Encoding orbital angular momentum of light in magnets

Abstract

Breaking the diffraction limit and focusing laser beams to subwavelength scale are becoming possible with the help of recent developments in plasmonics. Such subwavelength focusing bridges different length scales of laser beams and matter. Here we consider optical vortex, or laser beam carrying orbital angular momentum (OAM) and discuss potential subwavelength magnetic phenomena induced by such laser. On the basis of numerical calculations using Landau-Lifshitz-Gilbert equation, we propose two OAM-dependent phenomena induced by optical vortices, generation of radially anisotropic spin waves and generation of topological defects in chiral magnets. The former could lead to the transient topological Hall effect through the laser-induced scalar spin chirality, and the latter reduces the timescale of generating skyrmionic defects by several orders compared to other known means.PhySH: Angular momentum of light, Ultrafast phenomena, Spin waves, Skyrmions, LLG Introduction-Realization of ultrashort laser pulses with femto-to pico-second width now offers a powerful tool for the study of non-equilibrium, ultrafast phenomena in solids. Since the pioneering observation of ultrafast demagnetization in Nickel by Beaurepaire et al in 1996 [1], such highly non-equilibrium, laser-induced physics is one of the most important research subjects in the field of magneto-optics [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16].So far, most of ultrafast magneto-optical phenomena in solids are explored using laser beams with Gaussian spatial profile. In 1992, however, Allen et al. proposed [17] a new type of laser beam, now called optical vortex [18]. Optical vortex is a beam carrying orbital angular momentum (OAM), which can be transferred to physical systems as, for example, mechanical rotation of (semi-) classical particles [19][20][21][22]. In the past decades, many applications such as super-resolution microscope [23] and optical ablation [24,25] are developed. However, the use of optical vortex for controlling microscopic magnetic degrees of freedom of solids is almost unexplored.The main difficulty in is in the mismatch between the timescales. Typically, the timescale of spin dynamics is in the Tera Hz (THz) or Giga Hz region, but the wavelength of optical vortex of such frequencies is too large for individual spins to "feel" its characteristic spatial profile. Using heating effect, as discussed in Ref. 26 is one option to resolve this issue, but here we take another approach: breaking the diffraction limit [27][28][29][30] of THz beams to realize the subwavelencth optical vortices [31]. Stimulated by the enormous successes for the microwave and visible light, breaking the diffraction limit of THz beams is actively explored and the technologies are rapidly developing recently with the help of plasmonics. We can now design the spatial profile of THz beams at the subwavelength scale [32] and actually focus the THz optical vortices [33] using designed metallic structures such as an array of antennas. Although the technology is s...

Magnetic Skyrmion Interacting with Optical Skyrmion

“…The same asymmetry appears in the magnetic response: Fig. 3(f) shows that the OSk excites an asymmetric multi-lobe spin-wave pattern even in a uniformly magnetized film without any MSk [42], evidencing an azimuthally nonuniform momentum transfer to the magnetic medium. By contrast, a single LG mode yields a fully symmetric profile (see Supplemental Material).…”

Magnetic Skyrmion Interacting with Optical Skyrmion

“…The same asymmetry appears in the magnetic response: Fig. 3(f) shows that the OSk excites an asymmetric multi-lobe spin-wave pattern even in a uniformly magnetized film without any MSk [42], evidencing an azimuthally nonuniform momentum transfer to the magnetic medium. By contrast, a single LG mode yields a fully symmetric profile (see Supplemental Material).…”

Fundamental theory of current-induced motion of magnetic skyrmions

“…The discussion based on the Thiele equation is also applicable to the skyrmion dynamics caused by these various physical mechanisms. In addition, it has turned out that skyrmions can be generated by various external stimuli such as magnetic fields [19,30,211,212], electric fields [183,184,188,[213][214][215][216][217][218][219], spin-polarized STM [99][100][101], light-induced local Joule heating [220][221][222][223][224][225], current-induced local Joule heating [226], pressures [227], tensile strains [228,229], circularly polarized microwave fields [230], and light with an angular momentum [231,232]. The Thiele equation is expected to provide important insights into the initial process of generation and the behaviors of skyrmions after generation.…”

Photonic orbital angular momentum transfer and magnetic skyrmion rotation

“…Optical vortex can transfer its OAM to skyrmions via a coherent Zeeman coupling between photon magnetic fields and local magnetic moments. One thus expect a skyrmion rotation after it absorbs the OAM of twisted lights, besides a possible ultrafast generation of skyrmions [45,46], because a rotating skyrmion manifests itself carrying orbital angular momentum [26,47]. In the following, we address this idea both theoretically and numerically.…”

Magnetic Skyrmion Interacting with Optical Skyrmion

“…The same asymmetry appears in the magnetic response: Fig. 3(f) shows that the OSk excites an asymmetric multi-lobe spin-wave pattern even in a uniformly magnetized film without any MSk [42], evidencing an azimuthally nonuniform momentum transfer to the magnetic medium. By contrast, a single LG mode yields a fully symmetric profile (see Supplemental Material).…”

Fundamental theory of current-induced motion of magnetic skyrmions

“…The discussion based on the Thiele equation is also applicable to the skyrmion dynamics caused by these various physical mechanisms. In addition, it has turned out that skyrmions can be generated by various external stimuli such as magnetic fields [19,30,211,212], electric fields [183,184,188,[213][214][215][216][217][218][219], spin-polarized STM [99][100][101], light-induced local Joule heating [220][221][222][223][224][225], current-induced local Joule heating [226], pressures [227], tensile strains [228,229], circularly polarized microwave fields [230], and light with an angular momentum [231,232]. The Thiele equation is expected to provide important insights into the initial process of generation and the behaviors of skyrmions after generation.…”

Photonic orbital angular momentum transfer and magnetic skyrmion rotation

“…Optical vortex can transfer its OAM to skyrmions via a coherent Zeeman coupling between photon magnetic fields and local magnetic moments. One thus expect a skyrmion rotation after it absorbs the OAM of twisted lights, besides a possible ultrafast generation of skyrmions [45,46], because a rotating skyrmion manifests itself carrying orbital angular momentum [26,47]. In the following, we address this idea both theoretically and numerically.…”

Magnetic Skyrmion Interacting with Optical Skyrmion

“…The same asymmetry appears in the magnetic response: Fig. 3(f) shows that the OSk excites an asymmetric multi-lobe spin-wave pattern even in a uniformly magnetized film without any MSk [42], evidencing an azimuthally nonuniform momentum transfer to the magnetic medium. By contrast, a single LG mode yields a fully symmetric profile (see Supplemental Material).…”

Fundamental theory of current-induced motion of magnetic skyrmions

“…The discussion based on the Thiele equation is also applicable to the skyrmion dynamics caused by these various physical mechanisms. In addition, it has turned out that skyrmions can be generated by various external stimuli such as magnetic fields [19,30,211,212], electric fields [183,184,188,[213][214][215][216][217][218][219], spin-polarized STM [99][100][101], light-induced local Joule heating [220][221][222][223][224][225], current-induced local Joule heating [226], pressures [227], tensile strains [228,229], circularly polarized microwave fields [230], and light with an angular momentum [231,232]. The Thiele equation is expected to provide important insights into the initial process of generation and the behaviors of skyrmions after generation.…”

Photonic orbital angular momentum transfer and magnetic skyrmion rotation

“…Optical vortex can transfer its OAM to skyrmions via a coherent Zeeman coupling between photon magnetic fields and local magnetic moments. One thus expect a skyrmion rotation after it absorbs the OAM of twisted lights, besides a possible ultrafast generation of skyrmions [45,46], because a rotating skyrmion manifests itself carrying orbital angular momentum [26,47]. In the following, we address this idea both theoretically and numerically.…”