Andrea Blanco-Redondo

Observation of soliton compression in silicon photonic crystals

Solitons are nonlinear waves present in diverse physical systems including plasmas, water surfaces and optics. In silicon, the presence of two photon absorption and accompanying free carriers strongly perturb the canonical dynamics of optical solitons. Here we report the first experimental demonstration of soliton-effect pulse compression of picosecond pulses in silicon, despite two photon absorption and free carriers. Here we achieve compression of 3.7 ps pulses to 1.6 ps with <10 pJ energy. We demonstrate a ~1-ps free-carrier-induced pulse acceleration and show that picosecond input pulses are critical to these observations. These experiments are enabled by a dispersion-engineered slow-light photonic crystal waveguide and an ultra-sensitive frequency-resolved electrical gating technique to detect the ultralow energies in the nanostructured device. Strong agreement with a nonlinear Schrödinger model confirms the measurements. These results further our understanding of nonlinear waves in silicon and open the way to soliton-based functionalities in complementary metal-oxide-semiconductor-compatible platforms.

Coupling mid-infrared light from a photonic crystal waveguide to metallic transmission lines

We propose and theoretically study a hybrid structure consisting of a photonic crystal waveguide (PhC-wg) and a two-wire metallic transmission line (TL), engineered for efficient transfer of mid-infrared (mid-IR) light between them. An efficiency of 32% is obtained for the coupling from the transverse magnetic (TM) photonic mode to the symmetric mode of the TL, with a predicted intensity enhancement factor of 53 at the transmission line surface. The strong coupling is explained by the small phase velocity mismatch and sufficient spatial overlapping between the modes. This hybrid structure could find applications in highly integrated mid-IR photonic-plasmonic devices for biological and gas sensing, among others.

Topological Optical Waveguiding in Silicon and the Transition between Topological and Trivial Defect States

One-dimensional models with topological band structures represent a simple and versatile platform to demonstrate novel topological concepts. Here we experimentally study topologically protected states in silicon at the interface between two dimer chains with different Zak phases. Furthermore, we propose and demonstrate that, in a system where topological and trivial defect modes coexist, we can probe them independently. Tuning the configuration of the interface, we observe the transition between a single topological defect and a compound trivial defect state. These results provide a new paradigm for topologically protected waveguiding in a complementary metal-oxide-semiconductor compatible platform and highlight the novel concept of isolating topological and trivial defect modes in the same system that can have important implications in topological physics.

Pure-quartic solitons

Temporal optical solitons have been the subject of intense research due to their intriguing physics and applications in ultrafast optics and supercontinuum generation. Conventional bright optical solitons result from the interaction of anomalous group-velocity dispersion and self-phase modulation. Here we experimentally demonstrate a class of bright soliton arising purely from the interaction of negative fourth-order dispersion and self-phase modulation, which can occur even for normal group-velocity dispersion. We provide experimental and numerical evidence of shape-preserving propagation and flat temporal phase for the fundamental pure-quartic soliton and periodically modulated propagation for the higher-order pure-quartic solitons. We derive the approximate shape of the fundamental pure-quartic soliton and discover that is surprisingly Gaussian, exhibiting excellent agreement with our experimental observations. Our discovery, enabled by precise dispersion engineering, could find applications in communications, frequency combs and ultrafast lasers.

Controlling free-carrier temporal effects in silicon by dispersion engineering

Nonlinear silicon photonics will play an important role in future integrated opto-electronic circuits. Here we report temporal pulse broadening induced by the dynamic interplay of nonlinear free-carrier dispersion coupled with group-velocity dispersion in nanostructured silicon waveguides for the first time, to the best of our knowledge. Further, we demonstrate that the nonlinear temporal dynamics are supported or countered by third-order dispersion, depending on the sign. Our time-domain measurements of the subpicojoule pulse dynamics are supported by strong agreement with numerical modeling. In addition to the fundamental nonlinear optical processes unveiled here, these results highlight dispersion engineering as a powerful tool for controlling free-carrier temporal effects.

Nonlinear silicon photonics analyzed with the moment method

We apply the moment method to nonlinear pulse propagation in silicon waveguides in the presence of two-photon absorption (TPA), free-carrier dispersion, and free-carrier absorption (FCA). The evolution equations for pulse energy, temporal position, duration, frequency shift, and chirp are obtained. We derive analytic expressions for the free-carrier induced blueshift and acceleration and show that they depend only on the pulse peak power. Importantly, these effects are independent of the temporal duration. The moment equations are then numerically solved to provide fast estimates of pulse evolution trends in silicon photonic waveguides. We find that group-velocity and free-carrier dispersion dominate the pulse dynamics in photonic crystal waveguides. In contrast, two-photon and FCA dominate the temporal dynamics in silicon nanowires. To the best of our knowledge, this is the first time the moment method is used to provide a concise picture of free-carrier effects in silicon photonics. The treatment and conclusions apply to any semiconductor waveguide exhibiting TPA.

Photonic quantum walks with symmetry protected topological phases

We experimentally study the effect of the lattice topology on photonic quantum walks. We generate correlated photon pairs in an array of silicon nanowaveguides making use of the high-nonlinearity available in the system. By using single-photon measurements and propagation simulations we demonstrate the transition between the characteristic path entanglement in random quantum walks and topological localization of the quantum states. Further, we show that this topological localization is robust against disorder that preserves the chiral symmetry of the system.We experimentally study the effect of the lattice topology on photonic quantum walks. We generate correlated photon pairs in an array of silicon nanowaveguides making use of the high-nonlinearity available in the system. By using single-photon measurements and propagation simulations we demonstrate the transition between the characteristic path entanglement in random quantum walks and topological localization of the quantum states. Further, we show that this topological localization is robust against disorder that preserves the chiral symmetry of the system.

Soliton dynamics in semiconductor photonic crystals

Semiconductor optical waveguides have been the subject of intense study as both fundamental objects of study, as well as a path to photonic integration. In this talk I will focus on the nonlinear evolution of optical solitons in photonic crystal waveguides made of semiconductor materials. The ability to independently tune the dispersion and the nonlinearity in photonic crystal waveguides enables the examination of completely different nonlinear regimes in the same platform. I will describe experimental efforts utilizing time-resolved measurements to reveal a number of physical phenomena unique to solitons in a free carrier medium. The experiments are supported by analytic and numerical models providing a deeper insight into the physical scaling of these processes.

Local Field Enhancement of Mid-Infrared Light in an Integrated Photonic-Plasmonic Structure

We numerically study the local field enhancement of mid-infrared light beating the diffraction limit in an integrated photonic-plasmonic structure. The light is locally transferred from a photonic crystal waveguide to a metallic transmission line on top of it. The transmission line connects the two sections of the photonic crystal waveguide in a passage configuration. The field intensity is locally enhanced in the transmission line passage by a factor larger than 50, with a power transfer efficiency of 33%. This passage structure holds the promise of enabling highly sensitive miniaturized sensing schemes and mid-infrared spectroscopy equipment.

High-energy ultra-short pulses from pure-quartic solitons

Conventional optical solitons are remarkably stable pulses that arise from the balance of the nonlinearity and the anomalous quadratic dispersion in the medium in which they propagate, such as optical fibres [1] or silicon chips [2], Due to their self-reinforced stability, they are ideal candidates to generate transform-limited pulses from simple laser architectures [3], However, generating ultrashort high-energy pulses from soliton lasers is difficult, due to the spectral sideband generation of periodically perturbed solitons [4], which limits the shortest width, and the soliton energy scaling, which limits the energy at a given width. Consequently, laser architectures had to become more complex, with additional recompressing stages [5], Here, by means of experimental measurements, nonlinear Schrödinger equation (NLSE) simulations, and analytical developments, we show that the recently discovered pure-quartic solitons (PQS) [6] have the potential to outperform conventional solitons in yielding high-energy ultrashort pulses and bring soliton lasers back to the forefront of ultrafast laser research.