Coherent pumping of high-momentum magnons by light

Abstract

We propose to excite a large number of coherent magnons with high momentum in optical cavities. This is achieved by two counterpropagating optical modes that are detuned by the frequency of a selected magnon, similar to stimulated Raman scattering. In sub-mm size yttrium iron garnet spheres, a mW laser input power generates 10 6 − 10 8 coherent magnons. The large magnon population enhances Brillouin light scattering, a probe suitable to access their quantum properties.Magnets are crucial for fast, non-volatile, and robust data storage as well as candidate materials for logic devices and interconnects [1]. Magnetic insulators, such as yttrium iron garnet (YIG) [2], are interesting since they can transport information over long distances via spin waves quantized into magnons [3,4], without the Ohmic dissipation of spin transport in metals. The magnons couple to microwaves [5][6][7], electric currents [1,3,8], mechanical motion [9][10][11][12], and light [13,14]. The high crystal quality of YIG promises long coherence times [6,7], opening prospects for 'quantum magnonics ' [15], the field that strives to employ magnons to store, process, and transfer information in a quantum coherent manner. Photons can become a coherent interface to manipulate and probe these magnons.The GHz magnons in ferro(ferri)magnets interact with light by inelastic (Brillouin) light scattering (BLS) [13]. By selecting the wave vector of the input and output photons, e.g. by an optical cavity, specific magnons modes can be excited [16]. The interaction can be large enough [17,18] to cool [19] or herald (generating single magnon states) [20] them, making BLS a promising probe into their quantum nature. Present experiment focus on the long wavelength 'Walker' (including the 'Kittel') magnons in optical resonators [21][22][23][24][25]. These have a small overlap with the light fields and corresponding low intrinsic scattering efficiency, but become observable because a large magnon density can be resonantly excited by microwaves. On the other hand, magnons with wavelengths ∼ 100 − 500 nm in the dipolar-exchange regime have almost perfect overlap with the photon modes in magnetic spheres [18], but couple only very weakly to microwaves (as do the relevant magnons in magnetic vortices [17]).Here, we propose to coherently pump a large number (∼ 10 6 − 10 8 ) of high-momentum magnons by optical lasers, similar to the resonant excitation of Kittel magnons by microwaves. We exploit the torques exerted by light on the magnetization by the inverse Faraday and Cotton-Mouton effects [26], which are proportional to the intensity of the electric field component [26] or, more precisely, the product of the photon numbers at the incident and scattered frequencies. Exposing the sample to two phase-coherent lasers that differ in frequency by a 1. A (massive) sphere of a magnetic insulator, such as YIG, with a proximity optical coupler, such as a fiber or a prism. Two oppositely propagating laser beams excite two whispering gallery modes with decay rates κ r,b . Th...

Cavity magnonics

“…Light is much less used to control the magnetic order [153]. The ability to manipulate magnetism in the strong optical interaction limit would enable a number of interesting fundamental experiments, such as optical cooling [264] or driving selected magnon modes [265].…”

Non‐Classical Correlations between Single Photons and Magnons

“…[26,27] Cavity optomagnonics, which support both photon modes and magnon modes in ferrimagnetic crystal YIG, provide a platform for studying magnon-photon interaction. [6,7,22,23,25,[28][29][30][31][32][33][34][35][36][37][38][39] Such interaction has been explored to demonstrate Brillouin light scattering [21][22][23][24][25] and microwave-to-optics frequency conversion. [28,40] However, the interaction strength is much smaller than the decay rates of both optical photon and magnon modes, which limits the microwave-to-optics frequency conversion efficiency.…”

Waveguide cavity optomagnonics for microwave-to-optics conversion

“…YIG exhibits very low dissipation for all these information carriers and in particular, as an optical material, it shows very low optical loss in the telecom c-band (0.13 dB/cm) [30]. Because of these advantages, the feasibility of using single crystalline YIG for realizing microwave-to-optical conversion is of significant interest [22,[31][32][33][34][35][36]. However, previous studies in this area mainly focused on bulk crystals with the uniform magnon mode (Kittel mode) [37], resulting in the low magneto-optical interaction strength and limited conversion bandwidth [18-21, 26, 38].…”