The stimulated emission and photoluminescence of ultrathin GaN quantum wells with a nominal thickness of 1.5 – 2 monolayers (MLs) and AlN barrier layers 4 – 6.66 ML thick, obtained by plasma-activated molecular beam epitaxy on c-sapphire substrates, are studied. The stimulated emission of TE polarisation in ultrathin GaN/AlN quantum wells is obtained under pumping directly into quantum wells. The wavelength of stimulated emission varied from 262 to 290 nm, depending on the thickness of the wells and barriers. It is shown that stimulated emission is achieved on localised GaN states with a thickness of 2 and 3 ML in ultrathin quantum wells with a nominal thickness of 1.5 and 2 ML, respectively. The minimum excitation threshold of stimulated emission was 700 kW cm−2 at λ = 270 nm.

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We have demonstrated optically pumped room-temperature pulse lasing at 300.4 nm from an AlGaN-based multiplequantum-well (MQW) structure grown by plasma-assisted molecular beam epitaxy on a c-sapphire substrate. The lasing was achieved at the threshold peak power of $12 MW/cm 2 . The MQW structure involved AlGaN/AlN short-period superlattices to decrease the threading dislocation densities from 10 11 down to 10 9 -10 10 cm À2 . Studies of time-resolved photoluminescence (TRPL) spectra and cw PL temperature dependences (10-300 K) of different MQW structures, as well as numerical calculations of the optical gain and confinement in the laser structure allowed us to conclude about the optimum design of AlGaN-based MQW structures for the lower threshold UV lasing.

The photoluminescence of barium thiogallates doped with Eu 2+ , Ce 3+ , and Eu 2+ + Ce 3+ ions is studied over a wide range of excitation levels (10 -3 -10 6 W/cm 2 ). Introduction of 3 at.% Eu and 3 at.% Ce instead of 5 at.% Eu into a BaGa 2 S 4 matrix doubles the luminescence quantum yield of the phosphor. Doped BaGa 2 S 4 exhibits a high linearity in its luminescence intensity as a function of excitation level (a constant efficiency) up to 2⋅10 4 W/cm 2 for excitation pulse durations of 8 ns, which corresponds to cw pumping at a power density of about 5⋅10 2 W/cm 2 in terms of the concentration of excited ions. It is shown that using BaGa 2 S 4 :Eu,Ce along with the "yellow" phosphor of a Nichia NS6L083 LED may increase its color rendering index from 0.64 to 0.80 with no reduction in its luminous efficiency.