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Optical devices which show different behaviors depending on the propagation direction of the light cannot be realized just by using optical passive systems in the linear regime because of the Lorentz reciprocity theorem (LRT). However, by using a non-Hermitian and nonlinear Taiji microresonator (MR) based on a silicon oxynitride (SiON) MR with an embedded S-shaped branch (see Fig. 1 (a) ) we have demonstrated non-reciprocity [1] , [2] . In this work, we study the role of the bus waveguide and we show that it is possible to break LRT for different propagation directions. In particular, we evidence the role of the bus waveguide position asymmetry with respect to the Taiji MR and of the Fabry-Perot (FP) oscillations generated by the waveguide facets.

The possibility to perform broad wavelength generation and conversion is of great interest in classical and quantum optics. With the introduction of integrated nonlinear silicon photonics, these tasks have been made even more viable thanks to the combination of the high nonlinear refractive index of silicon with the high confinement provided by the photonic waveguides. Exploiting this platform, several solutions for wavelength conversion have been investigated, for both optical processing [1] and applications in the Mid Infrared (MIR) [2]. The possibility to perform broad and controllable wavelength generation can be exploited also for creating integrated sources of quantum states of light based on single photons [3] or correlated photon pairs [4]. Both the generation and conversion can be accomplished by means of Four Wave Mixing (FWM). FWM is a nonlinear optical process in which two pump photons at frequency are converted into signal and idler photons at frequencies and ω/ respectively (with ω I /ω S ). In the particular case of wavelength conversion, FWM is stimulated by a seed wave at the same wavelength of the signal (idler) that will be converted to the wavelength of the idler (signal). FWM has to satisfy the energy conservation, according to which the relation ω P + ω P = ω S + ω I holds, and its efficiency is ruled by the phase matching condition. This latter condition is related to the phase mismatch among the waves involved in the FWM process, that is evaluated as Δk = k p + k p −k S −k I , where k P , k S , k I are the wavevectors for the pump, the signal and the idler, respectively. When Δk = 0, the phase matching condition is fulfilled and the efficiency of FWM is maximized. What is important for wavelength generation and conversion is the spectral position of the phase matching condition, which determines the wavelength at which the maximum efficiency of the conversion or generation process is achieved. Because of this, techniques for the control of the phase matching condition have been developed, focusing on the possibility to tailor the Group Velocity Dispersion (GVD) through a proper engineering of the waveguide geometry [5]. While these techniques consider only the first order waveguide mode, here we propose to exploit the higher order modes propagating inside a multimode waveguide. The dispersion of the higher order modes can be used to tune the spectral position of the phase matching condition for the FWM process, achieving controllable and large spectral translation in both Stimulated Four Wave Mixing (sFWM) and Spontaneous Four Wave Mixing (SFWM).

Silicon has a centrosymmetric crystalline structure that leads to a null second order nonlinear χ(2) coefficient. On the other hand, χ(2) effects are highly sought in Si to develop low power and fast electro-optic modulators. Recently, it has been reported that by stressing a Si waveguide with a silicon nitride over-layer, a sizeable χ(2) is induced. In [1] Second Harmonic Generation (SHG) measurements were reported in large area strained Si waveguides. However, the SHG data were affected by different contributions caused by the interfaces, the free carrier induced internal electric fields, the inhomogeneous strain and the uncontrolled modal structure of the waveguides. Here, we present SHG in Silicon On Insulator (SOI) strained Si waveguides, designed by a nonlinear propagation model. The design maximizes the SHG efficiency by a detailed simulation of the waveguide modal structure to achieve proper phase matching: n eff (λ p ) = n eff (λ p /2), (n eff is the effective refractive index and λ p is the pump wavelength). Since the refractive index is dispersive, this condition is never satisfied for the same optical mode. However, the phase matching condition can be satisfied by different optical modes (modal phase matching). The cross section of the resulting waveguides is sketched in Fig.1(a). These are SOI waveguides with different widths around 2 μm, an height of 250 nm and a 140-nm-thick overlayer of silicon nitride, which induces a tensile stress of +1.25 GPa in Si. Waveguides of different widths have been measured for different pump powers, polarizations and wavelengths. The laser source was a —100 fs tunable laser operating at 1 kHz repetition rate. The temporal width of the laser pulses was tailored to about 13 ps, by means of a pulse shaper, in order to avoid walk-off effects within the waveguide. The operating wavelengths were (2.4 : 2.6) μm, to eliminate two photon absorption. A SPAD InGaAs detector, externally triggered by the pump laser led to a lowest detectable power of P ∼ 0.01 fW. The experimental results are reported in Fig. 1(b). The SHG generated power scales with the square of the pump power, as expected. The optical modes involved in this process are: TBI for the pump and TM5 for SHG. No interface charge effects were observed and, therefore, we isolated in the measurements the effects of strain. We estimate a low value for the strain induced second order nonlinearity of χ2 = (0.304 ± 0.025) pm/V, which is actually an underestimation due to experimental issues. This value fits with other high frequency observations of the electro-optic effect in strained silicon [2].

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 χ(2) = (8 ± 3) pm/V to the value of the strain induced χ eff,zzz (2) tensor component. This is an order of magnitude lower than the previously reported values for static electro-optic measurements.

In this paper we demonstrate design, fabrication and characterization of both amorphous silicon (a-Si) and polycrystalline silicon (poly-Si) photoconductive detectors integrated on top of passive silicon oxynitride (SiON) circuit. The devices show the best responsivity of 0.5mA/W and 0.8A/W (under broadband illumination in the UV-VIS-NIR spectral region) and dark current of less than 0.23nA and 270nA at the bias of 5V for amorphous and polycrystalline silicon, respectively. The applicability of devices for lab-on-chip biosensor operating at 850nm has been proved by demonstration the ability of the detector to reproduce the sensor spectral response.

Tensile strained-silicon waveguides were produced by LPCVD (780 °C) by depositing a 150 nm-thick Si 3 N 4 overlayer. Nonlinear nanosecond and femtosecond transmission experiments indicated the presence of a significant χ(2) in the bulk silicon.

Q-factor tuning in stress-engineered microresonators is presented. Bent-edge microdisks (µ-kylix), where highest Q's are 60 nm blue-shifted with respect to flat disks, are used to probe Purcell enhancement of Si-nc emission rate in a wide spectral range.

We report on visible light emission from Si quantum dot (QD)-based optically active microdisk resonators with highest quality factors of up to 7000 at visible wavelengths, where Si QDs absorb strongly. Generally, contributions from various loss channels within optically active micro-resonator devices lead to limited spectral windows, referred to as Q-bands, where highest mode Q-factors manifest. Here we describe a strategy for tuning Q-bands using a new class of micro-resonators, in which engineered stress within an initially flat disk results in either concave or convex devices after post formation. To shift the Q-band by 60nm towards short wavelengths in conventional flat micro-disks, for example, a 50% diameter reduction is required, which causes severe radiative losses, suppressing Q's. With our bent devices we achieve similar tuning and even higher Q's by two orders of magnitude smaller diameter modification (0.4%). The phenomenon relies on geometry-induced smart interplay between modified dispersions of material absorption and radiative loss-related Q-factors. Importantly, this tuning scheme does not require larger device sizes, but rather utilizes self-adjustment properties of originally stressed resonator core. Remarkably, the bent resonators benefit from unmodified free-spectral range and cleaner WGM spectra due to the absence of higher order mode families.

We report on visible light WGM emission from Si-nanocrystal-based all-active microdisk resonators. The observed significant, 13-fold mode narrowing (Q~2800) at lowest excitation powers is attributed to the attenuation of pump-induced excited carrier absorption loss mechanism.