Substrate Interference and Strain in the Second-Harmonic Generation from MoSe₂ Monolayers

Area of Science:

• Materials Science
• Condensed Matter Physics
• Optics

Background:

• Atomically thin 2D materials, particularly non-centrosymmetric transition metal dichalcogenides, exhibit tunable nonlinear optical properties.
• The second-order nonlinear susceptibility (χ(2)) in these materials can be modulated by applied strain, but reports on strain's effect are conflicting.

Purpose of the Study:

• To investigate the impact of controlled biaxial strain on the nonlinear optical response of molybdenum diselenide (MoSe2) monolayers.
• To resolve conflicting reports regarding strain's influence on second-order nonlinear susceptibility (χ(2)).

Main Methods:

• Growth of high-quality MoSe2 monolayers on different substrates (SiO2 and Si3N4) to induce controlled biaxial strain.
• Experimental characterization of linear and nonlinear optical responses.
• Theoretical calculations incorporating dielectric interference effects.

Main Results:

• Observed up to a 15-fold enhancement in second-harmonic generation (SHG) intensity for MoSe2 monolayers on SiO2 compared to Si3N4.
• Attributed a factor of 2 enhancement in χ(2) to biaxial strain, independent of substrate interference effects.
• Demonstrated that substrate interference and strain are distinct mechanisms for tuning SHG strength.

Conclusions:

• Biaxial strain is a powerful tool for enhancing the nonlinear optical properties of 2D materials.
• Substrate engineering, in conjunction with strain, offers independent control over second-harmonic generation intensity.
• These findings provide a pathway for designing advanced nonlinear optical devices based on 2D materials.