Abnormal Spectral Behavior of Trivalent Neodymium Ions in Potassium Yttrium Fluoride Crystals
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Abnormal Spectral Behavior of Trivalent Neodymium Ions in Potassium Yttrium Fluoride Crystals Chunlai Yang and Baldassare Di Bartolo Department of Physics, Boston College, Chestnut Hill, MA 02467 ABSTRACT Nd3+ ions in KYF4 can occupy different sites. The energy transfer processes that take place between them are responsible for the unusual behavior of Nd3+. From the spectroscopic measurements at different temperatures we found an anomalous spectroscopic behavior in the low temperature range (20K - 100K). Based on an energy transfer model, we proposed an explanation to this abnormal behavior. We derived the formula to describe the deexcitation process and make a data fitting on our dynamic decay measurements.
INTRODUCTION The optical activity of lanthanides as impurities in various host materials is centered on the states of the 4f shell. These states are strongly screened from the influence of the environment by the 5s25p6 shells and therefore are not greatly influenced by the nature of host materials. Thus it is generally believed that the luminescence of these materials may be simply interpreted. This , however, is not true in the case of KYF4. The laser transition of Nd3+ laser materials originates in an 4F3/2 energy level. Between the 4F3/2 state and the next lower state 4I15/2 there is a large energy gap of about 6000cm1. Nonradiative transition between these levels are improbable because, in solid state hosts, the upper energy of the phonon spectrum is only a few hundred cm-1. In the case of fluoride hosts it is about 400 cm-1, one fifteenth of the energy gap. As a consequence the lifetime of 4F3/2 state is expected to be temperature independent. This general feature is not present in KYF4 crystals. Our spectroscopic measurements on Nd3+ doped KYF4 crystal at different temperatures show a complicated and anomalous behavior in a low temperature range (T < 100K): at ~ 50K, the lifetimes and the emission intensity undergoes dramatic changes with the temperatures.
EXPERIMENTS Three Nd3+ doped KYF4 crystals were used in this research with different Nd3+ concentrations (2%, 3%, and 4%). These samples were provided by CREOL (Center for Research in Electro-Optics and Laser) of the University of Central Florida. Related experiments for laser performance were also performed there [1]. The crystal structure of KYF4 is reported by Y. L. Fur et al [2]. Luminescence spectra were carried out by using a Omnichrome 532 Argon laser to excited the sample and a SPEX 1800 3/4 - meter Czerny Turner Spectrometer as the G2.8.1
optical analyzer. A RCA 7102 photomultiplier tube and a EG&G P.A.R. 5101 lock-in amplifier were used to detect the signal. We used a Leybold-Heraeus refrigerator and a Lake Shore Cryotronics 805 temperature controller to set the sample at different temperatures from 20K to 300K. The excitation source for the lifetime measurements was a Molectron DL-12 pulsed tunable dye laser, which was pumped by a Molectron UV-12 Nitrogen laser. The excitation wavelength was chosen to correspond to the strong transition fr
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