D light field. Recently, a detailed physical optics model of the light field microscope has been derived, which led to the development of a deconvolution algorithm that reconstructs 3-D volumes with high spatial resolution. However, the spatial resolution of the reconstructions has been shown to be non-uniform across depth, with some z planes showing high resolution and others, particularly at the center of the imaged volume, showing very low resolution. In this paper, we enhance the performance of the light field microscope using wavefront coding techniques. By including phase masks in the optical path of the microscope we are able to address this non-uniform resolution limitation. We have also found that superior control over the performance of the light field microscope can be achieved by using two phase masks rather than one, placed at the objective's back focal plane and at the microscope's native image plane. We present an extended optical model for our wavefront coded light field microscope and develop a performance metric based on Fisher information, which we use to choose adequate phase masks parameters. We validate our approach using both simulated data and experimental resolution measurements of a USAF 1951 resolution target; and demonstrate the utility for biological applications with in vivo volumetric calcium imaging of larval zebrafish brain.

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Mechanical displacement in commonly used piezoelectric materials is typically restricted to linear or biaxial in nature and to a few percent of the material dimensions. Here, we show that free-standing BaTiO3 membranes exhibit non-conventional electromechanical coupling. Under an external electric field, these superelastic membranes undergo controllable and reversible "sushi-rolling-like" 180° folding-unfolding cycles. This crease-free folding is mediated by charged ferroelectric domains, leading to a giant > 3.8 and 4.6 µm displacements for a 30-nm thick membrane at room temperature and 60 °C, respectively. Further increasing the electric field above the coercive value changes the fold curvature, hence augmenting the effective piezoresponse. Finally, it is found that the membranes fold with increasing temperature followed by complete immobility of the membrane above the Curie temperature, allowing us to model the ferroelectric-domain origin of the effect.The electromechanical power conversion of piezoelectrics is the basis for a broad range of sensing, actuating, and communication technologies, including ultrasound imaging and cellular phones. 1-3 Recent interest in electromechanical energy harvesting 4,5 as well as in flexible electronics for wearable devices, 6,7 nano motors, 8 and medical applications 9-11 raises a need for flexible piezoelectric materials and devices. Modern applications of piezoelectrics hinge on thin films, 12-14 however, the substrate in such geometries is typically rigid, preventing the development of flexible devices. Flexible piezoelectric devices are therefore typically based on either nanowires 4 or on thin-film systems, but with substrates that have been designed especially for such applications. 15,16 Most piezoelectric applications rely on lead-based materials, which exhibit strong piezoelectric coefficients. Nevertheless, the toxicity of these materials is undesirable for environmental considerations, while it also disqualifies them for medical or wearable applications. Likewise, traditional thin-film geometries limit the electromechanical excitation modes. That is, usually, uniaxial electric field results in either parallel or perpendicular uniaxial or biaxial mechanical deformation (or vice versa).Nevertheless, the interest in flexible-electronic technologies raises a need for advanced electromechanical excitation modes, e.g., for motorized devices, including microscale aerial vehicles. 17 Substrate removal for piezoelectric films or membranes augments their functional properties, 18-21 mainly thanks to mechanically-induced ferroic-domain reorganization. 22 However, the preparation of completely stand-alone substrate-free films has remained a challenge. Lu et al. 23 demonstrated lately a general method to prepare oxide materials in the form of membranes, i.e., continuous free-standing thin films with no substrate. More recently, Dong et al. 24 used this method to process BaTiO3 membranes, which is a well-known lead-free piezoelectric and ferroelectric material. This work show...

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