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We developed a new fluorene-based chromophore for a degradation-recoverable polydimethylsiloxane (PDMS) dye laser. The chromophore has dimethylsiloxane chains to enhance its solubility in the PDMS matrix. The spectroscopic and mobile characteristics were evaluated, and the attaching of the siloxane/silyl chains improved solubility in PDMS without influencing the laser property. It extended the durability by a factor of 20 for shots in an index-type Bragg grating/PDMS complex laser waveguide compared with a similar fluorene-based chromophore in PMMA laser waveguides. This molecular diffusion not only increased durability but also provided detailed information about dye degradation in waveguides.
©2013 Optical Society of America
1. Introduction
Several research works have reported on blue-emitting dyes and polymers [1–12], because they have various potential applications, such as in integrated optics, owing to their ease of fabrication, flexibility, and compatibility with functional organic molecules. However, organic solid-state lasers emitting in the blue-violet region have a serious durability problem because an ultraviolet (UV) laser is used as the pump source. Although various blue laser dyes have been developed to extend durability as a “chemical” approach, blue-emitting dyes suffer from very rapid degradation in plastic materials and solutions with UV laser pumping.
To extend the durability of these materials, some attempts have been made. As a “structural” approach, liquid-state dye lasers exhibit a relatively high durability even in integrated optofluidic devices [13]; however, disturbance and scattering during pumping are unavoidable. As a “physicochemical” approach, enhancing the thermal and optical properties of these hybrid matrices using polyhedral oligomeric silsesquioxanes (POSS), which significantly improves lasing properties, has been reported [14,15]. Recently, we reported a degradation-recoverable solid-state PDMS dye laser as a new “nanostructural-physicochemical” approach [16]. In this work, the nanopores in PDMS provide mobility to doped dye molecules and degraded dye molecules in the lasing area can be replaced with fresh dye molecules from outside the lasing area. Upon using this approach, the effective durability of the PDMS distributed feedback (DFB) laser was found to be 20.5 times that of a polymethylmethacrylate (PMMA)-based waveguide DFB laser using pyrromethene597 (P597). However, PDMS has a very low solubility in most polar fluorescent laser dyes. We confirmed that only some types of dipyrromethene boron difluoride (BODIPY, pyrromethene series) can be dissolved at sufficient concentration for laser action. Laser dyes such as coumarin, stilbene, and fluorene, which cover a shorter-wavelength region, showed insolubility in the preliminary experiment.
In the present work, we developed a new blue chromophore for a recoverable laser waveguide by modifying a previously studied laser chromophore. For a relatively stable blue laser dye, we selected a fluorine-based chromophore with an alkyl-group (2C6BPF) [7] and attempted to append the solubility on PDMS by replacing or attaching compatible groups such as poly-dimethylsiloxane or trimethylsilyl. The effects that we investigated are as follows:
- ● Solubility and mobility in PDMS given by the modification
- ● Influence of the modification on lasing properties
- ● Degradation process in the waveguide with UV laser pumping
2. Chromophore for PDMS lasers
Figures 1(a) and 1(b) show the structural formulas of the chromophore used in this study. The dye 2,7-bis-biphenyl-4-y1-dihexylfluorene (2C6BPF) in Fig. 1(a) was reported in [7,17] with a durability of 3.0 × 103 shots in a laser with the PMMA matrix. The absorption cross section at a wavelength of 335 nm and the peak of the emission cross section are 2.9 × 10−16 cm2 and 2.7 × 10−16 cm2, respectively. The emission lifetime and quantum yield are 0.93 ns and 0.82, respectively. However, 2C6BPF in Fig. 1(a) shows a very low solubility (<0.1 mM) in the PDMS matrix. This is not sufficient for laser action. Figure 1(b) shows a newly developed chromophore that was designed for improved solubility in PDMS. It is based on 2C6BPF, and the two hexyl groups that can be seen in Fig. 1(a) were substituted with chained dimethylsiloxane (DMS) groups such as (CH2)3(SiMe2O)mSiMe3. The number of DMS chains, m, can affect both solubility and molecular mobility in the PDMS matrix. The chromophore with m = 4 experimentally showed a saturated concentration at 6.0 mM, which is a sufficient concentration for laser action. Therefore, in this study, the chromophore with m = 4 (termed “KIDL-F1”) was adopted to realize laser action of a fluorene derivative in the PDMS matrix. For this purpose, the molecular mobility behavior needed to be investigated for chromophore inter-circulation for extending the durability. To synthesize KIDL-F1, a 100-mL four-necked round-bottom flask was thoroughly flushed with nitrogen and charged with 2,7-dibromo-9,9-bis[3-(1,1,3,3,5,5,7,7,9,9,9-undecamethyl-1,3,5,7,9-pentasiloxan-1-yl)propyl]fluorene (2.02 g, 1.76 mmol, obtained by Pt-catalyzed hydrosilylation of 9,9-diallyl-2,7-dibromofluorene), biphenyl-4-boronic acid (0.807 g, 4.08 mmol), tetrakis(triphenylphosphine)palladium(0) (98.0 mg, 0.085 mmol), and 1,2-dimethoxyethane (13 mL). To the stirred mixture, an aqueous solution of potassium carbonate (848 mg, 6.14 mmol in 5 mL H2O) was added. Then, the mixture was heated under nitrogen at 84°C for 7 h. Conventional work-up and subsequent purification by preparative high-performance liquid chromatography (HPLC) afforded a colorless liquid. The synthesis yield was 51%.
Fig. 1 Chemical structure of (a) 2C6BPF and (b) KIDL-F1. (c) Absorption and emission cross sections of KIDL-F1 and 2C6BPF.
Figure 1(c) shows the absorption and emission cross sections of KIDL-F1 and 2C6BPF. These cross sections of KIDL-F1 were measured with KIDL-F1-doped PDMS at room temperature. The absorption cross sections were estimated from the measured transmittance using the Lambert-Beer law. The stimulated emission cross sections were also estimated using the following formula:
where E(λ) is the normalized fluorescence spectrum, ϕ is the quantum yield, τ is the lifetime, and n is the refractive index. The quantum yield and lifetime were 0.89 and 0.88 ns, respectively. These values are almost identical to those of 2C6BPF. This explains the reason that no quenching due to the PDMS matrix was observed. The absorption cross section at a wavelength of 335 nm and the peak of the stimulated emission cross section are estimated as 2.8 × 10−16 cm2 and 3.0 × 10−16 cm2, respectively. A comparison of these results with the absorption and emission cross sections of 2C6BPF in Fig. 1(c) shows that the fundamental compositions of the spectra are the same; further, the spectral shapes and peak-to-peak widths are in agreement. This means that the DMS group does not affect the spectra, because the delocalization of π electrons is allowed across all adjacent aligned p orbitals in only the bis-diphenyl-fluorene group, which has a conjugate system.3. Experiments and discussion
3.1 Mobility in PDMS
The nanoporous structure of PDMS allows the mobility of laser dyes. However, there was a possibility of that the laser size with the modified chromophore would limit the mobility of the molecule. Therefore, the mobility of KIDL-F1 was evaluated in comparison with that of P597, which we demonstrated with PDMS [16]. Figure 2 shows the mobility characteristics of KIDL-F1 and P597. The diffusion coefficients of KIDL-F1 and P597 were measured through their permeation into a PDMS film from the film edge, as shown in Fig. 2(a). Dye-doped DMS at a concentration of 3.0 mM was placed in a container, and a PDMS film was pasted onto the wall of the container. The samples were then kept for 24 hours at each temperature. Then, the concentrations of the dye-permeated PDMS film were obtained using the Lambert-Beer law with the measured transmittance. Additionally, diffusion coefficients were obtained by fitting the measured concentration distributions with the diffusion equation. Figure 2(b) shows the measured and fitted concentration distributions of KIDL-F1- and P597-permeated PDMS films at 25°C. There is a difference in diffusion speed between KIDL-F1 and P597 at the same temperature of 25°C. The diffusion coefficient of dye molecules in PDMS is determined by the interactions between PDMS and the dye molecules, and the size of the dye molecules is the most important factor. The molecular sizes of KIDL-F1 and P597 were approximately 2.6 nm and 1.3 nm, respectively. Thus, KIDL-F1, which is has a larger molecular size, shows a smaller diffusion coefficient in PDMS.
Fig. 2 (a) Dyes permeating the PDMS film. (b) Diffusion of P597 and KIDL-F1 in PDMS at 25°C after 24 h. (c) Dependence of diffusion coefficient on temperature.
Figure 2(c) shows the dependences of the diffusion coefficients of KIDL-F1 and P597 on temperature. It indicates that the logarithmic diffusion coefficient has a linear relationship with the inverse of the temperature. This is in agreement with the following Arrhenius equation for gas:
where D is the diffusion coefficient, A is the pre-exponential factor (or simply the prefactor), Ea is the activation energy, R is the universal gas constant, and T is the temperature. The ln(D) of the dye molecules in PDMS is linear for 1/T, as shown in Fig. 2(c). The diffusion coefficient fitted using Eq. (2) in Fig. 2(c) shows that the Ea/R of KIDL-F1 is smaller than that of P597. This indicates that the diffusion coefficient of KIDL-F1 is larger than that of P597 above 223°C. As mentioned above, an extreme lack of the mobility was not confirmed. Therefore, KIDL-F1 has sufficient mobility, for a degradation-recoverable solid-state dye laser. Additionally, these mobility characteristics lead to a direct rapid doping into cured PDMS or a mobility control.3.2 Lasing characteristics
We fabricated a multilayer waveguide laser containing KIDL-F1:PDMS as the laser medium, as shown in Fig. 3(a) . This structure is an unsymmetrical slab waveguide consisting of an active clad layer, a DFB core layer, and an air clad layer. Some waveguides have an active clad layer [18–21], and our waveguide is based on that of the evanescent dye lasers [18] we reported on previously. The active clad layer of KIDL-F1:PDMS has a concentration of 6.0 mM and a thickness of 100 μm. A relatively high rate of internal circulation was expected because the layer was thick. A PDMS oligomer (SIM-360, Shin-Etsu Chemical Co., Ltd.) and KIDL-F1 were mixed without any solvent and cured at 72°C on a PMMA substrate with a size of 1.0 cm × 1.0 cm. The propagation losses of KIDL-F1:PDMS at 355 nm and 401 nm were 0.20 dB/cm and 0.07 dB/cm, respectively. These losses were separate from the reflection loss and the absorption influence of KIDL-F1. After curing, to ensure distributed feedback, a coating layer was fabricated on the KIDL-F1:PDMS layer by spin coating. The coating layer was composed of poly 2,2,2-trifluoroethylmethacrylate (PTFEMA), and its refractive index was designed to be 1.416, which is close to that of PDMS (1.408). The thickness of the DFB layer required to be only 2.5 μm to obtain a single vertical mode. Finally, DFB structures were recorded at a wavelength of 403.6 nm using the second harmonic generation (SHG) of a continuous wave (CW) Ar+ laser (BeamLok 2060 and WaveTrain, Spectra Physics). An index-type Bragg grating was recorded in the PTFEMA layer. The effective refractive index neff and grating pitch Λ ( = mλlaser/2neff, m = 1) were 1.410 and 143 nm, respectively. The contrast Δn of the refractive index on the DFB structure was measured and estimated at ~4.0 × 10−3.
Fig. 3 (a) Structure of KIDL-F1:PTFEMA/PDMS laser, (b) electric field at TE0 mode, and (c) DFB laser beam profile.
Figure 3(b) shows the calculated electric field of the TE0 mode based on the refractive index profile. The emission from KIDL-F1:PDMS is amplified as the DFB laser by the evanescent component of 3.5 μm in KIDL-F1:PDMS. Finally, the evanescent component of the DFB laser is amplified to the depth of 17 μm at 1/e intensity from the border of the PTFEMA layer and the KIDL-F1:PDMS layer, as shown in Fig. 3(c) [18]. In fact, the spectra of the beam from the KIDL-F1:PDMS layer consist of the DFB laser component. This depth of 17 μm agrees with the absorption length of the KIDL-F1:PDMS.
Subsequently, the input-output characteristics were evaluated; they are shown in Fig. 4(a) . A passively Q-switched and frequency-tripled Nd:YAG laser was used as the pumping source. The maximum pulse energy and repetition rate were 5.08 μJ (1.01 mJ/cm2) and 10 Hz, respectively, and the size of the sheet-shaped pumping beam was estimated to be 50 μm × 10 mm on the PDMS waveguide. The PDMS laser sample was mounted under atmospheric conditions at 25°C in these experiments. In the DFB laser operation, the maximum output energy and slope efficiency were approximately 0.56 μJ and 16.8%, respectively. Although the DFB laser threshold was estimated at 1.74 μJ (0.35 mJ/cm2), shown by the straight red line in Fig. 4(a), the measured threshold decreased to 0.48 μJ (0.10 mJ/cm2). The DFB laser can be described as a microresonator laser under the pump energy of 3.5 μJ (0.70 mJ/cm2) [12]. In amplified spontaneous emission (ASE) operation, the maximum output energy and slope efficiency were approximately 0.44 μJ and 12.4%, respectively. The ASE was also operated as a microresonator laser under the pump energy of 3.5 μJ (0.70 mJ/cm2). Figure 4(b) shows the spectrum of the DFB laser and the ASE. The DFB laser oscillated with a single mode at a wavelength of 403.6 nm. The full width at half maximum (FWHM) of the spectral width was 0.11 nm. Then, the peak wavelength and FWHM of the spectral width about the ASE were found to be 402.1 nm and 2.26 nm, respectively. Although the spectrum was similar to that of random lasing, no random lasing could be observed due to the low scattering loss of the PDMS waveguide. Since similar spectra are often observed, they likely occur due to scattering at the rough-treated ends of waveguide or surface microcracks.
3.3 Increased durability
Finally, we performed a durability test under a low pumping condition with the aim of increasing durability. Figure 5(a) shows the durability of the KIDL-F1:PDMS/PTFEMA DFB laser and the ASE. In this investigation, the output of the laser and ASE were measured under a soft-pumping condition with a pulse energy of 1.2 μJ (0.24 mJ/cm2) and a repetition rate of 10 Hz. Here, the structure of the ASE sample was the same as that of the PDMS DFB laser sample, but it did not include an index-type Bragg grating in PTFEMA layer. Intensities of the laser and ASE were increased up to 2.0 × 103 shots and 7.0 × 104 shots, respectively. In both cases, the influence from a high concentration of dye, such as self-absorption, can be slightly relieved. Furthermore, the initial damage caused by DFB recording must be simultaneously recovered for the laser. For the laser and ASE, 6.1 × 104 shots (total pump dosage of 15 J/cm2) and 2.1 × 105 shots (total pump dosage of 50 J/cm2), respectively, were required for the laser intensity to be reduced by half. The durability of the KIDL-F1:PDMS laser is 20-fold for the number of shots (60-fold for the total pump dosage) higher than 3.0 × 103 shots (total pump dosage of 0.25 J/cm2) for a blue laser using 2C6BPF, which has a similar fluorene group [7]. Then, we also confirmed that the durability of ASE for KIDL-F1 and 2C6BPF were nearly the same in a PMMA matrix. In addition, the accelerated test under the hard pumping condition of 5.0 μJ (1.0 mJ/cm2) in Fig. 5(b) shows that KIDL-F1 provides durability of only 1/23 in shots (1/6 in total pump dosage) without molecular diffusion. These indicate that the mobility of KIDL-F1 molecules must be dominant for extending the durability. However, there is a 3.4-fold increase in the durability between the laser and the ASE, as shown in Fig. 5(a). This increase was caused by the quenching of the index-type Bragg grating in the PTFEMA layer. In other words, quenching by absorption in the pumping pulse disrupted the laser oscillation, thereby limiting the increase in durability. In fact, laser oscillation of the finished sample did not occur only in the pumped area, despite the fact that bleached dyes in the pumping area were replaced with new dyes at 75°C. By using a PDMS matrix, this quenching phenomenon of the index-type Bragg grating was confirmed for the first time. There is a potential to extend the durability up to that of ASE by imprinting an embossed-type Bragg grating onto PDMS or by etching an embossed-type Bragg grating onto PTFEMA. The 1/2 degradation in the energy dosage of 50 J/cm2 by the KIDL-F1:PDMS ASE corresponds to the 1/e degradation in the energy dosage of 58 J/cm2. This 1/e degradation in energy dosage is 1.1-fold higher than that of 54 J/cm2 about an oligofluorene laser [12]. It indicates that the KIDL-F1:PDMS laser has a potential to demonstrate the highest durability in solid-state organic dye lasers in the blue region. Additionally, the KIDL-F1:PDMS laser is recoverable because of diffusion phenomena. Figure 5(b) shows degradation-recoverable ASE operation by hard pumping at 5.0 μJ (1.0 mJ/cm2). After the intensity decreased to half in a degradation cycle, the operation had a refresh time of ~24 h to exchange bleached dyes with active dyes in the pumping area (50 μm × 10 mm). The average durability at half the intensity in the two cycles, as shown in Fig. 5(b), was 9.0 × 103 shots (total pump dosage of 9.0 J/cm2), and the total durability for the sample with a size of 1.0 cm × 1.0 cm was estimated at 1.8 × 106 shots (total pump dosage of 1.8 kJ/cm2), assuming that no diffusion phenomena occurred as a result of hard pumping.
Fig. 5 (a) Increased durability of the PDMS DFB laser and ASE pumped by a low pulse energy of 1.2 μJ, and (b) degradation-recoverable operation of ASE.
4. Conclusion
We reported the first degradation-recoverable solid-state dye laser in the blue region. A new biphenylfluorene-based chromophore with solubility in PDMS was synthesized by attaching dimethylsiloxane and trimethylsilyl termination. Even with a relatively large molecular size of 2.6 nm, the mobility could be confirmed experimentally. The dependence of the mobility of the modified-biphenylfluorene on temperature was evaluated, and it was found that the diffusion coefficient of KIDL-F1 agreed with the Arrhenius equation. Compared with case of normal-biphenylfluorene, the new dye extends durability by a factor of 20 for the number of shots in an index-type Bragg grating/PDMS complex laser waveguide. Additionally, it was found that the disappearance of the index-type Bragg grating structure is also a critical factor in a UV-pumped waveguide dye laser with an index-type DFB.
Acknowledgment
This work was supported by JSPS KAKENHI, a Grant-in-Aid for JSPS Fellows, 23·3687.
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