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.
In this work, resonance phenomena in a negative photonic crystal are experimentally detected and discussed. Localized surface modes and guided mode resonances appear in the reflection spectrum of a photonic crystal slab interacting with external infrared radiation and can be connected with the negative refractive index of the sample . These phenomena can provide an efficient way to confine the radiation into the structure, with an high field enhancement and a strong sensibility of the resonance position to the refractive index variations.
Diatoms are monocellular algae responsible of 20-25% of the global oxygen produced by photosynthetic processes. The protoplasm of every single cell is enclosed in an external wall made of porous hydrogenated silica, the frustule. In recent times, many effects related to photonic properties of diatom frustules have been discovered and exploited in applications: light confinement induced by multiple diffraction, frustule photoluminescence applied to chemical and biochemical sensing, photonic-crystal-like behavior of valves and girdles. In present work we show how several techniques (e.g. digital holography) allowed us to retrieve information on light manipulation by diatom single valves in terms of amplitude, phase and polarization, both in air and in a cytoplasmatic environment. Possible applications in optical microsystems of diatom frustules and frustule-inspired devices as active photonic elements are finally envisaged.
Photonic crystal metamaterial can exhibit negative index properties and this behaviour is well described by a resonator model. In this work, we present the experimental evidence that a Lorentz resonator correctly reconstruct data obtained with a negative refracting Photonic Crystal (PhC) by using a standard optical technique, such as ellipsometry. In particular we show that, in the frequency range in which the effective refractive index, neff, is equal to -1, the incident light couples efficiently to the guided modes in the top surface layer of the PhC metamaterial. These modes resemble surface plasmon polariton resonances. In add we present measurements by using standard technique of prism coupling evanescent wave. Once again the presence of localized plasmon-like modes at the surface of a silicon two-dimensional photonic crystal slab is demonstrated. Also in this case, in analogy with surface plasmons supported in metals in a photonic crystal metamaterial, the electromagnetic surface waves arise from a negative effective permittivity. These results opens new strategies in light control at the nanoscale, allowing on chip light manipulation in a wide frequency range and avoiding the intrinsic limits of plasmonic structures due to absorption losses in metals. Such negative index PhC materials may be of use in biosensing applications.
Porous silicon (PSi) is by far a very useful technological platform for optical monitoring of chemical and biological
substances and due to its peculiar physical and morphological properties it is worldwide used in sensing experiments. On
the other hand, we have discovered a natural material, the micro-shells of marine diatoms, ubiquitous unicellular algae,
which are made of hydrated amorphous silica, but, most of all, show geometrical structures made of complex patterns of
pores which are surprisingly similar to those of porous silicon. Moreover, under laser irradiation, this material is
photoluminescent and the photoluminescence is very sensitive to the surrounding atmosphere, which means that the
material can act as a transducer. Starting from our experience on PSi devices, we explore the optical and photonic
properties of marine diatoms micro-shells in a sort of inverse biomimicry.
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 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 this communication, we report some new results obtained in our laboratories in design, fabrication and
characterization of silicon-based optical structures and devices, including metamaterials, raman light amplifiers, and
biomatter-silicon interfaces for sensors and biochips.
Self-assembled monolayers are surfaces consisting of a single layer of molecules on a substrate: widespread examples of chemical and biological nature are alkylsiloxane, fatty acids, and alkanethiolate which can be deposited by different techniques on a large variety of substrates ranging from metals to oxides. We have found that a self-assembled biofilm of proteins can passivate porous silicon (PSi) based optical structures without affecting the transducing properties. Moreover, the protein coated PSi layer can also be used as a functionalized surface for proteomic applications.
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.
Micro-ring resonators have been widely employed, in recent years, as wavelength filters, switches and frequency
converters in optical communication circuits, but can also be successfully used as transducing elements in optical sensing
and biosensing. Their operation is based on the optical coupling between a ring-shaped waveguide and one or more
linear waveguides patterned on a planar surface, typically an input and an output waveguide. When incoming light has a
wavelength which satisfies the resonance conditions, it couples into the micro-ring and continuously re-circulates within
it. A fraction of this resonant light escapes the micro-ring structure and couples into the output waveguide. The presence
of a target analyte over the top surface of the micro-ring (i.e. within the evanescent field) changes the effective refractive
index of the mode propagating into the structure, thus causing a shift in resonance wavelength which can be determined
by monitoring the spectrum at the output port. Proper functionalization of the micro-ring surface allows to add selectivity
to the sensing system and to detect specific interaction between a bioprobe and its proper target (e.g. protein-ligand,
DNA-cDNA interactions). We present our preliminary results on the design of micro-ring resonators on silicon-on-insulator
substrate, aimed at selective detection of several biomolecules. The design of the structure has been
accomplished with the help of FDTD 2D numerical simulations of the distribution of the electromagnetic fields inside
the waveguides, the micro-ring and near the micro-ring surface. Furthermore, all the functionalization reactions and the
bio/non-bio interfaces have been studied and modelled by means of spectroscopic ellipsometry.
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. Frustules
from centric diatoms are characterized by a radial disposition of areolae and exhibit several optical properties, such as
photoluminescence, lens-like behavior and, in general, photonic-crystal-like behavior as long as confinement of
electromagnetic field is concerned. In particular, intrinsic photoluminescence from frustules is strongly influenced by
the surrounding atmosphere: on exposure to gases, the induced luminescence changes both in the optical intensity and
peaks positions. To give specificity against a target analyte, a key feature for an optical sensor, a biomolecular probe,
which naturally recognizes its ligand, can be covalently linked to the diatom surface.
We explored the photoluminescence emission properties of frustules of Coscinodiscus wailesii centric species,
characterized by a diameter of about 100-200 μm, on exposure to different vapours and in presence of specific bioprobes
interacting with target analytes. Very high sensitivities have been observed due to the characteristic morphology of
diatoms shells. Particular attention has been devoted to the emission properties of single frustules.
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.
The development of label-free optical biosensors could have a great impact on life sciences as well as on screening
techniques for medical and environmental applications. Peptide nucleic acid (PNA) is a nucleic acid analog in which the
sugar phosphate backbone of natural nucleic acid has been replaced by a synthetic peptide backbone, resulting in an
achiral and uncharged mimic. Due to the uncharged nature of PNA,
PNA-DNA duplexes show a better thermal stability
respect the DNA-DNA equivalents. In this work, we used an optical biosensor, based on the porous silicon (PSi)
nanotechnology, to detect PNA-DNA interactions. PSi optical sensors are based on changes of reflectivity spectrum
when they are exposed to the target analytes. The porous silicon surface was chemically modified to covalently link the
PNA which acts as a very specific probe for its ligand (cDNA).
A direct laser writing process has been exploited to fabricate a high order Bragg grating on the surface of a porous
silicon slab waveguide. The transmission spectrum of the structure, characterized by a pitch of 10 µm, has been
investigated by end-fire coupling on exposure to vapor substances of environmental interest. The analyte molecules
substitute the air into the silicon pores, due to the capillary condensation phenomenon, and the transmitted spectrum of
the grating shifts towards higher wavelengths. The experimental results have been compared with the theoretical
calculations obtained by using the transfer matrix method together with the slab waveguide modal calculation.
Micro-total-analysis-systems and lab-on-chip are more than promises in lot of social interest applications such as clinical
diagnostic or environmental monitoring. There is an increasing demand of new and customized devices with better
performances to be used in very specific applications. Nanostructured Porous silicon is a functional material and a
versatile platform for the fabrication of integrated optical microsystems to be used in biochemical analysis. Our research
activity is focused on the design, the fabrication and the characterization of several photonic porous silicon based
structures, which are used in the sensing of specific molecular interactions. To integrate the porous silicon based optical
transducer in biochip devices we have modified standard micromachining processes, such as anodic bonding and photo-patterning,
in order to make them consistent to the utilization of biological probes.
In this work, an integrated optical microsystems for the continuous detection of flammable liquids has been fabricated
and characterized. The proposed system is composed of a the transducer element, which is a vertical silicon/air Bragg
mirror fabricated by silicon electrochemical micromachining, sealed with a cover glass anodically bonded on its top. The
device has been optically characterized in presence of liquid substances of environmental interest, such as ethanol and
isopropanol. The preliminary experimental results are in good agreement with the theoretical calculations and show the
possibility to use the device as an optical sensor based on the change of its reflectivity spectrum.
The interaction between an analyte and a biological recognition system is normally detected in biosensors by the
transducer element which converts the molecular event into a measurable effect, such as an electrical or optical signal.
Porous silicon microstructures have unique optical and morphological properties that can be exploited in biosensing. The
large specific surface area (even greater than 500 m2/cm3) and the resonant optical response allow detecting the effect of
a change in refractive index of liquid solutions, which interact with the porous matrix, with very high sensitivity.
Moreover, the porous silicon surface can be chemically modified to link the bioprobe which recognize the target
analytes, in order to enhance the selectivity and specificity of the sensor device. The molecular probe we used was
purified by an extremophile organism, Thermococcus litoralis: the protein is very stable in a wide range of temperatures
even if with different behavior respect to the interaction with the ligand.