Spectroscopic Properties of Sm3+-Doped Lanthanum Borogermanate Glass

Ultraviolet–visible–near infrared (UV–vis–NIR) absorption and photoluminescence of (25-x) La2O3–25B2O3–50GeO2 glass series have been studied with different concentrations (x=0.1–1.0 wt%) of Sm2O3 as an optically active dopant. The values of Judd–Ofelt (JO) parameters (Ot) follow the trend O2>O4>O6. Visible emission and decay times from the 4G5/2 level and its relative quantum efficiencies are measured. Intense reddish-orange emission corresponding to 4G5/2?6H7/2 transition has been observed in these glasses under 488 nm excitation. A decrease in the quantum yield is observed with increasing Sm3+ ion concentration beyond 1% doping level

Enhanced luminescence efficiency in Eu-doped GaN superlattice structures revealed by terahertz emission spectroscopy

Eu-doped Gallium nitride (GaN) is a promising candidate for GaN-based red light-emitting diodes, which are needed for future micro-display technologies. Introducing a superlattice structure comprised of alternating undoped and Eu-doped GaN layers has been observed to lead to an order-of-magnitude increase in output power; however, the underlying mechanism remains unknown. Here, we explore the optical and electrical properties of these superlattice structures utilizing terahertz emission spectroscopy. We find that ~0.1% Eu doping reduces the bandgap of GaN by ~40 meV and increases the index of refraction by ~20%, which would result in potential barriers and carrier confinement within a superlattice structure. To confirm the presence of these potential barriers, we explored the temperature dependence of the terahertz emission, which was used to estimate the barrier potentials. The result revealed that even a dilutely doped superlattice structure induces significant confinement for carriers, enhancing carrier recombination within the Eu-doped regions. Such an enhancement would improve the external quantum efficiency in the Eu-doped devices. We argue that the benefits of the superlattice structure are not limited to Eu-doped GaN, which provides a roadmap for enhanced optoelectronic functionalities in all rare-earth-doped semiconductor systems.Murakami F., Takeo A., Mitchell B., et al. Enhanced luminescence efficiency in Eu-doped GaN superlattice structures revealed by terahertz emission spectroscopy. Communications Materials 4, 100 (2023); https://doi.org/10.1038/s43246-023-00428-6

Multilayer aberration correction for depth-independent three-dimensional crystal growth in glass by femtosecond laser heating

Focused femtosecond lasers are known for their ability to modify transparent materials well below the surface with 3D selectivity, but spherical aberration causes degraded focal intensity and undesirable absorption conditions as focal depth increases. To eliminate such effects we have implemented an aberration correction procedure that accounts for multiple refracting layers in order to crystallize LaBGeO5 glass inside a temperature-controlled microscope stage via irradiation through a silica glass window. The correction, applied by a spatial light modulator, was effective at removing the focal depth-dependent degradation and achieving consistent heating conditions at different depths, an important consideration for patterning single-crystal architecture in 3D. Additional effects are noted, which produce a range of crystal cross-section shapes and varying degrees of partial crystallization of the melt

Stability and Charge Transfer Levels of Extrinsic Defects in LiNbO₃

The technologically important incorporation of extrinsic defects (Mg2+, Fe2+, Fe3+, Er3+, and Nd3+) in LiNbO3 is investigated using density-functional theory combined with thermodynamic calculations. Defect energies, the charge compensation mechanisms, and charge transfer levels, are determined for congruent and stoichiometric compositions. In general, under congruent (Nb2O5-rich) conditions impurities occupy lithium sites, compensated by lithium vacancies. Under stoichiometric (Li2O-rich) conditions, impurities occupy both lithium and niobium sites. The effects of the concentration of Mg on the dominant defect and site occupancy are analyzed. In addition, the thermal ionization energy and relative defect stability order for Fe2+ and Fe3+ are evaluated. The charge transfer levels of impurities with regard to the band structure, and their influences on the optical properties of the material are elucidated

Picosecond time-resolved dynamics of energy transfer between GaN and the various excited states of Eu3+ ions

To elucidate the energy transfer and reexcitation processes in Eu-doped GaN layers that are used in recently developed, highly efficient red light-emitting diodes, a systematic series of photoluminescence and time-resolved photoluminescence (TR-PL) measurements was performed. Critical insights on how “slow” Eu processes (∼µs) can compete against fast semiconductor processes (∼ps) are revealed using TR-PL with a high temporal resolution, as it is found that the initial energy transfer from GaN to the Eu3+ ions takes place rapidly, on a timescale of \u3c100 ps. Below band-gap resonant excitation was used to identify the states into which the energy transfer occurs. For the most efficient Eu defect complexes, this transfer dominantly occurs directly into the 5 D0 state of Eu3+. Less efficient complexes also exhibit transfer into the 5 D2 state, the emission of which can be detected using photoluminescence at low temperature, indicating the importance of the excitation pathway on device efficiency. Under high excitation intensity, reexcitation can also occur, leading to a redistribution of population into the 5 D2, 5 D1, or 5 D0 states

Enhancement of Radiative Efficiency with Staggered InGaN Quantum Well Light Emitting Diodes

The technology on the large overlap InGaN QWs developed in this program is currently implemented in commercial technology in enhancing the internal quantum efficiency in major LED industry in US and Asia. The scientific finding from this work supported by the DOE enabled the implementation of this step-like staggered quantum well in the commercial LEDs

Direct laser-writing of ferroelectric single-crystal waveguide architectures in glass for 3D integrated optics

Direct three-dimensional laser writing of amorphous waveguides inside glass has been studied intensely as an attractive route for fabricating photonic integrated circuits. However, achieving essential nonlinear-optic functionality in such devices will also require the ability to create high-quality single-crystal waveguides. Femtosecond laser irradiation is capable of crystallizing glass in 3D, but producing optical-quality single-crystal structures suitable for waveguiding poses unique challenges that are unprecedented in the field of crystal growth. In this work, we use a high angular-resolution electron diffraction method to obtain the first conclusive confirmation that uniform single crystals can be grown inside glass by femtosecond laser writing under optimized conditions. We confirm waveguiding capability and present the first quantitative measurement of power transmission through a laser-written crystal-in-glass waveguide, yielding loss of 2.64 dB/cm at 1530 nm. We demonstrate uniformity of the crystal cross-section down the length of the waveguide and quantify its birefringence. Finally, as a proof-of-concept for patterning more complex device geometries, we demonstrate the use of dynamic phase modulation to grow symmetric crystal junctions with single-pass writing