Explaining interactions between light, heat, and charge carriers in silicon photonic microresonators

In an article appearing in the March 28th issue of Physical Review Letters, researchers from Northwestern University working with researchers from the NIST Center for Nanoscale Science and Technology (CNST) have presented a new analysis that accurately describes the behavior of silicon photon microresonators in the nonlinear regime, where the amount of light exiting the system is not directly proportional to the amount of light entering it. Their work includes simple equations to provide physical intuition and scaling rules that can be used to design new chip-scale photonic devices, including optically-driven oscillators and switches with potential applications as optical components in computing and communication systems.

The researchers studied a 10 µm diameter, 260 nm thick silicon microdisk resonator fabricated in the CNST NanoFab and optically probed with a continuous infrared laser. When the laser light entering the device is at low intensity, the fraction of that light exiting the device remains constant over time. However, as predicted by the researchers' theory, when the incoming laser intensity is increased past a certain threshold, the exiting light begins to oscillate. Its intensity varies periodically in time. Its frequencies spread across several hundred megahertz and consist of narrow spectral lines which are called a "frequency comb spectrum" because of their resemblance on a graph to the teeth of a comb.

The system's behavior is driven by two-photon absorption, a process by which light is absorbed in the silicon with a strength that depends on the square of the incoming light's intensity. This absorption heats the silicon, changes its refractive index, and generates free charge carriers (e.g., electrons), which further change the refractive index of the silicon and cause additional absorption. This overall interplay between light, heat, and the free charge carriers in the disk leads to a range of interesting behavior, including the experimentally observed oscillations and frequency comb spectrum.

The authors' new theoretical analysis of this behavior combines detailed numerical simulations with a combination of semi-analytic techniques that use approximations to simplify the equations describing the system. This analysis yields simple, yet experimentally verified, expressions for critical quantities such as the frequency comb spacing.

According to the researchers, future work will focus on experimentally studying other regimes of device behavior predicted by the theory and exploring the potential role of such devices in metrology applications.