Showing papers by "Pietro Ferraro published in 2008"

Super-resolution in digital holography by a two-dimensional dynamic phase grating.

Abstract: An approach that uses an electro-optically tunable two dimensional phase grating to enhance the resolution in digital holographic microscopy is proposed. We show that, by means of a flexible hexagonal phase grating, it is possible to increase the numerical aperture of the imaging system, thus improving the spatial resolution of the images in two dimensions. The augment of the numerical aperture of the optical system is obtained by recording spatially multiplexed digital holograms. The grating tuneability allows one to adjust the intensity among the spatially multiplexed holograms maximizing the grating diffraction efficiency. Furthermore we demonstrate that the flexibility of the numerical reconstruction allows one to use selectively the diffraction orders carrying useful information for increasing the spatial resolution. The proposed approach can improve the capabilities of digital holography in three-dimensional imaging and microscopy.

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

Abstract: 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 circuitless 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.

Full Color 3-D Imaging by Digital Holography and Removal of Chromatic Aberrations

Abstract: In color digital holography the recording and reconstruction of three holograms, by an optical setup operating in multi-wavelength mode, is necessary. Incorrect superposition of different images at various wavelengths may occur due to the presence of chromatic aberrations. We demonstrate that the compensation of the chromatic aberrations can be achieved by using all of the information contained in the multi-wavelength digital holograms. Chromatic aberration is extracted from phase maps reconstructed at different wavelengths, resulting in perfect full color amplitude reconstructions. The method has important applications in color 3-D holographic imaging and display.

Dielectrophoretic trapping of suspended particles by selective pyroelectric effect in lithium niobate crystals

Abstract: We have developed a dielectrophoretic approach for trapping suspended dielectric particles. The electric forces were produced by using the pyroelectric effect in periodically poled lithium niobate substrates. Complex electric field distributions can be built up through an appropriate control of the temperature and the particles can be distributed according to the geometry of the reversed ferroelectric domain structure. The electrode-less configuration makes the technique easier to accomplish compared to conventional dielectrophoretic devices, with interesting possibilities of applications in the field of biology and biomedicine.

Surface-Charge Lithography for Direct PDMS Micro-Patterning

Abstract: 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

Abstract: 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.

Design and test of a laser-based optical-fiber Bragg-grating accelerometer for seismic applications

Abstract: We report on a proof-of-principle work aimed at the development of fast-response fiber-optic accelerometers for seismic monitoring. The system is based on a semiconductor diode-laser source that interrogates a newly devised two-dimensional inertial sensor suitable for measurement of horizontal ground accelerations. Plane acceleration components of the sensor's mass are detected by two fiber Bragg gratings anchored to its structure. Calibration and comparison with a commercial accelerometer are presented. A great potential, in terms of frequency response and sensitivity, is demonstrated in view of possible field applications in active seismic areas.

In situ investigation of periodic poling in congruent LiNbO3 by quantitative interference microscopy

Abstract: The dynamic evolution of the periodic poling into resist patterned LiNbO3 samples is investigated here by measuring the variation of the optical phase shift distribution across domain walls. In situ quantitative and high-contrast imaging is obtained through an interference microscope based on digital holography. Phase map movies of the evolving domains are presented and discussed. The method provides remarkable in situ information about the poling of ferroelectric crystals in the case of insulating constraints, under different voltage and resist conditions, providing a deeper understanding of the electric field periodic poling.

Numerical multiplexing and demultiplexing of digital holographic information for remote reconstruction in amplitude and phase.

Abstract: We investigate the possibility to multiplexing (Mux) and demultiplexing (de-Mux) numerically digital holograms (DHs) with the aim of optimizing their storage and/or transmission process. The DHs are multiplexed and demultiplexed thanks to the unique property of the digital holography to numerically manage the complex wavefields. We show that it is possible to retrieve correctly quantitative information about the amplitude and phase of one hundred DHs. This result can be useful to transmit efficiently, in terms of reduced amount of data, the DHs from the recording head to a remote display unit.

Self-organized and high-density filamentous nanodomain patterns fabricated in lithium niobate by discharge poling

Abstract: We report on spontaneous formation of nanoscale surface domains obtained in lithium niobate substrates by electric field discharge poling. A constant high voltage was applied at room temperature to virgin and unpatterned crystals in air, by using metal tip-shaped electrodes. An enhancement of the electric field in correspondence of the electrode tips was expected due to the fringing field effect, and electric discharge was observed during the process. The standard wet etching in hydrofluoric acid solution revealed the formation of nanoscale domain structures that exhibit filamentous self-organized geometries. Topographic measurements of the fabricated structures were performed by an atomic force microscope, and the corresponding results are presented and discussed. The investigation provides remarkable information about the shortest domain size possibly obtainable by electric field poling.

Digital holography in combination with diffraction grating to get super-resolution

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. Demonstration that super-resolution is possible have been also established in interference microscopy. We have studied the possibility to use diffraction gratings, in different experimental configurations for increasing the aperture of an optical imaging system. 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. Various attempts have been performed and results are presented and discussed. The approaches could be used for metrology and imaging application in various fields of engineering and biology.

Manipulating Thin Liquid Films for Tunable Microlens Arrays

Abstract: Nano-optics: The coupling of micro-fluidics and array imaging opens a fascinating area of research.

Optical characterization of erbium doped LiNbO3 poling properties

Abstract: We investigate the characteristics of the poling process of two samples of Er:LiNbO3 grown by the Chzocralski technique by comparing their optical properties. The first has a multidomain structure and shows some optical inhomogeneities probably due to lattice defects; the second, poled at the high temperature just after the growth, has a ferroelectric monodomain structure. Moreover, in order to highlight the effect of the high temperature poling, a further poling process was performed on both the samples at room temperature and was monitored in real time through a simple optical setup in order to evidence whether the erbium inclusion influences the poling process and which are the differences between the poling dynamics of the two samples. The results show that the erbium causes a change of the coercive field value with respect to the pure congruent LiNbO3 crystals, and that the multidomain structure influences strongly the poling dynamics.

Phase map retrieval in digital holography: avoiding the under-sampling effect by a lateral shear approach

Abstract: In Digital Holography (DH) the numerical reconstruction of the whole wavefront refracted or reflected by a sample object allows one to extract the wrapped phase map mod, 2π. In fact, since the hologram is coded numerically as a digitized image, both the wavefront amplitude and phase can be reconstructed simultaneously to provide amplitude and phase contrast imaging. The resolution in the image plane is the reconstruction pixel size that depends on wavelength, reconstruction distance and the size of the CCD recording area. Efforts to improve the resolution of DH reconstructions have been accomplished, following various strategies: increasing of the hologram aperture by moving the camera in different positions or even by using synthetic aperture approaches, using a diffraction grating to record digital holograms with a wider solid angle in the object beam, or using multiple sources and/or multiple acquisitions. Although all of these methods allow one to increase the spatial resolution, one more complication exists concerning the loss of resolution that occurs in the usual DH reconstruction approaches. It can occur that the reconstructed wrapped phase map in the image plane is undersampled because of the limited pixel size which limits the spatial bandwidth of the reconstructed image. In such a case the phase distribution cannot be retrieved correctly by the usual unwrapping procedures. We show that the use of the digital Lateral-Shearing Interferometry (LSI) approach in DH provides the correct reconstruction of the phase map in the image plane, even in extreme cases where the phase profile changes very rapidly. We demonstrate the effectiveness of the method in a particular case where the profile of a highly curved silicon micro-electromechanical system membrane has to be reconstructed.

Ultra-high sensitivity frequency-comb-referenced multi- parametric sensors based on 1-D photonic components

Abstract: A novel generation of sensors of strain, temperature, absolute and relative molecular concentration is reported. Such devices, based on 1-D photonic structures, rely on ultrastable laser sources, referenced to a fiber-based optical frequency comb synthesizer (OFCS). In particular, recent advances in the realization of two complementary laser sensors are presented. One is a spectroscopic facility which exploits frequency mixing in a periodically-poled LiNbO3 crystal to generate highly coherent (a few hundred kHz linewidth) infrared radiation tunable in the 2.9-3.5 micron wavelength range. Such radiation can be coupled to high-finesse enhancement cavities to detect trace amounts of gases, including rare isotopes in natural abundance. The other system, making use of fiber Bragg grating components, provides strain and temperature sensing with extremely high sensitivities (about 100 fe, i.e. 10-13 ΔL/L). Due to the remoteness guaranteed by the fiber coupling, these two systems can both be used in difficult environments and inserted in a multi-parametric network for real-time and continuous monitoring of large areas. Prospects for application in volcanic areas are also discussed.

Activation and control of microlens liquid arrays on functionalized polar electric crystal substrates by electro-wetting effect and temperature

Abstract: In recent years a variety of liquid bases optical elements have been conceived, designed and fabricated even for commercial products like digital cameras o cellular phone cameras. The impressive development of microfluidic systems in conjunction with optics has led to the creation of a completely new Science field of investigation named optofludics. Optofludics, among others topics, deals with investigation and methods for realizing liquid micro-lenses. A variety of liquid micro-lenses have been designed and realized by using different configurations. We demonstrate that a lensing effect can be obtained in an open microfluidic system by using a thin layer of liquid on a polar electric crystal such as Lithium Niobate (LiNbO3). Electrowetting patterning on LiNbO3 surface is obtained by pyroelectric effect consisting in a simple but reliable electrodes-less and circuit-less configuration. The electrodes are intrinsically embedded into the substrate. The material is functionalised by means of a micro-engineering electric filed poling process. Lens array with variable focus has been demonstrated with a large number of lens elements (10x10) on micrometric scale (aperture of single lens 100 microns).

Interferometric characterization of pyroelectrically activated micro-arrays of liquid lenses in lithium niobate crystals

Abstract: In recent years a wide variety of liquid based optical elements have been conceived, designed and fabricated even for commercial products like digital cameras. The impressive development of microfluidic systems in conjunction with optics has led to the creation of a completely new field of investigation named optofludics . Among other things, the optofluidic area deals with the investigation and the realization of liquid micro-lenses. Different methods and configurations have been proposed in literature to achieve liquid variable micro-lenses. This paper reports about the possibility to achieve lensing effect by a relatively easy to accomplish technique based on an open microfluidic system consisting of a tiny amount of appropriate liquid manipulated by the pyroelectric effect onto a periodically poled LiNbO 3 substrate. Basically, an electrowetting process is performed to actuate the liquid film by using the surface charges generated pyroelectrically under temperature variation. The configuration is electrode-less compared to standard electrowetting systems, thus improving the device flexibility and easiness of fabrication. The curvature of the liquid lenses has been characterized by interferometric techniques based on the evaluation of the phase map through digital holography. The results showing the evolution of the lens curvature with the temperature variation will be presented and discussed.

Pyroelectric effect inducing trapping of particles on periodically poled lithium niobate crystals

Abstract: The possibility to use periodically poled lithium niobate structures for dielectrophoretic trapping of dielectric particles was investigated. The trapping effect was achieved through the development of opposite electrical charges on z+ and z- faces of a crystal when subject to temperature variation. Appropriate oil suspensions of dielectric particles were used for the experiments which show the possibility to trap those particles by means of the pyroelectric effect in correspondence of the reversed ferroelectric domain areas. Results are presented while perspectives of exploitation in different fields are illustrated.

Tunable liquid microlens array driven by pyroelectric effect: full interferometric characterization

Abstract: Liquid lenses with adjustable focal length are of great interest in the field of microfluidic devices. They are, usually, realized by electrowetting effect after electrodes patterning on a hydrofobic substrate. Applications are possible in many fields ranging from commercial products such as digital cameras to biological cell sorting. We realized an open array of liquid lenses with adjustable focal length without electrode patterning. We used a z-cut Lithium Niobate crystal (LN) as substrate and few microliters of an oily substance to obtain the droplets array. The spontaneous polarization of LN crystals is reversed by the electric field poling process, thus enabling the realization of periodically poled LN (PPLN) crystals. The substrate consists of a two-dimensional square array of reversed domains with a period around 200 μm. Each domain presents an hexagonal geometry due to the crystal structure. PPLN is first covered by a thin and homogeneous layer of the above mentioned liquid and therefore its temperature is changed by means of a digitally controlled hot plate. During heating and cooling process there is a rearrangement of the liquid layer until it reaches the final topography. Lenses formation is due to the superficial tension changing at the liquid-solid interface by means of the pyroelectric effect. Such effect allows to create a two-dimensional lens pattern of tunable focal length without electrodes. The temporal evolution of both shape and focal length lenses are quantitatively measured by Digital Holographic Microscopy. Array imaging properties and quantitative analysis of the lenses features and aberrations are presented.

Novel concept in electro-wettability patterning with electrodes-less configuration to activate and control Liquid Microlens Arrays on functionalized polar electric substrates

Abstract: Liquid microlens array can be obtained by using a thin layer of liquid on a functionalized LiNbO3 substrate. The array is activated and controlled by electro wetting patterning and is driven by pyroelectric effect.