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Journal of the Optical Society of America B

Gain dynamics in a highly ytterbium-doped potassium double tungstate epitaxial layer

Yean-Sheng Yong, S. Aravazhi, Sergio Andrés Vázquez-Córdova, Jennifer L. Herek, Sonia M. García‐Blanco, Markus Pollnau

Published Aug 17, 2018• 7 citations
Publisher

Abstract

Active media with high rare-earth concentrations are essential for small-footprint waveguide amplifiers. When operating at high population inversion, such devices are often affected by undesired energy-transfer processes and thermal effects. In this work, we study a 32-μm-thick epitaxial layer of KGd0.43Yb0.57(WO4)2, representing an Yb3+ concentration of ∼3.8×1021 cm−3, grown on an undoped KY(WO4)2 substrate. The pump absorption, luminescence decay, and small-signal gain are investigated under intense pumping conditions. Spectroscopic signatures of an energy-transfer process and of quenched ions, as well as thermal effects, are observed. We present a gain model which takes into account excessive heat generated due to the abovementioned experimental observations. Based on finite-element calculations, we find that the net gain is significantly reduced due to, first, a fraction of Yb3+ ions not contributing to stimulated emission, second, a reduction of population inversion owing to a parasitic energy-transfer process and, third, degradation of the effective transition cross-sections owing to device heating. Nevertheless, a signal enhancement of 8.1 dB was measured from the sample at 981 nm wavelength when pumping at 932 nm. The corresponding signal net gain of ∼800 dB/cm, which was achieved without thermal management, is promising for a waveguide amplifier operating without active cooling.

DOI
10.1364/josab.35.002176

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Active media with high rare-earth concentrations are essential for small-footprint waveguide amplifiers. When operating at high population inversion, such devices are often affected by undesired energy-transfer processes and thermal effects. In this work, we study a 32-μm-thick epitaxial layer of potassium gadolinium ytterbium double tungstate with a high Yb content of 57at.%, representing an Yb3+ concentration of ~3.8 × 1021 per cubic centimeter, grown onto an un-doped KY(WO4)2 substrate. The pump absorption, luminescence decay, and small-signal gain are investigated under intense pumping conditions. Spectroscopic signatures of an energy-transfer process and of quenched ions, as well as thermal effects are observed. We present a gain model which takes into account excessive heat generated due to the abovementioned experimental observations. Based on finite-element calculations, we find that the net gain is significantly reduced due to, firstly, a fraction of Yb3+ ions not contributing to stimulated emission, secondly, a reduction of population inversion owing to a parasitic energy-transfer process and, thirdly, degradation of the effective transition cross-sections owing to device heating. Nevertheless, a signal enhancement of 8.1 dB was measured from the sample at 981 nm wavelength when pumping at 932 nm. The corresponding signal net gain of ~800 dB/cm, which was achieved without thermal management, is promising for waveguide amplifier operating without active cooling.

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Yttrium Aluminum Garnet (YAG, Y_3 〖Al〗_5 O_12) crystals doped with Erbium have posed an interesting position in the field of rare earth solid state lasers as they possess the property of self-saturation, in which the upper energy level in a laser has a much shorter lifetime than that of the lower level. This is the case of the 2.94μm transition in Er:YAG. That property at first seems to make this material unfit for use as a laser gain medium, however further research into these classes of rare earth materials revealed interesting non-liner energy transfer mechanisms that allow it to be a useful mid wave infra-red (MWIR) coherent source in the continuous wave (CW) and pulsed regimes. This came to fruition due to decades of modeling and spectroscopic research investigating the non-linear energy transfer mechanisms of excited state absorption (ESA), energy transfer up-conversion (ETU), and cross relaxation (XR). These effects show up in the rise and fall times of the levels observed through fluorescence. Resulting in the non-exponential rise and fall characteristics and having a squared or even cubed relation to the population. The population evolution for each of the lasing levels is now affected by ions recycling energy and in turn cause the lifetimes of the levels to “effectively” change to a point where simple linear models do not adequately describe the system and its performance. The non-linear energy transfer dynamics of Er:YAG are modeled under high resonant pump conditions. By pumping with selective resonant pumps, the interaction dynamics of the (_ ^4)I_(11/2) and the (_ ^4)I_(13/2) levels in the Erbium ion reveal the contributions from generally ignored non-linear energy transfer mechanisms. Specifically, the multi-photon effect known as excited state absorption (ESA) is modeled by measuring and characterizing the cross section in single crystal Er:YAG samples utilizing a pump and probe technique coupled with transient fluorescence measurements. The measured ESA cross section is then included in the rate equation modeling.

•Jan 1, 2018•0 citations
Active media with high rare-earth concentrations are essential for small-footprint waveguide amplifiers. When operating at high population inversion, such devices are often affected by undesired energy-transfer processes and thermal effects. In this work, we study a 32-μm-thick epitaxial layer of KGd0.43Yb0.57(WO4)2, representing an Yb3+ concentration of ∼3.8×1021 cm−3, grown on an undoped KY(WO4)2 substrate. The pump absorption, luminescence decay, and small-signal gain are investigated under intense pumping conditions. Spectroscopic signatures of an energy-transfer process and of quenched ions, as well as thermal effects, are observed. We present a gain model which takes into account excessive heat generated due to the abovementioned experimental observations. Based on finite-element calculations, we find that the net gain is significantly reduced due to, first, a fraction of Yb3+ ions not contributing to stimulated emission, second, a reduction of population inversion owing to a parasitic energy-transfer process and, third, degradation of the effective transition cross-sections owing to device heating. Nevertheless, a signal enhancement of 8.1 dB was measured from the sample at 981 nm wavelength when pumping at 932 nm. The corresponding signal net gain of ∼800 dB/cm, which was achieved without thermal management, is promising for a waveguide amplifier operating without active cooling.
Active media with high rare-earth concentrations are essential for small-footprint waveguide amplifiers. When operating at high population inversion, such devices are often affected by undesired energy-transfer processes and thermal effects. In this work, we study a 32-μm-thick epitaxial layer of potassium gadolinium ytterbium double tungstate with a high Yb content of 57at.%, representing an Yb3+ concentration of ~3.8 × 1021 per cubic centimeter, grown onto an un-doped KY(WO4)2 substrate. The pump absorption, luminescence decay, and small-signal gain are investigated under intense pumping conditions. Spectroscopic signatures of an energy-transfer process and of quenched ions, as well as thermal effects are observed. We present a gain model which takes into account excessive heat generated due to the abovementioned experimental observations. Based on finite-element calculations, we find that the net gain is significantly reduced due to, firstly, a fraction of Yb3+ ions not contributing to stimulated emission, secondly, a reduction of population inversion owing to a parasitic energy-transfer process and, thirdly, degradation of the effective transition cross-sections owing to device heating. Nevertheless, a signal enhancement of 8.1 dB was measured from the sample at 981 nm wavelength when pumping at 932 nm. The corresponding signal net gain of ~800 dB/cm, which was achieved without thermal management, is promising for waveguide amplifier operating without active cooling.
Radiation trapping occurs in rare-earth-doped active media with strong spectral overlap of luminescence and ground-state absorption. It is demonstrated experimentally that a confocal measurement mitigates the influence of radiation trapping on the measured luminescence lifetime, hence allowing for direct extraction of the lifetime from the measured decay curves. The radiation trapping effect is largely suppressed by probing a small sample volume and rejecting the photons reemitted from the unpumped region. This non-destructive measurement method is applied to ytterbium (Yb3+) activated potassium double tungstate crystalline layers with Yb3+ concentrations ranging from 1.2 at.% up to 76 at.% (~8 × 1019 - 5 × 1021 cm-3). The measured lifetime values are comparable to the results reported for Yb3+-doped potassium double tungstate powder diluted in liquid.
Summary form only given: Amplification in rare-earth doped solid state planar optical structures has attracted a lot of interest in recent years due to the fact that the rare earth ions show sharp absorption and emission bands and relatively long luminescence lifetimes. In particular the erbium (Er) ion is very attractive because of its intra 4f-transition from the first excited state to the ground state around 1.53 mum (4I13/2-4I15/2). The main problem of the erbium doped waveguide amplifiers (EDWAs) is that high values of the Er3+concentration are required to reach high gain figures in a compact length, but in this way the concentration quenching effects become very considerable and detrimental to the optical gain, since they reduce the population in the first excited state. A good solution to overcome this dramatic drawback is to consider erbium doped photonic crystal amplifier (EDPCA). In fact the small group velocity of electromagnetic eigenmodes brings about the enhancement of stimulated emission if the photonic crystals are doped by active media as the rare earth ions E3+r or Yt3+. This phenomenon originates from the long interaction time between radiation field and the matter caused by the small group velocity near the edge of the photonic band gap. The efficient confinement of light is achievable not only by tuning the input signal at the band edge of the gap of the photonic crystal but it is also obtainable by introducing local defects inside photonic crystals. The density of electromagnetic states inside a cavity is significantly modified and the spontaneous emission of atoms in a cavity can be either enhanced or inhibited. Erbium doped silica-titania one-dimensional photonic crystal microcavities have been analyzed by solving the erbium rate equations and the Maxwell equations by means of an auxiliary differential equation finite difference time domain technique. Since the performance of optical amplifiers depends on the populations of the energy levels participating in radiative transitions, determined principally by the distribution and concentration of the rare-earth ions and by the pumping conditions, the relations between these parameters and the transmittance of the EDPCMA have been studied
Yb:Er: codoped CaF2 monocrystalline films have been grown by molecular beam epitaxy (MBE) using YbF3, ErF3, and CaF2 evaporation on CaF2 substrates. Luminescence and excitation of luminescence at 25 K are measured in order to investigate the role of Yb3+ codoping and the effect of the growth conditions during the MBE process on the incorporation of active rare-earth species in CaF2. It is found, from a comparison with earlier work on singly doped thin films, that the energies of the levels of Er3+ ions are not modified by the double doping, whereas mixed complex centers are formed. Growth conditions are found to be important controlling factors for the incorporation of Yb3+ as a sensitizer, and, consequently, for the transfer efficiency. From excitation and absorption spectra it is shown that the lowest component of the Er:4I11/2 manifold has an energy higher than that of Yb:2F5/2 and the consequences are discussed.
The enhanced laser cooling performance of rare-earth-ion-doped nanocrystalline powders is predicted, compared to the bulk material, using Yb3+:Y2O3{\mathrm{Yb}}^{3+}:{\mathrm{Y}}_{2}{\mathrm{O}}_{3}Yb3+:Y2​O3​ as the model material. This is achieved by enhancing the off-resonance, phonon-assisted absorption, which is proportional to the three factors considered in this paper: the dopant concentration, the pumping field energy, and the excitation coefficient. Using the energy transfer theory for concentration quenching, the optimum concentration corresponding to the maximum cooling power is found to be considerably larger than the currently used value, suggesting noticeable enhancement effects for laser cooling. The pumping field energy is enhanced in random nanopowders compared with bulk crystals under the same irradiation, due to the multiple scattering of photons. Photons are thus localized in the medium and do not propagate through, increasing the photon absorption of the pumping beam (and it is shown that the reabsorption of the fluorescence is negligible). Using molecular dynamics simulations, the phonon density of states (DOS) of the nanopowder is calculated, and found to have broadened modes, and extended small tails at low and high frequencies. The second-order electronic transition rate for the anti-Stokes luminescence is calculated using the Fermi golden rule, which includes the influence of this phonon DOS, and is shown to have enhancement effects on the laser cooling efficiency using nanopowders. It is finally concluded that these three enhancement mechanisms are essentially to increase the population of the three participating carriers (electron, photon, and phonon) in the interacting volume, and this also points out directions for enhancing laser cooling performance in bulk materials.
Yttrium Aluminum Garnet (YAG, Y_3 〖Al〗_5 O_12) crystals doped with Erbium have posed an interesting position in the field of rare earth solid state lasers as they possess the property of self-saturation, in which the upper energy level in a laser has a much shorter lifetime than that of the lower level. This is the case of the 2.94μm transition in Er:YAG. That property at first seems to make this material unfit for use as a laser gain medium, however further research into these classes of rare earth materials revealed interesting non-liner energy transfer mechanisms that allow it to be a useful mid wave infra-red (MWIR) coherent source in the continuous wave (CW) and pulsed regimes. This came to fruition due to decades of modeling and spectroscopic research investigating the non-linear energy transfer mechanisms of excited state absorption (ESA), energy transfer up-conversion (ETU), and cross relaxation (XR). These effects show up in the rise and fall times of the levels observed through fluorescence. Resulting in the non-exponential rise and fall characteristics and having a squared or even cubed relation to the population. The population evolution for each of the lasing levels is now affected by ions recycling energy and in turn cause the lifetimes of the levels to “effectively” change to a point where simple linear models do not adequately describe the system and its performance. The non-linear energy transfer dynamics of Er:YAG are modeled under high resonant pump conditions. By pumping with selective resonant pumps, the interaction dynamics of the (_ ^4)I_(11/2) and the (_ ^4)I_(13/2) levels in the Erbium ion reveal the contributions from generally ignored non-linear energy transfer mechanisms. Specifically, the multi-photon effect known as excited state absorption (ESA) is modeled by measuring and characterizing the cross section in single crystal Er:YAG samples utilizing a pump and probe technique coupled with transient fluorescence measurements. The measured ESA cross section is then included in the rate equation modeling.