Spatial resolution limit of single pixel imaging of complex light fields.

Complex light fields with arbitrary amplitudes and phases can be measured by sampling them with an orthogonal basis (i.e., canonical, Hadamard) and performing single pixel interferometric measurements of the focused modes. In this work, we show that when the spatial resolution of the sampling basis is coarser than the spatial resolution of the phase in the complex field, the measured reconstructed amplitude exhibits cross talk with the phase, i.e., phase information appears in the amplitude. To demonstrate this phenomenon, we encode an arbitrary amplitude and a phase with a spatial light modulator and compare measurements with simulations.

Shaping the angular spectrum of a Bessel beam to enhance light transfer through dynamic strongly scattering media.

We prepare a quasi-non-diffracting Bessel beam defined within an annular angular spectrum with a spatial light modulator. The beam propagates through a strongly scattering media, and the transmitted speckle pattern is measured at one point with a Hadamard Walsh basis that divides the ring into N segments (N = 16, 64, 256, 1024). The phase of the transmitted beam is reconstructed with 3-step interferometry, and the intensity of the transmitted beam is optimized by projecting the conjugate phase at the SLM. We find that the optimum intensity is attained for the condition that the transverse wave vector k⊥ (of the Bessel beam) matches the spatial azimuthal frequencies of the segmented ring k ϕ. Furthermore, compared with beams defined on a 2d grid (i.e., Gaussian) a reasonable enhancement is achieved for all the k⊥ sampled with only 64 elements. Finally, the measurements can be done while the scatterer is moving as long as the total displacement during the measurement is smaller than the speckle correlation distance.

Interferometric measurement of arbitrary propagating vector beams that are tightly focused.

In this work, we demonstrate a simple setup to generate and measure arbitrary vector beams that are tightly focused. The vector beams are created with a spatial light modulator and focused with a microscope objective with an effective numerical aperture of 1.2. The transverse polarization components (Ex, Ey) of the tightly focused vector beams are measured with three-step interferometry. The axial component Ez is reconstructed using the transverse fields with Gauss's law. We measure beams with the following polarization states: circular, radial, azimuthal, spiral, flower, and spider web.

Comparison between Hadamard and canonical bases for in situ wavefront correction and the effect of ordering in compressive sensing.

In this work we compare the canonical and Hadamard bases for in situ wavefront correction of a focused Gaussian beam using a spatial light modulator (SLM). The beam is perturbed with a transparent optical element (sparse) or a random scatterer (both prevent focusing at a single spot). The phase corrections are implemented with different basis sizes (N=64,256,1024,4096) and the phase contribution of each basis element is measured with three-step interferometry. The field is reconstructed from the complete 3N measurements, and the correction is implemented by projecting the conjugate phase at the SLM. Our experiments show that in general, the Hadamard basis measurements yield better corrections because every element spans the relevant area of the SLM, thus reducing the noise in the interferograms. In contrast, the canonical basis has the fundamental limitation that the area of the elements is proportional to 1/N, and it requires dimensions that are compatible with the spatial period of the grating. In the case of the random scatterer, we only obtain reasonable corrections with the Hadamard basis and the intensity of the corrected spot increases monotonically with N, which is consistent with fast random changes in phase over small spatial scales. We also explore compressive sensing with the Hadamard basis and find that the minimum compression ratio needed to achieve corrections with similar quality to those that use the complete measurements depends on the basis ordering. The best results are achieved in the case of the Hadamard-Walsh and cake-cutting orderings. Surprisingly, in the case of the random scatterer we find that moderate compression ratios on the order of 10%-20% (N=4096) allow us to recover focused spots, although as expected, the maximum intensities increase monotonically with the number of measurements due to the non-sparsity of the signal.

Phase Dependent Vectorial Current Control in Symmetric Noisy Optical Ratchets.

In this work, we demonstrate single microparticle transport in a symmetric noisy optical ratchet made with a linear array of 20 optical potentials, where each potential is a spatially symmetric low power (<2.5 mW) three-dimensional trap. Both the external force F(t) and the depth V_{0i}(t) of the optical potentials are dynamic and change at the same frequency ν=2 Hz. The depths of the individual optical potentials are random (uncorrelated noise) distributed around a mean value V_{0}, ⟨V_{0i}(t)⟩=V_{0}, while the external force is periodic and unbiased ⟨F(t)⟩=0. The system is completely symmetric for times t≫1/ν. Directed transport is possible as a result of the symmetry being broken at times on the order of 1/ν. We find that the direction and speed of motion (current) are coupled to the phase difference between the noise in the optical potentials and the external periodic force.

Microparticle transport networks with holographic optical tweezers and cavitation bubbles.

Optical transport networks for active absorbing microparticles are made with holographic optical tweezers. The particles are powered by the optical potentials that make the network and transport themselves via random vapor-propelled hops to different traps. The geometries explored for the optical traps are square lattices, circular arrays, and random arrays. The degree distribution for the connections or possible paths between the traps are localized like in the case of random networks. The average travel times across n different traps scale as nb with exponents in the range of 2.06 and 2.31, in agreement with random walks on connected networks (upper bound ∝n3). Finally, a particle traveling the network attracts others as a result of the vapor explosions enhancing transport.

Noise-enabled optical ratchets.

In this contribution, we report on the implementation of a novel noise-enabled optical ratchet system. We demonstrate that, unlike commonly-used ratchet schemes-where complex asymmetric optical potentials are needed-efficient transport of microparticles across a one-dimensional optical lattice can be produced by introducing controllable noise in the system. This work might open interesting routes towards the development of new technologies aimed at enhancing the efficiency of transport occurring at the micro- and nanoscale, from novel particle-sorting tools to efficient molecular motors.

Characterization of periodic cavitation in optical tweezers.

Microscopic vapor explosions or cavitation bubbles can be generated repeatedly in optical tweezers with a microparticle that partially absorbs at the trapping laser wavelength. In this work we measure the size distribution and the production rate of cavitation bubbles for microparticles with a diameter of 3 µm using high-speed video recording and a fast photodiode. We find that there is a lower bound for the maximum bubble radius R(max)∼2 µm which can be explained in terms of the microparticle size. More than 94% of the measured R(max) are in the range between 2 and 6 µm, while the same percentage of the measured individual frequencies f(i) or production rates are between 10 and 200 Hz. The photodiode signal yields an upper bound for the lifetime of the bubbles, which is at most twice the value predicted by the Rayleigh equation. We also report empirical relations between R(max), f(i), and the bubble lifetimes.

A microscopic steam engine implemented in an optical tweezer.

The introduction of improved steam engines at the end of the 18th century marked the start of the industrial revolution and the birth of classical thermodynamics. Currently, there is great interest in miniaturizing heat engines, but so far traditional heat engines operating with the expansion and compression of gas have not reached length scales shorter than one millimeter. Here, a micrometer-sized piston steam engine is implemented in an optical tweezer. The piston is a single colloidal microparticle that is driven by explosive vapourization of the surrounding liquid (cavitation bubbles) and by optical forces at a rate between a few tens of Hertz and one kilo-Hertz. The operation of the engine allows to exert impulsive forces with optical tweezers and induce streaming in the liquid, similar to the effect of transducers when driven at acoustic and ultrasound frequencies.

Optical stacking of microparticles in a pyramidal structure created with a symmetric cubic phase.

We show a simple way to generate three dimensional optical potentials consisting of tightly localized high intensity spots arranged in a structure with a pyramidal geometry. The three dimensional patterns are created by focusing a Gaussian beam with a symmetric cubic phase abs((ax)3) + abs((ay)3) imprinted by a spatial light modulator. We show that it is possible to trap and stack around a hundred dielectric microspheres (silica mean diameter 2.47 µm) in pyramidal structures (characteristic dimensions H, W ∼ 15 - 20µm) held together by optical binding with moderate laser power (P < 20 mW). Axial stability is mainly provided by balancing the light scattering force with the axial gradient and gravity. The microparticle structures are sufficiently stable to be easily displaced by moving the microscope stage.

Fast temperature measurement following single laser-induced cavitation inside a microfluidic gap.

Single transient laser-induced microbubbles have been used in microfluidic chips for fast actuation of the liquid (pumping and mixing), to interact with biological materials (selective cell destruction, membrane permeabilization and rheology) and more recenty for medical diagnosis. However, the expected heating following the collapse of a microbubble (maximum radius ~ 10-35 µm) has not been measured due to insufficient temporal resolution. Here, we extend the limits of non-invasive fluorescence thermometry using high speed video recording at up to 90,000 frames per second to measure the evolution of the spatial temperature profile imaged with a fluorescence microscope. We found that the temperature rises are moderate (< 12.8°C), localized (< 15 µm) and short lived (< 1.3 ms). However, there are significant differences between experiments done in a microfluidic gap and a container unbounded at the top, which are explained by jetting and bubble migration. The results allow to safe-guard some of the current applications involving laser pulses and photothermal bubbles interacting with biological material in different liquid environments.

Birth and growth of cavitation bubbles within water under tension confined in a simple synthetic tree.

Water under tension, as can be found in several systems including tree vessels, is metastable. Cavitation can spontaneously occur, nucleating a bubble. We investigate the dynamics of spontaneous or triggered cavitation inside water filled microcavities of a hydrogel. Results show that a stable bubble is created in only a microsecond time scale, after transient oscillations. Then, a diffusion driven expansion leads to filling of the cavity. Analysis reveals that the nucleation of a bubble releases a tension of several tens of MPa, and a simple model captures the different time scales of the expansion process.

Observation of non-diffracting behavior at the single-photon level.

We demonstrate the generation of non-diffracting heralded single photons, i.e. which are characterized by a single-photon transverse intensity distribution which remains essentially unchanged over a significant propagation distance. For this purpose we have relied on the process of spontaneous parametric downconversion (SPDC) for the generation of signal and idler photon pairs, where our SPDC crystal is pumped by a Bessel-Gauss (BG) beam. Our experiment shows that the well-understood non-diffracting behavior of a BG beam may be directly mapped to the signal-mode, single photons heralded by the detection of a single idler photon. In our experiment, the heralded single photon is thus arranged to be non-diffracting without the need for projecting its single-photon transverse amplitude, post-generation, in any manner.

Motion of micrometer sized spherical particles exposed to a transient radial flow: attraction, repulsion, and rotation.

It is now accepted that the physical forces in ultrasonic cleaning are due to strongly pulsating bubbles driven by the sound field. Here we have a detailed look at bubble induced cleaning flow by analyzing the transport of an individual particle near an expanding and collapsing bubble. The induced particulate transport is compared with a force balance model. We find two important properties of the flow which explain why bubbles are effectively cleaning: During bubble expansion a strong shear layer loosens the particle from the surface through particle spinning and secondly an unsteady boundary layer generates an attractive force, thus collecting the contamination in the bubble's close proximity.

Red blood cell rheology using single controlled laser-induced cavitation bubbles.

The deformability of red blood cells (RBCs) is an important property that allows the cells to squeeze through small capillary vessels and can be used as an indicator for disease. We present a microfluidic based technique to quantify the deformability of RBCs by stretching a collection of RBCs on a timescale of tens of microseconds in a microfluidic chamber. This confinement constrains the motion of the cell to the imaging plane of the microscope during a transient cavitation bubble event generated with a focused and pulsed laser. We record and analyze the shape recovery of the cells with a high-speed camera and obtain a power law in time, consistent with other dynamic rheological results of RBCs. The extracted exponents are used to characterize the elastic properties of the cells. We obtain statistically significant differences of the exponents between populations of untreated RBCs and RBCs treated with two different reagents: neuraminidase reduces the cell rigidity, while wheat germ agglutinin stiffens the cell confirming previous experiments. This cavitation based technique is a candidate for high-throughput screening of elastic cell properties because many cells can be probed simultaneously in situ, thus with no pre-treatment.

Controlled manipulation and in situ mechanical measurement of single co nanowire with a laser-induced cavitation bubble.

The flow induced by a single laser-induced cavitation bubble is used to manipulate individual Co nanowires. The short-lived (<20 µs) bubble with a maximum size of 45 µm is created in an aqueous solution with a laser pulse. Translation, rotation, and radial motion of the nanowire can be selectively achieved by varying the initial distance and orientation of the bubble with respect to the nanowire. Depending on the initial distance, the nanowire can be either pushed away or pulled toward the laser focus. No translation is observed for a distance further than approximately 60 µm, while at closer distance, the nanowire can be bent as a result of the fast flow induced during the bubble collapse. Studying the dynamics of the shape recovery allows an estimation of the Young's modulus of the nanowire. The low measured Young's modulus (in a range from 9.6 to 13.0 GPa) of the Co nanowire is attributed to a softening effect due to structural defects and surface oxidation layer. Our study suggests that this bubble-based technique allows selectively transporting, orienting, and probing individual nanowires and may be exploited for constructing functional nanodevices.

Nonspherical laser-induced cavitation bubbles.

The generation of arbitrarily shaped nonspherical laser-induced cavitation bubbles is demonstrated with a optical technique. The nonspherical bubbles are formed using laser intensity patterns shaped by a spatial light modulator using linear absorption inside a liquid gap with a thickness of 40 microm. In particular we demonstrate the dynamics of elliptic, toroidal, square, and V-shaped bubbles. The bubble dynamics is recorded with a high-speed camera at framing rates of up to 300,000 frames per second. The observed bubble evolution is compared to predictions from an axisymmetric boundary element simulation which provides good qualitative agreement. Interesting dynamic features that are observed in both the experiment and simulation include the inversion of the major and minor axis for elliptical bubbles, the rotation of the shape for square bubbles, and the formation of a unidirectional jet for V-shaped bubbles. Further we demonstrate that specific bubble shapes can either be formed directly through the intensity distribution of a single laser focus, or indirectly using secondary bubbles that either confine the central bubble or coalesce with the main bubble. The former approach provides the ability to generate in principle any complex bubble geometry.

Acoustically driven cavitation cluster collapse in planar geometry.

We demonstrate the dynamics of arrays of transient cavitation bubbles exposed to a sound field in a planar geometry. Single, double, and complex configurations of cavitation bubbles are obtained by shaping a pulsed laser beam with a digital hologram and focusing it into a thin gap of liquid. The liquid is driven with an oscillating pressure field of variable phase and amplitude. We compare the dynamics of a single bubble recorded with high-speed photography with a two-dimensional Rayleigh model. For multibubble configurations we observe bubble-bubble interaction and coalescence which depends on the phase of the acoustic field. Larger clusters demonstrate drastically enhanced collapse for high-amplitude driving, enabling the study of artificial cavitation clusters under strong driving.

Cavitation bubble dynamics in microfluidic gaps of variable height.

We study experimentally the dynamics of laser-induced cavitation bubbles created inside a narrow gap. The gap height, h , is varied from 15 to 400 microm and the resulting bubble dynamics is compared to a semiunbounded fluid. The cavitation bubbles are created with pulsed laser light at constant laser energy and are imaged with a high-speed camera. The bubble lifetime increases with decreasing gap height by up to 50% whereas the maximum projected bubble radius remains constant. Comparing the radial dynamics to potential flow models, we find that with smaller gaps, the bubble-induced flow becomes essentially planar, thus slower flows with reduced shear. These findings might have important consequences for microfluidic applications where it is desirable to tune the strength and range of the interactions such as in the case of cell lysis and cell membrane poration.

Dynamics of magnetic bubbles in acoustic and magnetic fields.

We report on shelled bubbles that can be manipulated with magnetic fields. The magnetic shell consists of self-assembled magnetic nanoparticles. The magnetic susceptibility of the bubbles is proportional to the surface area, chi_{b}=(9+/-3x10;{-6} m)r;{2} where r is the radius. Magnetic bubbles are compressible in moderate acoustic fields. A bubble with a radius of 121 mum oscillates in resonance in a sound field of 27 kHz with a peak-to-peak radial amplitude of 1.7 mum. The bubble oscillations induce a microstreaming flow with a toroidal vortex at the upper pole of the bubble. Further findings are the longevity of the magnetic bubbles and the ease of manipulation with standard magnets.