Mid-Infrared, Green and Red Emission in Ho3+ - Er3+ Co-Doped Phosphate Glasses
Authors: Taherunnisa Sheik , Rami Reddy M. , Piasecki Michal , Veeraiah Nalluri , Brik Mikhail G. 
Published: December 2020

Over the past decades, phosphate glasses show ample advantages in laser technology because of their excellent optical properties like a low glass transition temperature, low phonon energy, large infrared transmission window, and high gain density. The inclusion of lead and alkali earth metals to these phosphate glasses also improves the chemical durability of glasses, which have more advantages in smart cards, medical applications, and micro-batteries. The optimum environment for chemical durability and incorporating rare earth ions can be achieved by the interaction between sulphate and phosphate ions. Moreover, phosphate glasses mixed with alkali sulphate provides scope for micro battery applications. The presence of network modifiers like Pb2+, Zn2+ in phosphate glasses introduces structural modifications and increases the number of non-bridging oxygens (NBOs) in glass network. The metal oxides like PbO, Bi2O3 makes phosphate glass suitable for rare earth ion incorporation by softening the glass and provides better distribution by controlling the formation of clusters. The wide variety of rare earth ions in the phosphate glasses leads to wide range of applications in optoelectronic devices.
To overcome the difficulty associated with lack of a useful absorption band at 980 nm of Ho3+ ion (easy excitation by commercial laser diode (LD)), rare earth ions such as Tm3+, Yb3+, and Er3+ can be introduced. Especially, among them, Er3+ exhibit strong absorption at 980 nm. Due to the narrower energy gap between Er3+: 4I13/2 and Ho3+: 5I7 states, the emission of Ho3+ ions can be enhanced by addition of Er3+. So, the Er3+/Ho3+ co-doped glasses have promising applications in mid-infrared (MIR) laser applications with 980 nm LD. In view of this, the set of the Er3+/Ho3+ co-doped sodium-sulfo lead phosphate glasses was prepared with the molar composition of (20-x-y) Na2SO4-20PbO-60P2O5-xEr2O3-yHo2O3 (x=0.5, y= 0.2, 0.4, 0.6, 0.8, 1.0 mol %) named as GEH-y glass and coded as GExHy. To explore various possible applications, all prepared glasses are characterised by EDS, XRD, FTIR, absorption, photoluminescence and lifetime profiles. All results are computed and few of them are compared with other glasses.
We present results focused on both mid infrared and visible emission properties of Er3+/Ho3+ions in sodium-sulfo lead phosphate (GExHy) glasses. These materials exhibit simultaneously DC (down-conversion) and UC (up-conversion) photoluminescence (PL) by excitation at 379 nm and 980 nm. The favourable concentration of Ho2O3 was estimated from the analysis of NIR PL spectra for Ho3+: 2.0 mm and Er3+: 1.5 mm. In GExHy glasses, the increase in intensity near 2.0 mm with decrease in intensity at 1.5 mm infers to energy transfer process between Er3+ and Ho3+ ions.
Under lexc= 379 nm the Er3+ ions are pumped to the 4G11/2 excited state and depopulate {via multi phonon relaxation MPR1} at lower 4S3/2,
2H11/2, and 4F9/2 levels and then relaxed to the 4I15/2 ground state by emitting weak and strong green emission bands along with weak red emission related to 4S3/2→4I15/2 (522 nm), 2H11/2→4I15/2 (544 nm), and 4F9/2→4I15/2 (657 nm) transitions, respectively. The up-conversion energy transfer (ET) analysis with 980 nm LD excitation, undergoes the following mechanism:

Ground state absorption (GSA) : Er: 4I15/2 + hν → Er: 4I11/2

Excited State Absorption
(ESA1) : Er: 4I11/2+ hν → Er: 4F7/2

Excited State Absorption
(ESA2) : Er: 4I13/2 +hν → Er: 4F9/2

CR1 (Cross Relaxation) : Er: 4I11/2+Ho: 5I6→ Er: 4I15/2+Ho: 5F4

CR2 (Cross Relaxation) : Er: 4I13/2+Ho: 5I6→ Er: 4I15/2+Ho: 5F5

ET1: Er: 2S3/2+Ho: 5I8→ Er: 4I15/2+Ho: 5S2+5F4
⇒ Ho: 5S2+5F4 → 5I8 (545 nm)

ET2 : Er: 4F9/2+Ho: 5I8→ Er: 4I15/2+Ho: 5F5 ⇒ Ho: 5F5→5I8 (659 nm)

MPR2: Ho: 5S2+5F4 → 5F5

ET3: Er: 4I11/2+Ho: 5I8→ Er: 4I15/2+Ho: 5I6 ⇒ Er: 4I13/2 → 4I15/2 (1554 nm)

ET4: Er: 4I13/2+Ho: 5I8→ Er:4I15/2+Ho

Fig. 1 Schematic energy level diagram of GE0.5H0.8 glass.
The maximum energy transfer efficiency of GExHy samples is 75.5% and with the increase of Ho3+ ions, the lifetime decay has been reduced from 5.06 ms to 1.24 ms. The absorption and stimulated emission cross-sections of Er3+:1.5 mm and Ho3+: 2.0 mm were calculated. Moreover, the FWHM × σemi was estimated for Ho3+:5I7→5I8 transition to estimate the probable MIR laser emission. The CIE color coordinates were estimated from both DC and UC PL spectra. All the results indicated that the prepared GExHy glasses have promising material applications in mid-infrared solid-state lasers and telecommunications. In addition, efficient green and red emission makes these glasses attractive for the phosphor applications.

Figure 1