In this work we present a detailed analysis of the infrared to visible upconversion in Nd3+-doped KPb2Br5 low phonon crystal by using both steady-state and time-resolved luminescence spectroscopy. Efficient blue, green, orange, and red emissions have been observed under excitation into the 4F5/2 and 4F3/2 states. The low phonon energy of this crystal leads to a significant reduction of the multiphonon relaxation rates which allows most of the excited states to relax radiatively. To investigate the nature of the upconversion processes, emission spectroscopy and lifetime measurements for the visible fluorescence, performed by using one photon excitation, have been compared to the upconverted emissions and lifetimes obtained under infrared pulsed excitation. The analysis of the experimental results indicates that the upconverted emissions occur via energy-transfer upconversion.
©2006 Optical Society of America
Frequency upconversion of infrared (IR) into visible (VIS) light in rare-earth (RE) doped solids has been intensively investigated due to the possibility of infrared-pumped visible lasers and their potential applications such as color display, optical storage, and medical applications . Recently, the search for all-solid compact laser devices operating in the blue-green region and the availability of powerful near-infrared laser diodes as pumping sources has increased the interest in upconversion emission. Among rare-earth ions, neodymium has been recognized as one of the most efficient rare-earth ions for solid-state lasers in different hosts [2, 3] due to its intense emission at 1.06 μm. In addition to the infrared emissions, Nd3+ ion could be also considered as a good candidate for upconversion fluorescence and lasers [4–8]. In order to investigate new upconversion materials with high luminescence efficiency, hosts with low phonon energies are required. The advantage of sulfide and halide (chloride, bromide,..) based hosts over the most extensively studied fluoride compounds is the lower phonon energy which leads to a significant reduction of the multiphonon relaxation rates Unfortunately, one drawback of chloride and bromide systems is that these materials usually present poor mechanical properties, moisture sensitivity, and are difficult to synthesize. An important advance on the search for new low phonon energy materials has been the identification of potassium lead halide crystals KPb2X5 (X=Cl, Br) as new low-energy phonon hosts for rare-earth ions. These crystals are non-hygroscopic and readily incorporate rare-earth ions [9–18]. Efficient IR to VIS upconversion in Pr3+-doped and Yb3+- Pr3+;-codoped KPb2Cl5 , in Er3+-doped  KPb2Cl5, and KPb2Br5 , and Nd3+-doped  KPb2Cl5 has been recently reported by some of the present authors and the mechanisms responsible for these upconversion processes have been investigated. Potassium lead bromide crystal KPb2Br5 (KPB) presents similar properties to KPb2Cl5 but with the advantage of even lower phonon energies, due to the higher mass of the bromine constituent. According to Raman-scattering measurements the maximum phonon energy, measured at the highest energy peak of the spectrum, is 138 cm-1 . The crystal is monoclinic  (space group P21/c) with lattice parameters a=0.9264 nm, b=0.8380 nm, c=1.3063 nm, and β=90.06°.
Similarly to RE doped KPb2Cl5, Pb2+ ions occupy two non-equivalent lattice sites, one site Pb(1) is a distorted octahedron and the second site Pb(2) is a distorted trigonal prism. It was supposed that RE ions substitute the lead (Pb2+) ions, and potassium (K+) vacancies were assumed to provide charge compensation . Very recently, some of the authors have demonstrated the existence of three different local environments around the RE ions in KPb2Cl5 by using the fluorescence line narrowing technique. The study reveals that the RE ions may occupy both the Pb and K sites but the luminescence results suggest that RE ions occupying the Pb(2) site is most likely to occur .
The IR spectroscopic properties of KPb2Br5:Nd3+ have been recently investigated showing laser action at 0.97 μm, 1.07 μm, and 1.18 μm [15, 19]. It is worthy to mention that the low phonon energy of this crystal leads to a significant reduction of the multiphonon relaxation rates which allows the excited states to get longer lifetimes and relax radiatively or serve as intermediate states for further upconversion. For example, excitation in the 4F5/2 state is followed by fast nonradiative relaxation into 4F3/2 state in fluoride matrices, with this level acting as intermediate state for upconversion. This is different in KPB crystal where multiphonon relaxation of the 4F5/2 state is negligible. In this work we present a detailed analysis of the infrared to visible upconversion processes in an Nd3+-doped KPb2Br5 crystal following excitation in the 4F3/2 and 4F5/2 levels by using both steady-state and time-resolved luminescence spectroscopy. We have observed blue, green, orange, and red emissions at room temperature and low temperature. The mechanisms leading to these emissions have been investigated by studying the dependence of the upconversion luminescence on the wavelength and intensity of the IR pump light as well as their temporal behaviour. The excitation wavelength dependence together with the temporal evolution of the upconverted emissions indicate that energy transfer upconversion processes are responsible for the observed emissions.
2. Experimental techniques
Single crystals of non-hygroscopic ternary potassium-lead bromide KPb2Br5 doped with Nd3+ ions were grown by the Bridgman technique. The Nd3+ content in the melt was 1 wt%. The Nd3+ concentration was measured to be ≈ 0.14 wt% using inductively coupled plasma optical emission spectroscopy (ICP-OES).
The sample temperature was varied between 77 K and 295 K with a continuous flow cryostat. Conventional absorption spectrum was performed with a Cary 5 spectrophotometer. The upconversion emission and excitation measurements were made by using a Ti-sapphire ring laser (0.4 cm-1 linewidth) in the 780-920 nm spectral range. The excitation beam was focused on the crystal by using a 50 mm focal lens. The fluorescence was analyzed with a 0.25 m monochromator, and the signal was detected by a Hamamatsu R928 photomultiplier and finally amplified by a standard lock-in technique. Direct emission spectra after one photon excitation were performed with a dye laser pumped by a pulsed nitrogen laser and detected by an EGG-PAR optical multichannel analyzer. Upconversion emission was also detected by an EGG-PAR optical multichannel analyzer. Lifetime measurements were obtained by exciting the sample with a dye laser pumped by a pulsed nitrogen laser and a Ti-sapphire laser pumped by a pulsed frequency doubled Nd:YAG laser (9 ns pulse width), and detecting the emission with Hamamatsu R928 and R5509-72 photomultipliers. The data were processed by a Tektronix oscilloscope.
Visible upconversion has been observed under continuous wave (cw) and pulsed laser excitation in the 4F5/2 and 4F3/2 levels. As an example, Fig. 1 shows the upconversion emission spectra in the 350-700 nm region at 295 K and 77 K obtained by exciting the 4F5/2 and 4F3/2 multiplets. Figures 1(a) and 1(c) present room temperature upconverted emission spectra obtained by exciting the sample at 813 nm and 886 nm in resonance with the 4I9/2→4F5/2 and 4I9/2→4F3/2 transitions respectively. As can be seen, the spectra are similar with emissions in the blue, green, orange, and red regions of the spectrum. It is worthy to mention that, for both excitations, the visible luminescence is very intense and the sample looks yellow-orange with naked eye. At low temperature, new very weak emissions appear at shorter wavelengths around 365, 388, and 420 nm corresponding to the 4D3/2→4IJ (J=9/2, 11/2, and 13/2) whereas the blue, green, and red emissions strongly decrease. In addition, the high energy components of the green, orange, and red emissions disappear at 77 K, which could indicate that the observed emissions at room temperature come from two closely spaced energy levels that are thermalized at 295 K but not at 77 K. At low temperature, only the lower lying energy level is populated, and thus only emissions at the longer wavelength remain.
According to the energy level diagram of Nd3+ in KPB crystal, the blue emission located at around 436 nm can be attributed to the 2P1/2→4I9/2 transition, whereas the emission at 479 nm can be due to the 2G9/2→4I9/2 transition. In contrast with the fact that the assignments of the blue emission bands are relatively simple, the green, orange, and red emissions are more complicated to be assigned due to the complexity of the energy levels of Nd3+ ions which allows accidental coincidences of fluorescence wavelengths resulting from transitions between different energy levels. For example, as it is shown in Fig. 2, the green emission band from 510 to 550 nm could be attributed to the (2G9/2-4G11/2)→4I11/2 and (4G7/2-4G9/2)→4I9/2 transitions, whereas the orange emission from 570 to 620 nm and the red emission from 620 to 690 nm could be assigned to the (2G9/2-4G11/2)→4I13/2,15/2, (4G7/2-4G9/2)→4I11/2,13/2, and (4G5/2-2G7/2)→4I9/2,4I11/2 transitions. The energy level diagram in Fig. 2 has been obtained from the RT absorption spectrum and the energies correspond to the barycenters of the absorption bands. As we can see many of the emissions that arise from the high energy levels overlap, and in addition several levels have energy separations small enough to produce thermalization of the second level at room temperature.
To identify which are the emitting levels responsible for the upconverted green, orange, and red emissions we have performed the emission spectra of the sample upon one photon (OP) excitation of the high energy levels of Nd3+ ion, and we have measured their intrinsic lifetimes. Concerning the green, orange, and red emission bands, we have recorded the emission spectra by exciting at 479, 532, and 592 nm, in resonance with the 4I9/2→2G9/2, 4I9/2→4G7/2, and 4I9/2→4G5/2 transitions respectively at 295 K and 77 K. These spectra are shown in Figs. 3 and 4 together with the upconversion emission spectra obtained at 813 nm for comparison. The emission spectrum obtained under excitation at 479 nm into the 4I9/2→2G9/2 absorption band shows green, orange, and red bands. The orange and red bands are similar to those found in the upconversion emission spectra, whereas the green one is sharper and shifted to shorter wavelengths. Upon excitation at 532 nm in resonance with the 4I9/2→4G7/2 absorption band, the emission spectrum shows the same features as the upconverted emissions; however the upconverted green emission is broader. A more detailed analysis of the green emission (see Fig. 5) shows that this upconverted emission is due to the overlapping of the (2G9/2-4G11/2)→4I11/2 and (4G7/2-4G9/2)→4I9/2 transitions. The assignment of the orange and red emissions is more complex because, as we can see in Figs. 3 and 4, similar orange and red emission bands are observed for the three excitation wavelengths. Therefore, we can not rule out the contribution of any among these groups of levels: (2G9/2-4G11/2), (4G7/2-4G9/2), and (4G5/2-2G7/2) to the upconverted emissions. Moreover, the intrinsic lifetimes of these emitting levels obtained under one photon (OP) excitation at room temperature are 18 μs (2G9/2), 16 μs (4G7/2), and 18 μs (4G5/2) respectively making it difficult to distinguish the emitting level on the basis of lifetime measurements.
To investigate which are the mechanisms for populating the upconversion emitting levels, we have measured the excitation spectra of the upconverted emissions, their pump power dependence, and lifetimes. Regarding the pump power dependence of the upconverted emissions, we measured the intensity of the visible bands by pumping the sample at different powers at 813 nm, in resonance with the 4I9/2→4F5/2 transition. The logarithmic plot of the emissions at 535, 598 and 660 nm as a function of the laser intensity shows a nearly quadratic dependence which indicates a two photon upconversion process. The same behavior is observed for the blue emission from level 2P1/2, which confirms that a two-photon upconversion process is also responsible for the blue emission.
To further investigate the nature of the upconversion processes in this crystal, excitation spectra of the upconverted emissions at 388, 436, 476, 535, 598, and 660 nm were performed at 77 and 295 K in the spectral ranges corresponding to the 4I9/2→4F52, 4F3/2 transitions.Figure 6 shows, as an example, the excitation spectra for the upconverted blue and orange emissions by monitoring the upconverted fluorescence at 436 nm and 598 nm at 295 K, together with the squared ground state absorption spectrum for comparison. The excitation spectra show two peaks similar to those observed in the squared ground state absorption spectrum. The first one corresponds to the (4F5/2, 2H9/2) levels, while the second one corresponds to the 4F3/2 level. Similar excitation spectra were obtained by monitoring all the visible emissions at 295 K. Figure 7 shows the low temperature excitation spectra monitored at 388, 436, 535, and 598 nm. No excitation lines corresponding to any excited-state absorption are observed. This result rules out the possibility of sequential absorption processes for the upconversion mechanisms.
We also investigated the temporal evolution of the upconverted emissions and the infrared emissions of the 4F5/2, 4F3/2 → 4I11/2 transitions by exciting the sample in the 4F5/2 and 4F3/2 levels at 813 nm and 870 nm respectively with a Ti:sapphire laser pumped by a frequency-doubled Nd:YAG laser. Figure 8 shows the time evolution of the upconverted blue emission from level 2P1/2 at room temperature obtained under OP excitation at 429 nm [Fig. 8(a)] and under infrared excitation at 813 nm [Fig. 8(b)]. The insets show the same decays but in a semilogarithmic representation. The decay obtained under infrared excitation in resonance with the 4I9/2→4F5/2 transition shows a rise and a decay with a lifetime much longer than that of the 2P1/2 level under OP excitation (55 μs). The same behavior is observed in the case of green, orange, and red emissions. In all cases, the temporal evolution of the upconverted emissions shows a rise and a decay with a lifetime much longer than the one obtained under OP excitation. As we have mentioned before, the upconverted green, orange, and red emissions can be attributed to the overlapping of emissions from different levels. In the case of the green emission, the comparison between the upconverted emission spectrum and the direct green emissions obtained by exciting levels (4G11/2-2G9/2) and (4G9/2,4G7/2) shows that the upconverted green emission can be attributed to the radiative transitions (4G11/2-2G9/2)→4I11/2 and (4G9/2,4G7/2)→4I9/2, whereas, the orange and red emissions can be assigned to the (2G9/2-4G11/2)→4I13/2,15/2, (4G7/2-4G9/2)→4I11/2,13/2, and (4G5/2-2G7/2)→4I9/2,4I11/2 transitions.
An analysis of the upconverted decay curves of the green, orange, and red emissions shows that the decays are not single exponential and present two different components. The orange and red emissions show a similar temporal behavior with rise times ≈ 10 μs and double exponential decays. As an example, Fig. 9 (a) shows the time evolution of the orange emission at 598 nm obtained under infrared excitation at 813 nm. The two components of the decay obtained under IR excitation are specially noticeable in the semilogarithmic plot. The fast component has a lifetime of ≈ 35 μs and the lifetime of the long component is about 80 μs.
Concerning the green emission monitored at 535 nm, the temporal evolution under infrared excitation shows a different behavior from the orange and red emissions. Figure 9(b) shows the time evolution of the green emission obtained under IR excitation at 813 nm. As can be seen, the rise and decay times are shorter than in the case of the orange and red emissions, suggesting that the green emission monitored at 535 nm could be attributed to the 2G9/2→4I11/2 transition. The temporal evolution of this emission shows a rise time of about 4 μs and a double exponential decay with a fast component of ≈ 18 μs and a long component of ≈ 63 μs. It is worthy to mention that a similar temporal behavior has been observed by monitoring the upconverted emission at 479 nm which corresponds to the 2G9/2→4I9/2 emission and at 576 nm, at the shoulder of the orange emission, suggesting that there is an overlapping of emissions from 2G9/2 and 4G7/2 levels. Similar results were obtained by exciting the 4F3/2 level. Table 1 shows the lifetime values obtained for both IR excitations at 295 K and 77 K.
Referring to the UV emission which is only observed at low temperature, the time evolution of the 388 nm emission also shows a rise time and a decay with a lifetime longer than the one of level 4D3/2.
As we have seen in the previous section 2P1/2, (2G9/2-4G11/2), (4G7/2-4G9/2), and (4G5/2-2G7/2) groups of levels are responsible for the observed upconverted emissions at room temperature. Disregarding the presence of avalanche upconversion, two possible mechanisms can be responsible for the observed luminescence: excited state absorption (ESA) and energy transfer upconversion (ETU) [20,21]. There are two ways of discerning between these two processes: upconversion luminescence excitation spectra and the temporal evolution of the upconversion luminescence after pulsed excitation. In the first mechanism (ESA) a single ion is involved, whereas two ions are involved in the second one (ETU). The ESA upconversion process requires an exact resonance between the ground state absorption (GSA) and an excited state absorption. Therefore, the ESA upconversion excitation spectrum is the product of the GSA and ESA spectra and it may differ from the GSA of the intermediate state. An ETU process can be phonon assisted and it requires no exact energy resonance between the intermediate and upper states involved in the upconversion process. In this case, the excitation spectrum is essentially the squared ground state absorption spectrum of the excited state into which the upconversion excitation occurs.
The time evolution of the upconversion luminescence after an excitation pulse provides a useful tool in discerning which is the operative mechanism. The radiative ESA process occurs during the excitation pulse and leads to an immediate decay of the upconversion luminescence after excitation. Upconversion by energy transfer leads to a time-dependent emission that shows a rise of the upconverted population after the laser pulse, followed by a decay of the population with a lifetime longer than the one after direct excitation. This distinction is possible when the pulse width is much shorter than the time constant of the relevant energy transfer step.
Concerning the blue emission coming from level 2P1/2, the similarity between the excitation spectrum and the squared ground state absorption spectrum as well as the features shown by the decay of the upconverted luminescence, with a rise time and a longer lifetime than the intrinsic lifetime of the emitting level, point out to an ETU as the mechanism responsible for this emission. This level can be populated through interaction of two ions in the 4F5/2 and/or 4F3/2 levels. It is worthy to mention that in this low phonon crystal the lifetimes of the 4F5/2 and 4F3/2 levels are 125 μs and 150 μs respectively. In KPB:Nd3+ crystal the reduction of multiphonon relaxation rates increases the lifetime of the 4F5/2 level which can act as an intermediate state for upconversion. Moreover, when pumping the 4F5/2 level at room temperature we have emissions from both 4F5/2 and 4F3/2 levels, which indicates that at room temperature both levels are populated. The inspection of the energy level diagram of Nd3+ in this crystal, suggests two possible energy transfer upconversion processes to populate the 2P1/2 level. One is (4F5/2,4F5/2)→(4I11/2, 2P1/2), namely ETU I in Fig. 10. In this process, when two Nd3+ ions are excited by an IR photon directly to the 4F5/2 state, a transfer occurs by which one ion loses energy and goes to the ground state whereas the other one gains energy and goes to the 2P1/2 state. This process is non resonant with an energy mismatch of -349 cm-1. Another possibility is an energy transfer process involving two Nd3+ ions in the 4F3/2 level (populated directly or through 4F5/2 at room temperature), one that decays to the 4I9/2 state transferring its energy to a nearby ion in the 4F3/2 level, which is promoted to the 2P1/2 level. The energy mismatch for such a process is -403 cm-1, (ETU II in Fig. 10). These mismatches correspond to the difference between the barycenters of the absorption bands. In both processes, the additional energy needed to reach the 2P1/2 level could be provided by the absorption of lattice phonons. This could explain the decrease of the blue emission at low temperature.
The excitation spectra of the green, orange, and red emissions, similar to the squared ground state absorption spectrum, as well as the decay of the upconverted luminescence showing longer time components than the intrinsic lifetime of the emitting levels, point out to ETU as the responsible mechanisms for these emissions. The emitting levels can be populated through interaction of two ions in the 4F5/2 or 4F3/2 levels. The ions in the 4F5/2 intermediate level can interact and reach the (2G9/2-4G11/2) and (4G7/2 - 4G9/2) levels through transitions like (4F5/2, 4F5/2)→ (2G9/2, 4I13/2) or (4F5/2, 4F5/2)→ (4G7/2, 4I15/2) (ETU III and IV in Fig. 10) respectively. Both processes are non resonant with energy mismatches of -277 and - 239 cm-1 respectively. When exciting the 4F3/2 level, we observe the same emission spectra. In this case the ETU processes that populate the emitting levels can be (4F3/2, 4F3/2)→ (2G9/2, 4I11/2) (ETU V) and (4F3/2, 4F3/2)→ (4G7/2, 4I13/2) (ETU VI). The energy mismatches for these processes are -251 and -229 cm-1. Another possible mechanism is the ETU process (4F3/2, 4F3/2)→ (4G5/2, 4I15/2). This process is nearly resonant with an energy mismatch of 33 cm-1 (ETU VII in Fig. 10). As can be observed in Table 1, the long components of the decays of the upconverted emissions are about half the values of the 4F5/2 and 4F3/2 levels which agree with the presence of ETU processes involving two Nd3+ ions in these levels .
In addition, taking into account that at room temperature, after excitation in the 4F5/2 level, both 4F5/2 and 4F3/2 levels are populated, we can not disregard other possible ETU mechanisms to populate the (2G9/2-4G11/2) and (4G7/2 - 4G9/2) levels that involve levels 4F5/2 and 4F3/2. One possibility consists of an energy transfer process involving one neodymium ion in the 4F5/2 state that decays to the 4I11/2 level and transfers its energy to a nearby ion in the 4F3/2 state, which is promoted to the (2G9/2-4G11/2) levels. The energy mismatch for such a process is +128 cm-1, which could be easily bridged by the emission of one phonon at room temperature. A similar ETU process could populate the (4G7/2 - 4G9/2) levels. In this case, two ions interact, one of them in the 4F5/2 state that decays to the 4I13/2 level and the other one in the 4F3/2 state which gains the energy and goes to the (4G7/2-4G9/2) levels. This process is also energetically favorable with an energy mismatch of +362 cm-1.
Concerning the emission from level 4D3/2 (not shown in Fig. 10) only observed at low temperature, this level can not be reached by means of two photon processes. The similarity of the excitation spectrum recorded at 388 nm with the squared ground state absorption spectrum indicates that the most likely upconversion mechanism is ETU. This is confirmed by the time evolution of the luminescence coming from the 4D3/2 level: the upconverted decay shows a rise time (≈26 μs) and a decay (≈58 μs) much longer than the natural lifetime of the level, and supports the hypothesis of an ETU mechanism for populating this level.
Finally, the efficient room temperature visible upconversion observed in KPB:Nd3+ crystal (Fig. 11) suggests that this crystal could be a potential candidate to obtain laser emission as a result of upconversion.
The IR to VIS upconversion processes of Nd3+ ion in the KPb2Br5 low phonon energy crystal have been investigated under continuous-wave and pulsed laser excitation in the 4F5/2 and 4F3/2 states. Similar upconversion emission spectra were found for both excitation energies. The comparison between the emission spectra of the high energy levels of Nd3+ ions obtained under one photon excitation and under IR excitation shows that the blue emission at 436 nm originates from level 2P1/2, whereas the green emission is due to the overlapping of the (2G9/2-4G11/2)→4I11/2 and (4G7/2-4G7/2-4G9/2)→4I9/2 transitions. In the case of the orange and red emissions, their analysis, points out to a mixed contribution of the groups of levels: (2G9/2-4G11/2), (4G7/2-4G9/2), and (4G5/2-2G7/2).
The dynamic behavior of the upconverted luminescence together with its dependence on the excitation wavelength and pump intensity show that the most likely mechanisms for the upconversion emissions are ETU processes involving two ions in the 4F5/2 or 4F3/2 levels.
This work was supported by the Spanish Government MEC (MAT2004-03780) and Basque Country University (UPV13525/2001).
References and links
2. A. A. Kaminskii, in Laser Crystals, 2nd ed., D. L. MacAdam , ed. Springer series in Optical Sciences Vol. 14 (Springer-Verlag, Berlin, 1990).
3. M. J. Weber, “Science and technology of laser glass,” J. Non-Cryst. Solids 123 , 208–222 (1990) [CrossRef]
4. S. Guy, M. F. Joubert, B. Jacquier, and M. Bouazaoui, “Excited-state absorption in BaY2F8:Nd3+,” Phys. Rev. B 4711001–11006 (1993). [CrossRef]
5. Y. Guyot, H. Manaa, J. Y. Rivoire, R. Moncorgé, N. Garnier, E. Descroix, M. Bon, and P. Laporte, “Excited-state-absorption and upconversion studies of Nd3+-doped single crystals Y3Al5O12, YLiF4, and LaMgAl11O19,” Phys. Rev. B 51, 784–799 (1995). [CrossRef]
6. T. Chuang and H. R. Verdún, “Energy-transfer up-conversion and excited state absorption of laser radiation in Nd-YLF crystals,” IEEE J. Quantum Electron. 32, 79–91 (1996). [CrossRef]
7. R. M. McFarlane, F. Fong, A. J. Silversmith, and W. Lenth, “Violet cw neodymium upconversion laser,” Appl. Phys. Lett. 521300–1302 (1988) [CrossRef]
8. R. Balda, M. Sanz, A. Mendioroz, J. Fernändez, L. S. Griscom, and J. L. Adam, “Infrared-to-visible upconversion in Nd3+-doped chalcohalide glasses,” Phys. Rev. B 64, 14410111–1441018 (2001). [CrossRef]
9. M. C. Nostrand, R. H. Page, S. A. Payne, W. F. Krupke, P. G. Schunemann, and L. I. Isaenko, “Spectroscopic data for infrared transitions in CaGa2S4:Dy3+ and KPb2Cl5: Dy3+,” OSA TOPS 19, 524–528, 1998.
10. L. I. Isaenko, A. P. Yelisseyev, V. A. Nadolinny, V. I. Pashkov, M. C. Nostrand, R. H. Page, S. A. Payne, and R. Solarz, “Spectroscopic investigation of rare-earth doped chloride single crystals for telecommunication amplifiers,” in Solid State Lasers VII, R. Scheps , ed, Proc. SPIE3265, 242–249 (1998). [CrossRef]
11. M. C. Nostrand, R. H. Page, S .A. Payne, W. F. Krupke, P. G. Schunemann, and L. I. Isaenko,“Room temperature CaGa2S4:Dy3+ laser action at 2.43 and 4.31 μm and KPb2Cl5: Dy3+ laser action at 2.43 μm,” OSA TOPS 26, 441–449, 1999.
12. R. Balda, J. Fernändez, A. Mendioroz, M. Voda, and M. Al-Saleh, “Infrared-to-visible upconversion processes in Pr3+/Yb3#x002B;-codoped KPb2Cl5,” Phys. Rev. B 68, 1651011–16510172003. [CrossRef]
13. R. Balda, A. J. Garcia-Adeva, M. Voda, and J. Fernändez, “Upconversion processes in Er3+-doped KPb2Cl5,” Phys. Rev. B 69, 2052031–2052038 (2004). [CrossRef]
14. A. Mendioroz, R. Balda, M. Voda, M. Al-Saleh, and J. Fernändez, “Origin of the infrared to visible upconversion mechanisms in Nd3+-doped potassium lead chloride crystal,” Opt. Mater 27, 1704–1710 (2005). [CrossRef]
15. K. Rademaker, W. F. Krupke, R. H. Page, S. A. Payne, K. Petermann, G. Huber, A. P. Yelisseyev, L. I. Isaenko, U. N. Roy, A. Burger, K. C. Mandal, and K. Nitsch, “Optical properties of Nd3+ and Tb3+-doped KPb2Br5 and RbPb2Br5 with low nonradiative decay,” J. Opt. Soc. Am. B 21, 2117–2129 (2004). [CrossRef]
16. H. P. Beck, G. Clicqué, and H. Nau, “A Study on AB2X5 Compounds (A: K, In, Tl; B: Sr, Sn, Pb; X: Cl, Br, I),” Z. Anorg. Allg. Chem 536, 35–44 (1986). [CrossRef]
17. A. J. García-Adeva, R. Balda, J. Fernändez, E. E. Nyein, and U. Hömmerich, “Dynamics of the infrared-to-visible upconversion in an Er3+-doped KPb2Br5 crystal,” Phys. Rev. B 72, 1651161–16511611 (2005). [CrossRef]
18. C. Cascales, R. Balda, and J. Fernändez , “Investigation of site-selective symmetries of Eu3+ ions in KPb2Cl5 by using optical spectroscopy,” Opt. Express 13, 2141–2152 (2005). [CrossRef] [PubMed]
19. K. Rademaker, E. Heumann, G. Huber, S. A. Payne, W. Krupke, L.I. Isaenko, and A. Burger, “Laser activity at 1.18, 1.07, and 0.97 μm in the low-phonon-energy hosts KPb2Br5 and RbPb2Br5 doped with Nd3+,” Opt. Lett. 30, 729–731 (2005). [CrossRef] [PubMed]
20. F. Auzel, “Materials and devices using double-pumped phosphors with energy transfer,” Proc. IEEE 61, 758–7861973. [CrossRef]
21. J. C. Wright, “Up-conversion and excited state energy transfer in rare earth doped materials,” in Radiationless processes in molecules and condensed phases, F. K. Fong , ed. (Springer-Verlag, Heidelberg, 1976), pp. 239–295.