Mui Viet Luong

Numerical simulation of ultraviolet picosecond Ce:LiCAF laser emission by optimized resonator transients

We perform numerical simulations to investigate the spectro-temporal evolution of broadband ultraviolet (UV) laser emission from Ce:LiCAF, modeled as a system of two homogeneous broadened singlet states, using the resonator transient method. Results show that a short-pulse Ce:LiCAF UV laser emission can be achieved by choosing a cavity with the appropriate photon cavity lifetime and by controlling the pump energy. Numerical simulations were performed to determine the optimum Q-value and cavity length that will make the photon cavity lifetime small compared to the pump pulse duration. A single 31.5 ps laser pulse with about 10 µJ output energy can be generated in principle from a 1 mm, 1 mol % Ce3+-doped LiCAF crystal using a low-Q (30% output coupler reflectivity), short cavity (2 mm cavity length) oscillator pumped by 75 ps laser pulses and 140 µJ pump energy. With these transient cavity parameters, a slope efficiency of about 10% is feasible. This technique can be extended to other fluoride-based solid-state laser gain media.

High pressure band gap modification of LiCaAlF6

First-principles density functional calculations together with experimental measurements demonstrate that pressure (uniform and uniaxial) increases the band gap of a perfect lithium hexafluoroaluminate (LiCaAlF6, LiCAF) crystal. As fluoride crystals can be highly transmitting at vacuum ultraviolet wavelengths, crystal modifications that further increase the band gap are highly sought after for future Vacuum ultraviolet applications. Through an extensive series of density functional theory simulations, we demonstrate that the band gap increases monotonically from 12.2 eV to 14.1 eV with the application of uniform pressure. Through joint theoretical and experimental investigation, we explore different uniaxial compressions that can be achieved through cutting-edge laser-shock compression. We find that uniaxial pressure also increases the LiCAF band gap by 0.3 and 0.4 eV for a- and c-axis compressions, respectively.

Gamma-ray irradiation effects on the optical properties of bulk ZnO single crystals

Hydrothermal-grown bulk ZnO single crystals are investigated before and after gamma-ray irradiation. The irradiation does not alter the optical transparency in the visible region. The gamma rays only induce modified near-band-edge UV emission with blue-shifted peaks and shortened response times. From the initial values before irradiation, the peaks shift by 5 to 6 nm, and the response times shorten by 140 to 440 ps. We attribute these observations to the radiation-induced defects on the bulk crystals. Our results nevertheless lead to the realization of short-wavelength ZnO scintillators that can be utilized in high-energy-radiation environments.

Comparison of the electronic band structures of LiCaAlF6 and LiSrAlF6 ultraviolet laser host media from ab initio calculations

We report the electronic structures and density of states (DOS) of perfect LiCAF and LiSAF crystals calculated from density functional theory (DFT) with local density approximation (LDA) using optimized lattice constants. DOS calculations reveal that the valence band is mainly derived from F 2p, thereby resulting to a very narrow valence band manifold. Meanwhile, the conduction band is mainly derived from Ca 4s or Sr 5s resulting to Sr having a broader band dispersion compared to Ca. Both fluoride compounds have indirect band gaps with LiCAF having a band gap of 8.02 eV and LiSAF a band gap of 7.92 eV. This is, to the best of our knowledge, the first report on the electronic structure of LiSAF calculated using DFT with LDA. Our results suggest that when doped with Ce3+, the shorter 5d-conduction band distance in Ce:LiSAF combined with the difficulty of growing high-purity crystals lead to the more pronounced excited state absorption (ESA) and solarization effect experimentally observed in Ce:LiSAF, limiting its potential as a laser material compared with Ce:LiCAF.

Electronic states of trivalent praseodymium ion doped in 20Al(PO 3)3-80LiF glass

We investigate the photoluminescence (PL) and photoluminescence excitation (PLE) spectra of 20Al(PO3)3–80LiF+Pr glass (APLF+Pr) and Pr3+-doped LiCaAlF6 crystal (Pr:LiCAF) in order to determine the electronic states of Pr3+ in APLF glass host and to improve APLF+Pr scintillation properties. Ultraviolet (UV) emission bands at around 250 and 340 nm were observed from both materials and these can be ascribed to 4f5d→4f2 transitions in Pr3+. Emission at around 400 nm was also obtained and is principally attributed to 1S0→4f2 transition. Difference in the emission profiles of these two materials was found to be due to the extent of the 5d band and its position relative to the 1S0 state. Increasing the concentration of Pr3+ up to 2 mol % was found to improve UV emission ratio due to the faster cross-relaxation of 4f states. This could improve the quantum efficiency of APLF+Pr as a neutron scintillator for scattered-neutron diagnostics in laser fusion research.

Optical characterization of Nd 3+ :LiCaAlF 6 in the vacuum ultraviolet region at low temperature

Solid-state vacuum ultraviolet (VUV) lasers are currently being developed because these are more robust, easier to maintain, and more cost-efficient compared to conventional light sources such as excimer lasers, synchrotron radiation, and free electron lasers. Fluorides are excellent solid-state laser host materials for this purpose because of their wide bandgaps, making them transparent down to nearly 100 nm. Moreover, interconfigurational 4fⁿ⁻¹ to 4fⁿ transitions in rare-earth-doped fluorides result to broad fluorescence bandwidths. Previous work has shown that LiCaAlF<inf>6</inf> is an excellent laser host when doped with Ce³+ ions [1]. However, emission from Ce³⁺ activator ions cannot extend below 265 nm. In this work, we investigate the optical properties of Nd³⁺-doped LiCaAlFe (Nd:LiCAF). Nd³⁺-doping results to emissions in the VUV region.