On the role of longitudinal currents in radiating systems of charges.

The time derivative of a charge density is linked to a current density by the continuity equation. However, it features only the longitudinal part of a current density, which is known to produce no radiation. This fact usually remains unnoticed and may appear puzzling at first, suggesting that the temporal variation of a charge density should be also irrelevant to radiation.

All-optical switching of liquid crystals at terahertz frequencies enabled by metamaterials.

Nematic liquid crystals integrated with metallic resonators (metamaterials) are intriguing hybrid systems, which not only offer added optical functionalities, but also promote strong light-matter interactions. In this report, we show with an analytical model that the electric field generated by a conventional oscillator-based terahertz time domain spectrometer is strong enough to induce partial, all-optical switching of nematic liquid crystals in such hybrid systems. Our analysis provides a robust theoretical footing for the mechanism of all-optical nonlinearity of liquid crystals, which was recently hypothesised to explain an anomalous resonance frequency shift in liquid crystal-loaded terahertz metamaterials.

Anomalous resonance frequency shift in liquid crystal-loaded THz metamaterials.

Babinet complementary patterns of a spectrally tunable metamaterial incorporating a nematic liquid crystal is normally assumed to exhibit the same tuning range. Here we show that for a hybrid, liquid crystal-loaded metamaterial, the sensitivity of its terahertz resonances to the variations of the refractive index differs substantially for the two complementary patterns. This is due to a mismatch between the alignment of the liquid crystal and the direction of the local electric field induced in the metamaterial patterns.

Observation of a High-Energy Tamm Plasmon State in the Near-IR Region.

We report on the experimental observation of a Tamm plasmon state in the near-IR region characterized by an anomalously high energy level located in the upper half of the photonic band gap. Such a "blue" Tamm plasmon was demonstrated at the interface between a conventional, completely periodic Bragg reflector and a nanostructured nonresonant thin gold grating. We study the effect of the grating period on the characteristics of the anomalous state and show that the anomaly results from a nontrivial topology of the nanograting's optical near field, which cannot be captured by the effective medium approach and transfer matrix method commonly employed in the analysis of Tamm plasmons.

Decay rate enhancement of diamond NV-centers on diamond thin films.

We demonstrate experimentally two-fold enhancement of the decay rate of NV° centers on diamond/Si substrate as opposed to a bare Si substrate. We link the decay enhancement to the interplay between the excitation of substrate modes and the presence of non-radiative decay channels. We show that the radiative decay rate can vary by up to 90% depending on the thickness of the diamond film.

Point-dipole approximation for small systems of strongly coupled radiating nanorods.

Systems of closely-spaced resonators can be strongly coupled by interactions mediated by scattered electromagnetic fields. In large systems the resulting response has been shown to be more sensitive to these collective interactions than to the detailed structure of individual resonators. Attempts to describe such systems have resulted in point-dipole approximations to resonators that are computationally efficient for large resonator ensembles.

Controlling Stiction in Nano-Electro-Mechanical Systems Using Liquid Crystals.

Stiction is one of the major reliability issues limiting practical application of nano-electro-mechanical systems (NEMS), an emerging device technology that exploits mechanical movements on the scale of an integrated electronic circuit. We report on a discovery that stiction can be eliminated by infiltrating NEMS with nematic liquid crystals. We demonstrate this experimentally using a NEMS-based tunable photonic metamaterial, where reliable switching of optical response was achieved for the entire range of nanoscopic structural displacements admitted by the metamaterial design.

Focused electromagnetic doughnut pulses and their interaction with interfaces and nanostructures.

"The "focused doughnut", a single-cycle electromagnetic perturbation of toroidal topology with inseparable time and spatial dependencies propagates at the speed of light in vacuum, as was shown by Hellwarth and Nouchi in 1996. While normal incidence reflection and refraction of conventional electromagnetic pulses in isotropic media do not lead to polarization changes, "focused doughnut" pulses undergo complex field transformations owing to the toroidal field structure and the presence of longitudinal components. We also demonstrate that "focused doughnuts" can interact strongly with structured media exciting dominant dynamic toroidal dipoles in spherical dielectric particles.

Toroidal lasing spaser.

Toroidal shapes are often found in bio-molecules, viruses, proteins and fats, but only recently it was proved experimentally that toroidal structures can support exotic high-frequency electromagnetic excitations that are neither electric or magnetic multipoles. Such excitations, known as toroidal moments, could be playing an important role in enhancing inter-molecular interaction and energy transfer due to its higher electromagnetic energy confinement and weaker coupling to free space. Using a model toroidal metamaterial system, we show that coupling optical gain medium with high Q-factor toroidal resonance mode can enhance the single pass amplification to up to 65 dB.

Design of plasmonic toroidal metamaterials at optical frequencies.

Toroidal multipoles are the subject of growing interest because of their unusual electromagnetic properties different from the electric and magnetic multipoles. In this paper, we present two new related classes of plasmonic metamaterial composed of purposely arranged of four U-shaped split ring resonators (SRRs) that show profound resonant toroidal responses at optical frequencies. The toroidal and magnetic responses were investigated by the finite-element simulations.

Nanohole array as a lens.

We demonstrate that a quasi-crystal array of nanoholes in a metal screen can mimic a function of the lens: one-to-one imaging of a point source located a few tens of wavelengths away from the array to a point on the other side of the array. A displacement of the point source leads to a linear displacement of the image point. Complex structures composed of multiple point sources can be faithfully imaged with resolutions comparable to those of high numerical aperture lenses.

Light-induced switching between structural forms with different optical properties in a single gallium nanoparticulate.

In a single gallium nanoparticulate, self-assembled (from an atomic beam) in a nanoaperture at the tip of a tapered optical fiber, we have observed reversible light-induced reflectivity changes associated with a sequence of transformations between a number of structural forms with different optical properties, stimulated by optical excitation at nanowatt power levels. The ability to change the optical properties of a nanoparticulate using structural transformations provides a new mechanism for photonic functionality on the nanoscale.