Concentration quenching of luminescence lifetime in ytterbium-doped potassium double tungstate waveguide amplifiers

Integrated optical amplifiers employing ytterbium-doped potassium double tungstates exhibit an ultra-high peak gain exceeding 1000 dB/cm, in addition to a broad gain of up to 150 dB/cm over a 55-nm wavelength range. Here we report a study of the luminescence lifetime in samples spanning a broad range of dopant concentrations (1.2−57.5 at.%). By use of the pinhole method, elongation of the luminescence lifetime due to radiation trapping is avoided, providing a more accurate analysis of concentration quenching of the luminescence lifetime, which directly influences the waveguide amplifier performance and can, in principle, inhibit scaling of the gain with increasing dopant concentration

Highly Yb-doped KGd(WO₄)₂ thin-film amplifier

We report record-high small-signal gain of 1050 dB/cm at 981 nm wavelength in a KGd0.425Yb0.575(WO4)2 thin film. The sensitivity of gain to the shift of beam-focus position, which is critical under non-waveguiding conditions, is investigated

Spiral amplifiers in a-Al₂O₃:Er on a silicon chip with 20 dB internal net gain

Spiral-waveguide amplifiers in erbium-doped amorphous aluminum oxide are fabricated by RF reactive co-sputtering of 1-µm-thick layers onto a thermally-oxidized silicon wafer and chlorine-based reactive ion etching. The samples are overgrown by a SiO2 cladding. Spirals with several lengths ranging from 13 cm to 42 cm and four different erbium concentrations between 0.5-3.0×10^20 cm^-3 are experimentally characterized. A maximum internal net gain of 20 dB in the small-signal-gain regime is measured at the peak emission wavelength of 1532 nm for two sample configurations with waveguide lengths of 13 cm and 24 cm and erbium concentrations of 2×10^20 cm^-3 and 1×10^20 cm^-3, respectively. The obtained gain improves previous results by van den Hoven et al. in this host material by a factor of 9. Gain saturation as a result of increasing signal power is investigated. Positive net gain is measured in the saturated-gain regime up to ~100 µW of signal power, but extension to the mW regime seems feasible. The experimental results are compared to a rate-equation model that takes into account migration-accelerated energy-transfer upconversion (ETU) and a fast quenching process affecting a fraction of the erbium ions. Without these two detrimental processes, several tens of dB/cm of internal net gain per unit length would be achievable. Whereas ETU limits the gain per unit length to 8 dB/cm, the fast quenching process further reduces it to 2 dB/cm. The fast quenching process strongly deteriorates the amplifier performance of the Al2O3:Er3+ waveguide amplifiers. This effect is accentuated for concentrations higher than 2×10^20 cm^-3

Exploring the transferability of large supramolecular assemblies to the vacuum-solid interface

We present an interplay of high-resolution scanning tunneling microscopy imaging and the corresponding theoretical calculations based on elastic scattering quantum chemistry techniques of the adsorption of a gold-functionalized rosette assembly and its building blocks on a Au(111) surface with the goal of exploring how to fabricate functional 3-D molecular nanostructures on surfaces. The supramolecular rosette assembly stabilized by multiple hydrogen bonds has been sublimed onto the Au(111) surface under ultra-high vacuum conditions; the resulting surface nanostructures are distinctly different from those formed by the individual molecular building blocks of the rosette assembly, suggesting that the assembly itself can be transferred intact to the surface by in situ thermal sublimation. This unanticipated result will open up new perspectives for growth of complex 3-D supramolecular nanostructures at the vacuum-solid interface

Direct confocal lifetime measurements on rare-earth-doped media exhibiting radiation trapping

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

Temperature-dependent absorption and emission of potassium double tungstates with high ytterbium content

We study the spectroscopic properties of thin films of potassium ytterbium gadolinium double tungstates, KYb0.57Gd0.43(WO4)2, and potassium ytterbium lutetium double tungstates, KYb0.76Lu0.24(WO4)2, specifically at the central absorption line near 981 nm wavelength, which is important for amplifiers and lasers. The absorption cross-section of both thin films is found to be similar to those of bulk potassium rare-earth double tungstates, suggesting that the crystalline layers retain their spectroscopic properties albeit having >50 at.% Yb3+ concentration. The influence of sample temperature is investigated and found to substantially affect the measured absorption cross-section. Since amplifiers and lasers typically operate above room temperature due to pump-induced heating, the temperature dependence of the peak-absorption cross-section of the KYb0.57Gd0.43(WO4)2 is evaluated for the sample being heated from 20 °C to 170 °C, resulting in a measured reduction of peak-absorption cross-section at the transitions near 933 nm and 981 nm by ~40% and ~52%, respectively. It is shown that two effects, the change of Stark-level population and linewidth broadening due to intra-manifold relaxation induced by temperature-dependent electron-phonon interaction, contribute to the observed behavior. The effective emission cross-sections versus temperature have been calculated. Luminescence-decay measurements show no significant dependence of the luminescence lifetime on temperature

A low-loss and broadband MMI-based multi/demultiplexer in Si3N4/SiO2 technology

A low-loss and broadband multimode interference (MMI)-based wavelength multi/demultiplexer in Si3N4/SiO2 technology for erbium-doped lasing and amplifying applications is presented. The structural parameters of a 2 × 1 Si3N4 MMI multi/demultiplexer are optimized to minimize losses. The design and analysis of the MMI multi/demultiplexer are carried out using a hybrid approach, which combines a modified effective index method, the 2D film mode matching method, and the 2D beam propagation method, with lower impact in the computing requirements and simulation time than 3D methods. Simulated total losses of 0.19 and 0.23 dB at 980 and 1550 nm, respectively were obtained for the optimized MMI multi/demultiplexer. The measurements of our fabricated couplers, with 110 nm thick Si3N4 layer, show good agreement with our design. As multiplexers, the average losses of the MMI were measured to be 0.4 ± 0.3 dB for both 976 and 1550 nm wavelengths, and less than 1 dB across the whole C-band. As demultiplexers, the measured average extinction ratio of the fabricated MMI was found to be 21.4 ± 1.2 and 26.3 ± 0.8 dB for pump and signal wavelengths, respectively