Efficient generation of blue light in oligothiophene chromophores doped in polymer films and fiber

Optical Materials 35 (2013) 2115–2121

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Efficient generation of blue light in oligothiophene chromophores doped in polymer films and fiber Christopher Dudley ⇑ Department of Physics and Materials Science Program, Washington State University Pullman, WA 99164-2814, United States

a r t i c l e

i n f o

Article history: Received 21 February 2013 Received in revised form 23 April 2013 Accepted 25 May 2013 Available online 28 June 2013 Keywords: Amplified spontaneous emission Blue light Fluorescent dye Two-photon absorption (TPA) Laser gain

a b s t r a c t A conjugated oligothiophene chromophore with large linear and nonlinear absorption cross-sections that emits light in the blue region is studied. Here measurements of the one- and two-photon absorption and fluorescence of an oligothiophene chromophore doped in polymethyl-methacrylate (PMMA) polymer is presented. It was observed that oligothiophene/PMMA fluoresces strongly and through amplified spontaneous emission (ASE), is an efficient source of blue light around 460 nm. Using these materials, a polymer fiber-based blue ASE light source is demonstrated. Blue emitting molecules (mainly in PMMA) are tabulated for comparison. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Blue emitting devices have been of interest for decades for microscopy, interferometry, general laser applications, underwater communication, remote sensing, data storage, organic light emitting diodes and other photo-luminescence or electroluminescent devices. The search for robust, efficipent and cost effective materials is ongoing. Even as the many available wavelengths in small footprint modules cover most of the spectrum, the need for even smaller scale fluorescent markers exist. Polymer fiber lasers have been considered since emission from chelate doped PMMA fibers (filaments) was reported in 1963 by Wolff and Pressley at 613 nm [1] and separately by Huffman at 545 nm [2]. Here a blue emitting dye that performs well compared to the historical standard Rh6G and is comparable to the blue emitting courmans. Most dyes characterized for absorption and emission do not include efficiency. Those that do, tend to be an order of magnitude lower than the chromophore studied here. Soffer and McFarland [6] reported on a tunable solid state dye laser in 1967 with output from 550–590 nm from rhodamine 6G in PMMA. Blue emitting polymer sources have been more recent [7]. A review of blue emitting dyes primarily in PMMA is given in Table 2. The cost of many light source options could be prohibitive for small ventures or academics. The ease and relative simplicity of dye doped polymers is practical for fundamental research and simple devices. ⇑ Tel.: +1 118502355341. E-mail address: [email protected] 0925-3467/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.05.033

Commercially available coumarin dyes account for a large number of blue emitting dyes in the literature [13–18]. Pi-conjugated molecules are another class that have been reported on for blue emission [20–28]. The factors commonly considered to improve lasing are the dye structure, host matrix and sample preparation. The dyes investigated here were designed to examine factors for increasing the two photon absorption (TPA) or cross section, rTPA [29]. Conjugated oligothiophene chromophores are known both for their optical and electrical properties. As such, they show promise for photonic and electronic applications [30]. Substituting constitutes on the base molecule affects absorption wavelength, fluorescence quantum yield, two-photon absorption efficiency and cross section. This work presents fluorescence, amplified spontaneous emission and two-photon absorption spectroscopy studies of a oligothiophene chromophore designed and synthesized by the Naval Research Laboratory. Additional historical background can be found in the literature [3–5,8–12]. 2. Experimental 2.1. Material preparation Both bulk and fiber samples were prepared and examined. To make the bulk materials, methyl-methacrylate (MMA) monomer, dyes, initiator, and chain transfer agent are mixed to the desired dye concentration. A test tube of the mixture is heated on a slow test-tube rocker setting. As the viscosity increases sufficiently the mobility of dye molecules is hindered even prior to full polymerization. The tubes are moved to a holding rack at the side of the oven at

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extruded fiber

Table 1 Dye doped PMMA parameters. Geometry NRL303 a Thick film Fiber Disk NRL303 b Thick film Fiber Disk Fluorescein-1 Disk Fluorescein-2 Disk Rhodamine 6G Disk Rhodamine 6G Disk

Dimensions (cm)

Concentration

3  2  0.1 2.5 cm  500 lm dia. 1.4  0.125

2.0  103 M (1.3  1018 molec/cm3)

1.35  1.75  0.08 2.5 cm  400 lm dia. 1.4  0.125

1.6  102 M (1.0  1019 molec/cm3)

nozzle

1.45 dia.  0.14 thick 1.45 dia.  0.14 thick 1.45 dia.  0.13 thick 1.45 dia.  0.14 thick

PMMA+dye

(3.17  1018 molecule/cm3) 4.8  103 M (2.93  1018 molecule/cm3) 9.9  104 M (5.93  1017 molecule/cm3) 2.8  104 M (1.7  1017 molecule/cm3) 4.33  105 M (2.6  1016 molecule/cm3)

plunger

Fig. 2. Extruding dye doped PMMA plug as a fiber.

90 °C for two days to fully cure the material (note: dye concentration and chemical properties affect polymerization time). The final plugs of PMMA and dye are scanned for consistent light absorption across the sample. Samples that vary are discarded. The dye-doped polymer material is then cut and pressed at a temperature of 150 °C to form a blank that can be sliced and polished to make thin disks. The cylinders can also be drawn into a fiber or cut into rectangular solids. Table 1 summarizes the composition, dimensions and concentration of the samples studied. The fluorescein chromophore was purchased from Alfa Aesar and the Rhodamine 6G chromophore was purchased from Aldrich as reference samples. The conjugated oligothiophene chromophore that was used in these studies is shown in Fig. 1. They were synthesized by O. Kim at the Naval Research Laboratory as described in the literature [30]. A fiber was made from NRL-303 in PMMA polymer as follows. A mold block with the dye-doped PMMA sample loaded (as shown in Fig. 2) was heated by ramping the temperature from 120°C to 190 °C over 15 min. The mold block (4.8 mm inner diameter) was removed from the oven and pressure applied to the plunger by hand. The extruded fiber was guided by hand and held at room temperature for a minute until it solidified. A drawing method was also used for making fibers. To do so, a 5.5 mm diameter perform cylinder was heated and pulled. The ends of the fibers made with both methods were polished with lapping paper. The extruded fiber was striated while the drawn fiber was smooth. 2.2. Setup for optical characterization A schematic of the experimental setup is provided by Fig. 3. (note the use two different lasers and various setups, so this figure may not portray all of the details in every experiment). The laser source (either a 35 ps pulse at a 10 Hz repetition rate from a Continuum model PY61 laser system or a 100 fs pulse from a 1 kHz rep rate Spectra Physics Hurricane system with a SHG package) passes through a polarizer, half-wave-plate then analyzer. The intensity of the beam reaching the sample is adjusted by computer using a stepper-motor-controlled half-wave plate. In the picosecond

Fig. 3. Experimental setup diagram.

experiments, a small fraction of the light is diverted to a photodetector that monitors the laser power passing the analyzer, and used to actively stabilize the laser power reaching the sample. This setup allows experiments to run over long periods of time without being affected by drift in the laser power.

Fig. 1. Naval Research Laboratory dye 303 molecular structure.

C. Dudley / Optical Materials 35 (2013) 2115–2121

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The light is focused to a point with a convex lens when studying fluorescence and two-photon absorption. For ASE studies, the light is focused to a line with a cylindrical lens and the spectrometer (Ocean Optics Model USB2000) is placed collinearly with the line so that ASE light traveling down the line makes it to the detector (as such, the detector axis is 90° to the laser pump beam). At low pump intensity, the fluorescence spectrum is observed. At high intensity, the light generated along the excitation line is amplified so ASE dominates. The length of the line is controlled using an aperture (not shown). In this way, as the line length changes, the intensity stays fixed but the total energy varies. In the two photon measurements (where the light is focused to a point), the spectrometer is placed approximately 45° to the beam axis and on the back side of the sample. This geometry is not necessary though being off axis is to prevent direct laser light or strong scattering being collected by the detector. 3. Results and discussion 3.1. Absorption and fluorescence Fig. 4. Linear absorption at 355 nm determined from the white light spectrum (open boxes) and from 355 nm laser light (solid lines). See Table 1 for dye concentrations.

3.1.1. Linear The linear absorption maximum of the sample was measured to be at 395 nm and the Fluorescence maximum at 491 nm. The difference between the wavelength of maximum absorption and the fluorescence peak was found to be 96 nm (the Stokes Shift). The difference from the absorption peak to the ASE peak is only 66 nm which is comparable to but smaller than that for coumarin laser dyes. The linear absorption value obtained from the white light absorption spectrum and from a 355 nm 35 ps 10 Hz laser source are compared in Fig. 4. The two methods for finding the linear absorbance pumped at 355 nm agree well for the concentrations of the NRL303 sample and the Rhodamine 6G. Absorption and fluorescence are well known optical phenomena. Absorbance is defined by

A  log

Fig. 5. One- and two-photon absorption and fluorescence.

ð1Þ

where I0 is the intensity incident on the sample and I is the transmitted intensity, which is a function of the depth of material. The intensity of light falls off exponentially with the distance propagated through the sample according to I = I0exp[ax], the relationship between A and the absorption coefficient a is given by,

a¼

Fig. 6. Absorbance, fluorescence and ASE in NRL 303.

  I0 ; I

1 A : L log e

Fig. 7. ASE from a NRL-303/PMMA fiber of 800 lm diameter.

ð2Þ

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C. Dudley / Optical Materials 35 (2013) 2115–2121

Fig. 8. Gain measurement geometry.

Fig. 9. Efficiency comparison for fiber and of two concentrations for an incident beam with kpump = 355 nm for 10 mm and 5 mm line lengths with a 500 lm width. Samples labeled (b) have higher concentration than (a), as listed in Table 1.

Fig. 11. Two-photon cross section for NRL303 compared to Rhodamine 6G. (a) 303 value measured by the author. (b) Rhodamine 6G from reference [31]. The Goeppert–Mayer unit, 1 GM = 1050 (cm4 s)/photon.

The linear absorption cross section, r was determined for NRL303 sample b from Eqs. (1) and (2), the absorbance A and the absorption coefficient is used to determine the linear absorption cross section given by Eq. (3). The result is a cross section of about 0.7Å2.

Fig. 10. Intensity of ASE at its peak wavelength and florescence at its peak wavelength as a function of time.

The linear absorption cross section r is defined as the ratio of the absorption coefficient to the number density N,

a r¼ : N

ð3Þ

3.1.2. Nonlinear It should be noted that the one photon absorption spectrum was obtained from a continuous white light source while the two photon absorption was measured at a finite number of points using a 100 fs Ti:sapphire pumped optical parametric amplifier (OPA) that provides light wavelengths from 630 nm to 790 nm. As such, the resolution of the two photon spectrum is low. Since the TPA spectrum is obtained from the integrated intensities of the twophoton fluorescence spectrum, and the two-photon spectrum is the same as the one photon fluorescence spectrum (see Fig. 5), it is clear that both fluorescence spectra result form de-excitation between the same states. It was observed that the florescence spectrum is approximately the mirror image of the absorption spectrum as to be expected. 3.2. Amplified spontaneous emission In the first set of experiments, measurements of the absorption spectrum, florescence spectrum and now ASE results are included. Fig. 6 shows a plot of these three measurements for the NRL-303

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C. Dudley / Optical Materials 35 (2013) 2115–2121 Table 2 Blue emitting dyes. Dye

Absorption Fluorescence ASE ASE Gain Fluorescence (nm) (nm) (nm) (cm1) efficiency /f

Linear cross- Two-photon sect r (cm2) cross-sect r2

Geometry Dimensions

NRL303

394

–

Thick film

487

460 2

–

Concentration M (molec/cm3)

3 cm  2 cm  0.1 cm 2  103 M

RefS. This work

18

–

–

Fiber

3

–

–

Thick film

–

4

–

–

Fiber

–

–

–

–

–

–

–

– –

– –

– –

– –

– –

– –

8–37 38

– –

(1.3  10 molec/ cm3) 2.5 cm  500 lm dia 2  103 M This work (1.3  1018 molec/ cm3) 1.35  1.75  0.08 1.6  102 M This work 19 (1.0  10 molec/ cm3) 2.5 cm  400 lm dia 1.6  102 M This work (1.0  1019 molec/ 3 cm ) – This work – – [31] – – [32]

–

–

–

–

–

–

0.075

–

–

–

–

530

556

NRL303

–

–

–

–

NRL303

–

490

–

NRL303

–

490

Fluorescein

–

Fluorescein Fluorescein 782 nm Pump Fluorescein 1064 nm Pump Fluorescein 1050 nm Pump Rh6G Rh6G Rh6G Rh6G (PMMA/EtOH) Rh6G (PMMA) Rh6G (MMA/EtOH) Rh6G (EtOH) BASF 339 KF241 PF-B C490 (pmma/etoh) C490 (MMA) C490 (mma/EtOH) C490 (EtOH) Coumarin 1 (EtOH) (Coumarin460) Coumarin 1 (PMMA +EtOH) Coumarin 1 (MMA) Coumarin 1 (MMA+EtOH) Coumarin 1 (PMMA) Coumarin 503 (PMMA+EtOH) Coumarin 503 (MMA+EtOH) Coumarin 503 (EtOH) Coumarin 503 (MMA) Coumarin 307 (PMMA+Dioxane) Coumarin 307 (MMA+Dioxane) Coumarin 307 (Dioxane)

0.88

–

[32]

0.23

[32]

– – 514

586

585 1.2

–

–

150 –

Rod

3 cm  1.5 cm dia.

50–100 M 6  103 M

This work [36] [31] [19]

526

566

–

–

–

–

Rod

3 cm  1.5 cm dia.

6  103 M

[19]

531

570

580 2.8

–

1.4  1018

–

Solution

1 cm

6  103 M

[19]

532

570

580 11

–

1.7  1018

–

Solution

1 cm

6  103 M

[19]

– –

16

3.5  10 2.4  1016

– –

0.95

– –

–

526 577 <470 382

605, 650 535, 575 480–550 472

– –

478 0.55

-

-

382

475

475 3.9

-

380

460

476 6.1

382 379

482 450

410

3

4

85  50  2.9 mm 85  50  2.9 mm3 1200 nm 3cm , 1cm dia.

10 M 104 M

-

Slab Slab Film Rod

2x10-2 M

[9] [9] [39] [15]

-

-

-

-

2x10-2 M

[15]

-

-

-

-

-

2x10-2 M

[15]

489 9 449 6.25

-

2.6 x 10-19 2.5 x 10-19

-

EtOH EtOH

1cm 1cm

2x10-2 M 3x10-2 M

[15] [16]

448

458 1.2

-

-

-

Disk

3x10-2 M

[16]

374

426

425 4

-

-

-

Solution

1mm thick, 2cm x 1cm dia. 1cm quartz cell

3x10-2 M

[16]

376

452

451 5.7

-

-

-

Solution

1cm quartz cell

3x10-2 M

[16]

406

424

425 0.9

-

-

-

1mm thick

3x10-2 M

[16]

448

485

505 0.2

-

-

-

Disk MMA solid Rod

-

3x10-2 M

[17]

445

482

504 9.8

-

-

-

Solution

-

3x10-2 M

[17]

445

489

504 15.8

-

-

-

Solution

-

3x10-2 M

[17]

443

470

473 8.2

-

-

-

Solution

-

3x10-2 M

[17]

387

467

491 3

-

-

-

Rod

-

30x10-2 M

[18]

385

469

480 4.9

-

-

-

Solution

-

-

[18]

380

476

480 3.6

-

-

-

Solution

-

-

[18] (continued on next page)

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C. Dudley / Optical Materials 35 (2013) 2115–2121

Table 2 (continued) Dye

Absorption Fluorescence ASE ASE Gain Fluorescence (nm) (nm) (nm) (cm1) efficiency /f

Linear cross- Two-photon sect r (cm2) cross-sect r2

Geometry Dimensions

Concentration M (molec/cm3)

RefS.

Coumarin 307 (MMA) Coumarin334 C334 / TPB C334 / TPB C334 / TPB TPB 5MeCz EtCz2 ADD-1 ADD-2 ADD-3 m-LPPP m-LPPP P1 PPV SP35 mppb PF3Cz CBP mCBP dCBP DPABP TPD a-NPD

385

471

476 6

-

-

-

Solution

-

-

[18]

450 480 371 364 374 425 450 265 440 350-355 370 -

455 465 480 380 420 425 420 443 455 490 425 460  420 393 400 409 419 424 445

430 490

-

-

2.5 x 1015

-

PMMA PMMA PMMA PMMA Film Film Film Rod Rod Rod Film

Film Film Film Film 700nm thick 700nm thick 2.5cm dia. 2.5cm dia. 2.5cm dia. 100-200nm

0 by wt 1/100 by wt 1/32 by wt 40% by wt. EtCz 1 to 7 x104 M 1 to 7 x104 M 1 to 7 x104 M conj. polymer

[35] [35] [35] [35] [35] [24] [24] [25] [25] [25] [26]

455 455 494 420 448 394 401 406 423 424 -

36 3.5 - 10 26 13 4 7.8 13 12 -

0.85 0.9 0.61 0.49 0.6 0.44 0.41 0.29

-

-

[25]

-

1mm thick 0.5mm dia 0-1000nm thick 200nm thick 100nm 100nm 100nm 100nm 100nm 100nm

2.8x10-3 M 1% by wt. mppb

2.8x1016 -

Solution Film Ps fiber ps Film Film Film Film Film Film Film Film

doped PMMA sample with a dye concentration of 1.3  1018 molecules/cm3. The absorption spectrum was measured with a white light source and an Ocean Optics Spectrometer. The emission experiments use either the second harmonic of a Ti:Sapphire laser at a wavelength of 400 nm and a pulse duration of 100 fs at average power of 0.2 W with a pulse rep rate of 1 kHz or a tripled modelocked and Q-switched Nd:YAG 35 ps laser at 355 nm running at a repetition rate of 10 Hz with an average pulse energy of 20 lW at the sample. The ASE peak grows at the expense of the fluorescence spectrum as the pump power is increased. Note that the peak ASE wavelength is at 461 nm for the Ti:Sapphire laser pump while it is shifted to 658 nm for the Nd:YAG laser. Fig. 7 shows the blue ASE light generated from the 800 lm diameter striated fiber. The striations in the extruded fiber run along its length; and, they can cause hot spots through multiple scattering that enhance the emission process. Indeed, larger conversion efficiency for the striated fiber was observed. However, since the striations are not regular, it is not an easy task to take into account their effect on the amount of light that is generated. As such, the smooth fibers were used for all quantitative fiber measurements, the results of which yield a lower amount compared to what could be obtained in the striated fiber. The gain was determined using the method of Shank et al. [33]. The ASE energy is measured for an excitation line of width 500 lm and lengths of 0.5 cm and 1 cm, both at the same intensity (so the power in the shorter line is half the power of the longer one). The intensity of the light I, entering a pinhole at a distance p from the sample is given by

IðkÞ ¼

Z

L

ecz ðp þ zÞ2

0

dz:

ð4Þ

Since p >> L, then Eq. (4) becomes

IðkÞ ¼

Z 0

L

ecz dz: p2

ð5Þ

Integration of Equation of (5) gives:

Z 0

L

ecz ecL  1 dz ¼ : p2 cp2

ð6Þ

(1.7x1017) -

[34] [25] [37] [38] [38] [38] [38] [38] [38]

The ratio between the output intensity recorded by the spectrometerof the dye’s ASE from a pump line length of L (IL) and of half of L I L is given by, 2

ðecL  1Þ IL : ¼  L c2 1 IL 2 cp2 e  1 1 cp2

ð7Þ

Upon factoring the numerator, we get:

IL L ¼ ec2 þ 1: IL

ð8Þ

2

Solving for the gain, c we find,

" # 2 IL c ¼ ln  1 ; L IL

ð9Þ

2

which is the standard definition of laser gain as given by Shank. Compared with the bulk, the gain in the fiber geometry saturates at lower pump power (15 kW for the fiber and 25 kW for the bulk) and at a higher gain (about 3.5 cm1 for the fiber and 2 cm1 for the bulk). Perhaps a more interesting measure of a material’s usefulness is the efficiency, which is defined as the fraction of the pump energy that is converted to ASE. The efficiency in the fiber is higher when compared with the bulk sample of the same concentration and reaches 25% for a peak pump power of 62 kW. For the fiber, the efficiency is linear over the useful range of the measurements. From the slope, we get the efficiency per unit of power of 0.75%/kW. (See Fig. 8 and Fig. 9). The reliability of the material is an important issue for applications. Depending upon the application, the quantities that are of importance are the stability of ASE and the stability of fluorescence. Fig. 10 shows a plot of the decay of each when illuminated with a line source. Note that the plots show the power emitted at the peak ASE wavelength and the peak Fluorescence wavelength as a function of time of the 10 Hz laser source. At high-enough power, ASE dominates, while at lower pump power only fluorescence is observed. The ASE signal, arising from a gain process, is a much more sensitive function of the dye concentration and the power than fluorescence. This decay over time was measured to see if ASE levels recovered after being left in the dark. No recovery was witnessed for these samples. No other decay studies were

C. Dudley / Optical Materials 35 (2013) 2115–2121

conducted. This dye in a pmma matrix would be suited for bright fluorescent markers or limited duration ASE devices. The fluorescence and ASE emission are linked too each other. When ASE dominates, it takes energy away from fluorescence, as can be surmised as displayed in Fig. 6. When the concentration of the NRL-303 chromophores decreases slightly, the ASE signal drops precipitously. It is important to point out that it is difficult to decouple the ASE signal from the fluorescence signal in the experimental design. Since the ASE signal is directed to the detector along the excitation line, strongly favoring that direction, as the ASE signal disappears, the sample fluoresces in all directions. As such, not all of the light that is lost to ASE is detected as fluorescence. It is clear, however, that the reliability of the material is better measured from the tail of the fluorescence spectrum. Since the pmma matrix seemed to fail prior to the dye in some test, it would be of intrest for furture work to use some of the techniqies to increase the damage threshold of pmma or increase scattering centers. Both are expected to increase output [11,12]. The 800 nm pumped results in Fig. 5 clearly yields good two-photon absorption. The two-photon cross section (Fig. 11) was found to be larger than that of Rhodamine 6G reported in the literature [31]. 4. Conclusion This work has reported on a material that efficiently generates blue light and demonstrated that it can be made into an optical fiber. The current approach offers a simple, rapid, inexpensive method of preparing many variant doped fiber optics for test samples and flexibility in device and components packaging. This process outlined is simplistic, inexpensive and yields relatively fast sample preparation times. The fiber showed signs of starting to give ASE under two-photon excitation at 800 nm, 100 fs pump. Each attempt with the present samples proved to be outside the damage threshold of the PMMA fibers. Experimental results have shown a blue emitting Chromophore (NRL303) made with symmetric acceptors and doped into PMMA surpasses the efficiency of Rh6G in PMMA, i.e. 25% vs. 21%, respectively, when both are doped in polymer hosts. When in an unmodified polymer matrix, the gain of NRL303 at 3–7 cm1 is better than that of Rh6G at 3 cm1. It is better than other emitters in the visible region of the spectrum listed in Table 2, especially when compared with blue emitters. Rh6G has been the standard benchmark of laser dyes in efficiency and gain. It is evident that the electronic matching of the chromophore p-center and linked acceptor/donor moiety is important for efficient photo-activity. The indications of two-photon ASE in this class of chromophore is of interest. Continued study of this class of chromophores should reveal insight for tailoring the synthesis for desired photonic and electrical properties. Acknowledgments I would like to thank OhKil Kim and the Naval Research Laboratory for providing the samples for this work. I also acknowledge support from the National Science Foundation for equipping the Washington State University femtosecond laser facility along with the WSU non linear optics group for allowing the use of space in their lab. References [1] N.E. Wolff, R.J. Pressley, Optical maser action in an Eu+3 -containing organic matrix, Appl. Phys. Lett. 2 (1963) 152. [2] E.H. Huffman, Stimulated optical emission of a terbium ion chelate in vinylic resin matrix, Nature 200 (1963) 158. [3] P.P. Sorokin, J.R. Lankard, Stimulated emission observed from an organic dye, chloro-aluminum phthalocyanine, IBM J. Res. Dev. 10 (1966) 162.

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[4] P.P. Sorokin, J.R. Lankard, Flashlamp excitation of organic dye lasers, IBM J. Res. Dev. 11 (1967) 148. [5] O.G. Peterson, B.B. Snavely, Stimulated emission from flashlamp-excited organic dyes in polymethyl methacrylate, Appl. Phys. Lett. 12 (7) (1968) 238–240. [6] B.H. Soffer, B.B. McFarland, Continuously tunable, narrow-band organic dye laser, Appl. Phys. Lett. 10 (10) (1967) 266–267. [7] A. Costela, I. Garcia-Moreno, J. Barroso, R. Sastre, Studies on laser action from polymeric matrices doped with coumarin 503, Appl. Phys. B 67 (1998) 167–173. [8] R. Reisfeld, E. Yariv, H. Minti, New developments in solid state lasers, Opt. Mater. 8 (1997) 31–36. [9] H. Manna, S.M. Al-Alawi, Optical gain measurements in polymethyl methacrylate plastic doped with perylimide dyes, J. Lumin. 94-95 (2001) 55– 58. [10] Costela, I. Garcia-Moreno, J. Barroso, R. Sastre, Studies on laser action from polymeric matrices doped with courmarin 503, Appl. Phys. B 67 (1998) 167– 173. [11] K.M. Dymaev, A.A. Manenkov, A.P. Maslyukov, G.A. Nechitalio, A.M. Prokorov, Dyes in modified polymers:problems of photostability and conversion efficiency at high intensities, Opt. Soc. B 9 (1) (1992) 143–150. [12] D.Y. Kim, H.N. Cho, C.Y. Kim, Blue light emitting polymers, Prog. Polym. Sci. 25 (2000) 1089–1139. [13] G.A. Reynolds, K.H. Drexhage, New coumarin dyes with rigidized structure for flashlamp-pumped dye lasers, Opt. Commun. 13 (3) (1975) 222–225. [14] E.J. Schmitschek, J.A. Trias, P.R. Hammond, R.A. Henry, R.L. Atkins, New laser dyes with blue–green emission, Opt. Commun. 16 (3) (1976) 313–316. [15] G. Somasundaram, A. Ramalingam, Gain studies in coumarin 490 dye-doped polymer laser, Chem. Phys. Lett. 324 (2000) 25–30. [16] G. Somasundaram, A. Ramalingam, Gain studies of coumarin 1 dye-doped polymer laser, J. Lumin. 90 (2000) 1–5. [17] G. Somasundaram, A. Ramalingam, Gain studies of coumarin 503 dye-doped polymer laser, Opt. Lasers Eng. 33 (2000) 157–163. [18] G. Somasundaram, A. Ramalingam, Gain studies of coumarin 307 dye-doped polymer laser, Opt. Laser Technol. 31 (1999) 351–358. [19] G. Somasundaram, A. Ramalingam, Gain studies of Rhodamine 6G dye doped polymer laser, J. Photochem. Photobiol. A: Chem. 125 (1999) 93–98. [20] V. Sharma, P. Sahare, A. Pandey, D. Mohan, Spectrochim. Acta Part A 59 (2003) 1035–1043. [21] A. Ramalingam, C. Vijila, J. Mol. Liq. 81 (1999) 237–244. [22] F. Hide, B. Schwartz, M. Diaz-Garcia, A. Heegar, Synth. Met. 91 (1997) 35–40. [23] M. Diaz-Garcia, F. Hide, B. Schwartz, M. Anderson, Q. Pei, A. Heegar, Synth. Met. 84 (1997) 455–462. [24] Castex, Olivero, Fischer, Mousel, Michelon, Ades, Siove, Polycarazoles microcavities: towards plastic blue lasers, Appl. Surf. Sci. 197-198 (2002) 22– 825. [25] V. Thiagarajan, C. Selvaraju, P. Ramamurthy, Excited behavior of dyes in PMMA matrix, inhomogeneous broading and enhancment of triplet, J. Photochem. Photobiol. A: Chem. 157 (2003) 23–31. [26] C. Zenz, G. Kranzelbinder, W. Graupner, S. Tasch, G. Leising, Blue green stimulated emission of a high gain conjugated polymer, Synth. Met. 101 (1999) 222–225. [27] M. Fakis, I. Polyzos, G. Tsigaridas, V. Giannetas, P. Perephonis, Laser action of two conjugated polymers in solution and in solid matrix: The effect of aggregates on spontaneous and stimulated emission, Phys. Rev. B 65 (2002) 195–203. [28] D. Katsis, Y.H. Geng, J.J. Ou, S.W. Culligan, A. Trajkoska, Spiro-linked ter-, penta-, and heptafluorenes as novel amorphous materials for blue light emission, Chem. Mater. 14 (2002) 1332–1339. [29] O.K. Kim, K.S. Lee, H. J Woo, K.S. Kim, G.S. He, J. Swiatkiewicz, P.N. Prasad, New class of two-photon-absorbing chromophores based on dithienothiophene, Chem. Mater. 12 (2000) 284–286. [30] O.-K. Kim, K.-S. Lee, Z. Huang, W.B. Heuer, C.S. Paik-Sung, Oligothiophene as photonic/electronic property modulator, Opt. Mater. 21 (2002) 559–564. [31] M. Albota, C. Xu, W.W. Webb, Two-photon fluorescence excitation cross secctions of biomolecular probes from 690 to 960nm, Appl. Opt. 37 (31) (1998) 7352–7356. [32] C. Xu, W.W. Webb, Measurment of two-photon excetation cross section of molecular fluorophors with data from 690 to 1050 mn, J. Opt. Soc. Am. B 13 (3) (1996) 481–490. [33] C.V. Shank, A. Dienes, W. Silfvast, Single pass gain of exciplex 4-MU and rhodamine 6G dye laser amplifiers, Appl. Phys. Lett. 17 (7) (1970) 307–309. [34] T. Kobayashi, W. Blau, Blue amplified spontaneous emission from a stilbenoidcompound-doped polymer optical fiber, Opt. Lett. 26 (24) (2001). [35] Jinman Huang, Vlasoula Bekiari, Panagiotis Lianos, Stelios Couris, Study of poly(methyl methacrylate) thin films doped with laser dyes, J. Lumin. 81 (1999) 285–291. [36] P. Kaantz, D. Shelton, Two-photon fluorescence cross-section measruments calibrated with hyper-rayleigh scattering, J. Opt. Soc. Am. B 16 (6) (1999) 998– 1006. [37] X. Liu et al., Low-threshold amplified spontaneous emission and laser emission in a polyfluorene serivative, Appl. Phys. Lett. 84 (15) (2004) 2722–2729. [38] Y. Kawamura et al., Ultraviolet amplified sponatneous emission from thin films of 4,4’-bis(9-carbazolyl)-2,2’-biphenyl and the derivates, Appl. Phys. Lett. 84 (15) (2004) 2724–2726. [39] Z. Yu et al., Highly flexible polymer light-emitting devices using carbon nanotubes as both anodes and cathodes, J. Photon. Energy 1 (2011).