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Investigation of Non-linear Energy Transfer Dynamics of Erbium in Yttrium Aluminum Garnet

Christian Vega

Published Jan 1, 2018• 0 citations
OpenAlex

Abstract

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.

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Yttrium Aluminum Garnet (YAG) 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.The non-linear energy transfer dynamics of Er:YAG are modeled under high resonant pump conditions. Specifically the multi-photon effect known as excited state absorption (ESA) is modeled by measuring the cross section in a single crystal Er:YAG sample utilizing a pump and probe technique. The measured ESA cross section is then included in the rate equation modeling that has typically been ignored such an effect when modeling the non-linear ion to ion interactions. By pumping with selective resonant pumps, the interaction dynamics of the upper and the lower laser levels in the Erbium ion reveal the contributions of the non-linear energy transfer mechanisms towards the populations of the levels respectively. This thesis work shows the added accuracy of the rate equation modeling by its inclusion of the measured ESA cross-section and its effect on the population evolution in the Er:YAG system. The model will be tested with measurements made with select Er:YAG crystals being pumped by combinations of resonant infra-red lasers with a chopper spinning at a rate on the order of magnitude of the lower lasing level lifetime.

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This research is an outgrowth of earlier experiments that demonstrated bistable luminescence in heavy metal halide crystals doped with trivalent ytterbium ions. This type of instability has importance as a fundamentally new physical phenomenon with a potential application for fast all-optical switching as well as a limitation on compact solid state laser performance. In this thesis, the investigation of luminescent instabilities is extended to bistable energy transfer processes in crystals and to the observation of bistable blackbody emission in rare earth nanopowders. High resolution laser spectroscopy was used to study bistable luminescence and energy transfer in Yb,Er:CsCdBr3 crystals at cryogenic temperatures. For the first time, it was found that bistable behavior associated with Yb 3+ ions was transferred to Er 3+ through resonant energy transfer. Bistability of the resulting sensitized luminescence caused sufficiently dramatic changes in the crystal dynamics so as to change the color of emission from yellow to green. This color changing phenomenon is fully explained in the present work and is referred to as chromatic switching. Temperature is a critical variable that is known to govern luminescent instabilities in all current theories. Therefore, in a search for new systems with luminescent instabilities at high temperatures, materials with extreme thermal properties were investigated as part of this research. Yb,Er:Y 2O3 nanopowders were selected for this purpose. Nanopowders exhibit greatly reduced thermal conductivity and were verified during the course of this work to cause enhanced absorption as the result of multiple scattering. Significant spectral differences between Yb,Er:Y2O 3 nanopowders and single crystals also emerged. Measurements of erbium upconversion luminescence versus pump intensity in resonance with the ytterbium absorption transition revealed striking new optical phenomena: strong luminescent quenching, intense bistable blackbody emission, and the formation of single crystal micro-tubes directly from powder samples. A simple theoretical model was successfully developed to explain all these optical effects by a detailed balance approach to thermal transport that explicitly included erbium and ytterbium atomic level occupation probabilities, blackbody radiation and radiation trapping. Calculations of nanopowder sample luminescence accurately reproduced experimentally observed quenching, the onset of blackbody emission, bistability and details of hysteresis loops. The model also predicted that absorbed powers of less than 15 mW could melt yttria nanopowders (melting point = 2410°C), again in agreement with experiments. This work is expected to enable new approaches to laser processing of ceramics and laser machining of reflective metals, notably aluminum, using low power lasers.

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Stimulated emission from Er3+ions in yttrium aluminum garnet crystals at λ = 2.94 μ

Evgeny V. Zharikov, V I Zhekov, L. A. Kulevskiǐ +7 more

Stimulated emission due to the 4 I 11/2 – 4 I 13/2 transition (λ = 2.936 μ) in yttrium aluminum garnet (Y 3 Al 5 O 12 ) doped with Er 3+ ions (30 wt%) was observed for the first time at 300°K. The output energy from crystals of 50 × 3 mm dimensions was 0.3 J under free-oscillation conditions and 0.05 J in the case of Q switching. The lifetime of the 4 I 11/2 level was determined for crystals with 3–30 wt% Er 3+ . The energy from the higher Er 3+ levels relaxed in a cascade manner through 4 I 11/2 to 4 I 13/2 .

Soviet Journal of Quantum Electronics•Aug 31, 1975•107 citations
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.
Yttrium Aluminum Garnet (YAG) 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.The non-linear energy transfer dynamics of Er:YAG are modeled under high resonant pump conditions. Specifically the multi-photon effect known as excited state absorption (ESA) is modeled by measuring the cross section in a single crystal Er:YAG sample utilizing a pump and probe technique. The measured ESA cross section is then included in the rate equation modeling that has typically been ignored such an effect when modeling the non-linear ion to ion interactions. By pumping with selective resonant pumps, the interaction dynamics of the upper and the lower laser levels in the Erbium ion reveal the contributions of the non-linear energy transfer mechanisms towards the populations of the levels respectively. This thesis work shows the added accuracy of the rate equation modeling by its inclusion of the measured ESA cross-section and its effect on the population evolution in the Er:YAG system. The model will be tested with measurements made with select Er:YAG crystals being pumped by combinations of resonant infra-red lasers with a chopper spinning at a rate on the order of magnitude of the lower lasing level lifetime.
This research is an outgrowth of earlier experiments that demonstrated bistable luminescence in heavy metal halide crystals doped with trivalent ytterbium ions. This type of instability has importance as a fundamentally new physical phenomenon with a potential application for fast all-optical switching as well as a limitation on compact solid state laser performance. In this thesis, the investigation of luminescent instabilities is extended to bistable energy transfer processes in crystals and to the observation of bistable blackbody emission in rare earth nanopowders. High resolution laser spectroscopy was used to study bistable luminescence and energy transfer in Yb,Er:CsCdBr3 crystals at cryogenic temperatures. For the first time, it was found that bistable behavior associated with Yb 3+ ions was transferred to Er 3+ through resonant energy transfer. Bistability of the resulting sensitized luminescence caused sufficiently dramatic changes in the crystal dynamics so as to change the color of emission from yellow to green. This color changing phenomenon is fully explained in the present work and is referred to as chromatic switching. Temperature is a critical variable that is known to govern luminescent instabilities in all current theories. Therefore, in a search for new systems with luminescent instabilities at high temperatures, materials with extreme thermal properties were investigated as part of this research. Yb,Er:Y 2O3 nanopowders were selected for this purpose. Nanopowders exhibit greatly reduced thermal conductivity and were verified during the course of this work to cause enhanced absorption as the result of multiple scattering. Significant spectral differences between Yb,Er:Y2O 3 nanopowders and single crystals also emerged. Measurements of erbium upconversion luminescence versus pump intensity in resonance with the ytterbium absorption transition revealed striking new optical phenomena: strong luminescent quenching, intense bistable blackbody emission, and the formation of single crystal micro-tubes directly from powder samples. A simple theoretical model was successfully developed to explain all these optical effects by a detailed balance approach to thermal transport that explicitly included erbium and ytterbium atomic level occupation probabilities, blackbody radiation and radiation trapping. Calculations of nanopowder sample luminescence accurately reproduced experimentally observed quenching, the onset of blackbody emission, bistability and details of hysteresis loops. The model also predicted that absorbed powers of less than 15 mW could melt yttria nanopowders (melting point = 2410°C), again in agreement with experiments. This work is expected to enable new approaches to laser processing of ceramics and laser machining of reflective metals, notably aluminum, using low power lasers.
Characteristics of ytterbium to erbium energy transfer in Yb:Er:phosphate glasses have been studied for a case when Nd phosphate glass laser is used as a pumping source. A theoretical model of the energy transfer was developed to reproduce the experimentally observed population dynamics of the Er/sup 3+/ upper and lower laser levels. The results show that saturation and frequency hole burning of the inhomogeneously broadened Yb/sup 3+/ absorption line are of primary importance in determining the dynamics and efficiency of the Yb/sup 3+/-Er/sup 3+/ energy transfer under Nd laser pumping. The influence of other loss channels that have been identified in previous studies is found to be relatively small. The model also yields the lifetime of the frequency hole, as well as the effective rates of energy transfer between groups of Yb/sup 3+/ and Er/sup 3+/ ions in phosphate glasses.>
Negative nonlinear absorption, which causes the transmission to decrease as the laser intensity increases, was observed in a 16 at. % Er:YAG crystal using a laser diode operating at 787 and 784 nm. This effect for 787 nm was strong because 787 nm was the center of an absorption peak that is related to the excited-state absorption from 4I13/2 to 2H11/2. It is suggested that these phenomena take place because the absorption cross section from 4I13/2 to 2H11/2 is larger than that from 4I15/2 to 4I9/2 using a double-absorption model.
Temperature dependence of an enhanced excited state absorption was investigated in erbium-doped garnet crystals using 787.3 and 787.8 nm laser diodes. The enhanced absorption was obtained at temperature ranges of 15–570 K, 15–420 K, and 15–1200 K in a yttrium aluminum garnet, a yttrium scandium gallium garnet, and a lutetium aluminum garnet, respectively. Absorption peaks corresponding to both the I1615/2−I49/2 and the I1613/2−I211/2 transitions of Er3+ in garnets were shifted from those for 300 K at high temperature. The enhanced absorption weakened at high temperature because an absorption cross section from the I413/2 level to the H211/2 level decreased due to the shift of the absorption peaks. It has been confirmed that a mechanism of the enhanced absorption can be explained by calculating the rate equations based on an analytical model for a five-level system of the Er3+ ion. It is suggested that the enhanced absorption weakened at temperatures lower than 12 K because the number of phonons decreased due to low temperature.
Stimulated emission due to the 4 I 11/2 – 4 I 13/2 transition (λ = 2.936 μ) in yttrium aluminum garnet (Y 3 Al 5 O 12 ) doped with Er 3+ ions (30 wt%) was observed for the first time at 300°K. The output energy from crystals of 50 × 3 mm dimensions was 0.3 J under free-oscillation conditions and 0.05 J in the case of Q switching. The lifetime of the 4 I 11/2 level was determined for crystals with 3–30 wt% Er 3+ . The energy from the higher Er 3+ levels relaxed in a cascade manner through 4 I 11/2 to 4 I 13/2 .