Fabio Ferri

High-resolution ghost image and ghost diffraction experiments with thermal light.

High-resolution ghost image and ghost diffraction experiments are performed by using a single source of thermal-like speckle light divided by a beam splitter. Passing from the image to the diffraction result solely relies on changing the optical setup in the reference arm, while leaving untouched the object arm. The product of spatial resolutions of the ghost image and ghost diffraction experiments is shown to overcome a limit which was formerly thought to be achievable only with entangled photons.

Coherent imaging with pseudo-thermal incoherent light

We investigate experimentally fundamental properties of coherent ghost imaging using spatially incoherent beams generated from a pseudo-thermal source. A complementarity between the coherence of the beams and the correlation between them is demonstrated by showing a complementarity between ghost diffraction and ordinary diffraction patterns. In order for the ghost imaging scheme to work it is therefore crucial to have incoherent beams. The visibility of the information is shown for the ghost image to become better as the object size relative to the speckle size is decreased, and therefore a remarkable tradeoff between resolution and visibility exists. The experimental conclusions are backed up by both theory and numerical simulations.

Modified version of the Chahine algorithm to invert spectral extinction data for particle sizing

A modified version of the nonlinear iterative Chahine algorithm is presented and applied to the inversion of spectral extinction data for particle sizing. Simulated data were generated in a λ range of 0.2-2 µm,and particle-size distributions were recovered with radii in the range of 0.14-1.4 µm. Our results show that distributions and sample concentrations can be recovered to a high degree of accuracy when the indices of refraction of the sample and of the solvent are known. The inversion method needs no a priori assumptions and no constraints on the particle distributions. Compared with the algorithm originally proposed by Chahine, our method is much more stable with respect to random noise, permits a better quality of the retrieved distributions, and improves the overall reliability of the fitting. The accuracy and resolution of the method as functions of noise were investigated and showed that the retrieved distributions are quite reliable up to noise levels of several rms percent in the data. The sensitivity to errors in the real and imaginary parts of the refraction index of the particles was also examined.

Use of a charge coupled device camera for low-angle elastic light scattering

A charge coupled device (CCD) camera has been successfully used to implement a low-angle elastic light scattering setup. A novel optical detection layout brings all the light scattered by the sample directly onto the CCD, optimizing the instrument sensitivity. The detectable angular range covers ∼2 decades, from ∼0.1° to ∼10°. The calibration of the instrument, as well as the estimate of its sensitivity, accuracy, dynamic range and linearity, can be carried out by using single pinholes. Experimental results from pinholes and diluted suspensions of polystyrene spheres are presented.

Fast multi-tau real-time software correlator for dynamic light scattering

We present a PC-based multi-tau software correlator suitable for processing dynamic light-scattering data. The correlator is based on a simple algorithm that was developed with the graphical programming language LabVIEW, according to which the incoming data are processed on line without any storage on the hard disk. By use of a standard photon-counting unit, a National Instruments Model 6602-PCI timer-counter, and a 550-MHz Pentium III personal computer, correlation functions can be worked out in full real-time over time scales of ~5 mus and in batch processing down to time scales of ~300 ns. The latter limit is imposed by the speed of data transfer between the counter and the PCs memory and thus is prone to be progressively reduced with future technological development. Testing of the correlator and evaluation of its performances were carried out by use of dilute solutions of calibrated polystyrene spheres. Our results indicate that the correlation functions are determined with such precision that the corresponding particle diameters can be recovered to within an accuracy of a few percent rms.

Backscattering differential ghost imaging in turbid media

In this Letter we present experimental results concerning the retrieval of images of absorbing objects immersed in turbid media via differential ghost imaging (DGI) in a backscattering configuration. The method has been applied, for the first time to our knowledge, to the imaging of thin black objects located inside a turbid solution in proximity of its surface. We show that it recovers images with a contrast better than standard noncorrelated direct imaging, but equivalent to noncorrelated diffusive imaging. A simple theoretical model capable of describing the basic optics of DGI in turbid media is proposed.

25 ns software correlator for photon and fluorescence correlation spectroscopy

A 25 ns time resolution, multi-tau software correlator developed in LABVIEW based on the use of a standard photon counting unit, a fast timer/counter board (6602-PCI National Instrument) and a personal computer (PC) (1.5 GHz Pentium 4) is presented and quantitatively discussed. The correlator works by processing the stream of incoming data in parallel according to two different algorithms: For large lag times (τ⩾100 μs), a classical time-mode (TM) scheme, based on the measure of the number of pulses per time interval, is used; differently, for τ⩽100 μs a photon-mode (PM) scheme is adopted and the time sequence of the arrival times of the photon pulses is measured. By combining the two methods, we developed a system capable of working out correlation functions on line, in full real time for the TM correlator and partially in batch processing for the PM correlator. For the latter one, the duty cycle depends on the count rate of the incoming pulses, being ∼100% for count rates ⩽3×104 Hz, ∼15% at 105 Hz, and ...

Small- and wide-angle elastic light scattering study of fibrin structure

We show how small- and wide-angle elastic light scattering (q ∼ 0.03-30 μm -1 ) can be used to quantitatively characterize the structure of polymeric gels made of semi-flexible entangled fibers. We applied the technique to the study of fibrin gels grown from the polymerization of fibrinogen (FG) macromolecular monomers following activation by the enzyme thrombin (TH), at different concentrations and under different physical-chemical conditions of the gelling solution. Our findings show that the gel can be imagined as a random network of fibers of size d and density p, entangled together to form densely packed blobs of mass fractal dimension D m and average size ξ, which may overlap by a factor η and exhibit a long-range order. Provided that d ≥50-100 nm, all of the above parameters can be recovered by the use of a global fitting function developed by us on the basis on the proposed gel model. When the fibers are thinner (d < ∼50 nm), only the fiber mass/length ratio μ ∼ ρd 2 can be retrieved instead of d and p. Our data confirm and quantify the major changes in the gel structure that can be obtained by varying either the salt concentration of the solution and/or the molar ratio TH/FG. Gels formed in Tris-HCl 50 mM/NaCl 150 mM, pH 7.4 and TH/FG = 0.01 are characterized by relatively small, fairly branched (D in ∼ 1.4-2.0) fibers with a mass/length ratio independent of concentration. On reducing the TH/FG ratio, the fibers become increasingly thicker, with d ∼ 90 nm at TH/FG = 10 -5 . When the salt concentration is reduced to NaCl 100 mM (TH/FG = 0.01) the fibers are less branched (D m ∼ 1.2-1.4), but much thicker, with μ increasing by an order of magnitude. These effects are quantitatively analyzed and discussed.

Coherent imaging of a pure phase object with classical incoherent light

A ghost imaging scheme is used to observe the diffraction pattern of a pure phase object . It is observed that when increasing the spatial coherence the diffraction pattern disappeared from the cross-correlation, while it appeared in the autocorrelation. The cross-correlation contains information about the phase object only when the light is spatially incoherent.

Longitudinal coherence in thermal ghost imaging

We show theoretically and experimentally that lensless ghost imaging with thermal light is fully interpretable in terms of classical statistical optics. The disappearance of the ghost image when the object and the reference planes are located at different distances from the source is due to the fading out of the intensity-intensity cross correlation between the two planes. Thus the visibility and the resolution of the ghost image are determined by the longitudinal coherence of the speckle beam, and no quantum explanation is necessary.