Coherent Random Lasing from Dye Aggregates in ... - ACS Publications

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Coherent Random Lasing from Dye Aggregates in Polydimethylsiloxane Thin Films Lihua Ye,*,† Yangyang Feng,† Zhixiang Cheng,† Chunlei Wang,† Changgui Lu,† Yanqing Lu,‡ and Yiping Cui† †

Advanced Photonics Center, School of Electronic Science and Engineering, Southeast University, Nanjing 210018, P. R. China College of Engineering, Applied Sciences and National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China

‡

ABSTRACT: The coherent random laser (CRL) from dyedoped polydimethylsiloxane (PDMS) has been investigated in both nanoparticle-doped (NP-doped) thin films and pure dye thin films. Compared with the literature, the pump threshold is only 1.5 mJ/cm2 in the pure dye thin film with a low dye concentration. The spontaneously formed micro-/nanocrystals of Pyrromethene 597 (PM597) dye support both gain and random feedback in the bulk of the PDMS during the sample preparation. When the SiO2 NPs were doped, the pump threshold was reduced to 0.75 mJ/cm2. The threshold increased after the film was peeled off from glass, which indicates that the photon localization effect of the leaky-waveguide structure plays an important role in the reduction of the CRL threshold. By a change in the pump stripe length or the thickness of the film, the peak wavelength red-shifts 6.7 or 5.93 nm, respectively. The PM597 dye molecule solubility changes, and they spontaneously aggregate in the process of toluene volatilization; the PDMS cures, which is the reason for the formation of PM597 micro-/nanocrystals. This thin film random laser with a low dye concentration can be used in integrated optoelectronics and display imaging. KEYWORDS: micro-/nanocrystals, PDMS thin film, multiple scattering, leaky-waveguide, random laser

1. INTRODUCTION Since Letokhov reported on random-laser action in the late 1960s,1 the development of the random laser has drawn much attention in the laser science field. The random laser can be divided into coherent or incoherent classes depending on whether the scattered light can go back to the original position,2,3 and the radiation distributions are random in terms of time, space, and frequency domains.4,5 A wide range of materials have been used to realize the random-laser emission, such as semiconductors,4 quantum dots,6 nanoparticles,7,8 liquid crystals,9−11 and biological tissue.12 Major advantages of random lasers are their technological simplicity and low fabrication cost.13 Polymers are promising materials for the further development of low-cost and tunable random lasing devices because of their easily tailored physicochemical properties. To date, random lasers have been achieved in various polymers, such as π-conjugated,14,15 dye-doped,16−18 and biologically inspired polymers.19−21 Of particular interest, the PDMS (polydimethylsiloxane) random laser has been realized in different systems,22−25 because of its high flexibility, easy processing, and unique optical properties. On the other hand, it was shown that RL (random lasing) can be achieved in dye aggregates dispersed in thin films,26,27 and the high concentration of dyes leads to their aggregation to form a micro-/nanocrystal; such aggregates are responsible both for the gain characteristic and © XXXX American Chemical Society

for multiple scattering, which in turn determines the random feedback. However, the loss induced by the self-absorption effect of the high concentration of dyes is increased, which thus leads to an increase in the lasing threshold. Recently, we demonstrated a liquid-waveguide CRL (coherent random laser) that consists of two PDMS plates and a highly transparent ultrathin liquid active layer embedded with SiO2 NPs (nanoparticles).28 In this work, we report the characterization of coherent random lasing action from the PDMS thin film. The optical feedback of our setup is provided by both multiple scattering and leaky-waveguide confinement mechanisms. The CRL spectra also can be observed in the pure dye thin film even at a low dye concentration, which is induced by the aggregation of PM597 (Pyrromethene 597) dye molecules to form micro-/nanocrystals. Compared with that for the pure dye thin film, the pump threshold was reduced by half after the SiO2 NPs were doped. The threshold increased after the film was peeled from the glass, which indicates that the photon localization effect of the leaky-waveguide structure plays an important role in the reduction of the RL threshold. The peak wavelength can be tunable by controlling the pump stripe Received: May 25, 2017 Accepted: July 24, 2017 Published: July 24, 2017 A

DOI: 10.1021/acsami.7b07464 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) Absorption spectra and fluorescence emission spectrum of the PDMS thin film. (b) Sketch of the random-laser experiment.

Figure 2. (a) Output spectrum dependent on the pump energy for the pure dye thin film. (b) Electron microscope image of PM597 micro-/ nanocrystals. Scanning electron microscope image of PDMS thin films (c) without SiO2 NP-doping and (d) with SiO2 NP-doping. oven at 80 °C for 6 h to complete the cross-linking polymerization and evaporate the solvent. After cooling to room temperature, a PDMS thin film (refractive index is n2 = 1.40) device was realized, as shown in the inset of Figure 1a. A step profiler with 0.3 μm step length was used to measure the thickness of the film, which was measured as 8.4 μm. The SiO2 NPs were synthesized by the Stöber method.30 The transmission electron microscopy (TEM) images of the SiO2 NPs are shown in our previous work.28 The diameter of SiO2 NPs is 698.4 nm with dynamic light scattering (DLS) measurements by use of a Malvern particle-size analyzer. The absorption peak and the fluorescence peak of PM597 in the PDMS thin film are 531 and 572 nm, respectively, as shown in Figure 1a. The sample was pumped by a frequency-doubled Nd:YAG laser system (PowerLite Precision II 8010) with 532 nm wavelength, 10 Hz repetition rate, and 8 ns pulse width. Figure 1b is the experiment setup schematic diagram. A cylindrical lens was aligned to sharpen a laser pump stripe whose length and width are 8 and 0.5 mm, respectively. The emitting signal is along the pump stripe, and is collected by an

length or the thickness of the film. Finally, the reason for the formation of PM597 dye micro-/nanocrystals was studied.

2. EXPERIMENTAL METHODS In the process of our experiment, we found that SiO2 NPs cannot be directly homogeneously dispersed in the PDMS,29 and we used the toluene-diluted Sylgard 184 PDMS prepolymer at a mass ratio of 2:3, stirring for 30 min. The as-synthesized SiO2 NPs were dispersed in the PDMS prepolymer toluene solution. Then, laser dye PM597 (from Exciton) was dissolved in the dilute solution. To prevent aggregation and sedimentation, we ultrasonically dispersed the mixed solution for about 10 min. The PM597 concentrations are 1, 2, and 3 mg/mL. The SiO2 NP concentration is 3.5 mg/mL. After that, the PDMS crosslinking catalyst was added and then stirred for 2 h to remove bubbles. The mass ratio of PDMS prepolymer to cross-linking catalyst is 10:1. The resulting homogeneous solution was dripped on the surface of glass substrate (refractive index n1 = 1.52) and was dried overnight at room temperature. Then, the sample was placed in a vacuum-drying B


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Figure 3. (a) Evolution of emission spectra as a function of the pump energy with SiO2 NP-doping. (b) The emission intensity dependent on the pump energy for both the pure dye thin film and the NP-doped thin film. Inset in part b shows a photograph of the emission when the pump energy is above the threshold.

Figure 4. Calculated results of power Fourier transforms of lasing emission spectra for (a) the NP-doped thin film and (b) the pure dye thin film.

Figure 3a depicts the output spectrum of PDMS thin films containing SiO2 NPs for different pump power levels. The concentration of PM597 dye molecules and SiO2 NPs are 3 and 3.5 mg/mL, respectively. The spectra show a broad spontaneous emission of PM597 molecules under a low pump energy. As the pump energy exceeds the threshold energy, well-distinguished sharp spikes with a line width less than 0.3 nm appear around 583 nm, indicating that the coherent random lasing occurs.3 The optical feedback of the random laser is provided by strong leaky-waveguide confinement, SiO2 NPs, and PM597 micro-/nanocrystal multiple scattering. The light is amplified by the PM597-doped SiO2 NPs through the confinement of light into the asymmetric slab optical waveguide as mentioned in ref 17, where light is confined by the film/air interface while the reflection at the film/substrate boundary is leaky. Nevertheless, the light is still highly confined because of the high reflectivity occurring at the grazing incidence.31 This leaky-waveguide structure is analogous to a random microcavity dramatically increasing light confinement and providing photon output paths in the sample; when the amplification along the microcavity exceeds the loss, random-laser operation can occur in the PDMS thin film. The relationships between the light emission intensity and the pump energy for both the pure dye thin film and the NP-doped thin film are illustrated in Figure 3b. It can be seen that the slope has an abrupt change, followed by a linear increase in the output signal as the excitation energy. The threshold of the random laser for pure dye thin films is about 1.5 mJ/cm2, which is much lower than that in the dye aggregation of thin films with a high dye concentration reported before.26 The threshold for NPdoped thin films is about 0.75 mJ/cm2, which is 1/2 that of pure dye thin films. This is due to the following: the multiple

optical multichannel analyzer (OMA) with a spectral resolution of 0.1 nm. The measured emission spectrum as shown is from a single shot of the light in operation as the integration time of 1 ms is used.

3. RESULTS AND DISCUSSION In the experiment, the emission spectra for various pump intensities were investigated for the pure dye thin film, as shown in Figure 2a. The PM597 dye concentration is 3 mg/ mL. The experimental results demonstrate that the CRL with line width less than 0.3 nm can be obtained. This may be because PM597 dye molecules form an aggregation. Such aggregations are responsible for both the gain characteristic and multiple scattering, which in turn determine the random feedback.26 However, in thin films with only Rh6G (Rhodamine 6G) dye crystals, no light amplification can be observed.27 For verification of our hypothesis, an electron microscope with 700 times magnification was used to observe the aggregate morphology of the scattering gain medium, as shown in Figure 2b. Various shapes of PM597 micro-/ nanocrystals exist in the PDMS thin film. Then, a scanning electron microscope (SEM) with 10 μm resolution was used to observe the sizes of the micro-/nanocrystals, as shown in Figure 2c. Figure 2c shows the PM597 micro-/nanocrystals without doped NPs, and the micro-/nanocrystals are randomly distributed. The size of micro-/nanocrystals is in the range 0.4−3.2 μm. Figure 2d shows the shapes of SiO2 NPs and PM597 micro-/nanocrystals. The SiO2 NPs show different diameters due to the fact that dye molecules cover the surface of the NPs. The particle morphologies between the pure dye thin film and the NP-doped thin film are obviously different, which indicates that the optical feedback in the pure dye thin film is provided by the PM597 dye micro-/nanocrystals. C


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Figure 5. (a) Evolution of emission spectra as a function of the pump energy in the NP-doped thin film without the leaky-waveguide structure. (b) The emission intensity dependent on the pump energy for different configurations. (c) The output spectrum dependent on the pump energy for the pure dye thin film without the leaky-waveguide structure.

Figure 6. Peak position of random-laser spectra dependent on (a) the pump stripe length and (b) the thickness of thin films. (c) Output intensity of the random laser as a function of the incident pump pulse intensity for different thicknesses of thin films.

experience strong amplification in the sample. However, the loss induced by the self-absorption effect of PM597 is large, which thus increases the stimulated emission threshold.

scattering provided by PM597 micro-/nanocrystals is weak; the amplifying random media system falls within the weakly scattering regime; and the majority of emitted photons do not D


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Figure 7. (a) Peak position of random-laser spectra for different different concentrations of PM597 dye in the pure dye thin film. The emission spectrum for (b) the pure dye solution and (c) the mixed solution contains the PDMS prepolymer.

threshold, which is 1.45 mJ/cm2. This phenomenon can be explained by the enhancement of the photon localization effect in the leaky-waveguide structure, causing a lower threshold. Therefore, the leaky-waveguide structure plays an important role in the reduction of the RL threshold. In addition, the pure dye thin film was also peeled from the glass, and we also observed the CRL emission spectrum, as shown in Figure 5c. It is noted that there is a bimodal phenomenon that exists in the laser emission spectrum when the pump energy is 2.6 mJ/cm2; this may be because there are multiple closed loop paths in the system, which serve as ring cavities for light. Along different light paths, the probability and distance of a photon scattering back to its starting point is different.4 Different closed loop paths correspond to different modes, which decide the frequency of the emission peaks, causing two different laser modes. However, the threshold (2.075 mJ/cm2) is higher than that in NP-doped thin films without the leaky-waveguide structure. The change of peak positions of the emission spectrum is an important characteristic of random lasers. To explore the effect of stripe length on lasing features, with an excitation of 0.9 mJ/ cm2, we try to decrease the excitation stripe length from 8 to roughly 5 mm, during which the peak intensity decreases dramatically, as shown in Figure 6a. The peak wavelength redshifts 6.7 nm. This phenomenon may be explained by the stronger reabsorption of the laser dye in the unpumped region, because the emitted photons are subjected to a strong reabsorption by the dye molecules in the unpumped region, and the emitted light far from the output end is more greatly reabsorbed by dye molecules than that near the output end.32 Thus, the amount of reabsorbed light increases with decreasing pump stripe length, resulting in the red-shift in the emission

For a derivation of the excited random ring cavity lengths from the parameters used in the research, the power Fourier transforms (PFTs) of the RL spectra of Figure 3a are calculated, and the results are presented in Figure 4a. It is well-known that Fourier analyses performed for random-laser spectra are represented on the 1/λ scale, so the frequencies of the obtained Fourier spectra are on the micrometer scale.17 The cavity length Lc is given by the following expression: Lc = πpm/ nm. Here, the terms are as follows: m is the order of the Fourier harmonic, Lc is the cavity path length, and n is the refractive index of the gain medium. The mean optical cavity length is Lc1 = 32.33 μm. Noticeably, that is about 4 times larger than the film thickness, which means that the feedback of the random laser takes effect in the waveguide plane. In addition, we also calculate the random cavity lengths of pure dye thin film RL spectra using PFT, which are shown in Figure 4b. The mean optical cavity length is Lc2 = 49.28 μm, which is larger than that in the NP-doped thin film. Therefore, a majority of emitted photons travel a longer distance in the pure dye thin film, and the emitted photons subjected a stronger self-absorption effect provided by the PM597 dye, thus leading to an increase in the lasing threshold. For verification of the influence of the leaky-waveguide structure on the random-laser operation, the PDMS thin film containing NPs was peeled from the glass using a knife, and the CRL phenomenon also appears, as shown in Figure 5a. The feedback is based on multiple scattering coming from NPs and micro-/nanocrystals presented in the gain medium. The relationships between the light emission intensity and the pump energy for both the leaky-waveguide structure and the thin film are illustrated in Figure 5b. Compared with the leakywaveguide structure, the thin film shows a higher laser E


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molecules do not aggregate. The ASE spectrum red-shifts with increasing dye concentration; this is because the absorption of dye molecules becomes stronger as the dye concentration increases, and the short wavelength of the emission spectrum closer to the dye absorption peak was absorbed, when the dye molecules were excited again, causing the peak wavelength to red-shift.35 In addition, we sealed the PM597/PDMS toluene solution for 48 h at room temperature, and the mixed solution could be cured. However, the aggregation of the dye molecules was not observed, which indicates that PM597 is well-dissolved in the mixed solution. In our experiment, the concentration of PM597 was 1 mg/mL, and there is still a formed micro-/ nanocrystal. On the basis of the above analysis, this is related to the polymer and solvent. Because the solubility of PM597 dye molecules in toluene was greater than that in PDMS, the PM597 dye molecule solubility changes, and they spontaneously aggregate in the process of toluene volatilization or during the period of time when the PDMS cures, which is the likely reason for the phase separation or microcrystallization.38 Additionally, this kind of feature was also mentioned in previous work.37,39

spectrum. Figure 6b shows the peak position of random-laser spectra for different thicknesses of the films. The peak wavelength red-shifts 5.93 nm with the increasing thickness of the thin films. At large thicknesses of the thin film, the multiple scattering provided by NPs and micro-/nanocrystals is enhanced, and a majority of emitted photons remain for a longer time inside the thin film; thus, the emission light is subjected to a strong reabsorption by the dye molecules, causing the random-laser spectrum to red-shift. The red-shift of peak positions of the emission spectrum could be widely used in photonic devices and special lighting.33,34 Figure 6c shows the pump threshold for different thicknesses of thin films, and other arrangements were kept unchanged. It can be clearly seen that the thickness of thin films obviously affects the threshold energy. When the thickness of the thin film was 9.6 μm, the lowest pump threshold energy (1.125 mJ/cm2) was obtained. In the PDMS thin film, the gain volume becomes large with the increasing thickness of the thin film from 7.7 to 9.6 μm. The scattering strength provided by NPs and micro-/nanocrystals increases, and there are more chances to form random cavities, thus decreasing the stimulated emission threshold.35 In the thicker film, the loss induced by the self-absorption effect of PM597 is large, leading to an increase in the lasing threshold. In the experiment, the reason for the formation of PM597 dye micro-/nanocrystals was studied. First, the influence of dye concentrations on the formation of PM597 dye micro-/ nanocrystals was investigated. Three material groups in the pure dye thin film for comparison were set with different concentrations of PM597 dye as shown in Figure 7a, and other arrangements were kept unchanged. It can be clearly seen that CRL appears even at lower concentrations (1 mg/mL) of PM597 dye, and the peak wavelength red-shifts 6.68 nm the increasing concentration of PM597 dye. The multiple scattering provided by the micro-/nanocrystals becomes stronger with increasing dye concentrations, and the emission light has a larger loss caused by the reabsorption of dye molecules. Thus, the random-laser spectrum shows a red-shift. However, in our previous work,36 2 mg/mL PM597 and 0.4 g of poly(methyl methacrylate) (PMMA) were dissolved in a cyclopentanone solution to form a thin film by spin-coating. The experiment results showed that there were no dye micro-/nanocrystals in the thin film. In our experiment, there are still micro-/ nanocrystals formed even at lower concentrations of dye-doped thin films. However, in ref 37, no micro-/nanocrystals of PM597 have been observed at a concentration of 2 mM (1.09 mg/mL), because the dye concentration in PDMS was commonly limited as the poor solubility and micro-/nanocrystals were observed in dye-doped PDMS samples at higher concentrations. The results indicate that the formation of PM597 dye micro-/nanocrystals has nothing to do with the concentration of dye, and it may be related to the polymer and solvent. To prove our hypothesis, we designed another experiment: two different configurations for comparison were set with various concentrations of PM597 in the sample bottles. The difference between our two configurations is whether the gain media contain PDMS prepolymer or not. For the prevention of the volatilization of toluene, all of the mixed solutions were sealed and placed for 48 h at room temperature; however, we have not observed curing of the mixed solutions. Figure 7b,c shows the output spectrum of two different configurations in the quartz cuvette. The emission spectrum shows an amplified spontaneous emission (ASE) property, which suggests that dye

4. CONCLUSIONS In conclusion, a CRL based on dye-doped PDMS thin films is demonstrated. The optical feedback of the CRL is provided by multiple scattering of NPs and micro-/nanocrystals as well as the confinement effect of the leaky-waveguide structure. The CRL can be obtained in the pure dye thin film because of the existence of spontaneously formed dye micro-/nanocrystals. Compared with the pure dye film, the SiO2 NPs can effectively reduce the threshold. The threshold increased after the film was peeled from glass, which indicates that the photon localization effect of the leaky-waveguide structure plays an important role in the reduction of the RL threshold. The emission peak maximum has a red-shift when the pump stripe length or the thickness of the film was changed, because of the change of PM597 dye molecule solubility, which contributed to the spontaneous aggregation of PM597 dye molecules and the formation of dye micro-/nanocrystals. This kind of randomlaser operation has prospects in a wide range of applications, such as integrated optoelectronics, wearable devices, and display imaging. However, many of the laser’s problems still remain unresolved, and more research is in strong demand.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Lihua Ye: 0000-0001-7000-2341 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China under Grants 11474052, 11174160, and 11274062.

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REFERENCES

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