Abstract

Silicon photonics underpins a wide range of applications spanning sensing, communication, 
computation, and emerging quantum technologies. A common thread across these platforms 
is their strong sensitivity to refractive index, with many applications critically relying 
on its dynamic modulation. Achieving reliable and predictable device operation, therefore, 
requires accurate characterization of refractive index changes and a clear understanding of 
the underlying physical mechanisms. In silicon, refractive index modulation arises from 
multiple contributions, including thermal effects, free-carrier dispersion, and the Kerr 
nonlinearity. While several techniques have been developed to probe these mechanisms, 
they are often limited in the quantities they measure, restricting their broader applicability.

Cavity-enhanced photothermal spectroscopy is a widely used method to characterize Kerr nonlinearity. 
It employs a pump tone and a probe tone, both tuned to different resonances of the same optical cavity. 
The pump intensity is harmonically modulated, and the resulting oscillations in the probe intensity 
are monitored. Different dispersion mechanisms are studied through the strength of oscillation transfer. 
Typically, probe amplitude data are analyzed using numerical fits; however, these fits can be 
non-unique and can be contaminated by experimental artifacts. Phase data, a complementary observable, 
are often affected by phase artifacts.

We present a method to remove these experimental artifacts and extract the oscillation-transfer phase 
difference. An Erbium-Doped Fiber Amplifier (EDFA) is introduced before the pump intensity modulator. 
The amplified spontaneous emission (ASE) passes through the intensity modulator and experiences the 
same envelope modulation as the pump. A fraction of this ASE propagates through the probe path. Since 
this ASE leakage undergoes the same experimental artifacts as the probe, its phase is measured. 
The intrinsic phase information of oscillation transfer is then obtained by subtracting the leakage 
phase from the probe phase. Using this

approach, we confirm the presence of free-carrier dispersion in a silicon-on-insulator ring resonator cavity. 
This method provides a complementary extension to an established technique and remains applicable in regimes 
where conventional pump-based phase extraction fails.

Next, we investigate the nonlinear properties of ReS₂ by studying its effects on SoI ring resonators. 
All-optical resonance shift measurements and cavity-enhanced photothermal spectroscopy are performed 
before and after transferring ReS₂ onto the resonator. Significant inter-device variability is observed 
in both resonance shift and photothermal response. Intra-device variability is also seen when the same 
device undergoes different surface processes. A possible explanation is proposed based on existing literature 
on linear absorption loss in silicon, its surface-dominated nature, and its sensitivity to surface chemistry.

Additionally, we study on-chip metal–semiconductor–metal (MSM) and graphene photodetectors. 
For MSM devices, current–voltage characteristics and photo-response are measured. 
The responsivity and dynamic range of photoresponse are characterized. We show that these MSM 
devices are suitable for integration with optomechanical systems. For graphene photoconductors, 
we demonstrate successful fabrication and performance comparable to that reported in the literature. 
Finally, we suggest a direction for further exploration through the fabrication of 
MSM graphene–silicon–graphene devices using atomic force microscope lithography.

This work extends a well-established technique and improves its robustness. It provides new 
insights into dispersion in silicon-on-insulator microring resonators, highlighting the sensitivity 
of micro-cavity behavior to surface effects. Finally, it validates multiple directions for future 
exploration of MSM and graphene photodetectors.