Advances in LIDAR-based methods have enabled the detection and reconstruction of images of static objects hidden from the direct line-of-sight [1, 2]. One of the drawbacks to the technology used in these demonstrations is the requirement for long acquisition times. More recently, Gariepy et al. have shown that it is possible to detect and track a moving hidden object, albeit with no information of the object’s form . Applications of this include, but are not limited to, search and rescue, and hazard detection.
We present a real-time tracking system that enables the detection of moving objects that are outside the direct line-of-sight. Our active imaging system is a single-pixel variant of the technology reported by Gariepy et al. It replaces the single-photon avalanche diode (SPAD) camera of 1024 pixels with a number of SPAD detectors to detect light back-scattered from the hidden object. The flexibility of the single-pixel detectors provides an increased field of view, allowing us to detect and simultaneously track with better precision with respect to a SPAD array. The use of single-pixel detectors also has the advantage of a high detection efficiency.
We perform two proof-of-concept experiments using three pixels and a single pulsed laser to interrogate a “room” for a hidden object. In the first experiment, we demonstrate that we can accurately locate the position of a hidden object. In the second experiment, we use the same system and demonstrate that we can accurately track the motion of a hidden object in real time.
The “room” is a purpose-built box measuring 102×102×77 cm. Optical access is provided by a 28×12 cm window. The target object is a 15×15 cm textured viewing screen that we move along a designated ground track outside the line-of- sight of our system. In our experiments, we send a train of light pulses through the window to the back of the room. The pulses scatter off the wall as a spherical wavefront that propagates in all directions. Some of this light reaches our hidden object and is scattered back again towards the rear wall where we image our three SPAD pixels. The SPAD detectors are capable of picosecond temporal resolutions. Our time-correlated single-photon counting system measures the photon arrival times (64 ps resolution) for the signal returning to each detector. A histogram is built up in one second of acquisition time over 80 million pulses. We use this temporal information in our target position retrieval of the hidden object.
We place the object at 11 positions in turn in a seven minute experiment, and localise its position. We then perform real-time tracking and move the object around the hidden scene for approximately one minute, processing the target position retrieval every 1.5 s.
The recent development of 2D arrays of single-photon avalanche diodes (SPAD) has driven the development of applications based on the ability to capture light in motion. Such arrays are composed typically of 32x32 SPAD detectors, each having the ability to detect single photons and measure their time of arrival with a resolution of about 100 ps. Thanks to the single-photon sensitivity and the high temporal resolution of these detectors, it is now possible to image light as it is travelling on a centimetre scale. This opens the door for the direct observation and study of dynamics evolving over picoseconds and nanoseconds timescales such as laser propagation in air, laser-induced plasma and laser propagation in optical fibres. Another interesting application enabled by the ability to image light in motion is the detection of objects hidden from view, based on the recording of scattered waves originating from objects hidden by an obstacle. Similarly to LIDAR systems, the temporal information acquired at every pixel of a SPAD array, combined with the spatial information it provides, allows to pinpoint the position of an object located outside the line-of-sight of the detector. A non-line-of-sight tracking can be a valuable asset in many scenarios, including for search and rescue mission and safer autonomous driving.
The ability to detect motion and to track a moving object that is hidden around a corner or behind a wall
provides a crucial advantage when physically going around the obstacle is impossible or dangerous. One recently
demonstrated approach to achieving this goal makes use of non-line-of-sight picosecond pulse laser ranging.
This approach has recently become interesting due to the availability of single-photon avalanche diode (SPAD)
receivers with picosecond time resolution. We present a time-resolved non-sequential ray-tracing model and its
application to indirect line-of-sight detection of moving targets. The model makes use of the Zemax optical
design programme's capabilities in stray light analysis where it traces large numbers of rays through multiple
random scattering events in a 3D non-sequential environment. Our model then reconstructs the generated
multi-segment ray paths and adds temporal analysis. Validation of this model against experimental results is
shown. We then exercise the model to explore the limits placed on system design by available laser sources and
detectors. In particular we detail the requirements on the laser's pulse energy, duration and repetition rate, and
on the receiver's temporal response and sensitivity. These are discussed in terms of the resulting implications
for achievable range, resolution and measurement time while retaining eye-safety with this technique. Finally,
the model is used to examine potential extensions to the experimental system that may allow for increased
localisation of the position of the detected moving object, such as the inclusion of multiple detectors and/or
There has been a considerable effort recently in the development of planar chiral metamaterials. Owing to the lack of inversion symmetry, these materials have been shown to display interesting physical properties such as negative index of refraction and giant optical activity. However, the biosensing capabilities of these chiral metamaterials have not been fully explored. Ultrasensitive detection and structural characterization of proteins adsorbed on chiral plasmonic substrates was demonstrated recently using UV-visible circular dichroism (CD) spectroscopy. Second harmonic generation microscopy is an extremely sensitive nonlinear optical probe to investigate the chirality of biomaterials. In this study, we characterize the chiral response of chiral plasmonic metamaterials using second harmonic generation microscopy and CD spectroscopy. These planar chiral metamaterials, fabricated by electron-beam lithography, consist of right-handed and left-handed gold gammadions of length 400 nm and thickness 100nm, deposited on a glass substrate and arranged in a square lattice with a periodicity of 800nm.
Using an electron multiplying CCD camera we observe both image plane (position) and far field (momentum) correlations between photon pairs produced from spontaneous parametric down-conversion when using a 201 x 201 bi-dimensional array of pixels and a flux of around 0.02 photons/pixel. After background subtraction we characterize the strength of signal and idler correlations in both transverse dimensions by applying entanglement and EPR criteria, showing good agreement with the theoretical predictions. The application of such devices in quantum optics could have a wide range, including quantum computation with spatial degrees of freedom of single photons.
The information carried by a photon can be encoded in one or more of many different degrees of freedom. Beyond the two-dimensional space of polarisation (spin angular momentum) our interest lies in the unbounded yet discrete state space of Orbital Angular Momentum (OAM). We examine how photon pairs can be generated and measured over a large range of OAM states.
We have developed a new approach to measuring the spatial position of a single photon. Using fibers of different
length, all connected to a single detector allows us to use the high timing precision of single photon avalanche diodes
(SPAD) to spatially locate the photon. We have built two 8-element detector arrays to measure the full-field quantum
correlations in position, momentum and intermediate bases for photon pairs produced in parametric down conversion.
The strength of the position-momentum correlations is found to be an order of magnitude below the classical limit.
We report a violation of the CHSH inequality for ghost-images. This is achieved by using two spatially separated
phase modulators within the context of a two-photon parametric down-conversion experiment. We obtain edge
enhanced images as a direct consequence of the quantum correlations in the orbital angular momentum (OAM)
of the down-converted photon pairs. For phase objects, with differently orientated edges, we show a violation of
the CHSH Bell-type inequality for an OAM subspace, thereby unambiguously revealing the quantum nature of
our ghost-imaging arrangement.
The angular profile and the orbital angular momentum of a light mode are related by Fourier transform. Any
modification of the angular distribution, e. g. via diffraction off a suitably programmed spatial light modulator,
influences the orbital angular momentum spectrum of the light. This holds true even at the single photon level.
We observe the influence of various angular masks on the orbital angular momentum spectrum, both in the near
and the far field, and describe the resulting patterns in terms of angular diffraction. If photons are entangled in
their orbital angular momentum, diffraction of one photon affects the orbital angular momentum spectrum of
its partner photon, and angular ghost diffraction can be measured in the coincidence counts. We highlight the
role of the angular Fourier relationship for these effects.
The Brownian dynamics of an optically trapped water droplet is investigated across the transition from over to
under-damped oscillations. The spectrum of position fluctuations evolves from a Lorentzian shape typical of overdamped
systems (beads in liquid solvents), to a damped harmonic oscillator spectrum showing a resonance peak.
In this later under-damped regime, we excite parametric resonance by periodically modulating the trapping
power at twice the resonant frequency. We also derive from Langevin dynamics an explicit numerical recipe
for the fast computation of the power spectra of a Brownian parametric oscillator. The obtained numerical
predictions are in excellent agreement with the experimental data.
In the 1970s, Jones demonstrated a photon drag by showing that the translation of a window caused a slight displacement
of a transmitted light beam. Similarly he showed that a spinning medium slightly rotated the polarization state. Rather
than translating the medium, the speed of which is limited by mechanical considerations, we translate the image and
measure its lateral delay with respect to a similar image that has not passed through the window. The equivalence, or
lack of it, of the two frames is subtle and great care needs to be taken in determining whether or not similar results are to
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
Central to the success of microfluidic systems has been the development of innovative methods for the
manipulation of fluids within microchannels. We demonstrate a method for generating flow within a
microfluidic channel using an optically driven pump. The pump consists of two counter rotating birefringent
vaterite particles trapped within a microfluidic channel and driven using optical tweezers. The transfer of spin
angular momentum from a circularly polarised laser beam rotates the particles at up to 10 Hz. We show the
that the pump is able to displace fluid in microchannels, with flow rates of up to 200 μm3 s-1 (200 fL s-1). The direction of fluid pumping can be reversed by altering the sense of the rotation of the vaterite beads. We also
incorporate a novel optical sensing method, based upon an additional probe particle, trapped within separate
optical tweezers, enabling us to map the magnitude and direction of fluid flow within the channel. The
techniques described in the paper have potential to be extended to drive an integrated lab-on-chip device,
where pumping, flow measurement and optical sensing could all be achieved by structuring a single laser
We demonstrate a technique for the multi-point measurement of fluid flow in microscopic geometries. The
technique consists of an array of microprobes can be simultaneously trapped and used to map out the fluid flow
in a microfluidic device. The optical traps are alternately turned on and off such that the probe particles are
displaced by the flow of the surrounding fluid and then re-trapped. The particles' displacements are monitored by
digital video microscopy and directly converted into velocity field values. The techniques described have potential
to be extended to drive an integrated lab-on-chip device, where pumping, flow measurement and optical sensing
could all be achieved by structuring a single laser beam.
We use holographic optical tweezers to create and monitor the liquid flow within a micro-fluidic device. Using the tweezers to both trap and spin micron-sized beads within a 10-20 micron wide channel creates a fluid flow of the order of 200 cubic microns/sec. We also use the optical tweezers to measure the fluid flow by trapping and releasing probe particles that are imaged with high temporal and spatial resolution. Using the multi-trap capability of the holographic optical tweezers we measure the transverse fluid velocity at many positions simultaneously with an accuracy of better than 1 micron/sec. Such studies are highly pertinent to lab-on-chip systems for various applications and studies within the biosciences.
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.
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.