Giant Helicity-Dependent Second Harmonic Generation in Type-2 Weyl Semimetal.

Area of Science:

• Condensed Matter Physics
• Materials Science
• Nonlinear Optics

Background:

• Second harmonic generation (SHG) probes Weyl semimetals' band structure via photocurrents.
• Circular photocurrents are sensitive to Weyl cone tilt, making SHG a probe of Weyl topology.
• WTe2 is a type-2 Weyl semimetal with potential for topological applications.

Purpose of the Study:

• Investigate the helicity-dependent nonlinear optical response in WTe2.
• Utilize equilibrium and ultrafast time-resolved SHG (TR-SHG) to probe topological attributes.
• Understand the role of photocurrent generation in SHG response.

Main Methods:

• Equilibrium and ultrafast time-resolved second harmonic generation (TR-SHG) measurements.
• Circular dichroism (CD) analysis of SHG signals.
• Quantum kinetic theory for quantitative analysis of helicity-dependent signals.
• Control experiments on TaAs (type-1 WSM).

Main Results:

• Observed 11% circular dichroism (CD) in equilibrium SHG, attributed to Weyl cone tilt.
• TR-SHG dynamics showed fast (~10 ps) and long-lived (~ns) components with A1 coherent optical phonon mode.
• Giant helicity-dependent modulation (CD ~40%) in TR-SHG dynamics observed.
• Injection current dominates SHG in overtilted Weyl cones, confirmed by control experiments on TaAs.

Conclusions:

• SHG is a powerful tool to monitor symmetry and topology evolution in real time.
• The observed helicity dependence in WTe2 is intrinsic to overtilted Weyl cones.
• Findings lay groundwork for helicity-controlled, ultrafast optoelectronic devices utilizing topological band structures.