Janus TMDs for light-based applications

1. From conventional TMDs to Janus materials

1.1 Conventional transition metal dichalcogenides

Transition metal dichalcogenides (TMDs) are a class of layered materials with chemical composition MX₂, where M is a transition metal such as molybdenum or tungsten and X is a chalcogen atom (typically sulfur, selenium, or tellurium). In their simplest form, a TMD monolayer consists of a single atomic plane of metal atoms sandwiched between two identical chalcogen layers. These monolayers are held together strongly in-plane, while adjacent layers interact only through weak van der Waals forces.

This layered structure makes TMDs particularly easy to exfoliate down to a single layer, where their properties change dramatically compared to the bulk. Many semiconducting TMDs become direct-bandgap materials at the monolayer limit, leading to strong absorption and emission of light. As a result, monolayer TMDs exhibit pronounced excitonic resonances and strong light–matter interaction, which have driven intense interest for applications in photonics, optoelectronics, and nonlinear optics.

A key characteristic of conventional MX₂ TMDs is their out-of-plane mirror symmetry: the top and bottom chalcogen layers are the same. This symmetry is essential in defining their electronic bands, optical selection rules, and nonlinear optical response.

1.2 New physics from stacking TMD monolayers

Additional functionality can be engineered by stacking TMD monolayers on top of one another. When layers are combined, the interaction between them can give rise to properties that do not exist in individual monolayers. Examples include interlayer excitons, tunable band alignments, and collective electronic states that depend on stacking order.

These van der Waals heterostructures demonstrate how breaking symmetry or introducing controlled interlayer coupling can be a powerful way to tailor material properties. While stacking multiple layers is one route to symmetry engineering, Janus TMDs take this idea a step further by embedding asymmetry directly into a single atomic layer.

1.3 Janus TMDs: intrinsic asymmetry at the atomic scale

Janus 2D TMDs are obtained by modifying conventional MX₂ monolayers through the replacement of one of the two chalcogen layers with a different chalcogen element. The resulting structure has the general formula MXY, where a single transition metal (usually Mo or W) is sandwiched between two distinct chalcogens (typically S, Se or Te). This elemental asymmetry gives rise to an intrinsically broken out‑of‑plane mirror symmetry in Janus monolayers.

This structural asymmetry has direct consequences. Janus TMDs exhibit a built‑in electric dipole perpendicular to the plane, which alters their electronic band structure and excitonic properties. Consequently, band energies, exciton binding, and light–matter interactions differ from those of conventional TMDs.

These modifications make Janus TMDs particularly sensitive to symmetry and excitation conditions, establishing them as a powerful platform for studying ultrafast and nonlinear optical processes.

1.4 Optical impact of broken symmetry of Janus TMDs

The absence of out-of-plane mirror symmetry, which is the defining characteristic of Janus TMDs, has a direct impact on their optical response. By presenting two chemically different surfaces, Janus TMD materials introduce an intrinsic dipole perpendicular to the crystal plane, which modifies the electronic bands and excitonic landscape compared to traditional MX₂ TMDs.

As a result, optical absorption and emission in Janus TMDs become more sensitive to excitation wavelength, polarization, and detection conditions. Excitonic resonances can shift, transition strengths can change, and optical responses may differ depending on whether the material is excited resonantly or off‑resonantly (at photon energies away from these excitonic resonances). Transitions that are weak or forbidden in MX₂ TMDs can then become optically active in Janus TMDs due to varied symmetry constraints.

Importantly, these symmetry‑driven effects are not purely static but are closely tied to the dynamics of excited states. Exciton formation, charge redistribution driven by the intrinsic dipole, and subsequent relaxation processes occur on ultrafast timescales, typically within femtoseconds to picoseconds after excitation. The optical response of Janus TMDs therefore depends not only on excitation energy, but also on how rapidly these processes evolve following photoexcitation.

These characteristics have important implications for how Janus TMDs are studied experimentally. Sensitivity to excitation wavelength, combined with ultrafast carrier and exciton dynamics, means that fully probing their optical behaviour requires both high temporal resolution and flexible control over excitation energy. Light sources that can be tuned across excitonic resonances and into off‑resonant regimes are therefore particularly well suited to exploring the symmetry‑driven properties of Janus TMDs.

In the following section, we discuss how wavelength tunable femtosecond sources, such as OPOs, support in-depth studies of these materials.

2. Optical excitation requirements for Janus TMD applications

Janus transition metal dichalcogenides exhibit optical properties that are fundamentally shaped by intrinsic out‑of‑plane asymmetry and strong excitonic effects. This combination leads to modified light–matter interactions, enhanced second‑order nonlinear responses, and ultrafast carrier dynamics that are highly sensitive to excitation wavelength and polarization. As a result, many application‑driven studies of Janus TMDs place stringent requirements on both temporal resolution and spectral control.

Ultrafast excitation is essential because key processes such as exciton formation, charge separation driven by built‑in dipoles, and relaxation dynamics occur on femtosecond to picosecond timescales. At the same time, these processes depend strongly on excitation energy. Resonant excitation of excitonic transitions is often required to maximize interaction strength and selectivity, while excitation away from resonances can highlight symmetry‑driven and nonlinear optical effects.

In addition to linear excitation regimes, Janus TMDs support strong nonlinear optical responses that arise directly from their broken mirror symmetry. Phenomena such as second‑harmonic generation exhibit a strong dependence on excitation wavelength and can be strongly enhanced near excitonic resonances. Studying these effects requires the ability to tune excitation wavelength systematically across relevant spectral regions.

These considerations highlight the importance of light sources that combine femtosecond pulse durations with broad, continuous wavelength tunability. Such sources provide the flexibility needed to address the diverse excitation regimes that underpin the most promising applications of Janus TMDs, which are discussed in the following section.

3. Applications of Janus TMDs enabled by ultrafast optics

3.1 Nonlinear and ultrafast photonics

One of the most immediate and actively explored application areas for Janus TMDs is nonlinear and ultrafast photonics. The intrinsic breaking of out‑of‑plane mirror symmetry gives rise to additional second‑order nonlinear susceptibility components that are absent in conventional MX₂ monolayers. As a result, Janus TMDs exhibit strong and anisotropic second‑harmonic generation, including out‑of‑plane contributions that enable new nonlinear optical functionalities.

The nonlinear optical response of Janus TMDs depends strongly on excitation wavelength and can be resonantly enhanced near excitonic transitions. Ultrafast, tunable excitation enables systematic study of this behaviour, allowing researchers to explore how nonlinear conversion efficiency, polarization dependence, and symmetry-specific tensor components evolve with excitation energy. These features make Janus TMDs attractive candidates for ultrathin frequency converters, ultrafast optical modulators, and integrated nonlinear photonic devices.

3.2 Optoelectronics and ultrafast photodetection

Beyond their nonlinear optical response, Janus TMDs also exhibit unique advantages in optoelectronic applications due to their intrinsic dipole and asymmetric structure.

This built-in electric field strongly affects charge separation and recombination after photoexcitation. Compared to symmetric TMD monolayers, this can lead to longer carrier lifetimes, spatial separation of electrons and holes and direction‑dependent responses.

These characteristics are of particular interest for ultrafast photodetectors and light‑harvesting devices. Femtosecond spectroscopy is an essential technique to understand the carrier dynamics, allowing time‑resolved measurements of exciton formation, charge transfer, and relaxation pathways. Moreover, wavelength tunability is key to select the right excitation of electronic states, providing valuable information into how excitation energy influences the optical response.

3.3 Valley‑selective and polarization‑dependent optical control

In addition to charge dynamics, Janus TMDs offer unique opportunities for controlling quantum degrees of freedom, particularly in the context of valley physics. Like conventional TMDs, Janus TMDs support valley‑selective optical excitation. However, the reduced symmetry and internal electric field in Janus structures modify optical selection rules and polarization‑dependent responses. This enables additional control over valley polarization dynamics and spin–orbit‑coupled excitonic transitions.

These properties are particularly relevant for emerging valleytronic applications, where information is encoded in the valley degree of freedom rather than charge or spin. In this context, the ability to control valley populations using the polarization and timing of optical excitation provides a powerful platform for ultrafast information processing. Time-resolved, polarization-controlled experiments lead to probe how valley polarization is created, evolves, and decays on femtosecond to picosecond timescales, offering important insights into scattering mechanisms, coherence lifetimes, and intervalley coupling processes.

This level of control is critical for the development of next-generation optoelectronic and quantum devices, such as valley-based logic elements, ultrafast optical switches, and polarization-sensitive photodetectors.

3.4 Energy conversion, photocatalysis, and sensing

Beyond optical and electronic functionalities, Janus TMDs are also gaining significant attention in energy conversion and sensing because their intrinsic electric field enhances charge separation and minimizes electron–hole recombination losses, addressing a key limitation in conventional photocatalytic and photovoltaic technologies.

This makes Janus TMDs particularly promising for solar fuel generation, where efficient charge separation is essential. Under illumination, electrons and holes are naturally driven toward opposite surfaces, which can be selectively functionalized with catalytic sites for specific reactions. This facilitates more efficient photocatalytic water splitting (hydrogen production) and CO₂ reduction, offering a pathway toward sustainable solar fuel generation. By minimizing recombination without the need for complex heterostructures, Janus materials provide a scalable approach to improving solar-to-chemical energy conversion efficiency.

In addition to energy applications, Janus TMDs are highly attractive for chemical and environmental sensing, as their distinct top and bottom surfaces can be independently functionalized for selective interactions. Changes in the local environment can be detected through, enabling sensitive, optically addressable sensing platforms.

Janus TMDs are also promising for chemical and environmental sensing, as their distinct top and bottom surfaces can be independently functionalized for selective molecular interactions. Changes in the local environment can be detected and monitored through variations in optical responses such as photoluminescence or nonlinear signals, making Janus TMDs a highly sensitive platform for chemical and environmental sensing.

4. Ultrafast light sources for Janus TMD research

The applications discussed in this white paper set clear requirements on the excitation source: femtosecond pulse durations, broad wavelength tunability, and long‑term stability.

Radiantis tunable femtosecond OPO systems meet these requirements, providing access to both resonant and off‑resonant excitation regimes across the visible, near‑infrared, and infrared spectral ranges, making them excellent tools for Janus TMD research.

TheInspire fs OPO offers gap-free tunability in the visible and near‑infrared (350-2500 nm), enabling resonant excitation of excitonic transitions, polarization studies and wavelength-dependent nonlinear measurements in Janus TMDs.

For nonlinear excitation schemes and off‑resonant studies, the ORIA IR provides broad tunability (1000-4000 nm) in the infrared, with the option to access shorter wavelengths by adding a SHG module, the ORIA VIS. This flexibility supports investigations of exciton‑enhanced nonlinear responses, symmetry‑dependent second‑order effects, and wavelength‑dependent harmonic generation in the near- and mid-IR.

5. Conclusions

Janus transition metal dichalcogenides introduce intrinsic out‑of‑plane asymmetry into atomically thin materials, giving rise to modified excitonic behaviour and enhanced nonlinear optical responses. These properties distinguish Janus TMDs from conventional MX₂ monolayers and make them particularly attractive for ultrafast and nonlinear photonics, optoelectronics, and emerging valley‑ and symmetry‑dependent applications.

Many of the most promising use cases rely on optical processes that are both ultrafast and strongly wavelength dependent, including exciton‑enhanced nonlinear effects and polarization‑selective dynamics. As a result, femtosecond excitation with flexible wavelength control plays a central role in both fundamental studies and application‑driven research on Janus TMDs.

As material quality and scalability continue to improve, Janus TMDs are expected to evolve from laboratory model systems toward functional photonic and optoelectronic components, with growing relevance for ultrafast and nonlinear photonics. Tunable femtosecond sources provide a versatile solution to support this development, making it possible to study resonant, off‑resonant, and nonlinear excitation regimes in these emerging two‑dimensional materials.

References

1. M. M. Petrić et al., Nonlinear Dispersion Relation and Out‑of‑Plane Second Harmonic Generation in MoSSe and WSSe Janus Monolayers, Advanced Optical Materials, 11, 2300958 (2023).
2. T. Zheng et al., Excitonic Dynamics in Janus MoSSe and WSSe Monolayers, Nano Letters, 21, 931–937 (2021).
3. Y. Wei et al., Second Harmonic Generation in Janus MoSSe Monolayer and Stacked Bulk with Vertical Asymmetry, Physical Chemistry Chemical Physics, 21, 21022–21030 (2019).
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