4,515 research outputs found
The transition from a coherent optical vortex to a Rankine vortex: beam contrast dependence on topological charge
Spatially coherent helically phased light beams carry orbital angular momentum (OAM) and contain phase singularities at their centre. Destructive interference at the position of the phase singularity means the intensity at this point is necessarily zero, which results in a high contrast between the centre and the surrounding annular intensity distribution. Beams of reduced spatial coherence yet still carrying OAM have previously been referred to as Rankine vortices. Such beams no longer possess zero intensity at their centre, exhibiting a contrast that decreases as their spatial coherence is reduced. In this work, we study the contrast of a vortex beam as a function of its spatial coherence and topological charge. We show that beams carrying higher values of topological charge display a radial intensity contrast that is more resilient to a reduction in spatial coherence of the source
Holographic optical trapping Raman micro-spectroscopy for non-invasive measurement and manipulation of live cells
We present a new approach for combining holographic optical tweezers with confocal Raman spectroscopy. Multiple laser foci, generated using a liquid-crystal spatial light modulator, are individually used for both optical trapping and excitation of spontaneous Raman spectroscopy from trapped objects. Raman scattering from each laser focus is spatially filtered using reflective apertures on a digital micro-mirror device, which can be reconfigured with flexible patterns at video rate. We discuss operation of the instrument, and performance and viability considerations for biological measurements. We then demonstrate the capability of the instrument for fast, flexible, and interactive manipulation with molecular measurement of interacting live cell systems
Measuring storage and loss moduli using optical tweezers: broadband microrheology
We present an experimental procedure to perform broadband microrheological
measurements with optical tweezers. A generalised Langevin equation is adopted
to relate the time-dependent trajectory of a particle in an imposed flow to the
frequency-dependent moduli of the complex fluid. This procedure allows us to
measure the material linear viscoelastic properties across the widest frequency
range achievable with optical tweezers.Comment: 5 pages, 3 figure
Optical tweezers: wideband microrheology
Microrheology is a branch of rheology having the same principles as
conventional bulk rheology, but working on micron length scales and micro-litre
volumes.
Optical tweezers have been successfully used with Newtonian fluids for
rheological purposes such as determining fluid viscosity. Conversely, when
optical tweezers are used to measure the viscoelastic properties of complex
fluids the results are either limited to the material's high-frequency
response, discarding important information related to the low-frequency
behavior, or they are supplemented by low-frequency measurements performed with
different techniques, often without presenting an overlapping region of clear
agreement between the sets of results. We present a simple experimental
procedure to perform microrheological measurements over the widest frequency
range possible with optical tweezers. A generalised Langevin equation is used
to relate the frequency-dependent moduli of the complex fluid to the
time-dependent trajectory of a probe particle as it flips between two optical
traps that alternately switch on and off.Comment: 13 pages, 6 figures, submitted to Special Issue of the Journal of
Optic
Development of a 3D printer using scanning projection stereolithography
We have developed a system for the rapid fabrication of low cost 3D devices and systems in the laboratory with micro-scale features yet cm-scale objects. Our system is inspired by maskless lithography, where a digital micromirror device (DMD) is used to project patterns with resolution up to 10 µm onto a layer of photoresist. Large area objects can be fabricated by stitching projected images over a 5cm2 area. The addition of a z-stage allows multiple layers to be stacked to create 3D objects, removing the need for any developing or etching steps but at the same time leading to true 3D devices which are robust, configurable and scalable. We demonstrate the applications of the system by printing a range of micro-scale objects as well as a fully functioning microfluidic droplet device and test its integrity by pumping dye through the channels
Sub-shot-noise shadow sensing with quantum correlations
The quantised nature of the electromagnetic field sets the classical limit to the sensitivity of position measurements. However, techniques based on the properties of quantum states can be exploited to accurately measure the relative displacement of a physical object beyond this classical limit. In this work, we use a simple scheme based on the split-detection of quantum correlations to measure the position of a shadow at the single-photon light level, with a precision that exceeds the shot-noise limit. This result is obtained by analysing the correlated signals of bi-photon pairs, created in parametric downconversion and detected by an electron multiplying CCD (EMCCD) camera employed as a split-detector. By comparing the measured statistics of spatially anticorrelated and uncorrelated photons we were able to observe a significant noise reduction corresponding to an improvement in position sensitivity of up to 17% (0.8dB). Our straightforward approach to sub-shot-noise position measurement is compatible with conventional shadow-sensing techniques based on the split-detection of light-fields, and yields an improvement that scales favourably with the detector’s quantum efficiency
Single-pixel imaging 12 years on: a review
Modern cameras typically use an array of millions of detector pixels to capture images. By contrast, single-pixel cameras use a sequence of mask patterns to filter the scene along with the corresponding measurements of the transmitted intensity which is recorded using a single-pixel detector. This review considers the development of single-pixel cameras from the seminal work of Duarte et al. up to the present state of the art. We cover the variety of hardware configurations, design of mask patterns and the associated reconstruction algorithms, many of which relate to the field of compressed sensing and, more recently, machine learning. Overall, single-pixel cameras lend themselves to imaging at non-visible wavelengths and with precise timing or depth resolution. We discuss the suitability of single-pixel cameras for different application areas, including infrared imaging and 3D situation awareness for autonomous vehicles
Adaptive foveated single-pixel imaging with dynamic super-sampling
As an alternative to conventional multi-pixel cameras, single-pixel cameras
enable images to be recorded using a single detector that measures the
correlations between the scene and a set of patterns. However, to fully sample
a scene in this way requires at least the same number of correlation
measurements as there are pixels in the reconstructed image. Therefore
single-pixel imaging systems typically exhibit low frame-rates. To mitigate
this, a range of compressive sensing techniques have been developed which rely
on a priori knowledge of the scene to reconstruct images from an under-sampled
set of measurements. In this work we take a different approach and adopt a
strategy inspired by the foveated vision systems found in the animal kingdom -
a framework that exploits the spatio-temporal redundancy present in many
dynamic scenes. In our single-pixel imaging system a high-resolution foveal
region follows motion within the scene, but unlike a simple zoom, every frame
delivers new spatial information from across the entire field-of-view. Using
this approach we demonstrate a four-fold reduction in the time taken to record
the detail of rapidly evolving features, whilst simultaneously accumulating
detail of more slowly evolving regions over several consecutive frames. This
tiered super-sampling technique enables the reconstruction of video streams in
which both the resolution and the effective exposure-time spatially vary and
adapt dynamically in response to the evolution of the scene. The methods
described here can complement existing compressive sensing approaches and may
be applied to enhance a variety of computational imagers that rely on
sequential correlation measurements.Comment: 13 pages, 5 figure
Optically controlled grippers for manipulating micron-sized particles
We report the development of a joystick controlled gripper for the real-time manipulation of micron-sized objects, driven using holographic optical tweezers (HOTs). The gripper consists of an arrangement of four silica beads, located in optical traps, which can be positioned and scaled in order to trap an object indirectly. The joystick can be used to grasp, move (lateral or axial), and change the orientation of the target object. The ability to trap objects indirectly allows us to demonstrate the manipulation of a strongly scattering micron-sized metallic particle
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