1,977 research outputs found
A conjugate gradient minimisation approach to generating holographic traps for ultracold atoms
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 figure
Multi-wavelength holography with a single spatial light modulator for ultracold atom experiments
The authors acknowledge funding from the Leverhulme Trust Research Project Grant RPG-2013-074 and from the EPSRC grant GR/T08272/01.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.PostprintPeer reviewe
Phase changes in neuronal postsynaptic spiking due to short term plasticity
In the brain, the postsynaptic response of a neuron to time-varying inputs is determined by the interaction of presynaptic spike times with the short-term dynamics of each synapse. For a neuron driven by stochastic synapses, synaptic depression results in a quite different postsynaptic response to a large population input depending on how correlated in time the spikes across individual synapses are. Here we show using both simulations and mathematical analysis that not only the rate but the phase of the postsynaptic response to a rhythmic population input varies as a function of synaptic dynamics and synaptic configuration. Resultant phase leads may compensate for transmission delays and be predictive of rhythmic changes. This could be particularly important for sensory processing and motor rhythm generation in the nervous system. © 2017 McDonnell, Graham
Configurational factors in the perception of unfamiliar faces
Young et al (1987) have demonstrated that the juxtaposition of top and bottom halves of different faces produces a powerful impression of a novel face. It is difficult to isolate perceptually either half of the 'new' face. Inversion of the stimulus, however, makes this task easier. Upright chimeric faces appear to evoke strong and automatic configurational processing mechanisms which interfere with selective piecemeal processing. In this paper three experiments are described in which a matching paradigm was used to show that Young et al's findings apply to unfamiliar as well as to familiar faces. The results highlight the way in which minor procedural differences may alter the way in which subjects perform face-recognition tasks
Feedback-enhanced algorithm for aberration correction of holographic atom traps
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 figure
Wavelength sensitivity of the speckle patterns produced by an integrating sphere
Funding: Leverhulme Trust (RPG-2017-197) and the UK Engineering and Physical Sciences Research Council (EP/P030017/1).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 four orders of magnitude above that of multimode fibres of a similar size, and discuss analogies with the transmission profile of a Fabry–Pérot interferometer.Publisher PDFPeer reviewe
Experimental Design for the Gemini Planet Imager
The Gemini Planet Imager (GPI) is a high performance adaptive optics system
being designed and built for the Gemini Observatory. GPI is optimized for high
contrast imaging, combining precise and accurate wavefront control, diffraction
suppression, and a speckle-suppressing science camera with integral field and
polarimetry capabilities. The primary science goal for GPI is the direct
detection and characterization of young, Jovian-mass exoplanets. For plausible
assumptions about the distribution of gas giant properties at large semi-major
axes, GPI will be capable of detecting more than 10% of gas giants more massive
than 0.5 M_J around stars younger than 100 Myr and nearer than 75 parsecs. For
systems younger than 1 Gyr, gas giants more massive than 8 M_J and with
semi-major axes greater than 15 AU are detected with completeness greater than
50%. A survey targeting young stars in the solar neighborhood will help
determine the formation mechanism of gas giant planets by studying them at ages
where planet brightness depends upon formation mechanism. Such a survey will
also be sensitive to planets at semi-major axes comparable to the gas giants in
our own solar system. In the simple, and idealized, situation in which planets
formed by either the "hot-start" model of Burrows et al. (2003) or the core
accretion model of Marley et al. (2007), a few tens of detected planets are
sufficient to distinguish how planets form.Comment: 15 pages, 9 figures, revised after referee's comments and resubmitted
to PAS
Asymmetric longitudinal optical binding force between two identical dielectric particles with electric and magnetic dipolar responses
Funding: This work was supported by Jiaxing Science and Technology Project Grants (No.2021AY10057), KD acknowledges support of the UK Engineering and Physical Sciences Research Council (grant EP/P030017/1).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 electric and magnetic 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.PostprintPeer reviewe
To focus-match or not to focus-match inverse spatially offset Raman spectroscopy : a question of light penetration
Funding: The work was supported by funding from the UK Engineering and Physical Sciences Research Council (EP/P030017/1 and EP/R004854/1) and the H2020 FETOPEN project “Dynamic” (EC-GA 863203).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.Publisher PDFPeer reviewe
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