Silvia Romano

Plasmon-like surface states in negative refractive index photonic crystals

In this paper, the presence of localized plasmon-like modes at the surface of a silicon two-dimensional photonic crystal slab is demonstrated. In analogy with surface plasmons supported in metals, we observe that, in a photonic crystal metamaterial, the electromagnetic surface waves arise from a negative effective permittivity. The proposed device is dimensioned in order to support surface states in a large spectral window (≃1550–1650 nm). The result opens 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.

Normal-State Optical Features Study of Nd123 and Gd1212 HTSC Materials for Photonics and Metamaterials Fabrication

YBCO-like Nd123 and Gd1212 superconductors in the normal state, in the visible and infrared regime, have been analyzed in this work. Superconductors, particularly high-temperature superconductors (HTSCs), hold indeed a great interest in the highly innovative research field of metamaterials and photonics with important applications. Indeed nanostructured devices suffer from increasing resistivity with frequency in metals, and superconducting materials provide a viable route to approach this problem. Moreover, HTSCs are plasmonic materials at nonzero temperature, and their features, not available in conventional materials, make them very attractive for metamaterials use. In particular, we investigate Nd123, for its high Tc, and Gd1212, for the coexistence, in its unitary cell, of magnetic order and superconductivity. Samples were extracted from top-seeded melt-textured growth bulk pellets and bars. Fourier transform infrared spectroscopy analysis, performed in the far-infrared, shows articulated peak structures with k <; kmin and a plateau for higher k, allowing for Nd123 and Gd1212 pseudogap measurement. Raman spectroscopy and microscopy provided a structural mapping of our Nd123 samples, allowing for distinguishing the superconducting matrix from nonsuperconducting phase inclusions (precipitates) and providing an instrument to discriminate among optical responses of the different compounds.

Superconductors in plasmonics and metamaterials : some experimental data

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.

Optical biosensors based on photonic crystals supporting bound states in the continuum

A novel optical label-free bio-sensing platform based on a new class of resonances supported in a photonic crystal metasurface is reported herein. Molecular binding is detected as a shift in the resonant wavelength of the bound states in the continuum of radiation modes. The new configuration is applied to the recognition of the interaction between protein p53 and its protein regulatory partner murine double minute 2 (MDM2). A detection limit of 66 nM for the protein p53 is found. The device provides an excellent interrogation stability and loss-free operation, requires minimal optical interrogation equipment and can be easily optimized to work in a wide wavelength range.

Patterning of electrically tunable light-emitting photonic structures demonstrating bound states in the continuum

The authors report a scalable process to fabricate electrically tunable light-emitting photonic structures made of optically active and electrically conductive erbium-doped zinc oxide (Er:ZnO) deposited by magnetron sputtering. Such structures are expected to produce a dramatic amplification of the erbium fluorescence due to enhanced light-matter coupling at topologically protected states called bound states in the continuum (BIC). Our patterning approach circumvents roughening of the Er:ZnO during plasma etching by employing a metallic mask and a lift-off process. Etching with a polymer mask resulted in an unacceptable increase of the surface roughness, from a root mean square (RMS) roughness of 0.5 nm for the as-deposited sample to a RMS roughness of 25 nm after etching. Such surface roughness proves detrimental to the photonic crystal resonances and to the BIC mode in particular. Using a metallic mask instead allowed for the etching of Er:ZnO with a modest roughness increase (RMS value 4.1 nm). This pa...

High field enhancement factors in photonic nanostructures

The possibility to excite high Q-factor resonances connected to high field enhancement factors in loss-free media, such as photonic crystal slabs, here is reported. In particular we demonstrate and experimentally prove the existence of this special type of resonances in the visible transmission spectrum of a very thin two-dimensional photonic crystal slab. For this purpose, we design and fabricate a square lattice array of holes in silicon nitride. Numerical simulations show that an extremely large field enhancement factor large as 700 times the amplitude of the incident wave, connected with high Q-factor resonances. The connection with the bound states in continuum phenomenon is highlighted.

High-field enhancement factor in dielectric photonic structures

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.

Dielectric negative index metamaterial as plasmonics devices

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.

New insight in guided resonances with negative refracting photonic crystals

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 [1] we propose a new physical model of the phenomenon. The experimental data shows an excellent agreement with mentioned theory.