First-principles identification of VI+Cui defect cluster in cuprous iodide: origin of red light photoluminescence

The γ-phase Cuprous Iodide (CuI) emerges as a promising transparent p-type semiconductor for next-generation display technology because of its wide direct band gap, intrinsic p-type conductivity, and high carrier mobility. Two main peaks are observed in its photoluminescence (PL). One is short wavelength (410-430 nm) emission, which is well attributed to the electronic transitions at Cu vacancy, whereas the other long wavelength emission (680-720 nm) has not been fully understood. In this paper, through first-principles simulations, we investigate the formation energies and emission line shape for various defects, and discover that the intrinsic point defect cluster V_I+Cu_i^(2+) is the source of the long wavelength emission. Our finding is further supported by the prediction that the defect concentration decreases dramatically as the chemical condition changes from Cu-rich to I-rich, explaining the significant reduction in the red light emission if CuI is annealed in abundant I environment.

Exceptionally bright optical emission from a rare and distant gamma-ray burst

Long gamma-ray bursts (GRBs) are produced by the dissipation of ultra-relativistic jets launched by newly-born black holes after the collapse of massive stars. Right after the luminous and highly variable gamma-ray emission, the multi-wavelength afterglow is released by the external dissipation of the jet in circumburst medium. We report the discovery of a very bright (10 mag) optical emission 28 s after the explosion of the extremely luminous and energetic GRB 210619B located at redshift 1.937. Early multi-filter observations allowed us to witness the end of the shock wave propagation into the GRB ejecta. We observed the spectral transition from a bright reverse to the forward shock emission, demonstrating that the early and late GRB multi-wavelength emission is originated from a very narrow jet propagating into an unusually rarefied interstellar medium. We also find evidence of an additional component of radiation, coming from the jet wings which is able explain the uncorrelated optical/X-ray emission.

Electrically tunable guided mode resonance grating for switchable photoluminescence

<p>We present a guided mode resonance grating based on the incorporation of an electro-optic material with monolayer WS₂. The grating is designed to exhibit highly selective directional photo-luminescent emission. We study the effect of doubling the grating period via the introduction of an alternating index perturbation. Using numerical simulations, we show that period doubling leads to formation of a photonic band gap and spectral splitting in the absorptivity (or emissivity) spectrum. We anticipate that this effect can either be used to switch on and off the emissivity at a fixed wavelength, or toggle between single- and double-wavelength emission.</p>

Electrically tunable guided mode resonance grating for switchable photoluminescence

<p>We present a guided mode resonance grating based on the incorporation of an electro-optic material with monolayer WS₂. The grating is designed to exhibit highly selective directional photo-luminescent emission. We study the effect of doubling the grating period via the introduction of an alternating index perturbation. Using numerical simulations, we show that period doubling leads to formation of a photonic band gap and spectral splitting in the absorptivity (or emissivity) spectrum. We anticipate that this effect can either be used to switch on and off the emissivity at a fixed wavelength, or toggle between single- and double-wavelength emission.</p>

Multi-wavelength observations of the Blazar 4C +28.07

The active galactic nucleus 4C +28.07 is a flat spectrum radio quasar, one of the brightest at γ-ray energies. We study its multi-wavelength emission by analysing ∼12.3 years of Fermi-LAT data in the γ-ray band and Swift-XRT/UVOT available data in X-ray and Optical-to-Ultraviolet bands. In the γ-ray band, five flaring periods have been detected, and quasi-simultaneously with these flaring times, the X-ray and UVOT data detected by Swift-XRT/UVOT have also been analysed. In one of the brightest flare periods (Flare 5; observed on Oct 12, 2018) the γ-ray flux reached (6.7 ± 0.81) × 10−6 photon cm−2 s−1 (∼31 × higher than the mean flux over 12.3 years) with detection significance of σ = 6.1. The estimated variability time(∼2 hours) constrains the γ-ray emitting region size to ≤9 × 1014 cm, which is close to the black hole radius. The spectral energy distributions (SEDs) in the γ-ray band for the ∼12.3 years of data show an early cut-off at ∼14 GeV; beyond ∼60 GeV, however, the spectrum hardens and is detected up to ∼316 GeV. Similar spectral behaviour is also noticeable for the SEDs of flares, which can be linked to the photon absorption by the emitting region’s internal and external narrow-band radiation fields. In the quiescent period, the γ-ray emission was described by the Synchrotron-Self-Compton scenario, while the external photons contributions from the Disk and the broad-line region were required to explain the short-term flaring γ-ray emission. Considering the significance of the obtained results from 4C +28.07, we compared the parameters with 3C 279 and M87, to motivate further studies.