Ultrasensitive Polarization-Resolved Probing of Transient Dynamics in MoS$_2$ on Silicon Nitride Microresonators

We present an ultrasensitive technique for probing transient optical changes in atomically thin molybdenum disulfide (MoS$_2$) layers integrated onto silicon nitride (Si$_3$N$_4$) ring resonators. The MoS$_2$ is illuminated by a femtosecond laser, while a tunable near-infrared (NIR) continuous-wave laser probes the microresonator resonance. The NIR light polarization can be adjusted to either transverse electric (TE, parallel to the 2D material) or transverse magnetic (TM, perpendicular), a configuration that is impossible to achieve with conventional normal-incidence pump-probe techniques. By capturing the transmitted signal on a fast oscilloscope, we detect transient optical shifts with unprecedented sensitivity, observing phenomena over time scales ranging from picoseconds to microseconds. Our results reveal both a rapid, carrier-induced nonlinear optical shift in the resonance, and a slower thermo-optic transient. The ability to simultaneously measure these fast and slow dynamics offers new insight into the complex optoelectronic behavior of 2D materials when integrated with microresonators. This method provides a significant advance over traditional pump-probe approaches, enabling the detection of exceedingly small transient signals and opening new avenues for exploring the optical properties of atomically thin materials. Our findings highlight the potential of this approach for investigating polarization-dependent nonlinear effects, with applications in photonics, sensing, and optoelectronics.

💡 Research Summary

In this work the authors present a novel, ultra‑sensitive pump‑probe methodology for probing the transient optical response of atomically thin molybdenum disulfide (MoS₂) that is integrated onto silicon nitride (Si₃N₄) microring resonators. The device consists of a 500 nm‑thick Si₃N₄ waveguide formed into a racetrack resonator (≈50 µm radius) with a 5 µm‑wide multimode section that accommodates an exfoliated MoS₂ flake (≈6 nm thick, ~8 µm long). The resonator supports both transverse‑electric (TE) and transverse‑magnetic (TM) guided modes around the 1550 nm probe wavelength, with distinct free‑spectral ranges (FSR_TE = 0.92 nm, FSR_TM = 0.94 nm) and quality factors (Q_TE ≈ 1.1 × 10⁵, Q_TM ≈ 4.0 × 10⁴).

The experimental scheme uses a femtosecond pump (800 nm, 50 fs, 1 kHz) incident normal to the chip to generate carriers in the MoS₂ layer, while a tunable continuous‑wave (CW) laser near the resonant wavelength is coupled into the waveguide to act as the probe. A fiber‑based polarization controller allows the probe to be launched in either TE or TM polarization, a capability that is impossible in conventional normal‑incidence pump‑probe setups. The transmitted probe signal is amplified by an erbium‑doped fiber amplifier, filtered, and detected simultaneously by two photodiodes: a 10 GHz detector (PD2) for fast dynamics (0–2 ns) and a 3 MHz detector (PD3) for slow dynamics (0–30 µs). A 13.6 GHz real‑time oscilloscope records both channels, while the pump‑trigger signal from a small fraction of the pump beam ensures precise timing.

Analysis of the fast channel reveals an initial blue‑shift of the resonant wavelength (Δλ < 0) followed by a slower red‑shift (Δλ > 0). The blue‑shift is attributed to free‑carrier plasma dispersion: photo‑generated electrons and holes reduce the effective refractive index, causing the resonance to move to shorter wavelengths. The peak of this blue‑shift occurs at t₁ ≈ 160 ps for TE and t₁ ≈ 63 ps for TM. The difference is not due to intrinsic material response but stems from the distinct cavity lifetimes set by the Q‑factors; the higher‑Q TE mode stores energy longer, delaying the observable response. The subsequent red‑shift is interpreted as a thermo‑optic (or other nonlinear) increase in refractive index, superimposed on the carrier‑induced effect. Because the detector bandwidth and cavity decay limit the resolution of the fastest processes, the exact magnitude of the carrier‑induced shift cannot be fully resolved, but the relative strengths of TE versus TM responses can be extracted by accounting for the differing lifetimes.

The slow channel captured with the 3 MHz detector shows a response of opposite sign, indicating a net red‑shift due to heating. This signal rises around t₃ ≈ 260 ns and decays over several microseconds. Detailed finite‑element thermal simulations suggest two distinct thermal peaks: an early peak arising from rapid heating at the MoS₂‑waveguide interface, and a later, smaller peak associated with heat diffusion into the Si₃N₄ core. Both TE and TM modes exhibit identical timing for the thermal peaks, confirming that the thermo‑optic dynamics are polarization‑independent.

Polarization‑resolved analysis demonstrates that the TE mode experiences a larger carrier‑induced nonlinear index change than the TM mode, consistent with the electric field being parallel to the 2D layer in TE. Conversely, the thermo‑optic contribution is identical for both polarizations, reflecting the isotropic nature of heat diffusion in the waveguide stack.

By integrating the pump‑probe scheme directly on a high‑Q microresonator and exploiting the resonator’s sensitivity to sub‑picometer wavelength shifts, the authors achieve an unprecedented detection limit for transient refractive‑index changes in 2D materials. The method simultaneously resolves dynamics spanning over five orders of magnitude in time (tens of picoseconds to tens of microseconds) and distinguishes polarization‑dependent carrier effects from polarization‑independent thermal effects—capabilities that are inaccessible to conventional free‑space pump‑probe techniques.

The study therefore provides a powerful platform for investigating ultrafast nonlinear optics, carrier relaxation, and thermal transport in atomically thin materials integrated with photonic circuits. Potential applications include polarization‑selective all‑optical modulators, ultra‑sensitive temperature or refractive‑index sensors, and hybrid photonic devices that leverage the strong light‑matter interaction of 2D semiconductors within scalable silicon‑compatible platforms.