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OPTICS LETTERS / Vol. 40, No. 7 / April 1, 2015
Distributed feedback and random lasing in DCNP aggregates dispersed in a polymeric layer Kacper Parafiniuk, Lech Sznitko, and Jaroslaw Mysliwiec* Advanced Materials Engineering and Modelling Group, Faculty of Chemistry at Wroclaw University of Technology, Wyb. Wyspianskiego 27, 50-370 Wroclaw, Poland *Corresponding author: [email protected] Received January 19, 2015; accepted February 24, 2015; posted March 6, 2015 (Doc. ID 232589); published March 30, 2015 Here, we report on the realization of random lasing (RL) and distributed feedback (DFB) lasing in a layer of luminescent 3-(1,1-dicyanoethenyl)-1-phenyl-4,5-dihydro-1H-pyrazole (DCNP) organic nonlinear optical dye that has been dispersed in a poly(methyl methacrylate) (PMMA) matrix. The RL phenomenon appears due to the presence of spontaneously formed micro- and nano-crystals of DCNP in the bulk of the PMMA during the sample preparation. DFB can be realized in an optical system by using degenerated two-wave mixing in the pumping beams. The period of the interference pattern can be easily changed by changing the intersection angle of the pumping beams, resulting in a real time, fully reversible method of DFB lasing emission tuning. Because of the two neighboring stimulated emission bands of DCNP, it is possible to tune the lasing wavelength over a long range of about 65 nm. © 2015 Optical Society of America OCIS codes: (140.2050) Dye lasers; (140.3490) Lasers, distributed-feedback; (140.3600) Lasers, tunable; (290.4210) Multiple scattering; (290.5850) Scattering, particles; (160.4890) Organic materials. http://dx.doi.org/10.1364/OL.40.001552
Nowadays, there are many ideas for the realization of distributed feedback (DFB) lasers in either a mono- and bilayer system with a permanent or temporary holographic pattern. Because feedback in DFB lasers can be provided by gain and/or refractive index periodic perturbations, there may be several approaches to their realization [1–3]. Two of the most popular techniques of DFB laser cavity fabrication are electron beam lithography based on photoresist modulation, and the inscription of a surface relief grating (SRG) on top of a layer of photochromic polymer [3–6]. These methods generally allow researchers to fabricate DFB lasers with a fixed wavelength of emission because the grating period is also fixed. However, it is possible to fabricate DFB lasers without introducing any parts of the physical periodic structure. Another approach utilizes the formation of temporary patterns based on either the refractive index and/or the gain coefficient modulation in the bulk of the material. One example is the bi-layer system in which azo dye molecules doped to the polymeric matrix are covered by another polymer doped with a luminescent dye. The materials used in the described setup provide for the formation of the temporary grating and allow for the tuning of the wavelength of the laser emission. [7] There are also a few examples where more simplified single-layer systems to achieving DFB lasing were used [8,9]. Another class of DFB lasers is random lasers (RLs), which utilize multiple scattering and disorder to achieve feedback [10,11]. In our work, we present the results of DFB lasing. We used an organic nonlinear optical dye doped to a polymeric film from which the DFB resonator was generated in one step with a noncontact, holographic, temporary, and reversible grating inscription in the matrix bulk. Our goal was to couple the DFB resonator based on the periodic modulation of the refractive index or gain coefficient to the material that showed the random lasing (RL) phenomenon based on multiple scattering from a recrystallized dye that supported both gain and random 0146-9592/15/071552-04$15.00/0
feedback. We have shown that gratings with different periods could be directly formed in a film by changing the incident angle of intersecting beams, thereby enabling the tuning of the emission wavelength in a real time. The scheme of DFB lasing and the RL induced by the recrystallized dye are shown in Fig. 1(a). As an active dye, we used the luminescent organic compound 3-(1,1-dicyanoethenyl)-1-phenyl-4,5-dihydro1H-pyrazole (DCNP), which possesses interesting nonlinear optical properties [12]. Single- as well as two-photon absorption-induced fluorescence was observed, and it was shown that efficient stimulated emission can be achieved from DCNP that has been dissolved in a solid polymeric matrix, such as poly(methyl methacrylate) (PMMA) [13–15]. DFB lasing was also observed, but only in a hybrid system based on a DCNP dye-doped biopolymer over-coated with a photochromic polymer in
Fig. 1. Schematic of the DFB and RL phenomena in the DCNP/ PMMA layer. (a) Bulk diffraction grating with periodic changes Λ of the refractive index and/or gain coefficient in the material provides feedback for DFB lasing. Multiple scattering on elements such as microcrystals results in RL. (b) Microscopic images that show the DCNP microcrystals present in the PMMA matrix. (c) Microscopic image taken in luminescence mode above the RL threshold. (d) Microscopic image taken in the same mode below the RL threshold. © 2015 Optical Society of America
April 1, 2015 / Vol. 40, No. 7 / OPTICS LETTERS
Fig. 2. (a) Experimental setup for the observation of the DFB lasing. (b) Scheme of real-time different periods grating formed by the stripe-like beams.
the angle of intersection between the beams [see Fig. 2(b)]. The emission was gathered from the edge of the excited sample by a fiber spectrometer (Andor Solis Shamrock 163, resolution: 0.1 nm). In order to measure DFB, two coherent beams could create an interference pattern with a period that was dependent on the angle between them. Due to the presence of two neighboring stimulated emission bands of DCNP (both of which have a similar position and shape to the DCNP single crystal emission: see Fig. 3) in the PMMA matrix, is possible to tune DFB lasing over a range of about 65 nm (λlas from 585 to 650 nm). According to the Bragg formula, the order of diffraction m can be obtained from the following equation: Λ
which the Bragg grating has been inscribed using an SRG formation in an azo-functionalized polymer [6]. We have prepared the layer of 2% weight for weight (w/w) DCNP/PMMA on a glass plate via the drop casting technique using a dye/polymer tetrahydrofuran (THF) solution. The drying process took two days under THF vapors. The sample then spent another week in an ambient atmosphere to ensure that no solvent remained. The thickness of the obtained layer was about 25 μm. The experimental effective refractive index of the planar waveguide was neff 1.513. In order to observe RL and real-time tunable DFB lasing, we have constructed the experimental setup that is schematically shown in Fig. 2(a). Cylindrical and spherical lenses were used to change the shape of the excitation beams from a spot to a stripe, the size of which was 1.5 cm × 0.1 cm. The doubled frequency light (λpump 532 nm) coming from the neodymium-doped yttrium aluminum garnet (Nd:YAG) nanosecond pulsed laser (with a pulse duration of 6 ns) was used both to excite the luminescence and to enable the grating inscription. The laser beam with the stripe shape was split into two beams, and, using two mirrors, was horizontally interfered in a sample volume. The distance between the mirrors was fixed, while their rotation around the vertical axes was utilized in order to change
1553
mλlas ; 2neff
(1)
and was chosen as m 2, giving the periodicity Λ in the range from 386.6–429.6 nm, which depends on the angle 2θ (86.9–76.5°) between the beams. DFB spectra were collected in a series of 50 accumulations. By exciting the sample with only one of the stripe-like beams and above a certain threshold, we were able to observe both coherent (represented as many sharp lines in the emission spectra) and incoherent (represented as smooth emission spectra, thereby reproducing the gain profile) RL phenomenon. For the coherent RL light, localization mechanisms have to be introduced to the system. In our case, rarely seen DCNP microcrystals of a size of a few tens of micrometers were responsible both for light trapping and enhancement. Microscopic images of one of the investigated layers are presented in Figs. 1(b)–1(d). DCNP microcrystals are clearly visible in Fig. 1(b). Some of them (indicated with white circles) can be involved in coherent RL. Figures 1(c) and 1(d) show microscopic images of the same part of the sample [see Fig. 1(b)] taken in luminescence mode below and above the RL threshold. The sizes of the microcrystals presented in Fig. 1(c) are approximately 110, 75, and 50 μm. The resolution of the images is 8 μm. The properties of the photoluminescence of the DCNP/ PMMA layer (absorption, emission, stimulated emission,
Fig. 3. (a) Normalized absorption and emission spectra of the DCNP/PMMA sample and the emission spectra of the single DCNP crystal measured at a temperature of 77 K. (b) Stimulated emission (SE), RL, and DFB lasing spectra of the DCNP/PMMA sample. The inset in (b) shows the DFB lasing threshold value estimation at the wavelength of the DFB emission λlas 630 nm.
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OPTICS LETTERS / Vol. 40, No. 7 / April 1, 2015
RL, and DFB lasing spectra), as well as a single DCNP crystal emission, are presented in Fig. 3. The stimulated emission spectrum was obtained from the excitation sample by one circular-shaped beam with an energy density ρ 5.6 mJ∕cm2 . The exemplary spectrum typical for coherent RL, which consists of many sharp modes [16], was gathered for pumping by a stripe-like beam with an energy density ρ 7.5 mJ∕cm2 . The DFB lasing spectrum presented in Fig. 3(b) was measured for the inter-beam angle 2θ, which was equal to 79.4°, thereby enabling us to achieve laser emission at the wavelength of λlas 630 nm with the excitation energy density ρ 5.5 mJ∕cm2 . The intensity of the emission of the sample as a function of the incident laser energy density is shown in the inset in Fig. 3(b). The estimated value of ρth 5.3 mJ∕cm2 corresponds to the DFB lasing threshold (the conditions for the lasing wavelength were set at λlas 630 nm). As demonstrated by the work in [17], it was crucial for the DCNP crystal emission to have trap states, in which the emission occurs exactly in the spectral region covered by the stimulated emission. As was reported in [15], no stimulated emission occurs in diluted solutions of DCNP; therefore, the DCNP micro- and nanocrystals are necessary to enable the stimulated emission. Recently, it was shown that emission amplification can only be achieved for THF solutions that have a high concentration of DCNP in the order of 10−2 mol∕l. At this concentration, recrystallization occurs [18]. Therefore, the stimulated emission can be achieved from relatively long-living trap states of emission of the DCNP crystal. We suppose that the presence of numerous small DCNP crystals are responsible for the multiple scattering, and that the stimulated emission no longer can be thought of as an amplified spontaneous emission, but rather as incoherent RL. The threshold of the last process is highly dependent on the geometry and size of the excited area and the amount of disorder [19]. A closer inspection of sample showed that we were able to find places in the sample that were illuminated by a single pumping beam and that show highly directional coherent RL [see Figs. 1(b)–1(d)]. In this case, two beams coupling with the fulfilled Bragg conditions for light amplification
resulted in the formation of DFB lasing and in a narrow emission line selection. The mechanism of the RL phenomenon is based on spontaneously formed microand nano-crystals in the matrix bulk, which appear during the slow solvent evaporation that occurs during the sample preparation. Critically for samples containing DCNP, such aggregates are responsible both for the constitution of gain and for the multiple scattering, which in turn determines the random feedback. The size distribution of the DCNP crystals determines whether the amplified light can be confined inside the crystals or not, and strongly affects both the threshold conditions and the emission spectra. The utilization of broadband-emitting DCNP crystal trap states for lasing gives the possibility of wide DFB lasing tunability. The emission wavelength λlas was tuned with the change of the incidence angle 2θ of the pumping beams in the sample. Figure 4(a) shows the normalized DFB emission spectra of the DCNP/PMMA layer for various intersection angles. The pumping energy density was found to be ρ 5.5 mJ∕cm2 . One great advantage of the studied system is that we can tune the emission wavelength in real time without any evidence of a permanent diffraction grating being recorded in the material. The series of spectra presented in Fig. 4(b) were recorded with the pumping energy density equal to ρ 15 mJ∕cm2 while one of pumping stripe-like beams was maneuvered to first increase and then decrease the angle of intersection. The spectrometer was triggered with the Nd:YAG laser so that each spectrum was gathered after each pumped pulse. The repetition rate was 10 Hz, so each presented series took about 30 seconds. Tuning is mostly limited by the spectral extent of the gain band of the DCNP dye [see Fig. 4(a)]. In Fig. 4(b), we show real-time tuning for over a dozen nanometers in range of one of the stimulated emission bands. Here, the tunability was limited by the spatial extent of the excited area where the pumping beams overlapped each other, so we were confined by the geometry of our setup, which did not allow us to cover the whole gain profile. We believe that by optimizing the experimental setup, we could achieve real time, tunable laser emission for the entire gain profile.
Fig. 4. (a) Tuning of the DFB lasing for the different intersection angles 2θ of the pumping beams crossing onto the sample. (B) Real time tuning of the laser emission wavelength.
April 1, 2015 / Vol. 40, No. 7 / OPTICS LETTERS
Finally, we must mention that our sample was not optimized in order to achieve the best lasing performance, thereby giving us the ability to reduce the thresholds of DFB lasing emission. We plan on studying the coupling of only the localized or extended modes of RL with a DFB resonator in detail in the future. Our goal was to show that such a simple system as was studied in this article can reveal many different features and phenomena that are interesting from the observational point of view. Therefore, the optimization of this system is still possible, and will be a really interesting issue to solve in our future work. In summary, we have shown that we can observe coupling between RL emissions in the same system. This is a consequence of multiple light scattering on the spontaneously formed micro- and nano-crystals of dye in the matrix bulk and of the DFB lasing based on the periodic refractive index and the gain modulation in the volume of the material. We have also shown the experimental results of real time, broadband tunable DFB lasing, which was achieved in a simple luminescent dye-doped polymeric system via a single-shot excitation. We believe that the utilization of unique and novel types of luminescent dyes that show efficient emission in an aggregated form can lead to the generation of highly tunable dye-doped solid state lasers, thereby merging the high efficiency and versatility of dye-doped polymers with the broadband emission of π-conjugated polymers. We would like to thank the Polish National Science Center for its support under Grant No. DEC-2013/09/D/ ST4/03780 and the Wroclaw University of Technology for financial support. We also thank Professor A. Miniewicz from the Wroclaw University of Technology for sharing the single DCNP crystal emission spectrum. References 1. H. Kogelnik and C. V. Shank, J. Appl. Phys. 43, 2327 (1972).
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2. H. Kogelnik and C. V. Shank, Appl. Phys. Lett. 18, 152 (1971). 3. I. D. W. Samuel and G. A. Turnbull, Chem. Rev. 107, 1272 (2007). 4. L. Rocha, V. Dumarcher, C. Denis, P. Raimond, C. Fiorini, and J.-M. Nunzi, J. Appl. Phys. 89, 3067 (2001). 5. J. Mysliwiec, L. Sznitko, A. Sobolewska, S. Bartkiewicz, and A. Miniewicz, Appl. Phys. Lett. 96, 141106 (2010). 6. L. Sznitko, J. Mysliwiec, P. Karpinski, K. Palewska, K. Parafiniuk, S. Bartkiewicz, I. Rau, F. Kajzar, and A. Miniewicz, Appl. Phys. Lett. 99, 031107 (2011). 7. T. Chida and Y. Kawabe, Opt. Mater. (Amsterdam) 36, 778 (2014). 8. V. Dumarcher, L. Rocha, C. Denis, C. Fiorini, J. M. Nunzi, F. Sobel, B. Sahraoui, and D. Gindre, J. Opt. A 2, 279 (2000). 9. N. Tsutsumi and A. Fujihara, Appl. Phys. Lett. 86, 061101 (2005). 10. S. Gottardo, R. Sapienza, P. D. Garcia, A. Blanco, D. S. Wiersma, and C. Lopez, Nat. Photonics 2, 429 (2008). 11. L. Cerdan, A. Costela, G. Duran-Sampedro, and I. GarciaMoreno, Appl. Phys. B 108, 839 (2012). 12. S. Allen, T. D. Mclean, P. F. Gordon, B. D. Bothwell, M. B. Hursthouse, and S. A. Karaulov, J. Appl. Phys. 64, 2583 (1988). 13. A. Miniewicz, S. Delysse, J. M. Nunzi, and F. Kajzar, Chem. Phys. Lett. 287, 17 (1998). 14. L. Sznitko, J. Mysliwiec, K. Parafiniuk, A. Szukalski, K. Palewska, S. Bartkiewicz, and A. Miniewicz, Chem. Phys. Lett. 512, 247 (2011). 15. J. Mysliwiec, L. Sznitko, A. Szukalski, K. Parafiniuk, S. Bartkiewicz, A. Miniewicz, B. Sahraoui, I. Rau, and F. Kajzar, Opt. Mater. 34, 1725 (2012). 16. H. Cao, Waves in Random Media 13, R1 (2003). 17. O. Morawski, A. L. Sobolewski, B. Kozankiewicz, L. Sznitko, and A. Miniewicz, Phys. Chem. Chem. Phys. 16, 26887 (2014). 18. L. Sznitko, K. Cyprych, A. Szukalski, A. Miniewicz, and J. Mysliwiec, Proc. SPIE 8983, 89830V (2014). 19. M. A. Noginov and V. S. Letokhov, Solid-State Random Lasers, Springer Series in Optical Sciences (Springer, 2005).
OPTICS LETTERS / Vol. 40, No. 7 / April 1, 2015
Distributed feedback and random lasing in DCNP aggregates dispersed in a polymeric layer Kacper Parafiniuk, Lech Sznitko, and Jaroslaw Mysliwiec* Advanced Materials Engineering and Modelling Group, Faculty of Chemistry at Wroclaw University of Technology, Wyb. Wyspianskiego 27, 50-370 Wroclaw, Poland *Corresponding author: [email protected] Received January 19, 2015; accepted February 24, 2015; posted March 6, 2015 (Doc. ID 232589); published March 30, 2015 Here, we report on the realization of random lasing (RL) and distributed feedback (DFB) lasing in a layer of luminescent 3-(1,1-dicyanoethenyl)-1-phenyl-4,5-dihydro-1H-pyrazole (DCNP) organic nonlinear optical dye that has been dispersed in a poly(methyl methacrylate) (PMMA) matrix. The RL phenomenon appears due to the presence of spontaneously formed micro- and nano-crystals of DCNP in the bulk of the PMMA during the sample preparation. DFB can be realized in an optical system by using degenerated two-wave mixing in the pumping beams. The period of the interference pattern can be easily changed by changing the intersection angle of the pumping beams, resulting in a real time, fully reversible method of DFB lasing emission tuning. Because of the two neighboring stimulated emission bands of DCNP, it is possible to tune the lasing wavelength over a long range of about 65 nm. © 2015 Optical Society of America OCIS codes: (140.2050) Dye lasers; (140.3490) Lasers, distributed-feedback; (140.3600) Lasers, tunable; (290.4210) Multiple scattering; (290.5850) Scattering, particles; (160.4890) Organic materials. http://dx.doi.org/10.1364/OL.40.001552
Nowadays, there are many ideas for the realization of distributed feedback (DFB) lasers in either a mono- and bilayer system with a permanent or temporary holographic pattern. Because feedback in DFB lasers can be provided by gain and/or refractive index periodic perturbations, there may be several approaches to their realization [1–3]. Two of the most popular techniques of DFB laser cavity fabrication are electron beam lithography based on photoresist modulation, and the inscription of a surface relief grating (SRG) on top of a layer of photochromic polymer [3–6]. These methods generally allow researchers to fabricate DFB lasers with a fixed wavelength of emission because the grating period is also fixed. However, it is possible to fabricate DFB lasers without introducing any parts of the physical periodic structure. Another approach utilizes the formation of temporary patterns based on either the refractive index and/or the gain coefficient modulation in the bulk of the material. One example is the bi-layer system in which azo dye molecules doped to the polymeric matrix are covered by another polymer doped with a luminescent dye. The materials used in the described setup provide for the formation of the temporary grating and allow for the tuning of the wavelength of the laser emission. [7] There are also a few examples where more simplified single-layer systems to achieving DFB lasing were used [8,9]. Another class of DFB lasers is random lasers (RLs), which utilize multiple scattering and disorder to achieve feedback [10,11]. In our work, we present the results of DFB lasing. We used an organic nonlinear optical dye doped to a polymeric film from which the DFB resonator was generated in one step with a noncontact, holographic, temporary, and reversible grating inscription in the matrix bulk. Our goal was to couple the DFB resonator based on the periodic modulation of the refractive index or gain coefficient to the material that showed the random lasing (RL) phenomenon based on multiple scattering from a recrystallized dye that supported both gain and random 0146-9592/15/071552-04$15.00/0
feedback. We have shown that gratings with different periods could be directly formed in a film by changing the incident angle of intersecting beams, thereby enabling the tuning of the emission wavelength in a real time. The scheme of DFB lasing and the RL induced by the recrystallized dye are shown in Fig. 1(a). As an active dye, we used the luminescent organic compound 3-(1,1-dicyanoethenyl)-1-phenyl-4,5-dihydro1H-pyrazole (DCNP), which possesses interesting nonlinear optical properties [12]. Single- as well as two-photon absorption-induced fluorescence was observed, and it was shown that efficient stimulated emission can be achieved from DCNP that has been dissolved in a solid polymeric matrix, such as poly(methyl methacrylate) (PMMA) [13–15]. DFB lasing was also observed, but only in a hybrid system based on a DCNP dye-doped biopolymer over-coated with a photochromic polymer in
Fig. 1. Schematic of the DFB and RL phenomena in the DCNP/ PMMA layer. (a) Bulk diffraction grating with periodic changes Λ of the refractive index and/or gain coefficient in the material provides feedback for DFB lasing. Multiple scattering on elements such as microcrystals results in RL. (b) Microscopic images that show the DCNP microcrystals present in the PMMA matrix. (c) Microscopic image taken in luminescence mode above the RL threshold. (d) Microscopic image taken in the same mode below the RL threshold. © 2015 Optical Society of America
April 1, 2015 / Vol. 40, No. 7 / OPTICS LETTERS
Fig. 2. (a) Experimental setup for the observation of the DFB lasing. (b) Scheme of real-time different periods grating formed by the stripe-like beams.
the angle of intersection between the beams [see Fig. 2(b)]. The emission was gathered from the edge of the excited sample by a fiber spectrometer (Andor Solis Shamrock 163, resolution: 0.1 nm). In order to measure DFB, two coherent beams could create an interference pattern with a period that was dependent on the angle between them. Due to the presence of two neighboring stimulated emission bands of DCNP (both of which have a similar position and shape to the DCNP single crystal emission: see Fig. 3) in the PMMA matrix, is possible to tune DFB lasing over a range of about 65 nm (λlas from 585 to 650 nm). According to the Bragg formula, the order of diffraction m can be obtained from the following equation: Λ
which the Bragg grating has been inscribed using an SRG formation in an azo-functionalized polymer [6]. We have prepared the layer of 2% weight for weight (w/w) DCNP/PMMA on a glass plate via the drop casting technique using a dye/polymer tetrahydrofuran (THF) solution. The drying process took two days under THF vapors. The sample then spent another week in an ambient atmosphere to ensure that no solvent remained. The thickness of the obtained layer was about 25 μm. The experimental effective refractive index of the planar waveguide was neff 1.513. In order to observe RL and real-time tunable DFB lasing, we have constructed the experimental setup that is schematically shown in Fig. 2(a). Cylindrical and spherical lenses were used to change the shape of the excitation beams from a spot to a stripe, the size of which was 1.5 cm × 0.1 cm. The doubled frequency light (λpump 532 nm) coming from the neodymium-doped yttrium aluminum garnet (Nd:YAG) nanosecond pulsed laser (with a pulse duration of 6 ns) was used both to excite the luminescence and to enable the grating inscription. The laser beam with the stripe shape was split into two beams, and, using two mirrors, was horizontally interfered in a sample volume. The distance between the mirrors was fixed, while their rotation around the vertical axes was utilized in order to change
1553
mλlas ; 2neff
(1)
and was chosen as m 2, giving the periodicity Λ in the range from 386.6–429.6 nm, which depends on the angle 2θ (86.9–76.5°) between the beams. DFB spectra were collected in a series of 50 accumulations. By exciting the sample with only one of the stripe-like beams and above a certain threshold, we were able to observe both coherent (represented as many sharp lines in the emission spectra) and incoherent (represented as smooth emission spectra, thereby reproducing the gain profile) RL phenomenon. For the coherent RL light, localization mechanisms have to be introduced to the system. In our case, rarely seen DCNP microcrystals of a size of a few tens of micrometers were responsible both for light trapping and enhancement. Microscopic images of one of the investigated layers are presented in Figs. 1(b)–1(d). DCNP microcrystals are clearly visible in Fig. 1(b). Some of them (indicated with white circles) can be involved in coherent RL. Figures 1(c) and 1(d) show microscopic images of the same part of the sample [see Fig. 1(b)] taken in luminescence mode below and above the RL threshold. The sizes of the microcrystals presented in Fig. 1(c) are approximately 110, 75, and 50 μm. The resolution of the images is 8 μm. The properties of the photoluminescence of the DCNP/ PMMA layer (absorption, emission, stimulated emission,
Fig. 3. (a) Normalized absorption and emission spectra of the DCNP/PMMA sample and the emission spectra of the single DCNP crystal measured at a temperature of 77 K. (b) Stimulated emission (SE), RL, and DFB lasing spectra of the DCNP/PMMA sample. The inset in (b) shows the DFB lasing threshold value estimation at the wavelength of the DFB emission λlas 630 nm.
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OPTICS LETTERS / Vol. 40, No. 7 / April 1, 2015
RL, and DFB lasing spectra), as well as a single DCNP crystal emission, are presented in Fig. 3. The stimulated emission spectrum was obtained from the excitation sample by one circular-shaped beam with an energy density ρ 5.6 mJ∕cm2 . The exemplary spectrum typical for coherent RL, which consists of many sharp modes [16], was gathered for pumping by a stripe-like beam with an energy density ρ 7.5 mJ∕cm2 . The DFB lasing spectrum presented in Fig. 3(b) was measured for the inter-beam angle 2θ, which was equal to 79.4°, thereby enabling us to achieve laser emission at the wavelength of λlas 630 nm with the excitation energy density ρ 5.5 mJ∕cm2 . The intensity of the emission of the sample as a function of the incident laser energy density is shown in the inset in Fig. 3(b). The estimated value of ρth 5.3 mJ∕cm2 corresponds to the DFB lasing threshold (the conditions for the lasing wavelength were set at λlas 630 nm). As demonstrated by the work in [17], it was crucial for the DCNP crystal emission to have trap states, in which the emission occurs exactly in the spectral region covered by the stimulated emission. As was reported in [15], no stimulated emission occurs in diluted solutions of DCNP; therefore, the DCNP micro- and nanocrystals are necessary to enable the stimulated emission. Recently, it was shown that emission amplification can only be achieved for THF solutions that have a high concentration of DCNP in the order of 10−2 mol∕l. At this concentration, recrystallization occurs [18]. Therefore, the stimulated emission can be achieved from relatively long-living trap states of emission of the DCNP crystal. We suppose that the presence of numerous small DCNP crystals are responsible for the multiple scattering, and that the stimulated emission no longer can be thought of as an amplified spontaneous emission, but rather as incoherent RL. The threshold of the last process is highly dependent on the geometry and size of the excited area and the amount of disorder [19]. A closer inspection of sample showed that we were able to find places in the sample that were illuminated by a single pumping beam and that show highly directional coherent RL [see Figs. 1(b)–1(d)]. In this case, two beams coupling with the fulfilled Bragg conditions for light amplification
resulted in the formation of DFB lasing and in a narrow emission line selection. The mechanism of the RL phenomenon is based on spontaneously formed microand nano-crystals in the matrix bulk, which appear during the slow solvent evaporation that occurs during the sample preparation. Critically for samples containing DCNP, such aggregates are responsible both for the constitution of gain and for the multiple scattering, which in turn determines the random feedback. The size distribution of the DCNP crystals determines whether the amplified light can be confined inside the crystals or not, and strongly affects both the threshold conditions and the emission spectra. The utilization of broadband-emitting DCNP crystal trap states for lasing gives the possibility of wide DFB lasing tunability. The emission wavelength λlas was tuned with the change of the incidence angle 2θ of the pumping beams in the sample. Figure 4(a) shows the normalized DFB emission spectra of the DCNP/PMMA layer for various intersection angles. The pumping energy density was found to be ρ 5.5 mJ∕cm2 . One great advantage of the studied system is that we can tune the emission wavelength in real time without any evidence of a permanent diffraction grating being recorded in the material. The series of spectra presented in Fig. 4(b) were recorded with the pumping energy density equal to ρ 15 mJ∕cm2 while one of pumping stripe-like beams was maneuvered to first increase and then decrease the angle of intersection. The spectrometer was triggered with the Nd:YAG laser so that each spectrum was gathered after each pumped pulse. The repetition rate was 10 Hz, so each presented series took about 30 seconds. Tuning is mostly limited by the spectral extent of the gain band of the DCNP dye [see Fig. 4(a)]. In Fig. 4(b), we show real-time tuning for over a dozen nanometers in range of one of the stimulated emission bands. Here, the tunability was limited by the spatial extent of the excited area where the pumping beams overlapped each other, so we were confined by the geometry of our setup, which did not allow us to cover the whole gain profile. We believe that by optimizing the experimental setup, we could achieve real time, tunable laser emission for the entire gain profile.
Fig. 4. (a) Tuning of the DFB lasing for the different intersection angles 2θ of the pumping beams crossing onto the sample. (B) Real time tuning of the laser emission wavelength.
April 1, 2015 / Vol. 40, No. 7 / OPTICS LETTERS
Finally, we must mention that our sample was not optimized in order to achieve the best lasing performance, thereby giving us the ability to reduce the thresholds of DFB lasing emission. We plan on studying the coupling of only the localized or extended modes of RL with a DFB resonator in detail in the future. Our goal was to show that such a simple system as was studied in this article can reveal many different features and phenomena that are interesting from the observational point of view. Therefore, the optimization of this system is still possible, and will be a really interesting issue to solve in our future work. In summary, we have shown that we can observe coupling between RL emissions in the same system. This is a consequence of multiple light scattering on the spontaneously formed micro- and nano-crystals of dye in the matrix bulk and of the DFB lasing based on the periodic refractive index and the gain modulation in the volume of the material. We have also shown the experimental results of real time, broadband tunable DFB lasing, which was achieved in a simple luminescent dye-doped polymeric system via a single-shot excitation. We believe that the utilization of unique and novel types of luminescent dyes that show efficient emission in an aggregated form can lead to the generation of highly tunable dye-doped solid state lasers, thereby merging the high efficiency and versatility of dye-doped polymers with the broadband emission of π-conjugated polymers. We would like to thank the Polish National Science Center for its support under Grant No. DEC-2013/09/D/ ST4/03780 and the Wroclaw University of Technology for financial support. We also thank Professor A. Miniewicz from the Wroclaw University of Technology for sharing the single DCNP crystal emission spectrum. References 1. H. Kogelnik and C. V. Shank, J. Appl. Phys. 43, 2327 (1972).
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