Zeno Gaburro

• Harvard University

This article deals with generalities and definitions of porous silicon (PSi): fabrication techniques, structural properties, chemical properties, electronic properties, electrical properties, optical properties, and actual or potential applications of PSi. Optical properties include light transport, photoluminescence, and electroluminescence.

An optical plate includes a substrate and a resonator structure formed on or in the substrate, wherein the resonator structure is configured to produce an abrupt change in phase, amplitude and/or polarization of incident radiation.

This chapter discusses a few models with varying complexity illustrating the physics behind the phase response of optical antennas. It describes 1D and 2D oscillator models considering explicitly the radiation damping effects besides the ohmic absorption losses for plasmonic antennas supporting one plasmonic mode and two independent, orthogonal pla...

Conventional optical components rely on the propagation effect to control the phase and polarization of light beams. One can instead exploit abrupt phase and polarization changes associated with scattered light from optical resonators to control light propagation. In this paper, we discuss the optical responses of anisotropic plasmonic antennas and...

A three-dimensional extension of the recently demonstrated generalization of the laws of refraction and reflection was investigated for both flat and curved metasurfaces. We found that out-of-plane refraction occurs for a metasurface that imparts a wavevector out of the plane of incidence onto the incident light beam. Metasurfaces provide arbitrary...

We demonstrate optically thin quarter-wave plates built with metasurfaces that generate high-quality circularly-polarized light over a broad wavelength range for arbitrary orientation of the incident linear polarization. The metasurface consists of an array of plasmonic antennas with spatially varying phase and polarization response. Experimentally...

The concept of optical phase discontinuities is applied to the design and demonstration of aberration-free planar lenses and axicons, comprising a phased array of ultrathin subwavelength spaced optical antennas. The lenses and axicons consist of radial distributions of V-shaped nanoantennas that generate respectively spherical wavefronts and non-di...

Conventional optical components such as lenses and holograms rely on gradual phase shifts accumulated during light propagation to shape light beam. New degrees of freedom in optical design are attained by introducing in the optical path abrupt phase changes over the scale of the wavelength. In this talk, we will discuss the results presented in [1]...

The manipulation of light by conventional optical components such as lenses, prisms, and waveplates involves engineering of the wavefront as it propagates through an optically thick medium. A unique class of flat optical components with high functionality can be designed by introducing abrupt phase shifts into the optical path, utilizing the resona...

Conventional optical components rely on gradual phase shifts accumulated during light propagation to shape light beams. New degrees of freedom are attained by introducing abrupt phase changes over the scale of the wavelength.

We report broadband birefringent metainterfaces comprising 2D arrays of phased optical antennas that can create light beams with arbitrary polarization states and arbitrary propagation directions over a wide spectral range (from λ5 to 14 μm).

3-dimensional laws of reflection and refraction are derived and demonstrated for thin interfaces that impart to the incident wavefront a phase gradient.The tangential wavevector provided by the interface creates out-of-plane reflection and refraction

We present a simplified numerical method to solve for the current distribution in a V-shaped antenna excited by an electric field with arbitrary polarization. The scattered far-field amplitude, phase, and polarization of the antennas are extracted. The calculation technique presented here is an efficient method for probing the large design paramete...

Experiments on ultrathin anisotropic arrays of subwavelength optical antennas display out-of-plane refraction. A powerful three-dimensional (3D) extension of the recently demonstrated generalized laws of refraction and reflection shows that the interface imparts a tangential wavevector to the incident light leading to anomalous beams, which in gene...

A flat optical device that generates optical vortices with a variety of topological charges is demonstrated. This device spatially modulates light beams over a distance much smaller than the wavelength in the direction of propagation by means of an array of V-shaped plasmonic antennas with sub-wavelength separation. Optical vortices are shown to de...

We demonstrate that plasmonic two-oscillator elements such as V- and Y-shaped antennas can locally tailor the phase of light over 2π. A theoretical model and full-wave simulations explain the measured spectral response of these elements.

Ultra-thin and integrated photonic devices based on optical phase discontinuities can generate optical vortices with a variety of topological charges, as well as sort out vortex beam topological charges by means of holographic gratings.

We explore the relationship between the near-field enhancement, absorption, and scattering spectra of localized plasmonic elements. A simple oscillator model including both internal and radiative damping is developed, and is shown to accurately capture the near- and far-field spectral features of linear optical antennas, including their phase respo...

The phase response of plasmonic antennas is used to create designer interfaces, based on which we demonstrated generalized laws of reflection and refraction in the presence of phase discontinuities and created optical vortex beams.

Conventional optical components rely on gradual phase shifts accumulated during light propagation to shape light beams. New degrees of freedom are attained by introducing abrupt phase changes over the scale of the wavelength. A two-dimensional array of optical resonators with spatially varying phase response and subwavelength separation can imprint...

Time-resolved luminescence measurements on silicon nanocrystal waveguides have revealed a fast recombination dynamics, related to population inversion which leads to net optical gain. The waveguide samples were obtained by thermal annealing of plasma enhanced chemical vapour deposited thin layers of silicon rich oxide Variable stripe length measure...

We report photoluminescence and electroluminescence at room temperature in diodes based on Si/SiO2 multilayers. The multilayers are fabricated by alternating Si and SiO2 layers, whose thickness is, respectively, 2 and 5 nanometers. In photoluminescence, a single band is observed, centered at 800 nm, which is due to electron-hole pair recombination...

We experimentally demonstrate that porous silicon optical microcavities can be effectively used as multi-parametric gas sensors. As known, the photoluminescence intensity and electrical conduction of porous silicon are strongly dependent on environmental properties, such as the dipole moment of molecules of surrounding gases. The sensitivity is lar...

A design is proposed that allows non-stationary field distribution with Bragg gratings in multiple slot waveguides. Selective coupling between modes is achieved or suppressed, according to controllable selection rules, based on mode symmetry. By applying such rules, backward pulling radiation pressure - i.e. toward the light source - can be obtaine...

Novel photonic devices, based on time-dependent dielectrics, to shift the optical frequency, may be conceived from two complementary principles, Doppler shift and time re- fraction, and possibly realized as as single cavities or as Coupled Resonator Optical Waveguides (CROWs). Simulations with the finite-difference time-domain method bore out these...

We review here our work on the photonic and sensor applications of nanostructured silicon. As we change the dimensionality of silicon very fascinating and new optical properties of the material appear. Light sources, modulators, waveguides, logical gates are a few examples of the various photonic devices which have been developed based on silicon n...

Electrical transport and light emission properties of plasma-enhanced chemical vapor deposition grown light emitting devices (LEDs) based on nanocrystalline silicon have been studied. Various active layer compositions have been used. Electroluminescence and current-voltage measurements have been performed on metal-oxide-semiconductor structures. We...

We demonstrate steady state pulsed pumping of a lithium niobate whispering gallery disk resonator. Coherent excitation is achieved by matching the repetition rate of a pumping mode-locked laser to a sub multiple of the free spectral range of one of the sets of modes of the disk. Once the disk is excited, we measure the resonator’s quality factor by...

We have excited a passive LiNbO3 whispering gallery resonator with an external mode-locked laser. This leads to a phase-coherent excitation of several modes of the resonator. The repetition rate is tuned to an integer submultiple 1/N of the free spectral range of the resonator. The output rate of the resonator is equal to the input rate multiplied...

The optical transmission through a porous superlattice is modulated by capillary condensation and evaporation of ethanol. The system exhibits optical bistability, with the two stable states characterized by a different amount of liquid ethanol in the pores. Switching between states is controlled by the input power of the incident light beam. By tun...

Electrical carrier injection into PECVD grown silicon-nanocrystals-based LEDs was examined by I-V, C-V, and impedance measurements. Electroluminescence was measured as a function of gate AC frequency. The correlations between conduction mechanism and electroluminescence are discussed.

The communications and computing industries are challenged to fasten and enlarge information exchange. The electrical chip routing challenges in the computer industry have driven the interest for silicon photonics. The idea is to integrate photonic circuits into a standard microelectronics process. In this paper we will review the status of the fie...

We show a vapor controlled one-dimensional photonic crystal built with a silicon-based dielectric mesoporous material, in which the refractive index can be continuously tuned by flowing organic vapors through the pores. Exposure of the crystal to vapors induces an inhomogeneous change in the refractive index through the depth and results in a tilte...

Photonic materials, which have optical properties that can be modulated by light, are extremely interesting both for their fundamental properties as well as for their potential in the applications of all-optical signal processing and possibly optical computing1. Earlier studies have been based on nonlinear photonic crystals, and have required relat...

Aqueous solutions of silicon nanocrystals have been obtained, by sonication, from porous Si (p-Si) aged in air for various times. The photoluminescence of these solutions changes with the aging time of p-Si. These changes correlate with nanocrystal core dimensions, i.e. with the oxidation of the nanocrystals. Infrared spectra show that the reaction...

We propose a time-dependent, spatially periodic photonic structure which is able to shift the carrier frequency of an optical pulse which propagates through it. Taking advantage of the slow group velocity of light in periodic photonic structures, the wavelength conversion process can be performed with an efficiency close to 1 and without affecting...

We study a vapor-controlled optical superlattice realized with a silicon-based dielectric mesoporous material. By flowing organic vapors through the nanometer-sized pores, the position dependent refractive index can be continuously tuned, resulting in a tilted photonic band structure. A careful design of pore size distribution, close to the critica...

We report on the observation of Zener tunnelling of light waves in spectral and time-resolved transmission measurements, performed on an optical superlattice of porous silicon with broken translational symmetry.

Nonlinear optical refraction and absorption have been measured on Si nanocrystals grown by plasma-enhanced- chemical-vapour-deposition. Strong nonlinearities were found at 830 nm and at 1552 nm. Different behaviours, depending on the pump-pulse duration, have been observed

A systematic study of nonlinear optical properties of silicon nanocrystals (Si-nc) grown by plasma enhanced chemical vapor deposition (PECVD) is reported. Nonlinear optical refraction and absorption have been measured by z-scan technique at three different time regimes and at different wavelengths to investigate both the thermal and electronic resp...

We study the effect on a LED of a controlled insertion of N in Si oxide containing Si nanocrystals. Nitrogen lowers the electronic potential barrier, thus increasing the carrier injection. Electroluminescence spectroscopy suggests that both electron and hole injection can be observed

We report on optical analogues of well-known electronic phenomena such as Bloch oscillations and electrical Zener breakdown. We describe and detail the experimental observation of Bloch oscillations and resonant Zener tunneling of light waves in static and time-resolved transmission measurements performed on optical superlattices. Optical superlatt...

We propose a time-dependent photonic structure, in which the carrier frequency of an optical pulse is shifted without changing its shape. The efficiency of the device takes advantage of slow group velocities of light attainable in periodic photonic structures. The frequency shifting effect is quantitatively studied by means of Finite Difference Tim...

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...

We present an experimental work on porous silicon-based optical devices. Notch filters and planar waveguides are fabricated and characterized. Three different types of filters are shown, the first one is a stop-band filter in the 1.5 mu m region, where improvements have been performed (smoothing of the index profile, apodization and index-matching)...

We report on optical analogues of well-known electronic phenomena such as Bloch oscillations and electrical Zener breakdown. We describe and detail the experimental observation of Bloch oscillations and resonant Zener tunneling of light waves in static and time-resolved transmission measurements performed on optical superlattices. Optical superlatt...

We report on the observation of time resolved Bloch oscillations of light waves in optical superlattice structures. The structures are series of coupled microcavities, which are grown in porous silicon with high control of optical parameters. A controlled linear gradient of refractive index along the growth direction was maintained to tilt the phot...

The electrical injection in porous silicon fabricated with heavily doped p-type silicon is very sensitive to NO2. The known effect is an injection increase associated to NO2. We show experimentally a strong correlation between two structural properties and the sensitivity of electrical injection to NO2. The first property is the microstructure, i.e...

We report an experimental study of porous silicon-based rugate filters. Possible optimisations that can improve different features of the filters are investigated. We demonstrate sidelobe attenuation by means of half-apodization of the structures with a sinusoidal window. Reduction of interference ripples are experi-mentally observed through the in...

After electrochemical etching, we have made a study of the effects generated on p(+)-type porous silicon layers when they are left in presence of the electrolyte for different post-etching times. Using an interferometric technique, we have monitored the change of its porosity during the post-etch process due to a chemical dissolution mechanism. The...

We report on the observation of Zener tunneling of light waves in spectral and time-resolved transmission measurements, performed on an optical superlattice made of porous silicon. The structure was designed to have two photonic minibands, spaced by a narrow frequency gap. A gradient in the refractive index was introduced to create two optical Wann...

We report on a polarization-sensitive optical gain in a blue-emitting Si/SiO(2) nanocrystalline system having a high degree of emission polarization memory. This system can show a positive optical gain or optical loss depending on the polarization state of the pump and emitted light. Under optical gain conditions, the degree of polarization of the...

Complementary metal-oxide-semiconductor-compatible tunable Fabry-Perot microcavities filled with liquid crystals (LCs) were realized and studied in the near-infrared region. The microcavities were produced by chip bonding technique, which allows one to infill LC between two [SiO2/Si]n lambda/4 (lambda = 1.5 mum) Dielectric Bragg Reflectors separate...

The electrical conductance of porous silicon fabricated with heavily doped p-type silicon is very sensitive to NO ₂. We show that the sign of the injection variations depends on the porous layer thickness. If the thickness is sufficiently low—of the order of few μm—the injection decreases instead of increasing. We discuss the effect in t...

The progress of silicon electronic industry is based on scaling down the minimum feature size of integrated circuits. Speed, density and costs of devices improve with scaling, but unfortunately the performance of interconnect worsens, both in terms of speed and power consumption. This issue -- the ‘‘interconnect bottleneck’’ -- is envisioned as a c...

Porous silicon (PS) conductometric gas sensors can exhibit large sensitivity to gases, due to the large surface versus volume ratio of porous silicon. A possible application is the detection of traces of nitrogen dioxide (NO2), an air pollutant. DeltaG/G signals in excess of 10 in the presence of concentrations as low as 50 ppb in dry air can be de...

We report on the observation of resonant Zener tunnelling of light waves in an optical superlattice. The one dimensional (1D) structures are made in free-standing porous silicon and are designed specifically to exhibit two photonic minibands. A controlled optical path gradient has been maintained over the sample thickness which resulted in tilting...

We report measurements of angle-resolved light scattering in porous silicon films. Scattering rings were observed, and their aperture allowed to measure optical birefringence. These values are confirmed with other techniques. We study birefringence changes when pores are filled with liquids, after thermal annealing, and when the porous silicon laye...

Electrical conductivity of porous silicon fabricated form heavily doped p-type silicon is very sensitive to NO$_2$, even at concentrations below 100 ppb. However, sensitivity strongly depends on the porous microstructure. The structural difference between sensitive and insensitive samples is independently confirmed by microscopy images and by light...

The electrical conductance of porous silicon fabricated with heavily doped p-type silicon is very sensitive to NO$_2$. A concentration of 10 ppb can be detected by monitoring the current injection at fixed voltage. However, we show that the sign of the injection variations depends on the porous layer thickness. If the thickness is sufficiently low...

The work function of nano Porous Silicon (PS) has been studied by the kelvin probe method as a function of the exposure to different gaseous species. Characterisation has been performed n dark and in presence of sub band and supra band gap light - Surface Photovoltage (SPV)measurements. Traces of ammonia and nitrogen dioxide change drastically the...

Silicon microphotonics, a technology which merges photonics and silicon microelectronic components, is rapidly evolving. Many different fields of application are emerging: transceiver modules for optical communication systems, optical bus systems for ULSI circuits, I/O stages for SOC, displays, .... In this review I will give a brief motivation for...

Porous silicon free-standing microcavity structures, with different layer designs, have been fabricated. Single microcavities show transmission resonances in the technologically relevant wavelength region of 1.55 μm with quality factors up to 3380. High-order cavities show sub-nm transmission peaks over the whole stop band. Coupled microcavity stru...

In this work we present a method for monitoring the optical parameters of a film during its etching process. Optical interferometry of two laser beams with different angles of incidence is observed to measure the index of refraction profile and the etch rate evolution simultaneously. With this technique we have measured the inhomogeneity in the etc...

We have fabricated a multiparametric gas sensor based on porous silicon. The sensing parameters are the electrical conductivity of a single porous Si layer and the resonance wavelength of a porous silicon microcavity, both fabricated on the same substrate. The electrical conductivity allows detection of 50 ppb of nitrogen dioxide in dry air. Althou...

We discuss the optical properties of one-dimensional complex dielectric systems, in particular the time-resolved transmission through thick porous silicon quasiperiodic multi-layers. Both in numerical calculations and experiments we find dramatic distortion effects, i.e. pulse stretching and coherent beatings, when band-edge states are resonantly e...

We have studied the properties of p+-type doped porous silicon, formed by electrochemical etching, when is left in presence of the electrolyte for different post-etching times. Using an interferometric technique, we monitored the formation of the porous silicon layer during the electrochemical treatment as well as the change of its porosity during...

We report a nitrogen dioxide sensor suitable for environmental control, based on the change in conductivity of a single meso-porous silicon (PS) layer with about 80% porosity. We present the characterization of the sensor in the presence of low concentrations of NO2 in dry air and study the influence of humidity on sensor behaviour. The sensor show...

The propagation of light in nonperiodic quasicrystals is studied by ultrashort pulse interferometry. Samples consist of multilayer dielectric structures of the Fibonacci type and are realized from porous silicon. We observe mode beating and strong pulse stretching in the light transport through these systems, and a strongly suppressed group velocit...

Photonics applications of silicon are presented. In particular it is demonstrated that silicon when rendered low dimensional, e.g. in form of nanocrystals or quantum wires, can be turned into an active photonic materials which shows light amplification characteristics, non-linear optical effects, photon confinement in both one and two dimensions, p...

Observation of optical gain in silicon nanocrystals (Si-nc) is critically dependent on a very delicate balance among the Si-nc gain cross-sections, the optical mode losses and confinement factors of the waveguide structures, the Si-nc concentration and the strongly competing fast non-radiative Auger processes. Here we report on optical gain measure...

We report the experimental characterization of porous silicon free-standing coupled microcavities. We have grown free-standing structures of up to 109 stacked layers. Free-standing structures are interesting because reflectance spectra can be measured on both sides of the samples. The comparison of reflectance spectra from the front and back side i...

The transport of light in complex dielectric materials is a rich and fascinating topic of research. With complex dielectrics we intend dielectric structures with an index of refraction that has variations on a length scales that is very roughly comparable to the wavelength. Such structures strongly scatter light. A possible building block for const...

Silicon is the electronic material per excellence. Integration and economy of scale are the two keys ingredients for the silicon technological success. Silicon has a band-gap of 1.12 eV, which is ideal for room temperature operation, and an oxide (SiO2), which allows the processing flexibility to place today more than 108 devices on a single chip....

CMOS circuitry dominates the current semiconductor market due to the astonishing power of silicon electronic integration technology. In contrast to the dominance of silicon in electronics, photonics utilises a diversity of materials for emitting, guiding, modulating and detecting light. In the last ten years a big research effort was aimed to rende...

We report the observation of strongly anisotropic scattering of laser light at oblique incidence on (100)-oriented porous silicon layers. We performed angle-resolved light scattering measurements and three concentric rings were observed. Modeling porous silicon by means of nanometric columnar air pores and an effective anisotropic uniaxial dielectr...

We report the experimental characterization of porous silicon free-standing coupled microcavities and the investigation of light transport properties in such structures, when ultrashort laser pulses are introduced and analyzed.

We report the observation of strongly anisotropic scattering of laser light at oblique incidence on a (100)-oriented porous silicon layer. The scattered light forms cones tangent to the incident and reflected beams. The conical pattern is caused by scattering on the vertical walls of pores, which are straight along the layer thickness. The light co...

Macroporous silicon with two-dimensional periodicity has been produced by electrochemical etching, using a p-type doped silicon substrate. The structure shows photonic energy gaps in the infrared region, as demonstrated by variable angle reflectance measurements. The agreement between measurement and band calculations confirms the high quality of t...

Optical gain has been recently observed in ion implanted Si nanocrystals (nc). Critical issues to the observation of optical gain are the formation of a waveguide structure to improve the mode confinement and a large nanocrystal area den-sity in the samples. Here we confirm these results by measuring optical gain by the variable stripe length (VSL)...

Nanostructuring silicon is an effective way to turn silicon into a photonic material. In fact, low-dimensional silicon shows light amplification characteristics, non-linear optical effects, photon confinement in both one and two dimensions, photon trapping with evidence of light localization, and gas-sensing properties.

Near-field optical spectroscopy has been used to investigate photoluminescence features of porous silicon microcavity samples with a subwavelength space resolution. The emission is found to be markedly dependent on the lateral position, with the presence of relatively narrow spectral features peaked at different wavelengths in the range 610-690 nm....

We provide a systematic study on the linear and nonlinear optical properties of silicon nanocrystals (Si-nc) grown by plasma-enhanced chemical vapour deposition (PECVD). Linear optical properties, namely absorption, emission and refractive indices are reported. The sign and magnitude of both real and imaginary parts of third-order nonlinear suscept...

The real and imaginary parts of third-order nonlinear susceptibility χ(3) have been measured for silicon nanocrystals embedded in SiO2 matrix, formed by high temperature annealing of SiOx films prepared by plasma-enhanced chemical vapor deposition. Measurements have been performed using a femtosecond Ti–sapphire laser at 813 nm using the Z-scan tec...

We investigated the possibility of using several sensing parameters from porous silicon in order to improve gas selectivity. By fabricating porous silicon optical microcavities, three independent quantities can be measured, i. e. the electrical conductance, the photoluminescence intensity, and the wavelength of the optical resonance. We monitored t...

Variable-angle reflectance performed on macroporous silicon photonic crystals yields the dispersion of two-dimensional photonic bands. A comparison with calculated optical spectra identifies the spectral structures, which mark the onset of a propagating photonic mode, as one-dimensional critical points. The experimental results agree with theoretic...

Nanostructuring silicon is an effective way to turn silicon into a photonic material. In fact low dimensional silicon shows light amplification characteristics, non-linear optical effects, photon confinement in both one and two dimensions, photon trapping with evidence of light localization, and gas sensing properties.

Infrared time-resolved interferometric transmission measurements have been performed on one dimensional porous silicon Fibonacci quasicrystals, obtained by electrochemical etching a p-type Si substrate, to address experimentally the problem of light transport and localization in deterministic aperiodic structures. Coherent beatings, pulse stretchin...

Optical gain has been recently observed in ion implanted Si nanocrystals (nc).1 Critical issues to the observation of opti-cal gain are the formation of a waveguide structure to improve the mode confinement and a large nanocrystal area den-sity in the samples. Here we confirm these results by measuring optical gain by the variable stripe length (VS...