Search

Optical tweezers are tools made of light that enable contactless pushing, trapping, and manipulation of objects ranging from atoms to space light sails. Since the pioneering work by Arthur Ashkin in the 1970s, optical tweezers have evolved into sophisticated instruments and have been employed in a broad range of applications in life sciences, physics, and engineering. These include accurate force and torque measurement at the femtonewton level, microrheology of complex fluids, single micro- and nanoparticle spectroscopy, single-cell analysis, and statistical-physics experiments. This roadmap provides insights into current investigations involving optical forces and optical tweezers from their theoretical foundations to designs and setups. It also offers perspectives for applications to a wide range of research fields, from biophysics to space exploration., Comment: 181 pages, 61 figures

In general,the optical binding force between identical particles is thought to be symmetric.However,we demonstrate analytically a counter-intuitively asymmetric longitudinal optical binding force between two identical dual dipolar dielectric particles.This homodimer is confined in two counter-propagating incoherent plane waves along the dimer's axis.The force consists of the electric dipolar,magnetic dipolar,and electric-magnetic dipolar coupling interactions.The combined effect of these interactions is markedly different than the expected behavior in the Rayleigh approximation.The asymmetric force is a result of the asymmetric forward and backward scattering of the particles due to the dipolar hybridization and coupling interactions.Consequently,it leads to a harmonic driving force on the pair,which decays with the interparticle distance to the first power.We show the rich nonequilibrium dynamics of the dimer and of the two particles impelled by the driving and binding forces and discuss the ranges of particle refractive index and size in which the asymmetric binding force arises.Our results open perspectives for nonequilibrium light-driven multiparticle transport and self-assembly., 9 pages, 10 figures

Highly resolved determination of refractive index is vital in fields ranging from biosensing through to laser range finding. Laser speckle is known to be a sensitive probe of the properties of the light and the environment, but to date speckle-based refractive index measurements have been restricted to 10

We demonstrate parametric feedback cooling of a levitated birefringent microsphere held in a circularly polarised Gaussian trap. We show efficient cooling of the birefringent microsphere to the temperature less than a millikelvin, which is orders of magnitude lower than that of a silica counterpart. We reveal that the transverse spin momentum present in circularly polarised light plays a role in the trapped particle dynamics, which exhibits a range of nonequilibrium phenomena, namely, (i) biased stochastic motion, (ii) coupling of mechanical modes, (iii) nonconservative rotation-translation coupling. We further discuss the temperature of the trapped particle in a nonconservative vacuum optical trap.

The physical origins of the transverse optical binding force and torque beyond the Rayleigh approximation have not been clearly expressed to date. Here, we present analytical expressions of the force and torque for a dual dipolar dielectric dimer illuminated by a plane wave propagating perpendicularly to the dimer axis. Using this analytical model, we explore the roles of the hybridized electric dipolar, magnetic dipolar, and electric-magnetic dipolar coupling interactions in the total force and torque on the particles. We find significant departures from the predictions of the Rayleigh approximation, particularly for high-refractive-index particles, where the force is dominated by the magnetic interaction. This results in an enhancement of the dimer stability by one to four orders of magnitude compared to the predictions of the Rayleigh approximation. For the case of torque, this is dominated by the coupling interaction and increases by an order of magnitude. Our results will help to guide future experimental work in optical binding of high-refractive-index dielectric particles., 14 pages, 10 figures

Speckle metrology is a powerful tool in the measurement of wavelength and spectra. Recently, speckle produced by multiple reflections inside an integrating sphere has been proposed and showed high performance. However, to our knowledge, a complete characterisation of speckle sensitivity to wavelength in that geometry has not been performed to date. In this work, we derive a general model predicting the variation in a speckle pattern as a result of a generic transformation. Applying this to a shift in the incident wavelength, we show that the speckle sensitivity is mainly governed by the radius and surface reflectivity of the sphere. We show that integrating spheres offer sensitivity three orders of magnitude above that of multimode fibres of a similar size, and discuss analogies with the transmission line of a Fabry-P\'erot interferometer., Comment: 7 pages, 4 figures

Highly-resolved determination of refractive index is vital in fields ranging from biosensing through to laser range-finding. Laser speckle is known to be a sensitive probe of the properties of the light and the environment, but to date speckle-based refractive index measurements have been restricted to $10^{-6}$ resolution. In this work we identify a strategy to optimise the sensitivity of speckle to refractive index changes, namely by maximising the width of the distribution of optical path lengths in the medium. We show that this can be realised experimentally by encapsulating the medium of interest within an integrating sphere. We demonstrate that variations of the refractive index of air as small as $4.5\times10^{-9}$ can be resolved with an uncertainty of $7\times10^{-10}$. This is an improvement of three orders of magnitude when compared to previous speckle-based methods., Comment: 7 pages, 5 figures

The fastest-spinning man-made object is a tiny dumbbell rotating at 5 GHz. The smallest wind-up motor is constructed from a DNA molecule. Picoliter volumes of fluids are remotely controlled and their viscosity precisely measured using microrheometers based on miniscule rotating particles. Theoretical predictions for extraordinarily weak forces related to the presence of dark matter, dark energy and vacuum-induced friction might be revealed, and the surprising properties of light have already been experimentally evidenced. All of these exciting landmarks have only been possible thanks to the torque exerted by light, which enables rotation of an optically trapped particle. Here, we review how light can impart torque on optically trapped particles, paying close attention to the design of the properties of both the particle and the light field. We detail how the maximum achievable rotation speed is limited by the environment, but can simultaneously be used to infer properties of the surrounding medium and of the light field itself. We also review the state-of-the-art applications of light-driven rotors, as well as proposals for the next generation of measurements, particularly at the classical-quantum interface, which can be performed using rotating optically trapped objects.

Many areas of optical science require an accurate measurement of optical spectra. Devices based on laser speckle promise compact wavelength measurement, with attometer-level sensitivity demonstrated for single wavelength laser fields. The measurement of multimode spectra using this approach would be attractive, yet this is currently limited to picometer resolution. Here, we present a method to improve the resolution and precision of speckle-based multi-wavelength measurements. We measure multiple wavelengths simultaneously, in a device comprising a single 1 m-long step-index multimode fiber and a fast camera. Independent wavelengths separated by as little as 1 fm are retrieved with 0.2 fm precision using Principal Component Analysis. The method offers a viable way to measure sparse spectra containing multiple individual lines and is likely to find application in the tracking of multiple lasers in fields such as portable quantum technologies and optical telecommunications., 5 pages, 6 figures

Laser speckle is generated by the multiple interference of light through a disordered medium. Here we study the premise that the speckle pattern retains information about the polarisation state of the incident field. We analytically verify that a linear relation exists between the Stokes vector of the light and the resulting speckle pattern. As a result, the polarisation state of a beam can be measured from the speckle pattern using a transmission matrix approach. We perform a quantitative analysis of the accuracy of the transmission matrix method to measure randomly time-varying polarisation states. In experiment, we find that the Stokes parameters of light from a diode laser can be retrieved with an uncertainty of 0.05 using speckle images of 150$\times$150 pixels and 17 training states. We show both analytically and in experiment that this approach may be extended to the case of more than one laser field, demonstrating the measurement of the Stokes parameters of two laser beams simultaneously from a single speckle pattern and achieving the same uncertainty of 0.05., 7 pages, 7 figures

Laser speckle can provide a powerful tool that may be used for metrology, for example measurements of the incident laser wavelength with a resolution beyond that which may be achieved in a commercial device. However, to realise highest resolution requires advanced multi-variate analysis techniques, which limit the acquisition rate of such a wavemeter. Here we show an arithmetically simple method to measure wavelength changes with dynamic speckle, based on a Poincar\`e descriptor of the speckle pattern. We demonstrate the measurement of wavelength changes at femtometer-level with a measurement time reduced by two orders of magnitude compared to the previous state-of-the-art, which offers promise for applications such as speckle-based laser wavelength stabilisation., Comment: 6 pages, 3 figures

Non-intrusive detection systems have the potential to characterise materials through various transparent glass and plastic containers. Food and drink adulteration is increasingly problematic, representing a serious health risk as well as an economic issue. This is of particular concern for alcoholic spirits such as Scotch whisky which are often targeted for fraudulent activity. We have developed a Raman system with a novel geometry of excitation and collection, exploiting the beam propagation from an axicon lens resulting in an annular beam that transforms to a Bessel illumination within the sample. This facilitates the efficient acquisition of Raman signals from the alcoholic spirit contained inside the bottle, while avoiding the collection of auto-fluorescence signals generated by the bottle wall. Therefore, this technique provides a way of non-destructive and non-contact detection to precisely analyse the contents without the requirement to open the bottle., Comment: 7 pages, 6 figures

The measurement of the wavelength of light using speckle is a promising tool for the realization of compact and precise wavemeters and spectrometers. However, the resolution of these devices is limited by strong correlations between the speckle patterns produced by closely-spaced wavelengths. Here, we show how principal component analysis of speckle images provides a route to overcome this limit. Using this, we demonstrate a compact wavemeter which measures wavelength changes of a stabilized diode laser of 5.3 am, eight orders of magnitude below the speckle correlation limit., Comment: 5 pages, 4 figures

Funding: UK Engineering and Physical Sciences Research Council (EPSRC) - EP/P030017/1. We demonstrate the thermal advantages of using low repetition rate, high peak power lasers for imaging in two-photon light sheet microscopy using a Bessel light beam. We compare the use of two ultrashort pulsed lasers in such an imaging system: a high repetition rate source operating at 80 MHz and a low repetition rate source operating at 1 MHz. The low repetition rate laser requires approximately one order of magnitude lower average power than the high repetition rate source to yield the same fluorescent signal. These lasers are used to image Zebrafish larvae and record their heart rates. The data show heart rate values 30% in excess of the ground truth baseline value when imaged with the high repetition rate source due to deleterious heating, whereas the low repetition rate source yields data only a few percent above this ground truth value. Publisher PDF

The speckle pattern produced when a laser is scattered by a disordered medium has recently been shown to give a surprisingly accurate or broadband measurement of wavelength. Here it is shown that deep learning is an ideal approach to analyse wavelength variations using a speckle wavemeter due to its ability to identify trends and overcome low signal to noise ratio in complex datasets. This combination enables wavelength measurement at high precision over a broad operating range in a single step, with a remarkable capability to reject instrumental and environmental noise, which has not been possible with previous approaches. It is demonstrated that the noise rejection capabilities of deep learning provide attometre-scale wavelength precision over an operating range from 488 nm to 976 nm. This dynamic range is six orders of magnitude beyond the state of the art., Comment: 23 pages, 7 figures

The speckle pattern produced when a laser illuminates a random medium can, with appropriate analysis, be used to uniquely identify the wavelength of the illuminating source. We have demonstrated that principal component analysis can be used as a very sensitive probe of the speckle pattern produced by random prisms [1] and integrating spheres [2]. However, to date, the state-of-the-art realisations of speckle spectrometers have been based on the use of multi-mode fibres as the scattering medium [3] and on transmission matrix analysis approaches, achieving a compact and stable device with picometre resolution. Here, we show that the speckle pattern produced by propagation through a metre-long step-index multi-mode optical fibre can be analysed with principal component analysis to achieve a femtometre-precision wavemeter, and present progress in the measurement of complete spectra, which demonstrates the applicability of our approach to many existing experiments. Moreover, we demonstrate that the speckle wavemeter can be used as part of a feedback loop to stabilise lasers to a fractional stability of 10-9. With the freedom to lock the laser at any user-desired frequency and a robust, compact setup, the method holds promise for the new generation of portable cold atom experiments currently being developed for quantum technology applications. [1] M Mazilu, et al. Opt Lett 39, 96 (2014) [2] N K Metzger, et al. Nat. Commun. 8, 15610 (2017) [3] H Cao, J. Opt. 19, 060402 (2017) Preprint

We demonstrate an approach to accurately measure the wavelength of light at femtometer resolution using the speckle output from a multimode fibre. Our approach can be used to measure a single wavelength, or distinguish between two wavelengths co-propagating through the fibre.

With the advent of spatially resolved fluorescence imaging in quantum gas microscopes, it is now possible to directly image glassy phases and probe the local effects of disorder in a highly controllable setup. Here we present numerical calculations using a spatially-resolved local mean-field theory, show that it captures the essential physics of the disordered system and use it to simulate the density distributions seen in single-shot fluorescence microscopy. From these simulated images we extract local properties of the phases which are measurable by a quantum gas microscope and show that unambiguous detection of the Bose glass is possible. In particular, we show that experimental determination of the Edwards-Anderson order parameter is possible in a strongly correlated quantum system using existing experiments. We also suggest modifications to the experiments which will allow further properties of the Bose glass to be measured., Comment: 7 pages, 4 figures

The lifetime of an atom trap is often limited by the presence of residual background gases in the vacuum chamber. This leads to the lifetime being inversely proportional to the pressure. Here we use this dependence to estimate the pressure and to obtain pressure rate-of-rise curves, which are commonly used in vacuum science to evaluate the performance of a system. We observe different rates of pressure increase in response to different levels of outgassing in our system. Therefore we suggest that this is a sensitive method which will be useful in applications of cold atom systems, in particular where the inclusion of a standard vacuum gauge is impractical., 5 pages, 6 figures

In recent years, light-induced atomic desorption (LIAD) of alkali atoms from the inner surface of a vacuum chamber has been employed in cold atom experiments for the purpose of modulating the alkali background vapour. This is beneficial because larger trapped atom samples can be loaded from vapour at higher pressure, after which the pressure is reduced to increase the lifetime of the sample. We present an analysis, based on the case of rubidium atoms adsorbed on pyrex, of various aspects of LIAD that are useful for this application. Firstly, we study the intensity dependence of LIAD by fitting the experimental data with a rate-equation model, from which we extract a correct prediction for the increase in trapped atom number. Following this, we quantify a figure of merit for the utility of LIAD in cold atom experiments and we show how it can be optimised for realistic experimental parameters., Comment: 10 pages, 8 figures

We demonstrate a method to independently and arbitrarily tailor the spatial profile of light of multiple wavelengths and we show possible applications to ultracold atoms experiments. A single spatial light modulator is programmed to create a pattern containing multiple spatially separated structures in the Fourier plane when illuminated with a single wavelength. When the modulator is illuminated with overlapped laser beams of different wavelengths, the position of the structures is wavelength-dependent. Hence, by designing their separations appropriately, a desired overlap of different structures at different wavelengths is obtained. We employ regional phase calculation algorithms and demonstrate several possible experimental scenarios by generating light patterns with 670 nm, 780 nm and 1064 nm laser light which are accurate to the level of a few percent. This technique is easily integrated into cold atom experiments, requiring little optical access.

Single-atom-resolved detection in optical lattices using quantum-gas microscopes has enabled a new generation of experiments in the field of quantum simulation. Fluorescence imaging of individual atoms has so far been achieved for bosonic species with optical molasses cooling, whereas detection of fermionic alkaline atoms in optical lattices by this method has proven more challenging. Here we demonstrate single-site- and single-atom-resolved fluorescence imaging of fermionic potassium-40 atoms in a quantum-gas microscope setup using electromagnetically-induced-transparency cooling. We detected on average 1000 fluorescence photons from a single atom within 1.5s, while keeping it close to the vibrational ground state of the optical lattice. Our results will enable the study of strongly correlated fermionic quantum systems in optical lattices with resolution at the single-atom level, and give access to observables such as the local entropy distribution and individual defects in fermionic Mott insulators or anti-ferromagnetically ordered phases., 7 pages, 5 figures; Nature Physics, published online 13 July 2015

We use a phase-only spatial light modulator to generate light distributions in which the intensity decays as a power law from a central maximum, with order ranging from 2 (parabolic) to 0.5. We suggest that a sequence of these can be used as a time-dependent optical dipole trap for all-optical production of Bose-Einstein condensates in two stages: efficient evaporative cooling in a trap with adjustable strength and depth, followed by an adiabatic transformation of the trap order to cross the BEC transition in a reversible way. Realistic experimental parameters are used to verify the capability of this approach in producing larger Bose-Einstein condensates than by evaporative cooling alone., 6 pages, 6 figures, minor changes to figures, sentence added at the end of section IV

We discuss the suitability of holographically generated optical potentials for the investigation of superfluidity in ultracold atoms. By using a spatial light modulator and a feedback enabled algorithm we generate a smooth ring with variable bright regions that can be dynamically rotated to stir ultracold atoms and induce superflow. We also comment on its future integration into a cold atoms experiment., 8 pages, 3 figures

We show that a phase-only spatial light modulator can be used to generate non-trivial light distributions suitable for trapping ultracold atoms, when the hologram calculation is included within a simple and robust feedback loop that corrects for imperfect device response and optical aberrations. This correction reduces the discrepancy between target and experimental light distribution to the level of a few percent (RMS error). We prove the generality of this algorithm by applying it to a variety of target light distributions of relevance for cold atomic physics., 5 pages, 4 figures

SUNcast Polyurethanes Inc. has developed a proprietary process technology that makes it possible to rebuild worn Polyurethane (PU) discs, cups and solid cast pipeline pigs. The technology creates a completely secure bond between the virgin and cured cast elastomer PU enabling rebuilt parts to perform “as new”. This paper addresses the mechanical characteristics of the bond possible with this technology and offers the results of a case study on a successful program to rebuild over fifty 24″ solid cast pigs for a crude oil pipeline. Cast elastomer PU is an expensive thermoset polymer which has not been readily recyclable. SUNcast’s technology offers pipeline operators, contractors and inspection companies an opportunity to recycle what is now waste back into high performance pigging products at significant cost savings.