In this talk we will review our recent demonstrations of mid-IR lasers grown on (001) Si or Ge substrates (diode lasers, interband cascade lasers, quantum cascade lasers) and compare their performance to those grown on their native substrates. We will demonstrate light coupling from lasers grown on patterned Si photonics wafers to passive SiN waveguides, with a coupling efficiency in line with simulations. Finally, we will discuss and evaluate strategies to enhance the coupling efficiency.
Simultaneous detection of different gas species represents an indispensable asset for several applications, such as instantaneous quantification of isotope concentration ratios, self-calibrating sensors, and monitoring of the temporal evolution of a chemical reaction. In this research work, a dual-gas quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor for a real-time analysis and in a continuous flow monitoring of one reactant and one product of a gas-phase chemical reaction involving nitrogen dioxide (NO2) and water vapor (i.e., H2O) – as reaction reactants – and nitrogen monoxide (NO) – as one of the reaction products – was realized. The QEPAS sensor implemented a spectrophone composed of a pair of metallic acoustic resonator tubes applied at both antinode points of a custom quartz tuning fork (QTF). In this configuration, two different quantum cascade lasers (QCLs) were used, having an emission wavelength centered at 5.26 μm – resonant with a nitrogen monoxide absorption feature located at 1,900.075 cm-1 – and at 6.25 μm – resonant with a nitrogen dioxide absorption feature located at 1,601.77 cm-1 –, respectively. The chemical reaction was studied by injecting in the gas line a certified concentration of 5,000 parts-per-million (ppm) of NO2:N2 and monitoring the QEPAS signals at four different total gas flow values, i.e., 10, 20, 30 and 50 standard cubic centimeters per minute (SCCM), respectively.
In this communication we will present the first semiconductor laser grown on a Si photonics platform in a butt-coupling configuration. A GaSb-based diode laser (DL) was grown on a patterned Si photonics wafer equipped with SiN waveguides. Growth and device fabrication challenges arising from the template architecture were overcome to demonstrate several mW outpower of emitted light in continuous wave operation at room temperature. In addition, around 10% of light was coupled into the SiN waveguides, in good agreement with theoretical calculations. This work paves the way to future on-chip sensors.
Optical devices requiring a compact and intelligent design find multifunctionality and reconfigurability to be of paramount importance. In this manuscript, we present a numerical investigation of a new design exhibiting these features. The proposed structure relies on the joint paradigm of 1D photonic crystals (PhC) and reconfigurable materials (graphene and liquid crystals). This composite structure can perform several reconfigurable narrowband optical functions such as notch filtering, amplitude modulation, and phase shifting. The effects on the behavior of this structure of monolayer graphene and liquid crystal polarization are independent of each other and allow control of absorbance intensity, phase shift action, and spectral position of resonance are detailed. This structure may find use in the design of smart reconfigurable metasurfaces, for optical modulators and beam-steering systems.
In this work, we demonstrate the modulation in optical response at visible wavelengths of a dielectric grating structure under a thermal stimulus. The grating structure is coated with a thin layer of vanadium dioxide (VO2) which undergoes a phase transition from an insulator to a metal at a temperature of ~ 68°C. We report on the design, simulations, and characterization of the proposed structure. Measured optical response through experiments finds a good agreement with the predictions made by numerical simulations.
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.
Mesoscopic photonic crystal based mirrors are exploited to theoretically conceive and analyse a novel high-Q factor Fabry-Perot-like cavity. Thanks to a beam focussing effect in reflection these mirrors efficiently confine and stabilise a mode inside a planar cavity, even for non-paraxial small beam sizes, mimicking the behaviour of curved mirrors. Furthermore, these mirrors show a higher reflectivity with respect to their standard distributed Bragg reflector counterparts, which allow these cavities to reach a maximum Q factor higher than 104 when 5-period-long mirrors are considered. Moreover, these cavities show other intriguing features as a beamforming behaviour and transverse translational invariance offered by the planar geometry. The latter opens interesting possibilities for lasing and biodetection. The optimization of the cavity size and the performances in terms of Q factor, energy storage and confinement are detailed.
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).
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