We review recent experimental and theoretical results of photon interferometry on rotating platforms. Quantum phenomena such as two-photon interference and entanglement can be controlled with mechanical rotations in a regime accessible to table-top experiments. We first discuss experiments demonstrating how low-frequency mechanical rotations affect the bunching behavior of frequency-entangled photon pairs. It was shown that low-frequency mechanical rotations can affect the temporal distinguishability of photons and can transform photonic behavior from perfectly indistinguishable (bosonic behavior) to perfectly distinguishable (fermionic behavior). We then give a future outlook for testing the generation of entanglement from mechanical rotation. A recent theoretical work showed that generating path-polarization entanglement from mechanical rotations could be verified with present technology. These works make a strong case for further exploration of quantum phenomena at the interface with non-inertial (rotational) motion.
Gas leaks pose a prevalent issue in industry and can have pressing impacts on individual safety and the environment. There is demand for new technologies that can ease, and reduce the cost of, detection of the source of leaks, both on a large and small scale. We present a device capable of visualizing the gas involved in the leaks allowing for an accessible tool in source location. Our current device can image methane leaks from ranges of up to 10m. By imaging a scene illuminated using a laser diode tuned to an absorption band of methane, followed by imaging at a similar but non-absorbed wavelength one can build a differential image of the scene and identify the presence of methane. This differential signal is then processed and assigned a false colour, in order to be overlaid upon an accompanying visible live feed. This system is adaptable and could be used to detect other gas species with modification to light source and detector. Future candidate gases would be based upon industry interest with acetylene, a common and flammable welding gas, being an example. The system is also robust enough to be drone mounted, we present data from conducted test flights. These flights demonstrate new ways in which the system can be used, such as in monitoring of difficult to access pipe geometries and for preset flight paths along expansive pipelines. This can allow for a more automated gas detection process, that is straightforward to review.
Methane gas is widespread in both industry and in domestic settings, but leaks into the environment pose a great risk both in terms of individual safety and climate. We previously presented a low cost methane imaging system that utilises backscattered illumination techniques to directly image methane gas in real time. This device was capable of imaging leaks with flow rates on the order of 0.05 L/min at a 3m range. The system illuminates a scene using tunable laser diodes centred at the methane absorption near 1653nm, with the backscattered light collected and imaged by a standard short-wave infrared focal plane array. Information obtained with the infrared images can be used in combination with a live visible feed to highlight exactly in the scene the source of the methane. We present a modified system for use on a UAV. Firstly the system, which utilises differential imaging, had to be altered in order to increase the image stabilisation to compensate for the increased amount of movement. Secondly, the impact of sunlight on the system was explored with imaging conducted both during the day and at night. We demonstrate the use of this system mounted to a UAV, imaging at an altitude of 3m and variety of horizontal range. It is demonstrated that performance is variable depending on surface-type, and sunlight level. It is envisaged that such a device would improve the ease of use for both routine facility or city pipeline inspections.
Biophotonics is becoming increasingly prominent in value and visibility. What was predominantly an advanced field of research is now being included increasingly in undergraduate and postgraduate programmes. To increase biophotonics-related STEM awareness, we developed a simple demonstration kit using LEDs and a camera that allows visualising of the blood vessels beneath the skin (specifically the blood vessels in the fingers). The kit uses inexpensive blue, green, red and near-infrared LEDs to show the absorption of shorter wavelengths and transmission of longer wavelengths in the skin. As an outreach and educational tool, this demonstration kit will illustrate the potential of using light for diagnostics.
Optical tweezers have propelled the advancement of micro-manipulation. Yet not all materials can be optically
tweezed, and high laser intensities can be harmful to living organisms. We propose a method for using optical tweezers to
indirectly control particles which are freely diffusing in water. By optically trapping and controlling specially designed
actuators, the surrounding fluid can be locally manipulated in a predictable manner. This, in turn, offers materialindependent
hydrodynamic control over nearby free objects. We experimentally demonstrate control over translational
and rotational motion of individual objects, and multiple particles simultaneously.
Alternatives to focal-plane detector arrays have emerged in recent years and are now being widely investigated. One of the most promising of these are single-pixel imaging techniques. Single-pixel techniques recreate a scene using the knowledge of projected patterns and the measured backscattered signals. This research investigates the implications of using idealised patterns, Fourier-transformed patterns and camera-captured patterns generated by plane wave decomposition methods. Hadamard patterns are projected into the far-field of the phased-array modulator source and used for robust reconstruction in a reflective arrangement. The choice of the optimal pattern sets from these sources are used for single-pixel imaging reconstruction and compressed sensing. The technique is robust to poor signal-to-noise conditions and is applicable in cases where a limited number of measurements are possible. Our technique and methodology can be further applied to any region of the electromagnetic spectrum where phased-array sources are available, such as in the radar regime.
Open-source technologies and solutions have paved the way for making science accessible the world over. Motivated to
contribute to the direction of open-source methods, our current research presents a complete workflow of building a microscope
using 3D printing and easily accessible optical components to collect images of biological samples. Further, these
images are classified using machine learning algorithms to illustrate both the effectiveness of this method and show the
disadvantages of classifying images that are visually similar. The second outcome of this research is an openly accessible
dataset of the images collected, OPEN-BIOset, and made available to the machine learning community for future research.
The research adopts the OpenFlexure Delta Stage microscope (https://openflexure.org/) that allows motorised control
and maximum stability of the samples when imaging. A Raspberry Pi camera is used for imaging the samples in a
transmission-based illumination setup. The imaging data collected is catalogued and organised for classification using
TensorFlow. Using visual interpretation, we have created subsets from amongst the samples to experiment for the best
classification results. We found that by removing similar samples, the categorical accuracy achieved was 99.9% and 99.59%
for the training and testing sets. Our research shows evidence of the efficacy of open source tools and methods. Future
approaches will use improved resolution images for classification and other modalities of microscopy will be realised based
on the OpenFlexure microscope.
Optical activity is a macroscopic property of chiral molecules which manifests as a rotation of the plane of linear polarization when light passes through a sample. We have developed a compact Bell inequality experiment for quantum probing of chiral liquids, based on polarization measurements. In particular, we show that we can use a Bell-type inequality configuration to measure the optical activity of D-Limonene, a chiral molecule which is a major component in the oil of citrus fruit peels.
Optical tweezers have contributed substantially to the advancement of micro-manipulation. However, they do have restrictions, mainly the limited range of materials that yield to optical trapping. Here we propose a method of employing optically trapped objects to manipulate the surrounding fluid and thus particles freely diffusing within it. We create and investigate a reconfigurable active-feedback system of optically trapped actuators, capable of manipulating translational and rotational motion of one or more nearby free objects.
Optical tweezers have played a significant role in the advancement of micro-manipulation. However, optically trappable objects are limited to a certain size and material range. To overcome these constraints, we propose a noncontact micro-manipulation technique, which uses optically trapped particles to locally manipulate the surrounding fluid and thus freely diffusing particles within it. We show that our method can be used to successfully suppress both translational and rotational Brownian motion of a free-floating object, using hydrodynamic interactions alone, in an easily reconfigurable setup.
Single-pixel cameras are both elegant and intellectually appealing, but can they outperform a camera based upon a focal-plane detector array? Single-pixel cameras use a digital mirror device to apply a time varying mask to an image of the scene, and a single-pixel detector to measure the light transmission through each of these masks. Given knowledge of the mask patterns and the measured transmissions, an image of the object is calculated.
Time-of-flight 3D imaging is an important tool for applications such as remote sensing, machine vision and autonomous navigation. Conventional time-of-flight three-dimensional imaging systems that utilize a raster scanned laser to measure the range of each pixel in the scene sequentially, inherently have acquisition times that scale directly with the resolution. Here we show a modified time-of-flight 3D camera employing structured illumination, which uses a visible camera to enable a novel compressed sensing technique, minimising the acquisition time as well as providing a high-resolution reflectivity map for image overlay. Furthermore, a quantitative assessment of the 3D imaging performance is provided.
Lorenz-Mie scattering theory allows to predict the field scattered by spherical objects illuminated by coherent light. By fitting the fringe pattern resulting from the interference of incident and scattered light, it is possible to track and size colloidal particles with a few nanometer precision.
Using digital holographic microscopy (DHM) we extend the applications of Lorenz-Mie theory to hollow spherical structures and to extremely high pressure conditions.
On the one hand, we geometrically and optically characterize complex colloids as polymer-shelled microbubbles, with high precision, low costs and short acquisition time. These microbubbles are likely to be unique tools for targeted drug delivery and are currently used as contrast agents for sonography. We measured size, shell thickness and refractive index for hundreds of polymeric microbubbles showing that shell thickness displays a large variation that is strongly correlated with its refractive index and thus with its composition.
On the other hand we demonstrate that DHM can be used for accurate 3D tracking and sizing of a holographically trapped colloidal probe in a diamond anvil cell (DAC). Polystyrene beads were trapped in water up to Gigapascal pressures while simultaneously recording in-line holograms at 1 KHz frame rate. This technique may potentially provide a new method for spatially resolved pressure measurements inside a DAC.
The mechanism by which light is slowed through ruby has been the subject of great debate. To distinguish between the two main proposed mechanisms, we investigate the problem in the time domain by modulating a laser beam with a chopper to create a clean square wave. By exploring the trailing edge of the pulsed laser beam propagating through ruby, we can determine whether energy is delayed beyond the input pulse. The effects of a time-varying absorber alone cannot delay energy into the trailing edge of the pulse, as a time-varying absorber can only attenuate a coherent pulse. Therefore, our observation of an increase in intensity at the trailing edge of the pulse provides evidence for a complicated model of slow light in ruby that requires more than just pulse reshaping. In addition, investigating the Fourier components of the modulated square wave shows that harmonic components with different frequencies are delayed by different amounts, regardless of the intensity of the component itself. Understanding the difference in delays of the individual Fourier components of the modulated beam reveals the cause of the distortion the pulse undergoes as it propagates through the ruby.
The linear Doppler shift forms the basis of various sensor types for the measurement of linear velocity, ranging from speeding cars to fluid flow. Recently, a rotational analogue was demonstrated, enabling the measurement of angular velocity using light carrying orbital angular momentum (OAM). If measurement of the light scattered from a spinning object is restricted to a defined OAM state, then a frequency shift is observed that scales with the rotation rate of the object and the OAM of the scattered photon. In this work we measure the rotational Doppler shift from micron-sized calcite particles spinning in an optical trap at tens of Hz. In this case the signal is complicated by the geometry of the rotating particle, and the effect of Brownian motion. By careful consideration of these influences, we show how the signal is robust to both, representing a new technique with which to probe the rotational motion of micro-scale particles.
Stereo-microscopy is a technique that enables a sample to be imaged from two directions simultaneously, allowing the tracking of microscopic objects in three dimensions. This is achieved by illuminating the sample from different directions, each illumination direction producing an individual image. These images are superimposed in the image plane but can be easily separated using a diffractive optical element in the Fourier plane of the imaging arm. Therefore this enables 3-dimensional coordinates to be reconstructed using simple 2-dimensional image tracking and parallax. This is a powerful technique when combined with holographic optical tweezers (HOT), where multiple objects can be trapped and tracked simultaneously in three dimensions. In this work, we extend this concept to four different illumination directions: quad stereo-microscopy. This allows us to measure the accuracy of tracking in three dimensions, and to optimise the system.
Diamond anvil cells can be used to study the behavior of materials at high pressure by compressing small samples
up to hundreds of GigaPascals. There is no mechanical access to the sample once the cell is pressurized but it
is possible to observe the sample through the diamond windows. Optical tweezers can be used to measure the
mechanical properties of fluids, such as viscosity, by trapping and monitoring micron sized spheres suspended in
the fluid. We use a diamond anvil cell within a modified optical tweezers instrument to measure the viscosity
of water as a function of pressure up to 1:3GPa. Development of this technique will allow investigations of the
mechanical changes in biological cells and other soft materials placed under high pressure.
We present a new type of stereo microscopy which can be used for tracking in 3D over an extended depth. The use of Spatial Light Modulators (SLMs) in the Fourier plane of a microscope sample is a common technique in Holographic Optical Tweezers (HOT). This set up is readily transferable from a tweezer system to an imaging system, where the tweezing laser is replaced with a camera. Just as a HOT system can diffract many traps of different types, in the imaging system many different imaging types can be diffracted with the SLM. The type of imaging we have developed is stereo imaging combined with lens correction. This approach has similarities with human vision where each eye has a lens, and it also extends the depth over which we can accurately track particles.
A high-intensity laser pulse can lead to a change of the group index of a material, so that the pulse within that
material is slowed to only hundreds of meters per second. This kind of slow-light phenomenon scales with the
optical intensity of the pulse. While previous experiments have produced this effect with an elliptical beam
passing through a spinning ruby window, a question remains as to whether the effect would be present in a
circular beam. Here we use two different methods of producing slow light in a round beam, showing that, while
less pronounced than the effect with an elliptical beam, a slow-light effect can be seen in a round beam.
Diamond anvil cells allow us to study the behaviour of materials at pressures up to hundreds of gigaPascals in a small and convenient instrument, however physical access to the sample is impossible once it is pressurised. Optical tweezers use tightly focussed lasers to trap and hold microscopic objects, and their ability to measure nanometric displacements and femtonewton forces makes them ubiquitous across the nano and bio sciences. We show that optical tweezers can be used to hold and manipulate particles in such a cell, in the ``macro tweezers'' geometry allowing us to use objective lenses with a higher working distance. Traps are structured to overcome the limitations imposed by the sample cell. Wedemonstrate the effectiveness of the technique by measuring water's viscosity up to 1.2 GPa. The maximum pressure reached was limited by the water crystallising under pressure.
In this proceedings paper we show describe how a microtool can be assembled, and tracked in three dimensions
such that its full rotational and translational coordinates, q, are recovered. This allows tracking of the motion
of any arbitrary point, d, on the microtool's surface. When the micro-tool is held using multiple optical traps
the motion of such a point investigates the inside of an ellipsoidal volume - we term this a `thermal ellipsoid. We
demonstrate how the shape of this thermal ellipsoid may be controlled by varying the relative trapping power
of the optical traps, and adjusting the angle at which the micro-tool is held relative to the focal plane. Our
experimental results follow the trends derived by Simpson and Hanna.
A spinning medium is predicted to induce a slight rotation in a transmitted image.
We amplify this effect by use of ruby as a slow light medium, giving image rotations of
several degrees. In terms of the orbital angular momentum such rotations are analogous to the
mechanical Faraday effect.
We present a holographic tweezers workstation to optically trap and spectroscopically characterise metal nanoparticles.
The versatility of the holographic tweezers allows us to create multiple traps manipulating several metal
nanoparticles simultaneously. We developed an imaging system to view the nanoparticles in a transmission darkfield
configuration without compromising the high numerical aperture of our trapping and viewing objective. In
addition we implemented single particle spectroscopy to interrogate the trapped particles' properties with the
ability to directly monitor the plasmonics coupling between particles while changing the distance between them.
We also demonstrate a laser based method to generate fixed arrays of nanoparticles.
The interactions between T cells and antigen-presenting cells (APCs) are crucial in triggering a successful antigenspecific,
adaptive immune response leading to protection against a particular pathogen or disease. At present very little is
known about the magnitudes of the forces involved in these interactions. We present results showing for the first time
that optical tweezers can be used to measure these cell-cell interaction forces. We were able to see a significant
difference in the force distributions taken with and without antigen, as evidenced by a Mann-Whitney U-test. The T cells
of interest were trapped directly and no exogenous beads were added to the sample. Interaction forces between T cells
and APCs in the presence of specific antigen ranged from 0-6.5 pN, whereas, when the specific antigen was absent the
interaction forces ranged from 0-1.5 pN. The accuracy of the system will be discussed in terms of how we tracked the
position of the optically trapped cell and the methods we used to minimise cell roll.
Holographic optical tweezers are fast becoming an established tool in micro- and nano-science. We present
developments in both the interface and the underlying technology as used in our lab, along with some of the
uses we have put them to. Real time particle tracking from stereoscopic video images enables us to determine
position to nanometre precision in 3D. This can be performed in real time at several kHz, allowing us to positionclamp
trapped objects in 3D and measure positions and forces with optically controlled tools. Our multi-touch
iPad interface gives interactive control over multiple traps at the same time, which opens new possibilities for
controlling tools, structures and other dynamic processes.
A Multitouch screen is an obvious choice for a holographic optical tweezers interface, allowing multiple optical traps to be
controlled in real-time. In this paper we describe the user interface used for our original multitouch system and demonstrate
that, for the user tasks performed, the multitouch performs better than a simple point-and-click interface.
Holographic optical tweezers are used to assemble and control probes made from high aspect-ratio CdS and SiO2 nanorods
and SiO2 microspheres. Analysis of the probe position allows for the measurement of forces experienced by the tip in a manner
analogous to existing scanning probe microscopy (SPM) techniques.
We present a comprehensive laser tweezers software package, comprising the software components for any laser
tweezers system. This includes fast hologram generation software, implemented on a graphics card, thereby
allowing 100 hundred independently moving traps at video frame rate. The software also includes comprehensive
camera suite and image recognition software for multi-particle tracking and analysis. The software is freely
available from the authors and online(http://www.physics.gla.ac.uk/Optics/).
We demonstrate the use of holographic optical tweezers to form arrangements of silica beads for trapping and
measuring the mechanical properties of micron sized objects, such as oil droplets and yeast cells. This allows
us to investigate the mechanical properties of the constrained object, which need not be optically trapped itself
(thus preventing radiation damage and allowing objects with a low refractive index to be constrained). By
compressing the object with the beads we are able to determine the size of the trapped object and show that
there is an elastic coupling between the beads due to the presence of a trapped object. We expect more detailed
analysis of the system will allow mechanical and frequency-dependent viscoelastic properties of objects to be
investigated.
Optical Tweezers have become a widespread tool in Cell Biology, microengineering and other fields requiring
delicate micromanipulation. But for those sensitive tasks, it remains difficult to handle objects without damaging
them. As the precision in position and force measurement increase, the richness of information cannot
be fully exploited with simple interfaces such as a mouse or a common joystick. For this reason, we propose
a haptic force-feedback optical tweezer command and a force-feedback system controlled by one hand. The
system combines accurate force measurement using a fast camera and the coupling of these measured forces
with a human operator. The overall transparency allows even the feeling of the Brownian motion.
In suspensions containing microspheres and a sub-micron species, such as nanoparticles or a polymer, an attractive force
can result between the microspheres. This attraction arises due to an entropic interaction, often referred to as a depletion
force. In this work we demonstrate an application of the depletion force to the controlled assembly of crystalline templates
for the production of photonic band gap (PBG) materials. The method makes use of holographic optical tweezers to
assemble crystalline arrays of silica or polystyrene microspheres, in which depletion interactions are used to stabilise the
structures being built. In addition, we use the holographic optical tweezers to characterise the attraction between pairs of
microspheres in the system.
CdS nanowires and silica microspheres are manipulated using optical traps into 1D and 2D structures. The bonding occurs through the use of biotin and streptavidin.
Holographic or diffractive optical components, such as a spatial
light modulator (SLM), can be used in optical tweezers for the
creation of multiple and modified optical traps. In addition to
this, SLMs can also be used to correct for aberrations within the
optical train resulting in an improved trapping performance.
Typically an electrically addressed SLM may deviate from flatness
by up to 4λ, dominated by astigmatism due to the overall
curvature of the SLM surface. This astigmatism may be corrected by
adding the appropriate hologram to the SLM display resulting in a
dramatic improvement in the fidelity of the focussed spot. The
impact that this correction has on the performance of the optical
trap is most noticeable for small particles. For the SLM used in
this study, the improvement in trap performance for a 0.8 μm
diameter particles can be in excess of 25%. However, for 5 μm
diameter particles our results show an improvement of less than
0.5%. This dependence upon particle size is most probably
associated with the relative size of the PSF and the trapped
particle. Once the PSF is significantly smaller than the particle
diameter, further reduction brings little improvement in trap
performance.
Optical tweezers use the electric-field gradient-force associated with tightly focused laser beams to trap micron-sized
objects at the beam focus. Over the last few years optical tweezers have been revolutionized by the addition of spatial
light modulators to split the laser beam into many traps that can be individually controlled; a technique called
holographic optical tweezers. However, the reliance of optical tweezers on the gradient-force largely restricts their
application to transparent objects that are not unduly sensitive to the effects of the laser light. Consequently, the
manipulation of metallic particles or sensitive biomaterials can be problematic. In this work we use a holographic
tweezers to position multiple silica beads acting as an optical gripper to lift, rotate and move micron-sized objects that
otherwise do not lend themselves to tweezers control. We illustrate the use of the optical gripper under real-time joystick
control to manipulate micron-sized metallic particles with nano-scale precision.
We present a method for aberration correction in a confocal microscope that successfully combines both a spatial light modulator and a deformable membrane mirror. An active locking technique is used that benefits from the fast update rate of the deformable membrane mirror and the large effective stroke of the spatial light modulator. Concentrating on defocus, we were able to track 'best focus' over a distance of 80 μm with a lock RMS precision of 57 nm. In principle, this technique can be applied to any Zernike mode or aberration that can be accurately reproduced on the deformable membrane mirror.
We have developed an interactive user-interface that can be used to generate phase holograms for use with spatial light modulators. The program utilises different hologram design techniques allowing the user to select an appropriate algorithm. The program can be used to generate multiple beams, interference patterns and can be used for beam steering. We therefore see a major application of the program to be within optical tweezers to control the position, number and type of optical traps.
Laguerre-Gaussian (LG) light beams possess discrete values of orbital angular momentum (OAM) of l&barh; per photon, where l is the azimuthal index of the mode. In principle l can take on any integer number, resulting in an unlimited amount of information that can be carried by any part of the beam - even a single photon. We have developed a technology demonstrator that uses OAM to encode information onto a light beam for free-space optical communications. In our demonstrator units both the encoding and decoding of the orbital angular momentum states is achieved using diffractive optical components (holograms). We use 9 different OAM values; one value is used for alignment purposes, the others carry data.
We use holographic optical tweezers to trap multiple micron-sized objects and manipulate them in 3-dimensions. Trapping multiple objects allow us to create 3-dimensional structures, examples of which include; simple cubes which can be rotated or scaled, complex crystal structures like the diamond lattice or interactive 3-dimensional control of trapped particles anywhere in the sample volume.
We have developed a field portable optical gas sensor for the ultra sensitive detection of ethane. The system is based on an adaptation of a commercially available system, which uses a cryogenically cooled, lead-salt, laser diode at 3.34 microns and a multi-pass astigmatic Herriott sample cell. We have adapted this system to a second derivative wavelength modulation scheme giving a lower detection limit of less than 100 parts per trillion for a one second measurement time. Our custom designed software controls every aspect of the instrument operation from spectral scanning of the laser diode, to automatic calibration, optical alignment, spectral analysis and complete data logging.
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