Black phase CsPbI3 is attractive for optoelectronic devices, while usually it has a high formation energy and requires an annealing temperature of above 300 °C. The formation energy can be significantly reduced by adding HI in the precursor. However, the resulting films are not suitable for light-emitting applications due to the high trap densities and low photoluminescence quantum efficiencies, and the low temperature formation mechanism is not well understood yet. Here, we demonstrate a general approach for deposition of γ-CsPbI3 films at 100 °C with high photoluminescence quantum efficiencies by adding organic ammonium cations, and the resulting light-emitting diode exhibits an external quantum efficiency of 10.4% with suppressed efficiency roll-off. We reveal that the low-temperature crystallization process is due to the formation of low-dimensional intermediate states, and followed by interionic exchange. This work provides perspectives to tune phase transition pathway at low temperature for CsPbI3 device applications.

We study the spectroscopic properties of thin films of potassium ytterbium gadolinium double tungstates, KYb_0.57Gd_0.43(WO₄)₂, and potassium ytterbium lutetium double tungstates, KYb_0.76Lu_0.24(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 KYb_0.57Gd_0.43(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.

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Low-cost, high-performance integration technologies are instrumental for active-passive integrated photonics devices. The monolithic integration of Al 2 O 3 and Si 3 N 4 is studied, enabling to combine the promising optical features of Si 3 N 4 with the excellent optical gain characteristics of rare-earth-ion doped Al 2 O 3 . The Al 2 O 3 and Si 3 N 4 layers are separated by a thin SiO 2 film and coupled by adiabatically width-tapered Al 2 O 3 and thickness-tapered Si 3 N 4 waveguides. In this paper, a detailed characterization of the couplers, as well as a study of the influence of the different design parameters and fabrication tolerances on the final device performance is presented. Test structures are characterized under transverse electric (TE) polarization. Measured loss per coupler is as low as 0.26 ± 0.03 dB at the wavelength of 1030 nm, and below 0.24 dB in the spectral window of 1460-1635 nm. Lateral misalignment of ±1 µm results in less than 0.6 dB increase of the coupler loss at 1030 nm, and the tolerance of misalignment goes up to 1.7 µm at the investigated longest wavelength of 1635 nm without introducing extra coupler losses. The reported integration technology paves the way toward a double-layer platform monolithically integrating Si 3 N 4 and rare-earth-ion doped Al 2 O 3 for active-passive photonic functionalities.

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A low-loss, broadband and high fabrication tolerant optical coupler for the monolithic integration of SiN and polymer waveguides is designed and experimentally demonstrated. The coupler is based on the adiabatic vertical tapering of the SiN waveguides. Low-loss operation is experimentally verified at both 976 and 1460-1635 nm wavelengths. Measured losses per coupler are as low as 0.12 and 0.14 dB at 976 and 1550 nm, respectively, and below 0.2 dB at both wavelengths for lateral misalignments between the SiN and polymer waveguides up to 1.0 μm.

Abstract:We report on the optical-gain properties of channel waveguides patterned into lattice-matched KGd x Lu y Er 1-x-y (WO 4 ) 2 layers grown onto undoped KY(WO 4 ) 2 substrates by liquid phase epitaxy. A systematic investigation of gain is performed for five different Er 3+ concentrations in the range of 0.75 to 10at.% and different pump powers and signal wavelengths. In pump-probe-beam experiments, relative internal gain, i.e., signal enhancement minus absorption loss of light propagating in the channel waveguide, is experimentally demonstrated, with a maximum value of 12 ± 5 dB/cm for signals at the peakemission wavelength of 1534.7 nm.

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