Julián L. Pita

Nonlinear carrier dynamics in silicon nano-waveguides

The understanding of free-carrier dynamics in silicon photonic nano-waveguides and micro-cavities is fundamental to several nonlinear optical phenomena. Through time-resolved pump and probe experiments, a complex and nonlinear carrier recombination dynamics is revealed. Our results show that the carrier lifetime varies as the recombination evolves, with faster decay rates at the initial stages (with lifetime of ∼800  ps) and much slower lifetimes at later stages (up to ∼300  ns). The large surface-to-volume ratio in nano-waveguides enables clear observation of the effect of carrier trapping, manifesting as a decay curve that is highly dependent on the initial carrier density. Further, we demonstrate faster recombination rates by operating at high carrier density. Our results, along with a theoretical framework based on trap-assisted recombination statistics applied to nano-waveguides, can impact the dynamics of several nonlinear nanophotonic devices in which free carriers play a critical role, and open further opportunities to enhance the performance of all-optical silicon-based devices.

Side-lobe level reduction in bio-inspired optical phased-array antennas

Phased arrays are expected to play a critical role in visible and infrared wireless systems. Their improved performance compared to single element antennas finds uses in communications, imaging, and sensing. However, fabrication of photonic antennas and their feeding network require long element separation, leading to the appearance of secondary radiation lobes and, consequently, crosstalk and interference. In this work, we experimentally show that by arranging the elements according to the Fermats spiral, the side lobe level (SLL) can be reduced. This reduction is proved in a CMOS-compatible 8-element array, revealing a SLL decrement of 0.9 dB. Arrays with larger numbers of elements and inter-element spacing are demonstrated through an spatial light modulator (SLM) and an SLL drop of 6.9 dB is measured for a 64-element array. The reduced SLL, consequently, makes the proposed approach a promising candidate for applications in which antenna gain, power loss, or information security are key requirements.

Optical free-carrier generation in silicon nano-waveguides at 1550 nm

We report on time-resolved pump and probe characterization of linear and nonlinear optical generation of free carriers in a silicon strip nano-waveguide at the 1550 nm communication band. Analytical expressions were developed to extract the carrier density averaged along the waveguide length from the measured free-carrier absorption for different input pump power levels. This allows us to discriminate the contributions from two-photon absorption (TPA) and single-photon absorption (SPA), obtaining TPA and SPA coefficients of (1.5 ± 0.1) cm/GW and (1.9 ± 0.1) m−1, respectively. Our results reveal that the effective TPA within the waveguide is higher than the value reported for bulk silicon. In addition, we find that for the waveguide under test, the carrier generation via SPA plays an important role up to ∼300 mW, and therefore, it must be taken into account to correctly assess free-carrier effects in silicon photonic devices.

Characterization of nonlinear carrier dynamics in silicon strip nanowaveguides

Nonlinear carrier recombination dynamics is characterized in a 450 nm × 220 nm silicon nanowire by employing a time-resolved pump-and-probe experiment. Our results show that the recombination rate is faster at the early stages of the decay as compared to the final stages, in agreement with trap-assisted mechanism. We have also demonstrated that by operating at high carrier density, faster excess carrier generation and recombination can be obtained, which we have used to improve the speed of an all-optical FCA based silicon switch from about 7 to 1 ns.

Lifetime dependence on carrier density in silicon nanowires

Free-carriers (FC) profoundly impact the performance of silicon nanophotonic devices including amplifiers, modulators, and ring microcavities [1]. Optimizing such devices relies on a good understanding of FC dynamics and how it depends on the FC density (N). This is particularly true in Si nanowaveguides, where surface recombination is important [2] and leads to an N-dependent lifetime [3]. Large density of surface-states enhances the N-dependence as compared to bulk, where recombination is impurity or defect dominated (as described by the Shockley-Read-Hall theory) while for very high N values, Auger process enters the picture [4]. The effect of carrier density on the recombination lifetime has been studied in silicon rib-waveguides [5], where surface effects are reduced somewhat relative to the case of nanowires considered in this paper, since carriers cannot diffuse away from the core region as in the rib. We used a pump-and-probe technique to investigate the recombination rate, and observe carrier lifetimes ranging over almost one order of magnitude for the range of N explored.