Metasurfaces are two-dimensional structures, arrays of scatterers with subwavelength separation or optically thin planar films, allowing light manipulation and enabling specific changes of optical properties, as for example beam-steering, anomalous refraction and optical-wavefront shaping. Due to the fabrication simplicity, the metasurfaces offer an alternative to 3-D metamaterials and providing a novel method for optical elements miniaturization. It has been demonstrated that a metasurface can support Bound States in Continuum (BIC), that are resonant states by zero width, due to the interaction between trapped electromagnetic. Experimentally, this involves very narrow coupled resonances, with a high Q-factor and an extremely large field intensity enhancement, up to 6 orders of magnitude larger than the intensity of the incident beam. Here, we demonstrate that the field enhancement in proximity of the surface can be applied to boost fluorescence emission of probe molecules dispersed on the surface of a photonic crystal membrane fabricated in silicon nitride. Our results provide new solutions for light manipulation at the nanoscale, especially for sensing and nonlinear optics applications.
The realization of miniaturized devices able to accumulate a higher number of information in a smallest volume is a challenge of the technological development. This trend increases the request of high sensitivity and selectivity sensors which can be integrated in microsystems. In this landscape, optical sensors based on photonic crystal technology can be an appealing solution. Here, a new refractive index sensor device, based on the bound states in the continuum (BIC) resonance shift excited in a photonic crystal membrane, is presented. A microfluidic cell was used to control the injection of fluids with different refractive indices over the photonic crystal surface. The shift of very high Q-factor resonances excited into the photonic crystal open cavity was monitored as a function of the refractive index n of the test liquid. The excellent stability we found and the minimal, loss-free optical equipment requirement, provide a new route for achieving high performance in sensing applications.
Dynamical diffraction in a deformed (often bent) crystal is described by the Takagi equations 1 which, in general, have to be solved numerically on a regular 2-D grid of points representing a planar cross section of the crystal in which the diffraction of an incident X-ray wavefront occurs . Presently, the majority of numerical approaches are based on a finite difference solving scheme2-4 which can be easily implemented on a regular Cartesian grid but is not suitable for deformed meshes. In this case, the inner deformed crystal structure can be taken into account, but not the shape of the crystal surface if this differs substantially from a planar profile 5,6.
Conversely, a finite element method (FEM) can be easily applied to a deformed mesh and serves very well to the purpose of modelling any incident wave on an arbitrarily shaped entrance surface 7 e.g. that of a bent crystal or a crystal submitted to a strong heat load 8-10. For instance, the cylindrical shape of the surface of a strongly bent crystal plate can easily be taken into account in a FEM calculation. Bent crystals are often used as focusing optical elements in Xray beamlines 11-13.
In the following, we show the implementation of a general numerical framework for describing the propagation of X-rays inside a crystal based on the solution of the Takagi equations via the COMSOL Multiphysics FEM software package (www.comsol.com). A cylindrically bent crystal will be taken as an example to illustrate the capabilities of the new approach.
We present a new approach to the numerical solution of Takagi equations, based on the Finite Element Method (FEM) that starting from a weak formulation of differential equations is much more versatile than the usual Finite Difference method (FDM). In particular, this is the only available approach that can easily handle with calculation where the free space propagation is required, as in the case of focusing by curved crystals, but also some crystals cut with special geometries that are useful for neutron monochromators.
Electromagnetic surface waves, analogous to the classic surface plasmons can be supported to any interface, providing that the effective permittivity have an opposite sign. Localized plasmonlike modes and guided mode resonances are established in a photonic crystal slab irradiated with out-of-plane incident radiation, making photonic crystals a very appealing alternative to plasmonic substrates, avoiding the limits of absorption losses in metals.
The existence of a special type of resonances in the visible transmission spectrum of a very thin two-dimensional photonic crystal slab is demonstrated. We illustrate a controlling mechanism that allows the stabilization of the field amplification in a thin layer lattice with low contrast dielectric. Numerical simulations show that an extremely large field enhancement, as large as 700 times the amplitude of the incident wave, connected with high Q-factor resonances can be axcited. The connection with the bound states in continuum phenomenon is highlighted.
A giant field enhancement, respect to the amplitude of the incident wave is achieved in a thin layer lattice with low contrast dielectric, is demonstrated. The key mechanism is a careful control of the parameters, which allows a stabilization of the coupling resonances.
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.
In this paper we propose an approach based on the Dynamical Diffraction Theory (DDT) is presented in order to derive
an analytic formulation of superprism effect that exhibit an extremely high angular dispersion. We apply the theory to a
one dimensional Photonic Crystal (1D-PhC) at the wavelength of 1.55μm. We demonstrate that it is possible to obtain an angular dispersion of 9.73°/nm by using a structure of Si/SiGe, which represent among the higher dispersion available in
In this paper we report a new set of accurate measurements of guided mode resonances coupled in a negative photonic
crystal slab. Narrow peaks are visible in the reflection spectrum with a full-width at half maximum (FWHM) of less than
2 nm. In addition to the traditional measurements of the reflected signal, we present the imaging of the coupled radiation
propagating into the slab. Finally, by comparison with the already known phenomenological analysis  we propose a
new physical model of the phenomenon. The experimental data shows an excellent agreement with mentioned theory.
In this work, we present a comparative theoretical study about the optical absorption coefficient calculated in ordered nanopillar and nanohole photonic crystal silicon structures for solar energy applications. In particular, we investigate the ultimate efficiency at normal incidence condition of such structures for several fill factors and lattice constants. We find that, except for small ranges of frequency where an inversion of tendency is observed, the total absorption coefficient in nanopillar arrays is greater than the one calculated in nanohole arrays. Moreover, optimized silicon nanopillar arrays show percentage improvements of the ultimate efficiency up to 138% with respect to the case of a silicon thin film of equivalent thickness. Finally, we report preliminary experimental results about the realization of a silicon photonic crystal with a nanopillar array structure to be exploited as an optical trapping film in solar cells.
High frequencies (visible and near infrared) applications of metamaterials and plasmonic structures are strongly limited by dissipative losses in structures, due to poor conductivity of most used metals in this frequency range. The use of high temperature superconductors (HTSC) is a possible approach to this problem, being HTSC plasmonic materials at nonzero temperature. Negative dielectic constant and variety of charge carriers (electrons or holes) are further very attractive features for plasmonic applications. Characterization of the high frequency response of these materials is then necessary in order to correctly understand the optical parameters of HTSC. We report on FTIR and ellipsometry measurements on NdBa2Cu3O7-δ (Nd123) and the ruthenocuprate superconductor GdSr2RuCu2O8-δ (Gd1212) in optical and near infrared regime. Among YBCO-like cuprate superconductors, Nd123 presents the highest Tc (96K), and the most interesting magnetic response properties. Even more interesting, in view of use for metamaterial, is Gd1212, whose main characteristic is the coexistence, in the same cell, of superconductivity and magnetic order below Tc: Ru ions intrinsic magnetic moments order themselves below 135K, whereas superconductivity onset is at about 40K, depending on fabrication details. We performed measurements on Melt-Textured bulk samples, which present the best superconducting properties. Results confirm the promising feature of the considered materials; further analyses, also on powders and films, are in progress.
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.
In a quasi- zero-average-refractive-index (QZAI) metamaterial, the light scattered out is extremely directive (Δθout =
0.06°), in despite of a divergent source at near infrared wavelength (λ=1.55 mm). In this paper we discuss and
experimentally demonstrate that this is possible when the light is coupled with diffraction order of a grating with
alternating complementary media. The experimental data prove with a high degree of accuracy also the strong vertical
confinement of the beam even in the air region of the metamaterial, where any simple vertical confinement mechanism is
absent. This extremely sensitive device works on a large contact area and open news perspective to integrated
In a zero-average refractive index metamaterial the light propagation is forbidden and a narrow full photonic bandgap is open in correspondence of the frequency where it vanishes and the refractive index is averaged over the volume. However, it should be underlined that Fabry-Pérot resonances can open a propagation state inside such a particular type of bandgap. Based on a photonic crystal exhibiting a negative refractive index, it was found that such Fabry-Pérot resonant states allow images transmission with subwavelength details.
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 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.
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.
Aberration effects in curved multilayers for hard X rays are studied using a simple analytical approach. The method is based on geometrical ray tracing including refraction effects up to the first order of the refractive index decrement δ. The interpretation of the underlying equations provides fundamental insight into the focusing properties of these devices. Using realistic values for the multilayer parameters the impact on spot broadening and chromaticity is evaluated. The work is complemented by a comparison with experimental focusing results obtained with a W/B4C multilayer mirror.
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.
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 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.
The beam at the exit surface of a Photonic Crystal (PhC) slab can be periodically modulated, in positive or in negative direction, changing the slab thickness. The thickness in negative refraction on PhC's is not always appropriately considered, in spite of an always increasing literature in this subject. This effect is well known in x-ray diffraction, in the most comprehensive version: the Dynamical Diffraction Theory (DDT). Thickness dependence is a direct result of the so-called Pendellosung phenomenon and is linked to a periodic exchange, inside the crystal, of the energy among direct beam (or positively refracted) and diffracted beam (or negatively refracted). It represents an outstanding example of the application of the result of DDT as a tool for the analysis of s electromagnetic interaction in PhC's.
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.
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.
In the hard x-ray range, optics based only on refraction, as in the case of visible optics, require extremely small (a few microns) bending radii of the crystal monochromators, since the deviation of the refraction index (δ = 1-n) from unity is of the order of 10-6. Based on the principle of a series of N refractive lenses, compound refractive lenses provide an appreciable focus at a reasonable distance, but the photon flux is limited by absorption because of the generally high value of N required. As the effect of refraction is very weak, x-rays deviate considerably when diffraction occurs in a crystal in Bragg geometry: this is the base of many crystal optics devices. Focusing with crystal optics is generally achieved bending the crystals to modify the orientation of the lattices planes or modulating the entrance surface of a flat or curved crystal, the so-called Bragg-Fresnel lens. Sagittal focusing can be also obtained using asymmetrically cut crystals. From a general point of view the focusing by means of bent crystals in Laue geometry is interesting when high energies are used, because the absorption due to the transmission in the crystals is very limited. The use of bent crystals has two big advantages: it allows to accept a great divergence of the incoming radiation, thus increasing the flux of the focused radiation and allowing at the same time to select its frequencies, owing to the Darwin width of the considered reflection. Indeed, the crystal bending enables the diffracting planes to be crossed at the Bragg angle corresponding to each ray of the incident beam and, at the same time, the diffraction process produces a monochromatic beam. Actually, a small Bragg angle implies a rather long beam path in the crystal in the case of Bragg geometry, whereas in Laue geometry the incidence is almost normal: the absorption is therefore minimized upon using a suitably thin crystal. We suggest here a method allowing to improve the quality of high energy polychromatic focusing by bent crystals in Laue geometry.