Component Of Spin Angular Momentum Research Articles

Plasmonic topological quasiparticle on the nanometre and femtosecond scales.

At the interface of classical and quantum physics, the Maxwell and Schrödinger equations describe how optical fields drive and control electronic phenomena to enable lightwave electronics at terahertz or petahertz frequencies and on ultrasmall scales1-5. The electric field of light striking a metal interacts with electrons and generates light-matter quasiparticles, such as excitons6 or plasmons7, on an attosecond timescale. Here we create and image a quasiparticle of topological plasmonic spin texture in a structured silver film. The spin angular momentum components of linearly polarized light interacting with an Archimedean coupling structure with a designed geometric phase generate plasmonic waves with different orbital angular momenta. These plasmonic fields undergo spin-orbit interaction and their superposition generates an array of plasmonic vortices. Three of these vortices can form spin textures that carry non-trivial topological charge8 resembling magnetic meron quasiparticles9. These spin textures are localized within a half-wavelength of light, and exist on the timescale of the plasmonic field. We use ultrafast nonlinear coherent photoelectron microscopy to generate attosecond videos of the spatial evolution of the vortex fields; electromagnetic simulations and analytic theory confirm the presence of plasmonic meron quasiparticles. The quasiparticles form a chiral field, which breaks the time-reversal symmetry on a nanometre spatial scale and a 20-femtosecond timescale (the 'nano-femto scale'). This transient creation of non-trivial spin angular momentum topology pertains to cosmological structure creation and topological phase transitions in quantum matter10-12, and may transduce quantum information on the nano-femto scale13,14.

Inversion of the longitudinal component of spin angular momentum in the focus of a left-handed circularly polarized beam

It has been shown theoretically and numerically that in the sharp focus of a circularly polarized optical vortex, the longitudinal component of the spin angular momentum vector is inverted. Moreover, if the input light to the optical system is left-hand circularly polarized, it has been shown to be right-hand polarized in the focus near the optical axis. Since this effect occurs near the focus where a backward energy flow takes place, such an inversion of the spin angular momentum can be used to detect the backward energy flow.

Spin angular momentum density in the tight focus of a light field with phase and polarization singularities

For a light field with both phase and polarization singularities at its center, expressions are obtained that describe the distribution of the spin angular momentum (SAM) density in the sharp focal spot of an aplanatic system. These expressions include the radial, azimuthal, and longitudinal SAM components. As special cases, focusing of optical vortices with radial, azimuthal, and saddle polarizations is studied. Using the Bessel beam as an example, it is shown that in some areas in the focal plane the longitudinal SAM component is zero (resulting in a photonic wheel), while in others it is an order of magnitude less than the transverse component.

Pseudo-topological property of Julia fractal vector optical fields.

Polarization singularities have topological properties, because they can maintain their features invariably during propagation. The topological property can be destroyed by shifting the polarization singularities away from the central axis, and this destruction originates from the space separation of spin angular momentum components. We find that paired centrosymmetric off-axis polarization singularities can recover the topological property in the Fourier plane (reciprocal space), which belongs to the pseudo-topological property. We reveal that the pseudo-topological property is related to the invisible redistribution of both spin and orbital angular momentum states. We experimentally generate a series of Julia fractal vector optical fields with the pseudo-topological property. They may have potential applications in optical encryption and quantum information.

Mass-polariton theory of sharing the total angular momentum of light between the field and matter

Light propagating in a nondispersive medium is accompanied by a mass density wave (MDW) of atoms set in motion by the optical force of the field itself [Phys. Rev. A 95, 063850 (2017)]. This recent result is in strong contrast with the approximation of fixed atoms, which assumes that atoms are fixed to their equilibrium positions when light propagates in a medium and which is deeply rooted in the conventional electrodynamics of continuous media. In many photonic materials, the atoms carry the majority of the total momentum of light and their motion also gives rise to net transfer of medium mass with a light pulse. In this work, we use optoelastic continuum dynamics combining the optical force field, elasticity theory, and Newtonian mechanics to analyze the angular momentum carried by the MDW. We show that accounting for the MDW in the analysis of the angular momentum gives for the field's share of the total angular momentum of light a quantized value that is generally a fraction of $\hbar$. In contrast, the total angular momentum of the mass-polariton (MP) quasiparticle, which is a coupled state of the field and the MDW, and also the elementary quantum of light in a medium, is an integer multiple of $\hbar$. The same coupling is found for orbital and spin angular momentum components. The physical picture of the angular momentum of light emerging from our theory is fundamentally more general than earlier theoretical models, in which the total angular momentum of light is assumed to be carried by the electromagnetic field only or by an electronic polariton state, which also involves dipolar electronic oscillations. These models cannot describe the MDW shift of atoms associated with light. We simulate the MDW of Laguerre-Gaussian pulses in silicon and present a schematic experimental setup for measuring the contribution of the atomic MDW to the total angular momentum of light.

Parallel sorting of orbital and spin angular momenta of light in a record large number of channels.

Parallel sorting of orbital and spin angular momentum components of structured optical beams is demonstrated. Both spin channels are multiplexed within the novel orbital angular momentum (OAM) sorter, reducing the size, weight, and number of elements. The sorted states are linearly spaced over 70 topological charge values. We experimentally and theoretically evaluate the operational range and crosstalk between neighboring channels and find that 30 orbital angular momentum states are available per spin channel for quantum communication or cryptography. This is achieved using an angular momentum sorter that we developed based on geometric phase optical elements. We present two devices consisting of liquid crystal polymer films photoaligned with complex two-dimensional patterns.

Angular momentum switching and orthogonal field construction of C-points.

In this Letter, we take on the non-trivial problem of transforming a C-point singularity into its orthogonal state by switching its angular momentum components. For homogeneous distribution, orthogonal transformation is a trivial operation using a single half-wave plate. For C-point singularity, this entails a change in the handedness without disturbing the index, followed by rotation of the state of polarizations in the distribution. Swapping the spin angular momentum (SAM) components of C-point singularities leads to index and handedness inversion, whereas, switching of orbital angular momentum (OAM) components results only in handedness inversion. By changing the SAM and OAM components in sequence, a C-point can be transformed into its orthogonal state. While experimentally demonstrating this, a spiral phase plate, which is a phase element, is shown to perform polarization transformation operation.

Orbital angular momentum of Laguerre–Gaussian beams beyond the paraxial approximation

We derive a full field solution for Laguerre-Gaussian beams consistent with the Helmholtz equation using the angular spectrum method. Field components are presented as an order expansion in the ratio of the wavelength to the beam waist, f=λ/(2πw₀), which is typically small. The result is then generalized to a beam of arbitrary polarization. This result is then used to reproduce the signature angular momentum properties of Laguerre-Gaussian beams in the paraxial limit. The subsequent higher-order term is similarly obtained, which does not display a clear separation of orbital and spin angular momentum components.

Approximate Good Quantum Numbers in Cs Stark States**The project supported by National Natural Science Foundation of China

Using Cs atom zero-field wavefunctions from a kind of atomic potential model, and the mean value of -component of spin angular momentum in each Stark state, we numerically investigate the approximate good quantum numbers in Cs Stark states. Our results quantitatively confirm the concept that and , can be taken as the approximate good quantum numbers in two cases, except for some Stark states in a few special field points or ranges; however we also find that and , cannot be treated as the approximate good quantum numbers for these special Stark states.

High Magnetic Field Spin-Lattice Relaxation Rates in the Ground Quartet State of a Cr3+ Ion in Ruby

Relaxation-rates in 4 A 2 state of a Cr 3+ ion in ruby have been studied by optical observation on population changes caused by application of magnetic field of rectangular pulses at 4.2 K. The direct one phonon emitting process is dominant in the field range from 47 kOe to 128 kOe. No transition occurs between the levels \(M_{S}{=}\frac{1}{2}\) and \({-}\frac{1}{2}\), where M S is the component of spin angular momentum along the field. The two types of transitions, Δ M S =2 and Δ M S =1, are found to constitute the spin-lattice relaxation. The rates of these transitions are (7.66±0.15)×10 -4 H 2 ms -1 , and (7.65±0.20)×10 -5 H 2 ms -1 , resrectively, where H is the magnetic field strength in kOe.

Ion Polarization Via Penning Collisions with Optically Pumped Metastable Helium

Transfer of spin polarization from optically pumped metastable helium atoms to the ions of Cd, Sr, and Zn following Penning collisions has been demonstrated indicating that a component of spin angular momentum is conserved. The ion polarization, detected optically, exceeds 10% at ion concentrations on the order of ${10}^{10}$-${10}^{11}$ ${\mathrm{cm}}^{\ensuremath{-}3}$.

Nuclear Spin-Lattice Relaxation in Magnetic Insulators

First-order nuclear spin-lattice relaxation arises in magnetic insulators when a nuclear spin directly interacts with one or more spin waves via the hyperfine interaction. The direct process, in which a single magnon is emitted, is not ordinarily allowed on the basis of energy conservation, since the energy of a nuclear spin flip is considerably lower than the minimum energy of a spin wave. When the hyperfine interaction is isotropic and the axes of quantization of the nuclear and electronic spins are collinear, the conservation of the $z$ component of spin angular momentum forbids the Raman process, in which a thermal magnon is scattered to a state of different wave vector, accompanied by a nuclear spin flip. The three-magnon process is usually allowed. We consider here some second-order two- and three-magnon processes which arise when a virtual magnon, emitted in the direct process, is scattered by thermal magnons via the exchange interaction or the magnetic dipole-dipole interaction. This is possible, since (1) the magnon spectrum is lifetime broadened to overlap the nuclear spin resonance frequency, and (2) the dipole interaction does not conserve spin angular momentum. The second-order three-magnon process, arising from the four-magnon exchange interaction, enhances the three-magnon process relaxation rate by a factor of 8 in ferromagnets, and results in a temperature-dependent enhancement of about one order of magnitude in antiferromagnets. Three-magnon terms in the dipole-dipole interaction may induce a two-magnon process in both ferromagnets and antiferromagnets, which is of significance when the first-order Raman process is forbidden. We also calculate the relaxation rate due to second-order exchange-scattering-induced two-magnon process which is often more important than the first-order process in canted antiferromagnets of both the easy-axis and the hard-axis anisotropy type.