No abstract

The linear dichroism (LD) transition within anisotropic photonic materials displays promising prospects for applications in polarization-wavelength-selective detectors, optical switching, and optical communication. In conventional two-dimensional (2D) anisotropic materials, the LD is predominantly uniaxial over a broad spectrum of wavelengths and arises principally from the reduced symmetry of the materials. However, the LD transition behavior in crystalline 2D materials remains elusive. Here, we demonstrate the observation of a unique LD conversion phenomenon at a wavelength of 472 nm in palladium diselenide (PdSe2) using polarization-resolved absorption spectroscopy. This material exhibits prominent anisotropic responses and a high absorption ratio of α y /α x ≈ 1.11 at 364 nm, 1.15 at 532 nm, and 0.84 at 633 nm. We propose that this abnormal LD conversion behavior originates from the forceful localization rules at different parallel energy bands that exist within this material. Furthermore, the robust periodicity of Ag and B1g modes in polarization-resolved Raman spectroscopy is in good agreement with the theoretical structure symmetry analysis. This indicates the strong intrinsic LD effect in the anisotropic nature of PdSe2, which offers a macrolevel determination of crystal orientations. Such unique LD conversion features, in combination with strong LD effects, enable the air-stable PdSe2 to be a potential candidate for technological innovations in multispectral imaging, sensing, and polarization-sensitive and wavelength-controllable photoelectronic applications.

Nonlinear optical effects in layered two-dimensional transition metal chalcogenides have been extensively explored recently because of the promising prospect of the nonlinear optical effects for various optoelectronic applications. However, these materials possess sizable bandgaps ranging from visible to ultraviolet region, so the investigation of narrow-bandgap materials remains deficient. Here, we report our comprehensive study on the nonlinear optical processes in palladium diselenide (PdSe2) that has a near-infrared bandgap. Interestingly, this material exhibits a unique thickness-dependent second harmonic generation feature, which is in contrast to other transition metal chalcogenides. Furthermore, the two-photon absorption coefficients of 1–3 layer PdSe2 (β ~ 4.16 × 105, 2.58 × 105, and 1.51 × 105 cm GW−1) are larger by two and three orders of magnitude than that of the conventional two-dimensional materials, and giant modulation depths (αs ~ 32%, 27%, and 24%) were obtained in 1–3 layer PdSe2. Such unique nonlinear optical characteristics make PdSe2 a potential candidate for technological innovations in nonlinear optoelectronic devices.