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Journal of Applied Physics

Coherent emission from Ho3+ ions by pumping into the YIG absorption band

Richard G. Stafford, Hiromitsu Masui, Roger L. Farrow, Richard K. Chang, LeGrand G. Van Uitert

Published Jun 1, 1976• 5 citations
Publisher

Abstract

Coherent emission from Ho3+ ions (2.1 μm) in a YIG host at 77 °K has been observed by optical pumping into the electronic absorption bands of the YIG host material, using (i) a pulsed Nd : glass laser to pump into the tail of the 6A1g→4T1g band (which has an absorption coefficient α=0.18 cm−1 at 1.06 μm) or (ii) a pulsed ruby laser (0.6943 μm) to pump the 6A1g→4T2g band near its peak (where α=300 cm−1). The Ho3+ emission from an illuminated slab 30–40 μm was investigated as a function of input pump energy. The electronic absorption bands of YIG are strong, broad, and have good energy-transfer efficiency to the upper laser level of the Ho3+ ions.

DOI
10.1063/1.322962

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Physical Review•Aug 15, 1961•384 citations
Coherent emission from Ho3+ ions (2.1 μm) in a YIG host at 77 °K has been observed by optical pumping into the electronic absorption bands of the YIG host material, using (i) a pulsed Nd : glass laser to pump into the tail of the 6A1g→4T1g band (which has an absorption coefficient α=0.18 cm−1 at 1.06 μm) or (ii) a pulsed ruby laser (0.6943 μm) to pump the 6A1g→4T2g band near its peak (where α=300 cm−1). The Ho3+ emission from an illuminated slab 30–40 μm was investigated as a function of input pump energy. The electronic absorption bands of YIG are strong, broad, and have good energy-transfer efficiency to the upper laser level of the Ho3+ ions.
Optical absorption and emission spectra are used to determine the energy levels of the 5I8 ground state and 5I7 excited state of Ho3+ in diamagnetic yttrium gallium garnet (YGaG) and ferrimagnetic yttrium iron garnet (YIG). The results for YGaG are consistent with the D2 site symmetry of the dodecahedral site in the garnet structure. Energy levels in YIG that are separated by site inequivalency are identified from spectra observed as the Fe3+ magnetization is rotated in the (11̄0) plane. Stimulated emission is observed from Ho3+ in YGaG at 2.086 and 2.114 μ and from Ho3+ in YIG at 2.086, 2.089, and 2.107 μ. Each of the lines in YIG is composed of two superimposed electronic transitions. The exchange field in YIG makes possible magnetic tuning of coherent emission over a range of 70 Å. This paper is a brief description of work that will be treated in full elsewhere.
Optical absorption and emission spectra are used to determine the energy levels of the 5I8^{5}I_{8}5I8​ ground state and 5I7^{5}I_{7}5I7​ excited state of Ho3+{\mathrm{Ho}}^{3+}Ho3+ in diamagnetic yttrium gallium garnet (YGaG) and ferrimagnetic yttrium iron garnet (YIG). The results for YGaG are consistent with the D2{D}_{2}D2​ site symmetry of the dodecahedral site in the garnet structure. Energy levels in YIG which are separated by site inequivalency are identified from spectra observed as the Fe3+{\mathrm{Fe}}^{3+}Fe3+ magnetization is rotated in the (11‾01\overline{1}0110) plane. Stimulated emission is observed from Ho3+{\mathrm{Ho}}^{3+}Ho3+ in YGaG at 2.086 and 2.114 \ensuremath{\mu} and from Ho3+{\mathrm{Ho}}^{3+}Ho3+ in YIG at 2.086, 2.089, and 2.107 \ensuremath{\mu}. Each of the lines in YIG is composed of two superimposed electronic transitions. The exchange field in YIG makes possible magnetic tuning of coherent emission over a range of 70 \AA{}.
Absorption transitions between two excited optical states in ruby have been studied by comparing the absorption coefficients measured under optically pumped and unpumped conditions. The spectrum has been determined in the π-and σ-polarizations in the photon energy range from 5,500 to 45,000 cm -1 . Several absorption bands have been found which arise from the transitions from the excited states t 2g 3 2 E g and t 2g 3 2 T 1g of the Cr 3+ ion. From the locations and intensities of these bands, the terminal states of the observed transitions are assigned to the charge transfer state, the t 2g 3 2 T 2g state, and the doublet states belonging to the electron configuration t 2g 3 e g . Calculated energies between these states and the metastable states, t 2g 3 2 E g and t 2g 3 2 T 1g , are in good agreement with the peak energies of the observed absorption bands. The present experiment gives new information inaccessible by the ordinary ground state absorption measurement, such as the locations of several doublet states with the configuration t 2g 2 e g and the intensities of doublet-doublet transitions in ruby.
To overcome the ETU-induced heating and realize efficient and enhanced laser cooling in Ho3+-doped solids, in Section 4.3, we will provide a modified excitation scheme based on ground-state absorption (GSA) of 5I8 Æ 5I7 at around 2.07 μm and ESA of 5I5 Æ 5F5 at around 2.28 μm. Compared with the conventional pumping scheme, the co-pumping scheme ameliorates the cooling performance significantly through alleviating the upconversion-quenching-induced heat [12], that is, the heat-producing 5I5 Ho3+ions are pumped to the 5F5 state by the laser at 2.28 μm. Fortunately, the 5F5 Ho3+ ions barely transit to the lower nearby state and tend to decay radiatively. Therefore, major heat produced in the conventional laser cooling of Ho3+-doped solids is avoided, and both cooling efficiency and cooling power density can be improved. In Section 4.4, we broaden the research domain of optical refrigeration to rare earth ion co-doped systems. Specifically, when Ho3+–Tm3+ co-doped YLiF4 crystals are pumped at around 2.07 μm, Ho3+ ions will populate at the 5I7 manifold. Meanwhile, Tm3+ ions will also be sensitized by absorbing energy transferred from 5I7 Ho3+ ions. Since the average energy of fluorescence photons generated by Tm3+3F4 Æ 3H6 radiative transitions is higher than that generated by Ho3+5I7 Æ 5I8 radiative transitions, energy transfer (ET) from Ho3+ to Tm3+ ions will induce annihilation of host phonons, and bring enhancement in both the cooling efficiency and the cooling power. Therefore, this cooling mechanism is referred next as energy transfer-enhanced laser cooling (ETLC) of solids [11]. 4.2 Conventional Laser Cooling of Holmium- The main upconversion process and the anti-Stokes fluorescence cooling (ASFC) processes of Ho3+-doped fluoride crystals are illustrated in Fig. 4.1. The ground-state (5I8) Ho3+ ions are pumped to the first excited state (5I7) with laser excitations of energy hν (solid red arrow on the left) and then relax thermally among the Stark levels in a timescale of picoseconds with host phonon absorption. Part of the Ho3+5I7 ions de-excite by emitting fluorescence photons of mean energy hν10 (solid blue arrow, ν10 > ν), which accounts for the ASFC process. The rest of the 5I7 Ho3+ ions de-excited and transfer their energy to the neighboring 5I7 Ho3+ ions, causing population accumulation at the third excited state (5I5) with host phonon absorption (solid green arrows). Comparison between the energy mismatch and the host phonon energy (for YLiF4 crystal host, ħω ≈ 450 cm-1) indicates that about two host phonons are annihilated for each ETU process (see Table 4.1).
Several experiments on optically excited ruby are described. First, absorption transitions initiated from the metastable states,t\min{2}\max{3}2Eandt\min{2}\max{3}2T1, are investigated in the photon energy range from 5500 to 45 000 cm-1. The absorption spectrum of these excited states is determined in both\pi-and σ-polarizations and assignments for the observed absorption bands are achieved. Obtained results are compared with other experiments reported in the literature. Next, emission properties of pink ruby are studied in the region near theRlines. The emission spectrum is determined point by point by means of intense flash excitation. The observed weak bands are ascribed to theR'and phonon assistedRtransitions. Laser actions in these bands are discussed and experimental results on phonon terminated laser amplification are presented. An experiment using a giant pulse as an excitation source reveals that the transition time fromt\min{2}\max{3}2Etot\min{2}\max{3}2T1is shorter than a fraction of 1μs at room temperature.
Optical absorption cross sections and the fluorescent quantum efficiency in ruby have been determined. This data has been used to correlate calculations with the analysis of the preceding paper. Stimulated emission from ruby under pulsed excitation has been studied in some detail; the observations are found to depend strongly on the perfection of the particular crystal under study. A peak power output of approximately 5 kw, total output energy of near 1 joule, beam collimation of less than ${10}^{\ensuremath{-}2}$ rad, and a spectral width of individual components in the output radiation of about 6\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$A at 6943 A have been measured. It is suggested that mode instabilities due to temperature shifts and a time-varying magnetic field are contributing to an oscillatory behavior of the output pulse.