Porous Silicon Optical Microcavities Research Articles

Tunable characteristics of porous silicon optical microcavities by energetic N ion beam interactions

The present study demonstrates the tuning of optical characteristics of porous silicon (PSi)-based microcavities by N ion beam interactions. These optical microcavities are prepared by using electrochemical etching of heavily doped p+-type Si. The PSi microcavities were exposed to N ions of 200 keV and 1 MeV at an optimized ion fluence of 1 × 1015 ions cm−2. A significant red-shifting of 32 ∼ 60 nm in the resonance cavity mode was observed due to ion interaction. The experimental results are in good agreement with the transfer matrix simulations. A substantial modification of the PSi microcavity surface states is visualized through Raman and x-ray photoelectron spectroscopy (XPS) techniques. The Raman spectral results show modifications from crystalline Si to nanostructured Si and subsequently to amorphous Si. The XPS indicates the modification of Si–Si and Si–O bonds and the formation of new Si–N bonds, implying the presence of Si3N4. These experimental observations, along with analytical simulations and transfer-matrix method microcavity modeling, conclusively support the realization of cavity tunability and substantial modification in the optical field intensity and photon confinement within the spacer layer of the microcavity. These results suggest that ion beams are the effective tool to produce wider tunable optical properties in microcavities with highly stable designer optical structures suitable for photonic applications.

Porous silicon optical microcavity biosensor on silicon-on-insulator wafer for sensitive DNA detection

Silicon-on-insulator (SOI) wafer is one of the most appealing platforms for optical integrated circuit with the potential to realize high performance Ultra Large Scale Integration (ULSI) and device miniaturization. In this work, based on simulations to obtain appropriate optical properties of a porous silicon microcavity (PSM), we successfully fabricated a highly efficient PSM on SOI wafer by electrochemical etching for DNA detection at optical wavelength 1555.0nm. The narrow resonance peak with a full width at half maximum about 26.0nm in the reflectance spectrum gives a high Q factor which causes high sensitivity for sensing performance. The sensitivity of this sensor is investigated through 19-base pair DNA hybridization in the PSM by surface modification using a standard cross link chemistry method. The red shift of the reflectance spectra shows a good linear relationship with complementary DNA concentration, ranging from 0.625 to 12.500μM, and the detection limit is 43.9nM. This optical PSM on SOI is highly sensitive, fast responsive, easy to fabricate and low-costly, that will broadly benefit to develop a new optical label-free biosensor on SOI wafer and has a great potential for biochips based on integrated optical devices.

A porous silicon optical microcavity for sensitive bacteria detection

A porous silicon microcavity (PSM) is highly sensitive to subtle interface changes due to itshigh surface area, capillary condensation ability and a narrow resonance peak (∼10 nm). Based on the well-defined optical properties of a PSM, we successfully fabricated abacteria detection chip for molecular or subcellular analysis by surface modification usingundecylenic acid (UA), and the specific recognition binding of vancomycin tothe D-alanyl-D-alanine of bacteria. The red shift of the PSM resonance peakshowed a good linear relationship with bacteria concentration ranging from 100 to1000 bacteria ml − 1 at the level of relative standard deviation of 0.994 and detection limit of20 bacteria ml − 1. The resulting PSM sensors demonstrated high sensitivity, good reproducibility, fastresponse and low cost for biosensing.

Innovative design for optical porous silicon gas sensor

A simple system for the detection of analytes in the atmosphere employing a freestanding nanostructured porous silicon optical microcavity was designed and built. The system is based on the measurement of optical transmittance changes as a function of the incidence angle of a monochromatic laser beam. Using a matrix formalism and effective medium theories we calculate the optimal conditions for the system so as to obtain the best sensitivity of the sensor. We tested the system over a detection area of the porous silicon microcavity of 100 μm 2 with isopropyl alcohol vapor diluted in a N 2 stream. With the system proposed we were able to detect changes in the concentration of isopropyl alcohol as small as 1 ppm, which is equivalent to a change in the effective refractive index of about 3 × 10 −6. The response time is very fast, lower than 0.5 s.

Quantitative measurements of hydro‐alcoholic binary mixtures by porous silicon optical microsensors

AbstractIn this communication, we present a simple, completely reversible and well performing optical microsensor for the quantitative determination in liquid hydro‐alcoholic binary mixtures, based on the simple and low cost porous silicon (PSi) nanotechnology. In our sensor device, the transducer element is a PSi optical microcavity (PSMC) which is constituted by a Fabry‐Pérot layer between two distributed Bragg reflectors. When the device is exposed to the mixture, the partial substitution of air in the pores changes the average refractive index of PSMC, causing a red shift of its reflectivity spectrum, characteristic of the concentration of each component. We have characterized, in the 0‐100% composition range, three hydro‐alcoholic binary mixtures (water‐Ethanol, water‐ Methanol and water‐ Isopropanol) which exhibit very different behaviour. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Resonant cavity enhanced optical microsensor for molecular interactions based on porous silicon

AbstractThe molecular binding between the glutamine binding‐protein (GlnBP) from Escherichia coli and L‐glutamine (Gln) is detected by means of an optical biosensor based on porous silicon technology. The binding event is optically transduced in the wavelength shift of the porous silicon optical microcavity (PSMC) reflectivity spectrum. The hydrophobic interaction links the GlnBP, which acts as a molecular probe for Gln, to the hydrogenated porous silicon surface area. We can thus avoid any preliminary surface functionalization process. The protein infiltrated PSMC results stable to oxidation at least for few cycles of wet measurements. The penetration of the proteins into the pores of the porous silicon matrix has been optimized: a strong base post‐etch process increases the pore size and removes any nanostructure on top and inside the porous silicon multilayer while does not degrade the optical response and the quality of the microcavity. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical Sensors for Vapors, Liquids, and Biological Molecules Based on Porous Silicon Technology

The sensing of chemicals and biochemical molecules using several porous silicon optical microsensors, based both on single-layer interferometers and resonant-cavity-enhanced microstructures, is reported. The operation of both families of sensors is based on the variation of the average refractive index of the porous silicon region, due to the interaction with chemical substances either in vapor or liquid state, which results in marked shifts of the device reflectivity spectra. The well established single-layer configuration has been used to test a new chemical approach based on Si-C bonds for covalent immobilization of biological molecules, as probe, in a stable way on the porous silicon surface. Preliminary results on complementary oligonucleotide recognition, based on this technique, are also presented and discussed. Porous silicon optical microcavities, based on multilayered resonating structures, have been used to detect chemical substances and, in particular, flammable and toxic organic solvents, and some hydrocarbons. The results put in evidence the high sensitivity, the reusability, and the low response time of the resonant-cavity-enhanced sensing technique. The possibility of operating at room temperature, of remote interrogation, and the absence of electrical contacts are further advantages characterizing the sensing technique.

Time-resolved sensing of chemical species in porous silicon optical microcavity

Time-resolved measurements of red-shifts in the reflectivity spectra of porous silicon multi-layer microcavities, due to exposure to vapor of chemical species, give deep insight about the spatial concentration of the liquid condensed into the pores. Results clearly indicate that capillary condensation starts immediately and proceeds homogeneously all over the stack, yielding a uniform concentration distribution.

Smart optical sensors for chemical substances based on porous silicon technology.

A simple geometry optical sensor based on porous silicon technology is theoretically and experimentally studied. We expose some porous silicon optical microcavities with different porous structures to several substances of environmental interest: Very large red shifts in the single transmission peak in the reflectivity spectrum due to changes in the average refractive index are observed. The phenomenon can be ascribed to capillary condensation of vapor phases in the silicon pores. We numerically compute the peak shifts as a function of the liquid volume fraction condensed into the stack by using the Bruggeman theory. The results presented are promising for vapor and liquid detection and identification.

Optical microcavities with subnanometer linewidths based on porous silicon

We have fabricated a number of high-quality porous silicon optical microcavities operating in the near infrared that exhibit cavity resonances with subnanometer linewidths. This was achieved through the low temperature anodic oxidation of highly doped p-type silicon wafers. We have investigated the optical properties of these microcavities using reflectivity and photoluminescence measurements and compared our results with theoretical predictions. From our analysis, we conclude that, for the low temperature fabrication process, the refractive index difference between adjacent layers of the multilayered structure is maximized while optical losses in the cavity are minimized. Furthermore, by considering the origin of optical losses in these microcavities, we demonstrate that fluctuations in the position of the resonance wavelength and optical absorption play an important role in the realization of high-quality interferometric structures.

Multiparametric Porous Silicon Sensors

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 the change of these three parameters as a function of NO2 (0.5-5 ppm), ethanol (300-15000 ppm) and relative humidity (0-100%). Preliminary results confirm that the examined species affect the parameters in a different way, both as a relative change and as dynamic.

Multiparametric sensor for air pollutants based on a porous silicon optical microcavity

AbstractWe 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 large due to the large surface/volume ratio of porous silicon. While these effects can be observed in any porous silicon structure, microcavities of porous silicon allow an additional sensing parameter, i.e. the spectral position of the resonance cavity peak. The position of the peak depends on the index of refraction of the environment, and gives independent additional information. Moreover, we show that the dynamic response of the peak shift is much faster comparing the other sensing parameters. The combined effects on the peak position, luminescence intensity and electrical conduction can allow discrimination between different substances, and therefore porous silicon optical microcavities can work as multi-parametric optical/electrical sensors. We report detection of 1 ppm of NO2 and 500 ppm of ethanol at room temperature. With NO2, the electrical conduction increases and PL quenches, but the peak does not shift, whereas the peak shifts with ethanol (no significant PL quenching is observed at 500 ppm). This suggests that discrimination between different species can be achieved.

Enhancement of the Spontaneous Emission Rates in all Porous Silicon Optical Microcavities

AbstractThe emission properties of a porous silicon layer placed in an optical microcavity is investigated by photoluminescence and time resolved photoluminescence measurements. The microcavity is formed by an all porous silicon Fabry-Perot filter made by two distributed Bragg reflectors separated by a λ or λ/2 porous silicon layer. Our main findings are that the spontaneous emission spectrum is drastically modified: the linewidth is narrowed, the time decay of the emission is shortened by a factor of about 2/3 at room temperature and the peak emission intensity is increased by a factor of more than 10. These facts are caused by the redistribution of the optical modes in the microcavity due to the presence of the optical resonator and to the variation of the dielectric environment.