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+: 4I13/2→4I15/2, Ho3+: 5I7→5I8, Er3+: 4I11/2→4I13/2 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+: 4I13/2→4I15/2, 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 4I9/2 of Nd3+, 4I15/2 of Er3+ and 5I8 of Ho3+ are excited to the states 4F3/2 of Nd3+ and 4I9/2 of Er3+ and 5I5 of Ho3+, respectively. Due to smaller energy level difference between 4F3/2 of Nd3+ and 4I9/2 of Er3+, 4F3/2 of Nd3+ and 5I5 of Ho3+, the energy at the excited state (4F3/2) of Nd3+ is transferred (ET1,ET2) to 4I9/2 of Er3+ and 5I5 of Ho3+. For Er3+ ions, the electrons at the excited state relax into 4I11/2, then further transit into 4I13/2, 4I15/2, 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+, 4I15/2 of Er3+ and 5I8 of Ho3+ are excited to the states 3F4 of Tm3+ and 4I9/2 of Er3+ and 5I5 of Ho3+. Due to smaller energy level difference between 5I6 of Ho3+ and 4I13/2 of Er3+, 4I13/2 of Er3+ and 3H4 of Tm3+, a fraction of energy at 5I5 of Ho3+ is transferred (ET3) to 4I13/2 of Er3+, and a fraction of energy at 4I13/2 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 4I11/2, 4I13/2, then transit into 4I15/2, 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 4I13/2 of Er3+, 4I13/2 of Er3+ and 3H4 of Tm3+, a fraction of energy at 5I6 of Ho3+ is transferred into the 4I13/2 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 4I11/2 and 4I13/2 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 4I13/2-4I15/2, 4I11/2 -4I13/2 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.