Photon Momentum Transfer Research Articles

Nonlinear scaling in photon momentum transfer caused by two-electron effects

The photon momentum transfer in the two-photon double ionization of helium is theoretically studied based on the $ab\phantom{\rule{4pt}{0ex}}initio$ calculations. A nonlinear dependence of the transferred momentum on the photon energy is identified at relatively small photon energies. As expected, the linear dependence is recovered with the slope approaching 1.60 when the photon energy becomes sufficiently large. With a semianalytical model including the Coulomb screen effect in the two-electron ground state, we can qualitatively reproduce the nonlinear dependence and find that it is mainly contributed by the fast electron. However, compared with the results from the $ab\phantom{\rule{4pt}{0ex}}initio$ calculations, the quantitative discrepancies at relatively small photon energies can be accounted for only by the electron correlation in the intermediate state. The present study reveals the vital roles played by the two-electron effects for the complicated photon momentum transfer process in the few-photon and few-electron systems.

Nondipole effects on the double-slit interference in molecular ionization by xuv pulses.

The double-slit interference in single-photon ionization of the diatomic molecular ion H2 + is theoretically studied beyond the dipole approximation. Via simulating and comparing the interactions of the prealigned H2 + and the hydrogen atom with the xuv pulses propagating in different directions, we illustrate two kinds of effects that are encoded in the interference patterns of the photoelectrons from H2 +: the single-atom nondipole effect and the two-center-interference one, both associated with the finite speed of light. While the two effects could modify the maxima of the interference fringes, we show that the former one hardly affects the interference minima. Our results and analysis show that the interference minima rule out the influences of the photon-momentum transfer and, potentially, the multielectron effect, thus performing a better role in decoding the zeptosecond time delay for the pulse hitting one and the other atomic centers of the molecule.

Progress on the Starshot laser propulsion system.

The Breakthrough Starshot research program envisions a laser-propelled sail that will probe our neighboring stars within a human lifetime. Starshot spacecraft weigh no more than a few grams and are accelerated by photon momentum transfer with a beam generated by a kilometer-scale, ground-based 100 GW coherent phased-array laser. We describe some of the major challenges for the laser and early exploratory efforts to overcome them. While challenges remain, we find no physical or economic restrictions that would preclude such a laser for the Starshot mission.

New features in the surface plasmon induced photon drag effect in noble metal thin films

We study light–matter interactions leading to the generation of photon drag voltage under surface plasmon resonance conditions in noble metal thin films and observe important effects, which provide opportunity for condensed matter theorists to critically evaluate theoretical models. The drag voltage originates from a force that arises because of the transfer of momentum from incident light to electrons. This transfer of photon momentum leads to an electric current, which in turn results in the generation of drag voltage. The effect is particularly enhanced under surface plasmon resonance conditions and thereby reinforces the interaction between light and collective oscillations of the surface charges. We observe relatively high voltages with a nonlinear dependence on laser intensity. In disagreement with previous results, we do not observe a reversal in the sign of the voltage when the direction of the incident laser momentum is reversed. Qualitative analyses of the data show that the hydrodynamic model of laser-induced drag voltage does not work: the hydrodynamic model predicts voltages that are nine orders of magnitude lower than our measurements. However, there is reasonable consistency between the results of the measurements and numerical simulations.

Laser cooling with adiabatic passage for type-II transitions

We extend the idea of laser cooling with adiabatic passage to multi-level type-II transitions. We find the cooling force can be significantly enhanced when a proper magnetic field is applied. That is because the magnetic field decomposes the multi-level system into several two-level sub-systems, hence the stimulated absorption and stimulated emission can occur in order, allowing for the multiple photon momentum transfer. We show that this scheme also works on the laser-coolable molecules with a better cooling effect compared to the conventional Doppler cooling. A reduced dependence on spontaneous emission based on our scheme is observed as well. Our results suggest this scheme is very feasible for laser cooling of polar molecules.

A New Mechanism of Light-Induced Bubble Growth to Propel Microbubble Piston Engine.

Radiation pressure refers to the momentum transfer of photons during light "particles" impacting a surface. The force is too small to drive microengines. Different from the classical radiation pressure, the indirect radiation pressure (Fm ) is introduced, coming from the momentum change of light-induced bubble expansion. Fm is shown to obey Fm ∼ (I·rb )2 , behaving faster growth of indirect radiation pressure versus light intensity I and bubble radius rb . An effective bubble size range is identified for Fm to suppress other forces for bubble in liquid. The top laser irradiation on nanofluid is used in this experiment. A well-defined bubble pulsating flow, being a new principle of bubble piston engine, is demonstrated. During pulse on (≈ns scale), Fm exceeding other forces generates an extremely large acceleration, which is three to four orders larger than the gravity acceleration, propelling the bubble traveling downward. During pulse off, the bubble is floating upward due to the nonexistence of Fm . In such a way, the piston engine sustains the oscillating ranges of 38-347 µm for bubble diameters and 2.7-457.9 µm for traveling distances of piston. This work is useful to manipulate bubble dynamics in solar energy systems, and can find various applications in optofluidics.

Momentum-transfer model of valence-band photoelectron diffraction

Recent instrumental progress of valence-band photoemission in the X-ray range allows uncovering bulk- and surface-related electronic properties. Four-dimensional recording of energy and momentum-vector gives access to the complete spectral-density function. Systematic measurements for a number of transition metals between 15 eV-6 keV reveal unexpected strong intensity modulations due to photoelectron diffraction. Here, we present a graphical model that illustrates the role of momentum-conservation in Fermi’s Golden-Rule in an intuitive way. Intensity enhancement or reduction by factors >5 are confined to small energy- and momentum-intervals (widths 0.03 Å−1 and 200 meV). Laue-type diffraction involves the photon momentum and is intrinsic in the photoemission process, in accordance with Pendry’s final-state-model. At higher energies, Kikuchi-diffraction imprints additional modulations on valence-band-patterns and quasi-elastic background. The absence of photon-momentum transfer uncovers the extrinsic nature of Kikuchi-diffraction. For Re at 30 K and 3.4 keV the relative weight of the Kikuchi-branch is comparable to the Laue-branch, whereas at 6 keV the Kikuchi-branch prevails.

Longitudinal photon-momentum transfer in strong-field double ionization of argon atoms

We investigate the longitudinal photon-momentum transfer in nonsequential double ionization of argon atoms exposed to a $2.0\ensuremath{-}\ensuremath{\mu}\mathrm{m}$ linearly polarized strong laser field. The zero momentum of the photoelectron is precisely calibrated using the photoelectron-photoion coincidence spectrum. The sum-longitudinal momentum of two photoelectrons from the double ionization noticeably deflects along the laser propagation direction, which is opposite to and much larger than that of single ionization. By tracking the electron trajectories for double ionization in semiclassical simulations, we find that the Coulomb focusing of the ionic core with the assistance of the Lorentz pushing of the laser field dominates the observed longitudinal momentum deflection. Our results open the possibility to control the photon-momentum transfer by manipulating the waveform of the laser fields and thus the Coulomb and magnetic effects experienced by the photoelectrons.

Photon Momentum Transfer in Single-Photon Double Ionization of Helium.

We theoretically and experimentally investigate the photon momentum transfer in single-photon double ionization of helium at various large photon energies. We find that the forward shifts of the momenta along the light propagation of the two photoelectrons are roughly proportional to their fraction of the excess energy. The mean value of the forward momentum is about 8/5 of the electron energy divided by the speed of light. This holds for fast and slow electrons despite the fact that the energy sharing is highly asymmetric and the slow electron is known to be ejected by secondary processes of shake off and knockout rather than directly taking its energy from the photon. The biggest deviations from this rule are found for the region of equal energy sharing where the quasifree mechanism dominates double ionization.

Subcycle transfer of photon momentum in nondipole tunneling ionization

SynopsisWe study the subcycle transfer of photon momentum in tunneling ionization beyond the dipole approximation using the attoclock setup. We separate the momentum transfer during tunneling and propagation in continuum using the backpropagation method.

High-accuracy bulk electronic bandmapping with eliminated diffraction effects using hard X-ray photoelectron momentum microscopy

A key benefit of angle-resolved photoelectron spectroscopy (ARPES) in the X-ray range is the significant increase of the information depth, thanks to the large inelastic mean-free-path of the escaping photoelectrons. In practice hard X-ray ARPES (HARPES) faces severe challenges by low cross sections, large photon momentum transfer, and in particular strong phonon scattering and photoelectron diffraction effects. Here, we show that these challenges can be overcome by extending ultra-efficient time-of-flight momentum microscopy into the hard X-ray regime. Phonon scattering destroys the initial momentum distribution but subsequent diffraction at the lattice imprints a pronounced Kikuchi-type pattern on the background signal. Moreover, the pattern of the valence electrons is modulated by diffraction as well. For the examples of the medium-weight element materials Mo and layered TiTe2, we demonstrate how comprehensive valence-band and core-level photoemission data taken under identical conditions can be used to effectively remove photoelectron diffraction effects in HARPES band maps.

Photon momentum induced precision small forces: a static and dynamic check

Practical means of generation and calibration of the small precision forces in static and dynamic regimes around 1 Hz by the usage of radiation pressure effect from 1 W continuous wave visible (diode) laser light is presented. The additive effect of the transfer of photon momentum, caused by non-overlapping multiply reflecting laser beam locked within a quasi-passive and/or active macroscopic cavity system, is employed. The effective laser power (partially trapped experimentally) is amplified, such that the optically generated forces are increased from hundreds of pN to sub-N level. The results presented in this paper should be seen as a means for extending the edge of practically verifiable lower limits of SI-traceable force metrology.

Photon momentum transfer at water/air interfaces under total internal reflection

The transfer of photon momentum at the water/air interface is important for optical manipulation of minute particles and is at the heart of the Minkowski–Abraham controversy. We use photoacoustic (PA) detection of ultrasound waves generated when pulsed laser light meets the water/air interface at 3.9 °C (zero thermal expansion), to distinguish momentum transfer from thermoelastic effects. The PA waves dependence on the angle of incidence reveals that momentum transfer maximizes at the critical angle. Momentum transfer is most efficient when the photons travel in water and remain in water after total reflection at the interface, rather than when they cross the interface between dielectric media.

Nonlinear optical characterization of porous carbon materials by CARS, SHG and TPEF.

The Coherent anti-Stokes Raman scattering (CARS) microscopy is employed to characterize porous carbon materials. The slices of two dimensional (2D) CARS images are obtained, using Z-scan technology. Furthermore, the three dimensional (3D) image of CARS is reconstructed with the Z-scanned 2D CARS images. The visualizations of rotation for 3D CARS are also shown. The 2D and 3D CARS images with rotations can clearly characterize the structures and properties of porous carbon materials, including the surface and inner parts. Nonlinear optical second harmonic generation demonstrates the inversion symmetry breaking of porous carbon materials, which is not centrosymmetric material. The two-photon excitation fluorescence image reveals that porous carbon material is of weak auto-fluorescence. The clearly observed signal of two photon-excitation fluorescence provides direct evidence that photon momentum transfer to the electron system occurs. Experimental results demonstrate that the nonlinear optical microscopy is a great optical analysis method for the study of porous carbon materials, and can be potentially used for the optical characterization on the materials at micro-nano-scale.

Photon-momentum transfer in diatomic molecules: An ab initio study

For a molecule, the two-center interference and the molecular scattering phase of the electron are important for almost all the processes that may occur in a laser field. In this study, we investigate their effects in the transfer of linear photon momentum to the ionized electron by absorbing a single photon. The time-dependent Schr\"odinger equation of H$_2^+$ is numerically solved in {the multipolar} gauge in which the electric quadrupole term and the magnetic dipole term are explicitly expressed. This allows us to separate the contributions of the two terms in the momentum transfer. For different configurations of the molecular and the laser orientation, the transferred momentum to the electron is evaluated at different internuclear distances with various photon energies { and two-center interferences are identified in the whole region. At small electron energies and small internuclear distances, we find significant deviations from the prediction of the classical double-slit model due to the strong mediation of the Coulomb potential.} Finally, even for a large internuclear distance, our results show that a varying molecular scattering phase is important at all electron energies, which is beyond the simple prediction of the linear combination of the atomical orbitals.

Symmetry breakdown of electron emission in extreme ultraviolet photoionization of argon

Short wavelength free-electron lasers (FELs), providing pulses of ultrahigh photon intensity, have revolutionized spectroscopy on ionic targets. Their exceptional photon flux enables multiple photon absorptions within a single femtosecond pulse, which in turn allows for deep insights into the photoionization process itself as well as into evolving ionic states of a target. Here we employ ultraintense pulses from the FEL FERMI to spectroscopically investigate the sequential emission of electrons from gaseous, atomic argon in the neutral as well as the ionic ground state. A pronounced forward-backward symmetry breaking of the angularly resolved emission patterns with respect to the light propagation direction is experimentally observed and theoretically explained for the region of the Cooper minimum, where the asymmetry of electron emission is strongly enhanced. These findings aim to originate a better understanding of the fundamentals of photon momentum transfer in ionic matter.

Heavy Quarkonia in a Contact Interaction and an Algebraic Model: Mass Spectrum, Decay Constants, Charge Radii and Elastic and Transition Form Factors

For the flavor-singlet heavy quark system of bottomonia, we compute the masses of the ground state mesons in four different channels, namely, pseudo-scalar ($\eta_{b}(1S)$), vector ($\Upsilon(1S)$), scalar ($\chi_{b_0}(1P)$) and axial vector ($\chi_{b_{1}}(1P)$). We also calculate the weak decay constants of the $\eta_{b}(1S)$ and $\Upsilon(1S)$ as well as the charge radius of $\eta_{b}(1S)$. It complements our previous study of the corresponding charmonia systems: $\eta_c(1S)$, $J/\Psi(1S)$, $\chi_{c_0}(1P)$) and ($\chi_{c_{1}}(1P)$). The unified formalism for this analysis is provided by a symmetry-preserving Schwinger-Dyson equations treatment of a vector$\times$vector contact interaction. Whenever a comparison is possible, our results are in fairly good agreement with experimental data and model calculations based upon Schwinger-Dyson and Bethe-Salpeter equations involving sophisticated interaction kernels. Within the same framework, we also report the elastic and transition form factors to two photons for the pseudo-scalar channels $\eta_{c}(1S)$ and $\eta_{b}(1S)$ in addition to the elastic form factors for the vector mesons $J/\Psi$ and $\Upsilon$ for a wide range of photon momentum transfer squared ($Q^2$). For $\eta_{c}(1S)$ and $\eta_{b}(1S)$, we also provide predictions of an algebraic model which correlates remarkably well between the known infrared and ultraviolet limits of these form factors.

Photon-momentum transfer in molecular photoionization

In most models and theoretical calculations describing multiphoton ionization by infrared light, the dipole approximation is used. This is equivalent to setting the very small photon momentum to zero. Using numerical solutions of the (nondipole) three-dimensional time-dependent Schr\odinger equation for one electron in a ${\mathrm{H}}_{2}{}^{+}$ molecular ion we investigate the effect the photon-momentum transfer to the photoelectron in an ${\mathrm{H}}_{2}{}^{+}$ ion in various regimes. We find that the photon-momentum transfer in a molecule is very different from the transfer in atoms due to two-center interference effects. The photon-momentum transfer is very sensitive to the symmetry of the initial electronic state and is strongly dependent on the internuclear distance and on the ellipticity of the laser.

Publisher's Note: Photon-momentum transfer in one- and two-photon ionization of atoms [Phys. Rev. A 96, 043414 (2017)

Received 30 October 2017DOI:https://doi.org/10.1103/PhysRevA.96.059901©2017 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasAtomic & molecular processes in external fieldsAtomic, Molecular & Optical

Photon-momentum transfer in one- and two-photon ionization of atoms

The transfer of the linear photon momentum to the electron in the few-photon ionization of an atom is accurately computed through a numerical solution to the three-dimensional time-dependent Schr\odinger equation (TDSE) beyond the dipole approximation. The characteristics of the transfer is studied in detail by comparing the TDSE results with those calculated by the perturbation theory (PT) using the exact scattering states. Our fully quantum-mechanical calculations show a discernible photoelectron momentum shift along the laser's propagation direction, which is caused by the nondipole effects. The momentum transfer rule for the single-photon ionization is shown to agree with results obtained from the perturbation theory. We identify some extraordinary ``dips'' in the photon-momentum transfer in the case of two-photon ionization, which depend on the laser ellipticity and are caused by the suppression of the nondipole transitions. In addition, the Coulomb screening effect on the photon-momentum transfer has also been investigated and we find that the Coulomb tail is negligible in the single-photon ionization case. However, the potential in the short range near the Coulomb center may influence the initial state, which will change the amount of the momentum that can be transferred.