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

Abstract

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.

Figures (9)

• Figure 1: (a) Three-dimensional schematic of the Si$_3$N$_4$ microring resonator integrated with an MoS$_2$ flake. (b) Optical micrograph of the fabricated microring on a 500 nm Si$_3$N$_4$ layer deposited on a 3 µ m SiO$_2$ undercladding, showing an exfoliated MoS$_2$ flake (thickness $\sim 6$ nm, corresponding to $\sim9$--$10$ layers) positioned on top of the ring waveguide. (c) Magnified optical micrograph of the MoS$_2$ region. (d) Atomic force microscopy (AFM) image of the MoS$_2$ flake.
• Figure 2: (a) Measurement setup. (b) Normalized transmission spectra for TE (top) and TM (bottom) polarizations. FPC - fiber polarization controller, PD - photodetector, BS - beam splitter, SMF - singlemode fiber, SLD - superluminescent diode, EDFA - Erbium doped fiber amplifier.
• Figure 3: Normalized photoexcited transient changes in transmission ($\Delta T/T$, %) measured using (a) 10 GHz ($0\leq t \leq2~$ns) and (b) 3 MHz ($0\leq t \leq30~\upmu$s) photodetectors. The color plots in (i) and (ii) represent the transients as a function of wavelength and time for TE and TM polarizations, respectively. (iii) and (iv) represent the corresponding data at two different wavelengths ($\lambda_1<\lambda_r$, $\lambda_2>\lambda_r$) near resonant wavelength ($\lambda_r$). The time axis is shifted to start from 0 by subtracting the time delay between the ultrafast pulse and trigger.
• Figure 4: Extracted peak changes in photoexcited transient ($\Delta T/T$, %) measured across a single resonance for (a) TE and (b) TM polarizations. (i) Unexcited transmission spectra with theoretical fit, (ii) and (iii) represent the ($\Delta T/T$, %) recorded using fast (10 GHz) and slow (3 MHz) detectors, respectively. Both (ii) and (iii) are fitted with a theoretical perturbative resonator model (see Section \ref{['main:sup:perturbative']} of Supporting Information).
• Figure 5: Simulated and experimental transients for (a) TE and (b) TM polarizations, normalized to their respective TM responses. The operating wavelength is detuned near the FWHM ($\lambda < \lambda_r$).