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We describe a simple technique for measuring high (up to 0.16) time-averaged solids volumetric concentration in a two-phase flow. The technique is based on a properly modified version of the forward scattering of laser light. It is useful in a variety of practical configurations, and, in particular, it is instrumental in the diagnostics of particle flow in the free board of bubbling fluidized beds and in the circulating fluidized beds. A fallout of this work is the measurement of the extinction coefficient of the solid material tested. © 1990 Optical Society of America.

In this article, the realization and characterization of a new kind of resonant cavity enhanced photo-detector, fully compatible with silicon microelectronic technologies and working at 1.55 μm, are reported. The detector is a resonant cavity enhanced structure incorporating a Schottky diode, and its working principle is based on the internal photo-emission effect. A comparison between a Schottky diode (Al/Si or Cu/Si) and the Schottky diode fed on a high-reflectivity Bragg mirror is carried out. Considering Al as Schottky metal, no difference in responsivity is obtained; considering Cu as Schottky metal, a three-fold responsivity improvement is experimentally demonstrated.

The performances of a digital optical switch, based on the thermo-optic effect in amorphous silicon, are investigated for the first time to our knowledge. Numerical simulations shows that the strong thermo-optic effect of amorphous silicon combined with the possibility of realizing micrometric integrated structures, allow the realization of efficient waveguide devices capable of microsecond switching times. The switch, operating at the infrared communications wavelength of 1550 nm, can be easily integrated in silicon optoelectronic circuits and is thought for low-cost photonic applications.

It is known that a properly arranged distribution of nanoholes on a metallic slab is able to produce, in far field conditions, light confinement at sub-diffraction and even sub-wavelength scale. The same effect can also be implemented by the use of Optical Eigenmode (OEi) technique. In this case, a spatial light modulator (SLM) encodes phase and amplitudes of N probe beams whose interference is able to lead to sub-wavelength confinement of light focused by an objective. The OEi technique has been already used in a wide range of applications, such as photoporation, confocal imaging, and coherent control of plasmonic nanoantennas. Here, we describe the application of OEi technique to a single valve of a marine diatom. Diatoms are ubiquitous monocellular algae provided with an external cell wall, the frustule, made of hydrated porous silica which play an active role in efficient light collection and confinement for photosynthesis. Every frustule is made of two valves interconnected by a lateral girdle band. We show that, applying OEi illumination to a single diatom valve, we can achieve unprecedented sub-diffractive focusing for the transmitted light.

Evolution shows that photonic structures are a constituent part of many animals and flora. These elements produce structural color and are useful in predator-prey interactions between animals and in the exploitation of light for photosynthetic organisms. In particular, diatoms have evolved patterned hydrated silica external valves able to confine light with extraordinary efficiency. Their evolution was probably guided by the necessity to survive in harsh conditions of sunlight deprivation. Here, we exploit such diatom valves, in conjunction with structured illumination, to realize a biological super-resolving lens to achieve sub-diffractive focusing in the far field. More precisely, we consider a single diatom valve of Arachnoidiscus genus which shows symmetries and fine features. By characterizing and using the transmission properties of this valve using the optical eigenmode technique, we are able to confine light to a tiny spot with unprecedented precision in terms of resolution limit ratio, corresponding in this case to 0.21λ/NA.

The characterization of partially coherent light transmission by micrometer sized valves of marine diatoms is an interesting optical challenge and, from the biological point of view, is of outmost relevance in order to understand evolution mechanisms of such organisms. In the present work, we have studied the transmission of light coming from a monochromator through single valves of Coscinodiscus wailesii diatoms. Incoming light is confined by the regular pore pattern of the diatom surface into a spot of few microns, its dimensions depending on wavelength. The effect is ascribed to the superposition of wavefronts diffracted by the pores' edges. 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. © 2010 Optical Society of America.

Inspired by the concept of complementary media, we experimentally demonstrate that an engineered metamaterial made of alternating, stripe layers of negatively refracting (photonic crystals) and positively refracting (air) materials strongly collimates a beam of near-infrared light. This quasi-zero-average-index metamaterial fully preserves the beam spot size throughout the sample for a light beam traveling through the metamaterial a distance of 2 mm\char22{}more than 1000 times the input wavelength $\ensuremath{\lambda}=1.55\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$. These results demonstrate the first explicit experimental verification of optical antimatter as proposed by Pendry and Ramakrishna [J. Pendry and S. Ramakrishna, J. Phys. Condens. Matter 15, 6345 (2003)], using two complementary media in which each ${n}_{\mathrm{eff}}=\ensuremath{-}1$ layer appears to annihilate an equal thickness layer of air.

A new class of silicon electro-optical modulators for λ = 1.3 and 1.55 μm is proposed and analyzed. The devices, based on single- and multi-pass Fabry-Perot interferometry, can be built by standard microelectronic tecniques.

We report on our preliminary results in the realization and characterization of a porous silicon (PSi) resonant mirror (RM) for optical biosensing. We have numerically and experimentally studied the coupling between the electromagnetic field, totally reflected at the base of a high refractive index prism, and the optical modes of a PSi waveguide. This configuration is very sensitive to changes in the refractive index and/or in thickness of the sensor surface. Due to the high specific area of the PSi waveguide, very low DNA concentrations can be detected confirming that the RM could be a very sensitive and label-free optical biosensor.

In this paper, the design of a resonant cavity enhanced photodetector, working at 1.55 µm and based on silicon technology, is reported. The photon absorption is due to the internal photoemission effect over the Schottky barrier at the metal–silicon interface. The photodetector is composed of a silicon layer in between multiple layers of Si–SiO2, as a bottom mirror, and a thin Au film and dielectric coating, as a top mirror. In order to estimate the quantum efficiency, we take advantage of the analytical formulation of the internal photoemission effect (Fowler theory) and its extension for thin films, while for the optical analysis of the device, used to calculate mirror reflectivities and active layer absorptance, a numerical method based on the transfer matrix method has been implemented. Our numerical results prove a significant enhancement of the efficiency obtained at resonant wavelengths by a very thin absorbing layer.

In this paper, the design of a resonant cavity-enhanced (RCE) Schottky photodetector, based on internal photoemission effect and working at 1.55 μm, is presented. In order to estimate the theoretical quantum efficiency we take the advantage of analytical formulation of the internal photoemission effect (Fowler theory), and its extension for thin films, while for the optical analysis of device a numerical method, based on the transfer matrix method, has been implemented. Finally, we complete our design calculating bandwidth and bandwidth-efficiency product. Our numerical results prove that a quantum efficiency of 0.1% is obtained at resonant wavelength (1.55 μm) with a very thin absorbing metal layer (30 nm). Theoretical values of 100 GHz and 100 MHz were obtained, respectively, for the carrier-transit time limited 3-dB bandwidth and bandwidth-efficiency. The proposed photodetector can work at room temperature and its fabrication is completely compatible with standard silicon technology.

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. 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.

A non-perturbing, fast, low-cost probe for near-field measurements is presented in this paper. The new sensing technique's capabilities in antenna fault diagnosis and direct measurement of field intensity distribution at subwavelength distance from the sources are demonstrated by test measurement on different microwave antennas and applicators. © 2003 Wiley Periodicals, Inc. Microwave Opt Technol Lett 38: 95–98, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10981

A combined thermal and optical simulation study is performed on an all-silicon waveguide integrated thermo-optic Fabry-Perot modulator to assess the impact of the driving signal shape on the device speed performances. The results clearly indicate that the cooling phase after pulse constitutes the true speed-limiting factor the application of a heat of a device, which is otherwise in thermal equilibrium with the environment. Therefore, to enhance the heat exchange in this phase, we analyze the hypothesis of applying a thermal bias and hold the modulator at a higher average temperature with respect to the environment and the substrate heat sink. With this approach, a new driving signal is proposed and predicted to allow a transmission rate of 2.2 Mb/s, representing over a 30-fold bandwidth widening with respect to the standard modulation scheme. We also discuss the fabrication process of this kind of modulator, reporting on its first prototypal realization and preliminary optical characterization.

A novel amorphous-silicon-based photonic micromodulator is proposed which can be easily integrated into silicon low-cost optoelectronic appliances. The device is designed to operate at the two infrared communication wavelengths of 1300 and 1550 nm, either on broad or narrow spectrum sources, and is activated by visible light pulses controlling the infrared light absorption coefficient through the generation of free carriers. A planar waveguide integrated prototype has been fabricated on a silicon wafer by plasma enhanced chemical vapour deposition, and tested at bit rates up to 200 kbit/s.

Discusses a new photonic sensor for the microwave and millimeter-wave range. The probe is a silicon optical microsensor made of an interferometric Fabry-Perot microcavity, directly connected to a fiber optic for remote interrogation. When exposed to an EM field, the power dissipated in the semiconductor-sensing element induces the heating, and in turn, a change of the refractive index of the silicon by thermo-optic effect. This variation induces a modulation of the probing light intensity reflected by the interferometric cavity. The characterization of a nonoptimized prototype in the frequency range 2-18 GHz was carried out, and resolutions going well beyond those characterizing other nonperturbing sensors operating at the same frequency range were measured. A theoretical analysis, giving results in good agreement with experiments, was also carried out, showing the sensor capability to detect signals up to 100 GHz and the possibility of significantly increasing the sensitivity in optimized designs. In the communication, new preliminary results concerning the application of the proposed class of probes to indoor near-field antenna characterization are presented. In particular, the results we report have been carried out with a prototype realized starting from a 280-/spl mu/m-thick Si wafer.

A new non-disturbing photonic probe for measuring electromagnetic (EM) power is presented. The sensing element is made of a silicon Fabry–Perot optical microcavity connected to an optical fiber for remote interrogation. Preliminary measurements have been carried out in a non-optimized prototype in the frequency range 2–18 GHz. They have shown the capability to detect minimum EM power densities of ∼1.4 mW/cm 2 , and a flat response at frequencies greater than 10 GHz. A proper calibration makes the sensor useful as an E-field probe. In this case, resolutions as low as 8 V/m have been measured. A simple theoretical analysis giving results in fair agreement with the experiments is also reported. Simulations show that wide margins of optimizations of the sensor exist.

The present work reports on our recent achievements in the exploitation of a simple technology for the fabrication of hydrogenated amorphous silicon (a-Si:H) based low-loss rib waveguides. In particular, waveguides with various widths have been fabricated out of an a-SiC:H/a-Si:H stack deposited by plasma enhanced chemical vapor deposition at the relatively low temperature of 220 degrees C. The ribs were defined by an anisotropic, CH4-based, reactive ion etching process. The devices have been subsequently characterized by cut-back technique. Even though a dependence of attenuation parameter on the waveguide width was observed, propagation losses as low as 0.7 dB/cm could be measured at λ=1.3 μm, in good agreement with he theoretical estimations based on the intrinsic absorption of the material. Starting from the same structure, a Fabry- Perot thermo optical modulator has been also fabricated and tested at the communication wavelength of 1.3 μm.

Waveguides and interferometric light amplitude modulators for application at the 1.3- and 1.55-/spl mu/m fiber communication wavelengths have been fabricated with thin-film hydrogenated amorphous silicon and its related alloys. The technique adopted for the thin-film growth is the plasma- enhanced chemical vapor deposition, which has been shown to give the lowest defect concentration in the film. Consequently the proposed waveguiding structures take advantage of the low optical absorption shown by a-Si:H at photon energies below the energy gap. In addition a good radiation confinement can be obtained thanks to the bandgap tailoring opportunity offered by this simple and inexpensive technology. In particular rib waveguides, based on a a-SiC:H/a-Si:H stack, have been realized on crystal silicon, showing low propagation losses. Recently, however, a new interest as low as 0.7 dB/cm. The same structure has been utilized for the fabrication of thermooptic Fabry-Perot modulators with switching times of 10 /spl mu/s. Modulators based on the alternative waveguiding configuration ZnO/a-Si:H, giving comparable results, are also presented.

A new class of temperature silicon micro-sensors, based on an interferometric optical technique, is presented. The sensing element consists of a planar Fabry-Perot cavity defined on a silicon wafer by highly anisotropic reactive ion etching, and is therefore suitable for full integration with other standard opto- and micro-electronic devices. Preliminary temperature measurements have been performed with the temperature resolutions predicted by the theory. The limit performances, in terms of resolution, speed of operation and energy dissipation of this class of sensors are discussed in detail. In particular, a final temperature resolution of 0.064/spl deg/C is expected for a low loss interferometric cavity, with a settling time of 150 ns and a 0.2% readout error. An energy resolution as low as 30 nJ is also estimated.

The purpose of this work is to describe a direct method for determining thermal dependence of the refractive indices of bulk materials and waveguiding film structures. The technique is alternative to standard ones, which require prism-shaped specimens, and decidedly simple than other new methods based on planar grating structures.

A three-power-level method for obtaining efficient thermo- optical modulation in an all-silicon waveguide-integrated Fabry-Perot thermo-optic modulator is discussed by means of a thermo-optical ana- lytical model and demonstrated. The thermal system is represented as a two-pole model where, at every time, the temperature in the waveguide core is modified by means of a heater. This temperature is calculated and used in turn for calculating the refractive index. In this way, the impact of the driving signal shape on the device speed performance is assessed. Results clearly indicate that the application of a thermal bias holding the modulator at a higher average temperature with respect to the substrate heat sink allows increasing the modulator speed. An application-specific integrated circuit has been designed and developed in order to test the new modulation logic. The system's electronics is implemented in a 0.8 m, 5 V, CMOS process. The experimental results of this new three-power-level driver method are reported, showing the shortening of the characteristic transient times. © 2009 Society of Photo-Optical

A new non-disturbing optically interrogated silicon sensor for measuring electromagnetic power in the microwave and millimetre-wave range is presented. A prototype was realised and then tested in the microwave frequency range 2.5-18 GHz. A minimum detectable electromagnetic power density of /spl sim/1.4 mW/cm/sup 2/ was measured. Higher performances are expected, according to simulations, in optimised devices.

A theoretical study aimed at the optimum design of thermo- optic guided-wave switches realized in silicon-on-oxide technology is reported. The heat transfer processes occurring inside such devices are analyzed by means of a 3D finite element thermal simulator. In particular, in rib waveguiding structures, we demonstrate that the optimum values of the oxide layer thickness and of the outer rib height are a function of the heating pulse power used to drive the device. The theoretical results carried out are consistent with the experiments reported in literature.