We propose a novel approach in optical trapping exploiting mesoscopic photonic crystal microcavities. Full light confinement in mesoscopic photonic crystal membranes, forming a mesoscopic self-collimating 1D Fabry-Pérot cavity, was theoretically predicted and experimentally verified by the authors in previous papers. In this paper, we numerically demonstrate a high performance MPhC microcavity for optical trapping of fine particulate matter in air. The MPhC cavity has been simulated by 3D FDTD simulations while the trapping potential has been evaluated by means of the gradient force density convolution method. We numerically show that it is possible to obtain very high trapping potential for polystyrene particles having radii as small as 245 nm.
We present a strategy to control Fano resonances in hybrid graphene-silicon-on-insulator gratings. The presence of a mono- or few-layer graphene film allows to electrically and/or chemically tuning the Fano resonances that result from the interaction of narrow-band, quasi-normal modes and broad-band, Fabry-Perot-like modes. Transmission, reflection and absorption spectra undergo significant modulations under the application of a static voltage to the graphene film. In particular, for low values of the graphene chemical potential, the structure exhibits a symmetric Lorentzian resonance; when the chemical potential increases beyond a specific threshold, the grating resonance becomes Fano-like, hence narrower and asymmetric. This transition occurs when the graphene optical response changes from that of a lossy dielectric medium into that of a low-loss metal. Further increasing the chemical potential allows to blue-shift the Fano resonance, leaving its shape and linewidth virtually unaltered. We provide a thorough description of the underlying physics by resorting to the quasi-normal mode description of the resonant grating and retrieve perturbative expressions for the characteristic wavelength and linewidth of the resonance. The roles of number of graphene layers, waveguide-film thickness and graphene quality on the tuning abilities of the grating will be discussed. Although developed for infrared telecom wavelengths and silicon-on-insulator technology, the proposed structure can be easily designed for other wavelengths, including visible, far-infrared and terahertz, and other photonic platforms.
Ultra-short vertical plasmonic couplers were devised for the efficient excitation of long-range surface-polariton-plasmon mode, in the visible regime, between a polymeric waveguide and a plasmonic waveguide in two different configurations. Numerical simulations suggest the realization of coupling efficiencies as high as 90% and insertion losses as low as −5.5 dB , with a coupling length of few micrometers. Thus the proposed design clearly proves that is possible to optimize contemporaneously the coupling efficiency and the coupling length. Therefore the compactness and the lower fabrication requirements make the proposed device very promising in a variety of applications.
We experimentally demonstrate the possibility to implement an optical bio-sensing platform based on the shift of the
plasmonic band edge of a 2D-periodic metal grating. Several 2D arrangements of square gold patches on a silicon
substrate were fabricated using electron beam lithography and then optically characterized in reflection. We show that
the presence of a small quantity of analyte, i.e. isopropyl alcohol, deposited on the sensor surface causes a dramatic red
shift of the plasmonic band edge associated with the leaky surface mode of the grating/analyte interface, reaching
sensitivity values of ~650nm/RIU. At the same time, dark field microscopy measurements show that the spectral shift of
the plasmonic band edge may also be detected by observing a change in the color of the diffracted field. Calculations of
both the spectral shift and the diffracted spectra variations match the experimental results very well, providing an
efficient mean for the design of sensing platforms based on color observation.
In this paper we describe the fabrication of a periodic, two-dimensional arrangement of gold square patches on a Silicon
substrate, and highlight technological limitations due to the roughness of the metal layer. Scanning Electron Microscope
(SEM) and Atomic Force Microscope analyses are also reported showing that the geometrical parameters obtained are
almost identical to the nominal parameters of the simulated structure.
The device is functionalized by means of a conjugated rigid thiol forming a very dense, closely packed, reproducible 18
Å–thick, self-assembled monolayer. The nonlinear response of the 2D array is characterized by means of a micro-Raman spectrometer and it is compared with a conventional plasmonic platform consisting of a gold nano-particles ensemble on Silicon substrate, revealing a dramatic improvement in the Raman signal. The SERS response is empirically investigated using a laser source operating in the visible range at 633 nm. SERS mapping and estimation of the provided SERS enhancement factor (EF) are carried out to evaluate their effectiveness, stability and reproducibility as SERS substrate.
Moreover, we take advantage of the simple geometry of this 2D array to investigate the dependence of the SERS
response on the number of total illuminated nano-patches.
We investigate the transmission properties of arrays of three-dimensional (3-D) gold patches having one- and two-dimensional
(1- and 2-D) periodicities, and describe the interaction of cavity and surface plasmon modes. We vary the
main geometrical parameters to assess similarities and emphasize differences between 1-D and 2-D periodic patterns.
We analyze the spectral response as a function of incident angle and polarization to corroborate our findings. We will
also consider form and air filling factors of the grating to assess our ability to control the transmission spectrum. In
particular, we observe strong inhibition of the transmission when the impinging wave-vector parallel to the surface of the
metal matches the surface plasmon wave-vector of the unperturbed air-gold interface when added to the grating lattice
wave-vector. This phenomenon favors the opening of a plasmonic band gap, featuring the suppression of transmission
and simultaneous coupling to back-radiation (reflections) of the unperturbed surface plasmon. High-Q, resonating modes
occur at the edges of the forbidden band, boosting the energy transfer across the grating thus providing enhanced
transmission and broadside directivity at the exit side of the grating.
Recent scientific publications have highlighted the possibility of enhancing solar conversion efficiency in thin
film solar cells using surface plasmon (SP) waves and resonances. One main strategy is to deposit layers of
metal nanoparticles on the top of a thin film silicon solar cell which can increase light absorption and
consequently the energy conversion in the frequency range where the silicon intrinsic absorptance is low. In
this paper, we investigate the effects produced on the light absorption and scattering by silver nanoparticles,
arranged in a periodic pattern, placed on the top of amorphous silicon (α-Si) thin layer. We propose different
geometry of metal objects, quantifying the scattering (back and forward) determined by the nanoparticles in
dependence of their shapes and Si thickness. The analysis reveals that the thickness of the substrate has huge
influence on the scattering, in particular on the back one, when the nanoparticles have corners, whereas it
seems less dramatic when rounded profiles are considered (nanospheres).
Laser cavities emitting in the near and medium infrared wavelength range, made of rare earth doped optical fibers and
suitable pairs of integrated mirrors, are used in a large number of applications. Nowadays, the efficient employment of
near and medium infrared laser beams is largely widespread in the field of m*aterial processing, surgery, directed energy,
remote sensing, spectroscopy, imaging, and so on. In a lot of cases, the high conversion efficiency, the excellent beam
quality, the compactness and, the good heat dissipation capability make fiber lasers competitive and attractive with
respect to other light sources, such as ion-doped crystal and bulk glass lasers, optical parametric oscillators,
semiconductor and gas lasers. The paper aims to recall and/or briefly illustrate a few among the numerous strategies
recently followed by research laboratories and industries to obtain laser sources based on rare earth doped optical fibres.
A recall on the host materials and the dopants employed for their construction, and the corresponding applications is
given, too. Moreover, an example of near infrared (NIR) fiber optic laser development, by employing available on
market components is illustrated by underlining the possibility to easily obtain high beam quality.
The lasing characteristics of an erbium doped silica glass microsphere coupled to a tapered fiber are numerically
investigated in the third band of the optical fiber communication. In the model, the electromagnetic field profile of the
whispering gallery modes (WGMs) traveling in the microsphere is described by means of spherical Bessel functions for
the radial dependence and spherical harmonics for the angular dependence, at both pump and signal wavelengths.
Moreover, the microsphere laser operation has been simulated by taking into account the rare earth ion emission, via the
rate equations, and the coupling with the tapered fiber. A number of simulations have been performed in order to
demonstrate the feasibility of the active microspheres to be employed as distributed micro laser sources or to fabricate
We conducted a theoretical investigation of second harmonic generation and other nonlinear features that result from the magnetic Lorentz force, when a single aperture is cut on a thick, opaque palladium substrate. We studied the dependences of linear pump transmission and second harmonic generation near resonance conditions, and explored the different physical mechanisms and their dependences, for example, geometrical features. We found that it is possible to exploit field localization and surface plasmon generation to enhance second harmonic generation in the regime of extraordinary transmittance of the pump field. Both transmitted and backward second harmonic generation conversion efficiencies were investigated. The results reveal that it may be possible to access several potential new applications. In particular, we demonstrated that the exploitation of a combination of nonlinear effects and enhanced transmission makes possible a palladium-based device suitable for H2-leak-detection.
We theoretically investigate second harmonic generation that originates from the nonlinear, magnetic Lorentz force term
from single and multiple apertures carved on thick, opaque metal substrates. The linear transmission properties of
apertures on metal substrates have been previously studied in the context of the extraordinary transmission of light. The
transmission process is driven by a number of physical mechanisms, whose characteristics and relative importance
depend on the thickness of the metallic substrate, slit size, and slit separation. In this work we show that a combination
of cavity effects and surface plasmon generation gives rise to enhanced second harmonic generation in the regime of
extraordinary transmittance of the pump field. We have studied both forward and backward second harmonic generation
conversion efficiencies as functions of the geometrical parameters, and how they relate to pump transmission efficiency.
The resonance phenomenon is evident in the generated second harmonic signal, as conversion efficiency depends on the
duration of incident pump pulse, and hence its bandwidth. Our results show that the excitation of tightly confined modes
as well as the combination of enhanced transmission and nonlinear processes can lead to several potential new
applications such as photo-lithography, scanning microscopy, and high-density optical data storage devices.
A temperature sensor immune to electromagnetic noise is designed and fabricated. The sensor key element is a periodically poled lithium niobate (PPLN) substrate. PPLN allows a direct and efficient frequency conversion of lightwave through the quasi-phase matching (QPM) of the pump radiation propagating at the fundamental and second harmonic wavelengths. For these devices, the efficiency of second harmonic generation (SHG) depends on the QPM condition, and it strongly changes with respect to the wavelength and the temperature. The effect of temperature variation on the SHG in periodically poled lithium niobate annealed proton exchange (APE) channel waveguides (WG) is theoretically modeled via a home-made computer code and experimentally validated via a suitable measurement set-up. A lot of simulations have been performed to test the temperature sensor feasibility and to identify its optimal configuration. Another sensor configuration made by two waveguides with suitable gratings of inverted ferroelectric domains is designed and refined, too. For an optimised PPLN-WG device, which could be fabricated through electric field poling and annealed proton exchange or titanium diffusion, a sensitivity S≡0.03μW/°C for the temperature range equal to 100 °C is demonstrated by using an input power at a fundamental wavelength equal to 40 mW. Similar evaluations and measurements, performed on bulk substrates, allowed us to design a layout of a sensor particularly suited for rugged in-field applications.
In this paper we propose the design and the fabrication of 90° bend ridge waveguide (WG) assisted by a two-dimensional photonic crystal (2D-PC). 2D-PCs act as efficient mirrors along the boundaries of the bend ridge thus reducing the in-plane losses. The ridge waveguide consists of a 3 μm x 0.75 μm titanium dioxide core on a silica bottom cladding. The 2D-PC structure surrounding the bend waveguide is composed of a triangular array of circular dielectric pillars having a height of 0.75 μm. The titanium dioxide waveguiding core layer is covered with PMMA in order to create a quasi-symmetric structure. A photonic band gap centered around 1.3 μm is obtained by a PC radius r = 0.33a and lattice period a = 0.450 μm. The design of the whole structure is subsequently optimized by using a 3D Finite Difference Time Domain based computer code. The ridge waveguide assisted by a 2D-PC has been fabricated by using electron beam lithography and reactive ion etching. For the pattern transfer we have used about 50 nm thin layer Cr metal etch mask obtained by means of a lift-off technique based on the use of bi-layer resist (PMMA/MMA).
The presence of the 2D-PC around the bend waveguide leads to a sharp increase of the transmission efficiency around 1.3 μm for curvature radius of 2.5 μm. The bend transmission results to be in the range between 0.76 and 0.85 when the thickness of the ridge WG and of the 2D-PC pillars is between 0.75 and 1.3 μm. This value is more than twice with respect to the bend waveguide without 2D-PC.
A computer code for the design of optical sensors is developed, the electromagnetic model taking into account both lossy guided and leaky modes. For a concentration C = 700 ppm of toluene in water, the maximum absorbance of a sensor having length L=2 cm is A ≡ 10 for the guided quasi-TM<sub>00</sub> mode, while that pertaining to the leaky quasi-TM<sub>12</sub> mode is A ≈ 1200. The simulation results indicate that a selective excitation of the suitable propagation modes could enhance the sensor performance.
Periodically poled LiNbO<sub>3</sub> (PPLN) makes available direct and efficient frequency conversion of laser light through quasi-phase matched (QPM) nonlinear optical interactions. For a given PPLN waveguide, the efficiency of frequency conversion strongly depends on QPM, which can be optimized by tuning the wavelength or the temperature. We propose a guided wave device for temperature measurement based on second harmonic generation (SHG) in a PPLN waveguide. Modeling of the device and a preliminary test in a PPLN waveguide fabricated through electric field poling and (APE) annealed proton exchange are reported.
The feasibility of high gain planar waveguide amplifiers, made by Er-doped, silica-titania SiO$02) - TiO<SUB>2</SUB> thin films on silica SiO<SUB>2</SUB> glass substrate, is proved by means of an on purpose developed computer code. The computer code implemented for simulation is based on a complete model, which takes into account the secondary transitions pertaining to the ion-ion interactions, the pump and signal propagation, and the ASE. In the small signal operation, a gain close to 4.7 dB is theoretically demonstrated for a waveguide of 5 cm length, by using a pump power of 200 mW. A first set of waveguiding samples is fabricated and characterized.
The feasibility of a planar channel waveguide amplifier and laser made of praseodymium (Pr<SUP>3+</SUP>) doped chalcogenide glass, based on the binary composition GeS<SUB>2</SUB>, is investigated by means of an appositely implemented computer code. The buried channel waveguide amplifier, having both core and substrate doped with a dopant concentration Nequals3000 ppm, shows gain close to Gequals7 dB for an input signal power of P<SUB>s</SUB>equals1 (mu) W at the wavelength (lambda) <SUB>s</SUB>equals1310 nm and a pump power P<SUB>p</SUB>equals 200 mW at the wavelength (lambda) <SUB>p</SUB>equals1020 nm. The Fabry-Perot laser, based on the aforesaid buried channel waveguide terminated pump power P<SUB>th</SUB>equals16mW and a slope efficiency Sequals8%.
The guided and leaky mode propagation in a multilayered c-rotated anisotropic waveguide having an equatorial dielectric tensor configuration is examined. In particular, the electromagnetic behavior of titanium diffused lithium niobate (Ti:LiNbO<SUB>3</SUB>) and proton exchanged lithium niobate (PE:LiNbO<SUB>3</SUB>) waveguides is compared. The dependence of modal characteristics on the equatorial angle (gamma) is shown and the effects of different metal layers are investigated. The transition from leaky to guided modes occurs only for the PE:LiNbO<SUB>3</SUB> waveguide while the modes do not change their nature for the Ti:LiNbO<SUB>3</SUB> waveguide. The number of guided modes, for the PE:LiNbO<SUB>3</SUB> waveguide, increases by increasing the angle (gamma) . The first guided mode, having the real part of the refractive effective index n<SUB>r</SUB> equals 2.2701, appears at (gamma) equals 25 degree(s), while, for (gamma) > 73 degree(s) nine different guided modes exist.
The soliton emission phenomenon can be exploited to design ultrafast switches to be used in all-optical processing systems. When very high speed switching is required and unless high optical power is used, there are needed switching devices very short in length. The propagation characteristics of the soliton emission from a nonlinear interface is simulated using the Beam Propagation Method. The effects of the interaction between two solitons emitted from two different nonlinear interfaces and propagating in a common nonlinear substrate are examined. Based on these principles, new all-optical switching elements can be designed, which exhibit many important advantages on the classical electro-optical devices.
The resolution of a sensor designed to measure strain may be compromised by fluctuations in the environment temperature. This problem can be solved by considering a combined interferometnic-polanimetric measure: in fact if both the interferometric phases for the TE-like and TM-like polarizations are determined the solution of a system of two equations allows for the simultaneous recovery of strain and temperature. In this paper we discuss the possibility of making unambiguous measurements by using the integrated-optic Michelson interferometer consisting of a semi-asymmetric Xjunction made by titanium indiffused lithium niobate waveguides designed and built specifically for the stress temperature and microdisplacement measurements.
the soliton generation and trapping in nonlinear waveguide can be used to implement ultrafast, all-optical switches. In this paper, the phenomenon of the soliton emission from a nonlinear interface of a symmetrical waveguide and of the soliton trapping are analyzed by simulations performed using the beam propagation method. We show the correlation between the input optical power and the length at which the soliton generation occurs, along the propagation direction of the single-mode nonlinear waveguide. Based on these principles, new all-optical switching elements and logical gates can be designed, which exhibit many important advantages on the classical electro-optical devices.
An ultra-rapid all-optical switch useful for application in the future high-bit-rate systems is presented. Ti:LiNbO3 waveguides are considered owing to the acceptable substrate nonlinear relative dielectric constant (a 41 01 8 m2/V2) and the consolidated waveguide fabrication technology. Logic gates which can perform the XOR AND and NOT functions are investigated. The optical switching is provided via optical control pulses in one or both arms by exploiting either the stable or the unstable region of the waveguided optical power versus the effective refractive index.
An interferometer temperature sensor based on a semi-asymmetric X junction made of four single-mode Ti:LiNbO3 channel waveguides is described. For a fixed path length difference, the X-cut configuration exhibits a thermal sensibility greater than that of Z-cut interferometer. The X-cut version is found to be superior for both thermal stability and sensitivity.
Interferometric optical sensors have been shown to possess very high resolution due to the sensitivity of the optical path length to a variety of electrical, chemical and mechanical quantities. We present the design of an integrated optic microdisplacement sensor made of four single mode Ti:LiNbO3 channel waveguides on X-cut substrate shaped to form a semi-asymmetric X junction. The effects of the rotation of the waveguide axis with respect to the principal reference coordinate system and the influence of the fluctuations in the environment temperature have been accounted. The calculated effective index change is dnef/dT = 17.10-5oC-1 for EY-11 mode and dnef/dT = 5.3 .10-5 ° C-1 for Ex11 mode, respectively.
Interferometer sensors using optical waveguides have been shown to be suitable for sensing a vanety of
physical items such as temperature, strain, humidity, electric and magnetic fields, posItion. In this paper we study
an integrated optical microdisplacement sensor making use of a Michelson interferometric configuration. The
two-beam semi-asymmetric X junction is composed of four single-mode Ti diffused LiNbO3 channel waveguides
at ? = 633 nm. The design criteria stress the fact that the waveguide runs at a given angle with respect to the
principal reference system. The electromagnetic field evolution is obtained both by mode-matching and beam
An exact statement about single Ti:LiNbO3 waveguide phase-modulators made on Z-cut and X-cut
substrates is reported. The optimal placement of the electrodes with respect to the optical channel waveguide is
discussed. The optical analysis of channel inhomogeneous anisotropic waveguide is carried-out by the effective
refractive index method together with the transformation matrix approach. At the microwaves the analysis is done
by conformal mapping and multiple images methods. The results are given in a graphical form.