Tuning the strain-induced resonance shift in silicon racetrack resonators by their orientation

Integrated photonics interferometric interrogator for a ring-resonator ultrasound sensor

We present a compact integrated photonics interrogator for a ring-resonator (RR) ultrasound sensor, the so-called MediGator. The MediGator consists of a special light source and an InP Mach-Zehnder interferometer (MZI) with a 3 ×3 multi-mode interferometer. Miniaturization of the MZI to chip size enables high temperature stability and negligible signal drift. The light source has a -3 dB bandwidth of 1.5 nm, a power density of 9 dBm/nm and a tuning range of 5.7 nm, providing sufficient signal level and robust alignment for the RR sensor. The mathematical procedure of interrogation is presented, leading to the optimum MZI design. We measure the frequency response of the sensor using the MediGator, giving a resonance frequency of 0.995MHz. Further, high interrogation performance is demonstrated at the RR resonance frequency for an ultrasound pressure range of 1.47 - 442.4 Pa, which yields very good linearity between the pressure and the resulting modulation amplitude of the RR resonance wavelength. The measured signal time traces match well with calculated results. Linear fitting of the pressure data gives a sensor sensitivity of 77.2 fm/Pa. The MediGator provides a low detection limit, temperature robustness and a large measurement range for interrogating the RR ultrasound sensor.

On the origin of second harmonic generation in silicon waveguides with silicon nitride cladding

Strained silicon waveguides have been proposed to break the silicon centrosymmetry, which inhibits second-order nonlinearities. Even if electro-optic effect and second harmonic generation (SHG) were measured, the published results presented plenty of ambiguities due to the concurrence of different effects affecting the process. In this work, the origin of SHG in a silicon waveguide strained by a silicon nitride cladding is investigated in detail. From the measured SHG efficiencies, an effective second-order nonlinear susceptibility of ~0.5 pmV^-1 is extracted. To evidence the role of strain, SHG is studied under an external mechanical load, demonstrating no significant dependence on the applied stress. On the contrary, a 254 nm ultraviolet (UV) exposure of the strained silicon waveguide suppresses completely the SHG signal. Since UV irradiation is known to passivate charged defects accumulated in the silicon nitride cladding, this measurement evidences the crucial role of charged centers. In fact, charged defects cause an electric field in the waveguide that via the third order silicon nonlinearity induces the SHG. This conclusion is supported by numerical simulations, which accurately model the experimental results.