Showing papers by "Pietro Ferraro published in 2009"

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

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.

Controlling depth of focus in 3D image reconstructions by flexible and adaptive deformation of digital holograms

Abstract: We show here that through an adaptive deformation of digital holograms it is possible to manage the depth of focus in 3D imaging reconstruction. Deformation is applied to the original hologram with the aim to put simultaneously in focus, and in one reconstructed image plane, different objects lying at different distances from the hologram plane (i.e., CCD sensor). In the same way, by adapting the deformation it is possible to extend the depth of field having a tilted object entirely in focus. We demonstrate the method in both lensless as well as in microscope configuration.

Creating an extended focus image of a tilted object in Fourier digital holography

Abstract: We present a new method to numerically reconstruct images on a tilted plane by digital holography in Fourier configuration. The proposed technique is based on a quadratic deformation of spatial coordinates of the digital hologram. By this approach we demonstrate that it is possible to recover the extended focus image (EFI) of a tilted object in a single reconstruction step from the deformed hologram.

Investigation of angular multiplexing and de-multiplexing of digital holograms recorded in microscope configuration.

Abstract: We investigated a method for the angular multiplexing and demultiplexing of digital holograms recorded in microscope off-axis configuration. The multiplexing has been performed rotating numerically one hologram at different angles and adding all the rotated holograms to obtain a single synthetic digital hologram. Then the digital holograms were de-multiplexed thanks to the unique property of the digital holography to manage numerically the complex wavefields at different image planes. We show that it is possible to retrieve correctly quantitative information about the amplitude and phase maps. The obtained results can be useful to employ the multiplexing technique during the recording process by rotating the CCD array.

Correct self-assembling of spatial frequencies in super-resolution synthetic aperture digital holography

Abstract: Synthetic aperture enlargement is obtained, in lensless digital holography, by introducing a diffraction grating between the object and the CCD camera with the aim of getting super-resolution. We demonstrate here that the spatial frequencies are naturally self-assembled in the reconstructed image plane when the NA is increased synthetically at its maximum extent of three times. By this approach it possible to avoid the use of the grating transmission formula in the numerical reconstruction process, thus reducing significantly the noise in the final super-resolved image. Demonstrations are reported in 1D and 2D with an optical target and a biological sample, respectively.

Self-patterning of a polydimethylsiloxane microlens array on functionalized substrates and characterization by digital holography

Abstract: Microlens arrays are realized through a self-arrangement process of thin liquid polymeric polydimethylsiloxane (PDMS) film on a functionalized polar dielectric crystal substrate. The self-arrangement process is named the pyro-electro-wetting mechanism. The substrate, a LiNbO3 (LN) z-cut wafer, has been micro-engineered with periodically poled ferroelectric domains, with the aim to provide an appropriate wettability patterning induced by a thermal stimulus. Different experimental procedures have been explored demonstrating that arrays of thousands of microlenses, having a diameter size of 100 µm and focal lengths ranging between 300 and 1100 µm, can be fabricated. Furthermore, a microscope interference method based on digital holography is adopted for microlens characterization.

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

Abstract: 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.

Sum-frequency generation of cw ultraviolet radiation in periodically poled LiTaO 3

Abstract: We present cw sum-frequency generation of UV radiation at 355 nm based on a high-power laser at 1064 nm and a two-stage quasi-phase-matching nonlinear interaction in periodically poled LiTaO3 crystals. In a first stage, second harmonic at 532 nm is generated. Then, the outcoming IR and green light interact in a second nonlinear crystal to generate about 7 mW of ultraviolet radiation at the sum frequency.

Ferroelectric crystals for photonic applications : including nanoscale fabrication and characterization techniques

Abstract: Micro-structuring and ferroelectric domain engineering of single crystal lithium Niobate.- Fabrication and characterization of self-assembled ferroelectric linear and nonlinear photonic crystals: GaN and LiNbO3.- Sub-micron structuring of LiNbO3 crystals with multi-period and complex geometries.- Nonlinear optical waveguides in stoichiometric lithium tantalate.- 3-D integrated optical microcircuits in lithium niobate written by spatial solitons.- Light aided domain patterning and rare earth emission based imaging of ferroelectric domains.- Visual and quantitative characterization of ferroelectric crystals and related domain engineering processes by interferometric techniques.- New insights into ferroelectric domain imaging with piezoresponse force microscopy.- Structural characterization of periodically poled lithium niobate crystals by high resolution X-ray diffraction.- Nonlinear Interactions in Periodic and Quasi-Periodic Nonlinear Photonic Crystals.- Domain-engineered ferroelectric crystals for nonlinear and quantum optics.- Photonic and phononic band gap properties of lithium Niobate.- Lithium niobate whispering gallery resonators: applications and fundamental studies.- Applications of domain engineering in ferroelectrics for photonic applications.- Electro-optics effect in periodically domain-inverted ferroelectrics crystals: principles and applications.

Mid-infrared tunable two-dimensional Talbot array illuminator

Abstract: We report the realization and characterization of a tunable, two-dimensional Talbot array illuminator for mid-infrared (MIR) wavelengths. A phase array, prepared by deposing tin-doped indium oxide electrodes on a square-lattice-geometry poled LiNbO3 sample, is illuminated by a difference-frequency generator emitting at 3 μm. Then, combining the electro-optic with the Talbot effect allows generation of a variety of light patterns under different values of distance and external electric field. Several potential applications with great relevance to the MIR spectral region are discussed.

Full characterization of the photorefractive bright soliton formation process using a digital holographic technique

Abstract: An extensive characterization of the photorefractive bright soliton writing process in a lithium niobate crystal is presented. An interferometric approach based on a digital holographic technique has been used to reconstruct the complex wavefield at the exit face of the crystal. Temporal evolution of both intensity and phase profile of the writing beam has been analysed. The effective changes of the refractive index of the medium during the writing process and after the soliton formation are determined from the optical phase distribution. This method provides a reliable way to observe the process of soliton formation, whereas the determination of the intensity distribution of the output beam does not show clearly whether the soliton regime has been achieved or not. Furthermore, a detailed analysis of the soliton in a steady-state situation and under different writing conditions is presented and discussed.

Resolution-enhanced approaches in digital holography

Abstract: Resolution is an important issue in inspection of objects on microscopic scale. Various approaches have been investigated to increase the optical resolution behind the diffraction limit of an optical imaging system. However every time the optical resolution of a fixed optical system is overcome it is possible to speak of super-resolution. Demonstration that super-resolution have been deeply investigated in interference microscopy through various approaches. Here we discuss briefly the different techniques that have been adopted in interferometry and specifically in digital holography (DH). Then we illustrate a novel method that uses a dynamic diffraction phase-grating for increasing synthetically the aperture of a DH imaging system in lens-less configuration. Consequently the optical resolution of the DH systems can be increased of a factor of 3. The aim of the study is to demonstrate that super-resolution is possible and is a practical and viable method for a coherent optical microscope. We take benefit of the numerical reconstruction properties of DH in combination with diffraction grating to get super-resolution. The approaches could be used for metrology and imaging application in various fields of engineering and biology.

Non-Invasive Optical Detection of Submicron Domains in Lithium Niobate Crystals

Abstract: A non-invasive detection of shallow submicron-scale ferroelectric domains has been demonstrated by optical diffraction technique. Periodic sub-micron domains are of great interest to the field of optics and optoeletronics and non-destructive techniques for their accurate inspection are always desirable. Submicron-scale domains have been fabricated by electric field overpoling in lithium niobate substrates and successively inspected by visualizing the diffraction grating just after poling and destructive etching process. The results demonstrate the usefulness of the technique for a non-invasive and rapid inspection of submicron-scale domains otherwise not visible by the conventional non-destructive methods available nowadays.

Optical-frequency-comb stabilised lasers for interrogation of fibre-resonator strain sensors

Abstract: Quasi-static and dynamic strain sensing is performed by means of a fibre Bragg-grating resonator based on an optical-frequency-comb stabilised laser. The comb synthesizer, centred around 1560 nm, is a fs-pulsed fibre laser, which is phase-locked to an ultra-stable quartz oscillator, serving as an optical reference to minimise laser-frequency and phase noise both in the long and short-term operation. Detection limits in the order of a few peRMS/√Hz, in the Hz and sub-Hz range, are demonstrated, while a sub- pe sensitivity is expected at higher frequencies in the acoustic range.

Processing of optically-captured digital holograms for three-dimensional display

Abstract: In digital holography, holograms are usually optically captured and then two-dimensional slices of the reconstruction volume are reconstructed by computer and displayed on a two-dimensional display. When the recording is of a three-dimensional scene then such two-dimensional display becomes restrictive. We outline our progress on capturing larger ranges of perspectives of three-dimensional scenes, and our progress on four approaches to better visualise this three-dimensional information encoded in the digital holograms. The research has been performed within a European Commission funded research project dedicated the capture, processing, transmission, and display of real-world 3D and 4D scenes using digital holography.

Microfluidic system based on the digital holography microscope for analysis of motile sperm

Abstract: Digital holography (DH) has been employed in the retrieval of three dimensional images of bull's sperm heads. The system allows a three dimensional analysis of the sperm morphology by means of a Digital Holographic Microscope (DHM). Microscopic holography measurements are performed by projecting a magnified image of a microscopic hologram plane onto a CCD plane. This could constitute the basis of an alternative method for the zoothecnic industry aimed at the investigation of morphological features and the sorting of the motile sperm cells. Indeed, one of the main advantages of digital holography consists in its full non-invasivity and in the capability of investigating the shape of the sperm cells without altering their characteristics. In particular the proposed technique could be applied to investigate the frequency of aberrant spermatozoa. Until now, in fact, such industrial investigations have been mainly performed by means of specific painting probes: unfortunately this technique dramatically reduces the vitality of the sperm cells and can even cause chromosome aberration, making them useless for the zootechnical applications.

Aliasing-based incoherent optical spatial image processor

Abstract: We present a configuration for a real-time spatial image processor that is based upon an imaging setup in which a grating with Fourier coefficients with tunable phase is attached to the object plane. The illumination that is used for the proposed concept is spatially incoherent. By proper adjusting of the magnification of the imaging system to the spatial period of the grating and the sampling grid of the camera, the aliasing effect due to the digital sampling realizes a nonuniform and tunable spectral distribution (a filter) that is applied over the spectrum of the object. Preliminary numerical and experimental demonstration of the operation principle is provided with a spatial LiNbO3 hexagonal grating.

Incoherent Optical Spatial Image Processing

Abstract: In this paper we present a new configuration for real-time spatial image processor that is based upon an imaging setup in which a grating with Fourier coefficients with tunable phase is attached to the object plane. The illumination that is used for the proposed concept is spatially incoherent. By proper adjusting of the magnification of the imaging system to the spatial period of the grating and the sampling grid of the camera, the aliasing effect due to the digital sampling realizes a non uniform and tunable spectral distribution (a filter) that is applied over the spectrum of the object.

Adaptive deformation of digital holograms for full control of depth-of-focus in 3D imaging

Abstract: Through adaptive deformation of digital holograms it is possible to manage and control the depth of focus in 3D imaging. Objects lying at different distances can be set simultaneously infocus.

Numerical aperture increasing in digital holography by a two-dimensional grating

Abstract: We show how it is possible to increase synthetically the numerical aperture of an interferometric imaging system using a 2D diffraction grating, thus improving the spatial resolution of the reconstructed images up to three times.

Imaging 3-D Objects by Extending the Depth of Focus in Digital Holography

Abstract: Digital holography is an imaging technique offering both sub-wavelength resolution and real-time capabilities to record 3-D objects using the interference between an object wave and a reference wave captured by an image sensor such as a CCD array. The basic advantage of digital holography is that it can quantitatively extract the three-dimensional (3-D) information of the object from the numerical reconstruction of a single digitally recorded hologram [1,2,3]. Since the information of the optically interfering waves is stored in the form of matrices, the numerical reconstruction process enables full digital processing of the holograms and offers many more possibilities than conventional optical processing. It is possible to numerically focus on any section of the 3-D volume object without mechanical focusing adjustment. This opens the field to a variety applications, such as 3-D microscopic investigations and real-time imaging of biological specimens where wavelength-scanning digital interference holography [4] is used to reconstruct the 3-D volume from a set of scanning tomographic images, hybrid holographic microscopy [5], 3-D microscopy by optical scanning holography [6], phase shifting digital holog-

Self-assembled Liquid Microlens Arrays activated by pyroelectric effect

Abstract: We demonstrate that Tuneable Liquid Microlens Array are self-assembled and controlled by pyroelectric effect on functionalised LiNbO 3 substrates. Liquid microlens array having spherical, cylindrical and toroidal shape can be activated.