An international team of researchers recently discovered that when Janus transition metal dichalcogenides (TMDs)—atom-thick semiconductors—are triggered by light, they undergo a physical shift within their atomic lattice that may enable next-gen optical devices with tunable properties and behavior.
TMDs are two-dimensional (2D) materials and their electronic and optical properties strongly depend on its thickness. A molybdenum disulfide (MoS2) monolayer’s photoemission, for example, is much stronger than its bulk form. And the flexibility of a 2D monolayer allows it to be twisted and stacked to create more intriguing functionalities.
“Janus TMDs are special because the two chalcogen atoms ‘sandwiching’ the transition metal are different species,” explains Kunyan Zhang, a University of California, Berkeley postdoc. “One widely studied example is molybdenum sulfur selenide (MoSSe). This unique structure leads to an intrinsic electric field vertical to the 2D plane, which results in a range of properties absent in conventional TMDs.”
Second harmonic generation (SHG) is a process in which the frequency of light doubles after passing through a material. “This process is useful for converting light frequencies, probing material symmetry, and enabling quantum optical applications,” Zhang says. “While TMD materials exhibit particularly strong SHG responses, the way the SHG of Janus TMD materials can be tuned by different incident lights hadn’t been studied before.”
Deviation of symmetry
When the team measured the SHG signal of the Janus MoSSe/MoS2 structure, which is very sensitive to the crystal structure, it revealed a pattern different than the expected six-fold symmetry. This deviation of symmetry is further enhanced when the incident light energy matches the optical absorption of the material—and it suggests the light generates a force on the materials and moves the atoms slightly.
“It was surprising to see six-fold symmetry change as the incident wavelength changes, and this behavior only happens for MoSSe/MoS2 bilayers, not for MoSSe or MoS2,” says Zhang. “It suggests the coupling between MoSSe/MoS2 is essential for the observed light-induced behaviors.”
The symmetry change is driven by subtle strain effects, but the biggest challenge is to quantify strain accurately. “We used group theory, which connects structures with properties, to analyze the SHG signals and obtained the values of the small, directional strain in these ultrathin 2D layers,” says Zhang. “It involves careful calibration to ensure the interpretation is reliable.”
Simulations aided the team’s work. “We combined first-principles calculations with group-theory analysis to understand the symmetry breaking,” Zhang says. “The first-principles calculation results for different excitation wavelengths can be used to reconstruct the SHG signal of bilayer structures, which perfectly reproduce the experimental observations.”
Actively tune nonlinear optical responses
This work shows that “stacking asymmetry and interlayer coupling can be used to actively tune nonlinear optical responses,” says Zhang. “It opens a pathway toward programmable SHG devices, wavelength-dependent modulators, and ultrathin photonic components based on tailored 2D heterostructures.”
Next, the researchers plan “to explore mechanical control of the SHG patterns and extend the concept to other 2D materials,” Zhang says. “These tunable nonlinear responses have the potential to be integrated into compact photonic circuits.”
FURTHER READING
K. Zhang et al., ACS Nano, 19, 44, 38371–38380 (2025); https://doi.org/10.1021/acsnano.5c10861.
