Site-selective spectroscopy and stimulated emission experiments performed in the 4F3/2→4I11/2 laser transition of Nd3+-doped 0.8CaSiO3-0.2Ca3(PO4)2 eutectic glass are presented. The spectral features of the excitation spectra and those of spontaneous and stimulated emissions reveal the existence of a very complex crystal field site distribution for Nd3+ ions. As a consequence, the stimulated emission of Nd3+ in this glass shows a tunability of about 10 nm as a function of excitation wavelength.
©2009 Optical Society of America
The search for developing new solid state lasers based on rare earth (RE)-doped glass matrices still remains a field of a great interest. High power lasers for industrial applications or inertial confinement fusion research, glass fiber lasers and amplifiers for telecommunications as well as for biomedical and environment purposes, laser cooling, optical sensors, or ultrafast spectroscopy, among other applications, are nowadays the subjects of an intense research. The investigation of new laser wavelengths and/or new RE-doped glass matrices thus becomes a prior requirement aimed to satisfy the demand of further outcomings in the development of this kind of solid state lasers.
Since the first report on laser action in glass , most of the glass lasers have used trivalent lanthanides as the active ion, due to the low coupling between the rare earth and the host vibrations, which moderates the nonradiative emission from the excited electronic level. In addition, rare earth ions may offer some tunability as a result of inhomogeneous broadening of the optical transitions produced by the local field variation at the ion site and, moreover, the laser parameters can be somewhat controlled by varying the chemical composition of the host glass. Neodymium doped glasses are the most investigated and many of the trends in the behavior of their properties with composition observed for Nd3+ are also applicable to other rare earths [2,3]. For example, spontaneous emission probabilities from levels 4F3/2 of Nd3+ and 4I13/2 of Er3+ in oxide glass compositions increase with increasing packing ratio of the glass host [4,5]. The smaller the alkaline or alkaline-earth ions, the larger this packing ratio. Moreover, in silicates the packing ratio increases with the content of alkali modifier. As an example, referring to the limitations imposed by the former content, SiO2 content in silicate glasses ranges typically between 60 and 80 mol%. For lower silica contents the glass tends to undergo devitrification and for higher contents the melt viscosity increases. To overcome these problems, the addition of small amounts of P2O5 glass former permits to extend the glass forming region in the CaO-SiO2-P2O5 system up to very high modifier concentrations. The limit is close to the CaSiO3-Ca3(PO4)2 eutectic formulation where a glass with the maximum Ca/(Si+P) ≈1 and Si/P=2 ratios can be produced . This composition corresponds to an inverted glass because modifier content is larger than former content and belongs to the family of the well known bioactive glasses. The fabrication of high quality glasses of this composition is possible by using high solidification and cooling rates, as provided by the laser floating zone method. It was found that the lifetimes and emission cross-sections of the 1.06 μm (Nd3+) and 1.5 μm (Er3+) emissions in this glass are equivalent to those of the best commercially used alkaline-silicate glasses . In the present work we report the study of site-selective laser spectroscopy of Nd3+ ions in this glass together with a dynamical study of the laser emission properties. This study analyses the effect of the pumping wavelength on the laser output properties and presents a discussion on the influence of site-dependent effects on the stimulated emission properties.
2. Experimental details
Ceramic precursor rods, 3 mm in diameter and 50–100 mm in length, were prepared from the powder mixture of wollastonite (CS)-tricalcium phosphate (TPC) with the eutectic composition (80CaSiO3+20Ca3(PO4)2 in mol%) by pressureless sintering at 1200 °C for 10 h. Nd2O3 was added to the precursors to obtain the doped samples. Glass rods were then produced from the precursors by the laser floating zone method . The composition and doping concentration were determined by electron probe microanalysis (EPMA). This analysis gives a glass composition of 55.7CaO-35.4SiO2-8.9P2O5 in mol%. The Nd2O3 concentration in the studied sample is 2.8 wt% (3×1020 at/cm3 of Nd3+ ions). This inverted glass with a high content of CaO modifier presents a high transparency optical window from 0.35 to 4 μm and is not hygroscopic. Its refractive index is 1.65 .
The sample temperature was varied between 4.2 and 300 K in a continuous flow cryostat. Site-selective steady-state emission and excitation spectra were obtained by exciting the sample with a Ti-sapphire ring laser (0.4 cm-1 linewidth) in the 780–920 nm spectral range. The fluorescence was analyzed with a 0.25 m monochromator, and the signal was detected by a Hamamatsu R5509-72 photomultiplier and finally amplified by a standard lock-in technique.
3. Results and discussion
3.1 Site-selective spectroscopy
To investigate the crystal field site inhomogeneity of Nd3+ in the glass matrix we took advantage of the tunability and narrow bandwidth of the ring Ti-sapphire laser and performed site-selective excitation spectra of the 4I9/2→4F3/2 transition by collecting the luminescence at different wavelengths along the 4F3/2→4I11/2 transition. As an example, Fig. 1 shows the excitation spectra obtained at different emission wavelengths along the 4F3/2→4I11/2 transition measured at 4.2 K. These spectra show, as expected, two main broad bands associated with the two Stark components of the 4F3/2 doublet. However, the low energy component corresponding to the 4I9/2→4F3/2 doublet spectacularly narrows and blue-shifts, as the emission wavelength goes from low to high energy. In addition at the longest emission wavelength (lowest energy) at least two components are observed. This behavior is a consequence of contributions from Nd3+ ions in a multiplicity of environments. The monochromatic radiation excites an isochromat corresponding to a subset of sites, which may not be physically identical. Therefore, the emission line profile is a composition of emissions from two or more statistical site distributions, which may have different natural homogeneous linewidths.
The site-selective steady-state emission spectra of the laser transition were obtained at low temperature for different excitation wavelengths along the low energy component of the 4F3/2 level. As can be observed in Fig. 2 the shape, peak position, and linewidth of the emission band change as excitation goes from high to low energy. The spectra obtained under excitation at the low energy side of the 4I9/2→4F3/2 absorption band narrow and red shift.
These results show the rare inhomogeneous behavior of the crystal field felt by Nd3+ ions in this glass as well as the relative spectral isolation of the neodymium site distributions that can be excited at a given wavelength. This is most probably associated to the glass CaO-modifier rich composition, which allows a broader distribution of Nd3+ sites than would be obtained in SiO2-network-forming rich compositions. As can be seen in Fig. 2, only when we excite at the high energy side of the absorption band it is possible to cover the full spectral range of the Nd3+ emission probably helped by vibronic transitions. As we shall see in the next paragraph, this behavior is in the origin of the broad band laser tunability of Nd3+ in this glass.
3.2 Laser experiments under wavelength selective pumping
In order to investigate the influence of crystal field inhomogeneities on the laser spectral performance of this material under selective pumping conditions, we have used a 9 ns pulse-width Ti-sapphire laser of 0.1 nm spectral width and about 30 mJ pulse energy to pump the 4F5/2 level of Nd3+ ions, around 800 nm with a repetition rate of 1 pulse/s. The laser experiments were performed at room temperature by using a longitudinal pumping scheme in a 15 cm long symmetrical confocal resonator with high reflectivity coated mirrors. The 2 mm thick sample was placed at Brewster angle to minimize the resonator losses and situated slightly out of the pump focus to avoid optical damage. The excited area was approximately 0.1 mm2. The spectral detection of the laser pulse was performed by using a Jobin Ivon TRIAX 190 monochromator with a Hamamatsu InGaAs multichanel detector head giving a resolution of 0.5 nm, whereas the time evolution of the pumping and laser output was recorded with a fast fotodiode connected to a digital oscilloscope. The temporal width of the laser output was 30 ns.
According to the site-selective spectral results shown above in Figs. 1 and 2, the laser experiments also demonstrate a strong influence of the pumping wavelength on the spectral behavior of the laser emission. Figure 3 shows the measured laser spectra obtained at different excitation wavelengths along the 4I9/2→4F5/2 absorption band. As we can see the tunability of the laser emission as a function of the excitation wavelength is about 10 nm. However, the shift of the laser emission with excitation wavelength goes to lower or higher energies depending on the crystal field distribution of Nd3+ sites sensed by the pump wavelength. As can be seen, by pumping at the high energy side of the absorption band from 790 to 794 nm there is a small red shift of the emission peak, and the contrary occurs from 794 to 801 nm where the peak shifts to blue, and finally for wavelengths higher than 801 nm to red again. Figure 3 shows the movement of the laser peak barycenter as a function of the pumping wavelength.
These stimulated emission results are somewhat similar to those found by the authors in fluoride glasses  where laser action was obtained in two well defined spectral domains, separated by 8 nm, and attributed to the existence of broad site distributions of the rare earth in these glasses. It is worth mentioning that our results show for the first time, as far as we know, the presence of strong site effects in the laser emission of an oxide glass, which allows for a broad laser tunability domain.
The laser slope efficiency (output energy as a function of the absorbed energy), obtained by pumping at the peak position of the 4F5/2 electronic level, is around 20%. Its plot is shown in Fig. 4.
Finally, Fig. 5 displays a 3D picture of the excitation wavelength dependence of the laser spectra obtained by plotting the laser intensity as a function of both excitation and emission wavelengths for our eutectic glass.
The excitation spectra show the existence of a variety of crystal field site distributions for Nd3+ ions in this glass. Moreover, the spectral shift and narrowing observed in the steady-state emission spectra of the 4F3/2→4I11/2 transition under selective excitation, while wavelength increases along the 4I9/2→4F3/2 absorption band, reveals the existence of a huge variety of spectrally isolated distributions of Nd3+ sites in this glass. Laser emission under pulsed pumping has been observed which shows a behavior close to a Q-switch operation. Wavelength-resolved pump excitation of Nd3+ ions in this glass allows for a broad band tunability of the laser emission which is related with the above mentioned variety of quasi-isolated crystal field site distributions of Nd3+ ions in this glass matrix. This behavior, which is close to those occurring in disordered crystals or fluoride glasses , shows for the first time, as far as we know, that very strong crystal field inhomogeneities can be also found in an oxide eutectic glass.
On the other hand, these results open a broad field of applications of oxide glasses obtained from eutectic compositions to optoelectronic devices such as pump-wavelength tunable lasers, amplifiers, wavelength sensors, and others.
This work was supported by the Spanish Government under projects MAT2005-06508-C02-02, MAT2006-13005-C03-01, MAT2008-05921, and Consolider CSD2007-00013 (SAUUL), and the Basque Country Government (IT-331-07).
References and links
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