We present a degenerate four-wave mixing experiment on a silicon nitride (SiN) waveguide covered with gated graphene. We observe strong dependencies on signal-pump detuning and Fermi energy, i.e. the optical nonlinearity is demonstrated to be electrically tunable. In the vicinity of the interband absorption edge (2|EF | ≈ ω) a peak value of the waveguide nonlinear parameter of ≈ 6400 m −1 W −1 , corresponding to a graphene nonlinear sheet conductivity |σ

Silicon nitride (SiN) is emerging as a competitive platform for CMOS-compatible integrated photonics. However, active devices such as modulators are scarce and still lack in performance. Ideally, such a modulator should have a high bandwidth, good modulation efficiency, low loss, and cover a wide wavelength range. Here, we demonstrate the first electro-optic modulators based on ferroelectric lead zirconate titanate (PZT) films on SiN, in both the O-band and C-band. Bias-free operation, bandwidths beyond 33 GHz and data rates of 40 Gbps are shown, as well as low propagation losses (α ≈ 1 dB cm−1). A half-wave voltage-length product of 3.2 V cm is measured. Simulations indicate that further improvement is possible. This approach offers a much-anticipated route towards high-performance phase modulators on SiN.

The nonlinear optical properties of graphene have received significant interest in the past years. Especially third-order nonlinear effects have been demonstrated to be large. Recently several groups have shown, through four-wave mixing (FWM) and third harmonic generation (THG) experiments, that the optical nonlinearity of graphene can be tuned through electrostatic gating. These effects are quantified by a strongly tunable |σ s(3) |, with σ s (3) the complex third-order conductivity. Here, by simultaneously observing cross-phase and cross-amplitude modulation on a silicon nitride waveguide covered with gated graphene, we are able to confirm such strong tunability for these nonlinear effects as well. Moreover, we can separately quantify the real and imaginary parts of σ s(3) , which respectively represent nonlinear absorption and refraction. This unveils a tunability that is far more drastic than what could be observed through FWM or THG, including sign changes in both the nonlinear absorption and refraction. Our results are confirmed by a theoretical model for the optical nonlinearity of graphene. The ability to tailor the nonlinearity of graphene to this extent can lead to new opportunities, such as nanophotonic devices with electrically tunable nonlinear properties.

Abstract:We demonstrate class I excitability in optically injected microdisk lasers, and propose a possible optical spiking neuron design. The neuron has a clear threshold and an integrating behavior, leading to an output rate-input rate dependency that is comparable to the characteristic of sigmoidal artificial neurons. We also show that the optical phase of the input pulses has influence on the neuron response, and can be used to create inhibitory, as well as excitatory perturbations.

Abstract:Recently, we have theoretically demonstrated that optically injected microdisk lasers can be tuned in a class I excitable regime, where they are sensitive to both inhibitory and excitatory external input pulses. In this paper, we propose, using simulations, a topology that allows the disks to react on excitations from other disks. Phase tuning of the intermediate connections allows to control the disk response. Additionally, we investigate the sensitivity of the disk circuit to deviations in driving current and locking signal wavelength detuning. Using state-of-the-art fabrication techniques for microdisk laser, the standard deviation of the lasing wavelength is still about one order of magnitude too large. Therefore, compensation techniques, such as wavelength tuning by heating, are necessary.