Optomagnonic Whispering Gallery Microresonators

Magnons in ferrimagnetic insulators such as yttrium iron garnet (YIG) have recently emerged as promising candidates for coherent information processing in microwave circuits. Here we demonstrate optical whispering gallery modes of a YIG sphere interrogated by a silicon nitride photonic waveguide, with quality factors approaching $10^6$ in the telecom $c$ band after surface treatments. Moreover, in contrast to conventional Faraday setups, this implement allows an input photon polarized colinearly to the magnetization to be scattered to a sideband mode of orthogonal polarization. This Brillouin scattering process is enhanced through triply resonant magnon, pump, and signal photon modes within an “optomagnonic cavity.” Our results show the potential use of magnons for mediating microwave-to-optical carrier conversion.

Figure 1

(a) Schematic illustration of the magnon-photon interaction. The YIG sphere is biased by a magnetic field along the $z$ direction, while the WGMs propagate along the perimeter in the $x\text{−}y$ plane. The TM input light excites the $\pi$ WGM in the YIG sphere, which is scattered by the magnon into a ${\sigma }^{+}$ polarized photon and then converts to the TE output in the waveguide. (b) Energy level diagram of the magnon-photon interaction. (c) Triple-resonance condition for the enhanced magnon-photon interaction process in the optomagnonic resonator. $m$ represents the magnon resonance (at frequency ${\omega }_m$). ${\sigma }^{+}$ and $\pi$ represent the two optical resonances with orthogonal polarizations (at frequency ${\omega }_{{\sigma }^{+}}$ and ${\omega }_{\pi }$, respectively).

Figure 2

[(a) and (b)] Schematic and optical image of the experimental assembly of our optomagnonic device, respectively. (c) Scanning electron microscope image of the polished YIG sphere. The scale bar is $100\mu \mathrm{m}$. (d) The surface of the YIG sphere before and after our surface treatment process. Scale bars are $1\mu \mathrm{m}$. The submicrometer particles vanish after the surface treatment. (e) Optical image of the silicon nitride coupling waveguide chip with glued fibers on the two sides. The chip and the fibers are attached to a piece of glass holder for mechanical support and to reduce long-term drift.

Figure 3

(a) Magnon resonances measured on a $300\text{−}\mu \mathrm{m}$-diameter YIG sphere biased at 1840 Oe. (b) The zoomed-in spectrum of the fundamental magnon mode. (c) Optical WGMs for both polarizations (${\sigma }^{+}$ and $\pi$) measured on the same YIG sphere using the silicon nitride coupling waveguide. The large extinction ratio and periodic mode distribution are evident. (d) and (e) are the zoomed-in spectrum for the two polarizations, respectively.

Figure 4

(a) Optical pump transmission and the generated optical signal as a function of pump laser wavelength. The correspondence of the generated optical signal peak and the optical pump resonance dip indicates the satisfaction of the conservation conditions. (b) Microwave reflection and the generated optical signal as a function of the microwave frequency. (c) Optical spectrum of the device output when the triple-resonance condition is satisfied. The TM and TE components of the output light are separated by a polarization beam splitter. The TM component corresponds to the direct transmission of the pump light, while the TE component contains the scattered sideband. (d) Power dependence of the sideband on the input microwave power $P_{\mathrm{MW}}$. Inset: extracted sideband power as a function of the input microwave power.