We have investigated the stimulated emission properties of Nd3+ doped La2O2S powders at room temperature as a function of pumping energy density, excitation wavelength, and Nd3+ ion concentration. The absolute stimulated emission energy has been measured. Expressions for the slope efficiencies and lasing thresholds as a function of rare earth concentration and pumping wavelengths, which qualitatively agree with experimental observations, are discussed.
© 2012 OSA
The investigation of the luminescence properties of rare earths (RE) in inhomogeneous dielectric structures including highly scattering powders has received much attention because of its potential applications in lighting and displays in the visible spectrum. In particular, RE-activated oxides have been thoroughly investigated . As the wavelength of the emitted light depends smoothly on the host matrix, the luminescence color of the RE- activated phosphor depends on the RE ion used. Until recent advent of high optical quality RE-doped ceramic materials, less attention was paid to near infrared emitters based on Nd3+- activated powder oxides which have been proved to be suitable for solid state lasers [2, 3]. On the other hand, a growing interest has arisen nowadays in the study of lasing properties of activated powders for optoelectronic and medical applications requiring low coherence such as high resolution imaging and/or highly stable optical frequency among others [4–6]. In this type of cavityless lasers, multiple light scattering replaces the standard optical cavity of a conventional laser, and the interplay between gain and scattering determines the lasing properties. A detailed discussion about the theories concerning the mechanisms responsible for the so called random lasing (RL) can be found in Refs [4, 6–9]. Among RE-activated powders those based on Nd3+ have received much attention due to their low threshold and high efficiency in almost every host [10–13]. The history and the state of the art of these neodymium based random lasers were reviewed by Mikhail Noginov .
Among the RE-doped oxides, oxysulfides (RE2O2S) are one of the most efficient phosphors investigated for commercial television and lighting applications . In particular, lanthanum oxysulfide crystal matrix, a uniaxial wide-gap (36000 cm−1)  semiconductor material, is known as an excellent host lattice for trivalent RE ions . Each lanthanum atom is coordinated by four oxygen atoms and three sulfur atoms in its nearest neighborhood . The lanthanum site symmetry is C3v. The spectroscopic and laser properties of Nd3+-activated lanthanum oxysulfide crystal has been worked out by Alves and associates in , and twenty years later, Markushev and associates shortly presented preliminary results obtained at liquid nitrogen temperature on the stimulated emission kinetics of a La2O2S powder doped with 1% of Neodymium .
In this work, to our knowledge, lasing action from Nd3+ doped La2O2S powders at room temperature is presented for the first time. We have investigated the stimulated emission properties of this material as a function of pumping energy density, wavelength, and Nd3+ ion concentration and measured the absolute stimulated emission energy by assuming Lambertian emission. The most relevant results have been compared with those found in the stoichiometric borate NdAl(BO3)4, which is one of the most investigated random laser materials [6, 13, 18].
The synthesis procedure of the Nd-doped oxysulfide powders was the following: The starting materials La(NO3)3·6H2O (Alfa Aesar, 99,9%), Nd(NO3)3·H2O (Alfa Aesar, 99,99%) and thioacetamide CH3CSNH2 (Aldrich ≥ 99,9%) were dissolved with absolute ethanol (Prolabo, Normapur, 20 mL). Stoichiometric lanthanum nitrate/fuel molar ratio was used in all preparations. The solution was heated below 80°C to allow the dissolution of thioacetamide. The preparation was then introduced into a muffle furnace (Thermolyne 48000) pre-heated to 500°C. After the inflammation reaction of ethanol, a spontaneous combustion reaction characterized by a high flame temperature produces an expanded white solid. This product was finally ground and post-treated under H2S/N2 flow at 1000°C for 2 hours. FTIR analysis has been performed on the powders before and after H2S/N2 treatment in order to assess the purity of the powders. FTIR transmittance spectra of all samples present several absorption peaks between 400 cm−1 and 4000 cm−1. As reported in a previous study of La2O2S:Er3+,Yb3+ , the absorption peaks are assigned to water, carbonate and sulfate groups. After sulfurization, all these peaks disappear, except La−O and La−S vibrational bands situated at 400−550 cm−1. La2O2S powders containing 2, 3, 6, and 9 mol % of Nd3+ were prepared with a filling factor of 0.18, being their average size about one micron.
2. Slope and threshold measurements
2.1. Experimental conditions
The pumping source was a tunable 10 ns time width Ti:sapphire pulsed laser. The pump beam size over the sample surface was controlled by means of a 60 cm focal lens mounted in a movable holder. The emission was directly collected (without lens) on a 0.5 mm diameter optic fiber, vertically located at a distance of 12 cm over the sample; therefore, though the emission energy is given in arbitrary units all measurements for different pump beam sizes, concentrations or wavelengths may be compared with each other. The fiber drives the optical signal to a fast detector directly coupled to a 1 Ghz bandwidth digital oscilloscope. To remove the pump signal and to avoid measurement saturation, a long wave pass filter and neutral attenuation filters were intercalated. As it is well known, when the pumping energy is higher than the threshold one, the temporal length of the emitted pulse shortens from tens of microseconds corresponding to the spontaneous emission time, down to the nanosecond domain. Thus, in order to collect the stimulated emission the recording time basis was set to the nanosecond scale, because the narrowed emission is dominant when the material is lasing, and the residual spontaneous contribution is negligible.
The pumping energy was calibrated by comparing the measurement given by an optical head detector, Ophir PE25BF-V2 ROHS, located at the exact position where powder samples were pumped, and the one provided by a silicon detector which measures the light dispersed by the folder mirror used to address the pump beam. Thus it is possible to measure pumping and output pulse energies simultaneously.
The procedure to measure the pump beam size over the sample consisted in collecting the diffused pump radiation on the sample surface with a CCD camera (Newport LBP-3-USB) by means of a carefully focused imaging lens and filtering the sample emission at 1 μm. With the aim of verifying the accuracy of the measured beam sizes at different focalizations and to avoid the possible distorting effect of the diffusion of pump radiation in the sample, we have used polished metal samples, with the same setup configuration, and the obtained results were similar.
The measurements were carried out with 2, 3, 6, and 9 mol % Nd3+-doped samples of La2O2S, using different pump beam sizes. The optimal pumping wavelength was found to be 819 nm. The spectrum of the laser emission was centered at 1076 nm and its width was about 0.2 nm, which is the spectral resolution limit of our system.
2.2 Experimental results
Figure 1(a) displays the output energy as a function of incident pump energy for the different Nd3+ concentrations. As can be seen, the pump threshold energy for stimulated emission slightly decreases as a function of concentration, whereas the slope efficiency clearly rises with it. The output energy as a function of the incident pump energy for some different pump areas is shown in Fig. 1(b), where, as in Fig. 1(a), the maximum pump energy at different pump areas is limited by the damage threshold of the powders, about 10 mJ/mm2. As can be seen, the slope efficiency remains essentially constant whereas the threshold energy for stimulated emission is reduced as the pump area decreases. We have observed that this reduction is proportional to the pumping beam area, so the pumping threshold energy per unit area remains constant in the working range of Fig. 1(b).
With the aim of finding the limit to this behavior, we have reduced the pumping area down to sizes of 200 μm in diameter. Figure 2 shows the pump threshold energy (black dots) and pump threshold energy per unit area (red dots) as a function of the pumped area. As can be seen, the pump threshold energy is approximately linear whereas the threshold energy per unit area fits well to a horizontal line for beam areas above 0.5 mm2 (diameters above 0.8 mm). The conclusion is that the pump threshold energy per unit area remains constant for diameters above 0.8 mm. We have also done the same kind of measurements with NdAl3(BO3)4 stoichiometric sample, obtaining very similar results, which significantly differ from previously reported ones [18, 20, 21] by other authors and show the importance of a careful measurement of the pump beam size. The average grain size of all samples used (about one micron) is large enough to have no significant influence on the laser threshold .
The influence of pumping wavelength on the threshold energy and slope efficiency has also been investigated. Figure 3 shows the pump wavelength dependence of the laser threshold for two different pump beam sizes and Fig. 4 displays the output energy as a function of incident pump energy for two different pumping wavelengths in the Nd3+-doped sample with 9 mol%. The dependence of both parameters, slope efficiency and input threshold energy, on the pump wavelength seems to be related with the effective absorption of the pump energy by the sample. Thus, in order to explain the observed behavior, a precise knowledge of the absorption properties of the powders as a function of the concentration and pumping wavelength would be necessary because it is well known that the reflectivity of a powdered material largely changes by the absorption dependence on wavelength and/or concentration [22, 23]. With this aim, we have done the spectral diffuse reflectance measurements of the powders by using an integrated sphere device coupled to a Cary-5 Spectrometer, which are shown in Fig. 5 . By comparing this figure and Fig. 3, it is worth noticing that the pumping wavelengths corresponding to the minimal input threshold energies are those with maximal absorption. However, although the shape of the threshold curves as a function of wavelength seems to follow the one of the absorbance spectrum, their profile features are much less sharp.
3. Absolute energy measurements
As we have mentioned in the introduction, an important aim of this work was to obtain a precise estimation of the RL absolute energy emitted by the powder samples. For this purpose we have used a calibrated optical detector (Ophir PE10-SH) vertically placed over the sample giving the absolute value of the energy received. The emission is assumed to be Lambertian  (as confirmed by the lack of spatial coherence of the powder emission ) so the emitted absolute energy is given by , being R the distance from the sample to the surface of the detector, and r the radius of its circular area (in our case 120 and 6 mm respectively). Beside this detector, we have placed the fiber head used to collect the RL emission on the fast detector in such a way that simultaneous measurements can be performed by both detectors. As an example Fig. 6 shows the absolute energy calibration of the signal detected through the fiber head by the detector. The obtained scale factor to convert arbitrary units to absolute energy ones in the ordinate axis of Figs. 1 and 4 is 32 μJ by displayed unit. These results imply a maximum value of the slope efficiency, referred to the incident energy, larger than 15% and a maximum emitted energy of 2.5 mJ. On the other hand, the values of the same magnitudes measured for the stoichiometric borate were about 35% and 6 mJ respectively, in good accordance with previous results found in  but in less agreement with those given in [20, 21]. Moreover, as shown in Fig. 6, the intersection between the fitting line to the experimental points and the ordinate axis is very close to the origin in the case of the borate, but at a higher point in the case of oxysulfide. This effect can be explained by taking into account that the integration time of the calibrated optical detector is about one microsecond, and as a consequence, the stimulated emission is fully detected. However, this is not the case for the spontaneous emission, with a much longer decay time for this transition in Nd3+ doped oxysulfides than the integration time, so the optical detector will only register a fraction of the spontaneous emission which is represented by the Y-cut in Fig. 6. As can be seen, it is higher for this oxysulfide than for the stoichiometric borate, which is surely due to differences in their respective decay times: less than 5 μs  for this oxysulfide and 20 μs  for the borate.
The conclusions obtained from the experimental results can be summarized as follows:
- -The slope efficiency remains essentially constant with respect to changes in the pump beam size (above 0.8 mm in diameter) and tends to grow when concentration is increased.
- -The incident pump threshold energy per unit area also remains constant as a function of the pump beam size for diameters larger than 0.8 mm. However, though one would predict noticeable changes in the behavior of this magnitude as a function of concentration and wavelength, these were not so significant as expected.
4.1 Slope efficiencies
In order to explain the obtained results, we have compared the absolute slope efficiencies experimentally obtained with the corresponding absorbance of the powders for different concentrations and pumping wavelengths. The results which can be seen in Table 1 conclude that slope efficiencies are essentially given by the absorbance of powder multiplied by the ratio between the emission and pumping photon energies; in other words, all the absorbed pumping photons above threshold are mainly reemitted as stimulated emission. This would be extensible only to powder materials with spontaneous decay times much longer than pumping time, and with not too small pump beam sizes which would give different laser dynamics. Therefore, the following formula is applicable:Fig. 5 and given in Table 1, and νp and νem the photon frequencies corresponding to the pumping and stimulated emission radiation respectively.
As can be seen in Table 1, the predicted values of slope efficiencies fit m experimental values quite well. It is worth noticing that the experimental slope efficiency is always slightly lower than the one calculated from absorbance, which surely indicates spurious losses in stimulated emission.
4.2 Lasing threshold
In a simple diffusion model, the theory of random lasers shows that, in a first approximation, assuming slab geometry and pump beam sizes much larger than the absorption length, the laser threshold is reached at a critical thickness of pumped volume :Eqs. (2) and (3) are equal, so the condition li = π 2lg is accomplished, where
By the other hand, the population inversion N can be written as a function of the absorbed energy as:Eqs. (4) and (5), we obtain the incident threshold pumping energy by:6]). In our case, the relevant parameters are the concentration ρ, the absorption cross-section σabs, which depends on the pumping wavelength, and the absorbance η, depending both on pumping wavelength and concentration. In this way, Eq. (6) can be written as:
If we compare results obtained from the last Eq. (7) with our experimental results, the very slight changes of threshold pumping energy with concentration can be qualitatively explained. When concentration is increased, absorbance also grows finally resulting in very little changes in the threshold energy. The quantitative accordance between our experimental results and those predicted by this formula when concentration is changed is nevertheless rather rough.
However, changes of the threshold pumping energy as a function of pumping wavelength are satisfactorily compared. For example, the experimentally obtained threshold energies for the 9% doped sample using a pump beam area of 4.80 mm2, at pumping wavelengths of 819 nm and 808 nm are 9 mJ and 15 mJ respectively, (see Fig. 3), being 0.6 its ratio. If we do calculations from Eq. (7), by using the absorbance from Table 1, and the absorption cross-section corresponding to these wavelengths , we obtain the same value, with an error less than 5%, which is a is a very good agreement, and is confirmed by our tests in stoichiometric borate.
As a conclusion we propose Eq. (1) for the calculation of absolute slope efficiency and Eqs. (6)-(7) for the evaluation of the pumping threshold energy of a Nd3+ random laser (pumped by using beam sizes higher than 0.8 mm in diameter). The validity limit of these expressions can change as a function of material, but account taken of the good agreement among the different samples studied, including the stoichiometric borate, we would anticipate it might be quite general.
We have investigated the stimulated emission properties of Nd3+ doped La2O2S powders at room temperature as a function of pumping energy density, excitation wavelength, and Nd3+ ion concentration. The absolute stimulated emission energy of the doped powders has been measured. The maximum value of the slope efficiency, referred to the pump energy is larger than 15% being 2.5 mJ the maximum emitted energy. These values closely compare to the ones measured for the stoichiometric borate which are 35% and 6 mJ respectively.
The experimental results demonstrate that the slope efficiency remains essentially constant with respect to changes in the pump beam size (above 0.8 mm in diameter) and tends to grow when concentration is increased. Moreover, the incident pump threshold energy per unit area also remains constant as a function of the pump beam size for diameters larger than 0.8 mm.
We have obtained approximate expressions for the slope efficiencies and lasing thresholds as a function of rare earth concentration and pumping wavelengths. Though changes in threshold pumping energy with concentration can be only qualitatively explained, changes in threshold pumping energy as a function of pumping wavelength and slope efficiencies are satisfactorily compared.
This work was supported by the Spanish Government under projects FIS2011-27968 and Consolider CSD2007-00013 (SAUUL) and by the Basque Country Government (IT-331-07).
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