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Porous silicon micro-particles (micro-pSi) with size in the range of 1–10 μm are obtained by etching of silicon wafers followed by sonication. The derivatization of the micro-pSi surface by wet chemistry (silylation and coupling with a diamine) yields an interface, which exposes negative (carboxylic) or positive (amine) groups at pH 7.4. The surface modification, beyond the introduction of groups for the drug loading by covalent or electrostatic interactions, stabilizes the intense orange luminescence characteristic of the silicon nano-crystallites. Derivatization by amines introduces also a second emission in the blue region, which follows a different excitation pathway and can be attributed to the interface defects. The micro-pSi are efficiently internalized by human dendritic cells and do not show any toxic effect even at a concentration of 1 mg mL−1. The intrinsic luminescence of the differently functionalized micro-pSi is preserved inside the cells and permits the selective and efficient tracking of the microparticles without using molecular tags and thus leaving the organic coating available for the interaction with the drug. The results obtained suggest that the functionalized micro-pSi are an efficient platform for simultaneous imaging and delivery of therapeutic agents to the disease site.

A functionalization strategy, consisting of a silylation reaction by acrylic acid followed by diamine coupling, preserves and stabilizes the photoluminescence (PL) of porous silicon (pSi) microparticles suspended in ethanol. We found that under the condition of efficient amine coupling, besides the orange emission typical of the native pSi, an emission band in the blue region appears. The investigation of the interaction between pSi and diamine shows that diamine quenches and shifts the orange band meanwhile it induces an increase of the intensity of the blue one. PL lifetimes of the orange and blue bands are in the micro and nano second range, respectively. These values and their wavelength dependence clearly prove that the two bands have different origin: quantum confinement and nitrogen impurities introduced at silicon/silicon oxide interface, respectively. Thus, they can be used to discriminate between the pSi microparticles obtained by silylation, which expose carboxylic groups and the pSi microparticles after the diamine coupling, which bear amine functionalities at the surface. The increase in the stability of the PL emission of pSi in aqueous solution after functionalization, with quantum yields of the order of 1–2%, supports the use in biological systems of these brightly emitting, largely porous microparticles, bearing positive or negative surface charge.

Silicon is the most common semiconductor in technology. Its main limit is the lacking of efficient light emission properties. Here quantum effects are exploited to fabricate optically active silicon photonics elements. The manuscript reports recent results obtained in different research fields spanning from bio- to integrated-photonics and nonlinear optics.

Silicon nanocrystals (Si-nc) are reviewed here for their interesting optical applications. On the one hand, they do exhibit quantum confinement effects. This allows turning silicon into a light-emitting material where luminescence can be excited by electrical injection. On the other hand, small sizes, large surfaces, and dielectric mismatch between the core and the surrounding matrix increase dramatically the nonlinear optical coefficients. This allows using Si-nc as a nonlinear material in different waveguide configurations. In this paper, we discuss specifically two different applications of Si-nc: (i) as a nonlinear material in various devices, e.g., in bistable optical cavities, in waveguide optical mode monitors that are based on two-photon excited luminescence detection, and in wavelength shifters by using four-wave mixing (FWM); (ii) as an entropy source for quantum random number generation, the key device for cryptography. (a) Top view optical image of a Si-nc-based whispering-gallery mode (WGM) microdisk resonator, vertically coupled with a bus waveguide. (b) Detail of the coupling zone. (c) Normalized waveguide transmission spectrum over a wide spectral range. The inset shows a blow up of a resonance with the radial and azimuthal mode orders and the Q factor value indicated.

In this article we describe a reliable etching method to fabricate porous silicon free-standing membranes (FSMs) based on a self detachment of the porous layer in moderately doped n-type silicon substrates. We found that stable growth of smooth and straight pores is restricted to a narrow range of etching conditions and, unlike p-type substrates, the lift-off of the membrane is a self-limited process that does not require a large burst of current. The detachment of the porous membrane is independent of the structure of the already porosified layer, meaning that the average pore diameter can be tuned from nano to macro size within the same membrane. We also demonstrate that, despite their limited thickness, FSMs are quite robust and can sustained further processing. Thus, the etching receipt we are proposing here extends the range of sensors and filters that can be fabricated using porous silicon technology.

We report on a photonic sensor with an ultralow limit of detection (LoD) based on a phase interrogation readout scheme together with an anisotropic porous silicon (PSi) membrane. First, the fabrication of porous free-standing membranes from medium doped (100) surface oriented silicon, with pore diameters suitable for the infiltration of biomolecules, around 50 nm, is reported. Then, the phase interrogation scheme for characterizing the PSi membranes is presented whose results show that while volumetric sensitivity increases with the membrane thickness, the resolution in the birefringence measurements decrease dramatically due to depolarization effects. The best LoD was found to be equal to 6.25 ×10-6 RIU, from the thinnest 10-μm-thick membrane. Finally, the thermooptic coefficient of the 10 μm membrane was measured in an aqueous environment and shown to be equal to 8×10-4 rad/°C.

Silicon photonics has produced the first all-optical components intergrated into systems available already on the market. Nanosilicon, where low dimensional silicon allows new phenomena and devices, is a way to expand the reach of silicon photonics. In this article we review some of our past research activities focused on the demonstration of innovative photonic integrated circuits building blocks. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Porous silicon solid supports with pore diameter 0.5–1 μm, infiltrated with Ag nanostructures for surface enhanced Raman scattering (SERS) were prepared according to two procedures: spontaneous Ag+ reduction on the surface of freshly etched porous silicon immersed in Ag+ aqueous solutions, or anchoring colloidal Ag nanoparticles on the surface previously functionalized by aminosilane. Using Rhodamine 6G (RH6G) as analyte the detection limits were of the order of 20 μM and 20 nM with porous silicon metalized by the first and second procedure, respectively. This large increase of sensitivity notwithstanding a reduced surface density of Rhodamine 6G obtained on porous silicon metalized by the second procedure is discussed in terms of better hot spot efficiency and reduced aspecific binding out of the hot regions obtained depositing the colloids on the aminosilane functionalized surface.

Polymer pyrolysis to nanoceramics is employed for the production of photo-luminescent thin film samples, consisting of Si-nanocrystals dispersed in silica matrix. Thin films were deposited on Si wafer by spin-coating technique from a triethoxysilane-derived sol–gel solution. This is a fast process and a low cost technique. Samples were annealed at different temperatures between 600 °C and 1300 °C, under N 2 gas atmosphere. Structural evolution and luminescence from Si-ncs vs. temperature were investigated by FT-IR absorption spectroscopy and photo-luminescence. Absorption spectrum of As-Prep sample shows several vibrational bands due to Si–H, C–H x , and Si–O–Si structural units, without any appreciable luminescence. Silica phase segregation and intense PL (790 nm) due to Si-ncs were observed on annealing at T > 1000 °C.

We present a post-processing free quantum random number generator (QRNG) based on silicon nanocrystals (Si-NCs) LEDs as a source of randomness. The relatively simple setup for data extraction, a negligible bias measured from the datasets and applying no post-processing operations to the raw data are the main advantages of this QRNG . The obtained bit sequences pass all the NIST tests and the highest bit-rate achieved is 0.6 Mbps.

Here we report an investigation of the growth of picene by supersonic molecular beam deposition on thermal silicon oxide and on a self-assembled monolayer of hexamethyldisiloxane (HMDS). In both cases film morphology shows a structure with very sharp island edges and well-separated islands which size and height depend on the deposition conditions. Picene films growth on bare silicon covered with hydrophobic HDMS shows islands characterized by large regular crystallites of several micrometers; on the other hand, films growth on silicon oxide shows smaller and thicker islands. We analyzed the details of the growth model and describe it as a balancing mechanism involving the weak interaction between molecules and surface and the strong picene-picene interaction that leads to a different Schwoebel-Ehrlich barrier in the first layer with respect to the successive one. Finally, we study the charge transport properties of these films by fabricating field-effect transistors devices in both top and bottom contact configuration. We notice that substrate influences the electrical properties of the device and we obtained a maximum mobility value of 1.2 cm2 V-1 s-1 measured on top contact devices in air. © 2012 American Chemical Society.

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 layer is chemically etched in HF for different times. A decrease of optical anisotropy was observed in all cases.

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 the samples. The use of an optimized electrolyte allows the fabrication of very high quality samples, with high aspect ratio and low roughness both at the surface and on the pore walls. The best results are obtained with aprotic and protophilic solvents.

In this article we describe the fabrication of free standing n-type porous silicon microcavity (MC) and their properties as liquid sensors. We have optimized the etching recipe to keep both large pore size and high quality factor (Q-factor). Thus the fabricated porous layers have pore size in the range of 40 to 110 nm and are thus compatible with mass transport across the porous layer. We found that MC with a Q-factor of 60 can measure down to 1.1*10 -5 refractive index variations. Furthermore we analyze the role of non specific binding by comparing flow through versus flow over geometries. We compare these two approaches using different techniques and we show that flow over assay systematically overestimates the sensitivity of the device because of an inefficient rinse of the sample. Our work clearly indicates a limit in the reliability of measurements performed in flow over geometry unless specific controls are taken into account. Keywords: Porous silicon, Free standing membrane, Microcavity, Non specific signal, flow through, mass transport, electrochemical etching

The modeling, fabrication and characterization of PSi fabricated from both (110) and (100) surface oriented silicon for optical sensing is thoroughly reported. First, based on the generalized Bruggeman method, the birefringence and sensitivity of the fabricated membranes were calculated as a function of the fabrication parameters such as porosity and pore sizes; and external effects, such as the pores surface oxidation. Thereafter we report on the fabrication of PSi membranes from (110) and (100) surface oriented silicon with pore sizes in the range of 50 - 80 nm, and the characterization of their birefringence using a polarimetric setup. Their sensitivities were determined by filling the pores with several liquids having different refractive index. As a result, sensitivities as high as 1407 nm/RIU were obtained for the (110) samples at a 1500 nm wavelength and 382 nm/RIU for the (100) samples at the same wavelength.

In this work anisotropic porous silicon is investigated as a material for optical sensing. Birefringence and sensitivity of the anisotropic porous silicon membranes are thoroughly studied in the framework of Bruggeman model which is extended to incorporate the influence of environment effects, such as silicon oxidation. The membranes were also characterized optically demonstrating sensitivity as high as 1245 nm/RIU at 1500 nm. This experimental value only agrees with the theory when it takes into consideration the effect of silicon oxidation. Furthermore we demonstrate that oxidized porous silicon membranes have optical parameters with long term stability. Finally, we developed a new model to determine the contribution of the main depolarization sources to the overall depolarization process, and how it influences the measured spectra and the resolution of birefringence measurements.

Tensile strained-silicon waveguides were produced by LPCVD (780 °C) by depositing a 150 nm-thick Si 3 N 4 overlayer. Nonlinear nanosecond and femtosecond transmission experiments indicated the presence of a significant χ(2) in the bulk silicon.

Silicon photonics has produced the first all-optical components intergrated into systems available already on the market. Nanosilicon, where low dimensional silicon allows new phenomena and devices, is a way to expand the

Integrated silicon photonics is nowadays a mature technology and it is considered one of the most likely solution to overcome crucial limits of electronics systems. In fact photonics will enables the realization of integrated devices with greater data transmission capacity, reduced power consumption and smaller latency[1].

High formation yield and a meaningful cooled fraction of positronium below room temperature were obtained by implanting positrons in a silicon target in which well-controlled oxidized nanochannels (5-8 nm in diameter) perpendicular to the surface were produced. We show that by implanting positrons at 7 keV in the target held at 150 K, about 27% of positrons form positronium that escapes into the vacuum. Around 9% of the escaped positronium is cooled by collision with the walls of nanochannels and is emitted with a Maxwellian beam at 150 K. Because positronium quantum confinement limits the minimum achievable positronium energy, the tuning of the nanochannel's size is crucial for obtaining positronium gases in vacuum at very low temperature.

We report on a novel organic/inorganic hybrid waveguide approach, which is composed of a cladding of extremely low refractive index oxidized porous silicon formed on a bulk silicon substrate and of it, a polymeric (polymethylmethacrylate) core doped with a visible laser dye (Nile-Blue) was deposited by spin coating. The waveguiding properties of the structures have been characterised by means of the m-line technique, demonstrating that the use of oxidized porous silicon as a cladding can considerably improve the mode confinement factor of single-mode waveguides. The low refractive index achievable in the cladding (n=1.16) allows forming waveguides with a low index polymer cores. Variable stripe length (VSL) measurements have been also performed in order to characterise the amplification properties of the waveguides. We demonstrate a clear transition from losses to gain at 694nm with a pump threshold of 28mJ/cm 2 . Values of net optical gain up to 104dB/cm have been measured at this wavelength.

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 scattering behavior. A way to change the structure is by modifying the composition of the electrochemical solution. We have found that best results are achieved using ethanoic solutions with HF concentration levels between 13% and 15%., 3 pages, 4 figures, package SIunits required

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.

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. The continuous improvements of silicon technology have made possible to grow routinely 200 mm single silicon crystals at low cost, and even larger crystals are now under development. The high integration levels reached by the silicon microelectronic industry have permitted high-speed device performances and unprecedented interconnection levels [1]. However, today the required interconnections between devices are sufficient to cause critical propagation delays, over heating and information latency. To overcome this interconnection bottleneck is together the main motivation and opportunity for silicon Microphotonics, where attempts to combine photonics and electronic components on a single Si chip or wafer are strongly pursued. In addition, photonics aims to combine the power of silicon microelectronics with the advantages of photonics. In this way it is expected that the continuous increase of chip performances predicted by Moore’s law can be ultimately faced.

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 indicates that the porous silicon anodization process has a natural drift along the growth direction. However, we demonstrate that this drift can be compensated, showing a homogeneous structure of ten coupled microcavities, in which all ten resonance peaks are resolved in both transmission and reflection measurements.

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 cone defines structured light rings onto a screen normal to the cone axis. We explain the various structures by optical anisotropy of porous silicon. For the sample under analysis, we directly measure from the ring patterns a value of Δn/nord=8% of positive birefringence.

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.

We numerically investigate nonlinear self-polarization flipping in a silicon waveguide. We identify specific silicon waveguide geometries that enhance this effect to facilitate its fabrication and experimental demonstration by varying various parameters such as fabrication distortion, waveguide loss, dispersion and laser noise to design the silicon waveguide. In optimized waveguides, we show that nonlinear self-polarization flipping can be observed with few tens of watts peak power pulses with widths as short as 60 ps and laser noise level as large as 7%. Refereed/Peer-reviewed

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 nanocrystals. Needless to say, all these devices rely on quantum confinement and on the interplay between bulk and surface properties. Size effects and surface reconstructions are two critical issues which one has to master to employ silicon nanocrystals. Finally the use of silicon nanocrystals to develop innovative sensing mechanisms to reveal biomolecules or poisoning gasses will be 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 field and report on our progresses. In Trento we are using nanocrystalline silicon to change the dimensionality of silicon so that very fascinating and new optical properties of the material appear. In particular, light emission starts to be a very efficient process in nanostructured silicon and light emitting diodes with high efficiency have been fabricated. We will discuss some of the main research lines. These cover a wide spectral range spanning from visible to infrared. Light sources, modulators, waveguides, logical gates are a few examples of the various photonic devices which have been developed based on silicon nanocrystals.

We report on a new hybrid approach to realize optical slab waveguides for optical amplification purposes. The structure consists of a dye-doped polymer core (PMMA) deposited over an oxidized porous silicon (PS) cladding layer formed on a silicon wafer. The very low refractive index ( n = 1.16) achievable in the cladding allows obtaining monomodal behavior with high confinement factors ( Γ TE = 96%) even for very thin cores (400 nm). Optically excited guided luminescence shows stimulated emission, strong line narrowing and a clear threshold and superlinear behavior with pump energy. By means of the variable stripe length (VSL) technique, values of net optical gain up to 113 dB/cm (constant over 3 mm) and absolute amplification values up to 34 dB have been measured at 694 nm when pumping with 80 mJ/cm 2 energy pulses. These results validate the use of oxidized PS as a cladding layer in silicon photonics.

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.

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, photon trapping with evidences of light localization, and gas sensing properties.