Room Temperature Anisotropic Photoresponse in Low-Symmetry van der Waals Semiconductor CrPS$_4$

Abstract: The crystalline and optical anisotropy of low-symmetry two-dimensional (2D) materials can enable strong dichroic responses, enhancing polarization contrast for photonic and optoelectronic devices. Here, we unveil pronounced optical and optoelectronic anisotropy in chromium thiophosphate CrPS$_4$ arising from the strong coupling between light polarization and its intrinsic crystal symmetry. Linearly polarized reflectivity and scanning photocurrent measurements in the 1.37-2.48 eV range reveal a robust dichroic response. The linear dichroism in reflection RLD reaches ~50, while in photocurrent PCLD it increases to ~60, with a sign reversal of the RLD between 1.6-1.8 eV, enabling strong narrow-band polarization contrast at room temperature. We attribute these anisotropic responses to the interaction between polarized light and Cr$^{3+}$ d-orbital T$_1$ and T$_2$ transitions. Spatially resolved photocurrent mapping further shows that the photocurrent is strongly dependent on the crystallographic axis: a 3-fold enhancement is obtained along the b-axis compared to the a-axis, yielding a clear 180° modulation of photoresponse across different contact orientations. Together, our findings establish CrPS$_4$ as a highly anisotropic 2D semiconductor with strong linear dichroism and polarization-sensitive photoresponse at room temperature. These characteristics highlight CrPS4 as a promising platform for narrow-band polarized photodetectors, anisotropic photo-transport, and future 2D spintronic and magneto-optical devices.

• The paper reveals a strong room-temperature optical anisotropy in CrPS4, evidenced by up to 60% photocurrent linear dichroism driven by d-orbital transitions.
• The methodology employs polarized reflectivity, Raman spectroscopy, and scanning photocurrent microscopy to correlate crystallographic axes with the photoresponse.
• The findings indicate CrPS4’s potential for integration into polarization-resolved photodetectors and spintronic devices under ambient conditions.

Room-Temperature Anisotropic Photoresponse in Low-Symmetry van der Waals Semiconductor CrPS$_4$

Introduction

This work systematically investigates the room-temperature optical and optoelectronic anisotropy of chromium thiophosphate (CrPS$_4$), a low-symmetry two-dimensional (2D) van der Waals (vdWs) semiconductor. Leveraging the intrinsic monoclinic symmetry of CrPS$_4$, the study demonstrates robust linear dichroism (LD) in both reflectivity and photocurrent, enabling polarization-sensitive photodetector applications. The authors provide a quantitative analysis of reflection and photocurrent linear dichroism (RLD and PCLD) across 1.37–2.48 eV and examine the role of d-orbital transitions in the observed phenomena.

Crystallographic and Optical Anisotropy

CrPS$_4$ differentiates itself from hexagonal TMDs (e.g., MoS$_2$, WS$_2$) by its monoclinic structure (C2/m or potentially lower C2), leading to reduced lattice symmetry and pronounced in-plane anisotropies. Polarized Raman spectroscopy allows precise determination of the a- and b-crystallographic axes, facilitating correlation of optoelectronic response with lattice orientation. The strong coupling between the incident light polarization and the crystal axes underpins the observed dichroism, with reflectivity and polarized photocurrent both exhibiting distinct 180° modulation as a function of incidence polarization.

Spectral Properties and Mechanisms

Quantitative measurements at room temperature reveal that the RLD reaches approximately 50% and the PCLD up to 60% near the T$_1$ and T$_2$ d–d transitions of Cr$^{3+}$ (1.6–1.9 eV). Notably, the RLD sign-reversal within 1.6–1.8 eV supports the existence of pronounced, spectrally sharp polarization selectivity, with a maximal differential reflectivity $\approx$70% within a $_4$0100 meV window. This is directly attributable to direction-selective optical transitions between the $_4$1A$_4$2 ground state and $_4$3T$_4$4/$_4$5T$_4$6 excited states.

The polarization sensitivity is corroborated by photoluminescence excitation (PLE) spectroscopy, revealing corresponding absorption peaks and confirming the direct linkage of the LD to fundamental crystal field excitations. The PCLD maintains a constant sign, reflecting the irreversible nature of carrier generation and collection mechanisms, whereas the RLD’s sign inversion captures complex refractive index anisotropy.

Photocurrent Anisotropy and Scanning Photocurrent Mapping

Device-level transport investigations utilizing scanning photocurrent microscopy unveil a pseudo-uniaxial photoresponse: the photocurrent along the b-axis exceeds that along the a-axis by nearly a factor of three. This anisotropy persists across multiple contact configurations, confirming the intrinsic nature of directional photoconductivity. The amplitude of the room-temperature PCLD matches or exceeds the cryogenic state performance of leading low-symmetry 2D vdWs photodetectors while operating in technologically relevant conditions.

Contact-proximate illumination and the localization of photocurrent support a dominant role for both the photovoltaic effect (at the Schottky interface) and the photothermoelectric effect, though conclusive separation of contributions would require further power- and gating-dependent studies.

Implications for 2D Optoelectronics and Spintronics

The demonstrated strong, room-temperature, energy- and direction-selective photoresponse in CrPS$_4$7 has several key implications:

• Device Integration: The combination of robust dichroism, simple device architecture, and ambient operation conditions provides an overview route for narrow-band polarization-resolved photodetectors without the need for external polarizers.
• Heterostructure Engineering: CrPS$_4$8 can be combined with other 2D materials possessing different symmetries or broken inversion/time-reversal symmetry, facilitating control over nonlinear optical responses and enabling new regimes of valleytronic and spintronic proximity effects.
• Magneto-Optical and Spintronic Devices: Given the coupling between the optical response and crystallographic axes, as well as the previously reported magnetic ordering in CrPS$_4$9, the observed anisotropic photoconductivity could interface with spin excitations and magnon transport for applications in reconfigurable photonic and spin logic circuits.

Outstanding Questions and Future Directions

The work motivates deeper study of:

• The explicit separation and quantification of PTE and PVE contributions through power- and gate-dependent photocurrent spectroscopy.
• Systematic thickness-dependent and strain-tunable measurements to resolve the impact on T$_4$0/T$_4$1 transition energies and dichroic response.
• Investigation into coherent spin-photoresponse interplays, leveraging the unique directional sensitivity for spin-orbitronics and optically addressable quantum devices.
• Integration of CrPS$_4$2 into complex heterostructures with strong spin/valley coupling materials to engineer emergent multi-component functionalities.

Conclusion

This study establishes CrPS$_4$3 as a uniquely anisotropic 2D vdWs semiconductor with highly tunable, direction-selective room-temperature photoresponse, far exceeding the typical performance bounds of prior 2D photodetectors in both magnitude and operational practicality. The strong linear dichroism and crystallographically controlled optoelectronic properties present immediate utility for polarized photodetection and form a foundational platform for further exploration of anisotropic photo-transport and magneto-optical interactions in 2D materials science.