Optical spectroscopy of Yb/Er codoped NaY(WO4)2 crystal

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Abstract

Erbium and ytterbium codoped double tungstates NaY(WO4)2 crystals were prepared by using Czochralski (CZ) pulling method. The absorption spectra in the region 290–2000 nm have been recorded at room temperature. The Judd–Ofelt theory was applied to the measured values of absorption line strengths to evaluate the spontaneous emission probabilities and stimulated emission cross sections of Er3+ ions in NaY(WO4)2 crystals. Intensive green and red lights were measured when the sample were pumped by a 974 nm laser diode (LD), especially, the intensities of green upconversion luminescence are very strong. The mechanism of energy transfer from Yb3+ to Er3+ ions was analyzed. Energy transfer and nonradiative relaxation played an important role in the upconversion process. Photoexcited luminescence experiments are also fulfilled to help analyzing the transit processes of the energy levels.

Introduction

Nowadays, peoples show great interests in compact laser operating in the infrared (∼1.5 and 3 μm) for optical communications, medical and eye-safe light detecting and ranging (LIDAR) applications [1], [2], [3], and in the visible region (blue–green), for data storage, undersea communications, etc. Diode pumped solid-state lasers could provide compact and efficient devices with the advantage of easy coupling with fiber integrated optical systems. For diode pumped laser emission at mid infrared and visible regions (upconversion based lasers), Er3+ seems to be a natural candidate due to its 1.5 μm (4I13/2→4I15/2) and 2.8 μm emissions (4I11/2→4I13/2). Its green emission is at ∼0.54 μm (4S3/2→4I15/2) and its absorption bands at ∼0.8 and ∼0.98 μm. Among several techniques that are currently used to obtain compact visible lasers, the upconversion lasing is one of the most promising techniques since it does not require a nonlinear media for second harmonic generation [4]. The overlapping of 2F5/2 energy level of Yb3+ ions and 4I11/2 energy level of Er3+ ions produce efficient resonant transfer between both ions. So it is possible to perform selective excitation of the Yb3+ ions and realize energy transfer between the two ions. Upconversion laser performance of Er3+ has been observed in some crystals and glass fibers [5], [6], [7].

The hosts play important roles in the upconversion luminescence. Er/Yb:NaY(WO4)2 crystal is a new kind of crystal demonstrating good optical performance. NaY(WO4)2 crystal (here denoted as NYW) is classified among the disorder crystalline host for lasing rare-earth ions [8]. The disorder structure presents the broadening of the optical features in the absorption and emission spectrum even at low temperature. This fact has some interesting results on the optical properties of the materials. The addition of proper RE oxides to the starting mixture allows obtaining crystals suitable for spectroscopic experiments. NYW has been demonstrated to be a promising host lattice for lasing ions: laser action has been reported for Nd3+ doped in the matrix [9]. In this letter, we report the absorption spectra of Yb/Er codoped NYW and whose analysis on the basis of the Judd–Ofelt theory. The three phenomenological J–O intensity parameters Ωt (t=2, 4 and 6) of Er3+ ions in NYW crystals are determined by performing a least square fit of calculated and observed absorption line intensities. These intensity parameters are then used to determine the spontaneous emission probabilities and branching ratios. We measured the upconversion luminescence of Er3+/Yb3+ codoped NaY(WO4)2 crystal excited by 974 nm laser diode. The concentration of Er3+ and Yb3+ is about 0.5% and 2% respectively. The thickness of the sample is 3.5 mm and both sides are well polished.

Section snippets

Crystal growth

Single crystals of NYW are usually grown with CZ pulling method using a MCGS-3 RF single-crystal furnace.

The raw materials was prepared according to the following formula:Na2CO3+(1−x−y)Y2O3+xEr2O3+yYb2O3+4WO3→2NaY1−x−yErxYby(WO4)2+CO2↑In our experiment, the Er3+ and Yb3+ dopants concentrations are of 0.5% and 2% substituting for Y3+. The depth of the Pt crucible was 30 mm, and the diameter is 55 mm. Crystal growth began on a 〈101〉 oriented seed. The rotation rate is 55–60 rpm and pulling rate 1.5 

Absorption spectra and Judd–Ofelt analysis

The sample was cut along a-face and polished to optical grade. The polarized absorption spectra (π, E∥c-axis; σ, E⊥c-axis) at room temperature in 290–2000 nm of a 2.06 mm a-slice were measured on a Hitachi U-3500 spectrophotometer. The room temperature polarized absorption spectra of Er/Yb codoped NaY(WO4)2 crystal was shown in Fig. 1. The absorption bands of Yb3+ and Er3+ ions appear to be broader than expected for a crystalline material with ordered structure, due to the random distribution of

Conclusions

Er/Yb codoped NaY(WO4)2 crystals with good optical quality can be grown by using CZ pulling method. The optical parameters of Er3+ ions in the codoped NaY(WO4)2 crystals were obtained as Ω2=18.1×10−20cm2,Ω4=2.59×10−20cm2 and Ω6=1.21×10−20cm2. Up-conversion luminescences of 552 nm green light, 530 nm red light and 407 nm violet light were obtained when the sample was pumped by 974 nm LD. The red light and green light emissions all were two-photon processes and the 407 nm emission is three-photon

Acknowledgements

The author would like to thank Prof. Zhaohe Yang for transmittance measurements.

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