Open Access
Add to BibSonomyAbstract
Laser emission from leaky waveguides based in dye-doped organic gain media incorporating Polyhedral Oligomeric Silsesquioxanes (POSS) nanoparticles is reported. The samples consist of thin film gain media deposited onto glass substrate defining a planar asymmetric slab waveguide, which does not incorporate any resonant substructure. The presence of POSS results in additional amplified spontaneous emission (ASE) spectral narrowing, and conditions have been found for which directional multimode laser emission is achieved. The spectral narrowing is ascribed to the photon path enlargement caused by a non-resonant feedback mechanism provided by individual scatterers, which enhances incoherently the magnitude of the amplification process. On the contrary, the appearance of multimode lasing is attributed to coherent random lasing from a many scatterers collective effect.
©2010 Optical Society of America
1. Introduction
In a recent paper [1] we demonstrated for the first time that, under certain conditions, addition of nanometer-sized particles based on polyhedral oligomeric silsesquioxanes (POSS) to liquid and solid solutions of organic dyes can scatter light in a way that increases the efficiency of the material’s laser action. The POSS particles act as weak scattering centres in the Rayleigh limit (particle size much smaller than the emission wavelength), and the photon path enlargement caused by multiple scattering provides an extra feedback which enhances incoherently the magnitude of the amplification process, in what has been called “Non-Resonant Feedback Lasing (NRFL)” or “Incoherent Random Lasing” [2,3].
Waveguiding structures have attracted much attention over the last years for their potential applications in integrated photonics [4–18]. In a conventional planar waveguide, amplified spontaneous emission (ASE) is obtained when the gain medium is pumped above a certain threshold value. ASE is a type of mirrorless lasing strongly directional but broadband, which appears when there is a population inversion but there are no structures providing resonant feedback. Although ASE may be used in applications where good spatial coherence is required but low temporal coherence suffices, coherent laser emission is needed in more demanding optoelectronic applications. Thus, it would be of much interest to study if a NRFL effect similar to that observed in bulk liquid and solid dye solutions could also act in waveguiding thin films.
Although random lasing of structureless π-conjugated polymer thin films containing TiO2 nanoparticles has been observed [19,20], lasing action in organic materials structures has been mainly obtained by using different feedback designs, such as microcavities, distributed feedback structures, or ring microlaser structures [21]. Of these, distributed feedback resonators (DFB), which offer periodic modulation of either the gain or refractive index of the medium, have been generally preferred for solid state dye lasers, principally because of the compact design, high degree of spectral selection, edge-on or out-of-plane emission, narrow linewidth, and broad tunability [22–31]. In this paper we explore the effect of the presence of POSS nanoparticles in dye-doped organic media in the form of thin films deposited onto glass substrates to define a waveguiding structure. We demonstrate, for the first time to the best of our knowledge, that the presence of POSS in photosensitized polymeric planar waveguides allows obtaining laser emission without the need of incorporating complex resonant substructures in the material.
2. Experimental
Asymmetric slab optical waveguides consisting of dye-doped polymer films deposited onto glass substrates (standard glass microscope slides with thickness of 0.98 mm) were prepared by spin coating (2000-4000 rpm, 20 s). The polymer films consisted of poly(methyl methacrylate) (PMMA) of 25,000 molecular weight (Polysciences, Inc) in proportion 100-200 mg/mL, depending on the desired thickness, containing octa(propyl methacryl)-POSS (8MMAPOSS, Fig. 1 ) (Hybrid Plastics) in weight proportions from 0 to 50 wt%, and dye pyrromethene 597 (PM597) (laser grade from Exciton) at a concentration (with respect to total polymer) of 2.5 x10−2M. Although the PM597 concentration was rather high, the dye was completely solved and photophysical studies did not show any evidence of aggregation of the dye molecules. The casting solvent for spin-coating was chloroform (Merck).
Fig. 1 Molecular structures of octa(propyl-methacryl)-POSS (8MMAPOSS) and methyl methacrylate monomer (MMA).
After solvent evaporation at 50°C for 5min films with 2.1 ± 0.1 μm and 5.5 ± 0.1μm were obtained. The thickness of the films was determined by gravimetric method, taking into account the density of the polymeric film mixture (between 1.19 g/mL for pure PMMA and 1.218 g/mL for PMMA/8MMAPOSS-50 wt%). Films of 9 μm thickness were prepared by using the extender roller technique, where a few drops of the solution were first placed onto the substrate and then extended along it by using a calibrated rod (endless screw with calibrated thread) [32].
The thin film samples were pumped at 532 nm with 20 ns full width at half maximum (FWHM) pulses from a frequency-doubled Q-switched Nd:YAG laser (Lotis TII SL-2132). The pump radiation was horizontally polarized and the pulse energy incident onto the sample was controlled by insertion in the beam path of a half-wave plate (HWP) and a linear polarizer (LP) set with its polarization axis horizontal. By rotating the HWP the linear polarization of the input beam is rotated out of the horizontal, and the pump beam is blocked more or less by the LP, depending on the rotation angle introduced by the HWP. A combination of negative and positive cylindrical quartz lenses (f = −15 and + 15 cm, respectively), perpendicularly arranged, focused the pump beam to a narrow horizontal line onto the surface of the film. An adjustable slit was used to select only the central portion of the pump beam. A micrometer screw allowed to precisely selecting the width of the slit. The light incident on the sample was perpendicular to the film surface, defining an excitation stripe of ~150 μm × 2 mm. The sample was placed on a XY Motorized Translation Stage (MTS50XY, Thorlabs), computer controlled, to allow precise positioning. Measurements were performed with the excitation stripe end placed right up to the edge of the film or translated away from the edge of the sample in 2 mm steps, while keeping the pumped stripe length constant (Fig. 2 ). The distance between the end of the pump stripe and the edge of the waveguide from where the emission is collected, is denoted as z. The edge emission from the sample was collected by a fibre bundle and directed to a spectrograph/monochromator (SpectraPro-300i Acton Research) equipped with a thermoelectrically cooled CCD detector (SpectruMM:GS 128B). Emission spectra averaged over 15 pulses for values of z between 0 and 16 mm were recorded.
2. Results and discussion
The refractive index of the film material increased linearly with the content of 8MMAPOSS, from n1 = 1.4883 (MMA homopolymer, 0% 8MMAPOSS) up to n1 = 1.4977 (8MMAPOSS content of 50 wt%) [33]. As the refractive index of the polymer layer is lower than that of the glass substrate (measured refractive index, n2 = 1.5176), no total internal reflection takes place at the film/substrate interface, and the emitted light leaks into the substrate. A leaky waveguide or quasi-waveguide is obtained, where light is confined by the film/air interface while the reflection at the film/substrate boundary is leaky. Nevertheless, significant confinement of the light still takes place because of the high reflectivity occurring at grazing incidence [34]. The different transverse modes propagating within the gain layer have different reflectivity losses due to the Fresnel law. Although several low-loss leaky modes may exist when the thickness of the gain layer is large compared with the wavelength of the light [35], the fundamental mode is the one with the lower losses and, thus, light is amplified preferably in this mode [36]. As a result, a quasi-waveguide provides a much stronger self-mode restriction capability than conventional total internal-reflection waveguides.
Figure 3 shows the normalized emission spectra recorded for different values of z from 5.5 μm thin films with different wt% content of 8MMAPOSS at a pump intensity of 500 kW/cm2. In the waveguides with no POSS, the emission is just ASE with linewidths in the range 9-15 nm FWHM. The presence of 8MMAPOSS in 13 wt% proportion results in a slight decrease in the emission linewidth, now in the range 7-13 nm FWHM, a moderate increase in the intensity, and a red shift of the peak of the emission of about 2 nm. Increasing the proportion of 8MMAPOSS in the material to 20 wt% results in a further increase of the intensity, narrowing of the emission to 4-10 nm, and a red shift of the peak of the emission of about 5 nm with respect to the emission from the material with no POSS. In the three cases, the emission with the narrowest linewidth is observed at a distance z = 6 mm between the end of the pumped stripe and the waveguide edge from which emission is collected. This behavior could be indicating a transition from ASE emission to broadband laser emission.
Fig. 3 Normalized emission spectra recorded for different values of z from 5.5 μm thick films onto glass with different wt% content of 8MMAPOSS at a pump intensity of 500 kW/cm2.
The characteristics of the emission change dramatically when the content of 8MMAPOSS in the polymer increases to 50 wt%. In this case, multimode emission with narrow peaks on top of a globally narrowed ASE spectrum does appear (Fig. 3), the intensity of the emission increases by more than tenfold, and in the centre of the previous fringe shaped emission does appear a bright spot. The multimode emission is best established for z distances of 4 and 6 mm and is displaced to the red for up to 10 nm with respect to the emission from the material with no POSS. The linewidth of the peaks is smaller than 0.1 nm, which is the resolution of our detection system.
Illustrative photographs of the emission from 5.5 μm films with no POSS and with 50 wt% content of 8MMAPOSS are presented in Fig. 4 . Figure 4(a) shows the fringe shape emission from a sample without POSS particles at a distance of 20 cm from the sample. This image should be compared with Fig. 4(b), where the emission from a sample with 50 wt% of 8MMAPOSS at the same distance from the sample is presented. In this case a bright spot in the middle of the fringe appears, resembling laser emission. To better appreciate the brightness of the emission, Fig. 4(c) is a photograph of the same emission taken with the room fully lighted. The divergence of the emission was measured to be about 10 mrad and 6 mrad in the planes parallel and perpendicular to the film, respectively.
Fig. 4 Emission from a sample without POSS (a) and with 50 wt% content of 8MMAPOSS (b) at 20 cm from the waveguide edge. In (c) the picture is as in (b) but with the room fully lighted.
In Fig. 5 we have represented the evolution of the intensity of the main narrow peak in the emission from a sample with 50 wt% 8MMAPOSS with the pump intensity when the pumped spot was placed at z = 4mm from the waveguide edge. A clear pump threshold for the narrowband emission to appear is evident in the figure. Taken together, the narrow linewidth of the peaks in the multimode emission, the small divergence of the emission, and the existence of a pump threshold are evidences of the laser nature of the narrow emission lines.
Fig. 5 Dependence on pump intensity of the intensity of the main peak in the multimode emission from a 5.5 μm thick film with 50 wt% 8MMAPOSS for a pumped stripe placed at position z = 4mm.
Some years ago, Duarte and James [37] demonstrated an improvement in the beam divergence of the laser emission from a dye-doped organic medium incorporating silica nanoparticles, and explained this effect as due to the improved thermal characteristics of the medium resulting from the presence of the nanoparticles. Although the thermal conductivity of the materials here studied was determined to increase linearly with the amount of POSS [33], going from 0.182 Wm−1K−1 for PMMA up to 0.233 Wm−1K−1 for PMMA/8MMAPOSS-30 wt%, with an estimated value of 0,252 Wm−1K−1 for PMMA/8MMAPOSS-50 wt%, the effects observed cannot be attributed to just the improvement of the thermal properties, since no narrowband laser emission was observed in waveguides based in newly synthesized fluorinated polyimides doped with the same dye PM597 [38], whose thermal conductivity at least doubles that of the PMMA.
We ascribe the observed spectral narrowing and red shift to an effect of Non-Resonant Feedback (NRF) lasing present in weakly scattering systems [2], where diffusive photons experience multiple scattering. Each scatterer contributes incoherently to this feedback, elongating the light path inside the gain medium and, above a certain pump threshold, results in an additional spectral narrowing. According to this the higher the scatterer density (8MMAPOSS content), the narrower the emission spectrum. This dependence can be noticed in Fig. 3. In addition, the increasing red shift in the emission with 8MMAPOSS content can be understood as being due to two related effects: on the one hand, the self-absorption of the emitted radiation is bigger due to the increased photon path, and on the other hand, the scattering losses are higher for the shorter wavelengths.
Another feature of this multimode laser emission is the randomness of the peak wavelengths when moving the sample. This fact leads us to ascribe the multimode emission to a mechanism of coherent random lasing [3]. That is, the distribution of scatterers within the excited region, that conform random ring cavities, changes in a random fashion when the sample is moved, leading to different oscillation conditions (random cavities) in each sample region.
To gain insight into the excited random cavities we calculated the Power Fourier Transform (PFT) of the spectra in Fig. 3. The PFT of the emission spectrum (in κ = 2π/λ space) from a well-defined laser cavity shows peaks at Fourier components pm = mLC·n/π, where m is the order of the Fourier harmonic, LC is the cavity path length, and n is the refractive index of the gain medium [39]. Figure 6 shows the calculated ensemble-averaged PFT spectra of the emission spectra shown in Fig. 3. It is clearly seen that whenever sharp lines appear in the emission spectrum, clear peaks can be found in the PFT. In the case of 50 wt% 8MMAPOSS the first sharp peaks in the PFT spectra, corresponding to the fundamental Fourier component m = 1, give a mean cavity path length LC ≈45 μm (assuming n = 1.4977). The fact that LC is much shorter than the scattering length (ℓSC = 56 cm for 50 wt% 8MMAPOSS [1]) implies that only a very small amount of the scattered light is confined in the random cavity. This small confinement could explain the fact that multimode emission only appears under certain conditions of gain and scatterer concentrations.
Fig. 6 Ensemble-averaged PFT spectra of the emission spectra shown in Fig. 3. Amplitude of PFT (ordinates) in arbitrary units.
To assess the dependence of the studied phenomenon on the film thickness, samples with 2.1 μm and 9 μm thin films were prepared. When the volume of the gain region was reduced by decreasing the thickness of the films to 2.1 μm, the overall gain seen by the photons was reduced and as a result only fluorescence was detected in the samples without POSS (Fig. 7 ). Addition of 8MMAPOSS up to 50 wt% was not able to compensate for the lower overall gain in these thinner films, and only narrowed ASE emission was detected, albeit with some indication of incipient peaks (Fig. 7). Increasing the thickness of the films to 9 μm resulted in well established multimode emission from 13 wt% 8MMAPOSS content.
Fig. 7 Normalized emission spectra recorded for selected values of z from 2.1 μm thick films onto glass with different wt% content of 8MMAPOSS at a pump intensity of 500 kW/cm2.
An explanation of the different behavior of the multimode emission in films with different thicknesses is suggested from a close analogy to conventional lasing. The threshold condition in usual lasers depends coarsely on the overall medium gain and the cavity reflectivity: the higher the gain and reflectivity, the lower the threshold. On the other hand, a lower value in one of these parameters can be compensated by a higher value in the other. In our system, the effective medium gain depends directly on film thickness, and the effective cavity reflectivity is determined by the amount of 8MMAPOSS nanoparticles. In the thinner films the pump radiation is not efficiently absorbed, and consequently the effective gain is low; hence, a higher amount of 8MMAPOSS nanoparticles (reflectivity) is needed to achieve lasing. Content of more than 50 wt% of 8MMAPOSS cannot be achieved because of solubility problems, and thus, for the 2 μm thick films the multimode emission regime cannot be reached (Fig. 7). The opposite is true for the thicker films, and a small content of 8MMAPOSS (13wt% for 9 μm thick films) is enough to excite multimode emission. For the 5.5 μm thick films an intermediate situation happens and a moderate amount of 8MMAPOSS is enough for multimode lasing to occur (Fig. 3).
3. Summary and conclusion
The emission under 532 nm pumping from quasi-waveguides based in PM597-doped organic gain media incorporating 8MMAPOSS nanoparticles has been studied. Thin films with thickness between 2 and 9 μm and with weight proportions of 8MMAPOSS varying from 0 to 50% were prepared. The presence of 8MMAPOSS in the thin film composition resulted in a narrowing and red shift of the ASE emission from the samples. In the 5.5 μm films the presence of 8MMAPOSS in 50 wt% proportion resulted in multimode laser emission. When the thickness of the films was increased to 9 μm, laser emission was obtained with 8MMAPOSS content above 13 wt%. In the films 2 μm thick spectral narrowing was observed but multimode emission could not be obtained.
The spectral narrowing has been ascribed to a mechanism of Non-Resonant Feedback, where the photon path enlargement caused by multiple scattering provides an intensity feedback which enhances incoherently the magnitude of the amplification process. The presence of multimode laser emission without the need to modulate the substrate to obtain feedback can be understood in terms of coherent random lasing. The calculated mean cavity path length indicates that only a small amount of scattered light is confined within the random cavity, which entails that certain gain conditions must be met for multimode laser to appear.
Acknowledgements
This work was supported by the Spanish CICYT, Project MAT2007-65778-C02-01. L. C. thanks MICINN for a Predoctoral scholarship (FPI, cofinanced by Fondo Social Europeo).
References and links
1. A. Costela, I. García-Moreno, L. Cerdán, V. Martín, O. García, and R. Sastre, “Dye-Doped POSS Solutions: Random Nanomaterials for Laser Emisión,” Adv. Mater. 21(41), 4163–4166 (2009). [CrossRef]
2. M. A. Noginov, Solid-State Random Lasers (Springer, New York, 2005).
3. S. Takeda and M. Obara, “Extremely selective modal oscillation of random lasing induced by strong multiple scattering,” Appl. Phys. B 94(3), 443–450 (2009). [CrossRef]
4. K. P. Kretsch, C. Belton, S. Lipson, W. J. Blau, F. Z. Henari, H. Rost, S. Pfeiffer, A. Teuschel, H. Tillmann, and H.-H. Hörhold, “Amplified spontaneous emission and optical gain spectra from stilbenoid and phenylene derivative model compounds,” J. Appl. Phys. 86(11), 6155–6159 (1999). [CrossRef]
5. P. Yang, G. Wirnsberger, H. C. Huang, S. R. Cordero, M. D. McGehee, B. Scott, T. Deng, G. M. Whitesides, B. F. Chmelka, S. K. Buratto, and G. D. Stucky, “Mirrorless lasing from mesostructured waveguides patterned by soft lithography,” Science 287(5452), 465–467 (2000). [CrossRef] [PubMed]
6. S.-S. Yap, W.-O. Siew, T.-Y. Tou, and S.-W. Ng, “Red-green-blue laser emissions from dye-doped poly(vinyl alcohol) films,” Appl. Opt. 41(9), 1725–1728 (2002). [CrossRef] [PubMed]
7. Y. Oki, S. Miyamoto, M. Maeda, and N. J. Vasa, “Multiwavelength distributed-feedback dye laser array and its application to spectroscopy,” Opt. Lett. 27(14), 1220–1222 (2002). [CrossRef]
8. M. A. Reilly, C. Marinelli, C. N. Morgan, R. V. Penty, I. H. White, M. Ramon, M. Ariu, R. Xia, and D. D. C. Bradley, “Rib waveguide dye-doped polymer amplifier with up to26 dB optical gain at 625 nm,” Appl. Phys. Lett. 85(22), 5137–5139 (2004). [CrossRef]
9. W. Lu, B. Zhong, and D. Ma, “Amplified spontaneous emission and gain from optically pumped films of dye-doped polymers,” Appl. Opt. 43(26), 5074–5078 (2004). [CrossRef] [PubMed]
10. R. Kumar, A. P. Singh, A. Kapoor, and K. N. Tripathi, “Effect of dye doping in poly(vinyl alcohol) waveguides,” J. Mod. Opt. 52(10), 1471–1483 (2005). [CrossRef]
11. K. Geetha, M. Rajesh, V. P. N. Nampoori, C. P. G. Vallabhan, and P. Radhakrishnan, “Laser emission from transversely pumped dye-doped free-standimg polymer film,” J. Opt. A, Pure Appl. Opt. 8(2), 189–193 (2006). [CrossRef]
12. G. Jordan, M. Flämmich, M. Rüther, T. Kobayashi, W. J. Blau, Y. Suzuki, and T. Kaino, “Light amplification at 501 nm and large nanosecond optical gain in organic dye-doped polymeric waveguides,” Appl. Phys. Lett. 88(16), 161114 (2006). [CrossRef]
13. E. M. Calzado, J. M. Villalvilla, P. G. Boj, J. A. Quintana, R. Gómez, J. L. Segura, and M. A. Díaz García, “Amplified spontaneous emission in polymer films doped with a perylenediimide derivative,” Appl. Opt. 46(18), 3836–3842 (2007). [CrossRef] [PubMed]
14. H. Goudket, T. H. Nhung, B. Ea-Kim, G. Roger, and M. Canva, “Importance of dye host on absorption, propagation losses, and amplified spontaneous emission for dye-doped polymer thin films,” Appl. Opt. 45(29), 7736–7741 (2006). [CrossRef] [PubMed]
15. M. Djiango, T. Kobayashi, W. J. Blau, B. Cai, K. Komatsu, and T. Kaino, “Near-infrared luminescent polymer waveguide with a 20 dB small-signal gain,” Appl. Phys. Lett. 92(8), 083306 (2008). [CrossRef]
16. S. Yuyama, T. Nakajima, K. Yamashita, and K. Oe, “Solid state organic laser emission at 970 nm from dye-doped fluorinated-polyimide planar waveguides,” Appl. Phys. Lett. 93(2), 023306 (2008). [CrossRef]
17. D. Zhang, Z. Chen, and D. Ma, “White light emission on amplified spontaneous emission with dye content controlled polymer system,” J. Appl. Phys. 103(12), 123103 (2008). [CrossRef]
18. L. Cerdán, A. Costela, I. García-Moreno, O. García, and R. Sastre, “Waveguides and quasi-waveguides based on pyrromethene 597-doped poly(methyl methacrylate),” Appl. Phys. B 97(1), 73–83 (2009). [CrossRef]
19. C. Liu, J. Liu, J. Zhang, and K. Dou, “Random lasing with scatterers of diameters 20 nm in an active medium,” Opt. Commun. 244(1-6), 299–303 (2005). [CrossRef]
20. A. Tulek, R. C. Polson, and Z. V. Vardeny, “Naturally occurring resonators in random lasing of π-conjugated polymer films,” Nat. Phys. 6(4), 303–310 (2010), doi:. [CrossRef]
21. G. Kranzelbinder and G. Leising, “Progress in organic solid state lasers,” Rep. Prog. Phys. 63(5), 729–762 (2000). [CrossRef]
22. Y. Oki, S. Miyamoto, M. Maeda, and N. J. Vasa, “Multi-wavelength distributed-feedback dye laser array and its application to spectroscopy,” Opt. Lett. 44, 4965–4971 (2002).
23. X. Zhu and D. Lo, “Sol-gel glass distributed-feedback waveguide laser,” Appl. Phys. Lett. 80(6), 917–920 (2002). [CrossRef]
24. D. Lo, L. Shi, J. Wang, G. X. Zhang, and X. Zhu, “Zirconia and zirconia-organically modified silicate distributed feedback waveguide lasers tunable in the visible,” Appl. Phys. Lett. 81(15), 2707–3181 (2002). [CrossRef]
25. D. Schneider, S. Hartmann, T. Bernsten, T. Dobbertin, D. Heithecker, D. Metzdorf, E. Becker, T. Riedl, H.-H. Johannes, W. Kowalsky, T. Weimann, J. Wang, and P. Hinze, “Wavelength-tunable organic solid-state distributed-feedback laser,” Appl. Phys. B 77(4), 399–402 (2003). [CrossRef]
26. J. Wang, G.-X. Zhang, L. Shi, D. Lo, and X.-L. Zhu, “Tunable multiwavelength distributed-feedback zirconia waveguide lasers,” Opt. Lett. 28(2), 90–92 (2003). [CrossRef] [PubMed]
27. F. Chen, J. Wang, C. Ye, W. Ni, J. Chan, Y. Yang, and D. Lo, “Near infrared distributed feedback lasers based on LDS dye-doped zirconia-organically modified silicate channel waveguides,” Opt. Express 13(5), 1643–1650 (2005). [CrossRef] [PubMed]
28. H. Sakata and K. Natsume, “Distributed-feedback vertical cavity structures for optically pumped solid-state organic lasers,” Opt. Quantum Electron. 39(7), 577–583 (2007). [CrossRef]
29. M. Lu, B. T. Cunningham, S.-J. Park, and J. G. Eden, “Opt. Commun, “Vertically emitting, dye-doped polymer laser in the green (λ ~ 536 nm) with a second order distributed feedback grating fabricated by replica molding,” Opt. Commun. 281(11), 3159–3162 (2008). [CrossRef]
30. Y. Yang, G. Lin, H. Xu, M. Wang, and G. Qian, “Distributed feedback laser actions in zirconia-ORMOSIL waveguides based on energy transfer between co-doped laser dyes,” Opt. Commun. 281(20), 5218–5221 (2008). [CrossRef]
31. K. Yamashita, M. Arimatsu, N. Takeuchi, M. Takayama, K. Oe, and H. Yanagi, “Multilayerd solid-state organic laser for simultaneous multiwavelength oscillations,” Appl. Phys. Lett. 93(23), 233303 (2008). [CrossRef]
32. C. Lowe, Surface Coatings Technology (John Wiley & Sons, London 1997), vol.5, Chap. VI, p.60–62. [PubMed]
33. R. Sastre, V. Martín, L. Garrido, J. L. Chiara, B. Trastoy, O. García, A. Costela, and I. García-Moreno, “Dye-Doped Polyhedral Oligomeric Silsesquioxane (POSS)-Modified Polymeric Matrices for Highly Efficient and Photostable Solid-State Lasers,” Adv. Funct. Mater. 19(20), 3307–3316 (2009). [CrossRef]
34. T.-N. Ding and E. Garmire, “Measuring refractive index and thickness of thin films: a new technique,” Appl. Opt. 22(20), 3177-3181(1983). [CrossRef] [PubMed]
35. D. B. Hall and C. Ye, “Leaky waves in a heteroepitaxial film,” J. Appl. Phys. 44(5), 2271-2274 (1973). [CrossRef]
36. A. Kumar, V. Rastogi, and K. S. Chiang, “Large-core single-mode channel waveguide based on geometrically shaped leaky cladding,” Appl. Phys. B 90(3-4), 507–512 (2008). [CrossRef]
37. F. J. Duarte and R. O. James, “Tunable solid-state lasers incorporating dye-doped, polymer-nanoparticle gain media,” Opt. Lett. 28(21), 2088–2090 (2003). [CrossRef] [PubMed]
38. L. Cerdán, A. Costela, I. García-Moreno, O. García, R. Sastre, M. Calle, D. Muñoz, and J. de Abajo, “High-Gain Long-Lived Amplified Spontaneous Emission from Dye-Doped Fluorinated Polyimide Planar Waveguides,” Macromol. Chem. Phys. 210(19), 1624–1631 (2009). [CrossRef]
39. R. C. Polson, G. Levina, and Z. V. Vardeny, “Spectral analysis of polymer microring lasers,” Appl. Phys. Lett. 76(26), 3858–3860 (2000). [CrossRef]
