Lorenzo Dominici

Guided Bloch Surface Waves on Ultrathin Polymeric Ridges

We present a direct evidence of Bloch surface waves (BSWs) waveguiding on ultrathin polymeric ridges, supported by near-field measurements. It is demonstrated that near-infrared BSWs sustained by a silicon-based multilayer can be locally coupled and guided through dielectric ridges of nanometric thickness with low propagation losses. Using a conventional prism-based configuration, we demonstrate a wavelength-selective BSW coupling inside and outside the ridge. Such a result can open interesting opportunities in surface wave-mediated sensing applications, where light could be selectively coupled in specific regions defined by nanometric reliefs.

Thickness dependence of surface plasmon polariton dispersion in transparent conducting oxide films at 1.55 mu m

We experimentally demonstrate propagation of surface plasmon polaritons in the near-IR window lambda (1.45 microm,1.59 microm) at the interface of indium-tin-oxide films with different thicknesses deposited on glass. Dispersion of such polaritons is strongly dependent on the film thickness, putting into evidence a regime in which polaritons at both filmss interfaces are coupled in surface supermodes. The experimental data are shown to be in good agreement with the analytical model for thin and absorbing conducting films. Measurements on aluminum-doped zinc oxide, characterized by a redshifted plasma resonance, do not show any surface plasmon polariton excitation in the same wavelength window.

Experimental determination of the sensitivity of Bloch surface waves based sensors.

Detection of glucose in water solution for several different concentrations has been performed with the purpose to determine the sensitivity of Near Infrared Bloch Surface Waves (lambda = 1.55microm) upon refractive index variations of the outer medium. TE-polarized electromagnetic surface waves are excited by a prism on a silicon nitride multilayer, according to the Kretschmann configuration. The real-time reflectance changes induced by discrete variations in glucose concentration has been revealed and analyzed. Without using any particular averaging strategy during the measurements, we pushed the device detection limit down to a glucose concentration of 2.5mg/dL, corresponding to a minimum detectable refractive index variation of the water solution as low as 3.8.10(-6).

Near-field imaging of Bloch surface waves on silicon nitride one-dimensional photonic crystals

We perform a near-field mapping of Bloch Surface Waves excited at the truncation interface of a planar silicon nitride multilayer. We directly determine the field distribution of Bloch Surface Waves along the propagation direction and normally to the surface. Furthermore, we present a direct measurement of a near-field enhancement effect under particular coupling conditions. Experimental evidence demonstrates that a approximately 10(2) near-field intensity enhancement can be realistically attained, thus confirming predictions from rigorous calculations.

Coupling of surface waves in highly defined one-dimensional porous silicon photonic crystals for gas sensing applications

We describe the use of one-dimensional porous silicon (p-Si) photonic crystals for guiding TE-polarized surface electromagnetic waves (SEWs). Although bulk and interface roughnesses might deteriorate the optical response of photonic structures, we observed reflection spectra presenting narrow (≲6nm) reflectivity anomalies associated with SEWs. In analogy with surface plasmons, SEWs are strongly sensitive to surface modifications. As a proof of principle for a sensor, we provide a direct real-time monitoring of the reversible interactions of organic vapors with the p-Si multilayer. We highlight the higher sensitivity of the SEW-based detection scheme as compared to a method exploiting perturbations of waveguide modes.

Bloch surface waves in ultrathin waveguides: near-field investigation of mode polarization and propagation

In this work, we use a multi-heterodyne scanning near-field optical microscope to investigate the polarization and propagation of Bloch surface waves in an ultrathin (∼λ∕10) ridge waveguide. First, we show that the structure sustains three surface modes, and demonstrate selective excitation of each. Then, by numerically processing the experimental data, we retrieve the transverse and longitudinal components of each of the modes, in good agreement with the calculated fields. Finally, we provide an experimental estimation of the effective indices and the dispersion relations of the modes.

Two-dimensional optics on silicon nitride multilayer: Refraction of Bloch surface waves

When properly designed, a dielectric multilayer can sustain Bloch surface waves (BSWs). Using a multiheterodyne scanning near-field optical microscope that resolves phase and polarization, we will show that a thin dielectric structure deposited on the multilayer deflects the BSW propagation according to Snell’s law. Moreover, the mechanism involved in this process is a transfer of energy from the BSW state in the bare multilayer to the new BSW state generated by the presence of the thin dielectric structure. No relevant radiative counterpart occurs. This characteristic validates the treatment of BSWs at the surface of dielectric multilayers as a two-dimensional phenomenon.

Experimental observation of optical bandgaps for surface electromagnetic waves in a periodically corrugated one-dimensional silicon nitride photonic crystal

Dispersion curves of surface electromagnetic waves (SEWs) in 1D silicon nitride photonic crystals having periodic surface corrugations are considered. We experimentally demonstrate that a bandgap for SEWs can be obtained by fabricating a polymeric grating on the multilayered structure. Close to the boundary of the first Brillouin zone connected to the grating, we observe the splitting of the SEW dispersion curve into two separate branches and identify two regions of very low group velocity. The proper design of the structure allows the two folded branches to lie beyond the light line in a wide spectral range, thus doubling the density of modes available for SEWs and avoiding light scattering.

Plasmon polaritons in the near infrared on fluorine doped tin oxide films

Here we investigate plasmon polaritons in fluorine doped tin oxide (FTO) films. By fitting reflectance and transmittance measurements as a function of wavelength lambda epsilon [1.0microm, 2.5microm] we derive a Drude dispersion relation of the free electrons in the transparent conducting oxide films. Then we compute the dispersion curves for the bulk and surface modes together with a reflectance map over an extended wavelength region (lambda==>10microm). Although the surface polariton dispersion for a single FTO/air interface when neglecting damping should appear clearly in the plots in the considered region (since it is supposedly far and isolated from other resonances), a complex behaviour can arise. This is due to different characteristic parameters, such as the presence of a finite extinction coefficient, causing an enlargement and backbending of the feature, and the low film thickness, via coupling between the modes from both the glass/FTO and FTO/air interfaces. Taking into account these effects, computations reveal a general behaviour for thin and absorbing conducting films. They predict a thickness dependent transition region between the bulk polariton and the surface plasmon branches as previously reported for indium tin oxide. Finally, attenuated total reflection measurements vs the incidence angle are performed over single wavelengths lines R(theta) (lambda= 0.633,0.830,1.300,1.550microm) and over a two dimensional domain R(theta,lambda) in the near infrared region lambda epsilon [1.45microm, 1.59microm]. Both of these functions exhibit a feature which is attributed to a bulk polariton and not to a surface plasmon polariton on the basis of comparison with spectrophotometer measurements and modeling. The predicted range for the emergence of a surface plasmon polariton is found to be above lambda >or= 2.1microm, while the optimal film thickness for its observation is estimated to be around 200nm.

Room-temperature superfluidity in a polariton condensate

Superfluidity is a phenomenon usually restricted to cryogenic temperatures, but organic microcavities provide the conditions for a superfluid flow of polaritons at room temperature. Superfluidity—the suppression of scattering in a quantum fluid at velocities below a critical value—is one of the most striking manifestations of the collective behaviour typical of Bose–Einstein condensates1. This phenomenon, akin to superconductivity in metals, has until now been observed only at prohibitively low cryogenic temperatures. For atoms, this limit is imposed by the small thermal de Broglie wavelength, which is inversely related to the particle mass. Even in the case of ultralight quasiparticles such as exciton-polaritons, superfluidity has been demonstrated only at liquid helium temperatures2. In this case, the limit is not imposed by the mass, but instead by the small binding energy of Wannier–Mott excitons, which sets the upper temperature limit. Here we demonstrate a transition from supersonic to superfluid flow in a polariton condensate under ambient conditions. This is achieved by using an organic microcavity supporting stable Frenkel exciton-polaritons at room temperature. This result paves the way not only for tabletop studies of quantum hydrodynamics, but also for room-temperature polariton devices that can be robustly protected from scattering.