Exciton polaritons in two-dimensional photonic crystals

Experimental evidence of strong coupling between excitons confined in a quantum well and the photonic modes of a two-dimensional dielectric lattice is reported. Both resonant scattering and photoluminescence spectra at low temperature show the anticrossing of the polariton branches, fingerprint of strong coupling regime. The experiments are successfully interpreted in terms of a quantum theory of exciton-photon coupling in the investigated structure. These results show that the polariton dispersion can be tailored by properly varying the photonic crystal lattice parameter, which opens the possibility to obtain the generation of entangled photon pairs through polariton stimulated scattering.Comment: 5 pages, 4 figure

Highly Tunable Emission by Halide Engineering in Lead-Free Perovskite-Derivative Nanocrystals: The Cs2SnX6 (X = Cl, Br, Br/I, I) System

Nanocrystals of Cs2SnX6 (X = Cl, Br, Br0.5I0.5, and I) have been prepared by a simple, optimized, hot-injection method, reporting for the first time the synthesis of Cs2SnCl6, Cs2SnBr6, and mixed Cs2Sn(I0.5Br0.5)6 nanocrystalline samples. They all show a cubic crystal structure with a linear scaling of lattice parameter by changing the halide size. The prepared nanocrystals have spherical shape with average size from 3 to 6 nm depending on the nature of the halide and span an emission range from 444 nm (Cs2SnCl6) to 790 nm (Cs2SnI6) with a further modulation provided by mixed Br/I systems

Guiding and reflecting light by boundary material

We study effects of finite height and surrounding material on photonic crystal slabs of one- and two-dimensional photonic crystals with a pseudo-spectral method and finite difference time domain simulation methods. The band gap is shown to be strongly modified by the boundary material. As an application we suggest reflection and guiding of light by patterning the material on top/below the slab.Comment: 12 pages, 7 figure

Towards efficient near-infrared fluorescent organic light-emitting diodes

The energy gap law (EG-law) and aggregation quenching are the main limitations to overcome in the design of near-infrared (NIR) organic emitters. Here, we achieve unprecedented results by synergistically addressing both of these limitations. First, we propose porphyrin oligomers with increasing length to attenuate the effects of the EG -law by suppressing the non-radiative rate growth, and to increase the radiative rate via enhancement of the oscillator strength. Second, we design side chains to suppress aggregation quenching. We find that the logarithmic rate of variation in the non-radiative rate vs. EG is suppressed by an order of magnitude with respect to previous studies, and we complement this breakthrough by demonstrating organic light-emitting diodes with an average external quantum efficiency of ~1.1%, which is very promising for a heavy-metal-free 850 nm emitter. We also present a novel quantitative model of the internal quantum efficiency for active layers supporting triplet-to-singlet conversion. These results provide a general strategy for designing high-luminance NIR emitters

High-Frequency Light Rectification by Nanoscale Plasmonic Conical Antenna in Point-Contact-Insulator-Metal Architecture

Numerous efforts have been undertaken to develop rectifying antennas operating at high frequencies, especially dedicated to light harvesting and photodetection applications. However, the development of efficient high frequency rectifying antennas has been a major technological challenge both due to a lack of comprehension of the underlying physics and limitations in the fabrication techniques. Various rectification strategies have been implemented, including metal-insulator-metal traveling-wave diodes, plasmonic nanogap optical antennas, and whisker diodes, although all show limited high-frequency operation and modest conversion efficiencies. Here a new type of rectifying antenna based on plasmonic carrier generation is demonstrated. The proposed structure consists of a resonant metallic conical nano-antenna tip in contact with the oxide surface of an oxide/metal bilayer. The conical shape allows for an improved current generation based on plasmon-mediated electromagnetic-to-electron conversion, an effect exploiting the nanoscale-tip contact of the rectifying antenna, and proportional to the antenna resonance and to the surface-electron scattering. Importantly, this solution provides rectification operation at 280 THz (1064 nm) with a 100-fold increase in efficiency compared to previously reported results. Finally, the conical rectifying antenna is also demonstrated to operate at 384 THz (780 nm), hence paving a way toward efficient rectennas toward the visible range

Luminescent solar concentrators: boosted optical efficiency by polymer dielectric mirrors

We report on the optical efficiency enhancement of luminescent solar concentrators based on a push\u2013 pull fluorophore realized using high dielectric contrast polymer distributed Bragg reflectors as back mirrors. The Bragg stacks are obtained by alternating layers of cellulose acetate and thin films of a new stable and solution processable hydrated titania\u2013poly(vinyl alcohol) nanocomposite (HyTiPVA) with a refractive index greater than 1.9 over a broad spectral range. The results obtained with these systems are compared with enhancements provided by standard Bragg reflectors made of commercial polymers. We demonstrate that the application of the Bragg stacks with photonic band-gap tuned to the low energy side of the dye emission spectrum induces a 10% enhancement of optical efficiency. This enhancement is the result of a photon recycling mechanism and is retained even in a scaled-up device where the Bragg mirrors are used in a mosaic configuratio

Synergic combination of the sol-gel method with dip coating for plasmonic devices

Biosensing technologies based on plasmonic nanostructures have recently attracted significant attention due to their small dimensions, low-cost and high sensitivity but are often limited in terms of affinity, selectivity and stability. Consequently, several methods have been employed to functionalize plasmonic surfaces used for detection in order to increase their stability. Herein, a plasmonic surface was modified through a controlled, silica platform, which enables the improvement of the plasmonic-based sensor functionality. The key processing parameters that allow for the fine-tuning of the silica layer thickness on the plasmonic structure were studied. Control of the silica coating thickness was achieved through a combined approach involving sol-gel and dip-coating techniques. The silica films were characterized using spectroscopic ellipsometry, contact angle measurements, atomic force microscopy and dispersive spectroscopy. The effect of the use of silica layers on the optical properties of the plasmonic structures was evaluated. The obtained results show that the silica coating enables surface protection of the plasmonic structures, preserving their stability for an extended time and inducing a suitable reduction of the regeneration time of the chip

Кінетика сумісного виділення цинку і нікелю з розбавлених електролітів

Досліджені закономірності виділення цінку, нікелю і цинк-нікелевого сплаву з розведених електролітів, що містять в якості лігандів амінокислоту та аміак. Найкращі технологічні параметри та якість покрить отримані при спільному вмісті у розчині обох лігандів. Запропонований електроліт характеризується високою стабільністю, є технологічним та екологічно безпечним.The mechanisms of zinc, nickel and zinc-nickel alloy deposition from diluted electrolytes, containing amino acid or ammonia as a ligand, were investigated. The very technological characteristics and coatings quality were obtained if the electrolyte contained both of the ligands. The suggested electrolyte is characterized by high stability, processibility and it is ecologically safe

Optical response with threefold symmetry axis on oriented microdomains of opal photonic crystals

The paper deals with three-dimensional photonic crystals known as artificial opals, namely, fcc lattices of dielectric spheres: such systems have been the subject of numerous investigations. Opal photonic crystals viewed along the [111] direction of the fcc structure have a threefold symmetry axis; however this microscopic symmetry is difficult to observe in optical measurements performed on macroscopic areas containing microdomains with different orientations. In this work polarized transmittance measurements on [111]-stacked silica opals with single oriented microdomains, identified by field-emission scanning electron microscopy and laser-scanning confocal microscopy, demonstrate different optical response of twin structures with the two possible vertical stacking sequences. A detailed comparison with theory shows that microtransmittance experiments probe the photonic band structure along the Gamma-L-K and Gamma-L-U orientations of the Brillouin zone, respectively, thus giving conclusive evidence for macroscopic optical response related to the presence of a threefold (instead of a sixfold) symmetry axis in the photonic microstructure. The paper arises from a collaboration between the University of Pavia and the Politecnico di Torino