R. S. Penciu

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.

Left-handed metamaterials: detailed numerical studies of the transmission properties

Using numerical simulation techniques such as the transfer matrix method and the commercially available code Microwave Studio, we study the transmission properties of left-handed (LH) metamaterials and arrays of split-ring resonators (SRRs). We examine the dependence of the transmission through single- and double-ring SRRs on parameters of the system such as the size and shape of the SRRs, size of the unit cell, dielectric properties of the embedding medium where the SRRs reside, and SRR orientation relative to the incoming electromagnetic field. Moreover, we discuss the role of SRRs and wires on the electric cut-off frequency of the combined system of wires and SRRs, as well as the influence of the various system parameters on the LH transmission peak of a medium composed of SRRs and wires. Finally, demonstrating the disadvantages of the currently used SRR designs due to the lack of symmetry, we discuss more symmetric, multigap SRRs, which constitute very promising components for future two-dimensional and three-dimensional LH structures.

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.

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.

Magnetic response of nanoscale left-handed metamaterials

Using detailed simulations we investigate the magnetic response of metamaterials consisting of pairs of parallel slabs or combinations of slabs with wires including the fishnet design as the length scale of the structures is reduced from millimeter to nanometer. We observe the expected saturation of the magneticresonance frequency when the structure length scale goes to the submicron regime, as well as weakening of the effective permeability resonance and reduction in the spectral width of the negative permeability region. All these results are explained by using an equivalent resistor-inductor-capacitor circuit model, taking into account the current-connected kinetic energy of the electrons inside the metallic parts through an equivalent inductance, added to the magnetic field inductance in the unit cell. Using this model we derive simple optimization rules for achieving optical negative permeability metamaterials with improved performance. Finally, we analyze the magnetic response of the fishnet design and we explain its superior performance regarding the high attainable magnetic-resonance frequency, as well as its poor performance regarding the width of the negative permeability region.

Phonons in colloidal systems

Rich phonon spectra were observed experimentally by Brillouin spectroscopy in liquid, glassy, and crystalline state of colloidal systems of low and high elastic constant contrast. The nature of these phonons was elucidated by theoretical calculations of the single sphere scattering cross section, the energy density distribution, the light scattering intensity, and determination of the band structure by the multiple scattering method. Besides the ordinary acoustic phonon, localized optic-like modes, mixed modes, and Bragg induced modes were identified. Their relation to the physical state of a colloidal suspension sensitively depends on the micromechanical mismatch between particle and surrounding medium and the coherence of the crystalline structure. Polycrystalline colloidal suspensions show distinct acoustic excitations in the high wave vector–low frequency region.

Experimental demonstration of negative magnetic permeability in the far-infrared frequency regime

Using transmission and reflection measurements in a five layer micrometer-scale split-ring resonator (SRR) system, fabricated by a photolithography procedure, the authors demonstrate the occurrence of a negative magnetic permeability regime in that system at ∼6THz. The transmission and reflection were measured using oblique incidence, resulting to a magnetic field component perpendicular to the SRR plane, which excites the resonant circular currents constituting the magnetic resonance.

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.

Nonparaxial abruptly autofocusing beams.

We study nonparaxial autofocusing beams with pre-engineered trajectories. We consider the case of linearly polarized electric optical beams and examine their focusing properties, such as contrast, beam width, and numerical aperture. Such beams are associated with larger intensity contrasts, can focus at smaller distances, and have smaller spot sizes as compared to the paraxial regime.