Multiband infrared luminescence of Er³⁺-Ho³⁺-Nd³⁺/Tm³⁺-codoped telluride glasses

Abstract

This paper reports the simultaneous emissions around 1.53, 1.80, 2.10, 2.70 and 3.00 μm in Er³⁺-Ho³⁺ -Nd³⁺/Tm³⁺-codoped telluride glasses upon excitation of a conventional 808 nm laser diode. Both emission bands of 1.53 and 2.70 μm were assigned to the transitions of ⁴I_13/2 -⁴I_15/2, ⁴I_11/2 -⁴I_13/2 of Er³⁺ ions, respectively, the emission near 1.80 mm was assigned to the transition ⁴F₄ -⁴H₆ of Tm³⁺ ions, and the emissions at 2.10 and 3.00 mm arose from the transitions of ⁵I₇ -⁵I₈, ⁵I₆ -⁵I₇ of Ho³⁺ ions. The materials are promising for ultra-broad band amplified spontaneous emission optical sources at near and middle infrared region.

Keywords

multiband luminescence / Er³⁺-Ho³⁺-Nd³⁺/ Tm³⁺ / telluride glass

Introduction

Presently, near- and mid-infrared lasers operating in the wavelength region around 1.5–3.0 μm are attracting interest of researcher in fields of optical communication, medical surgery, eye-safe laser radar, remote sensing, atmosphere pollution monitoring, and so on. New materials, which are used to produce these lasers, are being searched. The glass, crystal and rare-earth ions should be considered because the hosts for dopants are cost-effective and easy to produce in large scale, and rare-earth materials are abundant in natural source. Rare-earth doped fluoride, fluoro-phosphate glasses [1–3], telluride and fluoro-telluride glasses [4–11], chalcohalide glasses [12,13] and other hosts [14–18] as well as fiber devices [19–23] for mid-infrared luminescence have been reported, and nearly all the literatures concentrated on the single emission at 2.70 μm or other wavelengths.

In our present work, we simultaneously observe the multiband emissions around 1.53, 2.10, 2.70 and 3.00 μm in Er³⁺-Ho³⁺-Nd³⁺-codoped telluride glasses, and the multiband emissions around 1.53, 1.80, 3.00 μm in Er³⁺-Ho³⁺-Tm³⁺-codoped telluride glasses with the excitation of 808 nm pump laser diode.

Fabrication and experiment

The Er³⁺-Ho³⁺-Nd³⁺-codoped system has the following compositions (in mol %): 70TeO₂-20ZnO-9.0CaO-0.6Er₂O₃-0.1Ho₂O₃-0.3Nd₂O₃ (EHN), and the Er³⁺-Ho³⁺ -Tm³⁺-codoped system (EHT) has the same compositions as EHN, but Nd₂O₃ is replaced by Tm₂O₃.

The samples were prepared with conventional melting method. Before melting powders in a Si-C electric resistance furnace, the compositions were weighted and mixed. The mixtures were melted and homogenized at 800°C–900°C in an Al₂O₃ crucible in the furnace. The melting was poured onto a preheated brass mold and annealed for 3 hours around 100°C in a muffle furnace, and then cooled inside the furnace by turning the power supply off. The samples were cut and polished to form a plate shape with the size 10 mm × 10 mm × 1 mm for emission spectra measurements.

The near- and mid-infrared (mid-IR) emission spectra of the samples were measured with excitation of an 808 nm laser diode at room temperature by a Traix 320 type spectrometer (Jobin-Yvon Co., France) with resolution of 1 nm and detectable wavelength range 1.0–3.0 μm.

Results and discussion

The near- and mid-IR photoluminescence spectrum of the EHN is shown in Fig. 1. As can be seen, its peaks are at 1.53, 2.10, 2.70 and 3.00 μm, which correspond to the transitions Er³⁺: ⁴I_13/2→⁴I_15/2, Ho³⁺: ⁵I₇→⁵I₈, Er³⁺: ⁴I_11/2→⁴I_13/2 and Ho³⁺: ⁵I₆→⁵I₇, respectively. The photoluminescence spectrum of the EHT is shown in Fig. 2, its peaks are at 1.53, 1.80, 3.00 mm, which correspond to the transitions Er³⁺: ⁴I_13/2→⁴I_15/2, Tm³⁺: ⁴F₄ -⁴H₆ and Ho³⁺: ⁵I₆→⁵I₇, respectively.

The processes of electron transitions and energy transfers in EHN and EHT systems are schematically shown in Figs. 3 and 4.

In EHN system, under the excitation of 808 nm laser diode, the electrons at the ground states ⁴I_9/2 of Nd³⁺, ⁴I_15/2 of Er³⁺ and ⁵I₈ of Ho³⁺ are excited to the states ⁴F_3/2 of Nd³⁺ and ⁴I_9/2 of Er³⁺ and ⁵I₅ of Ho³⁺, respectively. Due to smaller energy level difference between ⁴F_3/2 of Nd³⁺ and ⁴I_9/2 of Er³⁺, ⁴F_3/2 of Nd³⁺ and ⁵I₅ of Ho³⁺, the energy at the excited state (⁴F_3/2) of Nd³⁺ is transferred (ET1,ET2) to ⁴I_9/2 of Er³⁺ and ⁵I₅ of Ho³⁺. For Er³⁺ ions, the electrons at the excited state relax into ⁴I_11/2, then further transit into ⁴I_13/2, ⁴I_15/2, with the emissions around 2.70, 1.53 mm. For Ho³⁺ ions, the electrons at the excited state relax into ⁵I₆, then further transit into ⁵I₇, ⁵I₈, with the emissions around 3.00, 2.10 μm.

In EHT system, under the same excitation as the system above, the electrons at the ground states ³H₆ of Tm³⁺, ⁴I_15/2 of Er³⁺ and ⁵I₈ of Ho³⁺ are excited to the states ³F₄ of Tm³⁺ and ⁴I_9/2 of Er³⁺ and ⁵I₅ of Ho³⁺. Due to smaller energy level difference between ⁵I₆ of Ho³⁺ and ⁴I_13/2 of Er³⁺, ⁴I_13/2 of Er³⁺ and ³H₄ of Tm³⁺, a fraction of energy at ⁵I₅ of Ho³⁺ is transferred (ET3) to ⁴I_13/2 of Er³⁺, and a fraction of energy at ⁴I_13/2 of Er³⁺ is transferred (ET4) to ³H₄ of Tm³⁺. For Tm³⁺ ions, meanwhile, the electrons at the excited state relax into ³H₄, then further transit into the ground state ⁴H₆, with emissions around 1.80 mm. For Ho³⁺ ions, the electrons at the excited state relax into ⁵I₆, then further transit into ⁵I₇ with emissions around 3.00 mm. For Er³⁺ ions, the electrons at the excited state relax into ⁴I_11/2, ⁴I_13/2, then transit into ⁴I_15/2, with emissions around 1.53 μm.

Additionally, it is shown in Figs. 1 and 2 that the emission bands at 2.10 and 2.70 μm in EHN system disappear in EHT system. It can be explained that in the latter, due to smaller energy level difference between ⁵I₆ of Ho³⁺ and ⁴I_13/2 of Er³⁺, ⁴I_13/2 of Er³⁺ and ³H₄ of Tm³⁺, a fraction of energy at ⁵I₆ of Ho³⁺ is transferred into the ⁴I_13/2 of Er³⁺, and a fraction of whose energy is transferred into ³H₄ of Tm³⁺. Consequently, the electron population inversions between the ⁵I₇ and ⁵I₈ of Ho³⁺ and between the ⁴I_11/2 and ⁴I_13/2 of Er³⁺ are so weak that the emissions caused by the transitions are too weak to be detected.

Conclusions

In conclusion, we presented Er³⁺-Ho³⁺-Nd³⁺- and Er³⁺- Ho³⁺-Tm³⁺-codoped telluride glasses. With excitation of a conventional 808 nm laser diode, the multiband emissions around 1.53, 1.80, 2.10, 2.70 and 3.00 μm were observed. The emission bands 1.53 and 2.70 μm were assigned to the transitions of ⁴I_13/2-⁴I_15/2, ⁴I_11/2 -⁴I_13/2 of Er³⁺ ions, respectively, and 1.80 mm was assigned to the transition ⁴F₄ -⁴H₆ of Tm³⁺ ions, and the emissions at 2.10 and 3.00 mm arose from the transitions of ⁵I₇ -⁵I₈, ⁵I₆ -⁵I₇ of Ho³⁺ ions. The materials will be potential in use of ultra broad band amplified spontaneous emission optical sources at near- and mid-IR region.

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