Radiative Emission Rate Research Articles

Orientation Dependence of Plasmonically Enhanced Spontaneous Emission

We computationally explore how the orientation of dipolar emitters placed near plasmonic nanostructures affects their radiative enhancement and spontaneous emission rate. We demonstrate that the expressions for these quantities show a subtle dependence on the molecular orientation, and this information is lost when typical calculations assume a random orientation and perform an average over all directions. This orientation dependence is strongly affected by the location of the emitter, the emission wavelength, and the symmetry of the system. While the plasmonic nanostructure can significantly modify the far-field from a molecule in its vicinity, this modification is heavily dependent on both the wavelength and the orientation of the emitter. We show that if a fluorescent molecule can be constrained to emit in a specific direction, we are able to obtain far superior control over its spontaneous emission and decay rate than otherwise and discuss implications for single molecule experiments.

Enhancing the radiative emission rate of single molecules by a plasmonic nanoantenna weakly coupled with a dielectric substrate.

Enhancing the spontaneous emission of single emitters has been an important subject in nano optics in the past decades. For this purpose, plasmonic nanoantennas have been proposed with enhancement factors typically larger than those achievable with optical cavities. However, the intrinsic ohmic losses of plasmonic structures also introduce an additional nonradiative decay channel, reducing the quantum yield. Here we report on experimental studies of a weakly coupled dielectric substrate and a plasmonic nanoantenna for enhancing the radiative decay rate of single terrylene molecules embedded in an ultrathin organic film. We systematically investigate how the refractive index of the dielectric substrate affects the lifetime and the quantum efficiency and show that the coupled structure could moderately enhance the radiative decay rate while maintaining a high quantum efficiency.

Red-emitting LaOF:Eu3+ phosphors: Synthesis, structure and their Judd–Ofelt analysis for LED applications

In the present study, we have synthesized a series of La1−xEuxOF (0.01≤x≤0.09) phosphors by the conventional solid-state reaction route at relatively low temperature (500°C) and shorter duration of 2h. The compounds were crystallized in the rhombohedral structure with the space group R-3m (No. 166). Upon UV excitation (254nm), the photoluminescence spectra exhibit characteristic luminescence 5D0→7FJ (J=1, 2, 3, and 4) intra-4f shell Eu3+ ion transitions. An intense red emission peak at 610nm was observed due to electric dipole (5D0→7F2) transition. Judd–Ofelt theory was employed to evaluate various radiative parameters such as radiative emission rates, lifetime, branching and asymmetry ratios. CIE color coordinates confirmed the red emission of the phosphors. The luminescent results reveal that LaOF:Eu3+ phosphor can be used as potential candidate for developing red component in white LED applications.

Twenty-fold plasmon-induced enhancement of radiative emission rate in silicon nanocrystals embedded in silicon dioxide

We report on a 20-fold enhancement of the integrated photoluminescence (PL) emission of silicon nanocrystals, embedded in a matrix of silicon dioxide, induced by excited surface plasmons from silver nanoparticles, which are located in the vicinity of the silicon nanocrystals and separated from them by a silicon dioxide layer of a few nanometers. The electric field enhancement provided by the excited surface plasmons increases the absorption cross section and the emission rate of the nearby silicon nanocrystals, resulting in the observed enhancement of the photoluminescence, mainly attributed to a 20-fold enhancement in the emission rate of the silicon nanocrystals. The observed remarkable improvement of the PL emission makes silicon nanocrystals very useful material for photonic, sensor and solar cell applications.

Role of surface plasmon polaritons and other waves in the radiation of resonant optical dipole antennas.

The radiation of an electric dipole emitter can be drastically enhanced if the emitter is placed in the nano-gap of a metallic dipole antenna. By assuming that only surface plasmon polaritons (SPPs) are excited on the antenna, we build up an intuitive pure-SPP model that is able to comprehensively predict the electromagnetic features of the antenna radiation, such as the total or radiative emission rate and the far-field radiation pattern. With the model we can distinguish the respective contributions from SPPs and from other surface waves to the antenna radiation. It is found that for antennas with long arms that support higher-order resonances, SPPs provide a dominant contribution to the antenna radiation, while for other cases, the contribution of surface waves other than SPPs should be considered. The model reveals an intuitive picture that the enhancement of the antenna radiation is due to surface waves that are resonantly excited on the two antenna arms and that are further coupled into the nano-gap or scattered into free space. From the model we can derive a phase-matching condition that predicts the antenna resonance and the resultant enhanced radiation. The model is helpful for a physical understanding and intuitive design of antenna devices.

Effect of hydroxyl concentration on Yb^3+ luminescence properties in a peraluminous lithium-alumino-silicate glass

Ytterbium doped lithium-alumino-silicate glasses suitable for diode-pumped laser applications were investigated concerning the hydroxyl quenching of the Yb3+ fluorescence. Glasses of the nominal composition 18 mol% Li2O, 22 mol% Al2O3 and 60 mol% SiO2 with variable OH concentrations NOH (between 0.04 and 6.01 ∙ 1019 cm−3) and Yb3+ concentrations NYb (between 0.1 and 9 ∙ 1020 cm−3) were produced and a direct correlation between spontaneous emission decay rate and the product NYb ∙ NOH was observed. The radiative spontaneous emission rate in the glass host is around 1,000 s−1 (radiative lifetime 1.0 ms) and the microparameter for Yb-Yb energy migration, CYb-Yb, was found to be 1.358∙10−38 cm6 s−1. It was calculated that on average 17% of the OH groups in the glass contribute to the quenching of the Yb3+ fluorescence. By analysis of the UV edge of the glass it was concluded that melting under inert conditions leads to reduction of iron impurities to Fe2+, which can act as quenching sites for the Yb3+ ions and therefore may additionally reduce the energy storage capability of the laser material.

Plasmon-Enhanced Upconversion.

Upconversion, the conversion of photons from lower to higher energies, is a process that promises applications ranging from high-efficiency photovoltaic and photocatalytic cells to background-free bioimaging and therapeutic probes. Existing upconverting materials, however, remain too inefficient for viable implementation. In this Perspective, we describe the significant improvements in upconversion efficiency that can be achieved using plasmon resonances. As collective oscillations of free electrons, plasmon resonances can be used to enhance both the incident electromagnetic field intensity and the radiative emission rates. To date, this approach has shown upconversion enhancements up to 450×. We discuss both theoretical underpinnings and experimental demonstrations of plasmon-enhanced upconversion, examining the roles of upconverter quantum yield, plasmonic geometry, and plasmon spectral overlap. We also discuss nonoptical consequences of including metal nanostructures near upconverting emitters. The rapidly expanding field of plasmon-enhanced upconversion provides novel fundamental insight into nanoscale light-matter interactions while improving prospects for technological relevance.

Temporal resolution in fluorescence imaging.

Temporal resolution is a key factor for imaging rapidly occurring events in biology. In this feature article, I investigate an approximate estimate for determining the temporal resolution limit. The condition that led to this limit is, the time taken by the ensemble (99.9%) of excited molecules to relax to ground state, assuming all the emitted photons are detected. In a simplistic three-level system, the temporal resolution is, ≈3τp, where τp = (loge10)/(kf + knr) and, kf and knr are respectively the radiative and non-radiative emission rates. This further assumes the ideal condition that, the quantum efficiency of the detector is unity and there are no other loses. We discuss few state-of-art microscopy techniques that are capable of high temporal resolution. This includes techniques such as multifocal multiphoton microscopy (MMM), multifocal plane microscopy, multiple excitation spot optical microscopy (MESO), multiplane microscopy and multiple light-sheet microscopy (MLSM).

Numerical Calculations of Radiative and Non-Radiative Relaxation of Molecules Near Metal Particles

The dependence of the radiative emission and the nonradiative (energy transfer to the metal) relaxation rates of a molecule near a small metal particle on the molecule-to-particle distance and on the molecular orientation is calculated using a numerical solution of the Maxwell equations for a model that described the metal as a dispersive dielectric particle and the molecule as an oscillating point dipole. The emission rate is obtained by evaluating the total oscillating dipole in the system, while the nonradiative rate is inferred from the rate of heat production on the particle. For the distance dependence of the nonradiative rate we find, in agreement with experimental observations, marked deviation from the prediction of the standard theory of fluorescence resonance energy transfer (FRET). In departure from previous interpretations, we find that electromagnetic retardation is the main source of this deviation at large molecule–particle separations. The radiative emission rate reflects the total dipole...

Photonic effects on the Förster resonance energy transfer efficiency

Förster resonance energy transfer (ET) between luminescent species is applied in bio-imaging, lighting and photovoltaics, and has an important role in photosynthesis. However, the fundamental question of whether ET rates and efficiencies can be tuned by the photonic environment remains under debate. Here we show that ET rates are independent of the photonic environment, using the model system of LaPO4 nanocrystals co-doped with Ce(3+) donors and Tb(3+) acceptors. Although the radiative emission rate of the Ce(3+) excited state increases with the refractive index of the solvent in which the nanocrystals are dispersed, the Ce(3+)-to-Tb(3+) ET rate does not. We demonstrate that, as a result, lower refractive index solvents enable higher ET efficiencies leading to higher Tb(3+) emission intensities. Furthermore, an analytical model for ET in (nano)crystalline host materials is presented, able to predict the dependence of ET efficiencies on the photonic environment and the concentration of acceptor ions.

Enhancing spontaneous emission rates of molecules using nanopatterned multilayer hyperbolic metamaterials

Plasmonic nanostructures have been extensively used to manipulate the spontaneous light emission rate of molecules and their radiative efficiency. Because molecules near a metallic surface experience a different environment than in free space, their spontaneous radiative emission rate is generally enhanced. Such enhancement, measured by means of the Purcell factor, arises as a consequence of the overlap between the surface plasmon mode frequency and the emission spectrum of the molecule. However, such overlap is available only for a few narrow bands of frequency due to the limited plasmonic materials existing in nature. Although this limitation can be overcome by using hyperbolic metamaterials (HMMs)—a type of nanoscale artificial material with hyperbolic dispersion relations—the Purcell factor and the radiative power have remained relatively low. Here, we show that by nanopatterning a hyperbolic metamaterial made of Ag and Si multilayers, the spontaneous emission rate of rhodamine dye molecules is enhanced 76-fold at tunable frequencies and the emission intensity of the dye increases by ~80-fold compared with the same hyperbolic metamaterial without nanostructuring. We explain these results using a dynamic Lorentzian model in the time domain.

Plasmonic engineering of spontaneous emission from silicon nanocrystals.

Silicon nanocrystals offer huge advantages compared to other semi-conductor quantum dots as they are made from an abundant, non-toxic material and are compatible with silicon devices. Besides, among a wealth of extraordinary properties ranging from catalysis to nanomedicine, metal nanoparticles are known to increase the radiative emission rate of semiconductor quantum dots. Here, we use gold nanoparticles to accelerate the emission of silicon nanocrystals. The resulting integrated hybrid emitter is 5-fold brighter than bare silicon nanocrystals. We also propose an in-depth analysis highlighting the role of the different physical parameters in the photoluminescence enhancement phenomenon. This result has important implications for the practical use of silicon nanocrystals in optoelectronic devices, for instance for the design of efficient down-shifting devices that could be integrated within future silicon solar cells.

Erbium doped tellurite glasses with improved thermal properties as promising candidates for laser action and amplification

The influence of composition on the thermal stability of tellurite glasses was investigated by using differential scanning calorimetry (DSC). The studied glasses were synthesized by conventional melting quenching method. The best thermal stability and poor crystallization tendency were obtained for the glass composed of 65TeO2–15ZnO–10Na2O–5BaO–3La2O3 doped with Er2O3 (2mol %). This glass will be referred, in this article, as TZNBL: Er3+ glass.The spectroscopic properties of the above glass are investigated based on the Judd–Ofelt and McCumber theories.The calculated intensity parameters (Ω2,4,6) are compared to those obtained for Er3+ in other glasses. The radiative emission rate has been calculated for the different Er3+emitting levels. The high values of Ω4 and Ω6 confirm the results of the DSC experiment concerning the rigidity of the studied glass.Absorption, emission and gain cross section of the 4I13/2↔4I15/2 (Er3+) transition in the studied glass are reported and the results are compared to those of other glasses.The 4I13/2↔4I15/2 (Er3+) absorption and emission cross sections derived by the application of the Mc Cumber’s theory corroborate the Judd–Ofelt results.The whole of results demonstrate that the new composition leads to good thermal and mechanical properties as well efficient Er3+ absorption, emission cross sections, which make this glass as a promising candidate for laser action and amplification.

Upconversion Luminescence of Er and Yb Codoped NaYF4 Nanoparticles with Metal Shells

Upconversion photoluminescence (PL) of a composite nanoparticle consisting of an Er and Yb codoped NaYF4 core and a Au shell is studied theoretically and experimentally. We first investigate the effects of a Au shell on the radiative and nonradiative emission rates of a dipole placed in a core, the absorption and scattering cross sections of a composite nanoparticle, and the electric field within a core at the excitation wavelength. We then synthesize the composite nanoparticle and study the PL properties. From the analyses of the PL data in combination with the data obtained by theoretical calculations, the mechanism of the enhancement and quenching of upconversion PL by the formation of a Au shell is studied.

Modification of Förster Resonance Energy Transfer Efficiencyat Interfaces

We present a theoretical study on the impact of an interface on the FRET efficiency of a surface-bound acceptor-donor system. The FRET efficiency can be modified by two effects. Firstly, the donor’s electromagnetic field at the acceptor’s position is changed due to the partial reflection of the donor’s field. Secondly, both the donor’s and the acceptor’s quantum yield of fluorescence can be changed due to the interface-induced enhancement of the radiative emission rate (Purcell effect). Numerical results for a FRET-pair at a glass-water interface are given.

Metal–Polymer–Metal Split‐Dipole Nanoantennas

The conjugated polymer semiconductor poly(3-hexylthiophene), (P3HT), is integrated directly into the slot region of resonant plasmonic split-dipole nanoantennas. The P3HT radiative emission rate is enhanced by a factor of up to 29, in experiment, and 550 for the ideal case, due to the large local density of optical states in the nanoantenna slot region. Additionally, the theoretical modified luminescence quantum efficiency is shown to increase from 1% to 45% for optimized nanoantenna parameters.

Mid-infrared luminescence and energy transfer of Tm3+/Yb3+ doped fluorophosphate glass

Tm3+/Yb3+-codoped fluorophosphate glass is prepared by conventional melt-quenching method and the thermal stability of the glass is analyzed. The Judd–Ofelt parameters, radiation emission rates, radiative lifetime and branching ratios of prepared samples are calculated based on the absorption spectra. Upon excitation of a conventional 980nm laser diode, 1.8μm emission is obtained from Yb3+/Tm3+ codoped fluorophosphate. Additionally, the energy transfer between Yb3+ and Tm3+ ions are quantitatively analyzed. Hence, this Yb3+/Tm3+ codoped fluorophosphate glass possessing high energy transfer efficiency and excellent thermal stability is a good candidate for efficient 1.8μm laser.

Optimum quantum dot size for highly efficient fluorescence bioimaging

Semiconductor quantum dots of few nanometers have demonstrated a great potential for bioimaging. The size determines the emitted color, but it is also expected to play an important role in the image brightness. In this work, the size dependence of the fluorescence quantum yield of the highly thermal sensitive CdTe quantum dots has been systematically investigated by thermal lens spectroscopy. It has been found that an optimum quantum yield is reached for 3.8-nm quantum dots. The presence of this optimum size has been corroborated in both one-photon excited fluorescence experiments and two-photon fluorescence microscopy of dot-incubated cancer cells. Combination of quantum yield and fluorescence decay time measurements supports that the existence of this optimum size emerges from the interplay between the frequency-dependent radiative emission rate and the size-dependent coupling strength between bulk excitons and surface trapping states.

Intense 2.0 μm emission properties and energy transfer of Ho3+/Tm3+/Yb3+ doped fluorophosphate glasses

Intense 2.0 μm emission has been obtained from Ho3+/Tm3+/Yb3+ triply doped fluorophosphate glass pumped by a conventional 980 nm laser diode. The spectroscopic properties and energy transfer mechanisms between Ho3+, Tm3+ and Yb3+ are analyzed. Based on the absorption spectra, the Judd-Ofelt parameters, radiation emission rates, radiative lifetime and branching ratios of Ho3+ ions are calculated. It is also found that the 2.0 μm emission of Ho3+ can be greatly enhanced under the excitation at 980 nm by incorporating Tm3+ and Yb3+ simultaneously. Additionally, the micro-parameters of the energy transfer processes are quantitatively analyzed. The energy transfer coefficient from Yb3+ to acceptors and from Tm3+ to Ho3+ can reach as high as 13.14 × 10−40 cm6/s and 23.39 × 10−40 cm6/s, respectively. This Ho3+/Tm3+/Yb3+ doped fluorophosphate glass possessing high energy transfer coefficient and excellent thermal stability is a promising candidate for efficient 2.0 μm laser.

Local-field effect on the spontaneous radiative emission rate

The effect of the dielectric medium upon the spontaneous emission rate was investigated using lead borate glass of various compositions lightly doped with the Eu3+ lanthanide ion. A dual approach used the variation of (i) electric dipole/magnetic dipole (ED/MD) emission rates from spectral integrations and (ii) absolute ED emission rates from lifetime measurements for comparison with the variation of the refractive index of the glass medium. Contrary to a previous study [Phys. Rev. Lett. 91, 203903 (2003)], the results show the relevance of the virtual-cavity model in this case. The MD emission rates were taken to follow the n3 increase in density of photon states in the medium. The justifications for employing Eu3+ for identifying the appropriate model are given.