Terahertz magnon-polaritons control using a tunable liquid crystal cavity

TL;DR

This work demonstrates remote electrical control of terahertz magnon–polaritons in a Fabry–Perot cavity by embedding a highly birefringent liquid-crystal layer with a NiO antiferromagnetic slab. By biasing the liquid crystal, the dielectric environment and thus the spatial overlap between cavity modes and the NiO layer are tuned, modulating the magnon–photon coupling without current through the magnetic medium or external magnetic fields. The authors extract voltage- and temperature-dependent coupling strengths using a four-level coupled-oscillator model and corroborate the results with transfer-matrix simulations, revealing a route to voltage-programmable THz magnonic devices and noninvasive control strategies for spin-based information processing. The findings offer a practical platform for fast, room-temperature control of THz information transfer in opto-spintronics with potential wide applicability across insulating antiferromagnets.

Abstract

Strong coupling of light to a collective spin excitation in antiferromagnets gives rise to hybrid modes called magnon-polaritons. They are highly promising for data manipulation and transfer at terahertz rates, much faster than in the case of ferromagnetic magnon-polaritons, which operate at GHz frequencies. Yet, control of terahertz magnon-polaritons by the voltage, i.e. without ohmic dissipation losses, remains challenging. Here, we showcase the ability to remotely control antiferromagnetic magnon-polaritons at room temperature using an electric field by integrating a highly birefringent liquid crystal layer into a terahertz Fabry-Pérot cavity containing an antiferromagnetic crystal. Positioned several millimeters from the magnetic material, the liquid crystal allows for electrical manipulation of the cavity's photonic environment by control of its dielectric constant. This adjustment, in turn, influences the extent of magnon dressing by cavity photons, thereby controlling the vacuum Rabi oscillations of the magnon resonance coupled to a particular cavity mode. Our approach enables reversible tuning of magnon-photon hybridization that can be triggered without direct electrical contact or alteration of the magnetic medium. These findings pave the way for voltage-programmable terahertz magnonic devices and open new avenues for noninvasive control strategies in spin-based information processing technologies.

Figures (6)

• Figure 1: (a) Schematic of the Fabry-Perot cavity incorporating a liquid crystal cell and an antiferromagnetic NiO crystal. (b) Time-domain reflection traces from the Fabry-Perot cavity for two selected values of the voltage bias. (c) Schematic of the quasi-optical setup. (d) Schematic of the liquid crystal cell, with electrodes made of conducting polymer Electra 92 highlighted in yellow and layers of the PMMA resist in green.
• Figure 2: Reflection spectra of the Fabry-Perot cavity embedding a liquid crystal layer and an antiferromagnetic NiO layer plotted as a function of $T$ for voltage bias of $U=0$ V. The red dashed lines indicate the magnon-polariton energies obtained from the fit (see text), while the white and black dashed lines represent the energy of uncoupled magnon and consecutive Fabry-Perot cavity modes, respectively.
• Figure 3: Reflection spectra of the Fabry-Perot cavity embedding a liquid crystal layer and an antiferromagnetic NiO layer plotted as a function of voltage bias at $T=353$ K. The red, black, and white dashed lines indicate the magnon-polariton, uncoupled FP modes, and uncoupled magnon energies obtained from the fit.
• Figure 4: Temperature-dependent reflection spectra of the Fabry-Perot cavity for the voltage bias values selected in the range from $U=0$ V to $U_s=30$ V. The $U$ values are indicated above each panel, along with a graphical representation of the respective orientation of the liquid crystal molecules relative to the cell plane. The red dashed lines represent the energies of magnon-polariton branches obtained from the fitting of eigenvalues of Hamiltonian $H$ (Eq. \ref{['Hamiltonian']}), while the white and black dashed lines represent the energy of the uncoupled magnon and modes of the Fabry-Perot cavity, respectively.
• Figure 5: (a) Solid lines: reflection spectra of the Fabry-Perot cavity for selected values of voltage bias $U$ at the temperature of 353 K. The spectra are shifted vertically for clarity. The MP$_3$ and MP$_4$ transitions are indicated with blue and red arrows, respectively. Dashed lines: results of the fitting. (b) Amplitude and (c) FWHM of the MP$_3$ and MP$_4$ transitions as a function of $U$, respectively. (d) Tuning of the interaction strength between the magnon and Fabry-Perot cavity modes, $\Omega_2$ and $\Omega_3$, by the applied voltage bias. (e) $H$-field distribution of $P_2$ and $P_3$ for two selected values of $U$.