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As in many fields, the most exciting endeavors in photon upconversion research focus on increasing the efficiency (upconversion quantum yield) and performance (anti-Stokes shift) while diminishing the cost of production. In this vein, studies employing metal-free thermally activated delayed fluorescence (TADF) sensitizers have garnered increased interest. Here, for the first time, the strategy of ternary photon upconversion was utilized with the TADF sensitizer 4CzIPN, resulting in a doubling of the upconversion quantum yield in comparison to the binary system employing p-terphenyl as the emitter. In this ternary blend, the sensitizer 4CzIPN is paired with an intermediate acceptor, 1-methylnaphthalene, in addition to the emitter molecule, p-terphenyl yielding a normalized upconversion quantum yield of 7.6% while maintaining the 0.83 eV anti-Stokes shift. These results illustrate the potential benefits of utilizing this strategy of energy-funneling, previously used only with heavy-metal based sensitizers, to increase the performance of these photon upconversion systems. This article is protected by copyright. All rights reserved.

Nanophotonic resonators offer the ability to design nanoscale optical elements and engineered materials with unconventional properties. Dielectric-based resonators intrinsically support a complete multipolar resonant response with low absorption, while metallic resonators provide extreme light confinement and enhanced photon–electron interactions. Here, we construct resonators out of a prototypical metal–insulator transition material, vanadium dioxide (VO2), and demonstrate switching between dielectric and plasmonic resonances. We first characterize the temperature-dependent infrared optical constants of VO2 single crystals and thin-films. We then fabricate VO2 wire arrays and disk arrays. We show that wire resonators support dielectric resonances at low temperatures, a damped scattering response at intermediate temperatures, and plasmonic resonances at high temperatures. In disk resonators, however, upon heating, there is a pronounced enhancement of scattering at intermediate temperatures and a substanti...

This article in the form of “reflections” is devoted mainly to the image-building of Heidegger in Russia as a thinker of Being, a conservative critic of late modernity and a “post” philosopher. On the one hand, due to French post-modernism influence on late Soviet and post-Soviet philosophy Russia has developed a special interest in Heidegger’s deconstruction theory. On the other hand, the reception of Heidegger’s criticism of European nihilism, totalitarianism and machine technology as the manifestations of modernity has shaped significantly the account of the “political”. The specifics of Russian reception of Heidegger can also be traced in the rejection of strict division between the “core” of Heidegger’s philosophy and “accidental” conditions related to social and political aspects of his work in 1930s. The current discussion about recently published “Black notebooks” shows not only the fact of existence of a separate language of description and analysis in Russian philosophical field but also an original tendency to holistic consideration of Heidegger’s thinking including not only his existential and historical reflections but also political passages and criticism of civilizational discourse. In addition to the reviews of “Black notebooks” by N.V. Motroshilova, V.V. Mironov and D. Mironova in the last year’s issues of the magazine “Voprosy filosofii” there are earlier works by N.V. Motroshilova, V.V. Bibikhin and A.V. Gulyga related to the debate on Heidegger’s political and ideological commitment which are in focus of attention.

Six experimental methods have been used to investigate the exciton diffusion length in materials with systematic chemical modifications. We find that exciton diffusion length correlates with molecular ordering. We discuss situations in which certain experimental techniques are more appropriate.

Addition of low concentrations (

Optical antenna metasurfaces have attracted substantial attention in recent years, as they may enable new classes of planar optical elements. However, actively tuning nanoantenna resonances, whether dielectric or plasmonic, remains an unresolved challenge. In this work, we investigate tuning mid-infrared (MIR) Mie resonances in semiconductor subwavelength particles by directly modulating the permittivity with free charge carriers. Using femtosecond laser ablation, we fabricate spherical silicon and germanium particles of varying sizes and doping concentrations. Single-particle infrared spectra reveal electric and magnetic dipole, quadrupole, and hexapole resonances. We first demonstrate size-dependent Si and Ge Mie resonances spanning the entire mid-infrared (2-16 μm) spectral range. We subsequently show doping-dependent resonance frequency shifts that follow simple Drude models. Taking advantage of the large doping dependence of Si and Ge MIR permittivities, we demonstrate a huge tunability of Mie resonance wavelengths (up to ~ 9 μm) over a broad 2-16 μm MIR range. This tuning range corresponds to changes of permittivity as large as 30 within a single material system, culminating in the emergence of plasmonic modes at high carrier densities and long wavelengths. We also demonstrate dynamic tuning of intrinsic semiconductor antennas using thermo-optic effects. These findings demonstrate the potential for actively tuning infrared Mie resonances, thus providing an excellent platform for tunable metamaterials.

A new, highly efficient green oxyfluoride phosphor family Sr2.975−xBaxCe0.025AlO4F (SBAF:Ce3+) has been developed as a component of solid state white light emitting diodes (LED). The phosphor emits with a maximum at 502 nm when excited by 405 nm excitation, with a quantum efficiency approaching 100%. When SBAF:Ce3+ (x = 1.0) is incorporated with encapsulant on an ultraviolet (405 nm) LED, greenish-white light with a color rendering index of 62 under a forward bias current of 20 mA is obtained. The results suggest that phosphors deriving from SBAF:Ce3+ have potential for incorporation in formulations for white LEDs and related applications. The preparation and structural and optical characterization of the phosphor family is described. Attempts to understand the origins of the high efficiency on the basis of the thermal quenching characteristics of SBAF:Ce3+ in comparison with related compounds are presented.

Here we report a new approach for the preparation of anode buffer layer for efficient polymer light-emitting devices (PLEDs) by using glycerol to modify relative low conductivity poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). This new type of anode buffer layer allows for a 50–90% increase in device performance for green emitting phosphorescent PLEDs in terms of luminous efficiency, and external quantum efficiency, while 90–150% in power efficiency, as compared to devices fabricated using commercially available PEDOT:PSS. The green emitting phosphorescent PLEDs with this modified anode buffer exhibit very high efficiencies, representing a significant step forward to matching and exceeding the efficiencies reported to date with vacuum-deposited small molecular devices. We anticipate that these findings can provide a simple experimental procedure for improvement of PLEDs.

We report a novel strategy for writing volume holograms by photoacid generation and subsequent acid-catalyzed degradation leading to increased free volume/refractive index modulation in the exposed regions of a cross-linked rigid polymeric matrix. This strategy offers nondestructive read out and high diffraction efficiency and allows optical-quality, millimeter thick films to be fabricated that possess excellent thermal and dimensional stability. A key feature of this approach is the efficient acid-catalyzed degradation of functional groups in the cross-linked matrix leading to release of volatile products which diffuse readily out of the thick films. Furthermore, the reported data storage material is lightweight and inexpensive and can be easily processed into different shapes, making it an attractive candidate for data storage applications.

We demonstrate tuning of infrared Mie resonances by varying the carrier concentration in doped semiconductor antennas. We fabricate spherical silicon and germanium particles of varying sizes and doping concentrations. Single-particle infrared spectra reveal electric and magnetic dipole, quadrupole, and hexapole resonances. We subsequently demonstrate doping-dependent frequency shifts that follow simple Drude models, culminating in the emergence of plasmonic resonances at high doping levels and long wavelengths. These findings demonstrate the potential for actively tuning infrared Mie resonances by optically or electrically modulating charge carrier densities, thus providing an excellent platform for tunable metamaterials.

A potential complication associated with the realization of the optical-gain regime in semiconductor nanocrystals (NCs) arises from the competing phenomenon of excited-state absorption. Here, we ap...

We demonstrate several femtosecond spectroscopic techniques utilizing intraband carrier reexcitation for studies of relaxation processes in semiconductor materials. By introducing an additional infrared post-pump pulse into a conventional transient absorption experiment, we are able to visualize carrier intraband dynamics and to detect weak conduction-to-valence band transitions unresolved in the traditional transient absorption configuration. Intraband reexcitation spectroscopy also allows us to study dynamics of radiative transitions by monitoring a post-pump-induced modulation of time-integrated photoluminescence.

Metal-organic semiconductor interfaces are important because of their ubiquitous role in determining the performance of modern electronics such as organic light emitting diodes (OLEDs), fuel cells, batteries, field effect transistors (FETs), and organic solar cells. Interfaces between metal electrodes required for external wiring to the device and underlying organic structures directly affect the charge carrier injection/collection efficiency in organic-based electronic devices primarily due to the mismatch between energy levels in the metal and organic semiconductor. Environmentally stable and cost-effective electrode materials, such as aluminum and gold typically exhibit high potential barriers for charge carriers injection into organic devices leading to increased operational voltages in OLEDs and FETs and reduced charge extraction in photovoltaic devices. This leads to increased power consumption by the device, reduced overall efficiency, and decreased operational lifetime. These factors represent a significant obstacle for development of next generation of cheap and energy-efficient components based on organic semiconductors. It has been noticed that introduction of organic materials with conjugated backbone and ionic pendant groups known as conjugated poly- and oligoelectrolytes (CPEs and COEs), enables one to reduce the potential barriers at the metal-organic interface and achieve more efficient operation of a device, however exact mechanisms of the phenomenonmore » have not been understood. The goal of this project was to delineate the function of organic semiconductors with ionic groups as electron injection layers. The research incorporated a multidisciplinary approach that encompassed the creation of new materials, novel processing techniques, examination of fundamental electronic properties and the incorporation of the resulting knowledgebase into development of novel organic electronic devices with increased efficiency, environmental stability, and reduced cost. During the execution of the project, main efforts were focused on the synthesis of new charge-bearing organic materials, such as CPEs and COEs, and block copolymers with neutral and ionic segments, studies of mechanisms responsible for the charge injection modulation in devices with ionic interlayers, and use of naturally occurring charged molecules for creation of enhanced devices. The studies allowed PIs to demonstrate the usefulness of the proposed approach for the improvement of operational parameters in model OLED and FET systems resulting in increased efficiency, decreased contact resistance, and possibility to use stable metals for fabrication of device electrodes. The successful proof-of-the-principle results potentially promise development of light-weight, low fabrication cost devices which can be used in consumer applications such as displays, solar cells, and printed electronic devices. Fundamental mechanisms responsible for the phenomena observed have been identified thus advancing the fundamental knowledgebase.« less

The development of optical gain in chemically synthesized semiconductor nanoparticles (nanocrystal quantum dots) has been intensely studied as the first step toward nanocrystal quantum dot lasers. We examined the competing dynamical processes involved in optical amplification and lasing in nanocrystal quantum dots and found that, despite a highly efficient intrinsic nonradiative Auger recombination, large optical gain can be developed at the wavelength of the emitting transition for close-packed solids of these dots. Narrowband stimulated emission with a pronounced gain threshold at wavelengths tunable with the size of the nanocrystal was observed, as expected from quantum confinement effects. These results unambiguously demonstrate the feasibility of nanocrystal quantum dot lasers.

A mechanistic study of the DNA interfacial layer that enhances the photoresponse in n-type field-effect transistors (FET) and lateral photoconductors using a solution-processed fullerene derivative embedded with disperse-red dye, namely PCBDR, is reported. Incorporation of the thin DNA layer simultaneously leads to increasing the electron injection from non-Ohmic contacts into the PCBDR active layer in dark and to increasing the photocurrent under irradiation. Such features lead to the observation of the enhancement of the photoresponsivity in PCBDR FETs up to 10(3) . Kelvin probe microscopy displays that in the presence of the DNA layer, the surface potential of PCBDR has a greater change in response to irradiation, which is rationalized by a larger number of photoinduced surface carriers. Transient absorption spectroscopy confirms that the increase in photoinduced carriers in PCBDR under irradiation is primarily ascribed to the increase in exciton dissociation rates through the PCBDR/DNA interface and this process can be assisted by the interfacial dipole interaction.

A novel cerium-substituted, barium yttrium silicate has been identified as an efficient blue-green phosphor for application in solid state lighting. Ba9Y2Si6O24:Ce(3+) was prepared and structurally characterized using synchrotron X-ray powder diffraction. The photoluminescent characterization identified a major peak at 394 nm in the excitation spectrum, making this material viable for near-UV LED excitation. An efficient emission, with a quantum yield of ≈60%, covers a broad portion (430-675 nm) of the visible spectrum, leading to the blue-green color. Concentration quenching occurs when the Ce(3+) content exceeds ≈3 mol %, whereas high temperature photoluminescent measurements show a 25% drop from the room temperature efficiency at 500 K. The emission of this compound can be red-shifted via the solid solution Ba9(Y(1-y)Sc(y))(1.94)Ce(0.06)Si6O24 (y = 0.1, 0.2), allowing for tunable color properties when device integration is considered.

We study different emission regimes in close-packed films of chemically synthesized CdSe nanoparticles [nanocrystal quantum dots (NQDs)]. We observe that the NQD photoluminescence is dominated by excitons and biexcitons, respectively, before and after the threshold for stimulated emission. Furthermore, we demonstrate the regime of microring lasing into sharp, whispering-gallery modes using NQD solids incorporated into microcapillary tubes. This result indicates a feasibility of miniature, solid-state laser devices based on chemically synthesized NQDs.

A solid solution strategy helps increase the efficiency of Ce{sup 3+} oxyfluoride phosphors for solid-state white lighting. The use of a phosphor-capping architecture provides additional light extraction. The accompanying image displays electroluminescence spectra from a 434-nm InGaN LED phosphor that has been capped with the oxyfluoride phosphor.

The objective of the proposed work was to develop the fundamental understanding and practical techniques for enhancement of Phosphorescent Organic Light Emitting Diodes (PhOLEDs) performance by utilizing radiative decay control technology. Briefly, the main technical goal is the acceleration of radiative recombination rate in organometallic triplet emitters by using the interaction with surface plasmon resonances in noble metal nanostructures. Increased photonic output will enable one to eliminate constraints imposed on PhOLED efficiency by triplet-triplet annihilation, triplet-polaron annihilation, and saturation of chromophores with long radiative decay times. Surface plasmon enhanced (SPE) PhOLEDs will operate more efficiently at high injection current densities and will be less prone to degradation mechanisms. Additionally, introduction of metal nanostructures into PhOLEDs may improve their performance due to the improvement of the charge transport through organic layers via multiple possible mechanisms ('electrical bridging' effects, doping-like phenomena, etc.). SPE PhOLED technology is particularly beneficial for solution-fabricated electrophosphorescent devices. Small transition moment of triplet emitters allows achieving a significant enhancement of the emission rate while keeping undesirable quenching processes introduced by the metal nanostructures at a reasonably low level. Plasmonic structures can be introduced easily into solution-fabricated PhOLEDs by blending and spin coating techniques and can be used for enhancement of performance in existing device architectures. This constitutes a significant benefit for a large scale fabrication of PhOLEDs, e.g. by roll-to-roll fabrication techniques. Besides multieexciton annihilation, the power efficacy of PhOLEDs is often limited by high operational bias voltages required for overcoming built-in potential barriers to injection and transport of electrical charges through a device. This problem is especially pronounced in solution processed OLEDs lacking the accuracy and precision of fabrication found in their small molecule counterparts. From this point of view, it seems beneficial to develop materials allowing reduction of the operation bias voltage via improvement of the charge injection. The materials sought have to be compatible with solution-based fabrication process and allow easy incorporation of metal nanostructures.

We analyze both theoretically and experimentally the response of individual gold nanoparticles and nanoparticle dimers to a near-field excitation with broad-band radiation ~a femtosecond white-light continuum! delivered through a subwavelength aperture. Because of the coherent superposition of the field emitted by the aperture and the secondary field reemitted by the nanostructure, the signals detected in the far zone exhibit a pronounced dependence on the phase of plasmon oscillations excited in the nanostructure. This phase sensitivity allows us to accurately determine positions of plasmon resonances not distorted by dielectric losses in a metal. In the near-field extinction spectra of individual nanoparticles, the plasmon resonance is observed as the ‘‘zero-extinction’’ point in which the transition from constructive ~lower spectral energies! to destructive ~higher spectral energies! interference occurs. By using spatially selective near-field excitation with a femtosecond white-light continuum, we are able to detect slight asymmetries in dimers composed of nominally identical nanoparticles.

We demonstrate a new nanoscale spectroscopic technique that combines subwavelength near-field imaging with broadband interference spectroscopy. We apply this technique to study phase spectra of surface plasmons in individual gold nanoparticles and nanoparticle dimers. Collective plasmon oscillations in selected nanostructures are excited by a femtosecond white-light continuum transmitted through a subwavelength aperture. The interference spectra detected in the far field result from the coherent superposition of the aperture field and the secondary field re-emitted by the nanostructure. The analysis of these spectra allows us to accurately measure the positions and damping constants of single-nanostructure plasmon resonances.

Summary form only given. Semiconductor quantum dots (QDs) promise the lowest lasing threshold for semiconductor media. Additionally, QDs in the strong confinement regime have an emission wavelength that is a pronounced function of size, adding the advantage of continuous spectral tunability simply by changing the dot radius. Lasing has previously been demonstrated for epitaxially grown III-V QDs. Large lateral dimensions and difficulties in size control limit their spectral tunability using quantum confinement effects. An alternative approach to fabricating QDs is through chemical synthesis which can produce semiconductor nanoparticles (colloidal QDs) with radii from 1 to 6 nm and with size dispersions as small as 5%. Such dots show strong quantum confinement and permit size-controlled spectral tunability over an energy range as wide as 1 eV. The combination of tunable electronic energies and chemical flexibility make colloidal QDs ideal building blocks for the bottom-up assembly of optical device structures, including optical amplifiers and lasers. However, despite more than a decade of effort, lasing in small-size colloidal nanoparticles has not been realized. To determine what hinders lasing action, we performed extensive dynamical studies of radiative and nonradiative processes in CdSe colloidal QDs.

We study optical processes relevant to optical amplification and lasing in CdSe nanocrystal quantum dots (NQD). NQDs are freestanding nanoparticles prepared using solution-based organometallic reactions originally developed for the Cd chalcogenides, CdS, CdSe and CdTe [J. Am. Chem. Soc. 115, 8706 (1993)]. We investigate NQDs with diameters ranging from 2 to 8 nm. Due to strong quantum confinement, they exhibit size-dependent spectral tunability over an energy range as wide as several hundred meV. We observe a strong effect of the matrix/solvent on optical gain properties of CdSe NQDs. In most of the commonly used solvents (such as hexane and toluene), gain is suppressed due to strong photoinduced absorption associated with carriers trapped at solvent-related interface states. In contrast, matrix-free close packed NQD films (NQD solids) exhibit large optical gain with a magnitude that is sufficiently high for the optical gain to successfully compete with multiparticle Auger recombination [Science 287, 10117 (2000)]. These films exhibit narrowband stimulated emission at both cryogenic and room temperature, and the emission color is tunable with dot size [Science 290, 314 (2000)]. Moreover, the NQD films can be incorporated into microcavities of different geometries (micro-spheres, wires, tubes) that produce lasing in whispering gallery modes. The facile preparation, chemical flexibility and wide-range spectral tunability due to strong quantum confinement are the key advantages that should motivate research into NQD applications in optical amplifiers and lasers.

We use a variety of femtosecond nonlinear optical techniques to study carrier relaxation and mechanisms for resonant optical nonlinearities in semiconductor quantum dots. Combining femtosecond inter- and intra-band probes we are able to separate electron and hole relaxation paths and to resolve single exponential dynamics of different multiparticle states.

We report on the optical characterization of a series of multi-layered organic superlattices made by polyelectrolyte self-assembly. Using a high molecular weight, water-soluble, anionic form of poly-phenylene vinylene, self-assembled films can be formed which show high photoluminescence quantum efficiency (QE). A dramatic red shift of the luminescence and increase in QE is observed as additional PPV layers are added. We attribute the red shift and increasing QE to a changing conformation of the polymer chains as the superstructure is assembled, together with efficient Frster energy transfer in a preferred direction away from the substrate toward layers with longer effective conjugation length. Upon adding a C60 top layer, the luminescence spectrum is strongly quenched. We attribute this to charge transfer of from the top-most polymer layer to the C60 layer. We discuss the possibilities of exploiting this directional charge transfer in an ultrafast holographic device along with other optics for increasing the temporal diffraction efficiency of polymer-based mediums.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Quantum dots have the potential to be used in solid state white lighting applications as an additional down-converting component to better represent wavelengths in the red spectral region, leading to higher quality white light with improved color rendering. In this contribution, we report on color characteristics of phosphor-converted white light-emitting diodes that utilize inorganic garnet-based phosphors, with quantum dots incorporated appropriately. Devices were fabricated using red-emitting CdSe/ZnS core/shell quantum dots (λem = 560 nm or λem = 590 nm) in conjunction with yellow-emitting Y3Al5O12:Ce3+ or green-emitting Lu3Al5O12:Ce3+ and a blue-emitting InGaN LED (λem = 450 nm). Several phosphor/quantum dot geometries were examined, including encapsulating the phosphor powder and quantum dots together in silicone resin, or in two separate layers of silicone resin, with either the quantum dots closest to the LED or the phosphor particles closest to the LED. The quantum dots were most efficient when encapsulated with the phosphor particles in the resin. Devices with Y3Al5O12:Ce3+ and quantum dots (λem = 590 nm) achieved a correlated color temperature of 4000 K, color rendering index of 81, and luminous efficacy of 57 lm W−1, while devices with Lu3Al5O12:Ce3+ and quantum dots (λem = 590 nm) achieved a correlated color temperature of 5700 K, a color rendering index of 90, and luminous efficacy of 22 lm W−1. The results obtained suggest that the use of quantum dots may allow for solid state white lighting devices with high color rendition.

A near-UV excited, oxyfluoride phosphor solid solution Sr1.975Ce0.025Ba(AlO4F)1−x(SiO5)x has been developed for solid state white lighting applications. An examination of the host lattice, and the local structure around the Ce3+ activator ions through a combination of density functional theory, synchrotron X-ray and neutron powder diffraction and total scattering, and electron paramagnetic resonance, points to how chemical substitutions play a crucial role in tuning the optical properties of the phosphor. The maximum emission wavelength can be tuned from green (λem = 523 nm) to yellow (λem = 552 nm) by tuning the composition, x. Photoluminescent quantum yield is determined to be 70 ± 5% for some of the examples in the series. Excellent thermal properties were found for the x = 0.5 sample, with the photoluminescence intensity at 160 °C only decreased to 82% of its room temperature value. Phosphor-converted LED devices fabricated using an InGaN LED (λmax = 400 nm) exhibit high color rendering white light with Ra = 70 and a correlated color temperature near 7000 K. The value of Ra could be raised to 90 by the addition of a red component, and the correlated color temperature lowered to near 4000 K.

Electroabsorption spectroscopy was used to directly probe the electric fields in a polymer light emitting diode that utilizes a conjugated polyelectrolyte electron transporting/injection layer. The electric field in the emitting layer was found to be negligible at applied biases greater than the built-in field of the device. Holes injected at these biases accumulate at the emitting layer/conjugated polyelectrolyte interface and screen the field from the emitting layer to the conjugated polyelectrolyte layer. In conjunction with mobile ions that redistribute the field in the conjugated polyelectrolyte layer, this leads to greatly improved electron injection from high work function cathodes.