Auger Upconversion Research Articles

New red phosphor ceramic K2SiF6:Mn4+

Abstract A new transparent ceramic phosphor for use in LED lighting has been fabricated. The previously reported and optimized narrow-emitting red phosphor, K2SiF6:Mn4+ (KSF), has been consolidated into a transparent ceramic phosphor for the first time, accomplished via hot-pressing the feedstock phosphor powder in a die under vacuum. KSF ceramics were fabricated with varying doping concentrations of Mn4+ and their properties studied. The absorption and emission spectra of the ceramics were identical to the feedstock phosphor powders and are ideal for LED lighting with strong absorption at 450 nm and narrow emission around 630 nm. The absorbance of the ceramics was directly proportional to the doping concentration. The ceramics were excited at various blue light fluxes and their emission intensities measured to study the effect of Mn4+ concentration on intensity-driven “droop” in the emission output. The ceramics with a lower Mn4+ doping were more efficient under higher light fluxes due to a decrease in Auger upconversion losses. KSF ceramics can allow a much longer path length of the diode light through the phosphor, as compared to phosphor-in-silicone, enabling the use of low optical absorption and the associated reduced activator concentration. The ceramics are measured to have a thermal conductivity of ~1.0 W/m-K, higher than that of phosphor-in-silicone or phosphor-in-glass. Several of these properties make KSF ceramics potentially desirable for use in white light LEDs. Greater thermal conductivity helps with heat dissipation, the lower surface area of the ceramic compared to the powder minimizes the environmental vulnerability of KSF, and the ability to lower the Mn4+ concentration reduces Auger recombination losses and mitigates the temperature rise, particularly at higher light flux.

Auger Up-Conversion of Low-Intensity Infrared Light in Engineered Quantum Dots.

One source of efficiency losses in photovoltaic cells is their transparency toward solar photons with energies below the band gap of the absorbing layer. This loss can be reduced using a process of up-conversion whereby two or more sub-band-gap photons generate a single above-gap exciton. Traditional approaches to up-conversion, such as nonlinear two-photon absorption (2PA) or triplet fusion, suffer from low efficiency at solar light intensities, a narrow absorption bandwidth, nonoptimal absorption energies, and difficulties for implementing in practical devices. Here we show that these deficiencies can be alleviated using the effect of Auger up-conversion in thick-shell PbSe/CdSe quantum dots. This process relies on Auger recombination whereby two low-energy, core-based excitons are converted into a single higher-energy, shell-based exciton. Compared to their monocomponent counterparts, the tailored PbSe/CdSe heterostructures feature enhanced absorption cross-sections, a higher efficiency of the "productive" Auger pathway involving re-excitation of a hole, and longer lifetimes of both core- and shell-localized excitons. These features lead to effective up-conversion cross-sections that are more than 6 orders of magnitude higher than for standard nonlinear 2PA, which allows for efficient up-conversion of continuous wave infrared light at intensities as low as a few watts per square centimeter.

Determining the average path length of amplified spontaneous emission in a four-level laser near the 1/3 mode-degeneracy cavity configurations

We determine the average path length ls of amplified spontaneous emission (ASE) by comparing the numerical slope of a straight line with the experimental slope in the graph of the square of relaxation oscillation frequency versus normalized pump ratio. The simple method is applied in an end-pumped Nd:YVO4 laser with the 1/3 mode-degeneracy cavity having the transverse mode spacing equal to 1/3 of the longitudinal mode spacing. We find that ls is larger at the degeneracy than that far from the degeneracy. This result indicates the existence of stronger ASE at the degeneracy, which is confirmed below the threshold. This is because many spontaneous emission photons can undergo amplification not only before escaping from the gain medium but also after leaving the gain medium, owing to cavity reflection. Our method can be applied in the situations where the Auger upconversion effect is absent, weak, or well-known.

Modeling population and thermal lenses in the presence of Auger Upconversion for Nd(3+) doped materials.

We present a theoretical model to thermal (TL) and population (PL) lenses effects in the presence of Auger upconversion (AU) for analysis of Nd(3+) doped materials. The model distinguishes and quantifies the contributions from TL and PL. From the experimental and theoretical results, the AU cannot be neglected because it plays an important role on the excited state population and therefore on the temperature and polarizability difference between excited and ground states. Considering the extensive use of these techniques, the model presented here could be useful for the investigation of materials and also to avoid misleading analysis of lenses transients.

Rigrod laser-pumped-laser resonator model: I. Theoretical considerations

Dilute laser materials whose ion doping is minimized are attractive because Auger up-conversion is minimized, substantially lower heat fractions result and transverse or longitudinal gain and heat deposition gradients are reduced. In addition, amplified spontaneous emission (ASE) effects are minimized as well. Lasers using dilute laser materials, however, tend to have lowered efficiencies due to the difficulty in obtaining efficient absorption in laser crystals of reasonable length. This is primarily because of the use of simple single or double-passed pump absorption schemes. While this difficulty may be overcome using multi-passed absorption pump beams, as has been successfully demonstrated in connection with the development of thin-disc lasers, higher absorption and efficiency are obtained with added complexity and cost. In this paper, we present an alternative method in which a dilute or thin absorbing laser material is placed intra-cavity to a pump laser resonator. With the absorbing laser crystal acting as an effective outcoupler, we find that the highest absorption and resonator extraction efficiencies are obtained with the most dilute or thin materials when resonator mirrors of near unity reflectivity are used. This counter-intuitive result is explained by a corresponding long photon lifetime inside the laser resonator, which effectively multi-passes the absorbing laser crystal, leading to a very high absorption efficiency.

Discrimination between thermal quenching of the fluorescence and Auger upconversion processes using thermal lens technique

In this work we used the Thermal Lens (TL) technique to discriminate two important processes responsible to reduce the upper-state population and fluorescence quantum efficiency (η) in Cr3+ doped colquiriite crystals: the thermal quenching of the fluorescence (TQF) and the Auger upconversion (ETU). We observed a nonlinear increase of the TL signal with laser power due to the decrease of η by ETU and/or TQF. The analysis of these curves allowed the determination of the thermal load, the increase of the crystal temperature as a function of the pump excitation and the discrimination between ETU and TQF processes.

Thermal lens and Auger upconversion losses' effect on the efficiency of Nd^3+-doped lead lanthanum zirconate titanate transparent ceramics

A thorough investigation of optical losses for the 1064nm emission in Nd3+-doped lead lanthanum zirconate titanate (PLZT) transparent ceramics is presented. Thermal lens experiments were carried out to evaluate thermo-optical properties and the fluorescence quantum efficiency of the emitting level 4F3/2. Excited-state absorption losses were measured in the emitting wavelength region, and the Auger upconversion energy transfer parameter γ was calculated. By using γ, the pump-intensity dependence of the optical gain at 1064nm, the fluorescence quantum efficiency, and the generation of heat in the ceramic were simulated for a high 803nm pump-power regime. Since the radiative and nonradiative losses in Nd:PLZT were verified to be considerably lower than in various commercial laser crystals and glasses, it is suggested that this material might become an interesting alternative for high-power laser emission.

Thermal lens and Z-scan measurements: Thermal and optical properties of laser glasses – A review

In this paper, the application of thermal lens and Z-scan techniques to the study of the thermo-optical and spectroscopic properties of solid-state laser glasses is described. The theoretical basis for quantitative measurements of thermal diffusivity and conductivity, temperature coefficient of the optical path length change, heat efficiency, fluorescence quantum efficiency, losses mechanisms (Auger upconversion and concentration quenching) and the line shape of the nonlinear refractive index are presented and discussed. The electronic contribution to the nonlinearity was investigated using the Z-scan technique in the time-resolved mode. The measurements were performed spectroscopically, allowing the determination of the line shapes of real and imaginary parts of the nonlinear refractive index, n2, in resonance with laser transitions. The results were interpreted by considering resonant and nonresonant contribution to n2. The magnitude of electronic and thermal contributions to the refractive index changes in solid-state laser glasses were also compared, and the thermal properties as a function of the temperature in the range of 20 up to 600K are presented.

Erratum: Fluorescence quantum efficiency and Auger upconversion losses of the stoichiometric laser crystalNdAl3(BO3)4[Phys. Rev. B72, 235111 (2005)

Received 27 February 2006DOI:https://doi.org/10.1103/PhysRevB.73.119902©2006 American Physical Society

Auger upconversion energy transfer losses and efficient 1.06 μm laser emission in Nd3+ doped fluoroindogallate glass

In addition to efficient continuous wave laser action at 1.06 μm (4F3/2→4I11/2 transition), with a slope efficiency 30% and 30 mW pump power threshold, this work presents a throughout study of the spectroscopic, and thermo-optical properties of a 2.0 mol % Nd3+ doped fluoroindogallate glass. The characterization of the sample using the pump probe experimental technique indicated the presence of two broad excited state absorption bands from 0.94 to 1.05 μm (4F3/2→2D3/2, 2G9/2, 4G11/2, 2K15/2) and from 1.16 to 1.42 μm (4F3/2→4G9/2, 4G7/2, 2K13/2), found not to interfere with the stimulated emission at 1.06 μm, with a peak cross section value σSE = 2.55×10-21 cm2 and a full width at half maximum of 22 nm. Thermal lens measurements were performed to quantify the Auger upconversion parameter (γ=(1.26±0.09)×10-16 cm3/s) and evaluate the thermal loading in the sample in comparison to other commercial glasses.

Fluorescence quantum efficiency measurements in the presence of Auger upconversion by the thermal lens method.

Mode-mismatched thermal lens (TL) measurements in Cr3+-doped fluoride crystals (LiSrAIF6 and LiSrGaF6) are reported. A nonlinear increase of the TL signal, and decrease of quantum efficiency, with increasing excitation power was observed and attributed to energy-transfer upconversion (ETU). Assuming an upconversion rate that is proportional to the excited-state population, Wup = gammaN(e), the theoretical model developed fits the experimental data well. The ETU parameters (gamma) were determined with unprecedented accuracy for low Cr concentrations. Thermo-optical parameters (K, ds/dT) were also obtained.

Determination of the energy diffusion and of the Auger upconversion constants in a Nd:YVO 4 standing-wave laser

The sudden translation at a continuous speed of the laser rod inside a single-mode standing-wave laser results in a transient oscillation of the output intensity. Using a first-order analytical rate equation model, the analysis of this transient behavior gives the values of the diffusion constant of the excited states, and of the upconversion Auger rate. We performed these measurements in a 1% doped Nd:YVO 4 laser.

Determination of the Auger upconversion rate in fiber-coupled diode end-pumped Nd:YAG and Nd:YVO 4 crystals

An analytical model is developed to study the influence of the Auger upconversion process on the thermal loading under lasing and nonlasing conditions. With the developed model, Auger upconversion rates can be determined by comparing theoretical calculations with experimental results for the ratio of the thermal loading under lasing and nonlasing conditions. The upconversion rates obtained with the present method are compared with the results measured from the fluorescence decay experiment.

Study of high-power diode-end-pumped Nd:YVO 4 laser at 1.34 μm: influence of Auger upconversion

We demonstrated an experimental study of the influence of the Nd 3+ concentration on scaling diode-pumped Nd:YVO 4 1.34-μm lasers to higher power. An output power of 5.1 W at 1.34 μm was obtained with a 0.5-at.% Nd-doped YVO 4 at 13.5 W of incident pump power. The strong dependence of the slope efficiency on the dopant concentration is attributed to an Auger upconversion process.

High-inversion densities in Nd:YAG-upconversion and bleaching

We report on the investigation of upconversion in Nd:YAG and its implications for intensely pumped devices. Analysis of lifetime measurements and the performance of a 1 at.% Nd-doped YAG amplifier give an Auger upconversion rate of 7/spl times/10/sup 3/ s/sup -1/. This is significantly smaller than previously reported, but modeling of the performance of Nd:YAG devices with high-inversion densities shows that even this rate can still seriously degrade the small-signal gain and significantly increase the thermal load. The variation of cross-relaxation and upconversion rates with doping level is also described. Finally, it is found that the effect of bleaching of the Nd:YAG absorption can lead to a reduced spatial overlap between the signal and inversion profiles and thus can also significantly reduce the gain.

Auger upconversion losses in Nd-doped laser glasses

We have measured the Auger upconversion constant, γ, of Nd 3+ ions in phosphate, silicate, and fluoride glasses. The Auger process involves two Nd 3+ ions initially in the 4F 3 2 metastable state that interact such that one ion is promoted to a higher lying state, while the other is forced to the 4I J levels of the ground state. γ values ranging from 2.6 to 8.1 × 10 -17 cm 3s -1 were measured, depending on the host medium and neodymium concentration. The product of the emission lifetime and the Auger coefficient was found to be in the range of γτ = (1 − 2) × 10 -20 cm 3 for the materials investigated. This suggests that Auger upconversion is an important problem for excited state densities approaching (5 − 10) × 10 19 cm -3.

Properties of Cr:LiSrAlF_6 crystals for laser operation

We have performed several physical and optical measurements on the Cr:LiSAF (LiSrAlF(6)) laser material that are relevant to its laser performance, including thermal and mechanical properties, water durabilities, and Auger upconversion constants. The expansion coefficient, Young's modulus, fracture toughness, thermal conductivity, and heat capacity are all used to determine an overall thermomechanical figure of merit for the crystal. An investigation of the water durability suggests that the cooling solution should be maintained at pH = 7 to ameliorate problems associated with water dissolution. The Auger constant was found to become much more significant at higher Cr doping, in which excited-state migration leads to a substantial increase in the upconversion rate. We propose a design for a 50-W Cr:LiSAF laser system that is based on a detailed knowledge of all the relevant material parameters.

Theory of Auger upconversion in quantum wells in a quantizing magnetic field

Scattering rates of Auger and electron-phonon processes between discrete Landau levels in a quasi-two-dimensional electron system are calculated. Using these rates the authors provide an understanding of the upconversion observed in the magneto-luminescence of one-side modulation doped GaAs/AlGaAs quantum wells; in particular, of its dependence on excitation power and magnetic field.

The mechanism of Tm to Ho energy transfer in LiYF4

The energy transfer properties of the Tm, Ho:LiYF4 are examined, and special attention is given to the Tm(3F4) to Ho(5I7) transfer dynamics and equilibration. The measured transfer times are compared with the values calculated on the basis of the static Forster-Dexter and the migration-assisted Burshtein energy transfer models (which require only spectral information and rare-earth concentrations as input data). The detection of anomalously fast measured transfer rates at high Tm doping levels is interpreted within the context of percolation theory, since migration pathways from the excited Tm ion to a Ho ion must necessarily exist above a particular threshold concentration. Lastly, it is suggested that the use of low Tm concentrations below the percolation threshold may serve to minimize the Auger upconversion losses, and also will shift the excited-state equilibrium to the Ho population; this type of scenario requires that the Tm ions be pumped directly into the 3F4 state with 1.68 mu m InGaAs diode sources.