Photodetectors are of great interest in several technological applications thanks to their capability to convert an optical signal into an electrical one through light-matter interactions. In particular, broadband photodetectors are used in multiple applications such as environmental monitoring, imaging, fire detection, and astronomical observations. We present a two-dimensional photodiode heterojunction based on reduced graphene oxide (rGO) deposited on an n-type Silicon substrate. We report on the electro-optical properties of the device that have been measured in dark and light conditions into a spectral range from UV to IR. The room temperature current–voltage (I–V) measurements of rGO/n-Si photodetector exhibits a reverse saturation current linearly dependent on the light power. The main figures of merit of the photodetector such as linearity and responsivity have been evaluated and compared with the recent progress obtained substituting the rGO with a graphene single layer (Gr) on the similar n-Si substrate. The photoconductive properties and analysis of the two devices are presented and discussed. Finally, the experimental results demonstrate the feasibility of the rGO/n-Si and Gr/n-Si device to detect light from UV to IR light, nominating graphene-based heterojunction as a novel candidate for the realization of new broadband photodetectors.
Nanostructured photoluminescent materials are optimal transducers for optical biosensors due to their capacity to convert molecular interactions in light signals without contamination or deterioration of the samples. In recent years, nanostructured biosensors with low cost and readily available properties have been developed for such applications as therapeutics, diagnostic and environmental. Zinc oxide nanowires (ZnO NWs) is material with unique properties and due to these they were widely studied in many fields as electronics, optics, and photonics. ZnO NWs can be either grown independently or deposited on solid support, such as glass, gold substrates and crystalline silicon. Vertical aligned ZnO forest on a substrate shows specific advantages in photonic device fabrication. ZnO NWs are typically synthesized by such techniques classified as vapour phase and solution phase synthesis. In particular, hydrothermal methods have received a lot of attention and have been widely used for synthesis of ZnO NWs. This technique shows more crystalline defects than others due to oxygen vacancies, so as the material shows intense photoluminescence emission under laser irradiation. ZnO NWs surface is highly hydrolysed, so it is covered by OH reactive groups, and standard biomodification chemistry can be used in order to bind bioprobes on the surface. In this work, we present our newest results on synthetic nanostructured materials characterization for optical biosensors applications. In particular, we characterize the ZnO NWs structure grown on crystalline silicon by SEM images and the biomodification by photoluminesce technique, fluorescence microscopy, water contact angle and FT-IR measurements.
A p-i-p configuration of an electro-optical modulator based on hydrogenated amorphous silicon (a-Si:H) is characterized and compared with an a-Si:H based p-i-n modulator. In particular, we estimate the performances in terms of optical losses, voltage-length product, and bandwidth at λ=1550 nm for waveguide-integrated p-i-p versus p-i-n configurations. Both devices are fabricated on a silicon substrate by plasma enhanced chemical vapor deposition at low temperature ensuring the back-end integration with a CMOS microchip. We demonstrate a factor of merit for the p-i-p waveguide integrated Fabry-Perot resonator of Vπ×Lπ=19 V×cm allowing the design of shorter devices with respect to p-i-n structure.
The realization of on-chip optical interconnects requires the integration of active micro-optical devices with
microelectronics. However, it is not clear yet how silicon photonics could be integrated within CMOS chips. In this
context the non-crystalline forms of silicon, such as laser-annealed polycrystalline and hydrogenated amorphous silicon
(a-Si:H), can deserve some advantages as they can be included almost harmlessly everywhere in a CMOS typical run-sheet,
yielding low-cost and flexible fabrication. In particular, a-Si:H can be deposited using the CMOS-compatible low
temperature plasma enhanced chemical vapour deposition (PECVD) technique, which brings clear advantages
particularly for a back-end photonic integrated circuit (PIC) integration. However, till now a-Si:H has been mainly
considered for the objective of passive optical elements within a photonic layer at λ=1.55 μm. Only a small number of
examples have been reported, in fact, on waveguide integrated active devices. In this paper we detail about an effective
refractive index variation obtained through an electrically induced carrier depletion in an as-deposited a-Si:H-based p-i-n
waveguiding device. For this device switch-on and switch-off times of ~2 ns were measured allowing a modulation rate
higher than 150 MHz.ÿÿ
Numerical analysis is implemented to investigate biological sample starting from Digital Holographic (DH) recording.
The aim is to improve visualization and detection of cow spermatozoa. Digital holograms are recorded in the off-axis
geometry where optical setup is a Mach-Zehnder interferometer. Then holograms are numerically manipulated to
retrieve, besides the usual Quantitative Phase Map (QPM), Differential Interference Contrast (DIC) visualization.
Furthermore, a new approach, named digital self-referencing holography, is described it's able to accomplish
quantitative phase analysis especially useful for specimen flowing in microfluidic channels.
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.
In this work, we have fabricated a porous silicon (PSi) Bragg reflectors microarray using a proper technological process
based on photolithography and electrochemical anodization of silicon. Each element of the array is characterized by a
diameter of 200 μm. The PSi structures have been used as platform to immobilize label-free DNA probe and a simple
optical method has been employed to investigate the interaction between probe-DNA and its complementary target. In
order to confirm the specificity of the DNA hybridization, we have also verified that the reaction of probe-DNA with
non-complementary DNA did not occur.
In the last few years, silicon photonics has been characterized by a wide range of applications in several fields, from
communications to sensing, from biophotonics to the development of new artificial materials. In this communication,
we report a review of the main results obtained in our laboratories in design, fabrication and characterization of new
silicon-based optical structures and devices, including metamaterials, photodetectors, raman light amplifiers, and
porous silicon based bio-chemical sensors and biochips. Future perspectives in integration of silicon based MEMS
and MOEMS are also presented.
In this work we investigate the possibility to use Zinc Oxide (ZnO) thin films, deposited by RF magnetron sputtering, for
the realization of integrated optical structures working at 1550 nm. Structural properties of sputtered zinc oxide thin
films were studied by means of X-ray Diffraction (XRD) measurements, while optical properties were investigated by
spectrophotometry and Spectroscopic Ellipsometry (SE). In particular, ellipsometric measurements allowed to determine
the dispersion law of the ZnO complex refractive index (see manuscript) = n - jk through the multilayer modeling using Tauc-Lorentz
(TL) dispersion model. We have found a preferential c-axis growth of ZnO films, with slightly variable deposition rates
from 2.5 to 3.8 Å/s. Conversely, the refractive index exhibits, from UV to near IR, a considerable and almost linear
variation when the oxygen flux value in the deposition chamber varies from 0 to 10 sccm. In order to realize a waveguide
structure, a 3-&mgr;m-thick ZnO film was deposited onto silicon single crystal substrates, where a 0.5-&mgr;m-thick thermal SiO2
buffer layer was previously realized, acting as lower cladding. Dry and wet chemical etching processes have been
investigated to achieve controllable etching rate and step etching profile, with the aim to realize an optical rib waveguide.
The etched surfaces were inspected using scanning electron microscopy (SEM) and optical microscopy. Moreover, we
carried out the experimental measurements of the fringes pattern and Free Spectral Range (FSR) of an integrated Fabry-
Perot etalon, obtained by cleaving of a single mode rib waveguide.
This work concerns the fabrication, optical characterization and potential applications of two types of microstructures manufactured in congruent lithium niobate. The first type consists of a simple 2D hexagonal lattice of inverted ferroelectric domains fabricated by standard electric field poling at room temperature. The second structure is the chemically etched version of the first one. Long etching in hot HF acid results in differential etching of opposite ferroelectric domain faces. In this way obtain a 3D structure is obtained in which the hexagonal domain array becomes an array of truncated pyramids.
Both these structures are characterized through a digital interferometric analysis. The samples are inserted in the arm of a Mach-Zenhder interferometer and the digital holograms acquired are used to numerically reconstruct both the amplitude and the phase of the wavefront transmitted by the sample.
Finally, we report on the possible applications of the fabricated structures. The hexagonally poled structure can be used as a variable binary phase array. In fact both sides of the poled sample are covered with a thin conductive layer (ITO), which acts as transparent electrode. By applying an external electric field it is possible to change the difference between the two phase levels, via the linear electro-optic effect, and, consequently, the distribution of light intensity in the diffracted orders.
On the other hand, the 3D structured etched sample can be used as an micrometer size integral imaging system.
A novel broad-band telecom laser source is used to realize a lateral-shear scanning-wavelength interferometer for measuring the thickness of thin plates. We show that the wide tunability range allows to detect samples down to tens of microns with a relative uncertainty of less than 0.5% and a resolution of about 1 nm. A comparable accuracy in the thickness characterization of double-layer structures is also demonstrated. In turn, the wide tunability range needs the dispersion law of the materials to be taken into account in the model for correct thickness evaluation.
The use of accelerometer based measurement techniques for evaluating bridge forced vibrations or to perform bridge modal analysis is well established. Some of the goals of this experimental approach and typical results are here reviewed. It is well known to all researchers who have experience in vibration measurements that values of acceleration amplitude can be very low at low frequencies and that a limitation to the use of accelerometer can be due to the threshold parameter of this kind of transducer. Under this condition the measurement of displacement seems more appropriate. On the other hand laser vibrometer systems detect relative displacements as opposed to the absolute measures of accelerometers. In this paper the results of some in field measurements on a highway little bridge, excited by traffic, under normal operating conditions, are analyzed. Vibrations have been measured simultaneously by a typical accelerometer for civil structures and by a laser vibrometer equipped with a fringe counter board in terms of velocity and displacements. The accelerations calculated from the laser vibrometer signals and the one directly measured by the accelerometer has been compared. The advantages of the use of a laser vibrometer at low frequency are relevant and it appears possible to open up new possibility to analyze bridge loads in operating conditions, monitoring bridge health state, predict fatigue life and improve the bridge design.
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