Kinetics of excitation transfer from Cr2+ to Fe2+ ions in co-doped ZnSe.

The transfer of electronic excitations from Cr2+ to Fe2+ ions in co-doped epitaxially grown ZnSe is studied by time-resolved photoluminescence (PL) spectroscopy with unprecedented sub-10 ns time resolution. Upon excitation of Cr2+ ions by a picosecond pulse at 2.05 µm wavelength, PL from Fe2+ ions displays a delayed onset and a retarded decay in comparison to Fe2+ PL directly excited at 3.24 µm. We measure an extremely rapid 60 ns buildup of the Fe2+ luminescence, which is followed by a slower relaxation on the few micrometer scale. The experimental results are analyzed in the framework of Förster radiationless resonant energy transfer. Directly connecting to the work of Fedorov et al. [Opt. Mater. Express9, 2340 (2019)10.1364/OME.9.002340], the 60-ns buildup time of energy transfer is found to correspond to a Cr2+-Fe2+ distance of 0.95 nm, close to the length of the space diagonal of the ZnSe unit cell. This result demonstrates a significant density of spatially correlated Cr2+-Fe2+ ion pairs at short distance, in parallel to ions with a random distribution at a larger mutual separation.

Origin of Band-Tail and Deep-Donor States in Cu2ZnSnS4 Solar Cells and Their Suppression through Sn-Poor Composition.

By comparing optical spectral results of both Sn-rich and Sn-poor Cu2ZnSnS4 (CZTS) with the previously calculated defect levels, we confirm that the band-tail states in CZTS originate from the high concentration of 2CuZn + SnZn defect clusters, whereas the deep-donor states originate from the high concentration of SnZn. In Sn-rich CZTS, the absorption, reflectance, and photocurrent (PC) spectra show band-tail states that shrink the bandgap to only ∼1.34 eV, while photoluminescence (PL) and PC spectra consistently show that abundant CuZn + SnZn donor states produce a PL peak at ∼1.17 eV and abundant SnZn deep-donor states produce a PL peak near 0.85 eV. In contrast, Sn-poor CZTS shows neither bandgap shrinking nor any deep-donor-defect induced PL and PC signals. These results highlight that a Sn-poor composition is critical for the reduction of band-tailing effects and deep-donor defects and thus the overcoming of the severe open-circuit voltage (Voc) deficiency problem in CZTS solar cells.

Dynamics of Broadband Lasing Cascade from a Single Dot-in-well InGaAs Microdisk.

The development of a fast semiconductor laser is required for the realization of next-generation telecommunication applications. Since lasers operating on quantum dot ground state transitions exhibit only limited gain due to the saturation effect, we investigate lasing from excited states and compare its corresponding static and dynamic behavior to the one from the ground state. InAs quantum dots (QDs) grown in dot-in-well (DWELL) structures allowed to obtain light emission from ground and three excited states in a spectral range of 1.0-1.3 µm. This emission was coupled to whispering gallery modes (WGMs) of a 6 µm microdisk resonator and studied at room temperature by steady-state and time-resolved micro-photoluminescence. We demonstrate a cascade development of lasing arising from the ladder of quantum dot states, and compare the lasing behavior of ground and excited state emission. While the lasing threshold is being increased from the ground state to the highest excited state, the dynamic behavior is improved: turn-on times and lifetimes of WGMs become shorter paving the way towards high frequency direct driven microlasers.

PbS:glass as broad-bandwidth near-infrared light source material.

Silicate- and borosilicate-based PbS:glass material and borosilicate-glass-based fibers are fabricated and analyzed. Optical properties including absorption and emission are characterized and related to growth and annealing conditions. In silicate glasses PbS volume fractions of exceeding 0.4 percent and almost octave-spanning emission spectra with a halfwidth of 940 nm are achieved. Fiber bundles with a core being surrounded by three PbS:Glass fibers are pulled. A confinement factor of Γ = 0.00406 is determined. Emission properties, in particular emission bandwidth, are subsequently tuned and spectrally widened by annealing fibers in a gradient furnace. The results pave the way towards optically pumped broad-bandwidth light emitters based either on 'bulk' PbS:glass or PbS:glass-based fibers.