AK Asundi

INO - Istituto Nazionale di Ottica, Florens, Tuscany, Italy

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Publications (8)1.2 Total impact

  • [Show abstract] [Hide abstract] ABSTRACT: An optical configuration is realized to obtain quantitative phase-contrast maps able to characterize particles floating in a microfluidic chamber by interference microscopy. The novelty is the possibility to drive the sample and measure it thorough the same light path. That is realized by an optical setup made of two light beams coming from the same laser source. One beam provides the optical forces for driving the particle along the desired path and, at same time, it works as object beam in the digital holographic microscope (DHM). The second one acts as reference beam, allowing recording of an interference fringe pattern (i.e., the digital hologram) in an out-of-focus image plane. This work finds application in the field of micromanipulation as, the devise developed allows to operate in microfluidic chambers driving samples flowing in very small volumes. Recently, the field of optical particle micro-manipulation has had rapid growth, due to Optical Tweezers development. A particle is trapped or moved along certain trajectories according to the intensity and phase distribution of the laser beam used. Here, particles freely floating are driven by optical forces along preferential directions and then analyzed by a DHM to numerically calculate their phase-contrast signature. The improvement is that one laser source is employed for making two jobs: driving and analyze the sample. We use two slightly off-axis laser beams coming from a single laser source. The interference between them gives the possibility to record in real-time a sequence of digital holograms, while one of the beam creates the driving force. By this method, a great amount of particles can be analyzed by a real-time recording of DH movies. This allows one to examine each particle at time and characterize it. The optical configuration and the working method are illustrated. Experimental results are shown for polymeric particles and in-vitro.
    No preview · Article · Apr 2012 · Proceedings of SPIE - The International Society for Optical Engineering
  • [Show abstract] [Hide abstract] ABSTRACT: Dielectrophoretic clustering is obtained both for liquid and solid matter thanks to light shaping performed by phase only Spatial Light Modulator (SLM). We present a procedure able to perform two functions: design polymeric stable structures usable as microfluidic channels and trapping micro objects. These two tasks are combined to realize a single device. The liquid matter is Polydimethylsiloxane (PDMS) and its patterning in microstructures is developed by means of photorefractive effect in a functionalized substrate. X-cut Iron-doped Lithium Niobate (LN) crystal is used as substrate while a thin film of PDMS is spin on it. When LN, covered by PDMS, is exposed to structured laser light, a space charge field arise that is able to induce self-patterning of the PDMS liquid film. The rearrangement of PDMS is due to the dielectrophoretic effect. Light structuring is achieved by a SLM positioned in the conjugated plane of the LN crystals. PDMS devices we realized are microfluidic channels. The first step of our procedure is the computing of a suitable Computer Generated Hologram (CGH) to be displayed by the SLM. An ideal target is designed and given as input to an Iterative Fourier Transform Algorithm (IFTA) to calculate the CGH. The IFTA used has been implemented for this particular application and it's tailored to generate a continuous light intensity profile in the LN plane. Then PDMS microstructures are cured to induce solidification. Such PDMS channels are then used to trap particles floating inside. Trapping is realized exploiting again dielectrophoresis induced by photorefractive effect. LN with PDMS channel is exposed to laser light which present, now, a periodic two-dimensional intensity profile. The charge distribution due to this second exposure is able to trap particle in the previously built channels. We realize a device with high degree of flexibility avoiding the need of moulds fabrication.
    No preview · Article · Apr 2012 · Proceedings of SPIE - The International Society for Optical Engineering
  • [Show abstract] [Hide abstract] ABSTRACT: We propose an alternative reconstructing strategy in digital color holography, based on the hologram stretching techniques. With a simple adaptive affine transformation on the digital color holograms and a correlation-matching procedure applied on their numerical reconstructions, we are able to manage the digital color reconstructions of the same object in order to obtain their perfect superimposition. We test our procedure in several experimental cases considering holograms recorded in both microscope configuration and lensless configuration. Finally we give a procedure, based on the National Television Systems Committee (NTSC) coefficients, to synthesize a single hologram that contains the information associated to the three colored numerical reconstructions. Numerical analysis and display tests are used to evaluate the effectiveness of the proposed method.
    No preview · Article · Apr 2012 · Proceedings of SPIE - The International Society for Optical Engineering
  • [Show abstract] [Hide abstract] ABSTRACT: In digital holography (DH) the hologram is recorded by a CCD while intensity and phase reconstruction are performed numerically. For imaging objects at different depths, holograms are numerically reconstructed in planes that are parallel to the hologram but at different distances 1. However, for objects having 3D extension or 3D shape, only some portions of the object can be in good focus in each of those planes 2-4. The limited depth of focus is affecting all optical and imaging systems. In classical optical microscopy the problem is resolved by scanning mechanically the 3D volume. By such a procedure it is possible to build-up a single image, named Extended Focus Image (EFI), in which all points of the object are in-focus 5. However, the problem remains unresolved for objects changing their shape during the measuring time (i.e. for dynamic events). Various solutions have been proposed adopting DH methods 6-10. In fact, since all DH methods are based on a single image acquisition, it is clear that those methods are useful for dynamic objects (i.e. objects that change their shape during the observation under the microscope). Here we show a new DH methods based on the hologram deformation 11. It has the advantage to be very simple and without additional computational efforts in respect to a standard reconstruction. The method is applied to maintain in focus the reconstructed image of a MEMS subjected to thermal load that changes its position during the hologram acquisition.
    No preview · Article · Apr 2010 · Proceedings of SPIE - The International Society for Optical Engineering
  • No preview · Book · Jan 2008
  • No preview · Book · Jan 2008
  • [Show abstract] [Hide abstract] ABSTRACT: In this paper is reported a method for measuring the thickness of a silicone nitride layers employed for fabricating silicon MEMS bi-morph structures. The method allows the precise evaluation of layer thickness by adopting Digital Holographic Microscope. The measurement is based on the fact that the silicon nitride layer is transparent to the visible light. The optical phase difference (OPD) between the light beam traveling through the layer and portion of the beam in air is measured exploiting an interferometric technique. The approach is very simple and can be utilized even for inspection of non-planar or stressed structures. Experimental values have been compared with ellipsometric measurements.
    No preview · Book · Apr 2006
  • [Show abstract] [Hide abstract] ABSTRACT: Digital Holographic Microscope has been employed to obtain an accurate characterization of a micro-hotplate for gas sensing applications. The fabrication of these sensors needs different materials, with different properties and different technological processes, which involve high temperature treatments. Consequently, the structure is affected by the presence of residual stresses, appearing in form of undesired bowing of the membrane. Moreover, when the temperature of the sensor increases, a further warpage of the structure is observed. DHM allows to evaluate, with high accuracy, deformations due to the residual stress and how these deformations are affected by thermal loads. In particular, profiles of the structure have been evaluated both in quasi-static condition and the profile variation due to the biasing of the heater resistor has been measured.
    No preview · Book · Aug 2004