Samuel Serna

Highly sensitive refractive index sensing by fast detuning the critical coupling condition of slot waveguide ring resonators

We experimentally investigate refractive index sensing in silicon slot waveguide ring resonators by the detection of the giant shift of the ring transmission spectrum envelope enabled by the following specific conditions: the slot waveguide cross section as well as the ring couplers have been designed to lead to a V-shaped microring resonator spectrum modulated by the classical frequency comb and exhibiting quality factor peaks of 2000-6000 around λ=1.5  μm. By tracking the spectrum envelope wavelength shift, sensitivity up to S=1,300  nm per refraction index unit (RIU) is reported when the slots are filled by liquids with refraction index values close to 1.33.

Analysis of silicon-on-insulator slot waveguide ring resonators targeting high Q-factors

Vertical slot waveguide micro-ring resonators in silicon photonics have already been demonstrated in previous works and applied to several schemes, including sensing and hybrid nonlinear optics. Their performances, first quantified by the reachable Q-factors, are still perceived to be restrained by larger intrinsic propagation losses than those suffered by simple Si wire waveguides. In this Letter, the optical loss mechanisms of slot waveguide micro-ring resonators are thoroughly investigated with a special focus on the coupler loss contribution that turns out to be the key obstacle to achieving high Q-factors. By engineering the coupler design, slotted ring resonators with a 50 μm radius are experienced with a loaded Q-factor up to 10 times improvement from Q=3,000 to Q=30,600. The intrinsic losses due to the light propagation in the bent slot ring itself are proved to be as low as 1.32±0.87  dB/cm at λ=1,550  nm. These investigations of slot ring resonators open high performance potentials for on-chip nonlinear optical processing or sensing in hybrid silicon photonics.

Nonlinear optimization of slot Si waveguides: TPA minimization with FOM(TPA) up to 4.25.

The χ(3) nonlinear properties of slotted crystalline silicon photonic waveguides filled with third-order nonlinear materials (NM) are studied by calculating the effective nonlinear susceptibilities associated to the silicon and cladding material, respectively. The adopted approach circumvents the assumptions that the introduced NM dominates the nonlinear behavior of the slotted waveguide and that strong light confinement in the slot allows neglecting the two-photon absorption (TPA) process in the silicon rails. Optimization of the geometry of silicon-slotted waveguides is performed on the basis of the nonlinear figure of merit (FOM(TPA)) of the guided mode, which is related to the balance between the Kerr and the TPA effects, allowing to reach a FOM(TPA)=4.25. The obtained results reveal the importance of properly choosing the waveguide width of the silicon rails in order to minimize the TPA effect even by tolerating a reduced overall nonlinearity.

Experimental GVD engineering in slow light slot photonic crystal waveguides

The use in silicon photonics of the new optical materials developed in soft matter science (e.g. polymers, liquids) is delicate because their low refractive index weakens the confinement of light and prevents an efficient control of the dispersion properties through the geometry. We experimentally demonstrate that such materials can be incorporated in 700 μm long slot photonic crystal waveguides, and hence can benefit from both slow-light field enhancement effect and slot-induced ultra-small effective areas. Additionally, we show that their dispersion can be engineered from anomalous to normal regions, along with the presence of multiple zero group velocity dispersion (ZGVD) points exhibiting Normalized Delay Bandwidth Product as high as 0.156. The reported results provide experimental evidence for an accurate control of the dispersion properties of fillable periodical slotted structures in silicon photonics, which is of direct interest for on-chip all-optical data treatment using nonlinear optical effects in hybrid-on-silicon technologies.

Enhanced nonlinear interaction in a microcavity under coherent excitation

The large field enhancement that can be achieved in high quality factor and small mode volume photonic crystal microcavities leads to strengthened nonlinear interactions. However, the frequency shift dynamics of the cavity resonance under a pulsed excitation, which is driven by nonlinear refractive index change, tends to limit the coupling efficiency between the pulse and the cavity. As a consequence, the cavity enhancement effect cannot last for the entire pulse duration, limiting the interaction between the pulse and the intra-cavity material. In order to preserve the benefit of light localization throughout the pulsed excitation, we report the first experimental demonstration of coherent excitation of a nonlinear microcavity, leading to an enhanced intra-cavity nonlinear interaction. We investigate the nonlinear behavior of a Silicon-based microcavity subject to tailored positively chirped pulses, enabling to increase the free carrier density generated by two-photon absorption by up to a factor of 2.5 compared with a Fourier-transform limited pulse excitation of equal energy. It is accompanied by an extended frequency blue-shift of the cavity resonance reaching 19 times the linear cavity bandwidth. This experimental result highlights the interest in using coherent excitation to control intra-cavity light-matter interactions and nonlinear dynamics of microcavity-based optical devices.

Integration of Carbon Nanotubes in Silicon Strip and Slot Waveguide Micro-Ring Resonators

Silicon photonics has emerged as a very promising technology platform for the implementation of high-performance, low-cost, ultra-compact circuits that can monolithically cointegrate electronic, opto-electronic and optic functionalities. However, Si neither has efficient light emission or detection in the telecom wavelength range, nor exhibits efficient electro-optic Pockels effect, hindering the implementation of integrated active devices like sources, detectors, or modulators. Current approaches relay on different materials to provide active functionalities in Si photonics, resulting in highly complex integration schemes that compromise cost-effectiveness. Semiconducting single-wall carbon nanotubes (SWNTs) are capable of emitting and detecting near-infrared light at room temperature and exhibit intrinsically fast electro-optic effects. They have also proven promising uses in micro-electronic devices, making them an ideal material to provide active functionalities in the Si photonic platform. In this work, we propose and experimentally validate the possible use of slot photonic waveguides to improve interaction between SWNTs and Si waveguide modes. Fabricated Si slot micro-ring shown an experimental ~ 60% photo-luminescence improvement compared to previous demonstration based on Si strip waveguide resonators. These results prove the potential of Si slot waveguides for the implementation of efficient SWNT-based Si photonic devices.

Nonlinear Properties of Ge-rich Si 1−x Ge x Materials with Different Ge Concentrations

Silicon photonics is a large volume and large scale integration platform for applications from long-haul optical telecommunications to intra-chip interconnects. Extension to the mid-IR wavelength range is now largely investigated, mainly driven by absorption spectroscopy applications. Germanium (Ge) is particularly compelling as it has a broad transparency window up to 15 µm and a much higher third-order nonlinear coefficient than silicon which is very promising for the demonstration of efficient non-linear optics based active devices. Si1−xGex alloys have been recently studied due to their ability to fine-tune the bandgap and refractive index. The material nonlinearities are very sensitive to any modification of the energy bands, so Si1−xGex alloys are particularly interesting for nonlinear device engineering. We report on the first third order nonlinear experimental characterization of Ge-rich Si1−xGex waveguides, with Ge concentrations x ranging from 0.7 to 0.9. The characterization performed at 1580 nm is compared with theoretical models and a discussion about the prediction of the nonlinear properties in the mid-IR is introduced. These results will provide helpful insights to assist the design of nonlinear integrated optical based devices in both the near- and mid-IR wavelength ranges.

Bi-directional top-hat D-Scan: single beam accurate characterization of nonlinear waveguides

The characterization of a third-order nonlinear integrated waveguide is reported for the first time by means of a top-hat dispersive-scan (D-Scan) technique, a temporal analog of the top-hat Z-Scan. With a single laser beam, and by carrying two counterdirectional nonlinear transmissions to assess the input and output coupling efficiencies, a novel procedure is described leading to accurate measurement of the TPA figure of merit, the effective two-photon absorption (TPA), and optical Kerr (including the sign) coefficients. The technique is validated in a silicon strip waveguide for which the effective nonlinear coefficients are measured with an accuracy of ±10%.

Experimental Investigation of Top Cladding on Properties of Silicon Slotted Photonic Crystal Waveguides

In this paper, we present an experimental study, validated by two different approaches, of the influence of the top cladding refractive index on the transmission and slow light properties of dispersion engineered silicon slot photonic crystal waveguides. We demonstrate that even though the operation wavelength is very sensitive to refractive index changes, the structures remain, nevertheless, robust in terms of group index, group velocity dispersion properties, and bandwidth figures of merit.

Silicon slot waveguide ring resonators: Can we target high Q factors?

We present our analysis and optimization of slot ring resonators by investigating the slotted coupler properties in order to achieve high Q ring resonators. Firstly, different ring resonators loaded by conventional straight bus ring resonator couplers with different gaps were fabricated with a cavity length of 828 μm and a bending radius of 50 μm. They were measured with an average Q-factor around 85,000 and proved to present large intrinsic coupler losses. We then proposed a new symmetric racetrack ring resonator coupler to reduce coupler loss thanks to a smoothed transition in coupling region between the coupled supermodes and achieved Q factor above 120,000 with total intrinsic loss around 2.2 dB/cm in a 50 μm radius ring resonator with total cavity length 326 μm by this approach. These optimized slot ring resonators could bring a new progress towards low loss silicon slot waveguiding structures for hybrid Si photonics including on-chip nonlinear optics and sensing in future.