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Ground-State Lifting – A New Method for Increasing the Stimulated-Emission Excitation Effectiveness of Activated Laser Crystals

Alexander A. Kaminskii, Konstantin K. Pukhov

Published May 16, 1991• 4 citations

Abstract

New ways are proposed of increasing broadband-lamp pumping effectiveness of laser crystals with trivalent lanthanides. The method is based on the broadening of lasing-ion absorption spectrum by the remove (excitation) of their considerable part from the ground state into one (some) of the upper states with the help of additional resonant bleaching laser add-pumping Advantages of the new pumping schemes are illustrated by the results, obtained during quantitative analysis of the necessary conditions of stimulated-emission excitation for four laser crystals (LaF3, LiYF4, BaY2F8, and YA103) with Pr3+ ions, which can lase at 3Po → 3F4,3,2, 3H6,5 channels at 300 K with lamp pumping. The method proposed is also suitable for laser effectiveness increase in crystals with other lanthanides activators.

DOI: 10.1002/pssa.2211250132

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New ways are proposed of increasing broadband-lamp pumping effectiveness of laser crystals with trivalent lanthanides. The method is based on the broadening of lasing-ion absorption spectrum by the remove (excitation) of their considerable part from the ground state into one (some) of the upper states with the help of additional resonant bleaching laser add-pumping Advantages of the new pumping schemes are illustrated by the results, obtained during quantitative analysis of the necessary conditions of stimulated-emission excitation for four laser crystals (LaF3, LiYF4, BaY2F8, and YA103) with Pr3+ ions, which can lase at 3Po → 3F4,3,2, 3H6,5 channels at 300 K with lamp pumping. The method proposed is also suitable for laser effectiveness increase in crystals with other lanthanides activators.

High resolution, absolute excited-state absorption (ESA) spectra, at room temperature, from the long-lived 3F4 energy level of several crystals doped with trivalent thulium (Tm3+) ions have been measured employing high-brightness narrowband (FWHM <30 nm) light emitting diodes (LEDs) as a probe wavelength. The aim of this investigation was to determine the strength of ESA channels at wavelengths addressable by commercially available semiconductor laser diodes operating around 630-680 nm. The favourable lifetime of the 3F4 manifold and negligible ground-state absorption (GSA) for the red-wavelength second-step excitation, ensures a direct and efficient route for a dual-wavelength pumping scheme of the thulium ion, which will enable blue-green laser emission from its 1G4 upper-laser level.

Among the many crystalline lasers based on insulating compounds doped with Ln 3+ ions, 1 only one laser is known to generate according to a classical three-level operating scheme, this being the “blue-green” lasing of LiYF 4 :Pr 3+ (its stimulated emission [SE] transition terminates at the lower [zero] Stark level of the ground 3 H 4 state of Pr 3+ ions). 2 For all others the terminal laser level usually has an energy E term ≫ kT. In these cases the number of excited Ln 3+ ions distributed over the working levels, mainly, the initial laser ones, is small and amounts to a few percent of the total number of generating ions in a crystal. This indicates that SE in those multilevel Ln 3+ systems proceeds with practically fully populated levels of the ground state (their usual population is N 0 = 10 19 to 10 21 cm −3 , which corresponds to Ln 3+ ion concentration amounting to C Ln = 0.1 to 10 at.%).

The impact of photodynamic processes induced by pump radiation in crystals doped with rare-earth (RE) ions on the possibility of generating stimulated radiation in the UV and VUV spectral ranges is analyzed. It is shown that, in addition to objective factors, one of the main causes preventing the lasing effect from being obtained using the interconfigurational 4ƒ n−15d-4ƒ n transitions of RE ions is insufficient consideration of the photodynamic processes involved in experiments. It is proposed to correct laser test techniques and “pumpprobe” experiments aimed at investigating active media appropriate for lasing in the UV and VUV ranges. Extended criteria oriented to search for new solid-state UV and VUV active media are formulated.

The possibility of using stimulated emission to improve the optical pumping of Tm3+ ions embedded in Tm3+:YAG to the 3F4 level is explored. The oscillator strengths and the frequencies of the transitions between the ground crystal field level of 3H4 to the different crystal field levels of 3F4 are measured at low temperature. Stimulated emission measurements at 1461 nm are performed to evaluate the optical pumping efficiency. Application to the spectral tailoring of Tm3+:YAG for quantum information applications is discussed.

A method of suppression of losses due to excited-state absorption (ESA) in laser crystals is proposed, based on the well-known phenomenon of electromagnetically induced transparency. Typical situations in which ESA prevents lasing are considered. Theoretical analysis shows that by using an additional driving laser field one can inhibit ESA and make lasing possible. On the basis of available spectroscopic information estimates for rare-earth-ion-doped laser materials are presented. They show the feasibility of obtaining optical gain at parity-allowed $4{f}^{n}\ensuremath{\leftrightarrow}4{f}^{n\ensuremath{-}1}5d$ transitions of rare-earth ions in the ultraviolet spectral region after suppression of ESA at an emission wavelength into the conduction band of a host material.