Energy Transfer and Photostability of Rh-6G and Rh-B Doped in Polyacrylamide Polymer T ⁎ Hamdan A.S

Optik - International Journal for Light and Electron Optics 182 (2019) 716–726

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Original research article Energy transfer and photostability of Rh-6G and Rh-B doped in polyacrylamide polymer T ⁎ Hamdan A.S. Al-shamiria,b, , M. Atta Khedrc, M.M. sabryc a Physics Department, Faculty of Science, University of Bisha, Bisha 61922, P.O. Box 551, Saudi Arabia b Physics Department, Faculty of applied Science, Taiz University, Yemen c National Institute of Laser Enhanced Sciences, Cairo University, Giza, 12613, Egypt

ARTICLE INFO ABSTRACT

Keywords: In this study, the polyacrylamide gel was chosen as a host material and the two rhodamine dyes: Polyacrylamide rhodamine 6 G (Rh-6 G) and rhodamine B (Rh-B) were chosen as guest molecules in order to Dye mixture prepare a novel guest–host systems (dye-loaded polyacrylamide gel) with promising optical Optical Properties properties. For the first time different concentration of binary mixture of dyes (Rh-6 G/Rh-B) Energy transfer doped polyacrylamide gel were prepared, the optical absorption and amplified spontaneous emission (ASE) of the dye doped polyacrylamide, which is pumped by the second harmonic (532 nm) of Nd-YAG laser, were measured. Energy transfer from Rh-6 G to Rh-B has been studied. 0 The critical transfer distance R0 has large value (12.5 A ). The gain and amplified spontaneous emission (ASE) efficiency for Rh-6 G and Rh-B doped polyacrylamide were measured under the − − same experimental condition. The optical gain was 4.5 cm 1 for Rh-6 G and 2.5 cm 1 for Rh-B and the amplified spontaneous emission (ASE) efficiency was 30% for Rh-6 G and 26% for Rh-B. The photostability for fresh samples of (Rh-6 G and Rh-B) and after 24 hours from the first use were studied. A self-healing phenomenon for the samples were observed.

1. Introduction

Up to now, the dye laser remains the only available multi-wavelength laser source for a variety of applications, particularly some medical applications, remote sensing and military applications, which require tunable high-power pulsed beams [1–3]. Over the last two decades there has been a renewed interest in the use of solid matrices containing lasing dyes to build practical tunable solid-state dye lasers, because of technical and economical advantages in comparison with classical liquid dye lasers. The main research has been devoted to material hosts which disperse well the dye molecules, and for which laser efficiency and operational photostability are high. It has been found that materials with an amorphous network structure like organic polymer polymethylmethacrylate (PMMA) and inorganic porous polymer sol–gel glass and their derivatives are good hosts for dye molecules [4–13]. A lot of work was devoted to incorporating different dyes into solid matrices in order to obtain solid-state laser emission from the near UV to the near IR light range like classical liquid dye laser emission [14–17]. Dyes, which are having low absorption cross-section at the pumping wavelength, are not suitable for efficient dye lasers. Increasing the concentration of the dye can overcome this difficulty but may cause concentration quench. A simple method is to use energy transfer from a dye, which can absorb the pump radiation efficiently. The use of dye mixtures as dopants in solid state dye laser (SSDL) materials has generated much research interest. In this type of laser, one kind of dye molecule, which is referred to as the donor dye, absorbs pumping light and is thus optically excited. The excited

⁎ Corresponding author at: Physics Department, Faculty of applied Science, Taiz University, Republic of Yemen. E-mail address: [email protected] (H.A.S. Al-shamiri). https://doi.org/10.1016/j.ijleo.2019.01.082 Received 22 May 2018; Received in revised form 22 January 2019; Accepted 25 January 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved. H.A.S. Al-shamiri et al. Optik - International Journal for Light and Electron Optics 182 (2019) 716–726 donor dye molecule relaxes by transferring the energy to another kind of dye molecule, which is referred to as the acceptor dye. The acceptor dye molecule is thus indirectly pumped by the pumping source. In this way, the acceptor dye molecules may act as a laser source. In the context of solid-state lasers, efficient energy transfer between numerous donor–acceptor (D–A) dye pairs has been reported to improve the laser performance and to broaden the spectral range of laser operation [18–26]. In D–A pairs, the emission region is shifted away from absorption and hence self-absorption is considerably reduced. It has been found that by controlling the interactions between various dyes co-doped into solid hosts, i.e. the energy transfer processes between dye molecules, improvement on laser efficiency, lifetime, threshold, and tunable range can be obtained [18–26]. In recent years, great progresses have been made in this field and the laser performances especially the photostability of solid- state dye laser (SSDL) materials, which have been the major obstacles for practical use, have been improved significantly by em- ploying various dyes and host media [7,9,27–30]. In this study, the polyacrylamide (PAAm) was chosen as a host material and the two Rhodamine dyes (Rh-6 G and Rh-B) were chosen as guest molecules in order to prepare a novel guest–host systems (dye-loaded polyacrylamide gel). Energy transfer from Rh-6 G to Rh-B in polyacrylamide matrices was studied using steady state emission measurements, the effect of donor concentration on energy transfer, the optical gain for Rh-6 G and Rh-B, the amplified spontaneous emission (ASE) efficiency, and the photostability for fresh samples and after 24 h were studied.

2. Experimental

2.1. Materials

The chemical materials which were used in this work are; Acrylamide, N, N-methylene bis acrylamide, ammonium persulfate (APS), and Ethelyn glycol, were used in synthesizing polyacrylamide gels. All the precursors were purchased from Aldrich and used as received without any further purification. The laser grade Rhodamine 6 G and Rhodamine B dyes were procured from Scientific limited Northhampton.uk and used without further purification. Fig. 1 shows the general structures of laser dyes Rhodamine 6 G and Rhodamine-B. Rhodamine dyes are important class of xanthene dyes. They have high quantum yield of fluorescence and high extinction coefficients that extensively have been studied due to their practical applications as active fluorescence agent in dye laser and probes.

2.2. Sample preparation

Polyacrylamide gels are synthesized by the standard free radical polymerization method [31,32]. The polyacrylamide gel was formed at room temperature from polymerization acrylamide (CH2 CHCCONH2) with cross linking agent N, N` methylene bis acrylamide as shown in Fig. 2. Acrylamide, N, N` methylene bis acrylamide and ammonium per sulfate were solved in ethylene glycol to form polyacrylamide solution. After less than two hours the polyacrylamide gel was formed. The resulting polymer network forms a rubbery solid which is 90–95 % base solvent by weight. A transmission spectrum of polyacrylamide gel revel good transparency down to 300 nm in 1 cm path length as shown in Fig. 3. Synthesis of polyacrylamide gel doped with laser dyes was performed by adding the dye to polyacrylamide solution before gelation. In this work several samples with diﬀerent concentration of dye doped polyacrylamide gel, (Rh-6 G, Rh-B and mixture of them) were prepared.

2.3. Methods

The solid-state dye laser (Rh-6 G, Rh-B and mixture of them doped polyacrylamide gel) were cast in Pyrex glass cuvette which had a cylindrical shape of 10 mm in diameter and 20 mm in length. A cut was made perpendicular to the axis of cylinder to ﬁxaﬂat surface glass windows of thickness 1 mm. Fig. 4 shows that the cuvette is transversely pumped by (532 nm), with 5 mJ, 7 ns pulses from frequency doubled Nd-YAG laser (Continuum PL7010) at a repetition rate of 5 Hz. The exciting pulse were directed towards the surface of solid samples with a combination of concave lens (f = 10 cm) and cylindrical lens with f = 10 cm. The cylindrical lens

Fig. 1. The molecular structures of the Rhodamine dyes.

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Fig. 2. The molecular structures of acrylamide monomer, methylene-bis- acrylamide and polyacrylamide (PAAm).

Fig. 3. Transmittance spectrum of the polyacrylamide gel.

Fig. 4. Experimental setup for measurement peak wavelength of the Ampliﬁed spontaneous emission.

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Fig. 5. Overlapping of the absorption spectra of Rh-B and Rh-6G with the fluorescence spectra of Rh-6G and Rh-B. (a) Absorption spectrum of Rh- 6G, (b) Absorption spectrum of Rh-B (c) Fluorescence spectrum of Rh-6G, (d) Fluorescence spectrum of Rh-B. perpendicular arranged focused the pump on the surface of the solid sample to form a line of ˜0.3 × 20 mm, so that the pump flunce − was 83 mJ.cm 2. The dye and pump laser pulses were characterized with the following instruments: Molectron EPM 2000 laser energy power meter, spectrometer and photodiode array (CCD). The amplified spontaneous emission (ASE) peak wavelength (super radiance), which emitted from the dye cell window perpendicular to the pumping beam were attenuated by neutral density filter and then entered the spectrograph through optical fiber. A CVI spectrograph Model 112 Sp.(1/8 meter) has grating with 1200 groves per − nm blazed at 250 nm, and reciprocal linear dispersion of 0.63 nm.mm 1 and attached by a CCD Camera (Apogee Model KX 85.), were used to process the spectrum. All the integrated signals were digitized and processed using a PC computer via a computer board interface. The pumping energy (input energy) was measured using beam splitter (4%) and Molectron energy meter (Model J3-09) detector head. The dye laser samples were irradiated as the same in Fig. 4 but amplified spontaneous emission (ASE) output were collected by convex lens with focal length (15 cm) on the energy detector head which was connected to the power meter for measuring the output energy of the laser dyes. The input energies and the concentrations of dye laser were varied to measure the effect of that on the gain and the amplified spontaneous emission (ASE) efficiency.

3. Results and discussion

The optical transmittance spectrum of the un-doped polyacrylamide gel in plastic cuvette with path length 1 cm and the spectral band pass of (1 nm) were represented in Fig. 3. The transmittance was excellent in the visible and expanded to UV- region, and there is no considerable absorption at (532 nm) of frequency doubled Nd-YAG laser when it is used as pumping source in this work. Absorption and fluorescence spectra were measured, respectively on a Perkin Elmer Lambda 35 UV–Vis spectrometer and on a Perkin Elmer LS-50B luminescence spectrometer. − − Fig. 5 shows the absorption and fluorescence spectra shapes of such polyacry-lamide gel doped with 1 × 10 5 mol. L 1 of Rh-6 G and Rh-B. The fluorescence spectrum of Rh-6 G overlap the absorption spectrum of Rh-B, allowing for electronic energy transfer from Rh-6 G to Rh-B, with Rh-6 G acting as donor and Rh-B acting as acceptor. The spectral overlapping in the wavelength band ranging from 520 to 600 nm between the fluorescence band of the Rh-6 G dye and the absorption band of the Rh-B is suitable for energy transfer from excited Rh-6 G donor molecules to Rh-B acceptor molecules. As noted earlier, a spectral overlap between the fluorescence spectrum of donor and absorption spectrum of acceptor is the prerequisite for any energy transfer experiment. The large area of overlap indicates that the energy transfer between the donor and the acceptor should be possible [33–35]. The degree of spectral overlap between the donor emission and the acceptor absorption is given by the overlap integral.

FυDA(¯)ε (¯) υ JDA (¯)υ = d(¯),υ ∫ (¯)υ (1) where fD(ʋ) is the spectral distribution of the donor ﬂuorescence intensity normalized to unity and εA(ʋ) the molar extinction

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− − coeﬃcient of the acceptor expressed in units of M 1 cm 1 [36,37]. The value of the spectral overlap integral for the rhodamine 6 G −14 3 −1 (donor) and rhodamine B (acceptor) doped polyacrylamide under study is (JDA(ῡ) = 5.92 × 10 cm . Mol ). The critical transfer distance R0 is deﬁned as the average distance between the donor molecule and acceptor molecule at which the probability of intermolecular energy transfer is just equal to the sum of probabilities for all de-excitation processes of the excited state donor and calculated according to [38].

2 ∞ 9000Ln 10 k ϕf Fυευ(¯) (¯) R 6 = DAdυ¯. 0 428πnN54 ∫ υ¯4 0 0 (2)

Where εA is the molar decadic extinction coeﬃcient of the acceptor; ύ the wave number; n the refractive index; N0 the Avogadro's number; FD (ύ) the spectral distribution of donor normalized to unity (i.e. ∫ FD(ύ)dύ =1); and k the orientation factor equal to 2/3 for isotropic media. The calculated value of critical transfer distance for (Rh-6 G/Rh-B) in polyacrylamide was 12.5 A0. This value is considerably 0 greater than those normally obtained for collision energy transfer in which R0 is in the range of (4–6A)[39]. The large value of critical transfer distance (R0) for Rh-6 G/Rh-B in polyacrylamide indicates that the dominance of long-range resonance energy transfer. Normally, excitation energy of the donor is transferred to the acceptor via an induced dipole–dipole interaction.

The energy transfer eﬃciency (ηT) from the Rh-6 G to Rh-B dyes in the polyacrylamide polymer host can be determined according to the data for the luminescence intensity using the following equation [40,41].

Is ηT =−1 Iso (3)

Where IS0 and IS are the luminescence intensity of Rh-6 G donor in the absence and presence of Rh-B, respectively. The energy transfer (ηT) value of the Rh-6 G to Rh-B can reach 57.9%. This result indicated that the energy transfer from Rh-6 G to Rh-B is highly eﬀective. The lasing wavelength may be tunable for some dyes by varying the laser cavity parameters, the dye concentration, the solvent (polarity and PH) and temperature [42–45]. The peak wavelengths of the ampliﬁed spontaneous emission (ASE) of each individual dye as a function of the dye concentration shown in Fig. 6, the samples were transversally pumped by 5 mJ of 532 nm at 10 Hz. − − − Increasing the dye concentration for Rh-6 G and Rh-B in the polyacrylamide polymer host from 1 × 10 4 to 5 × 10 3 mol. L 1 induced a red shift in the ASE peak wavelengths towards a longer wavelength. The induced shifts were found ranging from 577 to 597 nm for Rh-6 G samples, and from 577 to 627 nm for Rh-B samples. This is due to the singlet-singlet re-absorption (self-absorption and re-emission) process as discussed in Ref. [46,47]. This process is a result of the frequency overlap of the absorption and emission bands of the dye. In the overlap, the light that is emitted by the dye is self-absorbed and re-emitted to longer wavelengths. As it is well known, using an appropriate mixture of dyes can result in extended tuning range and shifting of the peak wavelength

Fig. 6. Peak wavelength of the ampliﬁed spontaneous emission as a function of laser dyes concentration.

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Fig. 7. Peak wavelength of the ampliﬁed spontaneous emission of binary mixture of Rh-6G/Rh-B with ratios are (1:1), (3:1), (5:1), (7:1) as a function of donor concentration. And ampliﬁed spontaneous emission for Rh-6G and Rh-B as a function of dye concentration. of the laser emission, depending on the nature, structure, and relative concentrations of the dyes in the mixture [48,49].

Fig. 7 the concentration effect of the donor (Rh-6 G) on the lasing wavelength (λmax) for different donor/acceptor ratios (i.e D/ A = 1, 3, 5, 7) the laser peak wavelength emission for (Rh-6 G and Rh-B) mixture in polyacrylamide lie between the peak wavelength emission of the (Rh-6 G) and (Rh-B) alone (i.e. red shift respect to donor and blue shift respect to acceptor), as the donor concentration increases (D/A = 1 → D/A = 7). The lasing wavelength (λmax) of the acceptor largely shifts toward shorter wavelength, i.e blue shift. This indicates that the gain enhanced of the acceptor (Rh-B) due to energy transfer which occurred efficiently [50]. The extension to higher concentration of the acceptor shows that efficient energy transfer excitation overcomes the losses arising in the high concentration region. Also, the energy transfer dye laser system can operate at low acceptor concentrations. These effects practically expand the spectra region of operation.

The dependence of λmax on the concentration of the acceptor was delineated from Fig. 7 and is given by Fig. 8. It is seen that the concentration of the acceptor increases, the λmax is red shifted. Again, as the donor concentration increases, the lasing wavelength of the acceptor shifts towards shorter wavelengths. Thus, most of the excitation energy absorbed by Rh-6 G is transferred to Rh-B as a useful pump power making the excitation transfer quite efficient. A single pass gain has been measured adopting the amplified spontaneous emission (ASE) method proposed by Shank et al. [51]. The ASE gain is defined as the increase in the ratio of the emitted to the incident light intensity per unit length of the pumped material and calculated according to [51].

G =(2/L)Ln(EL/EL/2 − 1) (4) where L is the length of the irradiated sample and EL,EL/2 are the resulting ASE energy from the length L and half-length L/2 of the sample at the peak wavelength. The computed gain for the sample length L = 20 mm for different concentrations of Rh-6 G and Rh-B doped polyacrylamide when pumped by 532 nm of 10 mJ. The Gain was found to depend on the dye concentration where the optimum gain per unit length was − − − − − 4.5 cm 1 for Rh-6 G samples at concentration of 7 × 10 4mol. L 1 and 2.5 cm 1 for Rh-B samples at concentrations 3 × 10 3 mol/ L. These were the concentrations which have the highest energy conversion. The value of gain initially increases to a maximum and then decreases with concentration as shown in Figs. 9 and 10. These decreases in the gain with increasing the concentrations beyond optimum are attributed to the concentration quenching and aggregation of dye molecules [17,52–54]. Amplified spontaneous emission (ASE) efficiency measurements for Rh-6 G and Rh-B doped polyacrylamide is carried out on 2 cm − − − − length of polymeric rod with (7 × 10 4 mol. dm 3) concentration of Rh-6 G and (3 × 10 3mol. dm 3) concentration of Rh-B which have a high gain as shown previously. The measured ASE energy versus the input pump energies are shown in Fig. 11 where the threshold energy was ˜1.1 mJ. Each data point was measured, using Molectron energy meter J3-09, as an average of five shots to minimize the probable thermo-optical distortion and dye degradation. The average energy efficiency extracted from this input–output energy measurements were 30% and 26% for Rh-6 G and Rh-B respectively.

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− − Fig. 8. Dependence of peak wavelength emission on the acceptor concentrations at 0.5, 0.7 and 1 × 10 3mol.dm 3 donor concentrations, when (D/A=1,3,5,7).

Fig. 9. The gain as a function of Rh-6G concentrations doped polyacrylamide.

Finally, the photostability of the Rh-6 G and Rh-B doped polyacrylamide are studied. Figs. 12 and 13 show the experimental results, where the amplified spontaneous (ASE) output energy (superradiance) is studied as a function of the number of pulses at repetition rat 5 Hz, pumping energy 5mj per pulse, for the two samples of (Rh-6 G and Rh-B) − − doped polyacrylamide with the same concentration (1 × 10 3 mol dm 3) both samples are capsulated by cylindrical glass cuvettes with length (2 cm) and transversely pumped bye the same number of pulses (35,000 pulse) at fixed position on the sample. It is observed that in case of Rh6 G Fig. 12 the amplified spontaneous emission output energy of the sample decreases to a half initial value after nearly (27,000 pulse) while in case Rh-B the output energy is decreased to a half initial value at (35,000 pulse).

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Fig. 10. The gain as a function of Rh-B concentrations doped polyacrylamide.

Fig. 11. Output energy as a function of Nd-YAG pumping energy for Rh-6G and Rh-B doped polyacrylamide.

After this number of pulses (35,000 pulse) the irradiated region of the samples presents some damage in the form of changing the color of the dye (appears as dark line), none emissive line in the pumped region disappear completely after several hours (12–24 hours) at room temperature. This means that "self -healing" of the solid dye's matrices may be diﬀusion of fresh dye molecules into photo-bleaching region. The self-healing is clear from use the samples (Rh-6 G, Rh-B) after 24 h and the same region on the samples pumped previously, observed that the samples (Rh-6 G, Rh-B) nearly have the same behavior before and after 24 h as shown in Figs. 12 and 13. This has never been seen with a laser dye in a polymer host, as photodegradation is normally permanent. In addition to the self-healing phenomena for laser dyes in polyacrylamide which has never been seen in any other polymer host,

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Fig. 12. Normalized laser output as a function of the number of pumping pulses at repetition rates of 5HZ.pumping energy(5mj) for Rh-6G doped acrylamide a) fresh b) after 24 a hour.

Fig. 13. Normalized laser output as a function of the number of pumping pulses at repetition rates of 5Hz, at pumping energy(5mj) for Rh-B doped polyacrylamide a) fresh b) after 24 h. the polyacrylamide gel has a good optical transparency at both pump and lasing wavelengths, good solubility of the dye in the material, and resistance to pump laser radiation. All these optical characteristics for polyacrylamide made it potential safely can- didate as host material for organic laser dyes with long life time operation.

4. Conclusion

Polyacrylamide gel is a very reliable and a good host material have been found and which could be used safely as host material for organic laser dyes with long lifetime operation. The mechanism of energy transfer between the investigated dyes (Rh-6 G and Rh-B) is

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long-range resonance energy transfer, this is clear from large values of the obtained transfer distance R0. Energy transfer between two dyes Rh-6 G/Rh-B extend the lasing wavelength ranges of dye laser. The amplified spontaneous emission (ASE) peak wavelength for Rh-6 G/Rh-B mixture depends on the donor to acceptor ratios, red-shift by increasing the acceptor (Rh-B) at a fixed concentration of donor (Rh-6 G), while blue shift by increasing the donor at a fixed concentration of the acceptor. The dye laser gain depends on dye concentrations where gain increases with increasing the dye concentration to a certain value and then decreases. Laser dyes Rh-6 G and Rh-B doped polyacrylamide have resistance to photobleaching and possess self- healing properties, which could be used safely with long life time operation.

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