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We present our work on high performance germanium (Ge) quantum dot (QD) photodetectors (PDs), fabricated on Si and Ge substrates, that operate via tunneling transport through a QD-containing active layer and feature high internal photoconductive gain. In the λ = 400–1100 nm range, the PDs fabricated on Si substrates exhibit room-temperature spectral responsivity ( R sp ) up to 4 A/W and internal quantum efficiency (IQE) up to 700%. At λ = 640 nm and 12 μ W of incident power, signal-to-noise ratio (SNR) of 7 × 10 6 and specific detectivity ( D * ) of 1.2×1011 cmHz1/2W−1 are obtained. The PDs demonstrate 3 dB bandwidths (f3dB) up to 10 MHz, corresponding to response times of ∼ 40 ns. When operated at 100 K temperature the performance improves, especially at low incident power, where at 10 nW D * increases to 2 × 10 13 cmHz1/2W−1, due to IQE in excess of 22000%. In order to extend the photoresponse into the near-infrared (near-IR), PDs were fabricated on Ge substrates, yielding room-temperature R sp = 1.5 A/W, IQE = 134% and f3dB = 10 kHz at the λ = 1550 nm telecommunication wavelength. Significant improvement to f3dB is expected in PDs employing thinner QD-containing layers. Lowering temperature to 50 K and incident power to 10 nW yield D* = 1013 cmHz1/2W−1, resulting from IQE exceeding 60000%. Based on the above figures of merit, as well as their Si technology compatibility, our Ge QD PDs appear promising for high-performance photodetectors working in the visible and near-IR.

There has been an urgent need to eliminate toxic lead from the prevailing halide perovskite solar cells (PSCs), but the current lead-free PSCs are still plagued with the critical issues of low efficiency and poor stability. This is primarily due to their inadequate photovoltaic properties and chemical stability. Herein we demonstrate the use of the lead-free, all-inorganic cesium tin-germanium triiodide (CsSn0.5Ge0.5I3) solid-solution perovskite as the light absorber in PSCs, delivering promising efficiency of up to 7.11%. More importantly, these PSCs show very high stability, with less than 10% decay in efficiency after 500 h of continuous operation in N2 atmosphere under one-sun illumination. The key to this striking performance of these PSCs is the formation of a full-coverage, stable native-oxide layer, which fully encapsulates and passivates the perovskite surfaces. The native-oxide passivation approach reported here represents an alternate avenue for boosting the efficiency and stability of lead-free PSCs., Replacing the toxic lead in the state-of-the-art halide perovskite solar cells is highly desired but the device performance and stability are usually compromised. Here Chen et al. develop inorganic cesium tin and germanium mixed-cation perovskites that show high operational stability and efficiency over 7%.

We report optical characterization and theoretical simulation of plasmon enhanced methylammonium lead iodide (MAPbI 3 ) thin-film perovskite solar cells. Specifically, various nanohole (NH) and nanodisk (ND) arrays are fabricated on gold/MAPbI 3 interfaces. Significant absorption enhancement is observed experimentally in 75 nm and 110 nm-thick perovskite films. As a result of increased light scattering by plasmonic concentrators, the original Fabry&ndash, P&eacute, rot thin-film cavity effects are suppressed in specific structures. However, thanks to field enhancement caused by plasmonic resonances and in-plane interference of propagating surface plasmon polaritons, the calculated overall power conversion efficiency (PCE) of the solar cell is expected to increase by up to 45.5%, compared to its flat counterpart. The role of different geometry parameters of the nanostructure arrays is further investigated using three dimensional (3D) finite-difference time-domain (FDTD) simulations, which makes it possible to identify the physical origin of the absorption enhancement as a function of wavelength and design parameters. These findings demonstrate the potential of plasmonic nanostructures in further enhancing the performance of photovoltaic devices based on thin-film perovskites.

We report on plasmon-enhanced hybrid organic–inorganic perovskite solar cells with methylammonium lead iodide (MAPbI3) as the active absorbing material. Three-dimensional finite-difference time-dom...

Methods to produce colored surfaces using physical mechanisms are proposed, with aluminum used as a demonstration vehicle. Our approach aims to achieve tunable, vibrant colors through the interaction of three well-established physical mechanisms of color formation, namely (1) resonant scattering and interference effects in a nano-structured surface (e.g. via anodization of aluminum), (2) plasmonic resonances in metal nanoparticles, and (3) spectrally selective absorption enhancement in ultrathin films, such as amorphous germanium and carbon. We provide proof-of-concept by fabricating and characterizing representative aluminum-based samples showing vivid colors throughout the visible spectral range.

Efficiency and response speed are key figures of merit for high-performance photodetectors, with high efficiency often obtained at the expense of slow photoresponse. Here, we report on germanium quantum dot photodetectors (Ge QD PDs) with a 25-nm-thick active layer that possesses both high internal quantum efficiency (IQE) and fast photoresponse, yet is still based on simple design and fabrication. We characterize these devices with continuous wave (CW) and pulsed excitation at room temperature as a function of incident power and applied bias. Under the reverse bias of –4 V, the IQE approaches ∼2000% over a broad spectral range (λ = 500–800 nm). The transient photoresponse speed to a 4.5 ns laser pulse at λ = 640 nm is under 20 ns. Furthermore, we observe an interesting phenomenon: by superimposing a weak CW HeNe laser beam ( λ = 632.8 nm) on the laser pulse, we obtain an optically tunable photoresponse while retaining fast speed. This study elucidates the role of photocarrier generation, trapping, and hopping in the percolative Ge QD oxide matrix and helps explain the observed high gain and fast response speed. The demonstrated IQE and nanosecond response time render our devices suitable for low-light detection and imaging.

Here we discuss methods that employ surface plasmons to measure and strongly modulate the degree of optical spatial coherence of light, paving the way for multifunctional optical elements beyond conventional refractive- and diffractive-based photonics metasurfaces.

We experimentally unveiled up to the sixth-order surface plasmon contributions to hole-groove plasmonic interferometry using discrete Fourier transform. This method is further extended to double-slit plasmonic structures to deconvolve competing interference effects from asymmetric interfaces.

A non-invasive method for the detection of glucose is sought by millions of diabetic patients to improve personal management of blood glucose over a lifetime. In this work, the synergistic advantage of combining plasmonic interferometry with an enzyme-driven dye assay yields an optical sensor capable of detecting glucose in saliva with high sensitivity and selectivity. The sensor, coined a “plasmonic cuvette,” is built around a nano-scale groove-slit-groove (GSG) plasmonic interferometer coupled to an Amplex–red/Glucose–oxidase/Glucose (AR/GOx/Glucose) assay. The proposed device is highly sensitive, with a measured intensity change of 1.7×105%/m (i.e., one order of magnitude more sensitive than without assay) and highly specific for glucose sensing in picoliter volumes, across the physiological range of glucose concentrations found in human saliva (20–240 μm). Real-time glucose monitoring in saliva is achieved by performing a detailed study of the underlying enzyme-driven reactions to determine and tune the effective rate constants in order to reduce the overall assay reaction time to ∼2 min. The results reported suggest that by opportunely choosing the appropriate dye chemistry, a plasmonic cuvette can be turned into a general, real-time sensing scheme for detection of any molecular target, with high sensitivity and selectivity, within extremely low volumes of biological fluid (down to femtoliters). Hereby, we present the results on glucose detection in artificial saliva as a notable and clinically relevant case study.

We compare the intensity modulation of passive light transmission and active fluorescence emission in planar plasmonic interferometers consisting of a nano-scale hole flanked by circular grooves etched in a silver film. Discrete fast Fourier transform applied to plasmonic interferograms – i.e., optical interferograms obtained by varying the propagation phase of surface plasmon polaritons (SPPs) – reveals higher-order interference effects that can be enhanced by optimizing in-plane SPP scattering and reflection. The experimental SPP dispersion relations agree with finite-difference frequency-domain calculations. Finally, we show that odd-order SPP contributions can be suppressed by reducing the spatial coherence of the incident beam.

This report will demonstrate broadband, wide-angle, and polarization-insensitive absorption enhancement in ultra-thin films resting on metal substrates that have been etched with arrays of shallow sub-wavelength cylindrical holes. Absorption enhancement will be studied as a function of array geometry, with particular emphasis given to quasiperiodic arrays (a class of deterministic aperiodic arrays that were originally developed to tessellate 2-D planes with regular polygons). Through simulations and experimental data, it was found that absorption enhancement is heavily dependent on the rotational symmetry of the pattern of holes, as well as the inter-hole distance.

Surface plasmon polaritons can make incoherent light coherent and vice versa., The degree of optical spatial coherence—a fundamental property of light that describes the mutual correlations between fluctuating electromagnetic fields—has been proven challenging to control at the micrometer scale. We use surface plasmon polaritons—evanescent waves excited on both surfaces of a thin metal film—as a means to mix the random fluctuations of the incident electromagnetic fields at the slit locations of a Young’s double-slit interferometer. Strong tunability of the complex degree of spatial coherence of light is achieved by finely varying the separation distance between the two slits. Continuous modulation of the degree of spatial coherence with amplitudes ranging from 0 to 80% allows us to transform totally incoherent incident light into highly coherent light and vice versa. These findings pave the way for alternative methods to engineer flat optical elements with multifunctional capabilities beyond conventional refractive- and diffractive-based photonic metasurfaces.

Physical coloration without chemicals offers a pathway to develop pollution-free coloration technology, and can be applied to colorimetric sensing of gases, toxic and chemical agents. In this paper, we report on realization of a high-purity red color using mechanism of mode-selective absorption in a thin-film optical cavity. By placing an ultra-thin absorber layer at the antinode of a targeted spectral band in a Fabry-Perot cavity, its otherwise conventional dichroic reflection spectrum is shaped into a broad rectangular flat-bottom one that gives the desired vivid red. The purity of our demonstrated red color reaches 76%, which is increased by 16% compared with those reported in prior thin-film structures. Our method of mode-selective absorption is adaptable to more general-purpose spectral shaping, and could be applied in producing other target colors as well as broadband light absorption for energy harvesting and infrared detection.Physical coloration without chemicals offers a pathway to develop pollution-free coloration technology, and can be applied to colorimetric sensing of gases, toxic and chemical agents. In this paper, we report on realization of a high-purity red color using mechanism of mode-selective absorption in a thin-film optical cavity. By placing an ultra-thin absorber layer at the antinode of a targeted spectral band in a Fabry-Perot cavity, its otherwise conventional dichroic reflection spectrum is shaped into a broad rectangular flat-bottom one that gives the desired vivid red. The purity of our demonstrated red color reaches 76%, which is increased by 16% compared with those reported in prior thin-film structures. Our method of mode-selective absorption is adaptable to more general-purpose spectral shaping, and could be applied in producing other target colors as well as broadband light absorption for energy harvesting and infrared detection.

Photonics is a promising candidate technology for information processing, communication and data storage. Essential building blocks, such as logic elements and modulators, have been demonstrated. However, because of weak nonlinear light–matter interactions, these components typically require high power densities and large interaction volumes, limiting their application in dense chip-based integration. A solution may be found in surface plasmon polaritons (SPPs), guided electromagnetic waves that propagate with high field confinement along a metal–dielectric interface. We demonstrate an all-optical modulator in which efficient interaction between two light beams at different wavelengths is achieved by converting them into co-propagating SPPs interacting by means of a thin layer of CdSe quantum dots (QDs). The high SPP field confinement and high QD-absorption cross-section enable optical modulation at low power densities (~10^2 W cm^(-2)) in micrometre-scale planar devices.

In this work we have studied the structural, electrical and optical properties of MOS devices, where the dielectric layer consists of a SiOx (x < 2) thin film prepared by plasma enhanced chemical vapor deposition and implanted with rare earth ions. As deposited SiOx films were annealed at high temperature (> 1000 degreesC) to induce the separation of the Si and SiO2 phases with the formation of Si nanocrystals embedded in the insulating matrix. Devices based on this system present a strong light emission at room temperature at a wavelength of about 900 nm. Devices emitting at different wavelengths have been fabricated by implanting SiOx films with Er or Tin. Devices based on Er-doped Si nanoclusters film exhibit an intense 1.54 mum room temperature electroluminescence (EL). We have calculated the excitation cross-section for Er ions in presence of Si nanoclusters under electrical pumping and the value is similar to1 x 10(-14) cm(2), comparable to the value found for the electrical excitation of undoped Si nanocrystals. Finally, devices based on Tm-doped Si nanoclusters exhibit two EL peaks at 0.78 and 1.7 mum.

In this work, we investigate the spectral response of metal-oxide- semiconductor photodetectors based on Ge nanoclusters (NCs) embedded in a silicon dioxide (SiO2) matrix. The role of Ge NC size and density on the spectral response was evaluated by comparing the performance of PDs based on either densely packed arrays of 2 nm-diameter NCs or a more sparse array of 8 nm-diameter Ge NCs. Our Ge NC photodetectors exhibit a high spectral responsivity in the 500-1000 nm range with internal quantum efficiency of ~ 700% at - 10 V, and with NC array parameters such as NC density and size playing a crucial role in the photoconductive gain and response time. We find that the configuration with a more dispersed array of NCs ensures a faster photoresponse, due to the larger fraction of electrically-active NCs and the partial suppression of recombination centers. The photoconduction mechanism, assisted by trapping of photo-generated holes in Ge NCs, is discussed for different excitation power and applied bias conditions. Our results provide guidelines for further optimization of high-efficiency Ge NC photodetectors. © 2013 Elsevier B.V. All rights reserved.

We report on the noise analysis of high performance germanium quantum dot (Ge QD) photodetectors with responsivity up to ∼2 A/W and internal quantum efficiency up to ∼400%, over the 400–1100 nm wavelength range and at a reverse bias of −10 V. Photolithography was performed to define variable active-area devices that show suppressed dark current, leading to a higher signal-to-noise ratio, up to 105, and specific detectivity D*≃6×1012 cm Hz 1/2 W−1. These figures of merit suggest Ge QDs as a promising alternative material for high-performance photodetectors working in the visible to near-infrared spectral range.

We describe the design, fabrication, and testing of a spectroscopic refractometer that employs plasmonic interferometry to measure the optical dielectric functions of materials in the visible range. The proposed device, dubbed a plasmonic refractometer, consists of an array of slit-groove plasmonic interferometers etched in a ∼300 nm-thick metal film (silver or gold) with arm lengths varying in steps of 25 nm up to ∼8 μm. The nano-groove in each interferometer is able to generate propagating surface plasmon polaritons efficiently in a broad wavelength range, without requiring prism- or grating-coupling configurations. An integrated microfluidic channel ensures uniform delivery of dielectric materials in liquid phase. Spectrally resolved plasmonic interferograms are generated by measuring light transmission spectra through the slit of each slit-groove plasmonic interferometer and plotting the normalized intensity as a function of arm length (0.26–8.16 μm) and incident wavelength (400–800 nm) for various co...

A plasmonic concentrator consisting of a quasi-periodic array of shallow sub-wavelength holes is considered for use in ultra-thin film organic solar cells. The level of absorption enhancement in the active layer will be studied as functions of array symmetry and planar hole density.

We report a systematic study of time-resolved and power-dependent photoresponse in high-efficiency germanium quantum dot photodetectors (Ge-QD PDs), with internal quantum efficiencies greater than 100 over a broad wavelength, reverse bias, and incident power range. Turn-on and turn-off response times (τ on and τ off) are shown to depend on series resistance, bias, optical power, and thickness (W QD) of the Ge-QD layer, with measured τ off values down to ∼40 ns. Two different photoconduction regimes are observed at low and high reverse bias, with a transition around -3 V. A transient current overshoot phenomenon is also observed, which depends on bias and illumination power. © 2012 American Institute of Physics.

A class of plasmonic interferometers consisting of a circular slit flanked by a concentric circular groove is demonstrated. Laying in-between the conventional bullseye and the linear slit-groove interferometers, these circular slit-groove interferometers show a polarization-insensitive optical response (thanks to the rotational symmetry imposed by the circular geometry), and overall higher light transmission (due to the absence of a cutoff condition in the circular subwavelength slit). Light transmission and sensitivity can be further enhanced by the presence of plasmonic resonant modes excited by the circular slit. The proposed circular plasmonic interferometers can be employed to develop improved biochemical sensors.

We report on high responsivity, broadband metal/insulator/semiconductor photodetectors with amorphous Ge quantum dots (a-Ge QDs) as the active absorbers embedded in a silicon dioxide matrix. Spectral responsivities between 1-4 A/W are achieved in the 500-900 nm wavelength range with internal quantum efficiencies (IQEs) as high as ∼700%. We investigate the role of a-Ge QDs in the photocurrent generation and explain the high IQE as a result of transport mechanisms via photoexcited QDs. These results suggest that a-Ge QDs are promising for high-performance integrated optoelectronic devices that are fully compatible with silicon technology in terms of fabrication and thermal budget. © 2011 American Institute of Physics.

Various approaches to the basic physics of optical transmission through subwavelength structures have led to diverse, conflicting interpretations, predictions. We present a series of experimental and analytical studies explaining essential transmission behavior through subwavelength slits.

Plasmonics has provided nanoscience researchers new control of optical dispersion and light localization at nanoscale dimensions. I will discuss plasmonic concepts that are yielding metamaterials designs and building blocks for chip-based optical device technology.

We report on the fabrication and photovoltaic characteristics of vertical arrays of silicon axial p-i-n junction nanowire (NW) solar cells grown by vapor-liquid-solid (VLS) epitaxy. NW surface passivation with silicon dioxide shell is shown to enhance carrier recombination time, open-circuit voltage (VOC), short-circuit current density (JSC), and fill factor (FF). The photovoltaic performance of passivated individual NW and NW arrays was compared under 532 nm laser illumination with power density of ∼10 W/cm2. Higher values of VOC and FF in the NW arrays are explained by enhanced light trapping. In order to verify the effect of NW density on light absorption and hence on the photovoltaic performance of NW arrays, dense Si NW arrays were fabricated using nanoimprint lithography to periodically arrange the gold seed particles prior to epitaxial growth. Compared to sparse NW arrays fabricated using VLS growth from randomly distributed gold seeds, the nanoimprinted NW array solar cells show a greatly increase...

We report local-field-enhanced light emission from silicon nanocrystals close to a film of nanoporous gold. We resolve photoluminescence as the gold-Si nanocrystal separation distance is varied between 0 and 20 nm and observe a fourfold luminescence intensity enhancement concomitant with increases in the coupled silicon nanocrystal/nanoporous gold absorbance cross section and radiative decay rate. A detailed analysis of the luminescence data indicated a local-field-enhanced quantum efficiency of 58% for the Si nanocrystals coupled to the nanoporous gold layer.

We have studied the electrical and optical properties of light emitting MOS devices based on Si nanostructures. In these devices the dielectric layer consists of a SiOx thin film prepared by plasma enhanced chemical vapor deposition. As-deposited SiOx films were annealed at high temperature to induce the separation of the Si and SiO2 phases with the formation of Si nanocrystals (nc) embedded in the insulating matrix. Devices based on Si nc present a strong light emission at room temperature at a wavelength of about 900 nm. Devices emitting at different wavelengths have been fabricated by implanting SiOx films with rare earth ions. Devices based on Er, Tm and Yb-doped Si nanoclusters exhibit an intense room temperature electroluminescence (EL) respectively at 1.54 μm, 0.78 μm and 0.98 μm. The EL properties of the devices have been optimized through the study of the structural properties of their active layers by means of the energy filtered transmission electron microscopy (EFTEM) technique. EFTEM has evidenced the presence of a relevant contribution of amorphous nanostructures, and has given a very reliable quantitative estimation of the size and density of the Si nanostructures, as well as of the concentration of Si atoms remaining dissolved in the matrix. In the case of RE-doped SiOx films, the role of amorphous nanostructures in sensitizing the RE luminescence has been clearly demonstrated. These results open the way to new interesting applications in optoelectronics, such as electrically driven optical amplifiers or light sources.

The incorporation of Si-nc in Er doped silica is known to strongly enhance the infrared luminescence of Er3+ at 1.54µm. The enhancement is believed to be due to an energy transfer process from Si-nc to Er. In this work we investigate the formation of Si nano-aggregates and their role in the energy transfer process to Er3+ ions for a multi-component glass host. These materials can offer better performances than silica in terms of Er solubility and band broadness for integrated Er-doped optical amplifiers and investigation is therefore very interesting for optoelectronic applications. Si and Er were co-implanted by choosing the implantation energies in order to optimize the overlap between the concentration profiles. The precipitation of Si and the enhancement of the 1.54 µm Er emission were studied for different post-implantation annealing temperatures. In particular the optical properties of the glass were investigated by means of photoluminescence (PL) spectroscopy and the results are discussed in relation to a standard silica substrate. These data are presented and related to the structural properties of the material. Moreover the implications on the future development of an Er doped optical amplifier are discussed.

The electroluminescence (EL) properties of Er-doped Si nanoclusters (nc) embedded in metal-oxide-semiconductor devices are investigated. It is shown that, due to the presence of Si nc dispersed in the SiO2matrix, an efficient carrier injection occurs and Er is excited producing an intense 1.54 μm room temperature luminescence. The EL properties as a function of the current density, temperature and time have been studied in details, elucidating the radiative and non-radiative de-excitation properties of the system. We have also estimated the excitation cross section for Er under electrical pumping finding a value of −1×10 cm. This value is two orders of magnitude higher than the effective excitation cross section of Er ions through Si nc under optical pumping, and quantum efficiencies of ∼1% are obtained at room temperature in these devices. These data will be presented and the impact on future applications discussed.

We have fabricated MOS devices where the dielectric layer consists of a substoichiometric SiO x (x thin film , annealed at 1100°C for 1 h to induce the separation of the Si and SiO 2 phases, with the formation of silicon nanocrystals (nc) embedded in the insulating matrix. We have studied the electroluminescence (EL) properties of such devices as a function of the current density and of the temperature. We have evaluated the excitation cross section of Si nc under electrical pumping at room temperature and at low temperature (12 K ) . Moreover, we have used the experimental EL intensities and decay times to evaluate the radiative rate as a function of the temperature.

In the last decade, a strong effort has been devoted towards the achievement of efficient light emission from silicon. Among the different approaches, rare-earth doping and quantum confinement in Si nanostructures have shown great potentialities. In the present work, the synthesis and properties of low-dimensional silicon structures in SiO 2 will be analyzed. All of these structures present a strong room temperature optical emission, tunable in the visible by changing the crystal size. Moreover, Si nanocrystals (nc) embedded in SiO 2 together with Er ions show a strong coupling with the rare earth. Indeed each Si nc absorbs energy which is then preferentially transferred to the nearby Er ions. The signature of this interaction is the strong increase of the excitation cross section for an Er ion in the presence of Si nc with respect to a pure oxide host. We will show the properties of Er-doped Si nc embedded within Si/SiO 2 Fabry–Perot microcavities. Very narrow, intense and highly directional luminescence peaks can be obtained. Moreover, the electroluminescence (EL) properties of Si nc and Er-doped Si nc in MOS devices are investigated. It is shown that an efficient carrier injection at low voltages and quite intense room temperature EL signals can be achieved, due to the sensitizing action of Si nc for the rare earth. These data will be presented and the impact on future applications discussed.

Sensitization of Er3+ photoluminescence in SiO2 layers by silicon nanocrystals has been investigated under resonant and nonresonant pulsed optical pumping. We observed that the very efficient channel of erbium excitation introduced by the nanocrystals can be saturated when very high pulsed photon flux is used. While this channel is saturated, tuning to the resonant excitation wavelength results in an increase of erbium-related photoluminescence, indicating that direct and indirect excitations can take place simultaneously.

In this work, the stationary and time-resolved electroluminescence (EL) properties of Si quantum dots embedded within a metal–oxide–semiconductor device are investigated. In particular, we measured the excitation cross section of Si nanocrystals under electrical pumping, finding a value of 4.7×10−14 cm2 which is two orders of magnitude higher with respect to the excitation cross section under 488 nm optical pumping. We also studied the radiative and nonradiative decay processes occurring in these devices by measuring the time evolution of the EL signal. We demonstrate that the mechanism responsible for the emission is the same under both electrical and optical pumping. The overall quantum efficiency of the electrical pumping is estimated to be two orders of magnitude higher than the quantum efficiency for optical pumping in all the studied temperature ranges.

Strong room temperature photoluminescence (PL) in the wavelength range 700–950 nm has been observed from Si nanocrystals (nc) obtained by high-temperature annealing of SiO x thin films prepared by plasma enhanced chemical vapor deposition (PECVD). A marked redshift of the luminescence peak has been detected by increasing the Si concentration of the SiO x films or the annealing temperature, due to the larger Si nc mean size obtained under these conditions. Narrower and more intense PL spectra are observed by decreasing the Si concentration of the SiO x samples or by increasing the annealing temperature, but the overall performances remain still far from those required for application of this material in optoelectronic devices. On the other hand, by embedding Si nc within Si/SiO 2 Fabry–Perot microcavities, extremely narrow (Δ λ ∼1.5 nm) and very intense PL peaks can be obtained. The emitted wavelength can be tuned within a wide range by properly varying the cavity resonance. The PL intensity is more than an order of magnitude above that of similar samples outside the cavity and the luminescence is strongly directional.

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 mum 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 similar to1x10(-14) cm(2). 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 similar to1% are obtained at room temperature in these devices.

We present a systematic study of light transmission through individual nanoscale apertures (rectangular slits and circular holes) etched in a 300-nm-thick silver film. Transmission spectra were obtained as functions of aperture shape and size, as well as the wavelength and polarization state of the normally incident light beam. By varying the wavelength of the incident light in the 550–750 nm range and the characteristic dimensions of the apertures from ∼100 nm to 10 μm, a universal behavior of light transmission is revealed. The role of incident polarization and aperture dimensions is investigated in detail, and a clear transition from the geometric regime of light transmission (large apertures) to the subwavelength regime is demonstrated experimentally. A quantitative analysis of the extinction coefficient is reported for rectangular slits, demonstrating that they can act as efficient linear polarizers with extinction ratios >100∶1. Finally, a method to convert far-field to near-field data is developed for circular apertures, revealing the contribution of surface plasmon polaritons to the decrease in light transmission for apertures below the cutoff condition.

Wavelength converters typically rely on inefficient nonlinear light–matter interactions or electro–optic effects. Researchers in the USA have now demonstrated a low-power and broadband all-optical wavelength shifter, which has the potential to fit on a single optical chip.

This report will present a generalized two-dimensional quasiperiodic (QP) tiling algorithm based on de Bruijn's "cut and projection" method for use in plasmonic concentrator (PC) / photovoltaic hybrid devices to produce wide-angle, polarization-insensitive, and broadband light absorption enhancement. This algorithm can be employed with any PC consisting of point-like scattering objects, and can be fine-tuned to achieve a high spatial density of points and high orders of local and long-range rotational symmetry. Simulations and experimental data demonstrate this enhancement in ultra-thin layers of organic photovoltaic materials resting on metallic films etched with arrays of shallow sub-wavelength nanoholes. These devices work by coupling the incident light to surface plasmon polariton (SPP) modes that propagate along the dielectric / metal interface. This effectively increases the scale of light-matter interaction, and can also result in constructive interference between propagating SPP waves. By comparing PCs made with random, periodic, and QP arrangements, it is clear that QP is superior in intensifying the local fields and enhancing absorption in the active layer.

We report on experimental absorption enhancement in 24 and 150-nm-thick bulk heterojunctions of the conducting polymer poly(3-hexylthiophene) and the fullerene (6,6)-phenyl C61 butyric acid methyl ester. By using periodic and quasiperiodic hole arrays as nanoengineered plasmonic concentrators milled in a silver film, a spectrally broad, omnidirectional, and polarization-insensitive absorption enhancement is observed over that of a reference layer deposited on a flat film. As the result of increased absorption, an enhancement in the polymer fluorescence intensity is also observed. This demonstrates the potential of plasmonic concentrators for improved energy harvesting in ultrathin film solar cells.

Time-resolved luminescence measurements on silicon nanocrystal waveguides have revealeda fast recombination dynamics, related to population inversion which leads to net optical gain.The waveguide samples were obtained by thermal annealing of plasma enhanced chemicalvapour deposited thin layers of silicon rich oxide Variable stripe length measurements performedon the fast emission signal have shown an exponential growth of the amplified spontaneousemission, with net gain values of about 10 cm -1 . Both the fast component intensity and itstemporal width revealed threshold behaviour as a function of the incident pump intensity. Amodelling of the decay dynamics is suggested within an effective four level rate equationtreatment including the delicate interplay among stimulated emission and Auger recombinations. INTRODUCTION One goal for silicon microphotonics consists in the demonstration of a laser device based onsilicon [1,2]. Following the initial observations of optical gain in highly packed siliconnanocrystals (Si-nc) prepared by ion-implantation [3], other works have recently demonstrated thepossibility of stimulated emission in Si-nc [4-6]. Nanosecond gain dynamics [4] and evidences forspeckle patterns [5] in the spatially coherent emission have been recently reported.As in other quantum dot based systems [7,8], a severe competition with efficient non-radiativeprocesses, mainly Auger type, is present in Si-nc, which causes very fast dynamics in the opticalgain. Although a clear understanding of the microscopic gain mechanism is still under debate, ithas been realised that interface radiative states associated with oxygen atoms can play a crucialrole in determining the emission properties of the Si-nc systems [9,10]. Here we report on lightamplification dynamic studies in Si-nc and we propose a simple phenomenological model toexplain our experimental results.

We have studied the structural, optical and electrical properties of MOS devices, where the dielectric layer consists of a substoichiometric SiOx (x < 2) thin film deposited by plasma enhanced chemical vapor deposition. After deposition the SiOx samples were annealed at high temperature (> 1000°C) to induce the separation of the Si and SiO2 phases with the formation of Si nanocrystals embedded in the insulating matrix. The effects of the Si concentration in the SiOx layer and of the annealing temperature on the electrical and optical properties of these devices are reported and discussed. It is shown that by increasing the Si content in the SiOx layers the operating voltage of the device decreases and the total efficiency of emission increases. In fact, devices having an active layer with high Si concentration (46 at.%) annealed at 1100 °C, exhibit the best performances (i.e. the highest electroluminescence intensity and the lowest operating voltage). Furthermore, we have observed that by decreasing the thickness of the SiOx layer from 75 to 25 nm it is possible to strongly reduce the operating voltage down to 4 V.