8 research outputs found
Theory and Experiments of Pressure-Tunable Broadband Light Emission from Self-Trapped Excitons in Metal Halide Crystals
Hydrostatic pressure has been commonly applied to tune broadband light
emissions from self-trapped excitons (STE) in perovskites for producing white
light and study of basic electron-phonon interactions. However, a general
theory is still lacking to understand pressure-driven evolution of STE
emissions. In this work we first identify a theoretical model that predicts the
effect of hydrostatic pressure on STE emission spectrum, we then report the
observation of extremely broadband photoluminescence emission and its wide
pressure spectral tuning in 2D indirect bandgap CsPb2Br5 crystals. An excellent
agreement is found between the theory and experiment on the peculiar
experimental observation of STE emission with a nearly constant spectral
bandwidth but linearly increasing energy with pressure below 2 GPa. Further
analysis by the theory and experiment under higher pressure reveals that two
types of STE are involved and respond differently to external pressure. We
subsequently survey published STE emissions and discovered that most of them
show a spectral blue-shift under pressure, as predicted by the theory. The
identification of an appropriate theoretical model and its application to STE
emission through the coordinate configuration diagram paves the way for
engineering the STE emission and basic understanding of electron-phonon
interaction
Photoacoustic Identification of Laser-induced Microbubbles as Light Scattering Centers for Optical Limiting in Liquid Suspension of Graphene Nanosheets
Liquid suspensions of carbon nanotubes, graphene and transition metal
dichalcogenides have exhibited excellent performance in optical limiting.
However, the underlying mechanism has remained elusive and is generally
ascribed to their superior nonlinear optical properties such as nonlinear
absorption or nonlinear scattering. Using graphene as an example, we show that
photo-thermal microbubbles are responsible for the optical limiting as strong
light scattering centers: graphene sheets absorb incident light and become
heated up above the boiling point of water, resulting in vapor and microbubble
generation. This conclusion is based on direct observation of bubbles above the
laser beam as well as a strong correlation between laser-induced ultrasound and
optical limiting. In-situ Raman scattering of graphene further confirms that
the temperature of graphene under laser pulses rises above the boiling point of
water but still remains too low to vaporize graphene and create graphene plasma
bubbles. Photo-thermal bubble scattering is not a nonlinear optical process and
requires very low laser intensity. This understanding helps us to design more
efficient optical limiting materials and understand the intrinsic nonlinear
optical properties of nanomaterials
The phase transition and optical properties of Cr2+-doped ZnSe under high pressure
The phase transition pressure, electronic structure, optical properties and stability for ZnSe and Cr2+:ZnSe with different doping concentrations were calculated by first-principles calculation based on density-functional theory. The phase transition pressure was calculated by enthalpy-pressure relation. The introduction of dopant (Cr2+) reduces the phase transition pressure, and the phase transition pressure decreases with the increase of doping concentration. The high pressure enhances the degeneracy of Cr-d orbitals. Under the high-pressure conditions, the absorption peak positions of Cr2+:ZnSe have obvious blue-shift. Meanwhile, the stability of structures for ZnSe and Cr2+:ZnSe were further confirmed by defect formation energy and elastic constants. Keywords: Phase transition pressure, Optical properties, Cr2+:ZnSe, First-principles calculatio
The effects of the impurity distribution on the electrical and optical properties of Cr2+:ZnSe nanowires: First-principles study
The structural, electrical and mid-infrared optical properties of wurtzite structured ZnSe nanowires with different Chromium impurity distribution are investigated using first-principles calculation based on density-functional theory (DFT). The formation energies have been calculated to study the relative stabilities of different Cr doping positions. It is shown that when the Cr doping position shifted from the center to the edge, the splitting energy between 5T2 and 5E levels of Cr d-orbitals is decreased and a redshift is observed in the calculated infrared absorption spectra. A probable reason for these effects of the impurity distribution is discussed. Keywords: First-principles, Nanowires, Impurity distribution, Cr-doped ZnS
Mid-infrared Fe2+:ZnSe semiconductor saturable absorber mirror for passively Q-switched Er3+-doped ZBLAN fiber laser
A mid-infrared (mid-IR) semiconductor saturable absorber mirror (SESAM) based on Fe2+:ZnSe for passively Q-switched Er3+-doped ZBLAN fiber laser has been demonstrated. Fe2+:ZnSe SESAM was fabricated by electron beam evaporation method. Fe2+ was innovatively doped into the reflective Bragg stack, in which ZnSe layer served as both doped matrix and high refractive layer during the fabricating process. By using the Fe2+:ZnSe SESAM, stable passively Q-switched pulses with the minimum pulse width of 0.43 μs under a repetition rate of 160.82 kHz were obtained. The recorded maximum average output power of 873 mW with a peak power of 12.59 W and pulse energy of 5.43 μJ were achieved. The results demonstrated a new method for fabricating Fe2+:ZnSe SESAM, which can be used in compact mid-IR Q-switched fiber laser