[Hg3Se2]2- cluster drives giant optical anisotropy and broad infrared transparency

Abstract

Optical anisotropy, a fundamental physical property for polarization control, has long presented a critical consideration in the development of optical materials, particularly in terms of its modulation mechanisms and performance optimization. In the mid- to far-infrared region, simultaneously achieving large birefringence and broad transparency within a single material remains a major challenge. Herein, we report the synthesis of Hg₁₈Ga₈Se₈Cl₃₂ (HGSC), a crystalline material featuring linear [Hg₃Se₂] structural units. HGSC demonstrates a large birefringence of 0.871 at 546 nm, accompanied by the broadest transparency window among Hg-based chalcogenide single crystals (0.4 to 25 µm). Theoretical calculations reveal that the significant birefringence of HGSC originates from the well-aligned linear [Hg₃Se₂]^2- clusters, which exhibit the highest polarizability anisotropy (δ = 430) among all reported birefringence-active functional units. The demonstration of [Hg₃Se₂]^2- clusters as an effective bifunctional unit offers new opportunities for designing infrared photonic materials.

Fig. 1. Crystal-structural features of Hg₁₈Ga₈Se₈Cl₃₂.

a Coordination environment of Hg atoms in HGSC. b A one-dimensional helical channel along the b-axis. c A chair-like conformation of Hg₈Se₆ along the b-axis. d Alternating antiparallel stacking of helical channels and chair-like conformations along the c-axis. e Crystal structure of HGSC viewed along the b-axis.

Fig. 2. Comprehensive characterization of the growth, electronic structure, optical properties, and thermal responses of Hg₁₈Ga₈Se₈Cl₃₂ single crystals.

a Schematic illustration of the single-crystal growth process via CVT. b UV-Vis-NIR diffuse-reflectance spectrum; the inset shows the optical bandgap estimated from the Kubelka–Munk function. c Transmittance spectrum. d XPS spectrum. e Thermochromic behavior observed upon cooling from 300 K to 77 K. f Temperature-dependent evolution of lattice parameters a, b, and c for HGSC single crystals. g Comparison between experimental and calculated Raman spectrum (λ = 633 nm, T = 298 K); the inset shows an optical micrograph of the HGSC crystal. h Temperature-dependent Raman spectra measured from 83 K to 423 K.

Fig. 3. Polarizing microscope-based birefringence characterization of Hg₁₈Ga₈Se₈Cl₃₂ (140 K).

a Schematic illustration of the optical setup for birefringence measurement using a polarizing microscope. b Polarizing microscopy images of the HGSC crystal under cross-polarized conditions at various rotation angles (0° to 180°, step = 15°). c Microscopic image of the HGSC sample used for measurement, with a thickness of 14 μm. d, e Images captured during positive and negative rotation of the compensator at the characteristic wavelength of 546.1 nm for accurate determination of the optical path difference.

Fig. 4. Polarization modulation of incident lasers at different wavelengths through HGSC crystals.

a Schematic illustration showing the polarization state change of the linearly polarized light propagating through the HGSC crystal. b–d Polarization-dependent intensity of the input light (blue curve) and output light for HGSC crystals with different thicknesses: red for the 12.9 μm sample and yellow for the 14.0 μm sample. Dots represent experimental data, and solid lines denote fitting curves.

Fig. 5. Theoretical calculation results of Hg₁₈Ga₈Se₈Cl₃₂.

a Band structure. b TDOS and PDOS plots. c Calculated birefringence. d Birefringence contributions of [Hg₃Se₂] and [Hg₂Se₂] units calculated via real-space atom-cutting method. e Comparison of polarizability anisotropy values among various linear units.

Fig. 6. Comparison of birefringence and optical transparency in Hg-based chalcogenides and benchmark crystals.

a The comparison of birefringence and transmission range of Hg-based chalcogenide single crystals (dark blue: high optical transparency regions; light blue: transparent regions with significant optical absorption). b Comparison of the birefringence of HGSC with those of representative commercial birefringent crystals and recently reported state-of-the-art materials, the yellow-shaded region denotes the mid-IR spectral range^,,,,–.