Paulo Dainese

Raman-like light scattering from acoustic phonons in photonic crystal fiber

Raman and Brillouin scattering are normally quite distinct processes that take place when light is resonantly scattered by, respectively, optical and acoustic phonons. We show how few-GHz acoustic phonons acquire many of the same characteristics as optical phonons when they are tightly trapped, transversely and close to modal cut-off, inside the wavelength-scale core of an air-glass photonic crystal fiber (PCF). The result is an optical scattering effect that closely resembles Raman scattering, though at much lower frequencies. We use photoacoustic techniques to probe the effect experimentally and finite element modelling to explain the results. We also show by numerical modelling that the cladding structure supports two phononic band gaps that contribute to the confinement of sound in the core.

Brillouin scattering self-cancellation

The interaction between light and acoustic phonons is strongly modified in sub-wavelength confinement, and has led to the demonstration and control of Brillouin scattering in photonic structures such as nano-scale optical waveguides and cavities. Besides the small optical mode volume, two physical mechanisms come into play simultaneously: a volume effect caused by the strain-induced refractive index perturbation (known as photo-elasticity), and a surface effect caused by the shift of the optical boundaries due to mechanical vibrations. As a result, proper material and structure engineering allows one to control each contribution individually. Here, we experimentally demonstrate the perfect cancellation of Brillouin scattering arising from Rayleigh acoustic waves by engineering a silica nanowire with exactly opposing photo-elastic and moving-boundary effects. This demonstration provides clear experimental evidence that the interplay between the two mechanisms is a promising tool to precisely control the photon–phonon interaction, enhancing or suppressing it.

Designing fiber dispersion for broadband parametric amplifiers

We present a method to determine the fiber chromatic dispersion curve that optimizes the gain bandwidth and ripple of fiber optical parametric amplifiers. We found that controlling high order dispersion terms a great enhancement of the gain spectrum is achieved. We provide a simulation example of a photonic crystal fiber, designed to control up to the sixth order dispersion parameter that gives 33 dB of gain from 1440 nm to 1600 nm, within 0.2 dB of gain variation.

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.

Fiber taper diameter characterization using forward Brillouin scattering

We propose a fast and non-destructive method to characterize the absolute diameter and uniformity of micrometer-scale fiber tapers using a pump and probe forward Brillouin scattering setup. The fundamental torsional-radial acoustic mode supported by the wire is excited using a pulsed pump laser and oscillates at a frequency that is inversely proportional to the taper waist diameter. This standing time-varying torsional-radial wave induces polarization modulation on a probe signal, whose spectrum structure reveals the sample diameter and its non-uniformity. By comparing our results with measurements using scanning-electron microscopy, a relative deviation of 1% or less was demonstrated, and diameter non-uniformity of less than 0.5% could be detected.

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.

Time-domain interferometric characterization of nonlinear and thermal-induced phase-shift in silicon waveguides

Time-domain interferometry is used to simultaneously characterize nonlinear and self-heating phase-shifts in silicon waveguides under long-pulse optical pumping. Applied to a strip waveguide, the method enabled measurements of both stationary phase-shifts and thermal time constant.