Anomalous In-Plane Anisotropic Harmonic Generation in Few-Layer PdSe₂

Area of Science:

• Materials Science
• Optics
• Condensed Matter Physics

Background:

• Anisotropic nonlinear optical effects in 2D materials are key for photonic devices.
• Understanding layer-dependent properties is crucial for device design.

Purpose of the Study:

• Investigate layer-dependent anisotropic second-harmonic generation (SHG) and third-harmonic generation (THG) in palladium diselenide (PdSe2).
• Explore the potential for controlling nonlinear optical responses in 2D materials.

Main Methods:

• Experimental investigation of SHG and THG in few-layer PdSe2.
• Tuning pump polarization and analyzing SHG orientation and ellipticity.
• Comparative analysis of THG amplitude with other 2D materials like molybdenum disulfide.

Main Results:

• Bilayer PdSe2 shows 4-fold symmetric polar response; four-layer PdSe2 exhibits a dumbbell-shaped polar pattern.
• Pronounced differences in SHG polarization orientation and ellipticity observed between bilayer and four-layer samples.
• Four-layer PdSe2 demonstrates strong coupling between SHG polarization and excitation ellipticity, yielding orthogonal orientations for circular polarizations.
• Significant layer-dependent THG anisotropy observed, with bilayer PdSe2 showing over 60-fold higher THG amplitude than monolayer MoS2.

Conclusions:

• PdSe2 exhibits unique layer-dependent nonlinear optical anisotropy.
• Intrinsic anisotropy allows direct control of SHG polarization via pump pulses.
• Findings advance the design of ultracompact nonlinear optical devices with tailored polarization properties.