Domenico Pacifici

Plasmonic Nanostructure Design for Efficient Light Coupling into Solar Cells

We demonstrate that subwavelength scatterers can couple sunlight into guided modes in thin film Si and GaAs plasmonic solar cells whose back interface is coated with a corrugated metal film. Using numerical simulations, we find that incoupling of sunlight is remarkably insensitive to incident angle, and that the spectral features of the coupling efficiency originate from several different resonant phenomena. The incoupling cross section can be spectrally tuned and enhanced through modification of the scatterer shape, semiconductor film thickness, and materials choice. We demonstrate that, for example, a single 100 nm wide groove under a 200 nm Si thin film can enhance absorption by a factor of 2.5 over a 10 microm area for the portion of the solar spectrum near the Si band gap. These findings show promise for the design of ultrathin solar cells that exhibit enhanced absorption.

Role of the energy transfer in the optical properties of undoped and Er-doped interacting Si nanocrystals

In this article the luminescence properties of Si nanocrystals (nc) formed by plasma enhanced chemical vapor deposition and their interaction with Er ions introduced by ion implantation are investigated in detail. Si nc with different size distributions and densities were produced and all show quite intense room temperature luminescence (PL) in the range 700–1100 nm. It is shown that the time-decay of the luminescence follows a stretched exponential function whose shape tends towards a single exponential for almost isolated nc. This suggests that stretched exponential decays are related to the energy transfer from smaller towards larger nc. Indeed, by comparing samples with similar nc size distributions, but with very different nc densities, it is demonstrated that the PL has a quite strong redshift in the high density case, demonstrating a clear energy redistribution within the sample. Excitation cross sections have been measured in all samples yielding a value of ∼1.8×10−16 cm2 for isolated nc excited w...

Electrooptic Modulation in Thin Film Barium Titanate Plasmonic Interferometers

We demonstrate control of the surface plasmon polariton wavevector in an active metal-dielectric plasmonic interferometer by utilizing electrooptic barium titanate as the dielectric layer. Arrays of subwavelength interferometers were fabricated from pairs of parallel slits milled in silver on barium titanate thin films. Plasmon-mediated transmission of incident light through the subwavelength slits is modulated by an external voltage applied across the barium titanate thin film. Transmitted light modulation is ascribed to two effects, electrically induced domain switching and electrooptic modulation of the barium titanate index.

Electroluminescence at 1.54 μm in Er-doped Si nanocluster-based devices

The electroluminescence (EL) properties of Er-doped Si nanoclusters (NC) embedded in metal–oxide–semiconductor devices are investigated. Due to the presence of Si NC dispersed in the SiO2 matrix, an efficient carrier injection occurs and Er is excited, producing an intense 1.54 μm room temperature EL. The EL properties as a function of the current density, temperature, and time have been studied in detail. We have also estimated the excitation cross section for Er under electrical pumping, finding a value of ∼1×10−14 cm2. This value is two orders of magnitude higher than the effective excitation cross section of Er ions through Si NC under optical pumping. In fact, quantum efficiencies of ∼1% are obtained at room temperature in these devices.

Sensitizing properties of amorphous Si clusters on the 1.54-μm luminescence of Er in si-rich SiO2

In this letter, the role of amorphous Si clusters in the excitation of Er implanted in substoichiometric SiOx films will be elucidated. It will be shown that the temperature of the SiOx thermal process prior to Er implantation is crucial in determining the luminescence properties of the samples. In particular, the luminescence intensity at 1.54 μm is almost constant for SiOx samples not annealed or pre-annealed at temperatures lower than 800 °C, reaches the maximum at 800 °C, and decreases at higher temperatures. The structural properties of these samples have been studied by energy filtered transmission electron microscopy. It will be shown that for annealing temperatures lower than 1000 °C, only amorphous Si nanoclusters are present. We demonstrate that a large density of small amorphous Si clusters produces the best luminescence performance and enhances the fraction of optically active Er.

Dynamics of stimulated emission in silicon nanocrystals

Time-resolved luminescence measurements on silicon nanocrystal waveguides obtained by thermal annealing of plasma-enhanced chemical-vapor-deposited thin layers of silicon-rich oxide have revealed fast recombination dynamics related to population inversion which leads to net optical gain. Variable stripe length measurements performed on the fast emission signal have shown an exponential growth of the amplified spontaneous emission with net gain values of about 10 cm−1. The fast emission component is strongly dependent on the pumping length for fixed excitation intensity. In addition, both the fast component intensity and its temporal decay revealed threshold behavior as a function of the incident pump intensity.

Universal optical transmission features in periodic and quasiperiodic hole arrays

We investigate the influence of array order in the optical transmission properties of subwavelength hole arrays, by comparing the experimental spectral transmittance of periodic and quasiperiodic hole arrays as a function of frequency. We find that periodicity and long-range order are not necessary requirements for obtaining enhanced and suppressed optical transmission, provided short-range order is maintained. Transmission maxima and minima are shown to result, respectively, from constructive and destructive interference at each hole, between the light incident upon and exiting from a given hole, and surface plasmon polaritons (SPPs) arriving from individual neighboring holes. These SPPs are launched along both illuminated and exit surfaces, by diffraction of the incident and emerging light at the neighboring individual subwavelength holes. By characterizing the optical transmission of a pair of subwavelength holes as a function of hole-hole distance, we demonstrate that a subwavelength hole can launch SPPs with an efficiency up to 35%, and with an experimentally determined launch phase phi = pi /2, for both input-side and exit-side SPPs. This characteristic phase has a crucial influence on the shape of the transmission spectra, determining transmission minima in periodic arrays at those frequencies where grating coupling arguments would instead predict maxima.

Nanoscale plasmonic interferometers for multispectral, high-throughput biochemical sensing.

In this work, we report the design, fabrication, and characterization of novel biochemical sensors consisting of nanoscale grooves and slits milled in a metal film to form two-arm, three-beam, planar plasmonic interferometers. By integrating thousands of plasmonic interferometers per square millimeter with a microfluidic system, we demonstrate a sensor able to detect physiological concentrations of glucose in water over a broad wavelength range (400-800 nm). A wavelength sensitivity between 370 and 630 nm/RIU (RIU, refractive index units), a relative intensity change between ~10(3) and 10(6) %/RIU, and a resolution of ~3 × 10(-7) in refractive index change were experimentally measured using typical sensing volumes as low as 20 fL. These results show that multispectral plasmonic interferometry is a promising approach for the development of high-throughput, real-time, and extremely compact biochemical sensors.

Er3+ ions–Si nanocrystals interactions and their effects on the luminescence properties

A detailed investigation on the interaction mechanisms between Er ions and Si nanocrystals (nc) is reported. Silicon nc were produced by high-temperature annealing of substoichiometric SiOx thin films grown by plasma-enhanced chemical vapor deposition. Subsequently, some of the samples were implanted by Er. These samples show intense room-temperature luminescence at both 1.54 and 0.98 μm. High-resolution luminescence spectra of Er-implanted Si nc suggest that the emitting Er ions are located in the SiO2 or at the Si nc/SiO2 interface. The pump-power dependence and the time decay of the 1.54 μm emission in Si nc with different Er contents have evidenced the presence of several nonradiative decay processes due to Er–Er and Er–Si nc interactions. Moreover, the number of Er ions per Si nc is shown to be a quite critical parameter in determining the final properties of the overall system.

Quantitative determination of optical transmission through subwavelength slit arrays in Ag films: Role of surface wave interference and local coupling between adjacent slits

Measurement of the transmitted intensity from a coherent monomode light source through a series of subwavelength slit arrays in Ag films, with varying array pitch and number of slits, demonstrate enhancement (suppression) by as much as a factor of 6 (9) when normalized to that of an isolated slit. Pronounced minima in the transmitted intensity were observed at array pitches corresponding to λSPP, 2 λSPP and 3λSPP where λSPP is the wavelength of the surface plasmon polariton (SPP). Increasing the number of slits to more than four does not increase appreciably the per-slit transmission intensity. These results are consistent with a model for interference between SPPs and the incident wave that fits well the measured transmitted intensity profile.