Permanent mitigation of loss in ultrathin SOI high-Q resonators using UV light

TL;DR

This paper demonstrates a method to permanently reduce optical losses in ultrathin SOI high-Q resonators by using UV light to neutralize charge-related absorption, significantly boosting cavity quality factors.

Contribution

The study introduces UV light treatment to mitigate free-carrier absorption in ultrathin SOI resonators, achieving a substantial increase in quality factor from 60,000 to 500,000.

Findings

Loss reduced from 5 dB/cm to 0.9 dB/cm after UV treatment

Cavity Q-factor increased from 60,000 to 500,000

UV exposure causes permanent charge neutralization in Si$_3$N$_4$ layers

Abstract

In this paper, we demonstrate strip-loaded guiding optical components realized on a 27 nm ultra-thin SOI platform. The absence of physically etched boundaries within the guiding core suppresses majorly the scattering loss, as shown by us previously for a silicon nitride (Si$_3$N$_4$) platform [Stefan \textit{et. al.}, OL 40, 3316 (2015)]. Unexpectedly, the freshly fabricated Si devices showed large losses of 5 dB/cm, originating from absorption by free carriers, accumulated under the positively charged Si$_3$N$_4$ loading layer. We use 254 nm ultraviolet (UV) light exposures to neutralize progressively and permanently silicon nitride's bulk charge associated with diamagnetic K+defects. This in turn leads to a net decrease of electron concentration in the SOI layer, reducing thus the propagation loss down to 0.9 dB/cm. Detailed MOS-capacitance measurements on test samples were performed to monitor the UV-induced modification of the electronic properties of the system. The evolution of loss mitigation was directly monitored both by Beer-Lambert approach in waveguide transmission experiments, as well as through more accurate cavity linewidth measurements. In the last case, we demonstrate how intrinsic cavity $Q$'s boost from 60,0000 to up to 500,000 after UV treatment. Our results may open routes towards engineering of new functionalities in photonic devices employing UV-modification of space charges and associated local electric fields, unveil the origin of induced optical nonlinearities in Si$_3$N$_4$/Si micro-photonic systems, as well as envisage possible integration of these with ultra-thin SOI electronics.