Two serial Dy3+-doped and Dy3+, Tm3+-codoped (100-x)(0.8GeS2·0.2Ga2S3)·xCdI2 (0≤x≤20) chalcohalide glasses were prepared and characterized. By analyzing the absorption and emission measurements of the two serial chalcohalide glasses with the Judd-Ofelt analysis, we were able to calculate its Judd-Ofelt strength parameters Ωt (t = 2, 4, 6), transition probabilities, exited state lifetimes, branching ratios, and emission cross-sections. In the Dy3+-doped glasses, increase of CdI2 had positive effect up to 1330nm fluorescence and the emission cross section (σemi) was estimated to be 4.19 × 10−20cm2 for the 0.2wt% Dy3+-doped 64GeS2·16Ga2S3·20CdI2 glass. In the Dy3+, Tm3+-codoped glasses, increase of CdI2 diminished the amount of ethane-like units [S3(Ga)Ge-Ge(Ga)S3], improved the Tm3+: 3F4→ Dy3+: 6H11/2 energy transfer efficiency and intensified the mid-infrared emissions. The emission cross sections (σemi) of the 2900 and 4300nm fluorescences were estimated to be 1.68 × 10−20 and 1.20 × 10−20cm2 respectively for the 0.2wt% Dy3+ and 0.5wt% Tm3+ codoped 64GeS2·16Ga2S3·20CdI2 glass. These novel chalcohalide glasses are promising candidate materials for fiber-amplifiers and mid-infrared laser devices.
©2009 Optical Society of America
2~5μm mid-infrared lasers are located in the three atmospheric transmission windows and cover a great number of important molecular characteristic spectral lines; therefore they have extensive application prospects in the fields of remote sensing, range finding, environmental monitoring, bio-engineering, medical treatment, and etc [1–3].
One efficient and feasible method of generating the 2~5μm mid-infrared lasers is pumping the rare-earth ions in proper hosts by using a near-infrared laser diode (LD). This method promotes a good prospect of all-solid-state mid-infrared laser possessing many advantages such as high power output, good stability, low cost in production and operation, and etc. However, because of the small gaps between the involved and intermediate energy levels of the rare-earth ions, non-radiative quenching of the mid-infrared emissions is substantial. Low phonon-energy materials are required to act as laser working medium.
In the past several years, chalcogenide, fluoride glasses or crystals, as well as chloride crystals have received considerable interests for their low phonon-energy. The longest laser wavelength of 7.24μm-produced in rare-earth ions doped solid-state laser working at room temperature was obtained in the Pr3+-doped LaC13 crystal . But the crystals are restricted to poor chemical durability. For the glass medium, the only continuous excitation at room temperature presently is the 3.5μm laser produced by Er3+-doped ZBLAN fiber , and longer wavelength mid-infrared lasers are hard to be generated. The Ho3+-doped ZBLAN fiber laser can generate 3.95μm laser, but must be operated at low temperature to avoid non-radiative quenching . Finding other more suitable glass materials as laser working medium which can generate 2~5μm waveband mid-infrared laser at room temperature is the main emphasis of our research. Chalcogenide and chalcohalide glasses presently seem to be the best candidate materials due to their unique features.
Compared with other glass hosts, Chalcogenide and chalcohalide glasses exhibit two distinctive features which are the higher refractive indices and lower phonon energies. The high refractive indices of greater than 2.1 result in large stimulated emission cross sections and the typically low phonon energies of 250~450cm−1 result in low non-radiative decay rates.
Many works in previous reports have proved the probability of mid-infrared light emitted in some well-established chalcogenide glasses such as Ge16.5As18.5Ga0.5Se64.5 , Ge30As10S60 , Ge25Ga5S70  and 70Ga2S3·30La2S3 , but little effort was put on the compositional optimization since else sulfides or halides introduced in glasses can largely effect their physical and optical properties as well as their micro-structures.
In this paper, we selected Ge-Ga-S-CdI2 glass as the host system. The spectral properties of Dy3+-doped and Dy3+/Tm3+-codoped glasses excited at 808nm are presented. Judd-Ofelt strength parameters Ωt (t = 2, 4, 6), the transition probabilities, exited state lifetimes, branching ratios, and emission cross-sections are calculated from absorption and emission measurements by using the Judd-Ofelt analysis. Effects of CdI2 content and Tm3+ level on the intensities of the 2.9μm (Dy3+: 6H13/2→6H15/2) and 4.3μm (Dy3+: 6H11/2→6H13/2) mid-infrared emissions are investigated and discussed. The glasses in Ge-Ga-S-CdI2 system were chosen because of their wide transparency in the visible region and good thermal stability . With addition of the heavy metal iodide CdI2, the density and refractive index increase almost linearly, therefore larger stimulated emission cross section, lower non-radiative decay rate and higher radiative quantum efficiency in mid-infrared waveband are expected.
Our work was aimed at the elucidation of the glass composition’s influence on the spectroscopic properties in this novel chalcohalide system and the discovery of a new material for applications in fiber-amplifier and mid-infrared laser devices.
2. Experimental procedure
The compositions of the host chalcohalide glasses were (100-x)(0.8GeS2·0.2Ga2S3)·xCdI2 with x = 5, 10, 15 and 20, while x is the mole percent. More precisely, these glasses were labeled as GGC5, GGC10, GGC15, GGC20, respectively. All investigated samples were prepared by heating a mixture (typically 6 grams) of the required amounts of the elemental materials (5N for Ge, Ga and S, 3N for CdI2, Dy2S3 and Tm2S3) at 950°C in an evacuated quartz ampoule (10mm inner diameter ampoule) for 12 hours and were subsequently quenched in an ampoule containing the liquid into a cold water bath. After being annealed near the glass transition temperature for 2 hours, the glass rod was cut into plates. Both surfaces of the plates were polished to mirror smoothness. The thicknesses of the obtained glass samples were 4.0mm.
All optical and spectroscopic measurements were carried out at room temperature. Absorption spectra of the samples were recorded using the Shimadzu UV-3101PC spectrophotometer between 400 to 3000nm wavelength. Fluorescence spectra were obtained using 808nm excitation from a laser diode (LD) (the average power is 1W). An InGaAs detector (Judson, U.S.A.) and an InSb detector (Judson, U.S.A.) cooled with liquid nitrogen were used to measure the intensities of fluorescence within the wavelength region of 1000~1500nm and 1650~4700nm, respectively. During the whole test, the locations of samples and the pumping source were kept unchanged and the excitation power was unvaried.
3.1 Dy3+-doped glasses
3.1.1 Absorption spectra and Judd-Ofelt calculation
In this section, we will focus on the optical properties of Dy3+ sole-doped GGC chalcohalide glasses. The Dy3+ doping content is fixed to 0.2wt%. Figure 1 is the Vis-NIR absorption spectrum of the GGC5 glass in the wavelength region of 500~3000nm. The short-wavelength absorption edge of the glass lies at about 505nm. Good visible light transmittance is helpful for being pumped with proper excitation source. The absorption bands corresponding to internal 4f-4f electronic transitions of Dy3+ are indicated in the figure, in which the intensities of 910, 1108, 1298, 1700, 2826nm are strong, whereas that of 808nm is weak. This indicates that the pumping efficiency of 808nm LD will be low. On the other hand, as for the (100-x)(0.8GeS2·0.2Ga2S3)·xCdI2 (x = 5, 10, 15 and 20) serial glasses, the absorption coefficients of Dy3+ bands increase linearly as CdI2 content increases. This slightly increases the absorption and efficiency of the pumping.
The oscillator strengths of the GGC glasses were derived by integrating each absorption band area, and the results are summarized in Table 1 . Some representative host glasses are also listed for comparison. It is apparent that the oscillator strengths of chalcogenide and chalcohalide glasses are significantly larger than those of oxide and fluoride glasses. Particularly, in the oxide and fluoride glasses, oscillator strengths related to Dy3+ energy levels are not quite different. Whereas in the chalcogenide and chalcohalide glasses, oscillator strength of the electric dipole transition 6H15/2→6H9/2 + 6F11/2 is much greater than other transitions, therefore the intensity of the corresponding absorption at 1298nm is the greatest within the absorption spectrum, as shown in Fig. 1. Furthermore, the oscillator strength of this transition also increases with the CdI2 addition in the GGC serial glasses. It is obvious that the oscillator strength of the 6H15/2→6H9/2 + 6F11/2 transition has a strong compositional dependence.
Table 1 also compares the Judd-Ofelt intensity parameters of Dy3+ among various glass hosts. The calculated values of Ω 2 for Dy3+ in GGC glasses are larger than those of other investigated hosts; and with the increase of CdI2 content, the Ω 2 is enhanced rapidly from 11.82 × 10−20 to 14.26 × 10−20cm2. The value of Ω 6 also has a similar trend, whereas that of Ω 4 decreases markedly.
3.1.2 Fluorescence spectra and Radiative properties
The fluorescence spectra of Dy3+-doped GGC serial glasses under 808nm LD excitation in the ranges of 1000~1500 and 1650~1900nm are respectively shown in Fig. 2 . Three major emission bands were observed at around 1140, 1330 and 1750nm, corresponding to the optical transitions 6H7/2 + 6F9/2→6H15/2, 6H9/2 + 6F11/2→6H15/2 and 6H11/2→6H15/2, respectively. From GGC5 to GGC20, the intensities of the 1140 and 1330nm fluorescence increase. Simultaneously, the band center of the 1330nm fluorescence shifts to longer wavelength. The intensity and center location of 1750nm fluorescence have no obvious change. It should be noted that the effective line width (Δλeff) of 1330nm fluorescence also increases, while that of 1750nm has no obvious change. Unfortunately, the intensities of mid-infrared emissions at 2900 and 4300nm were too weak to be detected in the Dy3+ sole-doped glasses.
Radiative transition probabilities (A), branching ratios (β) and radiative lifetimes (τ rad) of the excited states of Dy3+ were evaluated following Judd-Ofelt theory and summarized in Table 2 .
The emission cross sections (σemi) of 1330 and 1750nm were calculated basing on the McCumber equation 
Radiative transition probabilities in the GGC glasses increase almost linearly with the addition of CdI2 and the values are similar to that of Ge25Ga5S70 glass but considerably greater than those of tellurite glasses. This is due to the higher refractive indices of GGC chalcohalide glasses.
3.2 Dy3+, Tm3+-codoped glasses
3.2.1 Absorption spectra
Figure 3 illustrates the Vis-NIR absorption spectra of the GGC20 glasses codoped with 0.2wt% Dy3+ and 0.3~0.7wt% Tm3+ in the wavelength region of 500~3000nm. Each assignment corresponds to the excited level of Dy3+ and/or Tm3+. It is apparent that the absorption at 808nm was enhanced with the addition of Tm3+.
3.2.2 Fluorescence spectra
Figure 4 shows the evolutions of fluorescence spectra with the increase of Tm3+ concentration in Dy3+, Tm3+-codoped GGC20 glasses. Emissions at 2900 and 4300nm due to the Dy3+: 6H13/2→6H15/2 and Dy3+: 6H11/2→6H13/2 transitions were clearly observed. The full widths at the half maximum (FWHM) intensities were ~170 and ~230nm, respectively. Introduction of Tm3+ also resulted in an obvious enhancement of 1330nm emission which is ascribed to Dy3+: 6H9/2 + 6F11/2→6H15/2 transition. On the other hand, when the Tm3+ amount was more than 0.5wt%, the intensity of emission at 1450nm due to the Tm3+: 3H4→3F4 transition shows no increase.
It should be noted that during the preparation of the experiment, we found that the glassy system became very unstable when the Tm3+ concentration was further increased, and the quartz ampoule would easily explode in the furnace. On the other hand, it can be seen from Fig. 4 that the intensities of emissions at 2900 and 4300nm were greatly improved when the Tm3+ concentration was enhanced from 0.3 to 0.5wt%, but no prominent improvements were achieved when the Tm3+ concentration was enhanced from 0.5 to 0.7wt%. Considering the above factors comprehensively, 0.5wt% is a suitable concentration for Tm3+. Therefore, in the following text for investigating the effect of CdI2 content on mid-infrared emissions, the concentrations of Dy3+ and Tm3+ were fixed to 0.2 and 0.5wt%, respectively.
The fluorescence spectra of Dy3+,Tm3+-codoped GGC serial glasses under 808nm LD excitation in the range of 1000~1600 and 1650~4700nm are shown in Fig. 5(a) and (b) respectively. It is worth noting that for the concerned two mid-infrared emissions at 2900 and 4300nm, the intensities’ evolutions have similar trends with that of the 1450nm emission, which is the intensities decrease first and then increase as the CdI2 content increasing. But the intensity ratio of I1450/I1800 decreases remarkably, as shown in Fig. 5(c). The calculated emission cross sections (σemi) of 2900 and 4300nm for GGC20 glass are 1.68 × 10−20 and 1.20 × 10−20cm2, respectively. These values are higher than those of the Dy3+-doped chalcogenide glasses in previous studies [7,9].
4.1 Oscillator strengths and intensity parameters
Oscillator strengths and intensity parameters are related to the local environments of rare-earths in hosts. Generally, lower symmetry of polyhedra surrounding the rare-earths and higher covalency of bonds inside the hosts result in larger oscillator strengths. Compared to oxide and fluoride glasses, oscillator strengths of Dy3+ ion in chalcogenide and chalcohalide glasses have larger values resulting from more significant distortion of sulfur-rare-earth polyhedra  and higher covalency of bonds. It can also be seen from Table 1 that in the GGC serial chalcohalide glasses, the values of Dy3+ oscillator strengths were enhanced almost linearly as the content of CdI2 increased. According to our previous work on GGC glasses’ structure , with the addition of CdI2, the amount of the original [GeS4] tetrahedra in glassy network decreases whereas the number of [S4-xGeIx] (x = 1, 2) tetrahedra becomes larger. The symmetry decreases and the covalence has also been reduced because of larger electronegativity of the I- ion. Considering the enhancement of oscillator strengths in the GGC serial glasses, the decrease of symmetry is therefore suggested to play a dominant role whereas the influence of covalence’s decrease can be ignored. Large Ω 2 values for GGC serial glasses also provide good evidence of the large asymmetry of the Dy3+ ion site.
Furthermore, value of Ω 6 increases with the addition of CdI2, reflecting the fact that more ionic of bonds and lower rigidity of glass are presented.
4.2 Effect of CdI2 content on the emission spectra of Dy3+-doped GGC glasses
From the fluorescence measurement, it can be seen that higher content of CdI2 in GGC glass system induces stronger fluorescence of Dy3+ ions, especially at 1330nm. This phenomenon accords with the variation of spontaneous emission probability summarized in Table 2 and has a similar trend with those of halogen or alkali halide introduced chalcogenide glasses . Lower nonradiative decay rate rooting from lower phonon energy and the enhancement of pumping efficiency rooting from the increase of Dy3+ absorption coefficients with the addition of CdI2 can effectively populate 6H7/2-6F9/ and 6H9/2-6F11/2 levels, resulting in the intensified fluorescences at 1140 and 1330nm.
The glassy structure’s evolution following with the CdI2 addition induces more stark level splitting in Dy3+ , resulting in the band center’s red-shift and shape’s broaden of 1330nm fluorescence.
4.3 Energy transfer and effect of Tm3+ concentration on the emission spectra of Dy3+, Tm3+-codoped GGC glasses
In the Dy3+ sole-doped glasses, we did not observe the 2900 and 4300nm emissions because of two main reasons. The intrinsical one is that the absorption of Dy3+at 808nm is quite low and the radiative transition probabilities of mid-infrared emissions are small. Also, the relatively low power of 808nm LD and low sensitivity of detecting equipment used in this experiment are further causes of the absences of 2900 and 4300nm emissions.
In the Dy3+, Tm3+-codoped GGC glasses, emissions at 2900 and 4300nm were clearly observed. Preliminary study by J. Heo’s indicated the presence of energy transfer process from the Tm3+: 3F4 to Dy3+: 6H11/2 (as shown in Fig. 6 ) and the increase of the 2900nm fluorescence accordingly . Evolutions of spectra in present work also proved this process. Furthermore, occurrence of another energy transferring from the Tm3+: 3H5 to Dy3+: 6H9/2 + 6F11/2 is suggested based on the obvious enhancement of 1330nm fluorescence shown in Fig. 4 (a). This provides a new idea for the development of a 1.3μm fiber amplifier. A cross relaxation process, i.e. 3H4, 3H6→3F4, 3F4 is well-known to exist in the high concentration Tm3+ doped chalcogenide glasses . The corresponding exhibition in fluorescence spectrum is that the intensity ratio of I1450/I1800 becomes smaller with increasing Tm3+ concentration. It can therefore be concluded from Fig. 4 that the 3H4, 3H6→3F4, 3F4 cross relaxation process has occurred in the 0.2wt% Dy3+ and 0.5wt% Tm3+ co-doped glasses. While, considering the aim of our present work is to obtain strong 2900 and 4300nm mid-infrared emissions, large electron populations of Tm3+: 3F4 is expected since it acts as source in the Tm3+: 3F4→ Dy3+: 6H11/2 energy transfer process. Therefore, in the Dy3+, Tm3+-codoped GGC glasses, the concentration of Tm3+ should be no less than 0.5wt%.
4.4 Effect of CdI2 content on the emission spectra of Dy3+, Tm3+-codoped GGC glasses
J.Heo et al.  figured out that the existence of the ethane-like units is responsible for the relatively higher solubility of rare-earth ions in Ge-Ga-S glasses as compared to the As-based glasses. Structural investigation of Ge-Ga-S-CdI2 glasses in our previous work based on the Raman spectral evolution indicated that in the glasses with little CdI2, some ethane-like units [S3(Ga)Ge-Ge(Ga)S3] exist because of the lack of sulfur, but the amount of these units will decrease with the addition of CdI2 . When the content of CdI2 is higher than 10mol%, the ethane-like units disappear. It makes the clustering degree of Tm3+ ions increases, resulting in the decrease of intensity ratio of I1450/I1800 from GGC5 to GGC20 glass. This is helpful for the Tm3+: 3F4→ Dy3+: 6H11/2 energy transfer process and relatively strong mid-infrared emissions are achieved in Dy3+, Tm3+-codoped GGC20 glasses.
In the search for novel materials with potential practical applications in fiber-amplifiers and mid-infrared laser devices, a serial Dy3+-doped and Dy3+, Tm3+-codoped (100-x)(0.8GeS2·0.2Ga2S3)·xCdI2 (0≤x≤20) chalcohalide glasses were elaborated. Judd-Ofelt strength parameters Ωt (t = 2, 4, 6) were evaluated based on the absorption spectra. The transition probabilities, exited state lifetimes and the branching ratios were calculated by using the Judd-Ofelt theory. For the Dy3+-doped glasses, increase of CdI2 had a positive effect on the 1330nm fluorescence, yielding from the lower non-radiative decay rate and higher pumping efficiency. The largest emission cross sections (σemi) of 1330nm were estimated to be 4.19 × 10−20cm2 for the 0.2wt% Dy3+-doped 64GeS2·16Ga2S3·20CdI2 glass. For the Dy3+, Tm3+-codoped glasses, an increase of CdI2 diminished the amount of ethane-like units [S3(Ga)Ge-Ge(Ga)S3], improved the Tm3+: 3F4→ Dy3+: 6H11/2 energy transfer efficiency and intensified the mid-infrared emissions. The largest emission cross sections (σemi) of 2900 and 4300nm were respectively estimated to be 1.68 × 10−20 and 1.20 × 10−20cm2 for the 0.2wt% Dy3+, 0.5wt% Tm3+-codoped 64GeS2·16Ga2S3·20CdI2 glasses.
This work was partially funded by the National Natural Science Foundation of China (NSFC, No. 10876009 and 60807034) and one Hundred Talents Programs of the Chinese Academy of Sciences.
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