Direct Momentum Transfer Research Articles

Optical Monitoring of the Didymos–Dimorphos Asteroid System with the Danish Telescope around the DART Mission Impact

The NASA’s Double-Asteroid Redirection Test (DART) was a unique planetary defence and technology test mission, the first of its kind. The main spacecraft of the DART mission impacted the target asteroid Dimorphos, a small moon orbiting the asteroid Didymos (65803), on 2022 September 26. The impact brought up a mass of ejecta which, together with the direct momentum transfer from the collision, caused an orbital period change of 33 ± 1 minutes, as measured by ground-based observations. We report here the outcome of the optical monitoring campaign of the Didymos system from the Danish 1.54 m telescope at La Silla around the time of impact. The observations contributed to the determination of the changes in the orbital parameters of the Didymos–Dimorphos system, as reported by Thomas et al., but in this paper we focus on the ejecta produced by the DART impact. We present photometric measurements from which we remove the contribution from the Didymos–Dimorphos system using an H–G photometric model. Using two photometric apertures we determine the fading rate of the ejecta to be 0.115 ± 0.003 mag day−1 (in a 2″ aperture) and 0.086 ± 0.003 mag day−1 (5″) over the first week postimpact. After about 8 days postimpact we note the fading slows down to 0.057 ± 0.003 mag day−1 (2″ aperture) and 0.068 ± 0.002 mag day−1 (5″). We include deep-stacked images of the system to illustrate the ejecta evolution during the first 18 days, noting the emergence of dust tails formed from ejecta pushed in the antisolar direction, and measuring the extent of the particles ejected Sunward to be at least 4000 km.

Exchange scaling of ultrafast angular momentum transfer in 4f antiferromagnets

Ultrafast manipulation of magnetism bears great potential for future information technologies. While demagnetization in ferromagnets is governed by the dissipation of angular momentum1–3, materials with multiple spin sublattices, for example antiferromagnets, can allow direct angular momentum transfer between opposing spins, promising faster functionality. In lanthanides, 4f magnetic exchange is mediated indirectly through the conduction electrons4 (the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction), and the effect of such conditions on direct spin transfer processes is largely unexplored. Here, we investigate ultrafast magnetization dynamics in 4f antiferromagnets and systematically vary the 4f occupation, thereby altering the magnitude of the RKKY coupling energy. By combining time-resolved soft X-ray diffraction with ab initio calculations, we find that the rate of direct transfer between opposing moments is directly determined by this coupling. Given the high sensitivity of RKKY to the conduction electrons, our results offer a useful approach for fine tuning the speed of magnetic devices.

Plasmonic drag photocurrent in graphene at extreme nonlocality

It is commonly assumed that photocurrent in two-dimensional systems with centrosymmetric lattice is generated at structural inhomogenities, such as p-n junctions. Here, we study an alternative mechanism of photocurrent generation associated with inhomogenity of the driving electromagnetic field, termed as 'plasmonic drag'. It is associated with direct momentum transfer from field to conduction electrons, and can be characterized by a non-local non-linear conductivity $\sigma^{(2)}({\bf q},\omega)$. By constructing a classical kinetic model of non-linear conductivity with full account of non-locality, we show that it is resonantly enhanced for wave phase velocity coinciding with electron Fermi velocity. The enhancement is interpreted as phase locking between electrons and the wave. We discuss a possible experiment where non-uniform field is created by a propagating graphene plasmon, and find an upper limit of the current responsivity vs plasmon velocity. This limit is set by a competition between resonantly growing $\sigma^{(2)}({\bf q},\omega)$ and diverging kinetic energy of electrons as the wave velocity approaches Fermi velocity.

Determining the momentum transfer in regolith-like targets using the TUM/LRT electro-thermal accelerator

Asteroids are small solid bodies that formed during the early stages of the solar system, even before the formation of our own planet. Asteroids have been known to strike our planet Earth ever since its formation, causing a great threat to all existing life forms. One of the means to avert a potential collision of an asteroid with Earth is to deflect the asteroid away from its trajectory towards Earth by means of an impacting spacecraft. This idea is already part of mission designs such as AIDA (Asteroid Impact and Deflection Assessment).The study of the surfaces of numerous asteroids through spacecraft, ground-based spectroscopic observations and other techniques has revealed that the surfaces of these small bodies are covered with loose, granular, heterogeneous geological material. This is called ‘regolith’. If an impactor were to collide with an asteroid, the physical phenomenon that needs to be understood is the reaction of the regolith to the impact. To understand the deflection of the asteroid, the transfer of momentum from the impactor to the asteroid needs to be characterised. The impact can produce ejecta which would also add momentum to the target. Therefore, there are two components to this momentum gained by target body: (1) direct transfer of momentum from the impactor to the asteroid, (2) momentum caused by ejecta flying in the direction opposite to that of the impactor. The momentum transfer can thus be described by the so-called momentum multiplication factor β, which is the ratio of the momentum gained by the target to the momentum of the impactor at the time of collision. In this work, small-scale impact experiments using polyvinylchloride (PVC) projectiles on regolith-like materials and simulants - sand, glass beads and the Johnson Space Center (JSC)-1A lunar regolith simulant - have been conducted. We used the electro-thermal accelerator of the Technical University of Munich (TUM)/Lehrstuhl für Raumfahrttechnik (LRT) to determine the momentum transfer from the projectiles onto these regolith targets. We derived relationships between the kinetic energy and the crater mass and characterized the ejecta cloud. We confirmed the dependency of β on the impact velocity of the projectile (impactor) - higher momentum transfer occurs at larger impact velocities. We have also seen a trend that β gets larger for a lower cohesion and higher porosity of the target material. Thus, a thorough understanding of the regolith properties of AIDA’s target asteroid will be crucial towards understanding and determining the momentum transfer.

Reactive Momentum Transfer Contributes to the Self-Propulsion of Janus Particles.

We report simulations of a spherical Janus particle undergoing exothermic surface reactions around one pole only. Our model excludes self-phoretic transport by design. Nevertheless, net motion occurs from direct momentum transfer between solvent and colloid, with speed scaling as the square root of the energy released during the reaction. We find that such propulsion is dominated by the system's short-time response, when neither the time dependence of the flow around the colloid nor the solvent compressibility can be ignored. Our simulations agree reasonably well with previous experiments.

Perspective on light-induced transport of particles: from optical forces to phoretic motion

Propulsive effects of light, which often remain unnoticed in our daily-life experience, manifest themselves on spatial scales ranging from subatomic to astronomical. Light-mediated forces can indeed confine individual atoms, cooling their effective temperature very close to absolute zero, as well as contribute to cosmological phenomena such as the formation of stellar planetary systems. In this review, we focus on the transport processes that light can initiate on small spatial scales. In particular, we discuss in depth various light-induced mechanisms for the controlled transport of microscopic particles; these mechanisms rely on the direct transfer of momentum between the particles and the incident light waves, on the combination of optical forces with external forces of other nature, and on light-triggered phoretic motion. After a concise theoretical overview of the physical origins of optical forces, we describe how these forces can be harnessed to guide particles either in continuous bulk media or in the proximity of a constraining interface under various configurations of the illuminating light beams (radiative, evanescent, or plasmonic fields). Subsequently, we introduce particle transport techniques that complement optical forces with counteracting forces of non-optical nature. We finally discuss particle actuation schemes where light acts as a fine knob to trigger and/or modulate phoretic motion in spatial gradients of non-optical (e.g., electric, chemical, or temperature) fields. We conclude by outlining possible future fundamental and applied directions for research in light-induced particle transport. We believe that this comprehensive review can inspire diverse, interdisciplinary scientific communities to devise novel, unorthodox ways of assembling and manipulating materials with light.

From Multiple- to Single-Pulse All-Optical Helicity-Dependent Switching in Ferromagnetic Co/Pt Multilayers

All-optical helicity-dependent switching in ferromagnetic Co/Pt multilayers is investigated using magneto-optical microscopy and anomalous Hall effect measurements. A state diagram is built by studying the effect of pulse duration, fluence, and spot size. We use numerical solutions of the three-temperature model to explain that the all-optical helicity-dependent switching mechanism relies on the spin bath reaching temperatures close to the Curie point. Further insights into the reversal process are provided by the experimental demonstration of significant helicity-dependent reversal after a single laser pulse that reveals the involvement of direct angular momentum transfer. Moreover, based on the observation that longer pulse durations and larger spot sizes lead to enhanced reversal efficiency, we identify experimental conditions that lead to saturated magnetization reversal after just a few tens of laser pulses.

Revisiting the Photon-Drag Effect in Metal Films.

The photon-drag effect, the rectified current in a medium induced by conservation of momentum of absorbed or redirected light, is a unique probe of the detailed mechanisms underlying radiation pressure. We revisit this effect in gold, a canonical Drude metal. We discover that the signal for p-polarized illumination in ambient air is affected in both sign and magnitude by adsorbed molecules, opening previous measurements for reinterpretation. Further, we show that the intrinsic sign of the photon-drag effect is contrary to the prevailing intuitive model of direct momentum transfer to free electrons.

Direct Transfer of Light's Orbital Angular Momentum onto a Nonresonantly Excited Polariton Superfluid.

Recently, exciton polaritons in a semiconductor microcavity were found to condense into a coherent ground state much like a Bose-Einstein condensate and a superfluid. They have become a unique testbed for generating and manipulating quantum vortices in a driven-dissipative superfluid. Here, we generate an exciton-polariton condensate with a nonresonant Laguerre-Gaussian optical beam and verify the direct transfer of light's orbital angular momentum to an exciton-polariton quantum fluid. Quantized vortices are found in spite of the large energy relaxation involved in nonresonant pumping. We identified phase singularity, density distribution, and energy eigenstates for the vortex states. Our observations confirm that nonresonant optical Laguerre-Gaussian beam can be used to manipulate chirality, topological charge, and stability of the nonequilibrium quantum fluid. These vortices are quite robust, only sensitive to the orbital angular momentum of light and not other parameters such as energy, intensity, size, or shape of the pump beam. Therefore, optical information can be transferred between the photon and exciton-polariton with ease and the technique is potentially useful to form the controllable network of multiple topological charges even in the presence of spectral randomness in a solid state system.

New methodology for computing tsunami generation by subaerial landslides: Application to the 2015 Tyndall Glacier landslide, Alaska

AbstractLandslide‐generated tsunamis pose significant hazards and involve complex, multiphase physics that are challenging to model. We present a new methodology in which our depth‐averaged two‐phase model D‐Claw is used to seamlessly simulate all stages of landslide dynamics as well as tsunami generation, propagation, and inundation. Because the model describes the evolution of solid and fluid volume fractions, it treats both landslides and tsunamis as special cases of a more general class of phenomena. Therefore, the landslide and tsunami can be efficiently simulated as a single‐layer continuum with evolving solid‐grain concentrations, and with wave generation via direct longitudinal momentum transfer—a dominant physical mechanism that has not been previously addressed in this manner. To test our methodology, we used D‐Claw to model a large subaerial landslide and resulting tsunami that occurred on 17 October 2015, in Taan Fjord near the terminus of Tyndall Glacier, Alaska. Modeled shoreline inundation patterns compare well with those observed in satellite imagery.

Surface reactions and structural evolution of organosilicate glass under Ar plasma bombardment

Ab initio molecular dynamics (AIMD) simulations with a variable time step setup were used to investigate collision reactions, collision cascades and subsequent structural changes of organosilicate glass (OSG) low-k dielectric surfaces under Ar plasma bombardment with kinetic energies up to 100eV. Reactions that result in carbon loss and glass network structure changes during plasma treatment were carefully examined, since carbon loss increases the dielectric constant of the low-k dielectrics and the susceptibility to water attack, thus adversely affecting the performance of these dielectric materials in microelectronic devices. Results from AIMD based collision cascade simulations have provided mechanistic understanding of collision reactions during plasma bombardment. It was found that Si–CH3 bond scission and CH3 abstraction reactions are the primary methods of carbon loss. Two distinct mechanisms were identified for CH3 removal from 100eV plasma bombardment, one due to direct momentum transfer from the high-energy incident Ar atom to a Si–O bond on the OSG surface, and the other due to generation of highly reactive atomic oxygen radicals during collision cascades. Atomic oxygen radicals generated during cascades were found to occur at a rate of 1–3 per 100eV Ar impact with radical kinetic energies ranging from 2.2 to 42.6eV, well above the minimum energy of 0.1eV for Si–CH3 bond breakage. Both reaction mechanisms were examined through atomic charge analysis and the results indicated that the Ar atom ionizes when it impacts the OSG surface, causing temporary charge transfer from the Ar atom to the Si–O bond and playing a role in CH3 removal. The cascading atomic oxygen radicals and the ionization of the Ar atom during the Si–CH3 bond scission and CH3 removal demonstrate the secondary effects of Ar bombardment which must be considered during plasma etching of low-k dielectrics.

Investigation of rotation at the plasma edge in TCABR

This paper summarizes experimental results from recent studies on intrinsic rotation at the plasma edge of the TCABR tokamak. These results were obtained after upgrading the number of channels of the rotation diagnostic to three. The measurements were carried out in the collisional (Pfirsch–Schluter) regime and the rotation profiles of the ions were obtained from the Doppler shifts of the impurity carbon lines, CIII (464.74 nm), and CVI (529.06 nm). Results on the correlation between toroidal rotation at the plasma edge and direction of gas injection are also presented. They indicate that the direction of gas injection has a small effect on rotation; the velocity of the background neutral hydrogen is affected by direct momentum transfer from the injected gas (also hydrogen), while the carbon ions' velocity is affected by inward radial friction force between the injected gas atoms and ions, increasing their velocity in the opposite sense of the plasma current.

Micromotors with asymmetric shape that efficiently convert light into work by thermocapillary effects.

The direct conversion of light into work allows the driving of micron-sized motors in a contactless, controllable and continuous way. Light-to-work conversion can involve either direct transfer of optical momentum or indirect opto-thermal effects. Both strategies have been implemented using different coupling mechanisms. However, the resulting efficiencies are always very low, and high power densities, generally obtained by focused laser beams, are required. Here we show that microfabricated gears, sitting on a liquid–air interface, can efficiently convert absorbed light into rotational motion through a thermocapillary effect. We demonstrate rotation rates up to 300 r.p.m. under wide-field illumination with incoherent light. Our analysis shows that thermocapillary propulsion is one of the strongest mechanisms for light actuation at the micron- and nanoscale.

Photon momentum transfer in inhomogeneous dielectric mixtures and induced tractor beams

The determination of optical force as a consequence of momentum transfer is inevitably subject to the use of the proper momentum density and stress tensor. It is imperative and valuable to consider the intrinsic scheme of photon momentum transfer, particularly when a particle is embedded in a complex dielectric environment. Typically, we consider a particle submerged in an inhomogeneous background composed of different dielectric materials, excluding coherent illumination or hydrodynamic effects. A ray-tracing method is adopted to capture the direct process of momentum transfer from the complex background medium, and this approach is validated using the modified Einstein–Laub method, which uses only the interior fields of the particle in the calculation. In this way, debates regarding the calculation of the force with different stress tensors using exterior fields can be avoided. Our suggested interpretation supports only the Minkowski approach for the optical momentum transfer to the embedded scatterer while rejecting Peierls's and Abraham's approaches, though the momentum of a stably moving photon in a continuous background medium should be considered to be of the Abraham type. Our interpretation also provides a novel method of realizing a tractor beam for the exertion of negative force that offers an alternative to the use of negative-index materials, optical gain, or highly non-paraxial or multiple-light interference. Support for the Minkowski formulation has been obtained by considering photon momentum transfer for a scatterer at a liquid–gas interface. To shed light on the century-old debate regarding the momentum of light in dielectric media (the so-called Abraham–Minkowski controversy), a team of scientists from Singapore, China and the USA investigated direct momentum transfer from the two media. To do this, they performed ray-tracing calculations combined with modified Einstein–Laub equations that consider only the fields inside the particle, thereby side-stepping problems arising from the use of different stress tensors for fields outside the particle. Their analysis also suggests an alternative way to realize tractor beams (that is, beams that pull rather than push a particle) other than using negative-index materials, optical gain, highly non-paraxial beams or multiple-light interference.

All-optical control of ultrafast photocurrents in unbiased graphene.

Graphene has recently become a unique playground for studying light-matter interaction effects in low-dimensional electronic systems. Being of strong fundamental importance, these effects also open a wide range of opportunities in photonics and optoelectronics. In particular, strong and broadband light absorption in graphene allows one to achieve high carrier densities essential for observation of nonlinear optical phenomena. Here, we make use of strong photon-drag effect to generate and optically manipulate ultrafast photocurrents in graphene at room temperature. In contrast to the recent reports on injection of photocurrents in graphene due to external or built-in electric field effects and by quantum interference, we force the massless charge carriers to move via direct transfer of linear momentum from photons of incident laser beam to excited electrons in unbiased sample. Direction and amplitude of the drag-current induced in graphene are determined by polarization, incidence angle and intensity of the obliquely incident laser beam. We also demonstrate that the irradiation of graphene with two laser beams of the same wavelength offers an opportunity to manipulate the photocurrents in time domain. The obtained all-optical control of the photocurrents opens new routes towards graphene based high-speed and broadband optoelectronic devices.

Experimental and numerical investigation of the effect of mixing section length on direct-contact condensation in steam jet pump

The phenomenon of direct-contact condensation is very important with respect to several industrial applications, such as steam jet pumps, direct-contact heat exchangers and nuclear reactor cooling systems. The direct and quick transfer of heat, mass and momentum across the steam–water interface makes the physics of direct-contact condensation very complex. In this work the phenomenon of direct-contact condensation, in steam jet pump, has been studied experimentally and computationally. The length of the mixing section has been varied to study its effect on the phenomenon occurring there. The experimental and CFD results obtained in this study match closely with each other and indicate towards an important process known as interface vibration process in direct-contact condensation of steam jet pump. It was found that this process has a strong dependence on the length of mixing section and plays an important role in producing high suction at the water nozzle outlet. As a result we get efficient transfer of heat, mass and momentum across the steam–water interface in steam jet pump.

Dissipation regimes for short wind waves

The dissipation processes affecting short wind waves of centimeter and decimeter scales are investigated experimentally in laboratory. The processes include damping due to molecular viscosity, generation of capillary waves, microbreaking, and breaking. The observations were made in a large wind wave tank for a wide range of fetches and winds, using a laser sheet and a high‐resolution video camera. The work aims at constructing a comprehensive picture of dissipative processes in the short wind wave field, to find for which scales particular dissipative mechanism may become important. Four distinct regimes have been identified. For capillary‐gravity wave fields, i.e., for dominant waves with scales below 4 cm, viscous damping is found to be the main dissipation mechanism. The gravity‐capillary wave fields with dominant wavelength less than 10 cm usually exhibit a train of capillary ripples at the crest wavefront, but no wave breaking. For such waves, the main dissipation process is molecular viscosity occurring through nonlinear energy cascade toward high‐frequency motions. Microscale breaking takes place for waves longer than 10 cm and manifests itself in a very localized surface disruption on the forward face of the crest. Such events generate turbulent motions in water and thus enhance wave dissipation. Plunging breaking, characterized by formation of a crest bulge, a microjet hitting the water surface and a splash‐up, occurs for short gravity waves of wavelength exceeding 20 cm. Macroscale spilling breaking is also observed for longer waves at high winds. In both cases, the direct momentum transfer from breaking waves to the water flow contributes significantly to wave damping.

The velocity decomposition method for second-order accuracy in stochastic parcel simulations

In Lagrangian Monte Carlo simulations of liquid disperse flows, the prediction of inter-droplet momentum transfer and kinetic energy dissipation strongly depends on the spatial resolution of the collision algorithm. These dependencies are caused by systematic errors induced into the relative velocity between the colliding droplets. In this study, a velocity decomposition method is proposed, which eliminates the erroneous component of relative velocity. The velocity decomposition method significantly reduces numerical dependencies in the prediction of the collision outcome, leading to second order convergence of relative velocities, and of direct momentum transfer and kinetic energy dissipation. This is proven in synthetic validation cases and demonstrated by an engine-spray simulation exemplarily.

Laser-induced ultrafast demagnetization in the presence of a nanoscale magnetic domain network

Femtosecond magnetization phenomena have been challenging our understanding for over a decade. Most experiments have relied on infrared femtosecond lasers, limiting the spatial resolution to a few micrometres. With the advent of femtosecond X-ray sources, nanometric resolution can now be reached, which matches key length scales in femtomagnetism such as the travelling length of excited 'hot' electrons on a femtosecond timescale. Here we study laser-induced ultrafast demagnetization in [Co/Pd](30) multilayer films, which, for the first time, achieves a spatial resolution better than 100 nm by using femtosecond soft X-ray pulses. This allows us to follow the femtosecond demagnetization process in a magnetic system consisting of alternating nanometric domains of opposite magnetization. No modification of the magnetic structure is observed, but, in comparison with uniformly magnetized systems of similar composition, we find a significantly faster demagnetization time. We argue that this may be caused by direct transfer of spin angular momentum between neighbouring domains.

Ion acceleration in bias-enhanced nucleation of diamond at relatively high pressures

In order to explain the bias-enhanced nucleation (BEN) of diamond at pressures of approximately 6 kPa, the flux and energy of hydrogen ions to a negatively biased substrate in microwave plasma chemical vapor deposition (MWP-CVD) were investigated. Ion flux increased rapidly upon increasing the negative bias voltage to greater than 150 V when the pressure was higher than 3 kPa. This phenomenon was attributed to the additional ionization in the vicinity of the substrate due to biasing. Optical emission spectroscopy at 6 kPa clarified that the emission intensity of the Swan band of C 2 radicals was reduced, and intramolecular temperatures were enhanced in this bias region. A numerical simulation on the basis of the measured ion flux and density revealed that the most frequent energy was less than 2 eV under the conditions of 200 V and 6 kPa, and that the average number of collisions in the plasma sheath reached 60. We propose, on the basis of these results, that the substrate biasing in BEN at pressures above 3 kPa causes the additional ionization and activation of neutral species in the sheath, while the direct momentum transfer of ions to the substrate is strongly suppressed in BEN in this pressure range.