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Journal of the Optical Society of America B

Excited-state absorption spectra and gain measurements of CaF_2:Sm^2+

J. K. Lawson, Stephen A. Payne

Published Jul 1, 1991• 42 citations
Publisher

Abstract

We have measured the pump-probe spectrum for CaF2:Sm2+ over a wide (360–1020 nm) spectral range. We observed gain throughout most of the 730-nm emission band, although there is some evidence of excited-state absorption (ESA) in this region. A much stronger ESA was observed near 500 nm. The temperature dependence of the ESA signal clearly indicates that the absorption is associated with divalent samarium. Because of the width and strength of this transition, we assign the ESA to a 4f55d → conduction band transition. We calculated the oscillator strength for this ESA band to be 0.10 and rationalized its magnitude based on an intensity-borrowing mechanism that involves the transition from the valence band to the conduction band.

DOI
10.1364/josab.8.001404

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We have measured the pump-probe spectrum for CaF2:Sm2+ over a wide (360–1020 nm) spectral range. We observed gain throughout most of the 730-nm emission band, although there is some evidence of excited-state absorption (ESA) in this region. A much stronger ESA was observed near 500 nm. The temperature dependence of the ESA signal clearly indicates that the absorption is associated with divalent samarium. Because of the width and strength of this transition, we assign the ESA to a 4f55d → conduction band transition. We calculated the oscillator strength for this ESA band to be 0.10 and rationalized its magnitude based on an intensity-borrowing mechanism that involves the transition from the valence band to the conduction band.
The optical-absorption transitions originating from the lowest 4f5d excited state of Pr3+{\mathrm{Pr}}^{3+}Pr3+ in Y3{\mathrm{Y}}_{3}Y3​Al5{\mathrm{Al}}_{5}Al5​O12{\mathrm{O}}_{12}O12​ have been studied. The experimental arrangement uses a 7-ns 266-nm light pulse to populate the lowest 4f5d state, and a spatially overlapped, time-sequenced pulse from a frequency-tunable dye laser to induce transitions from that excited state. The excited-state-absorption (ESA) spectrum covering the 900--300-nm probe wavelength range is characterized by two broad peaks centered at 700 and 350 nm on a slowly rising background. The peak ESA cross section at 355 nm is (1\ifmmode\pm\else\textpm\fi{}0.1)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}17}$ cm2{\mathrm{cm}}^{2}cm2. The position, shape, and strength of the spectrum suggest that the terminal states of the ESA transitions are the second 4f5d state of Pr3+{\mathrm{Pr}}^{3+}Pr3+ and the conduction band of the host lattice. The ESA measurements have been extended to transitions from the 3^{3}3P0{\mathit{P}}_{0}P0​ and 1^{1}1D2{\mathit{D}}_{2}D2​ excited states of the 4f2{\mathit{f}}^{2}f2 configuration of Pr3+{\mathrm{Pr}}^{3+}Pr3+ as well. The ESA cross sections at a probe wavelength of 532 nm are (8\ifmmode\pm\else\textpm\fi{}1)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}19}$ cm2{\mathrm{cm}}^{2}cm2 and (5.7\ifmmode\pm\else\textpm\fi{}0.6)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}19}$ cm2{\mathrm{cm}}^{2}cm2 for transitions from the 3^{3}3P0{\mathit{P}}_{0}P0​ and 1^{1}1D2{\mathit{D}}_{2}D2​ excited states, respectively. The terminal state of ESA transitions from these two levels is the lowest 4f5d state. The strong ESA completely inhibits the potential laser action based on emission from these states in this crystal.
The excited-state absorption (ESA) in Nd 3+ :SrF 2 crystals was investigated in the 1280 — 1320 nm spectral wavelength range by selective laser excitation. It was found that, on excitation at wavelengths of 743 and 800.5 nm, there is no ESA in the ranges 1304 — 1309 and 1295 — 1297 nm. Weak ESA bands are observed in the 1297 — 1304 and 1309 — 1320 nm ranges and the differential ESA cross section does not exceed 0.3 × 10 -21 cm 2 . The results indicate that the ESA does not preclude the use of an Nd 3+ :SrF 2 crystal as the amplifying medium in the wavelength range 1280 — 1320 nm.
Measurements are reported of absorption, emission, and activation spectra in CaF2{\mathrm{F}}_{2}F2​: Sm++{\mathrm{Sm}}^{++}Sm++, and also of fluorescence lifetime. A revised level scheme is proposed. Observations of optical maser effects were made at liquid hydrogen and liquid helium temperatures over a wide range of pumping intensities. For illumination in the 6400-A band, the threshold intensity of illumination was about 20 w ${\mathrm{cm}}^{\ensuremath{-}2}$. Observations are reported of the dependence of the intensity of the maser beam on the pumping intensity. Photographic observations, made both with or without a Fabry-Perot etalon, show that on one particular sample five distinct frequencies were present in the maser signal, and that the number of modes excited was of the order of 1000.
The spectral dependence of the photoionization energy of Eu2+:CaF2, Ce3+:YAG, and Sm2+:CaF2 systems have been measured and thresholds experimentally determined and compared with theoretical values calculated from electrostatic models. It is shown that the excited state absorption transitions or the persistent hole burning observed by other authors occur above the threshold energy of photoionization of the impurities and that the states of the crystal which form the bottom of the conduction band may play an important role in the strong probability of these processes. A review of thresholds now known is also given.
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.