Massimo Borghi

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.

Chaotic dynamics in coupled resonator sequences

We report on the generation of chaotic signals in sequences of integrated coupled silicon resonators at telecommunication wavelengths. These can pave the way to on chip all optical random bit generators naturally compatible with silicon photonics.

Homodyne Detection of Free Carrier Induced Electro-Optic Modulation in Strained Silicon Resonators

In the last few years, strained silicon has been proposed as a potential electro-optic material, paving the way to the realization of ultrafast modulators which are compatible with the CMOS fabrication technology. The linear Pockels effect has been used for measuring the magnitude of the induced (2) components, with values reaching hundreds of pm/V. Recently, it has been shown that these values could have been overestimated due to the contribution of free carriers to the electro-optic modulation. In this work, this hypothesis is validated by a series of experimental observations, which are performed on strained silicon racetrack resonators. These are fabricated with different waveguide widths and orientations. We use a low frequency (KHz) homodyne detection technique to monitor the electro-optic response of the devices. The results indicate that the modulation strength is not dependent on the waveguide geometry or direction. A lot of anomalies are encountered in the device response, which are not compatible with a modulation mechanism of (2) origin. To this purporse, a theory based on the nonlinear injection of free carriers inside the waveguide is presented. This is able to account for all the observed anomalies.

An All Optical Method for Fabrication Error Measurements in Integrated Photonic Circuits

We propose an optical method to quantify the level of fabrication imperfections in a Silicon On Insulator wafer. The method is based on the use of Side Coupled Integrated Spaced Sequence of Resonators (SCISSOR) as test devices. Fabrication induced fluctuations of the effective index and of the coupling coefficient are mapped by comparing different spectral responses of nominally identical samples taken from different dies in the wafer. Random variations of the resonators optical path are quantified in terms of standard deviations of normally distribuited variables by finding a statistical correlation with the coupled resonator induced transparency (CRIT) phenomena. We found a strong correlation between CRIT and fabrication errors. This led us to design a SCISSOR based test structure that allows to quantify the degree of local structural imperfections in a fast, accurate and non invasive way. Performances, possible applications and limitations are investigated with the help of transfer matrix simulations.

Thermo-optic coefficient and nonlinear refractive index of silicon oxynitride waveguides

Integrated waveguiding devices based on silicon oxynitride (SiON) are appealing for their relatively high refractive index contrast and broadband transparency. The lack of two photon absorption at telecom wavelengths and the possibility to fabricate low loss waveguides make SiON an ideal platform for on-chip nonlinear optics and for the realization of reconfigurable integrated quantum lightwave circuits. Despite this, very few studies on its linear and nonlinear optical properties have been reported so far. In this work, we measured the thermo-optic coefficient dn/dT and the nonlinear refractive index n2 of relatively high (n ∼ 1.83 at a wavelength of 1.55 μm) refractive index SiON by using racetrack resonators. These parameters have been determined to be dndT=(1.84±0.17)× 10−5 K−1 and n2 = (7 ± 1) × 10−16 cm2W−1.

Tuning the strain-induced resonance shift in silicon racetrack resonators by their orientation

In this work, we analyze the role of strain on a set of silicon racetrack resonators presenting different orientations with respect to the applied strain. The strain induces a variation of the resonance wavelength, caused by the photoelastic variation of the material refractive index as well as by the mechanical deformation of the device. In particular, the mechanical deformation alters both the resonator perimeter and the waveguide cross-section. Finite element simulations taking into account all these effects are presented, providing good agreement with experimental results. By studying the role of the resonator orientation we identify interesting features, such as the tuning of the resonance shift from negative to positive values and the possibility of realizing strain insensitive devices.

Intermodal four wave mixing in silicon waveguides for on-chip wavelength conversion and generation (Conference Presentation)

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.

Controlling stimulated and spontaneous four wave mixing in coupled microring resonators (Conference Presentation)

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.

Oblique beams interference for mode selection in multimode silicon waveguides

Here, we propose to use the interference pattern which arises from the superposition of two coherent free space tilted beams at the input facet of an optical waveguide to excite selectively a given optical mode. By tuning the period of the interference fringes, it is possible to select the excited mode in the waveguide by maximizing the overlap integral with the modal optical field. Our setup is based on a free space interferometer that is theoretically capable of selectively exciting higher order modes in a micron-sized waveguide with an average cross-talk of 37 dB and a mode selectivity higher than 90%. The system is easily reconfigurable and can be straightforwardly integrated on a chip to enhance miniaturization, compactness, and stability.

Time resolved electro-optic measurements in strained silicon racetrack resonators

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 χeff,zzz(2) tensor component. This is an order of magnitude lower than the previously reported values for static electro-optic measurements.