Enhanced emission of 2:7 μm pumped by laser diodefrom Er3�=Pr3�-codoped germanate glasses

Rongrong Xu,1,2 Ying Tian,1,2 Lili Hu,1 and Junjie Zhang1,*1Key Laboratory of Materials for High Power Lasers, Shanghai Institute of Optics and Fine Mechanics,

Chinese Academy of Sciences, Shanghai 201800, China2Graduate School of Chinese Academy of Sciences, Beijing 100039, China

*Corresponding author: jjzhang@mail.siom.ac.cn

Received January 12, 2011; revised February 24, 2011; accepted February 28, 2011;posted March 3, 2011 (Doc. ID 140785); published March 25, 2011

A novel Er3þ=Pr3þ codoped germanate glass was fabricated and analyzed. Efficient emission at 2:7 μm from the glasswas observed upon excitation of a conventional 980nm laser diode. The 2:7 μm emission characteristics and energytransfer (ET) were investigated. Population inversions between 4I13=2 and 4I11=2 levels have been achieved, and anenhanced emission from 2550 to 2800nm was obtained. Large ET efficiency of 95% indicates that the ET processfrom Er3þ to Pr3þ ð4I13=2; 3H4Þ → ð4I15=2; 3F3Þ is efficient and that Pr3þ can enhance the emission of 2:7 μm by quench-ing the lower laser level of Er3þ via ET. © 2011 Optical Society of AmericaOCIS codes: 140.3480, 140.3580, 140.5680, 140.3600, 160.3380, 160.2750.

In recent years, mid-IR lasers operating around 2:7 μmhave triggered increasing interest in coherent mid-IRlight sources for applications in spectroscopy [1] andmedicine [2,3] as well as in efficient and high-qualitypump sources for longer wavelength mid-IR lasers andoptical parametric oscillators [4]. Er3þ-doped fluorideglass shows promise as a compact and efficient sourceof 2:7 μm radiation, and many researchers have devotedthemselves to their study [4–6]. Usually, the mid-IRemission of Er3þ can hardly be observed in oxide glasses,whose phonon energy is large [7]. However, comparedwith fluoride glasses, oxide glasses provide easy prepa-ration process, good thermal stability and chemicaldurability, as well as good physical and mechanical per-formance [8]. Exploring mid-IR emitting from oxide glassbecomes a challenge to researchers. Er3þ is a well-known ion with transitions in the IRZ region around2:7 μm ð4I11=2 → 4I13=2Þ; however, the 2:7 μm emissioncannot be obtained efficiently, because the fluorescencelifetime of the lower level, 4I13=2, is considerably longerthan that of the upper level, 4I11=2. Fortunately, codopingPr3þ, Nd3þ, Cr3þ, Tm3þ, or Ho3þ ions with Er3þ has beenused as a feasible alternative to quench the lower level[7,9–11].To our knowledge, despite important characteristics

for device fabrication, there are few results focusingon the 2:7 μm emission in Er3þ=Pr3þ codoped germanateglass. The previous efforts have mainly paid attention tothe nonoxide glasses, such as fluoride [5,12] and chalco-genide glasses [13]. Germanate glass is chosen as thehost matrix due to its combination of good thermalstability, chemical durability, and high transparency ina wide wavelength range [8]. Compared to silicate orphosphate glasses, germanate glasses have the advantageof being characterized by significantly lower phononenergy. Therefore, the multiphonon de-excitation pro-cesses are less pronounced, and the vibronic absorptionbands appear in the mid-IR at a significantly higherwavelength. The aim of this study is to investigate thespectroscopic properties relating to the 2:7 μm emissionof a new kind of germanate glass and to demonstrate itsfuture applications in mid-IR lasers.

In this Letter, a novel germanate glass is composed of,in mol.%, GeO2 (55%–70%), BaO=BaF2 (10%–30%), Ga2O3(8%–10%), Na2O (5%–10%), La2O3 (3%–10%). Previouswork [8] has reported the characteristic optical proper-ties of barium gallo-germanate (BGG) glass as an exitwindow for high-energy lasers operating in the mid-IRwavelength region. It has been demonstrated [14,15] thatthe properties of BGG glass can also be modified byadding/substituting other components, such as La2O3and Na2O. F− could reduce the content of OH− groupsin glass, which will increase the emission intensity withan efficient energy transfer (ET) of rare-earth ions [16].The doping concentrations of Er2O3 and PrF3 in the sin-gle doped glasses were 2 and 0:2mol:%, respectively. Thecodoped samples were prepared with 2mol:% Er2O3 and0:2mol:% PrF3. Before melting the powders in a SiC-resistance electric furnace, the molar masses wereweighted, mixed, and dried. The mixtures were homoge-nized at 1350 °C in a platinum crucible in the furnace witha stream of dry air. The melt was poured onto a pre-heated brass mold and annealed for several hours at600 °C in a muffle furnace, and then allowed to coolslowly inside the furnace by turning the power supplyoff. The two-face polished sample has a plate shape with12mm × 12mm × 1mm for optical measurements.

The characteristic temperatures, including tempera-tures of glass transition, Tg, onset crystallization peak,Tx, and top crystallization peak, Tp, were measured bydifferential scanning calorimeter via a NETZSCH STA409PC/PG (NETSCH-Gerätebau GmbH, Germany). TheIR transmittance spectrum was measured with a Perki-nElmer 1600 series FTIR (PerkinElmer, USA) spectro-meter in a wave number region between 1500 and4000 cm−1, with a resolution of 2 cm−1. The analysis ofEr3þ=Pr3þ ion absorption was carried out using a Perki-nElmer-Lambda 900UV/VIS/NIR spectrophotometer inthe range of 350 to 2200 nm. Emission in bulk glasswas measured by using a laser diode (LD) to pump at980 nm. The near-IR emission spectra were measuredby a Traix 320 type spectrometer (Jobin-Yvon Co.,France) with resolution of 1nm. Excitation at 980 nmwas also used to measure the lifetime of the laser level.

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The lifetimes were calculated by fitting a single exponen-tial function to the measured data. All the measurementswere performed at room temperature.The values of Tg, Tx, and Tp are 618, 747, and 769 °C,

respectively. The glass formation factor, kgl ¼ðTx − TgÞ=ðTm − TgÞ, developed by Hruby [17], whereTm is the melting temperature of glass, is more suitablefor estimating the glass thermal stability thanΔTð¼ Tx − TgÞ. The existing ability criterion parameterskgl andΔT are 0:176° C and 129 °C, respectively. They arelarger than those of fluoride glasses [18]. The resultsreveal that the germanate glass has good forming abilityand chemical durability. Tg is an important factor forlaser glass; a high one, such as 618 °C, gives glass goodthermal stability to resist thermal damage at high pump-ing intensities.The IR transmittance spectrum of the prepared glass,

indicating the OH− content, is shown in Fig. 1(a). It canbe seen that the maximum transmittance reaches as highas 85%. The 15% loss contains the Fresnel reflections,dispersion, and absorption of the glass. As we know,the content of OH− in glass is related to the emissionefficiency of rare-earth ions, as the residual OH− groupswill participate in the ET of rare-earth ions and reducethe intensity of emission [16,19]. The content of OH−

groups in the glass can be expressed by the absorptioncoefficient of the OH− vibration band at 3 μm [20], whichcan be given by

αOH− ¼ lnðT0=TÞ=l; ð1Þwhere l is the thickness of the sample at 1mm, and T0and T are the transmitted and incident intensities,respectively. The absorption coefficient, αOH− at 3 μm,is 0:196 cm−1, which is much lower than that of telluriteglass [19]. The good IR transmission property proves thatthe germanate glass is a potential candidate for IR lasermaterials.The visible–near-IR absorption spectra of

Er3þ=Pr3þ-codoped and Pr3þ-doped germanate glasseswere shown in Fig. 1. This information allowed an esti-mation of the peak absorption cross section of the4I15=2-4I11=2 transition to be made from the absorption

spectra. The value 7:8 cm × 10−21 cm was found for theEr3þ=Pr3þ-codoped germanate glass. Figure 2 showsthe emission spectra of Er3þ- and Er3þ=Pr3þ-doped ger-manate glasses. It can be seen that the emission intensityat 2:7 μm from Er3þ=Pr3þ-codoped glass is quite strong,whereas emission at 1:5 μm is almost absent. Enhanced2:7 μm emission in the codoped glass is mainly attributedto the quenching effect of the Er3þ:4I13=2 by ET toPr3þ:3F3;4 or energy transfer upconversion (ETU) from4I13=2 [21]. A significant reduction of 1:5 μm emission in-tensity is observed between the Er3þ-doped andEr3þ=Pr3þ-codoped glasses, and the measured lifetimeof Er3þ:4I13=2 was decreased from 8.48 to 0:42ms. Itcan be deduced that the Pr3þ ions can be used effectivelyto depopulate the Er3þ:4I13=2 level. According to theFüchtbauer–Ladenburg theory and emission spectra,the 2:7 μm emission cross section, σe, can be calculated[22]. It is worth noting that the maximum of the calcu-lated emission cross section of Er3þ=Pr3þ-codoped glassis 7:02 cm × 10−21 cm at 2:7 μm, which is larger than thatof Er3þ-doped ZrF4 − BaF2 − LaF3 − AlF3 − NaF (ZBLAN)fiber [23]. It suggests that prepared glass with promisingproperties can be applied in high-power regime lasersystems.

To further understand the energy interaction mech-anisms, the visible upconversion fluorescence spectraof germanate glasses pumped at 980 nm are shown inFig. 3. It can be seen that the intensity of green emission(520 and 542 nm correspond to Er3þ:2H11=2 →

4I15=2and 2S3=2 → 4I15=2, respectively) decreases moderatelybut that of red emission (650 nm corresponds toEr3þ:4F9=2 →

4I15=2) decreases drastically when codopedwith Pr3þ. The energy level diagram of Er3þ and Pr3þ ionsevaluates the processes of ET, ETU, and cross-section[4,6]. It is worth noting that the ET process ð4I13=2; 3H4Þ →ð4I15=2; 3F3Þ is much more efficient than ð4I11=2; 3H4Þ →ð4I15=2; 1G4Þ, because the oscillator strength of the formerprocess is significantly larger than the latter [6]. The ETefficiency has been estimated from the lifetime values bythe following equation: [24]

ηt ¼ 1 −τEr=PrτEr

; ð2Þ

Fig. 1. Absorption spectra of Er3þ=Pr3þ-codoped germanateglass. Insets show (a) IR transmission spectrum and (b) absorp-tion spectra of Pr3þ-doped germanate glass.

Fig. 2. (Color online) (a) 2:7 μm and (b) 1:5 μm emission spec-tra of Er3þ-doped and Er3þ=Pr3þ-codoped germanate glassesunder 980nm excitation.

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where τEr=Pr and τEr are the Er3þ lifetimes monitored at1:5 μm, with and without Pr3þ ions, respectively. Thevalue of ηt from the ð4I13=2; 3H4Þ → ð4I15=2; 3F3Þ processis up to 95% for the Er3þ=Pr3þ-codoped germanate glass.It is demonstrated again that Pr3þ can efficiently quenchthe lower laser level of Er3þ by ET and enhance the emis-sion of 2:7 μm.In sum, a novel germanate glass with good thermal

stability and high transmittance around 3 μm was pre-pared and investigated. Large ET efficiency of 95% andan emission cross section of 7:02 cm × 10−21 cm giveevidence of enhanced emission at 2:7 μm. It has beendemonstrated that Er3þ=Pr3þ-codoped germanate glassis a promising candidate for optical sources for mid-IRlaser applications.

This work is financially supported by Chinese National863 Program (2007AA03Z441).

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Fig. 3. (Color online) Upconversion fluorescence spectra ofEr3þ-doped and Er3þ=Pr3þ-codoped germanate glass under980nm excitation. Insert shows partial energy level diagramof Er3þ and Pr3þ ions.

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