Koen Alexander

Excitability in optically injected microdisk lasers with phase controlled excitatory and inhibitory response

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.

Excitation transfer between optically injected microdisk lasers

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.

Electrically Tunable Optical Nonlinearities in Graphene-Covered SiN Waveguides Characterized by Four-Wave Mixing

Third order optical nonlinearities in graphene have been demonstrated to be large and have been predicted to be highly dependent on the Fermi energy of the graphene. This prediction suggests that graphene can be used to make systems with large and electrically tunable optical nonlinearities. In this work, we present what is to our knowledge the first experimental observation of this Fermi energy dependence of the optical nonlinearity. We have performed a degenerate four-wave mixing experiment on a silicon nitride (SiN) waveguide covered with graphene which was gated using a polymer electrolyte. We observe strong dependencies of the four-wave mixing conversion efficiency on the signal-pump detuning and Fermi energy, that is, the optical nonlinearity is indeed demonstrated to be electrically tunable. In the vicinity of the interband absorption edge (2|EF| ≈ ℏω), a peak value of the waveguide nonlinear parameter of ≈6400 m–1W−1, corresponding to a graphene nonlinear sheet conductivity |σs(3)| ≈ 4.3 × 10–19 A...

New materials for modulators and switches in silicon photonics (Conference Presentation)

In this presentation we will report on our recent work on new materials that can be monolithically integrated on high-index contrast silicon or silicon nitride photonic ICs to enhance their functionality. This includes graphene and other 2D-materials for realizing compact electro-absorption modulators and non-linear devices, ferroelectric materials for realizing phase modulators and adiabatic couplers for realizing bistable switches.

Hybrid graphene-silicon photonics devices

We will review state-of-the-art of hybrid graphene silicon photonics devices, discussing electro-absorption modulators, detectors and controllable saturable absorption.

Cascadable excitability in optically injected microdisks

All-optical spiking neural networks would allow high speed parallelized processing of time-encoded information, using the same energy efficient computational principles as our brain. As the neurons in these networks need to be able to process pulse trains, they should be excitable. Using simulations, we demonstrate Class 1 excitability in optically injected microdisk lasers, and propose a cascadable optical spiking neuron design. The neuron has a clear threshold and an integrating behavior. In addition, we show that the optical phase of the input pulses can be used to create inhibitory, as well as excitatory perturbations. Furthermore, we incorporate our optical neuron design in a topology that allows a disk 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. Finally, as the dynamical behavior of the microdisks is identical to the behavior in Semiconductor Ring Lasers (SRL), we compare the excitability mechanism due to optically injection with the previously proposed excitability due to asymmetry in the intermodal coupling in SRLs, as the latter mechanism can also be induced in disks due to, e.g., asymmetry in the external reaction. In both cases, the symmetry between the two counter-propagating modes of the cavity needs to be broken to prevent switching to the other mode, and allow the system to relax to its initial state after a perturbation. However, the asymmetry due to optical injection results in an integrating spiking neuron, whereas the asymmetry in the intermodal coupling is known to result in a resonating spiking neuron.