Spontaneous Radiative Rates Research Articles

X-Ray Emission from Atomic Systems Can Distinguish between Prevailing Dynamical Wave-Function Collapse Models.

In this work the spontaneous electromagnetic radiation from atomic systems, induced by dynamical wave-function collapse, is investigated in the x-ray domain. Strong departures are evidenced with respect to the simple cases considered until now in the literature, in which the emission is either perfectly coherent (protons in the same nuclei) or incoherent (electrons). In this low-energy regime the spontaneous radiation rate strongly depends on the atomic species under investigation and, for the first time, is found to depend on the specific collapse model.

Modeling of Output Spectra and Numerical Simulation of Thulium-doped Broadband Fiber Light Sources at 1400-1520nm, 1800-2000nm

Lasers have a great influence in industrial processing and biomedical fields. Thulium-doped fiber lasers have good prospects for communication transmission and polymer welding. In this paper, the research method is as follows: in the pump light at 0. 79um, the values of Thulium-doped fiber, including its core radius, spontaneous radiation rate, cross-sectional absorption area, etc., are preset in advance. Then, by changing the fiber length, pumping power, and doping concentration of the fiber, the curve between the output spectrum of ASE light and the wavelength of ASE light is demonstrated. Finally, the peak power of the Thulium-doped broadband light source fiber is about 1. 6× w at L=3m, N=4× / , Pp=0. 2w. The output spectrum of thulium-doped fiber is mainly concentrated around the 2000nm wavelength band, and examples of thulium-doped fiber lasers for modern medical and industrial fields are presented. The thulium-doped fiber is the main direction of future research and the outlook for the future.

Signatures of cooperative emission in photon coincidence: Superradiance versus measurement-induced cooperativity

Indistinguishable quantum emitters confined to length scales smaller than the wavelength of the light become superradiant. Compared to uncorrelated and distinguishable emitters, superradiance results in qualitative modifications of optical signals such as photon coincidences. However, recent experiments revealed that similar signatures can also be obtained in situations where emitters are too far separated to be superradiant if correlations between emitters are induced by the wave-function collapse during an emission-angle-selective photon detection event. Here, we compare two sources for cooperative emission, superradiance and measurement-induced cooperativity, and analyze their impact on time-dependent optical signals. We find that an antidip in photon coincidences at zero time delay is a signature of interemitter correlations in general but does not unambiguously prove the presence of superradiance. This suggests that photon coincidences at zero time delay alone are not sufficient and time-dependent data are necessary to clearly demonstrate a superradiant enhancement of the spontaneous radiative decay rate.

Temperature dependence of fluorescence dynamic behavior of wide-bandgap compounds

In this work, the transition properties of wide-bandgap compounds are studied by way of the temperature dependence of fluorescence dynamic curves. The YVO4, CaWO4, and CaMoO4 samples are selected in this study owing to their strong absorption in the ultraviolet (UV) region. The fluorescence decay curves of the three samples, in the range of 173–373 K, are measured under excitation by deep UV light-emitting diodes (LEDs). The temperature dependence of transition rate (A) is composed of a constant term, as well as a mono-exponential term which follows the Boltzmann distribution law. The constant term reflects a spontaneous radiative transition rate (WR), while the mono-exponential term represents the temperature-dependent nonradiative transition rate (WNR). The nonradiative relaxation is attributed to the upward thermal population rather than the downward multiphonon relaxation (MPR). The thermal populating process is supposed to originate from the bottom of the excited state to the intersection of the ground state and excited state by overcoming the potential barrier of activation energy (Ea), and the Ea values of YVO4, CaWO4, and CaMoO4 samples are calculated.

Low-threshold miniaturized core-shell GaAs/InGaAs nanowire/quantum-dot hybrid structure nanolasers

A core-shell GaAs/InGaAs nanowire/quantum dot hybrid structure nanolaser is proposed and studied by the finite-difference time-domain method. The results show that the coupled nanowire cavity-emitter system enables enhancement of the spontaneous radiation rate of quantum dots, resulting in a high Purcell factor of 15.5 at a diameter of 330 nm. Due to the quantum confinement and reduced propagation loss, the hybrid structure laser exhibits a threshold lower by 1–2 magnitudes than that of the pure nanowire laser. In addition, by incorporating multiple quantum dots, the lasing intensity is significantly enhanced at the expense of threshold. This work may pave the way for the development of ultra-small low-consumption near infrared lasers and related microsystems.

Universal lasing condition

Usually, the cavity is considered an intrinsic part of laser design to enable coherent emission. For different types of cavities, it is assumed that the light coherence is achieved by different ways. We show that regardless of the type of cavity, the lasing condition is universal and is determined by the ratio of the width of the atomic spectrum to the product of the number of atoms and the spontaneous radiation rate in the laser structure. We demonstrate that cavity does not play a crucial role in lasing since it merely decreases the threshold by increasing the photon emission rate thanks to the Purcell effect. A threshold reduction can be achieved in a cavity-free structure by tuning the local density of states of the electromagnetic field. This paves the way for the design of laser devices based on cavity-free systems.

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.

High-efficiency ∼2 µm CW laser operation of LD-pumped Tm3+:CaF2 single-crystal fibers.

The Tm:CaF2 single-crystal fibers were successfully grown by modified temperature gradient technique method. The J-O intensity parameters, spontaneous radiative transition rates, radiative lifetimes and fluorescence branching ratios of Tm3+ were calculated with Judd-Ofelt theory. A systematic study of the fluorescence characteristics has been carried out. Simulated emission cross-sections of the 3F4 → 3H6 transition were calculated to be 6.68×10-21 cm-2 and 4.65×10-21cm-2 for crystal doped with 3 at.% and 4 at.% Tm3+. The 64.4% slope efficiency with output power of 2.23W was achieved in 3 at.% Tm:CaF2 single-crystal fiber. The slope efficiency decreased to 44.5% and maximum output power decreased to 1.65W in 4 at.% Tm:CaF2 single-crystal fiber. Obtained results show that Tm-doped CaF2 single-crystal fibers are promising materials for IR laser action generation.

Coherent control of subluminal optical solitons by the incoherent pumping in a ladder-type atomic system

We investigate the coherent control of subluminal optical solitons in a ladder-type atomic system with an incoherent pumping. In the linear regime, we find that the quantum interference of the system remains destructive when the incoherent pumping is less than the spontaneous radiation rate of the intermediate state, and the incoherent pumping can introduce an active gain into the system under this condition without changing the linear coherent properties of the system. In addition, the group velocity of the probe field can be hugely suppressed. In the nonlinear regime, a giant Kerr nonlinearity can be achieved without linear absorption. Thus, the lossless slow light optical solitons can be produced and propagate over a long distance being stable. The coherent control of the subluminal optical solitons can be realized by changing the incoherent pumping.

Characterization of InN - In0.25Ga0.75N Quantum Well Laser Structure for 1330 nm Wavelength

In this work, a nitride based wurtzite-strained single quantum well laser has been designed and characterized at 1330 nm wavelength. Here, 12Å InN layer has been used as well material and 15Å In0.25Ga0.75N layer has been used as barrier material along with GaN as separate confinement heterostructure for better carrier and optical confinement. To determine the electronic properties of the designed structure, a developed self-consistent method has been solved. Conduction subband calculation has been performed with the solution of Schrodinger equation formed by single band Hamiltonian followed by Poisson's equation. 6-bands k . p Hamiltonian for wurtzite semiconductor has been solved for valence band Schrodinger's equation with Poisson's equation considering band mixing effect, strain due to lattice mismatch and spontaneous and piezoelectric polarization. The interband momentum matrix elements, optical gain, spontaneous emission rate, spontaneous radiative recombination rate and radiative current density have been calculated to analyze the optical properties of the designed laser and a good agreement with previously published works is obtained. All analysis has been performed using injection carrier density of 6×10-19 cm-3 at T = 300 K. Due to the strain effect, the wave-function overlap integral has been obtained as 43.27%. From the electronic properties analysis, it has been found that, the designed structure is TE polarized with C1-HH1 and C1-LH1 dominating transitions. From the analysis of optical properties, the obtained spontaneous emission rate is 5945.2 s-1 at 1329.55 nm and spontaneous emission rate per energy interval per unit volume is 7.21×1027 s-1cm-3eV-1 at same wavelength. The obtained radiative recombination rate is 7.77×1029 s-1cm-3 and radiative current density is 149.19 Acm-2. The optical gain for TE polarization of the designed QW structure is 5261.52cm-1 at 1336.7 nm. Figure 1

Calculation and analysis of the number of return photons from sodium laser beacon excited by the long pulse laser with circular polarization

The number of return photons from sodium laser beacon (SLB) greatly suffers down-pumping, recoil, and geomagnetic field when the long pulse laser with circular polarization interacts with sodium atoms in the mesosphere. Considering recoil and down-pumping effects on the number of return photons from SLB, the spontaneous radiation rates are obtained by numerical computations and fittings. Furthermore, combining with the geomagnetic field effects, a new expression is achieved for calculating the number of return photons. By using this expression and considering the stochastic distribution of laser intensity in the mesosphere under different turbulence models for atmosphere, the number of return photons excited by the narrow-band single mode laser and that by the narrow-band three-mode laser are respectively calculated. The results show that the narrow-band three-mode laser with a specific spectrum structure has a higher spontaneous radiation rate and more return photons than a narrow-band single mode laser. Of note, the effect of the atmospheric turbulence on the number of return photons is remarkable. Calculation results indicate that the number of return photons under the HV5/7 model for atmospheric turbulence is much higher than that under the Greenwood and ModHV models.

Surface Plasmon Enhanced Hot Exciton Emission in Deep UV‐Emitting AlGaN Multiple Quantum Wells

The surface plasmon (SP)‐exciton coupling effect has attractive potential applications in nitride semiconductor‐based light‐emitting diodes (LEDs) with improved performances by enhancing the spontaneous radiative recombination rate, especially for AlGaN‐based deep UV LEDs with an intrinsic low internal quantum efficiency (IQE). In this work, significant deep UV emission enhancement is demonstrated on AlGaN‐based multiple quantum well (MQW) structure by introducing the coupling of local surface plasmons (LSPs) from Al nanoparticles (NPs) with the excitons in AlGaN MQWs. By precisely manipulating the size of the Al NPs, the evolution of emission wavelength and enhancement ratio for the SP‐coupled AlGaN MQWs along with the change of the LSP properties is investigated in detail. With respect to the bare MQWs, strong emission enhancements accompanied by large shifts towards shorter wavelengths are observed from the AlGaN MQW structure decorated with Al NPs, which is proposed to originate from the suppressing of the ground state exciton emission and the strongly enhanced emission from the high‐order QW exciton states by the LSP–exciton coupling process. Theoretical calculations confirm that the higher energy sub‐band e2–hh2 transition from the MQWs is the maximum enhanced emission channel through the SP coupling process.

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.

Photoluminescence, recombination rate, and gain spectra in optically excited n-type and tensile strained germanium layers

We theoretically investigate the optical properties of photo-excited biaxially strained intrinsic and n-type doped Ge semi-infinite layers using a multi-valley effective mass model. Spatial inhomogeneity of the excess carrier density generated near the sample surface is considered. Strain effects on the band edges, on the band dispersions, and on the orbital compositions of the near gap states involved in radiative recombinations are fully taken into account. We obtain, as a function of the distance from the sample surface, the energy resolved absorption/gain spectra resulting from the contribution of the radiative direct and phonon-assisted band-to-band transitions and from the intra-band free carrier absorption. Photoluminescence spectra are calculated from the spatially dependent spontaneous radiative recombination rate, taking into account energy-dependent self-absorption effects. For suitable combinations of doping density, strain magnitude, pump power, and emitted photon polarization, we find gain values up to 5800 cm−1.

Cold ion-atom chemistry driven by spontaneous radiative relaxation: a case study for the formation of the YbCa+ molecular ion

Using both quantum and semi-classical methods, we calculate the rates for radiative association and charge transfer in cold collisions of Yb+ with Ca. We demonstrate the fidelity of the local optical potential method in predictions for the total radiative relaxation rates. We find a large variation in the isotope dependence of the cross sections at ultra-cold gas temperatures. However, at cold temperatures, 1 mK < T < 1 K, the effective spontaneous radiative rates for the different isotopes share a common value of about 1.5 × 10−15 cm3 s−1. It is about five orders of magnitude smaller than the chemical reaction rate measured in Rellergert et al (2011 Phys. Rev. Lett. 107 243201).

Tensile-strain and doping enhanced direct bandgap optical transition of n+ doped Ge/GeSi quantum wells

Band structures of tensile strained and n+ doped Ge/GeSi quantum wells (QWs) are calculated by multiple-band k·p method. The energy dispersion curves of the Γ and L conduction subbands are obtained. The effects of tensile strain and n+ doping in Ge on direct bandgap optical gain and spontaneous radiative recombination rate spectra are investigated including the electron leakage from Γ to L conduction subbands. Our results show that the optical gain and spontaneous radiative recombination rate can be significantly increased with the tensile strain, n-type doping concentration, and injection carrier density in the Ge QW. The free carrier absorption is calculated and cannot be ignored because of the heavily doped Ge. The pure TM mode polarized net optical gain up to 1153 cm−1 can be achieved for the Ge/Ge0.986Si0.014 QW with tensile strain of 1.61% and n-type doping concentration of 30 × 1018 cm−3.

Luminescence properties and energy transfer mechanism of Er3+/Tm3+ co-doped tellurite glasses

Er3+ single doped and Er3+/Tm3+ co-doped tellurite glasses with composition of TeO2–ZnO–Na2O were prepared by the melt-quenching method, and the absorption and fluorescence spectra were measured. The intensity parameters Ωt (t=2, 4, 6), spontaneous radiative transition rates A, fluorescence branching ratios β and radiative lifetimes τrad of Tm3+ ions were calculated from the measured absorption spectra based on the Judd–Ofelt theory, and the luminescence properties of samples with different doped concentration of Er3+–Tm3+ ions were investigated from the measured fluorescence emission spectra under the 800nm LD excitation. It was shown that the 1.53μm fluorescence emission intensity of Er3+ ions decreases with the increasing of Tm3+ ion doped concentration, while the 1.85μm fluorescence emission intensity of Tm3+ ions increases accordingly. The energy transfer mechanisms between Er3+ and Tm3+ ions were investigated to explain the fluorescence varying behavior, and it indicates that phonon-assisted non-resonant energy transfers from Er3+ to Tm3+ ions play a very important role in the 1.85μm fluorescence emission of Tm3+ ions. The energy transfer coefficient and phonon contribution for the existed energy transfer process between Er3+ and Tm3+ ions were calculated and analyzed. Meanwhile, the 1.85μm band stimulated absorption, emission cross-sections and gain coefficient of Tm3+:3F4→3H6 transitions were also obtained and analyzed. The research shows that Er3+/Tm3+ co-doped TeO2–ZnO–Na2O glass is a promising host material applied for 1.85μm band solid lasers and amplifiers.

Using BBGKY hierarchies to study the effect of the local field on the rate of radiative relaxation of quantum systems in a dielectric medium

We consider a model of atomic radiators in a dielectric medium and its optical characteristics. In the framework of the microscopic approach and the Bogolyubov-Born-Green-Kirkwood-Yvon method, we obtain a chain of coupled equations for reduced density matrices and correlation operators of atomic particles and modes of a quantized field. We derive a closed system of equations describing the evolution of radiators in the model medium under the action of external radiation. We calculate the effective spontaneous relaxation rate in the presence of the local field produced by the medium. For the first time, we demonstrate the relation between micro and macro approaches to the solution of the problem involving a change in the spontaneous radiation rate in the medium.

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.

Enhanced mid-IR emission in Yb 3+–Tm 3+ co-doped oxyfluoride glass ceramics

Tm 3+–Yb 3+ co-doped transparent oxyfluoride glass ceramics were prepared through thermal treatment of the as-prepared glasses. The precipitation of nanocrystals and the incorporation of Tm 3+ and Yb 3+ into the nanocrystals were confirmed by X-ray diffraction and absorption spectra. Based on the Judd–Ofelt theory, the J–O parameters Ω λ ( λ = 2, 4, 6), spontaneous radiative transition rates, radiative lifetimes and fluorescence branching ratios of Tm 3+ in both as-prepared glasses and glass ceramics were calculated. Intense mid-IR emission and upconversion luminescence in the Tm 3+ and Yb 3+ co-doped glass ceramics were observed under 980 nm excitation. Especially, compared with that of the as-prepared glasses, mid-IR luminescence intensity of Tm 3+ in the glass ceramics was greatly enhanced. Desirable spectroscopic characteristics suggest that these oxyfluoride glass ceramics may be promising mid-IR laser active medium.