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.

Bound electronic and free carrier nonlinearities in Silicon nanocrystals at 1550nm.

We present a detailed investigation of the different processes responsible for the optical nonlinearities of silicon nanocrystals at 1550 nm. Through z-scan measurements, the bound-electronic and excited carrier contributions to the nonlinear refraction were measured in presence of two-photon absorption. A study of the nonlinear response at different excitation powers has permitted to determine the change in the refractive index per unit of photo-excited carrier density sigma(r) and the value of the real bound-electronic nonlinear refraction n(2be) as a function of the nanocrystals size. Moreover at high excitation power, a saturation of the nonlinear absorption was observed due to band-filling effects.

Fast and Sensitive Detection of Paramagnetic Species Using Coupled Charge and Spin Dynamics in Strongly Fluorescent Nanodiamonds.

Sensing of a few unpaired electron spins, such as in metal ions and radicals, is a useful but difficult task in nanoscale physics, biology, and chemistry. Single negatively charged nitrogen-vacancy (NV-) centers in diamond offer high sensitivity and spatial resolution in the optical detection of weak magnetic fields produced by a spin bath but often require long acquisition times on the order of seconds. Here, we present an approach based on coupled spin and charge dynamics in dense NV ensembles in strongly fluorescent nanodiamonds (NDs) to sense external magnetic dipoles. We apply this approach to various paramagnetic species, including gadolinium complexes, magnetite nanoparticles, and hemoglobin in whole blood. Taking advantage of the high NV density, we demonstrate a dramatic reduction in acquisition time (down to tens of milliseconds) while maintaining high sensitivity to paramagnetic centers. Strong luminescence, high sensitivity, and short acquisition time make dense NV- ensembles in NDs a potentially promising tool for biosensing and bioimaging applications.

An all-optical single-step process for production of nanometric-sized fluorescent diamonds.

Nanodiamonds (NDs) containing negatively charged Nitrogen-Vacancy (NV) centers are promising materials for applications in photonics, quantum computing, and sensing of environmental parameters like temperature, strain and magnetic fields. However, the production of fluorescent NDs remains a technological challenge, requiring a complex multi-step process involving controlled introduction of substitutional nitrogen into the diamond lattice, annealing and fragmentation from macrocrystals to nanocrystals. Here, we report on a single-step, all-optical process for the production of nanometric-sized fluorescent diamonds based on laser ablation of a carbon substrate at low temperature (100 °C) under a nitrogen atmosphere. We demonstrate that this synthesis route yields fluorescent NDs with a concentration of native NV centers controlled by adjusting the experimental ablation conditions. Spin-polarization dependent optical-transitions are observed by optically detected magnetic resonance spectroscopy, thus providing strong evidence of the presence of negatively charged NV centers in the as-grown NDs. Finally, we propose a thermodynamic model able to describe the nucleation of NDs and the formation of NV centers in the present single-step optical process.

On the thermodynamic path enabling a room-temperature, laser-assisted graphite to nanodiamond transformation.

Nanodiamonds are the subject of active research for their potential applications in nano-magnetometry, quantum optics, bioimaging and water cleaning processes. Here, we present a novel thermodynamic model that describes a graphite-liquid-diamond route for the synthesis of nanodiamonds. Its robustness is proved via the production of nanodiamonds powders at room-temperature and standard atmospheric pressure by pulsed laser ablation of pyrolytic graphite in water. The aqueous environment provides a confinement mechanism that promotes diamond nucleation and growth, and a biologically compatible medium for suspension of nanodiamonds. Moreover, we introduce a facile physico-chemical method that does not require harsh chemical or temperature conditions to remove the graphitic byproducts of the laser ablation process. A full characterization of the nanodiamonds by electron and Raman spectroscopies is reported. Our model is also corroborated by comparison with experimental data from the literature.

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.