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Speckle patterns are a powerful tool for high-precision metrology, as they allow remarkable performance in relatively simple setups. Nonetheless, researchers in this field follow rather distinct paths due to underappreciated general principles underlying speckle phenomena. Here, we advise on a universal metric of intrinsic speckle sensitivity, and on the advantages and disadvantages of multiple scattering. This will catalyse progress in speckle metrology but will also translate to other domains of disordered optics which are undergoing rapid developments at present., Comment: 15 pages, 4 figures

Reliable identification of high-value products such as whisky is an increasingly important area, as issues such as brand substitution (i.e. fraudulent products) and quality control are critical to the industry. We have examined a range of machine learning algorithms and interfaced them directly with a portable Raman spectroscopy device to both identify and characterize the ethanol/methanol concentrations of commercial whisky samples. We demonstrate that machine learning models can achieve over 99% accuracy in brand identification across twenty-eight commercial samples. To demonstrate the flexibility of this approach we utilised the same samples and algorithms to quantify ethanol concentrations, as well as measuring methanol levels in spiked whisky samples. Our machine learning techniques are then combined with a through-the-bottle method to perform spectral analysis and identification without requiring the sample to be decanted from the original container, showing the practical potential of this approach to the detection of counterfeit or adulterated spirits and other high value liquid samples., Comment: 26 pages, 4 figures, 2 table

Near-field optics can overcome the diffraction limit by creating strong optical gradients to enable the trapping of nanoparticles. However, it remains challenging to achieve efficient stable trapping without heating and thermal effects. Dielectric structures have been used to address this issue, but they usually offer weak trap stiffness. In this work, we exploit the Fano resonance effect in an all-dielectric quadrupole nanostructure to realize a twenty-fold enhancement of trap stiffness, compared to the off-resonance case. This enables a high effective trap stiffness of $1.19$ fN/nm for 100 nm diameter polystyrene nanoparticles with 3.5 mW/$\mu$m$^{2}$ illumination. Furthermore, we demonstrate the capability of the structure to simultaneously trap two particles at distinct locations within the nanostructure array., Comment: 18 pages, 5 figures

We demonstrate all-optical sympathetic cooling of a laser-trapped microsphere to sub-Kelvin temperatures, mediate by optical binding to a feedback-cooled adjacent particle. Our study opens prospects for multi-particle quantum entanglement and sensing in levitated optomechanics., Comment: 3 pages, 2 figures

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

The non-conservative, azimuthal forces associated with inhomogeneous optical-spin angular momentum play a critical role in optical trapping. Intriguingly, birefringent microspheres can be stably levitated and rapidly rotated in circularly polarized optical traps in ultra-high vacuum whereas isotropic spheres are typically destabilized and expelled, even at relatively modest pressures. Here we show that the resolution of this apparent key paradox rests in the form of the orientationally averaged, effective forces acting on the spinning birefringent particle. In particular, the effective azimuthal component is heavily suppressed and highly non-linear. As a consequence, non-conservative effects are strongly, if imperfectly, inhibited. Their influence is apparent only at very low pressures where we observe the formation of noisy, nano-scale limit cycles or orbits. Finally, we show how parametric feedback can synthesize a form of dissipation, necessary to preserve limit cycle oscillation, without introducing additional thermal fluctuations. This allows the preparation of highly coherent, self-sustained oscillations with effective temperatures on the order of a milliKelvin. The tailoring of azimuthal spin forces through the material structure of a spinning, non-spherical particle opens up new opportunities for the design of ultra stable optical rotors. In addition, we have shown that the unique profile of the azimuthal force, featured in this work, allows for the formation of nano-scale limit cycles that can be stabilized and cooled. In principle, this approach could enable the cooling of limit cycles into the quantum regime, allowing for experimental realisation of quantum synchronization, or alternative ways of entangling mesoscopic bodies., Comment: Main: 10 pages, 5 figures. Supplementary Document: 10 pages, 7 figures

The ability to identify the contents of a sealed container, without the need to extract a sample, is desirable in applications ranging from forensics to product quality control. One technique suited to this is inverse spatially offset Raman spectroscopy (ISORS) which illuminates a sample of interest with an annular beam of light and collects Raman scattering from the centre of the ring, thereby retrieving the chemical signature of the contents while suppressing signal from the container. Here we explore in detail the relative benefits of a recently developed variant of ISORS, called focus-matched ISORS. In this variant, the Fourier relationship between the annular beam and a tightly focused Bessel beam is exploited to focus the excitation light inside the sample and to match the focal point of excitation and collection optics to increase the signal from the contents without out compromising the suppression of the container signal. Using a flexible experimental setup which can realise both traditional and focus-matched ISORS, and Monte-Carlo simulations, we elucidate the relative advantages of the two techniques for a range of optical properties of sample and container., Comment: 11 pages, 4 figures, plus 9 supplementary figures

Significance: Monte Carlo radiation transfer (MCRT) is the gold standard of modeling light transport in turbid media. Typical MCRT models use voxels or meshes to approximate experimental geometry. A voxel based geometry does not allow for the accurate modeling of smooth curved surfaces, such as may be found in biological systems or food and drink packaging. Aim: We present our algorithm which we term signedMCRT (sMCRT), a new geometry-based method which uses signed distance functions (SDF) to represent the geometry of the model. SDFs are capable of modeling smooth curved surfaces accurately whilst also modeling complex geometries. Approach: We show that using SDFs to represent the problem's geometry is more accurate and can be faster than voxel based methods. sMCRT, can easily be incorporated into existing voxel based models. Results: sMCRT is validated against theoretical expressions, and other voxel based MCRT codes. We show that sMCRT can accurately model arbitrary complex geometries such as microvascular vessel network using SDFs. In comparison to the current state-of-the-art in MCRT methods specifically for curved surfaces, sMCRT is up-to forty-five times more accurate. Conclusions: sMCRT is a highly accurate, fast MCRT method that outperforms comparable voxel based models due to its ability to model smooth curved surfaces. sMCRT is up-to three times faster than a voxel model for equivalent scenarios. sMCRT is publicly available at https://github.com/lewisfish/signedMCRT

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., Comment: 9 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

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., Comment: 14 pages, 10 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

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., Comment: 7 pages, 7 figures

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., Comment: 5 pages, 6 figures

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 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

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

The accurate determination and control of the wavelength of light is fundamental to many fields of science. Speckle patterns resulting from the interference of multiple reflections in disordered media are well-known to scramble the information content of light by complex but linear processes. However, these patterns are, in fact, exceptionally rich in information about the illuminating source. We use a fibre-coupled integrating sphere to generate wavelength-dependent speckle patterns, in combination with algorithms based on the transmission matrix method and principal component analysis, to realize a broadband and sensitive wavemeter. We demonstrate sub-femtometre wavelength resolution at a centre wavelength of 780 nm and a broad calibrated measurement range from 488 to 1064 nm. This is comparable with or exceeding the performance of conventional wavemeters. Using this speckle wavemeter as part of a feedback loop, we stabilize a 780 nm diode laser to achieve a linewidth better than 1 MHz., Comment: 13 pages, 8 figures

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

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., Comment: 7 pages, 5 figures; Nature Physics, published online 13 July 2015

We demonstrate a method to create arbitrary intensity distributions of multiple wavelengths of light, which can be useful for ultracold atom experiments, by using regional phase-calculation algorithms to find a single hologram which is illuminated with overlapped laser beams. The regionality of the algorithms is used to program spatially distinct features in the calculated intensity distribution, which then overlap in the Fourier plane due to the dependence of diffraction angle on wavelength. This technique is easily integrated into cold atom experiments, requiring little optical access. We demonstrate the method and two possible experimental scenarios by generating light patterns with 670nm, 780nm and 1064nm laser light which are accurate to the level of a few percent., Comment: 9 pages, 5 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., Comment: 5 pages, 4 figures

Direct minimisation of a cost function can in principle provide a versatile and highly controllable route to computational hologram generation. However, to date iterative Fourier transform algorithms have been predominantly used. Here we show that the careful design of cost functions, combined with numerically efficient conjugate gradient minimisation, establishes a practical method for the generation of holograms for a wide range of target light distributions. This results in a guided optimisation process, with a crucial advantage illustrated by the ability to circumvent optical vortex formation during hologram calculation. We demonstrate the implementation of the conjugate gradient method for both discrete and continuous intensity distributions and discuss its applicability to optical trapping of ultracold atoms., Comment: 11 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., Comment: 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 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., Comment: 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., Comment: 8 pages, 3 figures