Second order nonlinearities are inhibited in centrosymmetric crystals, like silicon. However, in the last ten years many attempts have been carried out to induce second order nonlinear susceptibility applying a stressing layer of silicon nitride on the top of a silicon waveguide. Succesful experiments showed both Second Harmonic Generation (SHG) or electro-optic modulation in strained silicon waveguide. In order to develop new devices, a full comprehension of the origins of such a nonlinearity is needed. In fact, a lot of estimations of the second order nonlinear coefficient have been given, all different from each other and, in some cases, even contradictory.
In this work, we perform SHG in multimodal phase-matched silicon waveguides. We propose a way to individuate the origin of the nonlinearity, discriminating among the break of the centrosymmetry, the presence of charged states at the interfaces between silicon and silicon nitride and the overlap of the optical mode with the silicon nitride. We estimated a value of the second order nonlinear coefficient of 0.5 pm/V, demonstrating that it results from the coupling of the silicon third order nonlinear coefficient with the electric field induced by the presence of the trapped charges at the core/cladding interface.
We also show preliminary results on SHG in strained silicon microring resonators. Our results open the door to interesting applications, going from broad frequency conversion, to generation of quantum states of light, up to the generation of octave spanning frequency comb based on second order nonlinearities.
Silicon photonics is currently moving towards the Mid Infrared (MIR), which attracts plenty of emerging technologies, from integrated spectroscopy to quantum communications. However, the development of MIR-photonics is hindered by the lack of efficient detectors and light sources. A possible solution could be an integrated system able to link the MIR with the near infrared, where detectors and light sources have been already developed for telecommunications. Because of this, the possibility to perform broad and tunable wavelength conversion and generation is of great interest. In particular, the generation and conversion can be accomplished by means of Four Wave Mixing (FWM), a nonlinear optical process in which two input pump photons are converted into signal and idler photons of different frequency.
Crucial for efficient FWM is the phase matching condition, which determines the spectral position of the maximum efficiency of the process. In order to achieve large spectral translation between signal and idler, we propose to use Intermodal FWM (IMFWM), which exploits the dispersion of the higher order waveguide modes to achieve the phase matching condition. In IMFWM, the pump, signal and idler propagate on different waveguide modes. With respect to common phase matching techniques, IMFWM does not require anomalous GVD, resulting in an easier handling of the phase matching condition. Moreover, due to the sensitivity of the higher order mode dispersion with the waveguide geometry, the spectral position of the intermodal phase matching can be easily tuned by engineering the waveguide cross-section, achieving also large detunings from the pump wavelength. Another advantage is the high tolerance to the fabrication defects, related to the large widths of the multimode waveguides used.
In our work, we report the first experimental demonstration of spontaneous and stimulated on-chip IMFWM using Silicon-On-Insulator (SOI) channel multimode waveguides. We used a pulsed pump laser at 1550 nm with 10 MHz repetition rate and 40 ps pulse width. The excitation of the higher order modes is attained by displacing horizontally the input tapered lensed fiber with respect to the center of the waveguide facet.
We investigated an intermodal combination involving the pump injected on both the first and second order modes, the signal on the second order mode and the idler on the first order mode, with transverse electric polarization.
We used a 3.8-um-wide waveguide, of 1.5 cm length, to perform a spectral conversion of 140 nm with -21 dB efficiency. With the same waveguide, we measured -85 dB between the pump and the spontaneously generated idler. The coupled peak pump power was about 2 W.
We then measured the spectral position of the idler as a function of the waveguide width, achieving a maximum wavelength detuning between the idler and the signal wavelengths of 861 nm in a 2-um-wide waveguide, corresponding to the generation of 1231 nm idler and 2092 nm signal.
IMFWM enables effective and viable wavelength conversion and generation. It also promotes the development of emerging technologies, like mode division multiplexing and modal quantum interference, whose working principle relies on the higher order waveguide modes.
In this paper, we report on time resolved electro-optic measurements in strained silicon resonators. Strain is induced by applying a mechanical deformation to the device. It is demonstrated that the linear electro-optic effect vanishes when the applied voltage modulation varies much faster than the free carrier lifetime, and that this occurs independently on the level of the applied stress. This demonstrates that, at frequencies which lie below the free carrier recombination rate, the electro-optic modulation is caused by plasma carrier dispersion. After normalizing out free carrier effects, it is found an upper limit of (8 ± 3) pm/V to the value of the strain induced χ<sup>(2)</sup><i><sub>eff, zzz</sub> </i>tensor component. This is an order of magnitude lower than the previously reported values for static electro-optic measurements.
We report on a joint theoretical and experimental study of an analogue of the Lamb shift in the photonic framework. The platform is an integrated photonic device consisting of a single mode waveguide vertically coupled to a disk-shaped microresonator. The presence of a neighboring waveguide induces a reactive inter-mode coupling in the resonator, an effect analogous to an off-diagonal Lamb shift from atomic physics. Waveguide mediated coupling of different radial families results in peculiar Fano lineshapes in the waveguide transmission spectra, which manifests for different relative frequency shifts of the modes at different azimuthal numbers. Finally, a non-linear model for the dinamic tuning of the Fano lineshape under continuous wave pumping conditions is proposed.