Single Speckle Research Articles

Coupling the dynamics of two actin networks--new views on the mechanics of cell protrusion.

We study how mechanical forces integrate spatially and temporally with regulatory signals at the leading edge of migrating cells. To probe the dynamics of this system, we developed quantitative fluorescent speckle microscopy, which maps out actin cytoskeleton transport, assembly and disassembly with high spatial resolution. Statistical processing of single speckle properties revealed two kinetically, kinematically and molecularly distinct, yet spatially overlapping, actin arrays at the leading edge of migrating epithelial cells. The first network, referred to as the lamellipodium, polymerizes and depolymerizes 1-2 microm from the edge in an Arp2/3 (actin-related protein 2/3)- and cofilin-dependent fashion. The second network, referred to as the lamella, exhibits Arp2/3-independent polymerization. To elucidate the dynamic relationship between the two networks, we have begun to examine how assembly and flow are temporally modulated with respect to a protrusion event. In control cells we found bursts of protrusion preceding bursts of F-actin assembly. The time lag disappears in cells where Arp2/3-function is impaired. This and other results allowed us to propose a model in which tropomyosin protects lamella filaments from branching and severing, and to conjecture that Arp2/3-mediated lamellipodium assembly is a natural consequence of lamella expansion, but not the initiator of cell protrusion.

Kinetic and fluid Langmuir wave nonlinearities driven by stimulated Raman scattering in a diffraction limited single-hot-spot

A diffraction limited focal spot is used to create a single speckle in a preformed plasma. The single speckle, or hot spot, is the simplest irreducible element for studying laser plasma interactions. Using the single hot spot configuration, the nonlinear behavior of SRS driven Langmuir waves has been investigated in both a fluid dominated and a kinetic regime. A transition between the two regimes has been observed as a function of kλD where k is the Langmuir wave-number and λD is the Debye length.

Enhanced detection of acousto-photonic imaging signals using a photorefractive crystal based system

Acousto-photonic imaging is a dual-wave sensing technique where diffuse coherent light wave interacts with a superimposed acoustic field. A phase-modulated light wave emanates from the interaction region and carries with it information about the local opto-mechanical properties of the insonated media. The coherent nature of the light produces a speckle field. The modulation of the speckle field is spatially incoherent, yielding a small modulation depth when collecting multiple speckles or extremely low light levels when detecting at the single speckle level. We report preliminary results from a new detection scheme where the scattered laser light is mixed with a reference beam in a photorefractive crystal. The crystal serves as a dynamic holographic medium and a photorefractive grating is formed from which the reference beam diffracts. The diffracted reference beam has the same spatial structure as the scattered light, and the two interfere at the photodetector where the phase modulation on the scattered beam is converted to an intensity modulation. Measurements of the signals are presented for gel phantoms seeded with suspended polystyrene beads. The results exhibit qualitative agreement with a simple theoretical model. [Work supported by the Center for Subsurface Sensing and Imaging Systems via NSF ERC Award No. EEC-9986821.]

Extraction of phase field from a single contoured correlation fringe pattern of ESPI

Speckle fringe patterns of ESPI are full of high-level speckle noise and normally are processed by phase shifting methods that require multi speckle fringe patterns. We propose a novel method to generate a speckle-noise-free fringe pattern from a single speckle fringe pattern and to extract the phase field from the new pattern. With the new method, the correlation between two speckle patterns is performed only within contour windows instead of rectangular windows and this contoured correlation results in a smooth, normalized fringe pattern without speckle noise. The new ESPI fringe patterns are speckle-noise-free and of comparable quality to that of moiré and hologram, which is unimaginable with traditional ESPI methods. In addition to the smoothness, the resultant fringe pattern is normalized automatically so that the full phase field can be extracted from this single fringe pattern by the single-image phase-shifting method.

Removing speckle noise and extracting the skeletons from a single speckle fringe pattern by spin filtering with curved-surface windows

Removing speckle noise in electronic speckle pattern interfer- ometry (ESPI) from a single speckle fringe pattern while keeping the fringe features is a difficult problem and remains unsolved. In this paper, the spin filtering proposed by the authors is developed further with curved surface windows to filter out speckle noise from a single speckle fringe pattern. With the large curved surface windows, the speckle noise can be removed nearly completely by the spin filtering. The most impor- tant advantage of the method is that no blurring effects occur for speckle fringes and the smooth fringe pattern of a phase field is retrieved and derived. The skeletons of the smoothed fringe pattern are extracted au- tomatically. © 2003 Society of Photo-Optical Instrumentation Engineers.

Experimental Investigation of Quantum Effects in Time-Resolved Resonant Rayleigh Scattering from Quantum Well Excitons

Resonant Rayleigh scattering from quantum well excitons is investigated using ultrafast spectral interferometry. We isolate the coherent Rayleigh scattering from incoherent luminescence in a single speckle. Averaging the resonant Rayleigh intensity over several speckles allows us to identify features in support of quantum corrections to the classical description of the underlying scattering process.

Interferometric analysis of resonant Rayleigh scattering from two-dimensional excitons

We have performed spectral interferometry with Rayleigh scattered light coming from a single speckle and emitted after resonant creation of excitons in GaAs quantum wells. The experiment confirms the coherence of the secondary emission at early times, regardless of sample parameters, but underlines the stochastic nature of a single speckle. The scattered electric field is calculated, correctly accounting for the center-of-mass quantization in the disordered potential. We find that the amplitudes of the electric fields emitted by different excitons are uncorrelated, so that the well-known Poisson distribution for speckle intensities applies.

Kinetic Effects on the Ion Sound Waves Generated by Stimulated Brillouin Scattering of a Spatially Smoothed Laser Beam

Stimulated Brillouin Scattering (SBS) can drive ion sound waves to amplitudes such that steepening and particle kinetic effects occur. Such phenomena have been studied recently in spatial volumes corresponding to a single laser speckle, in two (Cohen B I et al 1997 Phys. Plasmas 4 956), and three spatial dimensions (Vu H X 1997 Phys. Plasmas 4 1841). We analyze the results of 2D kinetic simulations modeling the evolution of SBS in the case of a spatially smoothed laser beam interacting with a plasma containing many speckles. For this purpose, we have coupled a 2D particle-in-cell code to a 2D non-paraxial electromagnetic wave solver. The generation of a fast ion population is observed to initially take place in the most intense hot spots, resulting in a decrease of the amplitude of the ion sound wave amplitude generated by SBS in these hot spots. SBS activity is consequently reduced in the most intense laser speckles, so that the overall backscattering SBS reflectivity drops considerably below the values expected from models that do not account for particle kinetics and/or nonlinear hydrodynamics.

Linear displacement transducers based on speckle interferometry: error avoidance techniques

Directionally sensitive in-plane displacement speckle interferometers in which the modulation of the intensity of a single speckle can be used to monitor the motion of a target surface were reported by Reynolds and Toal in 1992. Difficulties arise due to the phenomenon of speckle motion, which causes a randomly varying intensity at the detector, upon which the interferometric signal is superimposed. This, in turn, makes reliable digital counting systems difficult to implement. Here we report on two methods of overcoming these problems. One employs a simple digital electronic algorithm and the other uses an optical phase-shifting technique. Results obtained for all systems confirm the validity of the principles underlying their design and indicate possible avenues for further improvement.

Real-time holospeckle shear interferometry

A new technique, which offers the possibility of obtaining the slope and curvature information in real-time is presented. The technique is based on holospeckles. In this the original state of the object is reconstructed with a hologram illuminated with a reference beam, the polarisation of which is orthogonal to the state of polarisation at the time of recording. A single exposure speckle shearogram recorded yields all the information.

Phase-shifting speckle interferometry

Speckle patterns have high frequency phase data, which make it difficult to find the absolute phase of a single speckle pattern; however, the phase of the difference between two correlated speckle patterns can be determined. This is done by applying phase-shifting techniques to speckle interferometry, which will quantitatively determine the phase of double-exposure speckle measurements. The technique uses computer control to take data and calculate phase without an intermediate recording step. The randomness of the speckle causes noisy data points which are removed by data processing routines. One application of this technique is finding the phase of deformations, where up to ten waves of wavefront deformation can easily be measured. Results of deformations caused by tilt of a metal plate and a disbond in a honeycomb structure brazed to an aluminum plate are shown.

The application of high speed photography and white light speckle to the study of dynamic fracture

A white light speckle technique is combined with high speed photography to study the dynamic fracture of a sample of PMMA. A series of single exposure speckle patterns are photographically recorded at the rate of 2 × 10 5 frames s -1 during fracture of the sample. The displacement field around the tip of the propagating crack may then be determined by superimposing pairs of individual frames to form a ‘double-exposure’ speckle pattern. It is shown that the method gives values for the stress intensity factor K which are consistent with those obtained from other techniques.