Photon-efficient optical tweezers via wavefront shaping.

Optical tweezers enable noncontact trapping of microscale objects using light. It is not known how tightly it is possible to three-dimensionally (3D) trap microparticles with a given photon budget. Reaching this elusive limit would enable maximally stiff particle trapping for precision measurements on the nanoscale and photon-efficient tweezing of light-sensitive objects. Here, we customize the shape of light fields to suit specific particles, with the aim of optimizing trapping stiffness in 3D. We show, theoretically, that the confinement volume of microspheres held in sculpted optical traps can be reduced by one to two orders of magnitude. Experimentally, we use a wavefront shaping-inspired strategy to passively suppress the Brownian fluctuations of microspheres in every direction concurrently, demonstrating order-of-magnitude reductions in their confinement volumes. Our work paves the way toward the fundamental limits of optical control over the mesoscopic realm.

Modular light sources for microscopy and beyond (ModLight).

Modular light (ModLight) sources can be integrated into complex systems for microscopy, medical imaging, remote sensing, and many more. Motivated by the need for affordable and open-access alternatives that are globally relevant, we have designed and presented light devices that use simple, off-the-shelf components. Red, green, blue, white and near-infrared LEDs are combined using mirrors and X-Cube prisms in novel devices. This modular nature allows portability and mounting flexibility. The ModLight suite can be used with any optical system that requires single- or multi-wavelength illumination such as bright-field and epifluorescence microscopes.

Computational version of the correlation light-field camera.

Light-field cameras allow the acquisition of both the spatial and angular components of the light-field. The conventional way to perform such acquisitions leads to a strong spatio-angular resolution limitation but correlation-enabled plenoptic cameras have been introduced recently that relax this constraint. Here we use a computational version of this concept to acquire realistic light-fields images using a commercial DSLR Camera lens as an imaging system. By placing the image sensor in the focal plane of a lens, within the camera we ensure the acquisition of pure angular components together with the spatial information. We perform an acquisition presenting a high spatio-angular rays resolution obtained through a trade off of the temporal resolution. The acquisition reported is photo-realistic and the acquisition of diffraction limited features is observed with the setup. Finally, we demonstrate the refocusing abilities of the camera.

Simulated assessment of light transport through ischaemic skin flaps.

Currently, free flaps and pedicled flaps are assessed for reperfusion in postoperative care using colour, capillary refill, temperature, texture, and Doppler signal (if available). While these techniques are effective, they are prone to error due to their qualitative nature. In this research, different wavelengths of light were used to quantify the response of ischaemic tissue. The assessment provides indicators that are key to developing a point-of-care diagnostic device that is capable of observing reduced perfusion quantitatively. Detailed optical models of the layers of the skin were set up and appropriate optical properties assigned, with due consideration of melanin and haemoglobin concentration. A total of 24 models of healthy, perfused and perfusion-deprived tissue were used to assess the responses when illuminated with visible and near-infrared wavelengths of light. In addition to detailed fluence maps of photon propagation, a simple mathematical model is proposed to assess the differential propagation of photons in tissue; the optical reperfusion factor (ORF). The results show clear advantages of using light at longer wavelengths (red, near-infrared) and the inferences drawn from the simulations hold significant clinical relevance. The simulated scenarios and results consolidate the belief in a multi-wavelength, point-of-care diagnostic device, and inform its design to quantify blood flow in transplanted tissue. The modelling approach is applicable beyond the current research and can be used to investigate other medical conditions in the skin that can be mathematically represented. Through these, additional inferences and approaches to other point-of-care devices can be realised.

i-RheoFT: Fourier transforming sampled functions without artefacts.

In this article we present a new open-access code named "i-RheoFT" that implements the analytical method first introduced in [PRE, 80, 012501 (2009)] and then enhanced in [New J Phys 14, 115032 (2012)], which allows to evaluate the Fourier transform of any generic time-dependent function that vanishes for negative times, sampled at a finite set of data points that extend over a finite range, and need not be equally spaced. I-RheoFT has been employed here to investigate three important experimental factors: (i) the 'density of initial experimental points' describing the sampled function, (ii) the interpolation function used to perform the "virtual oversampling" procedure introduced in [New J Phys 14, 115032 (2012)], and (iii) the detrimental effect of noises on the expected outcomes. We demonstrate that, at relatively high signal-to-noise ratios and density of initial experimental points, all three built-in MATLAB interpolation functions employed in this work (i.e., Spline, Makima and PCHIP) perform well in recovering the information embedded within the original sampled function; with the Spline function performing best. Whereas, by reducing either the number of initial data points or the signal-to-noise ratio, there exists a threshold below which all three functions perform poorly; with the worst performance given by the Spline function in both the cases and the least worst by the PCHIP function at low density of initial data points and by the Makima function at relatively low signal-to-noise ratios. We envisage that i-RheoFT will be of particular interest and use to all those studies where sampled or time-averaged functions, often defined by a discrete set of data points within a finite time-window, are exploited to gain new insights on the systems' dynamics.

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.

Developing a portable gas imaging camera using highly tunable active-illumination and computer vision.

We have developed a portable gas imaging camera for identifying methane leaks in real-time. The camera uses active illumination from distributed feedback InGaAs laser diodes tuned to the 1653 nm methane absorption band. An InGaAs focal plane sensor array images the active illumination. The lasers are driven off resonance every alternate frame so that computer vision can extract the gas data. A colour image is captured simultaneously and the data is superimposed to guide the operator. Image stabilisation has been employed to allow detection with a moving camera, successfully imaging leaks from mains pressure gas supplies at a range of up to 3 m and flow rates as low as 0.05 L min-1.

What Caging Force Cells Feel in 3D Hydrogels: A Rheological Perspective.

It has been established that the mechanical properties of hydrogels control the fate of (stem) cells. However, despite its importance, a one-to-one correspondence between gels' stiffness and cell behavior is still missing from literature. In this work, the viscoelastic properties of poly(ethylene-glycol) (PEG)-based hydrogels are investigated by means of rheological measurements performed at different length scales. The outcomes of this work reveal that PEG-based hydrogels show significant stiffening when subjected to a compressional deformation, implying that conventional bulk rheology measurements may overestimate the stiffness of hydrogels by up to an order of magnitude. It is hypothesized that this apparent stiffening is caused by an induced "tensional state" of the gel network, due to the application of a compressional normal force during sample loading. Moreover, it is shown that the actual stiffness of the hydrogels is instead accurately determined by means of both passive-video-particle-tracking (PVPT) microrheology and nanoindentation measurements, which are inherently performed at the cell's length scale and in absence of any externally applied force in the case of PVPT. These results underpin a methodology for measuring hydrogels' linear viscoelastic properties that are representative of the mechanical constraints perceived by cells in 3D hydrogel cultures.

Dual-band single-pixel telescope.

Single-pixel imaging systems can obtain images from a wide range of wavelengths at low-cost compared to those using conventional multi-pixel, focal-plane array sensors, especially at wavelengths outside the visible spectrum. The ability to sense short-wave infrared radiation with single-pixel techniques extends imaging capability to adverse weather conditions and environments, such as fog, haze, or night time. In this work, we demonstrate a dual-band single-pixel telescope for imaging at both visible (VIS) and short-wave infrared (SWIR) spectral regions simultaneously under some of these outdoor weather conditions. At 64 × 64 pixel-resolution, our system has achieved continuous VIS and SWIR imaging of various objects at a frame rate up to 2.4 Hz. Visual and contrast comparison between the reconstructed VIS and SWIR images emphasizes the significant contribution of infrared observation using the single-pixel technique. The single-pixel telescope provides an alternative cost-effective imaging solution for synchronized dual-waveband optical applications.

Photon Bunching in a Rotating Reference Frame.

Although quantum physics is well understood in inertial reference frames (flat spacetime), a current challenge is the search for experimental evidence of nontrivial or unexpected behavior of quantum systems in noninertial frames. Here, we present a novel test of quantum mechanics in a noninertial reference frame: we consider Hong-Ou-Mandel (HOM) interference on a rotating platform and study the effect of uniform rotation on the distinguishability of the photons. Both theory and experiments show that the rotational motion induces a relative delay in the photon arrival times at the exit beam splitter and that this delay is observed as a shift in the position of the HOM dip. This experiment can be extended to a full general relativistic test of quantum physics using satellites in Earth's orbit and indicates a new route toward the use of photonic technologies for investigating quantum mechanics at the interface with relativity.

A compact acoustic spanner to rotate macroscopic objects.

Waves can carry both linear and angular momentum. When the wave is transverse (e.g. light), the angular momentum can be characterised by the "spin" angular momentum associated with circular polarisation, and the "orbital" angular momentum (OAM) arising from the phase cross-section of the beam. When the wave is longitudinal (e.g. sound) there is no polarization and hence no spin angular momentum. However, a suitably phase-structured sound beam can still carry OAM. Observing the transfer of OAM from sound to a macroscopic object provides an excellent opportunity to study the exchange of energy between waves and matter. In this paper we show how to build a compact free-space acoustic spanner based on a 3D-printed sound-guiding structure and common electronic components. We first characterise the sound fields by measuring both phase and amplitude maps, and then show a video of our free-space acoustic spanner in action, in which macroscopic objects spin in a circular motion and change direction of rotation according to the handedness of the OAM acoustic field.

Indirect optical trapping using light driven micro-rotors for reconfigurable hydrodynamic manipulation.

Optical tweezers are a highly versatile tool for exploration of the mesoscopic world, permitting non-contact manipulation of nanoscale objects. However, direct illumination with intense lasers restricts their use with live biological specimens, and limits the types of materials that can be trapped. Here we demonstrate an indirect optical trapping platform which circumvents these limitations by using hydrodynamic forces to exert nanoscale-precision control over aqueous particles, without directly illuminating them. Our concept is based on optically actuated micro-robotics: closed-loop control enables highly localised flow-fields to be sculpted by precisely piloting the motion of optically-trapped micro-rotors. We demonstrate 2D trapping of absorbing particles which cannot be directly optically trapped, stabilise the position and orientation of yeast cells, and demonstrate independent control over multiple objects simultaneously. Our work expands the capabilities of optical tweezers platforms, and represents a new paradigm for manipulation of aqueous mesoscopic systems.

Experimental demonstration of ray-rotation sheets.

We have built microstructured sheets that rotate, on transmission, the direction of light rays by an arbitrary, but fixed, angle around the sheet normal. These ray-rotation sheets comprise two pairs of confocal lenticular arrays. In addition to rotating the direction of transmitted light rays, our sheets also offset ray position sideways on the scale of the diameter of the lenticules. If this ray offset is sufficiently small so that it cannot be resolved, our ray-rotation sheets appear to perform generalized refraction.

Approach to classify, separate, and enrich objects in groups using ensemble sorting.

The sorting of objects into groups is a fundamental operation, critical in the preparation and purification of populations of cells, crystals, beads, or droplets, necessary for research and applications in biology, chemistry, and materials science. Most of the efforts exploring such purification have focused on two areas: the degree of separation and the measurement precision required for effective separation. Conventionally, achieving good separation ultimately requires that the objects are considered one by one (which can be both slow and expensive), and the ability to measure the sorted objects by increasing sensitivity as well as reducing sorting errors. Here we present an approach to sorting that addresses both critical limitations with a scheme that allows us to approach the theoretical limit for the accuracy of sorting decisions. Rather than sorting individual objects, we sort the objects in ensembles, via a set of registers which are then in turn sorted themselves into a second symmetric set of registers in a lossless manner. By repeating this process, we can arrive at high sorting purity with a low set of constraints. We demonstrate both the theory behind this idea and identify the critical parameters (ensemble population and sorting time), and show the utility and robustness of our method with simulations and experimental systems spanning several orders of scale, sorting populations of macroscopic beads and microfluidic droplets. Our method is general in nature and simplifies the sorting process, and thus stands to enhance many different areas of science, such as purification, enrichment of rare objects, and separation of dynamic populations.

Reversal of orbital angular momentum arising from an extreme Doppler shift.

The linear Doppler shift is familiar as the rise and fall in pitch of a siren as it passes by. Less well known is the rotational Doppler shift, proportional to the rotation rate between source and receiver, multiplied by the angular momentum carried by the beam. In extreme cases the Doppler shift can be larger than the rest-frame frequency and for a red shift, the observed frequency then becomes "negative." In the linear case, this effect is associated with the time reversal of the received signal, but it can be observed only with supersonic relative motion between the source and receiver. However, the rotational case is different; if the radius of rotation is smaller than the wavelength, then the velocities required to observe negative frequencies are subsonic. Using an acoustic source at [Formula: see text]100 Hz we create a rotational Doppler shift larger than the laboratory-frame frequency. We observe that once the red-shifted wave passes into the "negative frequency" regime, the angular momentum associated with the sound is reversed in sign compared with that of the laboratory frame. These low-velocity laboratory realizations of extreme Doppler shifts have relevance to superoscillatory fields and offer unique opportunities to probe interactions with rotating bodies and aspects of pseudorelativistic frame translation.

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.

Adaptive foveated single-pixel imaging with dynamic supersampling.

In contrast to conventional multipixel cameras, single-pixel cameras capture images using a single detector that measures the correlations between the scene and a set of patterns. However, these systems typically exhibit low frame rates, because to fully sample a scene in this way requires at least the same number of correlation measurements as the number of pixels in the reconstructed image. To mitigate this, a range of compressive sensing techniques have been developed which use a priori knowledge to reconstruct images from an undersampled measurement set. Here, we take a different approach and adopt a strategy inspired by the foveated vision found in the animal kingdom-a framework that exploits the spatiotemporal redundancy of many dynamic scenes. In our system, a high-resolution foveal region tracks motion within the scene, yet unlike a simple zoom, every frame delivers new spatial information from across the entire field of view. This strategy rapidly records the detail of quickly changing features in the scene while simultaneously accumulating detail of more slowly evolving regions over several consecutive frames. This architecture provides video streams in which both the resolution and exposure time spatially vary and adapt dynamically in response to the evolution of the scene. The degree of local frame rate enhancement is scene-dependent, but here, we demonstrate a factor of 4, thereby helping to mitigate one of the main drawbacks of single-pixel imaging techniques. The methods described here complement existing compressive sensing approaches and may be applied to enhance computational imagers that rely on sequential correlation measurements.

Real-time imaging of methane gas leaks using a single-pixel camera.

We demonstrate a camera which can image methane gas at video rates, using only a single-pixel detector and structured illumination. The light source is an infrared laser diode operating at 1.651µm tuned to an absorption line of methane gas. The light is structured using an addressable micromirror array to pattern the laser output with a sequence of Hadamard masks. The resulting backscattered light is recorded using a single-pixel InGaAs detector which provides a measure of the correlation between the projected patterns and the gas distribution in the scene. Knowledge of this correlation and the patterns allows an image to be reconstructed of the gas in the scene. For the application of locating gas leaks the frame rate of the camera is of primary importance, which in this case is inversely proportional to the square of the linear resolution. Here we demonstrate gas imaging at ~25 fps while using 256 mask patterns (corresponding to an image resolution of 16×16). To aid the task of locating the source of the gas emission, we overlay an upsampled and smoothed image of the low-resolution gas image onto a high-resolution color image of the scene, recorded using a standard CMOS camera. We demonstrate for an illumination of only 5mW across the field-of-view imaging of a methane gas leak of ~0.2 litres/minute from a distance of ~1 metre.

Improving the signal-to-noise ratio of single-pixel imaging using digital microscanning.

Single-pixel cameras provide a means to perform imaging at wavelengths where pixelated detector arrays are expensive or limited. The image is reconstructed from measurements of the correlation between the scene and a series of masks. Although there has been much research in the field in recent years, the fact that the signal-to-noise ratio (SNR) scales poorly with increasing resolution has been one of the main limitations prohibiting the uptake of such systems. Microscanning is a technique that provides a final higher resolution image by combining multiple images of a lower resolution. Each of these low resolution images is subject to a sub-pixel sized lateral displacement. In this work we apply a digital microscanning approach to an infrared single-pixel camera. Our approach requires no additional hardware, but is achieved simply by using a modified set of masks. Compared to the conventional Hadamard based single-pixel imaging scheme, our proposed framework improves the SNR of reconstructed images by ∼ 50 % for the same acquisition time. In addition, this strategy also provides access to a stream of low-resolution 'preview' images throughout each high-resolution acquisition.

Noninvasive, near-field terahertz imaging of hidden objects using a single-pixel detector.

Terahertz (THz) imaging can see through otherwise opaque materials. However, because of the long wavelengths of THz radiation (λ = 400 µm at 0.75 THz), far-field THz imaging techniques suffer from low resolution compared to visible wavelengths. We demonstrate noninvasive, near-field THz imaging with subwavelength resolution. We project a time-varying, intense (>100 µJ/cm(2)) optical pattern onto a silicon wafer, which spatially modulates the transmission of synchronous pulse of THz radiation. An unknown object is placed on the hidden side of the silicon, and the far-field THz transmission corresponding to each mask is recorded by a single-element detector. Knowledge of the patterns and of the corresponding detector signal are combined to give an image of the object. Using this technique, we image a printed circuit board on the underside of a 115-µm-thick silicon wafer with ~100-µm (λ/4) resolution. With subwavelength resolution and the inherent sensitivity to local conductivity, it is possible to detect fissures in the circuitry wiring of a few micrometers in size. THz imaging systems of this type will have other uses too, where noninvasive measurement or imaging of concealed structures is necessary, such as in semiconductor manufacturing or in ex vivo bioimaging.