Changsu Kim

Mid-IR random lasing of Cr-doped ZnS nanocrystals

The laser ablation method enabled fabrication of transition-metal (Cr, Co, Fe)-doped nanocrystals with a strong photoluminescence over a 2?5? ?m spectral range. The first room temperature mid-IR random lasing due to impurity intra-shell transitions in transition-metaldoped semiconductor nanocrystals is reported. Temperature dependences of the lasing spectra reveal diverse behaviour of the output spectra for powders of different grain sizes.

Mid-infrared Cr 2+ :ZnSe random powder lasers

We report simple methods of laser active Cr2+:ZnSe powder fabrication with average grain sizes of either approximately 10 or approximately 1 microm without crystal growth stage. Pure, uniformly mixed ZnSe and CrSe powders annealed at 1000 masculineC for 3 days in a sealed evacuated (approximately 10(-4) Torr) quartz ampoule exhibited middle-infrared laser action at room temperature under 1.56 microm excitation of D(2) Raman shifted radiation of Nd:YAG laser. The output-input characteristic clearly demonstrated the threshold-like behavior of the output signal with the threshold energy level of 0.5 and 3 mJ in 2.9 mm spot for 10 and 1 microm grain sizes, respectively.

Chromium doped ZnSe and ZnS gain media for optically and electrically pumped mid-IR lasers

We report methods of fabrication and laser-spectroscopic characterization of mid-IR gain media based on micron size Cr2+:ZnSe/ZnS powders, as well as Cr2+:ZnSe/ZnS doped fluorocarbon polymer films, and perfluorocarbon liquids. All samples demonstrated strong mid-IR luminescence over 2000-3000nm spectral range under optical 1700nm excitation. The random lasing of the doped liquids and polymer films was realized with pump energy density of 100 and 15mJ/cm2, respectively. Previously we have demonstrated mid-IR electroluminescence of Cr:ZnSe with n-conductivity provided by thermal diffusion of Al and Zn. However, the formation of conductivity was accompanied by compensation of the Cr2+ optical centers and relatively weak chromium electroluminescence. In this paper we report study of the Cr2+ compensation in the crystals co-doped with donor and acceptor impurities. Optical and electrical characterization of Cr:ZnSe crystals with Ag, Cu, Al, In, and Zn co-dopants were studied to optimize mid-IR electroluminescens of the Cr2+ ions. The best results were obtained with p-conductive Ag:Cr:ZnSe samples featuring a low 600 Ωcm resistivity. First mid-IR electroluminescence in presumable p-type Ag:Cr:ZnSe was demonstrated, which could prove valuable for developing laser diodes that function in this spectral region.

Mid-IR luminescence of nanocrystalline II-VI semiconductors doped with transition metal ions

A novel method of transition metal (TM) (Cr, Co and Fe) doped II-VI nanocrystalline quantum dots (NCD) fabrication based on laser ablation was demonstrated. For the first time mid-IR luminescence from TM:II-VI NCD is reported.

Mid-IR electroluminescence of Cr:ZnSe crystals co-doped with donor and acceptor impurities

Studies of divalent Chromium ions compensation in ZnSe crystals after thermo-diffusion of Al and Ag co-dopants are presented. We report 2-3mm Mid-IR electro-luminescence in the Al:Cr:ZnS and Ag:Cr:ZnSe bulk samples.

Room-temperature mid-infrared Cr2+:ZnSe and Cr2+:ZnS random powder lasers

We report a simple method for fabricating transition metal (TM) doped II-VI powders with average size of about 10- 20μm as well as room temperature mid-infrared (2-3 μm) random lasing based on Cr2+-doped ZnSe and ZnS powders prepared without crystal growth stage under optical intra-shell excitation of chromium. Fabrication of Cr2+-doped ZnSe and ZnS powders involved two simple stages. At the first stage, pure ZnSe, (ZnS) and CrSe, (CrS) (with a concentration of Cr2+ ion 2×10-19 cm-3 and 5 × 10-19 cm-3 for ZnSe and ZnS respectively) chemicals with an average grain size of 10μm were uniformly mixed by means of a mechanical shaker. At the second stage the obtained ZnSe/CrSe mixture was sealed into evacuated (~10-4 Torr) quartz ampoule and annealed either at 1200°C for 15 minute, or 1000°C for 3 days. In the case of ZnS/CrS mixtures the annealing was performed in evacuated quartz ampoule at 1000 °C for 14 days. After annealing, under 1560 nm excitation, the powders demonstrated room temperature middle-infrared luminescence of Cr2+ similar to Cr2+ emission in bulk ZnSe and ZnS. Moreover, the output-input characteristic clearly demonstrated the threshold-like behavior of the output signal with the threshold pump energy density of ~44.5 mJ/cm2 ~7.46 mJ/cm2, and 63.6 mJ/cm2 for Cr:ZnSe annealed for 15 min, 3 days, and Cr:ZnS respectively.

Mid-IR photoluminescence and lasing of chromium doped II-VI quantum dots

Here we report a new method for transition-metal (TM) doped II-VI Quantum Dots (QD) fabrication and first mid-IR (2-3 μm) lasing at 77K of Cr2+:ZnS QD powder (~ 27 nm grain size). Cr2+:ZnS nanocrystalline dots (NCDs) were prepared using laser ablation. The mid-IR photoluminescence (PL) and lasing were studied. The dependence of PL spectrum profile on pump energy demonstrated a threshold behavior accompanied by the appearance of a sharp stimulated emission band around 2230 nm. The stimulated emission band is shifted to the longer wavelength with respect to the spontaneous emission and corresponds to the peak of the Cr:ZnS gain spectrum. This was also accompanied by a considerable lifetime shortening.

Middle-IR Random lasing of Cr:ZnS nanocrystalline powder - from diffusion to photon localization regimes

First room temperature mid-IR lasing due to intra-shell transitions in the doped semiconductor nanocrystals (NC) is reported. The 27 nm Cr:ZnS NCs oscillate at 2200 nm with 325 mJ/cm2 laser threshold. Temperature dependence of photoluminescence decay, lasing threshold and lasing wavelength maxima of Cr:ZnS NCs are studied.