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Nonlinear Optical Characterization of Monolayer Janus TMDs: SHG and THG Measurements for MoSSe and WSSe

Nonlinear optical interactions are profoundly influenced by material symmetry. An example is second harmonic generation (SHG), a nonlinear optical process that exclusively occurs in materials exhibiting inversion asymmetry.

In SHG, two photons of identical wavelength interact with a material, giving rise to a photon with double the frequency. The efficiency of this nonlinear process correlates directly with the second-order nonlinear susceptibility (χ(2)) components of the material.

At present, commercially available media with significant nonlinear optical properties predominantly consist of bulk crystals like beta barium borate (BBO), lithium triborate (LBO), or potassium titanyl phosphate (KTP). However, the increasing need for miniaturization in photonic and optoelectronic devices motivates researchers to explore materials that can maintain robust nonlinearity coefficients while being scaled down in size.

Layered materials present a promising alternative to bulk crystals, offering two distinct advantages. Firstly, they exhibit a high level of compatibility with on-chip photonic architectures. Secondly, their atomically thin nature, coupled with a robust nonlinear response, enables efficient frequency conversion without the limitations of phase matching. Consequently, they present a viable pathway toward developing ultra-broadband classical and quantum light sources.

In this context, the research labs of Prof. Jonathan Finley at the Center of Nanotechnology and Nanomaterials within the Walter Schottky Institute (WSI) in Garching, Germany, and Prof. Giancarlo Soavi at Friedrich-Schiller-Universität in Jena, Germany, have been focusing on the characterization of the nonlinear response of a distinctive class of layered materials known as Janus transition metal dichalcogenides (Janus TMDs).

Monolayers of Janus TMDs offer distinct advantages over conventional layered materials. Their unique benefit lies in incorporating out-of-plane components in the nonlinear optical response. This characteristic not only allows their integration into vertical photonic structures but also broadens the spectral region of resonantly enhanced nonlinear optics, extending it into the visible and near-infrared spectral regions.

The work of Marko Petrić, Dr. Viviana Villafañe and Paul Herrmann, led by Dr. Matteo Barbone, within the labs of Prof. Jonathan Finley and Prof. Giancarlo Soavi recently conducted measurements of the SHG and third harmonic generation (THG) nonlinear coefficients (χ(2) and third χ(3) respectively) of molybdenum sulfide selenide (MoSSe) and tungsten sulfide selenide (WSSe), two different types of Janus TMD materials.

Figure 1 – (a) Schematic representation of MoSSe and WSSe Janus TMD monolayers. (b) Second harmonic (SH, blue), third harmonic (TH, purple) generation and two-photon photoluminescence (TP-PL, light blue) transitions under two- and -three-photon excitation. Adapted figure from the original publication (link)

For the MoSSe monolayer, all components of the second-order susceptibility tensor χ(2), including out-of-plane components, were measured by means of polarized-resolved spectroscopy. The χ(2)yyy was found to be the component that showed the highest nonlinear efficiency and hence was selected to be experimentally measured for both, MoSSe and WSSe monolayers, as a function of the excitation wavelength, at room temperature (300 K) and at cryogenic temperatures (7 and 10 K).

The experiment was performed across the spectral range 0.8 – 1.15 eV (1550 – 1078 nm), using Radiantis Ti:Sapphire-pumped INSPIRE (link) femtosecond (fs) optical parametric oscillator (OPO), gap-free tunable across 345 – 2500 nm, with 100 fs pulses and a repetition rate of 80 MHz. Figure 2 shows a simplified version of the experimental set-up.

Figure 2 – Simplified experimental set-up for the wavelength-dependent SHG measurements, and SHG and THG measurements, using Radiantis Ti:Sapphire-pumped INSPIRE femtosecond OPO.

For both MoSSe and WSSe monolayers, two exciton transitions were observed at 1.7 – 1.8 eV (730 – 689 nm) and 1.9 – 2.3 eV (650 – 540 nm), where a clear SHG intensity enhancement is also present. The SHG intensity ratio between the two exciton resonances, marked as A or B in Figure 3, together with the non-linear susceptibility dispersion coefficient χ(2)yyy, varied with the sample temperature.

Figure 3 – SHG enhancement at exciton resonances and c(2)yyy dispersion. a,b) SHG spectra from MoSSe Janus monolayer as a function of two-photon absorption energy (bottom x-axis) and pump energy (top x-axis) at a) T = 300 K and b) T = 7 K with c(2)yyy represented in gray points within a light gray error band. c,d) SHG spectra from WSSe Janus monolayer as a function of two-photon energy and pump energy at c) T = 300 K and d) T = 10 K with c(2)yyy dispersion represented in gray points within a light gray error band. Assignments of A and B excitons are denoted by arrows. Adapted figure from the original publication (link).

In the case of the MoSSe monolayer, the spectrum is predominantly governed by the B exciton at room temperature (300 K), whereas at cryogenic temperatures (7 K), both A and B excitons exhibit comparable intensities. Conversely, for the WSSe monolayer, the dominance of the A exciton is evident at room temperature, with no distinct B exciton discernible. However, at cryogenic temperature (10 K), A and B excitons display comparable characteristics.

Measured second-order susceptibility values (χ(2)yyy) for MoSSe and WSSe Janus monolayers were ≈ 150 pmV-1 and 200 pmV-1, respectively. Notably, these values represent some of the highest reported second-order susceptibilities for any TMD monolayer to date.

The effective third-order susceptibility factors (χeff(3)) of MoSSe and WSSe monolayers were measured under the same experimental conditions, utilizing pump energies of 0.80 eV (1550 nm) and 0.83 eV (1494 nm), respectively. The χeff(3) values were determined to be 1.55 x105 pm2V-2 and 1.52 x105 pm2V-2 respectively, comparable to χeff(3) values of other TMDs reported in the literature.

This work was published in the 11th volume of Advanced Optical Materials (October 2023), and it was selected to be on the inside back cover of the magazine! (link)

It serves as a robust foundation for understanding the optical properties of Janus TMD monolayers. These monolayers exhibit promising optical attributes, which positions them as viable materials for developing next-generation on-chip photonic devices.

Radiantis offers a variety of broadly tunable lasers with high peak power in the visible and infrared regions that are a fundamental tool for characterizing the non-linear optical properties of layered materials like TMD crystal monolayers. An alternative solution to the Inspire, also provided by Radiantis, is the Oria IR OPO (link) which offers higher peak powers in the IR range. Check out our website for more information.

We express our gratitude for the collaboration of Prof. Jonathan Finley’s and Prof. Giancarlo Soavi´s labs, with special thanks to Marko Petrić, Dr. Viviana Villafañe and Dr. Matteo Barbone, for their contributions to the preparation of this application note.

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