Single and multilayer metamaterials fabricated by nanoimprint lithography.

We demonstrate for the first time a fast and easy nanoimprint lithography (NIL) based stacking process of negative index structures like fishnet and Swiss-cross metamaterials. The process takes a few seconds, is cheap and produces three-dimensional (3D) negative index materials (NIMs) on a large area which is suitable for mass production. It can be performed on all common substrates even on flexible plastic foils. This work is therefore an important step toward novel and breakthrough applications of NIMs such as cloaking devices, perfect lenses and magnification of objects using NIM prisms. The optical properties of the fabricated samples were measured by means of transmission and reflection spectroscopy. From the measured data we retrieved the effective refractive index which is shown to be negative for a wavelength around 1.8 µm for the fishnet metamaterial while the Swiss-cross metamaterial samples show a distinct resonance at wavelength around 1.4 µm.

Multi-gap individual and coupled split-ring resonator structures.

We present a systematic numerical study, validated by accompanied experimental data, of individual and coupled split ring resonators (SRRs) of a single rectangular ring with one, two and four gaps. We discuss the behavior of the magnetic resonance frequency, the magnetic field and the currents in the SRRs, as one goes from a single SRR to strongly interacting SRR pairs in the SRR plane. We show that coupling of the SRRs along the E direction results to shift of the magnetic resonance frequency to lower or higher values, depending on the capacitive or inductive nature of the coupling. Strong SRR coupling along propagation direction usually results to splitting of the single SRR resonance into two distinct resonances, associated with peculiar field and current distributions.

Negative index short-slab pair and continuous wires metamaterials in the far infrared regime.

Using transmission and reflection measurements under normal incidence in one and three layers of a mum-scale metamaterial consisting of pairs of short-slabs and continuous wires, fabricated by a photolithography procedure, we demonstrate the occurrence of a negative refractive index regime in the far infrared range, ~2.4-3 THz. The negative index behavior in that system at ~2.4-3 THz is further confirmed by associated simulations, which are in qualitative agreement with the experimental results.

The spectrum of vibration modes in soft opals.

Numerous vibrational modes of spherical submicrometer particles in fabricated soft opals are experimentally detected by Brillouin light scattering and theoretically identified by their spherical harmonics by means of single-phonon scattering-cross-section calculations. The particle size polydispersity is reflected in the line shape of the low-frequency modes, whereas lattice vibrations are probably responsible for the observed overdamped transverse mode.

Magnetic response of split-ring resonators in the far-infrared frequency regime.

We report on the fabrication, through photolithography techniques, and the detailed characterization, through direct transmission measurements, of a periodic system composed of five layers of photolithographically aligned micrometer-sized Ag split-ring resonators (SRRs). The measured transmission spectra for propagation perpendicular to the SRRs plane show a gap around 6 THz for one of the two possible polarizations of the incident electric field; this indicates the existence of a magnetic resonance, which is verified by detailed theoretical analysis. To our knowledge this is the first time that a system of more than one layer of micrometer-sized SRRs has been fabricated. The measured optical spectra of the Ag microstructure are in very good agreement with the corresponding theoretical calculations.

Air bubbles in water: a strongly multiple scattering medium for acoustic waves.

Using a newly developed multiple scattering scheme, we calculate band structure and transmission properties for acoustic waves propagating in bubbly water. We prove that the multiple scattering effects are responsible for the creation of wide gaps in the transmission even in the presence of strong positional and size disorder.

Phonons in suspensions of hard sphere colloids: volume fraction dependence.

The propagation of sound waves in suspensions of hard sphere colloids is studied as a function of their volume fraction up to random close packing using Brillouin light scattering. The rich experimental phonon spectra of up to five phonon modes are successfully described by theoretical calculations based on the multiple scattering method. Two main types of phonon modes are revealed: Type A modes are acoustic excitations which set up deformations in both the solid (particles) and the liquid (solvent) phases; for type B modes the stress and strain are predominantly localized near the interface between the solid particles and the surrounding liquid (interface waves). While the former become harder (increase their effective sound velocity) as the particle volume fraction increases the latter become softer (the corresponding sound velocity decreases).