D. Navarro-Urrios

Porous silicon-based rugate filters

We report an experimental study of porous silicon-based rugate filters. We performed filter apodization, following a half-apodization approach, which successfully attenuated the sidelobes at both sides of the photonic stop band. We achieved successful reduction of interference ripples through the insertion of index-matching layers on the first and last interfaces. An apodized dielectric mirror and a rugate filter are compared: Appreciable differences in the harmonic presence and stop-band performance were observed and are commented on. Bandwidth control when index contrast is modified is also demonstrated. Finally, the possibility of combining different rugate filter designs to attain more complex responses is demonstrated by the achievement of a multi-stop-band filter. Numerical calculations for design optimization and comparison with experimental data are reported too.

Absorption cross section and signal enhancement in Er-doped Si nanocluster rib-loaded waveguides

Pump and probe experiments on Er3+ ions coupled to Si nanoclusters have been performed in rib-loaded waveguides to investigate optical amplification at 1.5μm. Rib-loaded waveguides were obtained by photolithographic and reactive ion etching of Er-doped silica layers containing Si nanoclusters grown by reactive sputtering. Insertion losses measurements in the infrared erbium absorption region allowed to gauge an Er3+ absorption cross section of about 5×10−21cm2 at 1534nm. Signal transmission under optical pumping at 1310nm shows confined carrier absorption of the Si nanoclusters. Amplification experiments at 1535nm evidence two pump power regimes: Losses due to confined carrier absorption in the Si nanoclusters at low pump powers and signal enhancement at high pump powers. For strong optical pumping, signal enhancement of about 1.2dB∕cm was obtained.

Optical gain in monodispersed silicon nanocrystals

Stimulated emission from silicon-nanocrystal planar waveguides grown via phase separation and thermal crystallization of SiO∕SiO2 superlattices is presented. Under high power pulsed excitation, positive optical gain can be observed once a good optical confinement in the waveguide is achieved and the silicon nanocrystals have proper size. A critical tradeoff between Auger nonradiative recombination processes and stimulated emission is observed. The measured large gain values are explained by the small size dispersion in these silicon nanocrystals.

Silicon Nanocrystals as an Enabling Material for Silicon Photonics

Silicon nanocrystals (Si-nc) is an enabling material for silicon photonics, which is no longer an emerging field of research but an available technology with the first commercial products available on the market. In this paper, properties and applications of Si-nc in silicon photonics are reviewed. After a brief history of silicon photonics, the limitations of silicon as a light emitter are discussed and the strategies to overcome them are briefly treated, with particular attention to the recent achievements. Emphasis is given to the visible optical gain properties of Si-nc and to its sensitization effect on Er ions to achieve infrared light amplification. The state of the art of Si-nc applied in a few photonic components is reviewed and discussed. The possibility to exploit Si-nc for solar cells is also presented. In addition, nonlinear optical effects, which enable fast all-optical switches, are described.

A one-dimensional optomechanical crystal with a complete phononic band gap

Recent years have witnessed the boom of cavity optomechanics, which exploits the confinement and coupling of optical and mechanical waves at the nanoscale. Among their physical implementations, optomechanical (OM) crystals built on semiconductor slabs enable the integration and manipulation of multiple OM elements in a single chip and provide gigahertz phonons suitable for coherent phonon manipulation. Different demonstrations of coupling of infrared photons and gigahertz phonons in cavities created by inserting defects on OM crystals have been performed. However, the considered structures do not show a complete phononic bandgap, which should enable longer lifetimes, as acoustic leakage is minimized. Here we demonstrate the excitation of acoustic modes in a one-dimensional OM crystal properly designed to display a full phononic bandgap for acoustic modes at 4 GHz. The modes inside the complete bandgap are designed to have high-mechanical Q-factors, limit clamping losses and be invariant to fabrication imperfections.

Distance dependent interaction as the limiting factor for Si nanocluster to Er energy transfer in silica

Si excess, Er content, and processing parameters have been optimized in a series of cosputtered oxide layers for maximizing Er emission and lifetime. The amount of excited Er as a function of the incident photon flux has been quantified for resonant (488nm) and nonresonant (476nm) excitations. Results show that a maximum of 3.5% of Er ions is excitable through the Si nanoclusters (Si-nc). This low value cannot be explained only by cooperative upconversion and/or excited state absorption. A short range (0.5nm) distance dependent interaction model is developed that accounts for this low Er population inversion. The model points to the low density of Si-nc [(3–5)×1017cm−3] as the ultimate limiting step for indirect Er excitation in this system.

Whispering-gallery modes and light emission from a Si-nanocrystal-based single microdisk resonator

We report on visible light emission from Si-nanocrystal based optically active microdisk resonators. The room temperature photoluminescence (PL) from single microdisks shows the characteristic modal structure of whispering-gallery modes. The emission is both TE and TM-polarized in 300 nm thick microdisks, while thinner ones (135 nm) support only TE-like modes. Thinner disks have the advantage to filter out higher order radial mode families, allowing for measuring only the most intense first order modal structure. We reveal subnanometer linewidths and corresponding quality factors as high as 2800, limited by the spectral resolution of the experimental setup. Moreover, we observe a modification of mode linewidth by a factor 13 as a function of pump power. The origin of this effect is attributed to an excited carrier absorption loss mechanism.

Quantification of the carrier absorption losses in Si-nanocrystal rich rib waveguides at 1.54μm

A detailed study of the carrier absorption (CA) mechanism in multilayered silicon-nanocrystals (Si-nc) rib waveguides is reported. A pump (532nm) and probe (1535nm) technique is used to assess two loss mechanisms due to optical excitation of the system: one characterized by slow (seconds) dynamics related to heating and the other characterized by fast (microsecond) dynamics associated to CA mechanisms within the Si-nc. CA losses increase with pumping flux of up to 6dB∕cm for 3×1020photons∕cm2s. By comparing the temporal dynamics of CA losses and time resolved photoluminescence, we suggest that both are determined by exciton generation and recombination.

Er-Coupled Si Nanocluster Waveguide

Rib-loaded waveguides containing Er3+-coupled Si nanoclusters (Si-nc) have been produced to observe optical gain at 1535 nm. The presence of Si-nc strongly improves the efficiency of Er3+ excitation but may introduce optical loss mechanisms, such as Mie scattering and confined carrier absorption. Losses strongly affect the possibility of obtaining positive optical gain. Si-nc-related losses have been minimized to 1 dB/cm by lowering the annealing time of the Er 3+-doped silicon-rich oxide deposited by reactive magnetron cosputtering. Photoluminescence (PL) and lifetime measurements show that all Er3+ ions are optically active while those that can be excited at high pump rates via Si-nc are only a small percentage. Er3+ absorption cross section is found comparable to that of Er3+ in SiO 2. However, dependence on the effective refractive index has been found. In pump-probe measurements, it is shown how the detrimental role of confined carrier absorption can be attenuated by reducing the annealing time. A maximum signal enhancement of about 1.34 at 1535 nm was measured

Refractive index dependence of the absorption and emission cross sections at 1.54μm of Er3+ coupled to Si nanoclusters

Absorption coefficient (αabs) of Er3+ ions coupled to Si nanoclusters (Si-nc) in SiO2 has been determined by optical transmission measurements in rib-loaded waveguides characterized by different refractive indices, thus gauging an Er3+ absorption cross section (σabs) of 0.4–1.2×10−20cm2 at 1534nm. Although no significant enhancement due to the presence of Si-nc was observed, a clear dependence on the refractive index has been found. Measurements of the decay lifetime permit one to model the behavior as due to both local and mean field variations caused by the composite nature of the core waveguide layer.