Nonvolatile Switching of Large Nonreciprocal Optical Absorption at Shortwave Infrared Wavelengths

We report large nonreciprocal optical absorption at shortwave infrared (SWIR) wavelengths in the magnetoelectric (ME) antiferromagnet (AFM) ${\mathrm{LiNiPO}}_4$. The difference in absorption coefficients for light propagating in opposite directions, divided by the sum, reaches up to $\sim 40\%$ at 1450 nm. Moreover, the nonreciprocity is switched by a magnetic field in a nonvolatile manner. Using symmetry considerations, we reveal that the large nonreciprocal absorption is attributed to ${\mathrm{Ni}}^{2+}$ $d\text{−}d$ transitions through the spin-orbit coupling. Furthermore, we propose that an even larger nonreciprocity can be achieved for a Ni-based ME AFM where electric dipoles of every ${\mathrm{NiO}}_6$ unit and ${\mathrm{Ni}}^{2+}$ spins are orthogonal and, respectively, form a collinear arrangement. This study provides a pathway toward nonvolatile switchable one-way transparency of SWIR light.

Figure 1

Semilogarithmic plot of the magnitudes ($|\mathrm{\Delta }\alpha /{\alpha }_0|$) of the spontaneous optical diode (OD) effect or closely related effects (nonreciprocal directional or linear dichroism and magnetochiral dichroism) in the visible to near-infrared regions for various materials including ${\mathrm{LiNiPO}}_4$ investigated here (see the text), ${\mathrm{MnTiO}}_3$ (wavelength of 577 nm, $|\mathrm{\Delta }\alpha /{\alpha }_0|=0.1\%$) [15], $\mathrm{Pb}(\mathrm{TiO}){\mathrm{Cu}}_4{({\mathrm{PO}}_4)}_4$ (700 nm, 2%) [16], ${\mathrm{Bi}}_2{\mathrm{CuO}}_4$ (750 nm, 20%) [17], ${\mathrm{GaFeO}}_3$ (992 nm, 0.08%) [6], ${\mathrm{CuB}}_2{\mathrm{O}}_4$ (883 nm, 50%) [7]. For each material, the largest magnitude of $|\mathrm{\Delta }\alpha /{\alpha }_0|$ is plotted. The asterisks on ${\mathrm{GaFeO}}_3$ and ${\mathrm{CuB}}_2{\mathrm{O}}_4$ mean that the magnitudes for these materials were obtained in a weak but finite applied magnetic field: For ${\mathrm{GaFeO}}_3$ a spontaneous $|\mathrm{\Delta }\alpha /{\alpha }_0|$ should be finite but was not reported in the literature [6], while for ${\mathrm{CuB}}_2{\mathrm{O}}_4$ $|\mathrm{\Delta }\alpha /{\alpha }_0|$ at a zero field is largely reduced compared to that in a weak field possibly due to randomization of magnetic domains [7]. The yellow background highlights the SWIR region. The inset illustrates the OD effect featured by a difference in the optical absorption between two counterpropagating light beams. The orientation and length of the red double headed arrows denote the polarization direction and the intensity of light, respectively.

Figure 2

(a),(b) Three-dimensional views of crystal and magnetic structures of ${\mathrm{LiNiPO}}_4$ for a pair of antiferromagnetic domains $\text{A}+$ (a) and $\text{A}-$ (b). Only Ni (green balls) and O atoms (black balls) are depicted for clarity. Red arrows denote ${\mathrm{Ni}}^{2+}$ spins. The unit cell (gray box) contains four distorted ${\mathrm{NiO}}_6$ octahedral units, which are crystallographically equivalent to each other. The two domains can exhibit the OD effect of opposite signs, as represented by purple and green arrows whose thickness denote the intensity of light. (c) A $b$-axis view of the crystal and magnetic structures, indicating that the symmetry of ${\mathrm{NiO}}_6$ is approximated as $2_z{m}_x{m}_y$, where $x$, $y$, and $z$ denote local coordinate axes on each ${\mathrm{NiO}}_6$ unit. ${\theta }_{cz}\approx 35°$ is an angle between $c$ and $z$. Each Ni spin is predominantly oriented along the $c$ axis but cants towards the $a$ axis by an angle ${\theta }_S=\pm (7.8°\pm 2.6°)$. (d) Geometrical relationship among a local electric dipole $\mathbf{p}$ along the $z$ axis (blue arrows), the ${\mathrm{Ni}}^{2+}$ spin $\mathbf{S}$ (red arrows), and a quantity $\mathbf{p}\times \mathbf{S}$ (green crosses). The quantity $\mathbf{p}\times \mathbf{S}$ on every ${\mathrm{NiO}}_6$ unit aligns uniformly along the $y$ axis.

Figure 3

The optical diode effect and its nonvolatile switching. (a) Absorption coefficient ($\alpha$) spectra for ${\mathbf{E}}^{\omega }\parallel a$ and $\mathbf{k}\parallel$$+b$. The red and the blue-colored data were obtained after the field cooling procedure with $H_{\mathrm{cool}}=+5(+H_{\mathrm{cool}})$ and $-5\mathrm{kOe}(-H_{\mathrm{cool}})$, respectively. The $H_{\mathrm{cool}}$ was removed before each measurement to examine a nonvolatile effect. (b) Subtraction absorption spectra $\mathrm{\Delta }\alpha =\alpha (+H_{\mathrm{cool}})-\alpha (-H_{\mathrm{cool}})$ for $+k$ ($\mathbf{k}\parallel$$+b$, red curve) and $-k$ ($\mathbf{k}\parallel$$-b$, blue curve). (c),(d) Spectra of $\alpha$ (c) and $\mathrm{\Delta }\alpha$ (d) at for $E^{\omega }\parallel c$ measured in the same manner as in (a) and (b), respectively. (e) $\mathrm{\Delta }\alpha /{\alpha }_0$ spectra at for ${\mathbf{E}}^{\omega }\parallel a$ (red), ${\mathbf{E}}^{\omega }\parallel c$ (blue) and unpolarized light (gray curve). (f) Isothermal magnetic-field ($H$) dependence of $\alpha$ for ${\mathbf{E}}^{\omega }\parallel a$ at 1400 nm in $\mathbf{H}\parallel c$, measured after the cooling procedure with $+H_{\mathrm{cool}}$. All the data presented here were measured at 5 K.

Figure 4

(a) Energy scheme of ${\mathrm{Ni}}^{2+}$ orbital states for $O_h$ and $C_{2v}$ partly reproduced from [29]. (b),(c) Spectra of $\alpha$ (b) and $\mathrm{\Delta }\alpha$ (c) for ${\mathbf{E}}^{\omega }\parallel a$ (red solid curve) and ${\mathbf{E}}^{\omega }\parallel c$ (blue dotted curve) measured at 5 K in the visible and near-infrared regions. The vertical dashed lines on absorption bands indicate the positions of respective ${\mathrm{Ni}}^{2+}$ orbital states shown in the panel (a).