Quantum Optoelectronics

Abstract:

Photoluminescence measurements in mono- and bilayer-MoS2 on SiO2 were undertaken to determine the thermal effect of the MoS2/SiO2 interface on the optical bandgap. The energy and intensity of the photoluminescence from monolayer MoS2 were lower and weaker than those from bilayer MoS2 at low temperatures, whilst the opposite was true at high temperatures above 200 K. Density functional theory calculations suggest that the observed optical bandgap crossover is caused by a weaker substrate coupling to the bilayer than to the monolayer.

Abstract:

We have successfully produced an ultrathin freely suspended GO film, which is a biomimetic structure inspired by the transparent dragonfly wing structure. Based on a colloidal self-assembly process over a large area, solvent evaporation was applied within a limited opening geometry. The free-standing GO film shows a significant enhancement of the nonlinear optical absorption, where saturable absorption and photoinduced absorption were observed at dramatically decreased excitation fluence compared with other work on GO films dispersed on substrates. Surprisingly, we also found that free-standing GO films are beneficial for compressing femtosecond pulses around 800 nm. Using a frequency-resolved optical gating as well as an open aperture Z-scan method, the origin was found to be associated with two effects. While the pulse shortening results from saturable absorption, the chirp effect is also suppressed due to the presence of an inflection point around 800 nm in the refractive index spectrum of free-standing GO film.

Abstract:

${\text{CoTi}}_2{\mathrm{O}}_5$ has the paradox that low temperature static magnetic order is incompatible with the crystal structure owing to a mirror plane that exactly frustrates magnetic interactions. Despite no observable structural distortion with diffraction, ${\text{CoTi}}_2{\mathrm{O}}_5$ does magnetically order below $T_N\sim 25\mathrm{K}$ with the breaking of spin ground state degeneracy proposed to be a realization of the spin Jahn-Teller effect in analogy to the celebrated orbital Jahn-Teller transition. We apply neutron and Raman spectroscopy to study the dynamics of this transition in ${\text{CoTi}}_2{\mathrm{O}}_5$ . We find anomalous acoustics associated with a symmetry breaking strain that characterizes the spin Jahn-Teller transition. Crucially, the energy of this phonon coincides with the energy scale of the magnetic excitations, and has the same symmetry of an optic mode, observed with Raman spectroscopy, which atypically softens in energy with decreasing temperature. Taken together, we propose that the energetics of the spin Jahn-Teller effect in ${\text{CoTi}}_2{\mathrm{O}}_5$ are related to cooperative magnetoelastic fluctuations as opposed to conventional soft critical dynamics which typically drive large measurable static displacements. Published by the American Physical Society 2025

Abstract:

We demonstrate a novel defect-mediated, thermally-activated emission mechanism in [(CH3)3NPh]2MnBr4 single crystals, driven by the coexistence of temperature-sensitive shallow traps and temperature-independent deep traps introduced by Br vacancies. Through comprehensive temperature-dependent photoluminescence (PL) and time-resolved PL measurements, combined with first-principles calculations, we reveal that the material exhibits exceptional thermal stability, retaining 67 % of its relative PL quantum yield at room temperature and achieving an absolute quantum yield of ∼38.9 % under optimal excitation conditions. The dual-component PL decay dynamics consist of a fast decay (∼hundreds of ps) governed by shallow traps and a long decay (∼350 μs) dominated by deep traps, creating an energy cascade that efficiently promotes radiative recombination while minimizing non-radiative losses. Our findings provide critical insights into defect-mediated, thermally-sensitive delayed emission mechanisms and establish [(CH3)3NPh]2MnBr4 as a lead-free, thermally stable material with high efficiency, making it an excellent candidate for next-generation optoelectronic applications, including solid-state lighting and temperature-sensitive devices.