A common strategy for increasing the efficiency of Stimulated and Spontaneous FourWave Mixing (SFWM) in integrated optical devices, is to enhance the intensity of the propagating optical field through nanoscale geometrical engineering. Typically, this is accomplished by confining light into microresonators or into sequences of mutually coupled cavities. Usually these structures are treated as a whole, with tens or hundreds of repeating units. In these structures, long grange periodicity is deliberately sought to tailor the frequency wavevector band diagram, in order to increase the group index while keeping the group velocity dispersion as low as possible. Having to deal with a large number of unit cells inherently precludes the study of the impact of the internal degree of freedom on FWM. The implementation of complex and extended structures precludes the observation of FWM regimes which are in general hidden, or overhung, by the long range periodicity, and that can emerge only by acting at the single resonator level. In this work, we study SFWM in a system made by two Silicon microring resonators (photonic dimer) of radius 7 um, quality factor Q = 10000 and separation 53.1 um, which are indirectly coupled by means of two waveguides with a coupling gap of 160 nm. We independently change the inter-cavity phase f, and the resonator eigenfrequency detuning d, by respectively implementing a Peltier cell and micro-heaters placed on the top of the resonators. We experimentally and theoretically demonstrate that, in the parameter space spanned by (f, d), the efficiency of SFWM can be enhanced, left unchanged or being completely suppressed with the respect to a single uncoupled resonator. This plethora of regimes can not be easily resolved and accomplished in large structures, where the structural periodicity makes slow light to overwhelm any other side effect. Here, a FWM enhancement of 7 dB with respect to each single constituent of the molecule is demonstrated, and attributed to the increase of internal field enhancement of one of the resonator induced by the presence of the other. We theoretically prove that this phenomenon is linked to the excitation of a sub-radiant mode in the structure. FWM suppression arises from the coherent destructive interference between the signal waves generated in the two resonators which are scattered into a common waveguide channel. We find that in the region where SFWM is enhanced, also the efficiency of Spontaneous Four Wave Mixing, the quantum counterpart of the stimulated process, is increased. This opens a plenty of possibilities for the implementation of this device for the generation of correlated photon pairs. We suggest that pairs could be deterministically bunched into a single scattering channel with a brightness that overcomes the one of a critically coupled resonator. This could beat the effective 50% of losses which suffer All-Pass and Add-Drop resonators, and could show superior brightness with respect to asymmetric microrings or dual Mach-Zehnder-microrings devices, whose maximum achievable field enhancement is inherently limited by the level attained at critical coupling.
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.
A re-visitation of the well known free space Mach Zehnder interferometer is here reported. Coexistence between one-photon and two-photons interference from collinear color entangled photon pairs is investigated. This is seen to arise from an arbitrarily small unbalance in the arm transmittance. The tuning of such asymmetry is reflected in dramatic changes in the coincidence detection, revealing beatings between one particle and two particle interference patterns. Our configuration explores new physics of the real Mach Zehnder interferometer especially useful for quantum optics on a chip, where the guiding geometry forces photons to travel in the same spatial mode.