Recently, unconventional bright magnetic dipole (MD) radiation
was observed from two-dimensional (2D) hybrid organic–inorganic
perovskites (HOIPs). According to commonly accepted HOIP band structure
calculations, such MD light emission from the ground-state exciton
should be strictly symmetry forbidden. These results suggest that
MD emission arises in conjunction with an as-yet unidentified symmetry-breaking
mechanism. In this paper, we show that MD light emission originates
from a self-trapped p-like exciton stabilized at energies below the
primary electric dipole (ED)-emitting 1s exciton. Using suitable combinations
of sample and collection geometries, we isolate the distinct temperature-dependent
properties of the ED and MD photoluminescence (PL). We show that the
ED emission wavelength is nearly constant with temperature, whereas
the MD emission wavelength exhibits substantial red shifts with heating.
To explain these results, we derive a microscopic model comprising
two distinct parity exciton states coupled to lattice distortions.
The model explains many experimental observations, including the thermal
red shift, the difference in emission wavelengths, and the relative
intensities of the ED and MD emission. Thermodynamic analysis of temperature-dependent
PL reveals that the MD emission originates from a locally distorted
structure. Finally, we demonstrate unusual hysteresis effects of the
MD-emitting state near structural phase transitions. We hypothesize
that this is another manifestation of the local distortions, indicating
that they are insensitive to phase changes in the equilibrium lattice
structure.
Light-matter interactions in semiconductor systems are uniformly treated within the electric dipole (ED) approximation, as multipolar interactions are considered "forbidden". Here, we demonstrate that this approximation inadequately describes light emission in novel two-dimensional hybrid organic-inorganic perovskite materials (2D HOIPs) -a class of solution processable layered semiconductor with promising optoelectronic properties. Consequently, photoluminescence (PL) spectra become strongly dependent on the experimental geometry, a fact that is often overlooked, though critical for correct optical characterization of materials. Using energy-momentum and time-resolved spectroscopies, we experimentally demonstrate that low-energy sideband emission in 2D HOIPs exhibits a highly unusual, multipolar polarization and angle dependence. Using combined electromagnetic and quantum-mechanical analyses, we attribute this radiation pattern to an out-of-plane oriented magnetic dipole transition arising from the 2D character of the excited and ground state orbitals. Symmetry arguments point toward the presence of significant inversion symmetry-breaking mechanisms that are currently under great debate. These results provide a new perspective on the origins of unexpected sideband emission in HOIPs, clarify discrepancies in previous literature, and generally challenge the paradigm of ED-dominated light-matter interactions in novel optoelectronic materials. 1 arXiv:1901.05136v2 [cond-mat.mtrl-sci]
Coherent optical links are becoming increasingly attractive for intra-data center applications as data rates scale. Realizing the era of high-volume short-reach coherent links will require substantial improvements in transceiver cost and power efficiency, necessitating a reassessment of conventional architectures best-suited for longer-reach links and a review of assumptions for shorter-reach implementations. In this work, we analyze the impact of integrated semiconductor optical amplifiers (SOAs) on link performance and power consumption, and describe the optimal design spaces for low-cost and energy-efficient coherent links. Placing SOAs after the modulator provide the most energy-efficient link budget improvement, up to 6 pJ/bit for large link budgets, despite any penalties from nonlinear impairments. Increased robustness to SOA nonlinearities makes QPSK-based coherent links especially attractive, and larger supported link budgets enable the inclusion of optical switches, which could revolutionize data center networks and improve overall energy efficiency.
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