Radiative Emission Rate Research Articles

Impact of Surrounding Environment on Hot‐Exciton Based Organic Emitters for TADF Applications

AbstractUnderstanding thermally activated delayed fluorescence (TADF) in solid‐state environments is crucial for practical applications. However, limited research focuses on how the medium affects TADF properties of hot‐exciton‐based emitters. In our study, we calculated and compared reverse intersystem crossing, radiative, and non‐radiative decay rates of TADF emitters in gas, solvent, and solid phases. The designed emitters have a donor‐acceptor‐donor (D‐A‐D) structure, with donors such as triphenylamine (TPA) and diphenylamine thiophene (ThPA), combined with acceptors such as benzothiadiazole (BT), pyridine thiadiazole (PT) and thiadiazolobenzopyridine (NPT). We model the solvent and solid phases with the polarizable continuum model (PCM) and quantum mechanical/molecular mechanics (QM/MM) methods, respectively. Using density functional theory (DFT) and time‐dependent DFT, we analyze how TADF emitters′ geometrical, electronic, and excited‐state properties vary in these phases. Our results show that the solid‐state environment significantly influences the geometry and TADF properties of emitters. In the presence of solid medium, our study indicates that non‐radiative decay rates tend to be slower. On the other hand, radiative emission rates were found to be less influenced by the properties of the surrounding medium. Overall, our study connects emitter chemical structure and the surrounding environment‘s impact on excited‐state characteristics and photochemical properties.

Emission enhancement of erbium in a reverse nanofocusing waveguide

Since Purcell’s seminal report 75 years ago, electromagnetic resonators have been used to control light-matter interactions to make brighter radiation sources and unleash unprecedented control over quantum states of light and matter. Indeed, optical resonators such as microcavities and plasmonic antennas offer excellent control but only over a limited spectral range. Strategies to mutually tune and match emission and resonator frequency are often required, which is intricate and precludes the possibility of enhancing multiple transitions simultaneously. In this letter, we report a strong radiative emission rate enhancement of Er3+-ions across the telecommunications C-band in a single plasmonic waveguide based on the Purcell effect. Our gap waveguide uses a reverse nanofocusing approach to efficiently enhance, extract and guide emission from the nanoscale to a photonic waveguide while keeping plasmonic losses at a minimum. Remarkably, the large and broadband Purcell enhancement allows us to resolve Stark-split electric dipole transitions, which are typically only observed under cryogenic conditions. Simultaneous radiative emission enhancement of multiple quantum states is of great interest for photonic quantum networks and on-chip data communications.

Enhancement of spontaneous emission from CdSe/ZnS quantum dots through silicon nitride photonic crystal cavity based on miniaturized bound states in the continuum.

Colloidal CdSe/ZnS quantum dots (QDs) exhibit excellent optical properties for wide potential applications in light-emitting diodes, solar concentrators, and single-photon sources. However, the ultra-thin films with low concentration of QDs still encounter inefficient photoluminescence (PL) and poor directionality of radiation, which need to be enhanced using nanophotonics device designs. Here we design and experimentally demonstrate an on-substrate silicon nitride (SiN) photonic crystal (PhC) microcavity encapsulated by a layer of PMMA hosting CdSe/ZnS QDs. The miniaturized bound states in the continuum (BIC) supported by our structures, provide high-Q resonant modes with highly-directional emission patterns. Experimental results show that the BIC mode in the microcavity has a Q-factor up to 7000 owing to the symmetric refractive index distribution along the Z-direction, rendering 8.5-fold enhancement of PL intensity and 8.4-fold acceleration of radiative emission rate. Our work provides a practical way for constructing efficient on-chip surface-emitting light sources on silicon-based integrated photonic devices.

Interfacial charge transfers and carrier regulation characteristics of narrow/wide band gap TMDs@Ga2O3 n-n heterojunction film

The application of transition metal dichalcogenides (TMDs) in many optical devices has been seriously limited by its narrow band-gap and high electron-hole recombination rate. Therefore, more studied are desired for exploring the novel TMDs/Ga2O3 heterostructures with excellent photoexcitation properties and interfacial dynamics. Herein, the WSe2/Ga2O3 and MoS2/Ga2O3 heterostructures were successfully prepared by a two-step radio frequency magnetron sputtering method. The growth mechanism and interfacial charge transfer of the wide and narrow TMDs/Ga2O3 heterostructures are considered. The X-ray diffraction results show that the (002) orientation of the WSe2 and MoS2 was suppressed, which can be attributed to interfacial interaction. Compared to pure TMDs, the A1 g was blue-shifted in the Raman spectrum for the proposed heterostructures, which is related to the decrease of the electron density. The photoluminescence emission of TMDs/Ga2O3 heterostructures was enhanced mainly due to the transition from trions recombination to exciton recombination. Meanwhile, the transmission properties of electrons under the built-in electric field are further confirmed by the transient absorption spectroscopy. Effective charge separation not only reduces radiation emission rate but also significantly enhances the nonlinear optical response of TMDs. An open-aperture Z-scan measurement was carried out to confirm that Ga2O3 is a promising candidate for enhancing the saturable absorption or reverse saturable absorption of TMDs.

All-Polymer Microcavities for the Fluorescence Radiative Rate Modification of a Diketopyrrolopyrrole Derivative.

Controlling the radiative rate of emitters with macromolecular photonic structures promises flexible devices with enhanced performances that are easy to scale up. For instance, radiative rate enhancement empowers low-threshold lasers, while rate suppression affects recombination in photovoltaic and photochemical processes. However, claims of the Purcell effect with polymer structures are controversial, as the low dielectric contrast typical of suitable polymers is commonly not enough to provide the necessary confinement. Here we show all-polymer planar microcavities with photonic band gaps tuned to the photoluminescence of a diketopyrrolopyrrole derivative, which allows a change in the fluorescence lifetime. Radiative and nonradiative rates were disentangled systematically by measuring the external quantum efficiencies and comparing the planar microcavities with a series of references designed to exclude any extrinsic effects. For the first time, this analysis shows unambiguously the dye radiative emission rate variations obtained with macromolecular dielectric mirrors. When different waveguides, chemical environments, and effective refractive index effects in the structure were accounted for, the change in the radiative lifetime was assigned to the Purcell effect. This was possible through the exploitation of photonic structures made of polyvinylcarbazole as a high-index material and the perfluorinated Aquivion as a low-index one, which produced the largest dielectric contrast ever obtained in planar polymer cavities. This characteristic induces the high confinement of the radiation electric field within the cavity layer, causing a record intensity enhancement and steering the radiative rate. Current limits and requirements to achieve the full control of radiative rates with polymer planar microcavities are also addressed.

Spontaneous emission enhancement by rotationally-symmetric optical nanoantennas: impact of radially and axially propagating surface plasmon polaritons.

The excitation and radiation properties of rotationally-symmetric optical nanoantennas are independent of the azimuth angle, which enables great convenience and superior performances in practical applications. However, for rotationally-symmetric nanoantennas, the physical mechanisms behind their resonance properties remain to be clarified. In this paper, firstly, for a simple single-nanocylinder-on-mirror antenna (S-antenna), we establish a first-principles-based semianalytical model of surface plasmon polariton (SPP) by considering an intuitive multiple-scattering process of the radially-propagating gap surface plasmon (RGSP) in the nanogap and the axially-propagating surface plasmon (ASP) on the nanocylinder. The model can comprehensively reproduce all the radiation properties of the S-antenna such as the total and radiative emission rates, SPP excitation rates, and far-field radiation pattern. The model indicates that when the antenna radius is small (respectively, large), the enhancement of spontaneous emission mainly results from the resonance of ASP (respectively, RGSP). To show the wide applicability of the SPP model along with its unveiled decisive role of the RGSP and ASP in the spontaneous emission enhancement for other rotationally-symmetric nanoantennas of cylindrical shapes, we extend the SPP model to a more complex ring-nanocylinder-on-mirror antenna (R-antenna) that supports two ASPs. Moreover, to provide an explicit explanation of the resonance properties of the R-antenna, we further establish a semianalytical model for the resonant modes (called quasinormal modes, QNMs) supported by the R-antenna based on the SPP model, which quantitatively reveals the role of the RGSP and ASP in forming the antenna resonant modes and the resultant enhancement of spontaneous emission.

Systematic measurements of M X‐ray components X‐ray fluorescence cross‐sections for the elements with 77 ≤ Z ≤ 92 by employing laboratory source based energy dispersive X‐ray fluorescence setup

AbstractThe differential X‐ray fluorescence (XRF) cross‐sections for (Mξ2, Mξ1, Mδ1), (Mδ2, Mα1,2 M5‐O3), (Mβ, M4‐O2,3), (Mγ, Mm2, M3‐N4, M5‐O2,3), (Mm1, M3‐N6,7, M3‐O4,5) and (Mm2, M2‐N6) group of M X‐rays components have been measured for the elements with 77 ≤ Z ≤ 92 following photoionization by Mn K X‐rays (EKαβ = 5.96 keV) obtained from 55Fe radioisotope. The measurements were performed in annular source geometry at 126° emission angle using a low‐energy Ge (LEGe) detector. The measured cross‐section values are compared with theoretical values calculated using available sets of Mi (i = 1–5) photoionization cross‐sections, radiative emission rates (Fij), Coster‐Kronig (fij), and fluorescence (ωi) yields. The measured XRF cross‐sections for the (Mξ2, Mξ1, Mδ1), (Mm1, M3‐N6,7) and (Mm2, M2‐N6) groups of X‐rays agree with the theoretical values within the experimental errors. The (Mβ, M4‐O2,3) group of X‐rays exhibit agreement with theoretical values within experimental uncertainty for all the elements under investigation except 79Au and 80Hg. The XRF cross‐section for the (Mδ2, Mα1,2) group of X‐rays are in general higher by ~20% for the elements with Z = 77–83 and exhibit agreement for the 90Th and 92U elements. For the (Mγ, Mm2, M3‐N4) X‐ray group, the measured values are generally higher than the theoretical values, but the deviations are within experimental uncertainties. The large deviation in measured XRF cross‐section for different M X‐ray components from the theoretical ones are attributed to (i) poor separation of M X‐ray components (ii) contribution of self‐resonant Raman scattering (RRS) process and (iii) self‐fluorescence of M5 subshell by Mi subshell X‐rays (i = 1–3).

Broadband Enhancement of the Spontaneous Emission by an Optical Dipole Nanoantenna on Metallic Substrate: an Intuitive Model of Surface Plasmon Polariton

Optical nanoantennas can achieve electromagnetic-field enhancement under far-field excitation or spontaneous-emission enhancement under excitation by radiating emitters. Among them, nanoantennas on a metallic substrate (i.e., the so-called nanoparticle-on-mirror antennas) have drawn great research interests due to their ease in forming metallic gaps of sizes down to a few nanometers or even subnanometer. Here we propose an optical dipole nanoantenna on a metallic substrate with a broadband enhancement of spontaneous emission. Its total and radiative emission-rate enhancement factors can be up to 5454 and 1041, respectively. In the near-infrared band, the wavelength range of spontaneous-emission enhancement (Purcell factor over 1000) can reach 260nm. By changing the width of the slit between the two antenna arms and changing the length of the antenna arms, the spontaneous-emission enhancement bandwidth and enhancement factors can be adjusted, respectively, which brings great freedom and simplicity to the design process. The antenna can achieve a strong far-field radiation within a central anglular zone (polar angle <i>θ</i>≤60°) corresponding to a certain numerical aperture of objective lens, and therefore can increase the intensity of the fluorescence collected by the objective lens. Based on the above performances, the antenna can provide a broadband enhancement of spontaneous emission for fluorescent molecules or quantum dots (whose fluorescence spectrum usually covers a certain wavelength range), which is of great significance for applications such as high-speed and super-bright nanoscale light sources and high-sensitivity fluorescent-molecule sensing.<br>To clarify the underlying physical mechanisms, we build up a semi-analytical model by considering an intuitive excitation and multiple-scattering process of surface plasmon polaritons (SPPs) that propagate along the antenna arms. All the parameters used in the model (such as the SPP scattering coefficients) are obtained via rigorous calculations based on the first principle of Maxwell’s equations without any fitting process, which ensures that the model has a solid electromagnetic foundation and can provide quantitative predictions. The SPP model can comprehensively reproduce all the radiation properties of the antenna, such as the total and radiative emission rates and the far-field radiation pattern. Two phase-matching conditions are derived from the model for predicting the antenna resonance, and show that under these conditions, the SPPs on the antenna arms form a pair of Fabry-Perot resonance and therefore are enhanced, and the enhanced SPPs propagate to the emitter in the nanogap (or scattered into the free space), so as to enhance the total spontaneous emission rate (or the far-field radiative emission rate). Besides, this pair of Fabry-Perot resonance result in a pair of resonance peaks close to each other, which then forms the broadband enhancement of spontaneous emission.

Broadband enhancement of spontaneous emission by optical dipole nanoantenna on metallic substrate: An intuitive model of surface plasmon polariton

<sec>Optical nanoantennas can achieve electromagnetic-field enhancement under far-field excitation or spontaneous-emission enhancement under excitation by radiating emitters. Among them, nanoantennas on a metallic substrate (i.e. the so-called nanoparticle-on-mirror antennas) have aroused great research interest due to their ease in forming metallic gaps of sizes down to a few nanometers or even subnanometer. Here we propose an optical dipole nanoantenna on a metallic substrate with a broadband enhancement of spontaneous emission. Its total and radiative emission-rate enhancement factors can reach up to 5454 and 1041, respectively. In the near-infrared band, the wavelength range of spontaneous-emission enhancement (Purcell factor over 1000) can reach 260nm. By changing the width of the slit between the two antenna arms and also the length of the antenna arms, the spontaneous-emission enhancement bandwidth and enhancement factors can be adjusted, respectively, which brings great freedom and simplicity to the design process. The antenna can achieve a strong far-field radiation within a central anglular zone (polar angle <i>θ</i>≤60°) corresponding to a certain numerical aperture of objective lens, and therefore can increase the intensity of the fluorescence collected by the objective lens. Based on the above performances, the antenna can provide a broadband enhancement of spontaneous emission for fluorescent molecules or quantum dots (whose fluorescence spectrum usually covers a certain wavelength range), which is of great significance for the applications such as in high-speed and super-bright nanoscale light sources and high-sensitivity fluorescent-molecule sensing.</sec><sec>To clarify the underlying physical mechanisms, we build up a semi-analytical model by considering an intuitive excitation and multiple-scattering process of surface plasmon polaritons (SPPs) that propagate along the antenna arms. All the parameters used in the model (such as the SPP scattering coefficients) are obtained via rigorous calculations based on the first principle of Maxwell's equations without any fitting process, which ensures that the model has a solid electromagnetic foundation and can provide quantitative predictions. The SPP model can comprehensively reproduce all the radiation properties of the antenna, such as the total radiative emission rate and the far-field radiation pattern. Two phase-matching conditions are derived from the model for predicting the antenna resonance, and show that under these conditions, the SPPs on the antenna arms form a pair of Fabry-Perot resonance and therefore are enhanced, and the enhanced SPPs propagate to the emitter in the nanogap (or scattered into the free space), so as to enhance the total spontaneous emission rate (or the far-field radiative emission rate). Besides, this pair of Fabry-Perot resonance results in a pair of resonance peaks close to each other, then enhancing the spontaneous emission with a broadband.</sec>

Spontaneous Emission Enhancement by a Rectangular-Aperture Optical Nanoantenna: An Intuitive Semi-Analytical Model of Surface Plasmon Polaritons

The spontaneous-emission enhancement effect of a single metallic rectangular-aperture optical nanoantenna on a SiO2 substrate was investigated theoretically. By considering the excitation and multiple scattering of surface plasmon polaritons (SPPs) in the aperture, an intuitive and comprehensive SPP model was established. The model can comprehensively predict the total spontaneous emission rate, the radiative emission rate and the angular distribution of the far-field emission of a point source in the aperture. Two phase-matching conditions are derived from the model for predicting the resonance and show that the spontaneous-emission enhancement by the antenna comes from the Fabry–Perot resonance of the SPP in the aperture. In addition, when scanning the position of the point source and the aperture length, the SPP model does not need to repeatedly solve the Maxwell’s equations, which shows a superior computational efficiency compared to the full-wave numerical method.

Development of radiation characteristics analysis code system for geological disposal and application to vitrified waste disposal with various LWR burnup conditions

The radiation characteristics analysis code system for geological disposal is developed to study the radiation influence of high-level waste on an engineered barrier system for the various conditions from a reactor burnup through geological disposal. The verification of the code system is performed by the comparisons with the data in the previous study which analyzed the radioactivity, the radiation emission rate, the radiation energy spectrum, and the radiation flux for the vitrified waste fabricated from spent UO2 fuel with 45 GWd/t discharge burnup. The calculation of neutron emission rate and energy spectrum shows the improvement by the introduction of updated data, and the calculation results of the code system show the good agreement with the reported data, considering the calculation conditions. The effect of the burnup condition of spent nuclear fuel in a light water reactor (LWR) on the radiation characteristics of vitrified waste is surveyed by the application of the code system. The numerical analyses are performed on the basis of the burnup conditions of two types of fuels (UO2 and MOX) and two burnup levels of current standard (45 GWd/t) and high burnup (70 GWd/t). The ratio of high-level liquid waste in vitrified waste is 20 wt% for all cases. The analysis results indicate that the utilization of MOX fuel increases the MA radioactivity drastically, thereby increasing the neutron emission rate of waste and the neutron flux on the inner surface of the overpack by a factor of over nine in comparison to the UO2 fuel waste disposal, whereas the utilization of high burnup fuel increases the emission rate and flux of neutron and γ-ray by 20 – 170 %. The radiation fluence is sufficiently small even by the change of burnup condition compared to the lifetime fluence of reactor pressure vessel employed in LWR.

Theoretical and experimental spectroscopic investigation of new Eu(III)-FOD complex containing 2-pyrrolidone ligand.

The preparation and photoluminescent properties of the new [Eu(FOD)3(2-Pyr)2] complex (FOD = 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octadionate; 2-Pyr = 2-pyrrolidone) are reported. The obtained complex was characterized by elemental analysis, complexometric titration using EDTA, infrared spectroscopy, and single-crystal X-ray diffraction studies. The coordination polyhedron of the complex is described as a distorted square antiprismatic with both 2-Pyr monodentate ligands coordinated to Eu(III) via the oxygen atoms, in neutral form, while the three FOD molecules are coordinated in the anionic form. Structural modeling at the PBE1PBE/SVP/MWB52 level of theory provided a geometry in excellent agreement with the one obtained experimentally. Spectroscopy properties such as intensity parameters (Ω2 and Ω4), radiative emission rate (Arad), and chemical partition of Arad for [Eu(FOD)3(2-Pyr)2] and [Eu(FOD)3(H2O)2] were calculated by using the QDC model with help of the semiempirical wavefunctions. The modeling of the ligand-to-metal energy transfer for both complexes was performed, allowing to obtain the theoretical emission quantum yield and to characterize the most relevant molecular orbitals involved.

Purcell radiative rate enhancement of label-free proteins with ultraviolet aluminum plasmonics

The vast majority of proteins are intrinsically fluorescent in the ultraviolet, thanks to the emission from their tryptophan and tyrosine amino-acid constituents. However, the protein autofluorescence quantum yields are generally very low due to the prevailing quenching mechanisms by other amino acids inside the protein. This motivates the interest to enhance the radiative emission rate of proteins using nanophotonic structures. Although there have been numerous reports of Purcell effect and local density of optical states control in the visible range using single dipole quantum emitters, the question remains open to apply these concepts in the UV on real proteins containing several tryptophan and tyrosine amino acids arranged in a highly complex manner. Here, we report the first complete characterization of the Purcell effect and radiative rate enhancement for the UV intrinsic fluorescence of label-free β-galactosidase and streptavidin proteins in plasmonic aluminum nanoapertures. We find an excellent agreement with a calibration performed using a high quantum yield UV fluorescent dye. Demonstrating and intensifying the Purcell effect is essential for the applications of UV plasmonics and the label-free detection of single proteins.

Angular selection of transmitted light and enhanced spontaneous emission in grating-coupled hyperbolic metamaterials.

We propose dielectric grating-coupled hyperbolic metamaterials as a functional device that shows angular selection of transmitted light and enhanced radiative emission rate. We numerically demonstrate that the surface plasmon polaritons in the hyperbolic metamaterials can be effectively outcoupled to the surrounding space by using gratings and facilitate control of the light transmission in the visible frequency. We confirm that the high density of states and the effect of outcoupled plasmonic modes of the proposed structure lead to the increase of Purcell factor and radiative emission. This work will provide multifunctionalities in sensing and imaging systems that use hyperbolic metamaterials.

Directional and Fast Photoluminescence from CsPbI3 Nanocrystals Coupled to Dielectric Circular Bragg Gratings.

Lead halide perovskite nanocrystals (NCs), especially the all-inorganic perovskite NCs, have drawn substantial attention for both fundamental research and device applications in recent years due to their unique optoelectronic properties. To build high-performance nanophotonic devices based on perovskite NCs, it is highly desirable to couple the NCs to photonic nanostructures for enhancing the radiative emission rate and improving the emission directionality of the NCs. In this work, we synthesized high-quality CsPbI3 NCs and further coupled them to dielectric circular Bragg gratings (CBGs). The efficient couplings between the perovskite NCs and the CBGs resulted in a 45.9-fold enhancement of the photoluminescence (PL) intensity and 3.2-fold acceleration of the radiative emission rate. Our work serves as an important step for building high-performance nanophotonic light emitting devices by integrating perovskite NCs with photonic nanostructures.

Frustules of Amphora sp. as a photonic crystal with photoluminescent CdS nanoparticles.

Diatom frustules have species-specific patterns of pores, striae, pores, and nanopores, periodically arranged on its silica surface, as sets of cavities that modify the vacuum electromagnetic density of states. Therefore, frustules may be considered photonic crystals; the interaction with light-emitting sources inside the pores may potentially result in enhancement or inhibition of their spontaneous radiative emission rate and frequencies. In this work, we studied the photoluminescence of cadmium sulfide nanoparticles (CdS-NP) deposited inside frustule cavities that conveyed evidence of cavity-NP interaction. We synthesized CdS-NP, a semiconductor compound achieving quantum dots small enough to impose confinement effects to the electronic states. CdS-NP and their clusters were physiosorbed onto the surface, striae, and predominantly inside the pores of the cleansed frustules of Amphora sp. A broad peak with a maximum intensity at 437 nm (2.84 eV) was recorded after excitation with a 375 nm light source, showing a large blue shift and signal amplification of the CdS-NP photoluminescence when these were embedded inside the pores of the silica frustule. Using the Brus equation, we estimated a NP size of 4.1 ± 0.2 nm for the CdS-NP snuggly packed inside the smaller pores of the frustule, of 10 ± 0.7 nm in average diameter, The emission Purcell enhancement factor for an emitting atom in a cavity was calculated. The obtained Q factor (c. 5) was smaller than typical Q factors for designed semiconductor cavities of similar dimensions, an expected situation if it is assumed that the pores are open-ended cavities.

Plasma environment effects on K lines of astrophysical interest

Aims.Within the framework of compact-object accretion disks, we calculate plasma environment effects on the atomic structure and decay parameters used in the modeling of K lines in lowly charged iron ions, namely Fe II–Fe VIII.Methods.For this study, we used the fully relativistic multiconfiguration Dirac–Fock method approximating the plasma electron–nucleus and electron-electron screenings with a time-averaged Debye-Hückel potential.Results.We report modified ionization potentials, K-threshold energies, wavelengths, radiative emission rates, and Auger widths for plasmas characterized by electron temperatures and densities in the ranges 105 − 107K and 1018 − 1022cm−3. In addition, we propose two universal fitting formulae to predict the IP and K-threshold lowerings in any elemental ion.Conclusions.We conclude that the high-resolution X-ray spectrometers onboard the future XRISM and ATHENA space missions will be able to detect the lowering of the K edges of these Fe ions due to the extreme plasma conditions occurring in the accretion disks around compact objects.

Broadband Enhancement of the Spontaneous Emission by a Split-Ring Nanoantenna: Impact of Azimuthally Propagating Surface Plasmon Polaritons

We propose a split-ring optical nanoantenna which can achieve enhancement factors of total and radiative emission rates up to high values of 7411 and 1941, respectively, with a tunable bandwidth of enhanced emission (Purcell factor over 1000) up to 221 nm at visible wavelengths. To clarify the underlying physical mechanism, we build up a semi-analytical model by considering an intuitive multiple-scattering process of surface plasmon polaritons (SPPs) that propagate azimuthally along the curved antenna arms. The SPP model provides an intuitive picture that under two phase matching conditions, the SPPs form paired and tunable whispering-gallery-like resonances and are then coupled to the emitter in the nano-gap or scattered into free space, which results in an enhancement of the total or radiative emission rate, respectively. Based on the SPP model, we further build up a semi-analytical model for the quasi-normal modes (QNMs) supported by the antenna, which provides a frequency analyticity and indicates that the broadband enhancement of the spontaneous emission rate results from an excitation of paired symmetric and anti-symmetric QNMs with different eigenfrequencies. The proposed models clarify the impact of azimuthally propagating SPPs on the antenna radiation and may be extended to other optical antennas with curved arms.

Underground test of gravity-related wave function collapse

Roger Penrose proposed that a spatial quantum superposition collapses as a back-reaction from spacetime, which is curved in different ways by each branch of the superposition. In this sense, one speaks of gravity-related wave function collapse. He also provided a heuristic formula to compute the decay time of the superposition—similar to that suggested earlier by Lajos Diósi, hence the name Diósi–Penrose model. The collapse depends on the effective size of the mass density of particles in the superposition, and is random: this randomness shows up as a diffusion of the particles’ motion, resulting, if charged, in the emission of radiation. Here, we compute the radiation emission rate, which is faint but detectable. We then report the results of a dedicated experiment at the Gran Sasso underground laboratory to measure this radiation emission rate. Our result sets a lower bound on the effective size of the mass density of nuclei, which is about three orders of magnitude larger than previous bounds. This rules out the natural parameter-free version of the Diósi–Penrose model.

External quantum efficiency determined by combined thermal lens and photoluminescence spectroscopy techniques: Application to Ce3+:YAG

In this work, thermal lens spectroscopy was performed simultaneously with photoluminescence experiments for determining the absolute external quantum efficiency (EQE) of luminescent materials. The procedure involved measurements of nonradiative (heat) and radiative (photon) emission rates after excitation of the material with a tunable laser. The main advantage of this combined methodology was that the amount of light absorbed by the sample was not required for EQE determination, which minimized the experimental error. In order to validate the methodology, the well-known Ce3+:YAG crystal was used for EQE determination, with the value obtained being in agreement with previous experiments. This shows that the method is a promising tool for accurate measurements of the EQE for materials with a single excited emission state.