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Rare earth doped PDMS elastomeric random lasers
Optical Materials 97 (2019) 109387
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Rare earth doped PDMS elastomeric random lasers a,b,e,∗
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A.R. Hlil , B.C. Lima , J. Thomas , J.-S. Boisvert , H. Iden , Y. Garcia-Puente , L.J.Q. Maia , Y. Ledemie, Y. Messaddeqe, A.S.L. Gomesc, R. Kashyapa,d a
Fabulas Laboratory, Department of Physics Engineering, École Polytechnique Montréal, P.O Box 6079, Station Centre-ville, Montreal, QC, H3T 1J4, Canada Département de chimie, Faculté des sciences et de génie Pavillon Alexandre-Vachon, 1045, avenue de la Médecine, Université Laval, Québec, G1V 0A6, Canada Departamento de Física, Universidade Federal de Pernambuco, Recife-PE, Brazil d Fabulas Laboratory, Department of Electrical Engineering, École Polytechnique Montréal, P.O Box 6079, Station Centre-ville, Montreal, QC, H3T 1J4, Canada e Centre d’Optique, Photonique et Laser, 2375 Rue de la Terrasse, Université Laval, Québec, QC, G1V 0A6, Canada f Grupo Física de Materiais, Instituto de Física, Universidade Federal de Goiás-UFG, Campus II, Av.Esperança 1533, 74690-900, Goiânia, GO, Brazil b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Elastomeric random laser PDMS Nd YAB nanoparticles Levy-like statistics
We report a very stable elastomeric random laser (SERL) system composed of two stable materials: inorganic Nd: YAB nanoparticles and polydimethylsiloxane (PDMS). Lasing at a wavelength of 1064.5 nm is observed when the samples are exposed to pulsed nanosecond excitation at 808 nm or 532 nm, with long term (days) operation without degradation. Contrary to other RLs based on polymers, this very stable RL is the first elastomeric system, hence tunable, which allows the systematic investigation of its dynamics over a long time, without the complication of gain reduction or deterioration of the polymer. The RL threshold is estimated to be around 1.0 mJ at 808 nm. The measured linewidth decreases from 3.2 nm to 0.5 nm. The spectral peak position as well as the intensity is conveniently tuned by stretching the elastomer polymer composite. As an application which requires long term operation, Levy-like statistics and replica symmetry breaking in this open cavity random laser were also demonstrated. These lasers have been used over several months without noticeable degradation.
1. Introduction Lasing in random nanostructures based on the different feedback and gain mechanism has been extensively investigated during the past decades in various materials such as dye-doped liquid crystals, semiconductor nanostructures, polymers etc. [1–3]. Random lasers work on the principle of multiple scattering with incorporated gain or separated gain in a dense media [4]. One of the challenges related with the random lasers is the tunability of the lasing emission owing to the absence of a well-defined cavity. The utilization of a stretchable substrate is a feasible method to generate tunable random lasers, and we recently reported our initial findings of an elastomeric RL system [5]. Another flexible materials system has also been reported on but with dyes, which are intrinsically short lived [2,3,6]. Stimulated emission has been demonstrated in different semiconducting polymer films such as functionalized polyphenylenevinylene (PPV), p-polyphenylene (PPP), and polyfluorene (PF) derivatives representing a wide variety of molecular structures with emission wavelengths covering almost the entire visible range [7]. The mechanical, electronic and optical properties of polymers can be easily modified by chemical substitutions or by doping
techniques. These advantages along with the stability of the matrix can lead to tunability of random lasers emission originating from the stretching of polymers [5,6]. We have been able to explore the tunability aspects of the RL system under different pumping regimes by stretching the PDMS composite, as well as explore the multi-directionality and multi-wavelength emission, using the same sample repeatably, something that has not been possible before. The random laser properties of Nd: YAB (NdxY 1-x Al3(BO3)4, with 0.05 ≤ x ≤ 1.00, nanocrystals (powder form) excited by a pulsed laser operating at 808 nm were previously investigated [8–11] showing several emission lines in the ultraviolet - near infrared range (340–1062 nm), depending on the excitation wavelength (700–900 nm), with the nanoparticles acting also as self-scattering media. Our recent work demonstrated a new material system using Nd: YAB (NdxY1-xAl3(BO3)4) nanoparticles with different concentrations of Neodymium and Rhodamine 6G incorporated in Polydimethylsiloxane (PDMS) polymer to evaluate their influence on random lasing [5]. In this work we report on a detailed study of random laser (RL) emission from elastomer polymeric composite films which contains Nd: YAB (Nd0.8Y0.2Al3B4O12) nanoparticles when pumped with pulsed
∗ Corresponding author. Fabulas Laboratory, Department of Physics Engineering, École Polytechnique Montréal, P.O Box 6079, Station Centre-ville, Montreal, QC, H3T 1J4, Canada. E-mail address: [email protected] (A.R. Hlil).
https://doi.org/10.1016/j.optmat.2019.109387 Received 11 July 2019; Received in revised form 15 August 2019; Accepted 14 September 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
Optical Materials 97 (2019) 109387
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nanosecond lasers at 808 nm and 532 nm. The combination of inorganic stable Nd:YAB nanoparticles which act as the lasing media and scatterers along with the PDMS offers long durability to the material system for performing experiments which requires long term operation. 2. Materials and methods The elastomer Sylgard 184 was used with the following procedure for the preparation of polymer composites with Nd: YAB nanoparticles (PC): Sylgard 184 elastomer was mixed in a 10:1 ratio with its curing agent and then placed in a desiccator for at least 30 min to remove bubbles created during mixing. Samples were then made by spinning the polymer on pre-cleaned glass slides and curing at 90 °C. We were able to improve the dispersity and avoid the aggregation of the nanoparticles powder by adding them to PDMS monomer prior to polymerization. The mixture was sonicated for 5 min to release the air bubbles. The composite was spin-coated with an initial rotary speed of 100 rpm for 30 s and a subsequent rotary speed of 200 rpm for 30 s, then cured at 90 °C for 1 h. The synthesized composite was investigated and characterized for the structural and optical properties. Our experiments were focused on optimizing the incorporation of the nanoparticles into the PDMS as host polymer (matrix). A Raman spectroscopic technique was used in order to understand the structural difference between the elastomer samples with and without nanoparticles. The unpolarized Raman spectra were measured in a micro-Raman spectrometer (LabRam HR Evolution, Horiba JobinYvon) equipped with a He–Ne laser at 633 nm (17 mW) and a Syncerity detector (Model 1024X256-OE, Horiba). The thermal stability behavior of the PDMS and PDMS-composites with nanoparticle were measured using the thermal gravimetric analysis (TGA). A Q500 TGA/DSC (Mettler-Toledo, Columbus, Ohio, USA) was used for these measurements. All measurements were made under a 50 mL-min−1 stream of nitrogen, and the samples were heated up to 800 °C using a constant heating ramp of 10 °C min−1. DSC analyses were conducted with a Netzsch DSC 404F3 Pegasus apparatus, equipped with a liquid nitrogen cooling unit, a highly sensitive type E sample carrier and a silver furnace. Samples of about 20 mg were loaded in aluminum pans and DSC traces were recorded at a heating rate of 10K/min under helium atmosphere. To observe random lasing emission due to the 4F3/2 to 4F11/2 transition in the Nd3+ ions, two wavelengths, 808 nm and 532 nm were used to excite the samples. For 808 nm excitation radiation, an optical parametric oscillator operating at 10 Hz, 3ns was used. A half-wave plate and polarizer were set to control the pulse energy. The polarizer was used to fix the pump to be p-polarized. A first lens (L1) focused the beam into the sample and L2 collimated and collected the emitted light, the lens L3 focused the signal into the aperture of the spectrometer, and an 808 nm wavelength notch filter (NF) placed in front of the aperture prevented the pump beam from entering the spectrometer (Fig. 1). The signal was measured with a CCD camera with a resolution of 0.024 nm determined by the discretization of the spectral bins. For each pump energy E, 1000 spectra were collected, with an integration time of 200 m s for each spectrum s γ (v ) .
Fig. 1. Experimental Setup used to excite the samples to collect the RL emission. The beam from an OPO is guided to the sample by two mirrors M1 and M2. The pump beam intensity is controlled through a half wave plate and a polarizer and is focused on the sample by the lens L1. The lens L2 and L3 collects the RL emission to the spectrometer, the signal is detected by a CCD camera. And a notch filter (NF) blocks the pump beam.
Fig. 2. Raman spectra of a) PDMS, b) Nd0.8Y0.2Al3B4O12, c) PDMS, and Nd0.8Y0.2Al3B4O12.
shows well defined peaks at 1191.85 and 1463.22 cm-1 attributed to the stretching of trigonal BO3 groups [8]. The new peaks at the range 2500–4000 cm−1 can be attributed to the presence of Nd0.8Y0.2Al3B4O12. The Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) characterizations shown in Fig. 3 confirmed the size and the chemical composition of the crystals in the polymer composite (PC) PDMS/Nd0.8Y0.2Al3B4O12. The surface and the cross section of the Nd0.8Y0.2Al3B4O12 particles embedded in the PDMS films were analyzed at different magnifications, revealing particle dimensions ranging between 100 and 300 nm. TGA of PDMS- composites showed that the nanocrystalline powder in the polymer matrix have enhanced the stability of PDMS. As an example of the thermal behavior of composites, for the samples with nanoparticles powder first weight loss peaks were only observed at 300 °C with 1.0% weight loss in the range of 400 °C–495 °C for 5–10% weight loss and at 800 °C (23–47% remaining). Substantial weight loss is accompanied by a decrease with increasing the ratio of nanocrystalline powder in the polymers [5]. Typical transition temperatures (Tg) measured with DSC showed no significant difference in the glass
3. Results and discussion 3.1. Structural properties The Raman spectrum of pure PDMS-Sylgard is shown in Fig. 2. It presents the typical vibrational modes for silicon, carbon, oxygen and hydrogen bonds; these include symmetric stretching (Si–O–Si, 487 cm−1), symmetric rocking (Si–CH3, 614 cm−1), (Si–C, 707 cm−1), asymmetric stretching (Si–C) and asymmetric rocking (CH3) overlapping at 789 cm−1 and symmetric stretching (CH3) at 2965 cm−1 and 2906 cm−1. These concur with those found in the literature results [12]. The Raman spectrum of the Nd0.8Y0.2Al3B4O12 nanoparticles 2
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Fig. 5. Decay-time of the 4 F below the RL threshold. Fig. 3. SEM (a, b) and TEM (c, d) images showing the morphology of filled (PC) PDMS/Nd0.8Y0.2Al3B4O12 at different magnifications.
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level for an excitation pulse energy (0.15 mJ)
emission with a narrow peak at 1064.5. The measured linewidth was observed to decreases from 3.8 nm to 0.8 nm, with a RL threshold of ~0.20 mJ. We recorded the emission from the excitation spot in different directions from the same sample with a pump energy of 0.30 mJ while moving the spectrometer (slit size, = 3.2 mm) horizontally, and the results are shown in Fig. 6(a). The observed emission occurs in multiple directions with different intensities. Since the direction of the scattered beam is random, the spatial modes are inhomogeneous and are highly irregular. We were able to observe spatially confined modes with higher intensity along with the extended modes. The lifetime of the 4F3/2 level was measured for pump power below the lasing threshold (in the spontaneous emission regime) and above the RL threshold. The decay time for an excitation energy below threshold is shown in Fig. 5. The obtained lifetime of ~18.3 +- 0.2 μs for the spontaneous emission is in good agreement with the previous reports of the Nd0.8Y0.2Al3B4O12 nanoparticles [8]. Certainly, the polymer composite, which contains the same nanoparticles, will also present a similar lifetime. In order to investigate the effect on the RL emission with respect to the stretching of the SERL system, we performed the test by loading the sample in a homemade stretching apparatus and using a pump wavelength of 532 nm for a fixed pump energy of 0.37 mJ. Fig. 6 (b) shows the spectroscopic response of the flexible random laser device for different amount of strain up to 150%. We could observe different random laser regimes by adjusting the density and the effective gain of scatterers by stretching. Stretching of the PDMS has three effects. Firstly, the density of scatterers per unit volume decreases. Secondly, the phase of the scattered light changes with each scatter event as a function of stretch. Thirdly, the orientation
transition temperature (Tg ~ −120 °C) between the PDMS and composite samples. It indicates that the introduction of nanoparticles with strong interaction with the polymer does not change the glass transition temperature. The crystallization event was not clearly observed which is in agreement with the literature where PDMS chains were reported to be unable to crystallize when they are adsorbed or close to the filler (particles) [13,14]. 3.2. Optical properties Fig. 4 (a) shows the average spectra (average intensity at each 1000 1 s (v ) = 1000 ∑γ = 1 s γ (v ) for pump energy wavelength indexed by (v ), ‾ below (0.2 mJ), near (1.0 mJ) and above (2.2 mJ) the laser threshold for a pump wavelength of 808 nm with the intensity normalized from 0 to 1 to better view the linewidth reduction. The RL threshold was estimated to be around 1.0 mJ from Fig. 4(b). The emitted intensity, and the linewidth reduction, obtained by the full width of half-maximum, are show in Fig. 4 (b). The measured linewidth decreases from 3.2 nm to 0.5 nm. Such measurement agrees with the spectral measurements shown in Fig. 4 (a). The elastomer-based polymer RL was also optically pumped by a frequency doubled QS Nd: YAG laser operating at 532 nm, exciting the 2G7 level at average pump energy varying from 0.007 mJ to 0.40 mJ with a spot size of 2 mm, 150 Hz, 10 ns pulse duration. Similar to excitation at 808 nm, broad emission spectrum was observed at 1064.5 nm for a pump energy of 0.007 mJ. Further increase in the pump energy up to 0.40 mJ resulted in an increase in peak intensity of
Fig. 4. (a) Normalized average emitted spectrum showing the linewidth reduction from below (0.2 mJ), near (1.0 mJ) and above (2.2 mJ) the threshold for a pump wavelength of 808 nm (b) FWHM (red squares) inferred by the average spectra, and maximum emitted intensity (blue circles) dependency on pump energy. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 6. (a) RL emission spectrum as the spectrometer is moved, pumped at 532 nm. (b) RL emission intensity as a function of strain ΔL/L applied to the PC elastomer.
energy increases, more electromagnetic modes are triggered and more competition for the gain occur. Such behavior is characterized as a photonic transition that is analogue to the thermodynamic spin glass transition [16] (see Fig. 7). From the experimental data, two statistical analyses were performed, alpha stable distribution [15] where the type of the statistic that characterizes the PDF P (I ) of the emitted intensity I at the laser emission wavelength, and replica symmetric breaking (RSB) by an analogy with the thermodynamic spin glass system [14], using the same set of measurement. The alpha stable distribution is described by the family of functions, with the Fourier transform in the k -space given by the characteristic function:
of Nd: YAB nanoparticles in the PDMS changes. Since the gain medium and the scatterer are at the same location, the phase of the scattered light plays an important role in the intensity of the output emission. It should be noted that the laser stops lasing for certain strain values and emits ASE instead. However, at other strain values the amplitude of the laser emission decreases, it does not stop lasing, but changes direction of emission as seen in Fig. 6(a). Also, the lasing intensity in different directions can go up or down depending on the position of the spectrometer, despite the pump power remaining constant, showing the multi-directional operation of the RL. It was not possible to ascertain whether the integrated emission over 4π remains constant due to the arrangement and complexity of the experiment. However, we believe that because random lasing stops under certain conditions (strain and pump power), the integrated emission will not remain constant. The phase of the scattered light changes as a function of the applied strain. For example, at 8% strain, a broad emission spectrum was observed and by increasing the strain to 15%, random lasing restarts and a narrow peak at 1064.5 nm emerges. Further increasing the strain to 25%–50%, results in a broad band emission centered at 1065 nm and simultaneous lasing at 1064.5 nm. Even though there was fluctuation in the emission intensity (increasing or decreasing), we could observe two narrow peaks at 1064.5 nm and 1065.4 nm simultaneously with 85% strain and again lasing was observed at only 1064.5 nm with strain between 86% and 88% but with higher intensity. Therefore, we demonstrated that different lasing regimes and random scatterer density can explain the random laser behavior with stretching. With a wider gain bandwidth medium, we should be able to see dramatic changes in the emission wavelength as a function of stretch.
P‾ (k ) = exp {iku − ck α [1 − iβsgn (k ) Φ]}
(1)
In Eq. (1) the most important parameter is the Lévy index α ∈ (0,2] as it determines the types of statistic that characterizes the intensity fluctuations of the system. The range 0 < α < 2 is associated to strong intensity fluctuations with relevant deviations from the Gaussian behavior. For α = 2 a Gaussian behavior with weak fluctuations is recovered, as result of the central limit theorem (CLT). The other parameters determines the asymmetry (β ∈ [−1,1]) , location (u ∈ (−∞, ∞)) and scale (c ∈ (−∞, ∞)) of the distribution, and Φ= tan(πα /2) if α ≠ 1, whereas Φ= −(2/ k ) ln|k| if α = 1. For the RSB analysis, each collected spectrum is considered as a replica of the experiment, i.e., a copy of the RL system under identical experimental conditions. The order parameter to quantify the overlap of the spectra intensity fluctuation between each replica spectrum γ and β is defined as [16]:
qγβ =
3.3. Statistical optical properties
∑ νΔγ (v ) Δβ (v ) ∑ νΔγ2 (v ) ∑ νΔβ2 (v )
(2)
where γ , β = 1, 2, …, 1000 . The intensity fluctuation in relation to average spectrum (‾ s (v )) with wavelength indexed by v is Δ γ (v ) = s γ (v ) − ‾ s (v ) . From the probability distribution P (qγβ ) , the qmax is defined as the modulus of the point of maximum chance of occurrence. If qmax = 0 the system behaves as a photonic paramagnetic uncorrelated system. When the parameter qmax ≠ 0 with the presence of a continuous curve in the P (qγβ ) , it describes a photonic spin-glass system. Fig. 7 (d) shown the α -stable parameter (blue circles) and qmax (red squares) as a function of the pump energy. A transition between the Gaussian to Lévy and back to Gaussian can be observed around 1.0 mJ, and the RSB transition from the photonic paramagnetic to the spin glass phase, that also coinciding with the threshold is present. This is related to the non-uniform distribution of the gain, cavity leakage and the disorder introduced in the system.
In order to exploit our novel RL device to one application which requires long term operation, we performed experiments on Lévy and photonic spin glass behavior of our RL, to verify how it behaves compared to other RL systems where such statistical physics behavior have been observed [9,15–18]. Although Lévy statistics have been reported on in other RL systems, this is the first study of the probability distribution function (PDF) of the emitted intensity in the SERL system. The PDF deviate from the Gaussian showing heavy tails, for pump pulses around the threshold. This behavior is known as Lévy flights [13]. RLs usually show transitions from Gaussian to Lévy and from Lévy to Gaussian in the emitted intensity probability distribution, as the pump energy increases. A possible explanation for the new Gaussian distribution of intensities far above the laser threshold is a self-averaged gain distribution among the active modes. Fig. 7 shows 1000 measured spectra for pump energy bellow 0.4 mJ (a), near 1.0 mJ (b) and above 2.2 mJ (c) the RL threshold. The intensity fluctuation of the emitted pulses at the lasing wavelength fluctuation is analyzed. Above the threshold, as the pump
3.4. Stability studies of the SERL system The long-term stability of the random laser was characterized by the 4
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Fig. 7. Fluctuation in the emitted intensity of the elastomeric random laser and the statistical analysis results using 808 nm. The set of N = 1000 spectra collected for pump energy of 0.4 mJ (a), 1.0 mJ (b) and 2.2 mJ (c), corresponding to the regime below, near and above the pump energy threshold, respectively, elucidate the intensity fluctuation behavior. The intensity is represented by the color bar. A large intensity fluctuation is observed in the Lévylike behavior near the threshold (b) when compared to the Gaussian regime far below (a) and far above (c). The qmax mode overlap parameter characterizing the transition from the photonic paramagnetic phase to the spin glass is shown (d) nicely coincides with the pump energy threshold and with the Lévylike behavior of the emission intensity fluctuations. The dashed lines are guide to the eyes. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
4. Conclusions In conclusion, a very stable elastomeric random laser (SERL) system made with Nd: YAB nanoparticles in an elastomeric polymer has been demonstrated. Random lasing at a wavelength of ~1064.5 nm was observed under excitation with 808 nm or 532 nm wavelength. The Nd: YAB nanoparticles act simultaneously as scattering centers and gain medium while the PDMS polymer provides as elastomeric host for the optical gain medium. This makes the PDMS/Nd0.8Y0.2Al3B4O12 elastomer composite a very promising candidate for the study of random lasing as well as future applications. The long lifetime and stable operation of this SERL system also makes it an excellent platform for investigating tunability and different RL phenomenon, using a variety of gain media. Funding Fig. 8. Influence on long-term stability of the RL intensity as a function of time of continuous operation. There is no noticeable effect of long term pumping on the RL intensity.
This work has been supported by Natural Sciences and Engineering Research Council of Canada (NSERC) Strategic Grants Program; Canadian Excellence Research Chair program (CERC) on Photonic Innovations; Canada Foundation for Innovation (CFI); Brazilian agencies: CAPES; CNPq and FAPEG.
measurement of the emitted intensity and the linewidth (FWHM) of the RL emission. A negligible reduction in intensity was observed in the polymer composite after continuous excitation of 8 h using the pump laser of 532 nm wavelength, 0.20 mJ (at the threshold), characterizing an important result for prolonged use of the investigated polymer composite for RL emission. The linewidth also remained the same as 0.8 ± 0.003 nm for continuous 8 h of excitation. Fig. 8 elucidates the long-term stability of the intensity as a function of the number of hours of excitation. We have used the same material over several months (off and on and continuously over a period of a week), without any noticeable degradation. An in-depth study is the subject of another article in preparation. Different RL regimes (Lévy or Gaussian) may be observed, by altering the density of scatterers and gain at a given location without interrogating different regions of the material in which the distribution of gain/scatterers varies from position to position. These studies are under investigation, and will also be reported in a future publication.
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Rare earth doped PDMS elastomeric random lasers a,b,e,∗
c
a
a
e
T a
f
A.R. Hlil , B.C. Lima , J. Thomas , J.-S. Boisvert , H. Iden , Y. Garcia-Puente , L.J.Q. Maia , Y. Ledemie, Y. Messaddeqe, A.S.L. Gomesc, R. Kashyapa,d a
Fabulas Laboratory, Department of Physics Engineering, École Polytechnique Montréal, P.O Box 6079, Station Centre-ville, Montreal, QC, H3T 1J4, Canada Département de chimie, Faculté des sciences et de génie Pavillon Alexandre-Vachon, 1045, avenue de la Médecine, Université Laval, Québec, G1V 0A6, Canada Departamento de Física, Universidade Federal de Pernambuco, Recife-PE, Brazil d Fabulas Laboratory, Department of Electrical Engineering, École Polytechnique Montréal, P.O Box 6079, Station Centre-ville, Montreal, QC, H3T 1J4, Canada e Centre d’Optique, Photonique et Laser, 2375 Rue de la Terrasse, Université Laval, Québec, QC, G1V 0A6, Canada f Grupo Física de Materiais, Instituto de Física, Universidade Federal de Goiás-UFG, Campus II, Av.Esperança 1533, 74690-900, Goiânia, GO, Brazil b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Elastomeric random laser PDMS Nd YAB nanoparticles Levy-like statistics
We report a very stable elastomeric random laser (SERL) system composed of two stable materials: inorganic Nd: YAB nanoparticles and polydimethylsiloxane (PDMS). Lasing at a wavelength of 1064.5 nm is observed when the samples are exposed to pulsed nanosecond excitation at 808 nm or 532 nm, with long term (days) operation without degradation. Contrary to other RLs based on polymers, this very stable RL is the first elastomeric system, hence tunable, which allows the systematic investigation of its dynamics over a long time, without the complication of gain reduction or deterioration of the polymer. The RL threshold is estimated to be around 1.0 mJ at 808 nm. The measured linewidth decreases from 3.2 nm to 0.5 nm. The spectral peak position as well as the intensity is conveniently tuned by stretching the elastomer polymer composite. As an application which requires long term operation, Levy-like statistics and replica symmetry breaking in this open cavity random laser were also demonstrated. These lasers have been used over several months without noticeable degradation.
1. Introduction Lasing in random nanostructures based on the different feedback and gain mechanism has been extensively investigated during the past decades in various materials such as dye-doped liquid crystals, semiconductor nanostructures, polymers etc. [1–3]. Random lasers work on the principle of multiple scattering with incorporated gain or separated gain in a dense media [4]. One of the challenges related with the random lasers is the tunability of the lasing emission owing to the absence of a well-defined cavity. The utilization of a stretchable substrate is a feasible method to generate tunable random lasers, and we recently reported our initial findings of an elastomeric RL system [5]. Another flexible materials system has also been reported on but with dyes, which are intrinsically short lived [2,3,6]. Stimulated emission has been demonstrated in different semiconducting polymer films such as functionalized polyphenylenevinylene (PPV), p-polyphenylene (PPP), and polyfluorene (PF) derivatives representing a wide variety of molecular structures with emission wavelengths covering almost the entire visible range [7]. The mechanical, electronic and optical properties of polymers can be easily modified by chemical substitutions or by doping
techniques. These advantages along with the stability of the matrix can lead to tunability of random lasers emission originating from the stretching of polymers [5,6]. We have been able to explore the tunability aspects of the RL system under different pumping regimes by stretching the PDMS composite, as well as explore the multi-directionality and multi-wavelength emission, using the same sample repeatably, something that has not been possible before. The random laser properties of Nd: YAB (NdxY 1-x Al3(BO3)4, with 0.05 ≤ x ≤ 1.00, nanocrystals (powder form) excited by a pulsed laser operating at 808 nm were previously investigated [8–11] showing several emission lines in the ultraviolet - near infrared range (340–1062 nm), depending on the excitation wavelength (700–900 nm), with the nanoparticles acting also as self-scattering media. Our recent work demonstrated a new material system using Nd: YAB (NdxY1-xAl3(BO3)4) nanoparticles with different concentrations of Neodymium and Rhodamine 6G incorporated in Polydimethylsiloxane (PDMS) polymer to evaluate their influence on random lasing [5]. In this work we report on a detailed study of random laser (RL) emission from elastomer polymeric composite films which contains Nd: YAB (Nd0.8Y0.2Al3B4O12) nanoparticles when pumped with pulsed
∗ Corresponding author. Fabulas Laboratory, Department of Physics Engineering, École Polytechnique Montréal, P.O Box 6079, Station Centre-ville, Montreal, QC, H3T 1J4, Canada. E-mail address: [email protected] (A.R. Hlil).
https://doi.org/10.1016/j.optmat.2019.109387 Received 11 July 2019; Received in revised form 15 August 2019; Accepted 14 September 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
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nanosecond lasers at 808 nm and 532 nm. The combination of inorganic stable Nd:YAB nanoparticles which act as the lasing media and scatterers along with the PDMS offers long durability to the material system for performing experiments which requires long term operation. 2. Materials and methods The elastomer Sylgard 184 was used with the following procedure for the preparation of polymer composites with Nd: YAB nanoparticles (PC): Sylgard 184 elastomer was mixed in a 10:1 ratio with its curing agent and then placed in a desiccator for at least 30 min to remove bubbles created during mixing. Samples were then made by spinning the polymer on pre-cleaned glass slides and curing at 90 °C. We were able to improve the dispersity and avoid the aggregation of the nanoparticles powder by adding them to PDMS monomer prior to polymerization. The mixture was sonicated for 5 min to release the air bubbles. The composite was spin-coated with an initial rotary speed of 100 rpm for 30 s and a subsequent rotary speed of 200 rpm for 30 s, then cured at 90 °C for 1 h. The synthesized composite was investigated and characterized for the structural and optical properties. Our experiments were focused on optimizing the incorporation of the nanoparticles into the PDMS as host polymer (matrix). A Raman spectroscopic technique was used in order to understand the structural difference between the elastomer samples with and without nanoparticles. The unpolarized Raman spectra were measured in a micro-Raman spectrometer (LabRam HR Evolution, Horiba JobinYvon) equipped with a He–Ne laser at 633 nm (17 mW) and a Syncerity detector (Model 1024X256-OE, Horiba). The thermal stability behavior of the PDMS and PDMS-composites with nanoparticle were measured using the thermal gravimetric analysis (TGA). A Q500 TGA/DSC (Mettler-Toledo, Columbus, Ohio, USA) was used for these measurements. All measurements were made under a 50 mL-min−1 stream of nitrogen, and the samples were heated up to 800 °C using a constant heating ramp of 10 °C min−1. DSC analyses were conducted with a Netzsch DSC 404F3 Pegasus apparatus, equipped with a liquid nitrogen cooling unit, a highly sensitive type E sample carrier and a silver furnace. Samples of about 20 mg were loaded in aluminum pans and DSC traces were recorded at a heating rate of 10K/min under helium atmosphere. To observe random lasing emission due to the 4F3/2 to 4F11/2 transition in the Nd3+ ions, two wavelengths, 808 nm and 532 nm were used to excite the samples. For 808 nm excitation radiation, an optical parametric oscillator operating at 10 Hz, 3ns was used. A half-wave plate and polarizer were set to control the pulse energy. The polarizer was used to fix the pump to be p-polarized. A first lens (L1) focused the beam into the sample and L2 collimated and collected the emitted light, the lens L3 focused the signal into the aperture of the spectrometer, and an 808 nm wavelength notch filter (NF) placed in front of the aperture prevented the pump beam from entering the spectrometer (Fig. 1). The signal was measured with a CCD camera with a resolution of 0.024 nm determined by the discretization of the spectral bins. For each pump energy E, 1000 spectra were collected, with an integration time of 200 m s for each spectrum s γ (v ) .
Fig. 1. Experimental Setup used to excite the samples to collect the RL emission. The beam from an OPO is guided to the sample by two mirrors M1 and M2. The pump beam intensity is controlled through a half wave plate and a polarizer and is focused on the sample by the lens L1. The lens L2 and L3 collects the RL emission to the spectrometer, the signal is detected by a CCD camera. And a notch filter (NF) blocks the pump beam.
Fig. 2. Raman spectra of a) PDMS, b) Nd0.8Y0.2Al3B4O12, c) PDMS, and Nd0.8Y0.2Al3B4O12.
shows well defined peaks at 1191.85 and 1463.22 cm-1 attributed to the stretching of trigonal BO3 groups [8]. The new peaks at the range 2500–4000 cm−1 can be attributed to the presence of Nd0.8Y0.2Al3B4O12. The Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) characterizations shown in Fig. 3 confirmed the size and the chemical composition of the crystals in the polymer composite (PC) PDMS/Nd0.8Y0.2Al3B4O12. The surface and the cross section of the Nd0.8Y0.2Al3B4O12 particles embedded in the PDMS films were analyzed at different magnifications, revealing particle dimensions ranging between 100 and 300 nm. TGA of PDMS- composites showed that the nanocrystalline powder in the polymer matrix have enhanced the stability of PDMS. As an example of the thermal behavior of composites, for the samples with nanoparticles powder first weight loss peaks were only observed at 300 °C with 1.0% weight loss in the range of 400 °C–495 °C for 5–10% weight loss and at 800 °C (23–47% remaining). Substantial weight loss is accompanied by a decrease with increasing the ratio of nanocrystalline powder in the polymers [5]. Typical transition temperatures (Tg) measured with DSC showed no significant difference in the glass
3. Results and discussion 3.1. Structural properties The Raman spectrum of pure PDMS-Sylgard is shown in Fig. 2. It presents the typical vibrational modes for silicon, carbon, oxygen and hydrogen bonds; these include symmetric stretching (Si–O–Si, 487 cm−1), symmetric rocking (Si–CH3, 614 cm−1), (Si–C, 707 cm−1), asymmetric stretching (Si–C) and asymmetric rocking (CH3) overlapping at 789 cm−1 and symmetric stretching (CH3) at 2965 cm−1 and 2906 cm−1. These concur with those found in the literature results [12]. The Raman spectrum of the Nd0.8Y0.2Al3B4O12 nanoparticles 2
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Fig. 5. Decay-time of the 4 F below the RL threshold. Fig. 3. SEM (a, b) and TEM (c, d) images showing the morphology of filled (PC) PDMS/Nd0.8Y0.2Al3B4O12 at different magnifications.
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level for an excitation pulse energy (0.15 mJ)
emission with a narrow peak at 1064.5. The measured linewidth was observed to decreases from 3.8 nm to 0.8 nm, with a RL threshold of ~0.20 mJ. We recorded the emission from the excitation spot in different directions from the same sample with a pump energy of 0.30 mJ while moving the spectrometer (slit size, = 3.2 mm) horizontally, and the results are shown in Fig. 6(a). The observed emission occurs in multiple directions with different intensities. Since the direction of the scattered beam is random, the spatial modes are inhomogeneous and are highly irregular. We were able to observe spatially confined modes with higher intensity along with the extended modes. The lifetime of the 4F3/2 level was measured for pump power below the lasing threshold (in the spontaneous emission regime) and above the RL threshold. The decay time for an excitation energy below threshold is shown in Fig. 5. The obtained lifetime of ~18.3 +- 0.2 μs for the spontaneous emission is in good agreement with the previous reports of the Nd0.8Y0.2Al3B4O12 nanoparticles [8]. Certainly, the polymer composite, which contains the same nanoparticles, will also present a similar lifetime. In order to investigate the effect on the RL emission with respect to the stretching of the SERL system, we performed the test by loading the sample in a homemade stretching apparatus and using a pump wavelength of 532 nm for a fixed pump energy of 0.37 mJ. Fig. 6 (b) shows the spectroscopic response of the flexible random laser device for different amount of strain up to 150%. We could observe different random laser regimes by adjusting the density and the effective gain of scatterers by stretching. Stretching of the PDMS has three effects. Firstly, the density of scatterers per unit volume decreases. Secondly, the phase of the scattered light changes with each scatter event as a function of stretch. Thirdly, the orientation
transition temperature (Tg ~ −120 °C) between the PDMS and composite samples. It indicates that the introduction of nanoparticles with strong interaction with the polymer does not change the glass transition temperature. The crystallization event was not clearly observed which is in agreement with the literature where PDMS chains were reported to be unable to crystallize when they are adsorbed or close to the filler (particles) [13,14]. 3.2. Optical properties Fig. 4 (a) shows the average spectra (average intensity at each 1000 1 s (v ) = 1000 ∑γ = 1 s γ (v ) for pump energy wavelength indexed by (v ), ‾ below (0.2 mJ), near (1.0 mJ) and above (2.2 mJ) the laser threshold for a pump wavelength of 808 nm with the intensity normalized from 0 to 1 to better view the linewidth reduction. The RL threshold was estimated to be around 1.0 mJ from Fig. 4(b). The emitted intensity, and the linewidth reduction, obtained by the full width of half-maximum, are show in Fig. 4 (b). The measured linewidth decreases from 3.2 nm to 0.5 nm. Such measurement agrees with the spectral measurements shown in Fig. 4 (a). The elastomer-based polymer RL was also optically pumped by a frequency doubled QS Nd: YAG laser operating at 532 nm, exciting the 2G7 level at average pump energy varying from 0.007 mJ to 0.40 mJ with a spot size of 2 mm, 150 Hz, 10 ns pulse duration. Similar to excitation at 808 nm, broad emission spectrum was observed at 1064.5 nm for a pump energy of 0.007 mJ. Further increase in the pump energy up to 0.40 mJ resulted in an increase in peak intensity of
Fig. 4. (a) Normalized average emitted spectrum showing the linewidth reduction from below (0.2 mJ), near (1.0 mJ) and above (2.2 mJ) the threshold for a pump wavelength of 808 nm (b) FWHM (red squares) inferred by the average spectra, and maximum emitted intensity (blue circles) dependency on pump energy. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 6. (a) RL emission spectrum as the spectrometer is moved, pumped at 532 nm. (b) RL emission intensity as a function of strain ΔL/L applied to the PC elastomer.
energy increases, more electromagnetic modes are triggered and more competition for the gain occur. Such behavior is characterized as a photonic transition that is analogue to the thermodynamic spin glass transition [16] (see Fig. 7). From the experimental data, two statistical analyses were performed, alpha stable distribution [15] where the type of the statistic that characterizes the PDF P (I ) of the emitted intensity I at the laser emission wavelength, and replica symmetric breaking (RSB) by an analogy with the thermodynamic spin glass system [14], using the same set of measurement. The alpha stable distribution is described by the family of functions, with the Fourier transform in the k -space given by the characteristic function:
of Nd: YAB nanoparticles in the PDMS changes. Since the gain medium and the scatterer are at the same location, the phase of the scattered light plays an important role in the intensity of the output emission. It should be noted that the laser stops lasing for certain strain values and emits ASE instead. However, at other strain values the amplitude of the laser emission decreases, it does not stop lasing, but changes direction of emission as seen in Fig. 6(a). Also, the lasing intensity in different directions can go up or down depending on the position of the spectrometer, despite the pump power remaining constant, showing the multi-directional operation of the RL. It was not possible to ascertain whether the integrated emission over 4π remains constant due to the arrangement and complexity of the experiment. However, we believe that because random lasing stops under certain conditions (strain and pump power), the integrated emission will not remain constant. The phase of the scattered light changes as a function of the applied strain. For example, at 8% strain, a broad emission spectrum was observed and by increasing the strain to 15%, random lasing restarts and a narrow peak at 1064.5 nm emerges. Further increasing the strain to 25%–50%, results in a broad band emission centered at 1065 nm and simultaneous lasing at 1064.5 nm. Even though there was fluctuation in the emission intensity (increasing or decreasing), we could observe two narrow peaks at 1064.5 nm and 1065.4 nm simultaneously with 85% strain and again lasing was observed at only 1064.5 nm with strain between 86% and 88% but with higher intensity. Therefore, we demonstrated that different lasing regimes and random scatterer density can explain the random laser behavior with stretching. With a wider gain bandwidth medium, we should be able to see dramatic changes in the emission wavelength as a function of stretch.
P‾ (k ) = exp {iku − ck α [1 − iβsgn (k ) Φ]}
(1)
In Eq. (1) the most important parameter is the Lévy index α ∈ (0,2] as it determines the types of statistic that characterizes the intensity fluctuations of the system. The range 0 < α < 2 is associated to strong intensity fluctuations with relevant deviations from the Gaussian behavior. For α = 2 a Gaussian behavior with weak fluctuations is recovered, as result of the central limit theorem (CLT). The other parameters determines the asymmetry (β ∈ [−1,1]) , location (u ∈ (−∞, ∞)) and scale (c ∈ (−∞, ∞)) of the distribution, and Φ= tan(πα /2) if α ≠ 1, whereas Φ= −(2/ k ) ln|k| if α = 1. For the RSB analysis, each collected spectrum is considered as a replica of the experiment, i.e., a copy of the RL system under identical experimental conditions. The order parameter to quantify the overlap of the spectra intensity fluctuation between each replica spectrum γ and β is defined as [16]:
qγβ =
3.3. Statistical optical properties
∑ νΔγ (v ) Δβ (v ) ∑ νΔγ2 (v ) ∑ νΔβ2 (v )
(2)
where γ , β = 1, 2, …, 1000 . The intensity fluctuation in relation to average spectrum (‾ s (v )) with wavelength indexed by v is Δ γ (v ) = s γ (v ) − ‾ s (v ) . From the probability distribution P (qγβ ) , the qmax is defined as the modulus of the point of maximum chance of occurrence. If qmax = 0 the system behaves as a photonic paramagnetic uncorrelated system. When the parameter qmax ≠ 0 with the presence of a continuous curve in the P (qγβ ) , it describes a photonic spin-glass system. Fig. 7 (d) shown the α -stable parameter (blue circles) and qmax (red squares) as a function of the pump energy. A transition between the Gaussian to Lévy and back to Gaussian can be observed around 1.0 mJ, and the RSB transition from the photonic paramagnetic to the spin glass phase, that also coinciding with the threshold is present. This is related to the non-uniform distribution of the gain, cavity leakage and the disorder introduced in the system.
In order to exploit our novel RL device to one application which requires long term operation, we performed experiments on Lévy and photonic spin glass behavior of our RL, to verify how it behaves compared to other RL systems where such statistical physics behavior have been observed [9,15–18]. Although Lévy statistics have been reported on in other RL systems, this is the first study of the probability distribution function (PDF) of the emitted intensity in the SERL system. The PDF deviate from the Gaussian showing heavy tails, for pump pulses around the threshold. This behavior is known as Lévy flights [13]. RLs usually show transitions from Gaussian to Lévy and from Lévy to Gaussian in the emitted intensity probability distribution, as the pump energy increases. A possible explanation for the new Gaussian distribution of intensities far above the laser threshold is a self-averaged gain distribution among the active modes. Fig. 7 shows 1000 measured spectra for pump energy bellow 0.4 mJ (a), near 1.0 mJ (b) and above 2.2 mJ (c) the RL threshold. The intensity fluctuation of the emitted pulses at the lasing wavelength fluctuation is analyzed. Above the threshold, as the pump
3.4. Stability studies of the SERL system The long-term stability of the random laser was characterized by the 4
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Fig. 7. Fluctuation in the emitted intensity of the elastomeric random laser and the statistical analysis results using 808 nm. The set of N = 1000 spectra collected for pump energy of 0.4 mJ (a), 1.0 mJ (b) and 2.2 mJ (c), corresponding to the regime below, near and above the pump energy threshold, respectively, elucidate the intensity fluctuation behavior. The intensity is represented by the color bar. A large intensity fluctuation is observed in the Lévylike behavior near the threshold (b) when compared to the Gaussian regime far below (a) and far above (c). The qmax mode overlap parameter characterizing the transition from the photonic paramagnetic phase to the spin glass is shown (d) nicely coincides with the pump energy threshold and with the Lévylike behavior of the emission intensity fluctuations. The dashed lines are guide to the eyes. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
4. Conclusions In conclusion, a very stable elastomeric random laser (SERL) system made with Nd: YAB nanoparticles in an elastomeric polymer has been demonstrated. Random lasing at a wavelength of ~1064.5 nm was observed under excitation with 808 nm or 532 nm wavelength. The Nd: YAB nanoparticles act simultaneously as scattering centers and gain medium while the PDMS polymer provides as elastomeric host for the optical gain medium. This makes the PDMS/Nd0.8Y0.2Al3B4O12 elastomer composite a very promising candidate for the study of random lasing as well as future applications. The long lifetime and stable operation of this SERL system also makes it an excellent platform for investigating tunability and different RL phenomenon, using a variety of gain media. Funding Fig. 8. Influence on long-term stability of the RL intensity as a function of time of continuous operation. There is no noticeable effect of long term pumping on the RL intensity.
This work has been supported by Natural Sciences and Engineering Research Council of Canada (NSERC) Strategic Grants Program; Canadian Excellence Research Chair program (CERC) on Photonic Innovations; Canada Foundation for Innovation (CFI); Brazilian agencies: CAPES; CNPq and FAPEG.
measurement of the emitted intensity and the linewidth (FWHM) of the RL emission. A negligible reduction in intensity was observed in the polymer composite after continuous excitation of 8 h using the pump laser of 532 nm wavelength, 0.20 mJ (at the threshold), characterizing an important result for prolonged use of the investigated polymer composite for RL emission. The linewidth also remained the same as 0.8 ± 0.003 nm for continuous 8 h of excitation. Fig. 8 elucidates the long-term stability of the intensity as a function of the number of hours of excitation. We have used the same material over several months (off and on and continuously over a period of a week), without any noticeable degradation. An in-depth study is the subject of another article in preparation. Different RL regimes (Lévy or Gaussian) may be observed, by altering the density of scatterers and gain at a given location without interrogating different regions of the material in which the distribution of gain/scatterers varies from position to position. These studies are under investigation, and will also be reported in a future publication.
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