High-frequency electro-optic measurement of strained silicon racetrack resonators.

The observation of the electro-optic effect in strained silicon waveguides has been considered a direct manifestation of an induced χ(2) nonlinearity in the material. In this work, we perform high-frequency measurements on strained silicon racetrack resonators. Strain is controlled by a mechanical deformation of the waveguide. It is shown that any optical modulation vanishes, independent of the applied strain, when the applied voltage varies much faster than the carrier effective lifetime and that the DC modulation is also largely independent of the applied strain. This demonstrates that plasma carrier dispersion is responsible for the observed electro-optic effect. After normalizing out free-carrier effects, our results set an upper limit of (8±3) pm/V to the induced high-speed effective χeff,zzz(2) tensor element at an applied stress of -0.5 GPa. This upper limit is about 1 order of magnitude lower than previously reported values for static electro-optic measurements.

Ultra-high-Q thin-silicon nitride strip-loaded ring resonators.

We report on the design, fabrication, and characterization of thin Si3N4 ultra-high-quality (UHQ) factor ring resonators monolithically integrated on a silicon chip. The devices are based on a strip-loaded configuration and operate at both near-infrared (NIR) and third-telecom wavelengths. This approach allows us to use a guiding Si3N4 core that is one order of magnitude thinner than what has been reported in the past for obtaining similar device performances. Our strip-loaded devices benefit from the absence of physically etched lateral boundaries to show minute light scattering and, therefore, reducing significantly scattering-related losses. Consequently, UHQs of 3.7×10(6) in the NIR and high-quality factors of up to 9×10(5) in the C-band were measured for the guiding material thickness of 80 nm and 115 nm, respectively. These first results are subject to further improvements that may allow employing strip-loaded resonators in nonlinear frequency conversion or quantum computing schemes within the desired spectral range provided by the material transparency.

Thermo-optical bistability with Si nanocrystals in a whispering gallery mode resonator.

We report on the observation of optical bistability in an integrated planar microresonator with embedded silicon nanocrystals (Si-ncs). The phenomenon originates from the thermo-optical modulation of the silica-embedded Si-ncs refractive index, which in turn alters the spectral position of the resonator mode. The estimated thermo-optical coefficient of the Si nanocrystalline material, dn/dT≈2.92×10(-5)> K(-1), is an order of magnitude lower than that of bulk silicon. Both time-resolved pump-and-probe experiments and numerical simulations confirm that the silica host is responsible for the heat dissipation from the resonator. Moreover, a negligible Q-factor degradation at pump powers as high as 100 mW, along with the absence of a fast component in time-resolved measurements, confirm the minute contribution from excited carriers effects. These observations, combined with the already published large third-order nonlinearities of Si-ncs (an order of magnitude larger than in bulk Si), make this system an outstanding candidate for low-power on-chip nonlinear comb generation.

Oscillatory vertical coupling between a whispering-gallery resonator and a bus waveguide.

We report on a theoretical and experimental study of the optical coupling between a whispering-gallery type resonator and a waveguide lying on different planes. In contrast to the usual in-plane geometry, the present vertical one is characterized by an oscillatory behavior of the effective coupling as a function of the vertical gap. This behavior manifests itself as oscillations in both the resonance peak waveguide transmission and the mode quality factor. An analytical description based on coupled-mode theory and a two-port beam-splitter model of the waveguide-resonator vertical coupling is developed for arbitrary phase-matching conditions and is successfully used to interpret the experimental observations.

Second-harmonic generation in silicon waveguides strained by silicon nitride.

Silicon photonics meets the electronics requirement of increased speed and bandwidth with on-chip optical networks. All-optical data management requires nonlinear silicon photonics. In silicon only third-order optical nonlinearities are present owing to its crystalline inversion symmetry. Introducing a second-order nonlinearity into silicon photonics by proper material engineering would be highly desirable. It would enable devices for wideband wavelength conversion operating at relatively low optical powers. Here we show that a sizeable second-order nonlinearity at optical wavelengths is induced in a silicon waveguide by using a stressing silicon nitride overlayer. We carried out second-harmonic-generation experiments and first-principle calculations, which both yield large values of strain-induced bulk second-order nonlinear susceptibility, up to 40 pm V(-1) at 2,300 nm. We envisage that nonlinear strained silicon could provide a competing platform for a new class of integrated light sources spanning the near- to mid-infrared spectrum from 1.2 to 10 µm.

Probing the spontaneous emission dynamics in Si-nanocrystals-based microdisk resonators.

As a possible cavity quantum electrodynamical system, unlike III-V quantum dots, Si-NCs are not considered ideal emitters for emission rate enhancement observations (Purcell effect). Here, we report on direct measurements of spontaneous emission rate enhancement of Si-NCs embedded in a whispering-gallery mode resonator at room temperature. Using time-resolved microphotoluminescence experiments, we demonstrate important lifetime reductions (approximately 70%) for Si-NCs coupled to cavity modes with respect to uncoupled ones. Comparing experiments with the theoretical Purcell enhancement in a bad emitter regime, we estimate effective linewidths of approximately 10 meV through which Si-NC emitters are coupled to cavity photons. Finally, our study provides an alternative method for the estimation of subnatural linewidths of quantum dots at room temperature.

Photoluminescence from (Si/SiO2)n superlattices and their use as emitters in [SiO2/Si]n SiO2 [Si/SiO2]m microcavities.

Si/SiO2 superlattices are recently under investigation to add optical functionality to silicon based microelectronics. In such superlattices quantum-confinement should drive Si to become a good light emitter. Emission properties can be further improved and controlled by placing the emitter in optical microcavities. In this paper emission properties of (Si/SiO2), superlattices grown by Low Pressure Chemical Vapour Deposition will be compared with the ones obtained by other growth techniques and the origin of the emission will be discussed. Emission properties can be further improved and controlled by placing the emitter in optical microcavities. Optical properties of microcavities produced with standard complementary metal-oxide-semiconductor techniques containing Si/SiO2 superlattices as light emitter will be reviewed and a comparison between properties estimated from calculations and experiments will be given.

Structural and optical properties of silicon nanocrystals grown by plasma-enhanced chemical vapor deposition.

Silicon nanocrystals (Si-nc) embedded in SiO2 matrix have been prepared by high temperature thermal annealing (1000-1250 degrees C) of substoichiometric SiOx films deposited by plasma-enhanced chemical vapor deposition (PECVD). Different techniques have been used to examine the optical and structural properties of Si-nc. Transmission electron microscopy analysis shows the formation of nanocrystals whose sizes are dependent on annealing conditions and deposition parameters. The spectral positions of room temperature photoluminescence are systematically blue shifted with reduction in the size of Si-nc obtained by decreasing the annealing temperature or the Si content during the PECVD deposition. A similar trend has been found in optical absorption measurements. X-ray absorption fine structure measurements indicate the presence of an intermediate region between the Si-nc and the SiO2 matrix that participates in the light emission process. Theoretical observations reported here support these findings. All these efforts allow us to study the link between dimensionality, optical properties, and the local environment of Si-nc and the surrounding SiO2 matrix.