Room-Temperature Anisotropic Plasma Mirror and Polarization-Controlled Optical Switch Based on Type-II Weyl Semimetal ${\mathrm{WP}}_2$

Anisotropy in electronic structures may ignite intriguing anisotropic optical responses, as has been well demonstrated in various systems including superconductors, semiconductors, and even topological Weyl semimetals. Meanwhile, it is well established in metal optics that the metal reflectance declines from one to zero when the photon frequency is above the plasma frequency ${\omega }_p$, behaving as a plasma mirror. However, the exploration of anisotropic plasma mirrors and corresponding applications remains elusive, especially at room temperature. Here, we discover a pronounced anisotropic plasma reflectance edge in the type-II Weyl semimetal ${\mathrm{WP}}_2$, with an anisotropy ratio of ${\omega }_p$ up to 1.5. Such anisotropic plasma mirror behavior and its robustness against temperature promise optical device applications over a wide temperature range. For example, the high sensitivity of polarization-resolved plasma reflectance edge renders ${\mathrm{WP}}_2$ an inherent polarization detector. We further achieve a room-temperature ${\mathrm{WP}}_2$-based optical switch, effectively controlled by simply tuning the light polarization. These findings extend the frontiers of metal optics as a discipline and promise the design of multifunctional devices combining both topological and optical features.

Figure 1

Structure characterizations of ${\mathrm{WP}}_2$. (a) Crystallographic structure. The black lines indicate a unit cell. (b) Optical images of the samples with different crystalline surfaces adopted for XRD and optical reflectance measurements. The orange squares in the background are $1\times 1{\mathrm{mm}}^2$. (c) TEM and selected area electron diffraction images along the [100] direction. The scale bar is 6 nm. (d) Atomic-resolution STM image of the cleaved (021) surface. The white rectangle denotes a surface unit cell. The scale bar is 1 nm. (e),(f) XRD patterns on the (001) and (010) surfaces, respectively. The insets show the rocking curves with a small full width at half maximum (FWHM).

Figure 2

Anisotropic reflectance and electronic structures of ${\mathrm{WP}}_2$. (a),(b) Measured reflectance spectra of the (001) and (010) surfaces, respectively. The light polarization $E$ in (a) is parallel to the $a$ (pink) and $b$ axes (blue), whereas $E$ in (b) is parallel to the $a$ (pink) and $c$ axes (yellow). The pink, blue, and yellow arrows indicate the reflectance valleys for $E||a$ ($V_a$), $E||b$ ($V_b$), and $E||c$ ($V_c$), respectively. The gray curves are the fitting curves based on the two-Drude model. (c) Plot of ${\omega }_p$ extracted from fitting the experimental data (circles) and from theoretical calculations (triangles), respectively. (d) Calculated reflectance spectra for $E||a$ (pink), $E||b$ (blue), and $E||c$ (yellow), respectively. The solid curves take into account both the intraband and interband excitations, whereas the dashed curves consider only interband excitations. (e) Calculated band structures along high-symmetry directions. (f) Calculated Fermi surfaces. The bow-tie-shaped closed pockets are electron Fermi surfaces, whereas the spaghetti-shaped open pockets are hole Fermi surfaces.

Figure 3

Polarization-resolved anisotropic reflectance of the ${\mathrm{WP}}_2$ (001) surface. (a) Reflectance spectra at various polarization angles $\theta$, where $\theta$ denotes the angle between the light polarization direction (direction of electrical field $E$) and the $a$ axis. Reflectance valleys $V_a$ and $V_b$ are indicated by the arrows. (b) Measured reflectances at the wave numbers of reflectance valleys $V_a$ ($\sim 3600{\mathrm{cm}}^{-1}$, red circles) and $V_b$ ($\sim 2700{\mathrm{cm}}^{-1}$, blue circles). The corresponding estimated reflectances are depicted as red and blue solid curves, respectively. (c) Polar plot of the measured and estimated reflectances from (b).

Figure 4

Polarization-controlled optical switch based on ${\mathrm{WP}}_2$. (a) Reflectance spectra of the (021) surface at various temperatures. The curves are shifted vertically for clarity. The light polarization is along the $a$ axis. (b) Schematic of the polarization-controlled optical switch apparatus. (c) Relative resistance change $\mathrm{\Delta }R/R$ of the ($\mathrm{Hg},\mathrm{Cd}$)$\mathrm{Te}$ thin film as a function of $\theta$, as induced by the reflected light of the ${\mathrm{WP}}_2$ (001) surface. The laser wavelength and wave number are $4.57\mu \mathrm{m}$ and $2188{\mathrm{cm}}^{-1}$, and the output power of the quantum cascade laser (QCL) is 20 mW (black circles) and 221 mW (blue circles), respectively. (d) Plot of $\mathrm{\Delta }R/R$ (blue circles) as a function of $\theta$, where laser power is 221 mW and measurements are taken at room temperature. The measured reflectance (red squares) of the ${\mathrm{WP}}_2$ (001) surface at $2188{\mathrm{cm}}^{-1}$ as a function of $\theta$ at room temperature.