Research · Publication & note
La nanofotonica in silicio e la fotonica con il nanosilicio: una piattaforma per ampliare il successo della fotonica in silicio
Il Nuovo Saggiatore · Journal article · 2012
Abstract
La fotonica integrata in silicio ha permesso di realizzare reti ottiche integrate in pochi mm² con velocità di trasmissione dati di più di 1 Tbps. Per poterne ampliare ulteriormente lo spettro di funzionalità ed applicazioni, a Trento applichiamo il paradigma delle nanotecnologie. Grazie alle nuove proprietà risultanti dal confinamento spaziale di portatori di carica (Nanosilicio) e di fotoni (Nanofotonica), abbiamo dimostrato una serie di funzionalità aggiuntive alla fotonica in silicio. Queste hanno permesso di realizzare amplificatori ottici, risonatori ottici, diodi elettroluminescenti, celle solari a resa elevata, biosensori biocompatibili e biodegradabili, interruttori tutti ottici superveloci, nodi di reti ultradensi, instradatori interferometrici, ed altro ancora.
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Research fields
Bottom-up
The article presents silicon nanocrystals and silicon colloids as material routes that introduce electronic or photonic confinement beyond bulk crystalline silicon. Deposition, annealing and particle formation are connected with room-temperature emission and wavelength-scale scattering.
Top-down
Planar waveguides, microcavities and resonators show how nanosilicon can be incorporated into silicon-photonic circuits. Device geometry supplies optical confinement while the nanostructured material adds emission or nonlinear functionality unavailable from passive bulk silicon alone.
RESEARCH TOPICS
CouplingThe review uses SCISSOR chains—side-coupled sequences of ring resonators—to show how weak exchange between each ring and a common guide can build a collective spectral response. With appropriate spacing, adjacent resonators also couple through the phase accumulated in the linking guide. This architecture supports coupled-resonator-induced transparency and wavelength routing, while preserving the compactness of silicon photonics. Coupling is presented as a circuit-design variable that connects isolated high-Q cavities into a functional filter rather than merely as an insertion loss.
EmissionSilicon nanocrystals overcome part of bulk silicon's poor radiative efficiency through quantum confinement, producing room-temperature visible photoluminescence. The review contrasts them with organic dyes: nanocrystals have microsecond decay times, broad absorption, chemically modifiable surfaces and better resistance to photobleaching. When embedded in a microdisk, their broad emission is sampled by narrow whispering-gallery resonances, as illustrated in Figure 3. Emission is therefore discussed from both the material side and the cavity-filtering side.
InterferenceSeveral devices rely on phase rather than absorption. Periodically spaced resonators can satisfy a Bragg condition and reflect strongly; two coupled SCISSOR arms can instead route a wavelength band according to constructive or destructive recombination at their outputs. The review also explains how an optically induced nonlinear phase shift can switch an interferometer between those conditions. These examples connect the abstract idea of interference to concrete silicon circuits for filtering and all-optical routing.
MaterialsThe material platform ranges from crystalline silicon waveguides to silicon-rich oxides containing nanocrystals. Nanosilicon supplies luminescence and a sizeable third-order response while remaining compatible with established microelectronic processing. The review also notes limitations visible over days, such as emission-intensity reduction and spectral blue shift in some colloidal systems. Presenting both useful optical properties and degradation mechanisms avoids treating “silicon photonics” as a single ideal material and shows why composition, confinement and surface chemistry matter.
MultimodeMicrodisks support many azimuthal whispering-gallery resonances, while coupled-ring chains create additional collective supermodes. Their narrow spectral spacing enables dense wavelength routing but also means that modal alignment, family selection and fabrication dispersion must be controlled. In the review, multimode behaviour is not an incidental complication: it is the basis for comb-like luminescence spectra, channel selection and coupled-resonator transparency. Device function comes from engineering which modes interact and which remain dark.
NonlinearAn optical control pulse changes refractive index through the third-order Kerr effect and thereby shifts a resonance or interferometric phase. Silicon nanocrystal composites are highlighted because their nonlinear response can enable fast all-optical switching at telecommunication wavelengths. The review distinguishes this desired ultrafast mechanism from slower thermal or carrier effects that can destabilise resonators. Nonlinearity is therefore framed both as a switching resource and as a dynamical constraint in densely integrated cavities.
QuantumQuantum confinement in nanometre-scale silicon changes the electronic density of states and relaxes the restrictions that make bulk silicon an inefficient light emitter. Size-graded quantum-dot ensembles can broaden absorption or tune electroluminescence, while individual nanocrystal populations supply red-visible emission. The discussion stays at the material-physics level: it does not claim single-photon operation, but explains how confinement provides optical functionality that ordinary crystalline silicon lacks.
ResonatorsWhispering-gallery microdisks store light around their perimeter and imprint narrow resonances on the broad nanocrystal luminescence. Chains of rings extend this single-cavity behaviour into filters and routers with coupled transparency windows. High Q enhances interaction and selectivity, but also amplifies sensitivity to loss, detuning and nonlinearity. The review uses this progression—from one emitting disk to interconnected SCISSOR circuits—to illustrate how resonators bridge material research and scalable silicon-photonic systems.