The authors report blue, green and yellow upconversion laser action in Er3+:YLiF4 following pumping at 1.500 mum below 80 K. For the blue transition the ratio of the output frequency to the input frequency is 3.3, the highest yet reported for an upconversion laser. In spite of the multiple upconversion steps, the thresholds for laser action were less than 80 mW. It is proposed that the nonlinear upconversion pumping proceeds via a sequence of several pairwise cross-relaxation steps.

We have shown that multiple-step upconversion pumping of Er:YLiF 4 at 1.50 μm into the metastable 4 I 13/12 level excites blue (469.7 nm), yellow (560.6 nm), and green (551 nm) laser emission from the 2 P 3/2 , 2 H 9/2 , and 4 S 3/2 levels, respectively. This requires the energy of at least five erbium ions pumped into 4 I 13/2 to populate the upper level for the blue transition, four for the yellow, and three for the green. We believe that the mechanism for upconversion consists of a sequence of pairwise energy transfer steps involving several metastable excited levels. Monolithic laser crystals, ~3 mm long and doped with 1% Er 3+ , were prepared with mirrors applied directly to the crystal surfaces. The thresholds for laser action were 25 mW, 50 mW, and 80 mW for the yellow, green, and blue lasers, respectively. This shows that high order excitation processes in upconversion lasers involving long lived (here ~10 ms) metastable levels can be rather efficient. Output powers of these lasers at 30 K reached 10 mW at 551 nm for 350 mW of pump light, 10 mW at 560.6 nm for 140 mW of pump light, and 0.7 mW at 469.7 nm for 350 mW of pump light. Given the quantum defect between the input and output photons, these are good overall efficiencies. Lasing was observed up to 80 K. This work extends an earlier study, 1 in which we obtained upconversion laser output at the three wavelengths by pumping in the near IR at 0.80 μm and 0.97 μm, to the observation of even higher order upconversion processes.

Uniaxial stress coefficients have been measured for the $^{3}\mathrm{H}_{4}$(1)${\ensuremath{\leftrightarrows}}^{1}$${D}_{2}$(1) transition of several ${\mathrm{Pr}}^{3+}$ centers in ${\mathrm{CaF}}_{2}$ doped with ${\mathrm{Pr}}^{3+}$ and ${\mathrm{D}}^{\mathrm{\ensuremath{-}}}$ ions. For the ${C}_{s}$(1) center at 6012.4 A\r{} these measurements were made using persistent spectral hole burning. The holes showed a stress shift of 1.8 GHz/kg ${\mathrm{mm}}^{\mathrm{\ensuremath{-}}2}$. This shift is somewhat inhomogeneous, which limits the magnitude of stress that can usefully be applied.

We have observed cw laser action on a new transition (4D3/2→4I11/2) of the Nd3+ ion at 380 nm. This was demonstrated in Nd:LaF3 using an upconversion pumping scheme in which near infrared (788 nm) and visible (591 nm) pump photons from cw dye lasers produce stepwise excitation of the Nd3+ ion. In addition, lasing was observed with a single pump source at wavelengths around 578 nm using doubly resonant, sequential absorption of two yellow photons. Single-mode, cw operation with an output power of 12 mW was measured at 20 K with 1% output coupling and pump powers of several hundred mW. At 77 K the maximum power dropped to 4 mW.

An upconversion pumping scheme was used to produce cw laser action at 0.55 μm in YAlO3:Er3+ at temperatures up to 77 K on the 4S3/2→4I15/2 transition. Two infrared dye lasers at 792.1 and 839.8 nm were used as the pump sources for stepwise two-photon excitation of the 4S3/2 upper laser level at 18 406 cm−1. The laser operates in the fundamental TEM00 mode, and a cw output power of ∼1 mW was achieved with ∼200 mW pump power from each infrared dye laser.

Temperature-dependent frequency shift and linewidth data of the Raman active 337-nm−1 CuO2 in-plane bond-bending mode have been measured for five different oxygen stochiometries between x = 7.00 and x = 6.14 in polycrystalline samples. Softening and line broadening effects observed for the x = 7.00 sample are sharply reduced for x = 6.87 even though the superconducting transition temperature is similar. With a further reduction of oxygen content below x = 6.68, both the soft mode behavior and the line broadening effects completely disappear. These unexpected results could suggest a large variation in the interaction between the electronic system and the lattice as the oxygen content is varied.

Upconversion excitation processes can be used to obtain laser operation at wavelengths that are shorter than that of the pump source. Efficient operation of such lasers is possible in materials with metastable intermediate energy levels that act as population storage reservoirs. Excitation of higher levels that emit short-wavelength radiation can occur by absorption of a second photon from the intermediate level, or by energy transfer processes involving two ions in an intermediate state. Stimulated emission by upconversion excitation using flashlamp pumping was fust reported in 1971.1

The recently discovered phenomenon of photon-gated spectral hole burning1,2 has been extended to transition metal compounds with trie observation of gated hole burning in LiGa5O8:Co2+.