Valves of Coscinodiscus wailesii diatoms, monocellular micro-algae characterized by a diameter between 100 and 200
μm, show regular pores patterns which confine light in a spot of few μm2. This effect can be ascribed to the
superposition of diffracted wave fronts coming from the pores on the valve surface. We studied the transmission of
partially coherent light, at different wavelengths, through single valves of Coscinodiscus wailesii diatoms. The spatial
distribution of transmitted light strongly depends on the wavelength of the incident radiation. Numerical simulations
help to demonstrate how this effect is not present in the ultraviolet region of the light spectrum, showing one of the
possible evolutionary advantages represented by the regular pores patterns of the valves.
In this paper the realization and the characterization of a resonant cavity enhanced photodetector (RCE), completely
silicon compatible and working at 1.55 micron, is reported. The detector is a RCE structure incorporating a Schottky
diode and its working principle is based on the internal photoemission effect. In order to obtain a fabrication process
completely compatible with standard CMOS silicon technology, a photodetector having copper (Cu) as Schottky metal
has been realized. Performances devices in terms of responsivity, free spectral range, finesse are reported.
Diatoms are monocellular micro-algae provided with external valves, the frustules, made of amorphous hydrated silica.
Frustules present patterns of regular arrays of holes, the areolae, characterized by sub-micrometric dimensions. In
particular, frustules from centric diatoms are characterized by a radial disposition of areolae and exhibit several optical
properties, such as photoluminescence variations in presence of organic vapors and photonic-crystal-like behaviour as
long as propagation of electromagnetic field is concerned.
We have studied the transmission of coherent light, at different wavelengths, through single frustules of Coscinodiscus
Walesii diatoms, a centric species characterized by a diameter of about 150 μm. The frustules showed the ability to
focalize the light in a spot of a few μm2, the focal length depending on the wavelength of the incident radiation. This
focusing effect takes place at the centre of the frustule, where no areolae are present and, as it is confirmed by numerical
simulations, it is probably due to coherent superposition of unfocused wave fronts coming from the surrounding areolae.
Diatoms-based micro-lenses could be used in the production of lensed optical fibers without modifying the glass core
and, in general, they could be exploited with success in most of the optical micro-arrays.
In this paper, the design of resonant cavity enhanced photodetectors, working at 1.55 micron and based on silicon
technology, is reported. The photon absorption is due to internal photoemission effect over the Schottky barrier at the
metal-silicon interface. A comparison is presented among three different photodetectors having as Schottky metal: gold,
aluminium or copper respectively. In order to quantify the performance of photodetector, quantum efficiency including
the image force effect, as a function of bias voltage is calculated.
Light passing through a photonic crystal can undergo a negative or a positive refraction. The two refraction states can be functions of the contrast index, the incident angle and the slab thickness. By suitably using these properties it is possible to realize very simple and very efficient optical components to route the light. As example we present two devices: a passive device acting as a polarizing beam splitter and a tunable switch. In the first device TM polarization is refracted in positive direction whereas TE component is negatively refracted, in the second device the light is positively refracted at room temperature and negatively refracted varying the local temperature of the device.
In this paper, a methodology for the analysis of a resonant cavity enhanced (RCE) photodetector, based on internal
photoemission effect and working at 1.55 &mgr;m, is reported. In order to quantify the performance of photodetector, quantum
efficiency including the image force effect, bandwidth and dark current as a function of bias voltage are calculated.
We propose a comparison among three different Schottky barrier Silicon photodetectors, having as metal layers gold, silver
or copper respectively. We obtain that the highest efficiency (0.2%) but also the highest dark current is obtained with metal
having the lowest barrier, while for all devices, values of order of 100GHz and 100MHz were obtained, respectively, for the
carrier-transit time limited 3-dB bandwidth and bandwidth-efficiency.
In this work, we have compared the optical characteristic of two different photonic dielectric multilayers based on the porous silicon technology. We designed and realized two models devices: a Bragg mirror and the S6 Thue-Morse sequence. Both the structures have the same thickness, the same porosity, and even the same number of the layers but differently spatially ordered. We demonstrate that the two arrangements of the layers influence not only the optical features of these interferometric devices but also their sensitivity when used as optical sensors. We have measured the change of the reflectivity spectra of the devices on exposure to several organic compounds. The experimental results demonstrated that the Thue-Morse aperiodic structure is more sensitive than the Bragg device due to a higher filling capability.
Complex micro- and nano-structured materials for photonic applications are designed and fabricated using top technologies. A completely different approach to engineering systems at the sub-micron-scale consists in recognizing the nanostructures and morphologies that nature has optimized during life's history on earth. In fact, biological organisms could exhibit ordered geometries and complex photonic structures which often overcome the products of the best available fabrication technologies. An example is given by diatoms. They are microalgae with a peculiar cell wall made of amorphous hydrated silica valves, reciprocally interconnected in a structure called the frustule. Valve surfaces exhibit specie-specific patterns of regular arrays of chambers, called areolae, developed into the frustule depth. Areolae range in diameter from few hundreds of nanometers up to few microns, and can be circular, polygonal or elongate. The formation of these patterns can be modeled by self-organised phase separation. Despite of the high level of knowledge on the genesis and morphology of diatom frustules, their functions are not completely understood. In this work, we show that the silica skeletons of marine diatoms, characterized by a photonic crystal-like structure, have surprising optical properties, being capable of filtering and focalizing light, as well as exhibiting optical sensing capabilities.
In this paper we present a general methodology for the design of resonant cavity enhanced (RCE) photodetectors based on the internal photoemission effect. In order to estimate the theoretical quantum efficiency we take advantage of the analytical formulation of the internal photoemission effect (Fowler theory), and its extension for thin films. In particular, the absorptance is numerically determined by means of an approach based on the transfer matrix method. Finally, we apply the proposed methodology to the design of a silicon RCE photodetector operating at 1.55μm, based on the internal
photoemission effect at an Au-Si schottky barrier.
In the last years there is a considerable interest in designing integrated optoelectronic or all-optical circuits based on photonic crystals (PhC). A PhC structure possess photonic band gap in which the light with a certain frequency range cannot propagate. However, the existence of linear defects causes dispersion relations in photonic band gaps. Light that satisfies in the dispersion relations decay except linear defects and can exist only in linear defects. Modifying some scatters it is possible to create a waveguide inside the PC. This waveguide have great potential in application for their ability to control light wave propagation and the possibilities of implementing PhC based optical devices. We propose a PhC diplexer based on a square lattice of silicon rods. The demupltiplexing mode is fed exploiting the different dispersion relation of the light in the three braches of a T-junction.
A difficult challenge is to realise active PhC devices. In order to achieve tunable photonic band gap devices, we investigate the possibility to use the thermo-optic effect and the Liquid Crystals (LCs). The main feature of LCs is the high sensitivity of their optical response to an applied electrical field. Moreover their ability to be micromanipulated, their low cost and the possibility for integration with silicon circuit technology make LCs particularly attractive in designing photonic devices.
It is well known by far that biological organisms could exhibit sophisticated optical system, which compete or overcame the top technology products available. The diatoms are microscopic algae enclosed in intricate amorphous silica cells, called frustules. In this work the optical reflectivity data, infrared spectroscopy, scanning electron microscopy and photoluminescence (PL) characterizations are presented for silica shells of Coscinodiscus wailesii, which is a centric diatom characterized from a diameter that varies in the range between 100 and 500 μm. Preliminary results suggest that the Coscinodiscus wailesii can be used as photonic material and sensor transducer.
Fluid-solid interfacial phenomena are a subject of much interest. In the adsorption phenomena, the adsorbent experiences the action of the molecular forces inducing strains.
In this paper, we experimentally investigate adsorption phenomena in porous silicon microcavities by spontaneous Raman scattering. Polarised Raman spectra are measured in a backscattering configuration using a diode laser at 404 nm. We observe a reversible blue shift of the Raman spectra exposing a porous silicon multilayer to air saturated with vapor of pentane or iso-propanol. We ascribe the shift of the Raman spectra to the strain in porous silicon due to the adsorption in the pore walls.
Silicon optical receivers, operating at the optical communication wavelengths in the 1.3-1.55 μm range, have attracted much research effort. Unfortunately, the performance of the devices proposed in literature are poor because this wavelength range is beyond the absorption edge of silicon. In order to extend the maximum detectable wavelength, the most common approach, in the realization of Si-based detectors, is the use of silicon-germanium layers on silicon, anyway, requiring processes non compatible with standard CMOS technology. In this paper, with the aim to extend the operation of silicon-based photo-detectors up to the 1.3-1.55 μm range, an alternative approach is investigated: we propose the design of a resonant cavity enhanced Schottky photodetector based on the internal photoemission effect. The device fabrication is completely compatible with standard silicon technology.
One of the most important optical properties of photonic crystals is that the waveguide dispersion relations can be tailored and allow for many non-conventional applications such as guiding and processing of the light signal. On the other hand, a variety of physical phenomena make liquid crystals (LC's) one of the most interesting subject of modern fundamental science. Moreover, in the last years, it has been proved that in order to obtain active tuning of the photonic crystals device a very promising approach can be achieved by infiltrating photonic structure with liquid crystals.
On this line of argument, in this paper, the design of an electro-optical switch based on 2D silicon photonic band-gap structure and using liquid crystals as active medium is presented. We consider a T-junction PhC diplexer in two dimensional photonic crystals composed of silicon rods with square lattices with nematic liquid crystals as background. We prove that a range of frequency can propagate in both left and right waveguide of T-junction or in only one of them by applying an external electric field reorienting the liquid crystal.
Two different original theoretical approach for the analysis of vapour sensors based on a porous silicon optical microcavity are presented. The devices under analysis are based on a cavity with a high porosity layer of optical thickness λB/2, where λB is the Bragg resonant wavelength. This is enclosed between two distributed Bragg reflectors with seven periods made of alternate low and high porosity layers. When such a porous silicon microcavity is exposed to chemical vapours, a marked red-shift of its resonant peak, ascribed to capillary condensation of vapour in the pores, is observed. According to the first approach, the features of porous silicon microcavities are analyzed looking at the correspondent band structure. In particular, the microcavity structure is viewed as a 1-D photonic crystal with a defect of optical thickness λB/2 giving rise to a narrow resonant transmittance peak at λB in a wide transmittivity stop-band. We then compare the derivation of the band structure with an original approach based on the dynamical diffraction theory, the same widely used in x-ray diffraction. Using this approach we get an analytical expression of the reflectivity, giving the position but also the shape of the resonant peak.
In this communication, the compatibility of porous silicon and anodic bonding technologies for the realization of sensing microcomponents in lab-on-chip applications has been demonstrated. The two techniques have been combined for the fabrication of a microsensor with biological and chemical molecules sensing capability, in view of its miniaturization and integration with smart micro-dosage systems.
Multi-layer structures, such as Bragg reflectors, rugate filters, and optical microcavities are widely used in optical sensing. They are characterised by a periodical modulation of the refractive index so that they can be classified as 1-D photonic crystals.
In this communication, the optical features of such a class of sensors are analyzed from the band structure point of view. This general approach is then applied to the case of vapour sensors based on a porous silicon microcavity. A numerical analysis of the photonic bands, when the porous microcavity is exposed at chemical vapours, is presented and discussed for design optimisation purposes. In particular, we investigate how the photonic band gap changes when a volatile substance condensates in the silicon pores inducing a variation of the refractive indices of the layers forming the microcavity. Results are also compared with those obtained by the usual optical transfer matrix method.
Photonic band gap Crystals (PhC) are usually analyzed using the analogy between photon propagation in artificial periodic structures and electron wave propagation in real crystals. The forbidden band of photons is regarded as equivalent to the energy gap that electrons experience in crystals because of the periodic potential.
On the other hand, electron propagation and electromagnetic wave diffraction in periodic solids, respectively developed into band-theory and Dynamical Diffraction Theory (DDT), are formally identical. It appears therefore natural to perform an analysis of the features of an electromagnetic phenomenon, as the PBG, in analogy to the most direct antecedent electromagnetic theory, the DDT, that historically has also represented the direct reference for the derivation of the band-theory of electrons.
In this communication, we introduce an analysis of the features of PhCs in analogy with the DDT, underlining the differences between DDT classical application to the x-ray diffraction from real crystals and that from artificial crystals at optical wavelengths. In particular, the high contrast of material refractive indices in PhC makes inapplicable some approximations generally used in x-ray diffraction analysis. Moreover, we discuss in which cases DDT has to be generalized in order to overcome such limitations.
The theoretical derivation carried out is validated by the good agreement with the experimental results obtained for very simple 1D photonic crystals, such as porous silicon multilayers and silicon nitride multilayers. The generalization of the proposed approach to the case of 2D and 3D photonic crystals is also discussed.
Recently, an increasing interest has been devoted to the use of porous silicon (p-Si) in photonics and in sensing fields. In particular, the great reactivity, mainly due to its large surface to volume ratio, has demonstrated to be promising in sensing applications for the detection of gases, vapors, and biochemical molecules. In this work, we present experimental and numerical results on p-Si optical microcavities as sensing transducers in biological and chemical fields. The measures are based on the change of the cavity reflectivity spectrum induced by the exposition to the bio-chemical specimen under test. The p-Si microcavity has a Fabry-Pèrot structure confined between two Distributed Bragg Reflectors (DBRs) with high reflectivity in the wavelength range of interest. The DBRs have been obtained modulating the porosity, therefore the refractive index, of p-Si layers during the silicon electro-chemical etching process. The optical thickness (nd) of each single-layer forming the DBR is l/4, where d is the layer physical thickness, n its refractive index and l is the Bragg wavelength. A l/2-thick layer placed between the top and bottom DBRs works as a microcavity resonating at the Bragg wavelength l. The realized sensors operate at the fiber optic communication wavelength of 1.55 mm. A complete experimental characterization of the devices as vapor and liquid sensor is reported. An analytical model, allowing the correct interpretation of the sensing dynamics, is also reported and discussed. Finally, preliminary results concerning DNA-probe immobilization in p-Si pores and consequent recognition of complementary DNA strands are presented.
Low original design of Resonant-Cavity-Enhanced photodetectors at 850 nm, realized in microcrystalline silicon by simpe and low-cost thin film deposition processes compatible with standard VLSI technologies is presented. The configuration allows high quantum efficiencies in thin active region. This increases the bandwidth reducign the carrier transit time in teh device. The wavelength selective behavior is a further characterization of high-quality distributed bragg reflectors, necessary to the microcavity definition and optimization, and of the active p-i-n structure are also reported.
On exposure at different chemical substances several physical quantities of porous silicon, such as reflectivity,
photoluminescence, and electrical conductivity, change drastically. In particular, we have used porous silicon microcavities as chemical sensors, measuring resonant peak shifts in the reflectivity spectra due to capillary condensation of the vapor in the silicon pores. Understanding sensor behaviour depends on the dielectric function model and on the interaction mechanism assumed. With proper choices, we can also quantitatively characterize the Porous Silicon Microcavity sensing device features.
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