Stroboscopic Technique Research Articles

Studies of Fluorescence Lifetimes of Biological Warfare Agents Simulants and Interferers Using the Stroboscopic Method

The fluorescence decays (FDs) of 27 dried vegetative bacteria, bacterial endospores, fungi, and pollens were measured and determined using a stroboscopic technique. Pulsed nanosecond LED sources, emitting light at wavelengths of 280, 340, and 460 nm, were used for the excitation of biological samples. The implicit advantages of the stroboscopic method are high sensitivity, speed of a single measurement (10–60 s), miniaturization of the device, and relatively low price compared to the typical lifetime methods. The Principal Component Analysis (PCA) method was used for chemometric analysis. It was found that the excitation at 340, 460, and data merged from 340 and 460 nm effectively separate individual groups of biological substances. These findings provide evidence that fluorescence decay data may allow the classification of the biological samples, and the FDs measurement method can be complementary to the study of fluorescence spectra.

In situ measurements of thermal-mechanical wear in blade-abradable liner contacts

The abradable coating on the casing of a jet engine minimises any air gaps by allowing blades to cut a path into the abradable, reducing efficiency losses. Unfortunately, abradable cutting performance varies significantly with rub conditions and abradable type, leading to poor cutting performance and potentially damaging the blades. This study further develops the abradable testing capabilities of a 200 m/s spindle test rig by applying previously researched stroboscopic imagining techniques to record the front of the blade alongside thermal imaging of the blade and the abradable. The front camera makes it possible to see how adhesions are forming along the width of the blade, and how the adhesion growth differs between materials. The thermal camera can then be used to identify hot spots on both the blade and the abradable, providing an insight into how hot spots relate to adhesions both spatially and temporally. These tools have been proven with testing of Metco 601 at a range of incursion rates. At the incursion rate of 0.2 μm/pass the tools were able to provide conclusive results, with video footage of the front, side, blade temperature and abradable temperatures being aligned temporally and spatially. Numerical data could then be extracted to produce rub maps of adhesion height, blade temperature and abradable temperature on separate plots which is a powerful tool when looking for relationships between the data sets, and for identifying time-based patterns. Further benefits of this tool set were shown when focussing on individual adhesions events within a rub, allowing for the delays between abradable heating, adhesions forming and blade heating to be observed and quantified. When applied to multiple materials and at different test conditions this tool will provide further insight into how adhesion formation differs, and potentially into why adhesions form.

Imaging and reconstruction of positive streamer discharge tree structures

Streamer discharges often exhibit branching, which can greatly affect their behavior and will lead to so-called streamer trees. In this work we present a methodology for investigating the structure of a streamer discharge tree by means of advanced imaging techniques. Stereoscopic and stroboscopic techniques augment the images with depth perception and temporal information relevant to study the inherently stochastic three-dimensional and transient streamers. A semi-automated post processing algorithm is developed to make a reconstruction of the streamer discharge tree formation. This results in a tree of streamer segments, separated by branching events, where velocities, diameters and trajectories are used to characterize the morphology. The workings of the algorithm is detailed using an exemplar measurement series of positive streamers in synthetic air at 233 mbar.

Two-Dimensional Programmable Tweezer Arrays of Fermions.

We prepare high-filling two-component arrays of tens of fermionic ^{6}Li atoms in optical tweezers, with the atoms in the ground motional state of each tweezer. Using a stroboscopic technique, we configure the arrays in various two-dimensional geometries with negligible Floquet heating. A full spin- and density-resolved readout of individual sites allows us to postselect near-zero entropy initial states for fermionic quantum simulation. We prepare a correlated state in a two-by-two tunnel-coupled Hubbard plaquette, demonstrating all the building blocks for realizing a programmable fermionic quantum simulator.

Real-time visualisation and optimisation of acoustic waves carrying orbital angular momentum

Travelling waves, such as light and sound, can carry angular momentum. Orbital angular momentum (OAM) is one of the components which is determined by the helicity of the phase fronts. The helical waveform is characterised in terms of an integer l and an azimuthal phase term of exp(−ilθ), but for |l| > 1 the resulting high-order beam structure is unstable to perturbation. In this work, using Fourier transform profilometry and stroboscopic imaging techniques, we demonstrate the real-time visualisation of the OAM-carrying acoustic waveform by imaging the pressure imprint of the acoustic wave on a thin rubber sheet. Furthermore, based on the visualised waveform, we are able to optimise high-order (|l| > 1) OAM states by controlling the individual elements of the acoustic source. Beyond the study of acoustic OAM, the real-time monitoring and optimising methods could be a benefit to other applications requiring acoustic waveform shaping, such as acoustic communications, acoustic holograms, etc.

Measurement of mechanical properties of liquid by observing droplet oscillation on substrates

In this paper, we introduce a method to measure the surface tension of a droplet on a solid substrate by observing the resonance oscillation excited by applying Maxwell stress using electric field tweezers in a noncontact manner. Additionally, we measured the frequency spectrum of the oscillation amplitude using a stroboscopic imaging technique. The resonance frequency of the droplet was inversely proportional to the 3/2th power of the droplet radius, with a contact angle of approximately π/2 rad. The acquired result is in good agreement with the theory derived by extending the established formula for a free-sphere droplet. Furthermore, the contact angle dependence of the resonance frequency can be qualitatively explained based on the behavior of waves on a confined liquid surface.

Investigation of wear mechanisms in AlSi-polyester abradable - Ti(6Al4V) blade contacts using stroboscopic imaging

Abradable linings in aero engines have been an area of research interest over the past few decades as small reductions in clearances between stationary and rotating parts can lead to large increases in engine efficiency. The work performed in this article focuses on characterising the blade wear behaviour in contacts between Ti (6Al 4V) blades and AlSi-polyester abradables. This was done by performing three abrasion tests on the new test rig developed at the University of Sheffield. Tests have been performed on the AlSi-polyester abradables of the same nominal hardness over two incursion rates – 0.02μm/pass and 0.2 μm/pass and two blade tip speeds – 85 m/s and 170 m/s. The front-on stroboscopic imaging technique was used for these tests, which allowed capturing images of the entire blade front for a number of blade strikes during a test. It was found that at the incursion rate of 0.02μm/pass, both adhesions to the blade surface and blade wear were observed across the blade width. It was observed that adhesions were more likely to gradually wear off rather than fracture at 0.02μm/pass, and, fracture at 0.2 μm/pass. Tested surface profiles were obtained using an Alicona non-contact measurement system. This allowed the comparison of the blade profile results from the blade images to the surface of the respective tested abradable sample. It was concluded that adhesions that fractured could contribute to the localized gaps between the final blade and the final abradable surface where such adhesions have fractured close to the end of a test. Further testing areas have been identified such as the investigation into the effects of parameters such as incursion rate of a blade into an abradable, blade tip speed and abradable hardness on the results. The developed front-on imaging system also opened a possibility to investigate in-situ the rub performance of blades of varying tip geometry.

Reconstruction of Periodic Signals From Asynchronous Trains of Samples

A short train of equidistant samples rarely suffices to compute a reasonable approximation to a continuous-time signal. Hence, there arises a need for signal reconstruction methods that exploit multiple trains of samples. If the starting times of the trains are known, then stroboscopic techniques can be used for signal reconstruction. If these times are not given, i.e., in the case of asynchronous trains of samples, it is still possible to profit from multiple trains provided that the starting times are uniformly random when considered modulo the period of the signal. For such a scenario, it is known that an infinite set of sample trains allows for reconstruction of the signal up to a time shift provided that the sampling period is small enough and that the derivative of the signal satisfies some regularity conditions. In this paper, we generalize that discovery by showing that the regularity conditions concerning the derivative of the signal can be dropped. The reconstruction of the signal is still possible even if the derivative of the signal does not exist. We also show how to estimate the signal from a finite set of asynchronous trains of samples. Eventually, we prove that the proposed signal estimator is consistent.

Precise control of magnetic fields and optical polarization in a time-orbiting potential trap

A time orbiting potential trap confines neutral atoms in a rotating magnetic field. The rotation of the field can be useful for precision measurements, since it can average out some systematic effects. However, the field is more difficult to characterize than a static field, and it makes light applied to the atoms have a time-varying optical polarization relative to the quantization axis. These problems can be overcome using stroboscopic techniques, where either a radio-frequency field or a laser is applied in pulses that are synchronized to the rotating field. Using these methods, the magnetic field can be characterized with a precision of 10 mG and light can be applied with a polarization error of $5\times 10^{-5}$.

Measurement of strain evolution in overloaded roller bearings using time-of-flight neutron diffraction

Neutron diffraction is an established method for non-destructively characterising residual stress or observing in situ strain during external stimuli. Neutron based stroboscopic techniques have previously been introduced for measuring strains undergoing cyclic processes but have not been used for tribological applications. This work presents a novel approach for measuring the evolution of radial strain in a rotating bearing through part of the component's lifetime. A cylindrical roller bearing was pre-overloaded to increase the probability of damage within a reasonable experimental time and to help develop further understanding of the influence such events have on bearing life, notably for the application of wind turbine gearbox bearing failure. The stroboscopic neutron diffraction technique was successful in measuring time-resolved contact strain, with a significant increase in compressive radial strain being observed after a suspected failure had been detected using condition monitoring techniques, implemented for validating damage propagation. Cyclic contact strains associated with rolling contact fatigue were also evaluated using neutron diffraction.

An instantaneous polarization phase-shifting interferometric system with stroboscopic illumination by using a photoelastic modulator

We present a phase-shifting interferometric system that uses the photoelastic modulator (PEM) with the stroboscopic illumination technique. The laser diode is controlled by a programmable pulse generator for triggering four short pulses at their specific temporal phase angles of a modulation cycle for phase-shifting operation. Thereby, the intensity variation of the PEM modulated signal can be freezed and the interferograms captured by a charge-coupled device. In this configuration, no mechanical operations are required in phase-shifting by using the stroboscopic illumination technique, and fast interference fringe measurements are possible. We performed experiments to demonstrate the phase map of two static samples and confirmed that the method is applicable to imaging the complex phase distributions of dynamic objects at speed of 2 Hz. The thickness deviation is almost less than 5% for an ultra-thin transparent film.

Unidirectional motion of magnetic domain walls: the experiment and numerical simulation

The results of study of unidirectional motion of topologically different domain structures under the influence of periodic bipolar and unipolar magnetic field pulses applied perpendicular to the sample plane of (111) iron garnet single crystal plate are presented. The response of the domain structure to the field pulses was studied by direct observations utilizing the stroboscopic technique. Experimentally obtained dependences of the speed of unidirectional motion of stripe domains on the parameters of external bipolar pulsed magnetic field are compared with the results of numerical simulations.

Selective hybrid spin interactions with low radiation power

We present a protocol for designing appropriately extended $\pi$ pulses that achieves tunable, thus selective, electron-nuclear spin interactions with low-driving radiation power. Our method is general since it can be applied to different quantum sensor devices such as nitrogen vacancy centers or silicon vacancy centers. Furthermore, it can be directly incorporated in commonly used stroboscopic dynamical decoupling techniques to achieve enhanced nuclear selectivity and control, which demonstrates its flexibility.

Combined frequency and time domain measurements on injection-locked, constriction-based spin Hall nano-oscillators

We demonstrate a combined frequency and time domain investigation of injection-locked, constriction-based spin Hall nano-oscillators by Brillouin light scattering (BLS) and the time-resolved magneto-optical Kerr effect (TR-MOKE). This was achieved by applying an ac current in the GHz regime in addition to the dc current which drives auto-oscillations in the constriction. In the frequency domain, we analyze the width of the locking range, the increase in intensity, and the reduction in the linewidth as a function of the applied direct current. Then, we show that the injection locking of the auto-oscillation allows for its investigation by TR-MOKE measurements, a stroboscopic technique that relies on a phase stable excitation, in this case given by the synchronisation to the microwave current. Field sweeps at different dc currents clearly demonstrate the impact of the spin current on the Kerr amplitude. Two-dimensional TR-MOKE and BLS maps show a strong localization of the auto-oscillation within the constriction, independent of the external locking.

In Situ Image Acquisition and Measurement of Microdroplets Based on Delay Triggering.

An in situ image acquisition apparatus based on delay triggering for visualizing microdroplets formation is described. The imaging system includes a charge-coupled device camera, a motion control card, a driving circuit, a time delay triggering circuit, and a light source. By adjusting the varying trigger delay time which is synchronized with respect to the signal for jetting, the steady sequential images of the droplet flying in free space can be captured real-time by the system. Several image processing steps are taken to measure the diameters and coordinates of the droplets. Also, the jetting speeds can be calculated according to the delay time interval. For glycerin/water (60:40, mass ratio), under the given conditions of the self-made pneumatically diaphragm-driven drop-on-demand inkjet apparatus, the average of diameter and volume are measured as 266.8 μm and 9944 pL, respectively, and the maximum average velocity of the microdroplets is 0.689 m/s. Finally, the imaging system is applied to measure the volume of 200 microsolder balls generated from the inkjet apparatus. The average diameter is 87.96 μm, and the relative standard deviation is 0.83%. The results show good reproducibility. Unlike previous stroboscopic techniques, the present in situ imaging system which is absence of instantaneous high intensity light employs two control signals to stimulate the microdroplet generator and the charge-coupled device (CCD) camera. Hence, the system can avoid the desynchronization problem of signals which control the strobe light-emitting diode (LED) light source and the camera in previous equipment. This technology is a reliable and cost-effective approach for capturing and measuring microdroplets.

Spin–phonon interactions in silicon carbide addressed by Gaussian acoustics

Hybrid spin-mechanical systems provide a platform for integrating quantum registers and transducers. Efficient creation and control of such systems require a comprehensive understanding of the individual spin and mechanical components as well as their mutual interactions. Point defects in silicon carbide (SiC) offer long-lived, optically addressable spin registers in a wafer-scale material with low acoustic losses, making them natural candidates for integration with high quality factor mechanical resonators. Here, we show Gaussian focusing of a surface acoustic wave in SiC, characterized by a novel stroboscopic X-ray diffraction imaging technique, which delivers direct, strain amplitude information at nanoscale spatial resolution. Using ab initio calculations, we provide a more complete picture of spin-strain coupling for various defects in SiC with C3v symmetry. This reveals the importance of shear for future device engineering and enhanced spin-mechanical coupling. We demonstrate all-optical detection of acoustic paramagnetic resonance without microwave magnetic fields, relevant to sensing applications. Finally, we show mechanically driven Autler-Townes splittings and magnetically forbidden Rabi oscillations. These results offer a basis for full strain control of three-level spin systems.

Single-shot real-time sub-nanosecond electron imaging aided by compressed sensing: Analytical modeling and simulation

Bringing ultrafast (nanosecond and below) temporal resolution to transmission electron microscopy (TEM) has historically been challenging. Despite significant recent progress in this direction, it remains difficult to achieve sub-nanosecond temporal resolution with a single electron pulse, in real-time (i.e., duration in which the event occurs) imaging. To address this limitation, here, we propose a methodology that combines laser-assisted TEM with computational imaging methodologies based on compressed sensing (CS). In this technique, a two-dimensional (2D) transient event [i.e. (x,y) frames that vary in time] is recorded through a CS paradigm, which consists of spatial encoding, temporal shearing via streaking, and spatiotemporal integration of an electron pulse. The 2D image generated on a camera is used to reconstruct the datacube of the ultrafast event, with two spatial and one temporal dimensions, via a CS-based image reconstruction algorithm. Using numerical simulation, we find that the reconstructed results are in good agreement with the ground truth, which demonstrates the applicability of CS-based computational imaging methodologies to laser-assisted TEM. Our proposed method, complementing the existing ultrafast stroboscopic and nanosecond single-shot techniques, opens up the possibility for single-shot, real-time, spatiotemporal imaging of irreversible structural phenomena with sub-nanosecond temporal resolution.

Determination of dynamic contact angles within microfluidic devices

Here, we report a single-point detection method for the determination of dynamic surface conditions inside microfluidic channels. The proposed method is based on monitoring fluorescence amplitude as a function of the convolution of a laser beam with segmented flow consisting of two immiscible liquids, one containing fluorescent dye. The fluorescence amplitude is determined by the flow rate and the droplet shape, which is affected by the channel surface properties. We modeled the interaction of a droplet and a laser beam via computer-aided design software, using the laser beam location in relation to the droplet shape as a parameter. The method was applied to fused silica capillaries with both unmodified and modified surfaces, with segmented flow exhibiting water contact angles of ≈ 30° and ≈ 100°, respectively. The method allows discrimination between hydrophillic and hydrophobic surfaces, as well as the quality of the treatment. The results were verified using fluorescence imaging of the droplets via a stroboscopic technique. We also applied this method to the analysis of microfabricated channels with non-circular cross sections. We demonstrated that the technique enables the determination of the hydrophobicity of channel surfaces, a crucial property required for the generation of segmented flow or emulsions for applications such as digital PCR.

GLOBAL ATTRACTABILITY AND PERMANENCE FOR A NEW STAGE-STRUCTURED DELAY IMPULSIVE ECOSYSTEM

In this paper, a new stage-structured delay ecosystem with impulsive effect is formulated and some dynamical properties of this system is investigated.By using comparison theorem and the stroboscopic technique, we prove the existence of the predator-extinction periodic solution of this system and obtain some sufficient conditions to guarantee the global attractivity of the prey-extinction periodic solution. In the final, we also obtain the permanence of this system. It should be pointed out that the new mathematical method used in this paper can also be applied to investigate such other ecosystems corresponding to both impulsive and delay differential equations.

Spontaneous avalanche dephasing in large Rydberg ensembles.

Strong dipole-exchange interactions due to spontaneously produced contaminant states can trigger rapid dephasing in many-body Rydberg ensembles [E. Goldschmidt et al., PRL 116, 113001 (2016)]. Such broadening has serious implications for many proposals to coherently use Rydberg interactions, particularly Rydberg dressing proposals. The dephasing arises as a runaway process where the production of the first contaminant atoms facilitates the creation of more contaminant atoms. Here we study the time dependence of this process with stroboscopic approaches. Using a pump-probe technique, we create an excess "pump" Rydberg population and probe its effect with a different "probe" Rydberg transition. We observe a reduced resonant pumping rate and an enhancement of the excitation on both sides of the transition as atoms are added to the pump state. We also observe a timescale for population growth significantly shorter than predicted by homogeneous mean-field models, as expected from a clustered growth mechanism where high-order correlations dominate the dynamics. These results support earlier works and confirm that the time scale for the onset of dephasing is reduced by a factor which scales as the inverse of the atom number. In addition, we discuss several approaches to minimize these effects of spontaneous broadening, including stroboscopic techniques and operating at cryogenic temperatures. It is challenging to avoid the unwanted broadening effects, but under some conditions they can be mitigated.