J. Opt. A: Pure Appl. Opt. 2 (2000) 279–283. Printed in the UK

PII: S1464-4258(00)10389-7

Polymer thin-film distributed feedback tunable lasers Vincent Dumarcher†, Licinio Rocha†, Christine Denis†, ´ Celine Fiorini†, Jean-Michel Nunzi†§, Frank Sobel‡, Bouchta Sahraoui‡ and Denis Gindre‡ † LETI (CEA—Technologies Avanc´ees), DEIN/SPE, Groupe Composants Organiques, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France ‡ Laboratoire des Propri´et´es Optiques des Mat´eriaux et Applications (POMA), Universit´e d’Angers, 2 Boulevard Lavoisier, 49045 Angers Cedex, France E-mail: [email protected] Received 14 December 1999, in final form 31 January 2000 Abstract. We report on measurements of laser emission from poly-methylmethacrylate and poly-vinyl carbazole polymer films doped with rhodamine-6G, DCM and coumarin laser dyes in an optically pumped distributed feedback scheme. We obtain tunability on a broad spectral range for all samples. We show the impact of waveguiding in the polymer film on reducing the laser threshold. We also show that the number of laser modes increases with the polymer film thickness, following the guided mode dispersion. Keywords: Luminescent polymer, organic laser, distributed feedback, thin-film waveguide

1. Introduction

Patterning and micro-structuring of functional polymers are key technologies employed in fabricating organic devices. A recent achievement in this respect is the patterning of photoinduced surface-relief gratings using the interference pattern between optical beams [1, 2]. This opens the route to molecular translation control using optical fields, in the same way as dual-frequency irradiation using appropriate combinations of circular beam polarizations has been demonstrated to enable full control of the molecular polar order [3, 4]. The well known azo-dye aromatic polymers have been shown to be among the most efficient materials for such structuring processes using light–matter interactions. A simple microscopic model accounting for the essential features of photoinduced surface-relief grating formation has been developed [5] and some of its peculiar features have been verified experimentally [6]. A challenging issue today is to pattern microstructures in organic devices in order to control the emission properties of polymer thin films such as the one used for electroluminescent diodes. Control of the radiation modes of electroluminescent diodes has already been demonstrated using planar microcavities [7–9]. Here we propose the use of a periodical excitation in a distributed feedback (DFB) scheme [10] to investigate the luminescence and lasing properties of dye-doped polymer thin films. The so-called organic lasers [11–15] have recently been revisited with the aim of building an organic semiconductor laser diode [16]. In this respect, single-crystal luminescent semiconductor § Author to whom correspondence should be addressed (also with POMA).

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© 2000 IOP Publishing Ltd

materials [17] which have large charge mobility should permit the transport of large enough currents to reach lasing threshold through electrical pumping in a DFB structure. In 1971, Kogelnik and Shank first demonstrated DFB dye-laser operation using a dye-doped gelatin film in which a grating had been previously printed optically [10]. They also showed that DFB laser action was possible by making a dynamic grating using the interference fringes from two pump beams inside a dye cell [18]. Narrow-band emission was tuned simply by changing the angle between the pump beams incident onto the dye cell. Efficient feedback is obtained from a spatial modulation of both the gain and index of refraction of the polymer film. Modulation is photoinduced with the interference pattern produced by the two coherent laser beams. Different interference schemes have already been proposed for DFB laser excitation: the so-called Lloyd’s mirror which permits stable operation in a compact setup [19] and a more sophisticated interferometer which permits a better control of the pump beam characteristics [20]. In this paper, we use the latter scheme to study DFB polymer dye-laser characteristics. In particular, we study effects related to waveguiding of the laser emission into the polymer thin film. This appears as a very important feature in order to control stimulated emission using optically confined structures. 2. Experiments

The samples used were poly-methylmethacrylate (PMMA) or poly-vinyl carbazole (PVK) polymer films spin-coated 279

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Figure 1. Set-up for DFB laser experiments.

on top of quartz or glass substrates. Polymer films were doped by dissolution of rhodamine-6G (Rh6G) or DCM dyes at concentrations ranging between 0.003 and 0.03 M. Such compositions have been widely studied for stimulated emission [21] and dye-laser action [22]. Coumarin-515 dye was blended at a concentration of 0.12 M in PVK. This latter blend is an organic semiconductor and has been used previously in electroluminescent diodes [23]. The polymer film thickness was chosen between 200 and 2000 nm. The pump source was a frequency doubled (for Rh6G and DCM dyes) or tripled (for coumarin dye) Q-switched mode locked Nd:YAG laser delivering 33 ps pulses at a 10 Hz repetition rate. The experimental set-up is represented in figure 1. A cylindrical lens is used before the beam splitter in order to enable efficient pumping over a long and narrow stripe. The spot size of the beam on the film is 2 × 10−3 cm2 . The pump beam is split in two parts by the two prisms P1 and P2 . The two resulting beams are directed to the film after reflection on two co-rotating mirrors M1 and M2 . The emitted beam is collected by a 5 cm focal length lens, into a optical fibre (core diameter 0.6 mm) coupled with a spectrometer. The light is then dispersed with a 0.12 m focal length monochromator, using a 1200 lines mm−1 grating, and detected with a thermoelectrically cooled charged coupled device. The dispersion of the spectrometer is 0.15 nm per pixel. The wavelength λL of the DFB laser at an incidence angle θ is given by λL =

nλP m sin θ

(1)

where λP is the pump wavelength, n is the refractive index of the dye, and m is the order of diffraction. In such a pumping scheme, both gain and index feedback are produced by the same excitation mechanism. For the studies reported here, we always use second-order diffraction on the DFB grating (m = 2), corresponding to θ angles around 40◦ . 280

3. Results and discussion

3.1. DFB laser action We studied the DFB effect on the transition from stimulated to laser emission using a 400 nm thick PMMA film doped with Rh6G. The pump energy was 5.9 µJ mm−2 , just above threshold. When the two pump beams do not interfere in the polymer film (figure 2(a)), we see the stimulated emission spectrum [21]. When the two beams poorly interfere, owing to a bad superposition, we see that most of the energy is emitted in a sharply peaked spectral region (figure 2(b)). When the two pump beams interfere into the polymer film, the DFB laser action is effective and we get a narrow emission with a 0.4 nm full width at half maximum which is mostly the resolution limit of the spectrometer (figure 2(c)). Notice the two orders of magnitude change in the vertical scale which show that most of the energy is emitted in the feedback direction. The emission wavelength λL is tuned with the incidence angle θ of the pump onto the film. Figure 3 shows the emission spectra of Rh6G in PMMA and DCM in PVK for various incidence angles. Tuning is mostly limited by the spectral extent of the gain band of the dye used [24]. 3.2. Optical confinement and waveguiding Confinement effects can be evidenced by use of the cylindrical lens in figure 1. The lens permits one to focus the interfering pump beams along a narrow stripe on the polymer film. Figure 4 shows the reduction of the pump fluence at laser threshold when the cylindrical lens is employed. Pumping over a long narrow stripe indeed permits one to improve the directivity of the stimulated emission and so, to reduce threshold intensity. Lateral confinement reduces the experimental threshold intensity by a factor of 3. Vertical confinement in a direction perpendicular to the film surface was also evaluated. The dyed polymer film can indeed serve as a waveguide for laser emission. We tested, comparatively, PMMA (n = 1.5) and PVK (n = 1.6) doped with DCM, using the same pump intensity above

Polymer thin-film distributed feedback tunable lasers

Figure 2. The DFB action on the transition from stimulated to laser emission in a PMMA film doped with Rh6G. The two pump beams do not interfere in (a). They poorly interfere in (b), and they interfere in (c), producing the distributed feedback.

Figure 3. Output spectra as a function of the incidence angle θ for DFB laser action of Rh6G in PMMA (a) and DCM in PVK (b).

Figure 4. Output spectrum of Rh6G in PMMA for different pump energy densities (fluence) before and after laser threshold, without (a) and with (b) cylindrical lens. The pump fluence values are: (a) 3.8 µJ mm−2 , (b) 3.1 µJ mm−2 , (c) 1.1 µJ mm−2 , (d) 0.7 µJ mm−2 .

threshold. With PMMA films deposited onto a glass substrate (figure 5(a)), laser emission is weak because the refractive indices of the substrate and of the PMMA film are very close. Losses through the substrate increase threshold. With a PVK film on glass, laser emission intensity is increased by two orders of magnitude for the same pump intensity (figure 5(b)). The reason for this is that the refractive index of PVK is larger that that of glass, so that laser emission can be guided inside the film. Guiding in the polymer film improves the confinement of the emission. The same situation (figure 5(c)) is realized for a PMMA film deposited on a fused quartz substrate (n = 1.45). We also studied the influence of the film thickness on the laser emission spectrum using PVK/DCM films on a glass substrate. When the film thickness is increased, the number of laser modes is increased (figure 6). The peaks correspond to propagating modes in a planar waveguide. Their wavelength is proportional to the effective indices of the guided modes in the polymer film [25]. Laser emission propagates under grazing incidence in the film and is reflected by the film–air and film–substrate interfaces. One mode appears with a 215 nm thickness film (figure 6(a)), two with

740 nm (figure 6(b)) and three with 1600 nm (figure 6(c)). The number of laser modes is limited by the stimulated emission bandwidth. 3.3. Index gratings We have studied laser emission of coumarin in PVK on a glass substrate. Excitation was performed with 355 nm wavelength, 1.2 mJ cm−2 intensity, and an incidence angle of around 33◦ . The film thickness was 430 nm. The laser emission spectrum is shown in figure 7. As can be seen in the spectrum, two laser lines with δλ ≈ 2 nm spacing appear in the emission spectrum. We attribute them to the DFB action of an index grating which opens a photon gap, as predicted by coupled mode theory [26]. In the small-gain limit, the width of the photon gap is δλ = δnλL

(2)

where δn is the amplitude of modulation of the index for second-order diffraction. We get δn ≈ 4 × 10−3 . Under larger pump intensities or prolonged excitation, a permanent grating which can be viewed using AFM microscopy is printed into the polymer film [25]. 281

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Figure 5. The effect of the refractive index difference between polymer and substrate on laser emission. (a) DCM in PMMA on a glass substrate, (b) DCM in PVK on a glass substrate and (c) DCM in PMMA on a fused-quartz substrate.

Figure 6. The evolution of the laser emission spectrum with the film thickness e, at a fixed incidence angle: (a) e = 215 nm, (b) e = 740 nm and (c) e = 1600 nm.

References

Figure 7. The laser emission spectrum of coumarin in PVK.

4. Conclusion

We have evidenced features related to optical confinement and waveguiding in polymer thin-film DFB lasers. Optical confinement is mandatory if electrically pumped polymer lasers are to be built [27]. Several luminescent polymers and waveguide configurations can be studied and evaluated easily using this technique [28]. The next step will be to draw permanent gratings in polymer films and to check their lasing properties. Interestingly, as has been demonstrated recently [29], permanent gratings also permit one to fabricate two-dimensional photonic structures.

Acknowledgment

Work performed at LETI was supported by ESPRIT-LTR project 28580 (LUPO). We thank Doctor Geoffrey Gale for the DFB setup used at LETI. 282

[1] Rochon P, Batalla E and Natansohn A 1995 Appl. Phys. Lett. 66 136 [2] Kim D Y, Li L, Kumar J and Tripathy S K 1995 Appl. Phys. Lett. 66 1166 [3] Fiorini C, Charra F, Nunzi J-M and Raimond P 1997 J. Opt. Soc. Am. B 14 1984 [4] Etil´e A-C, Fiorini C, Charra F and Nunzi J-M 1997 Phys. Rev. A 56 3888 [5] Lefin P, Fiorini C and Nunzi J-M 1997 Pure Appl. Opt. 7 71 Lefin P, Fiorini C and Nunzi J-M 1998 Opt. Mater. 9 323 [6] Fiorini C, Prudhomme N, Etil´e A-C, Lefin P, Raimond P and Nunzi J-M 1999 Macromol. Symp. 137 105 Fiorini C, Prudhomme N, de Veyrac G, Maurin I, Raimond P and Nunzi J-M 1999 Molecular migration mechanism for laser induced surface relief grating formation Synth. Met. at press Nunzi J-M, Fiorini C, De Veyrac G, Raimond P and Maurin I 1999 Evidence for photoinduced molecular migration mediated surface-relief grating formation in azo-dye polymers Mol. Cryst. Liq. Cryst. at press [7] Takada N, Tsutsui T and Saito S 1993 Appl. Phys. Lett. 63 2032 [8] Fisher T A, Lidzey D G, Pate M A, Weaver M S, Whittaker D M, Skolnick M S and Bradley D C C 1995 Appl. Phys. Lett. 67 1355 [9] Wittmann H F, Gruner J, Friend R H, Spencer G W C, Moratti S C and Holmes A B 1995 Adv. Mater. 7 541 [10] Kogelnik H and Shank C V 1971 Appl. Phys. Lett. 18 152 [11] Kuwata-Gonokami M, Jordan R H, Dodabalapur A, Katz H E, Schilling M L, Slusher R E and Ozawa S 1995 Opt. Lett. 20 2093 [12] Tessler N, Denton G J and Friend R H 1996 Nature 382 695 [13] Hide F, Diaz-Garcia M A, Schwartz B J, Anderson M R, Pei Q and Heeger A J 1996 Science 273 1833 [14] Sch¨ulzgen A, Spiegelberg Ch, Morell M M, Mendes S B, Kippelen B, Peyghambarian N, Nabor M F, Mash E A and Allemand P M 1998 Appl. Phys. Lett. 72 269 [15] Kozlov V G, Parthasarathy G, Burrows P E, Forrest S R, You Y and Thompson M E 1998 Appl. Phys. Lett. 72 144 [16] Lidzey D G, Bradley D D C, Alvarado S F and Seidler P F 1997 Nature 386 135

Polymer thin-film distributed feedback tunable lasers [17] Fichou D, Delysse S and Nunzi J M 1997 Adv. Mater. 9 1178 [18] Shank C V, Bjorkholm J E and Kogelnik H 1971 Appl. Phys. Lett. 18 395 [19] Maeda M, Oki Y and Imamura K 1997 IEEE Trans. Quantum Electron. 33 214 [20] Gale G M, Ranson P and Denariez-Roberge M 1987 Appl. Phys. B 44 221 [21] Huang J, Bekiari V, Lianos P and Couris S 1999 J. Lumin. 81 285 [22] Gromov D A, Dyumaev K M, Manenkov A A, Maslyukov A P, Matyushin G A, Nechitailo V S and Prokhorov A M 1985 J. Opt. Soc. Am. B 2 1028 [23] Gautier-Thianche E, Sentein C, Lorin A, Denis C, Raimond P and Nunzi J M 1998 J. Appl. Phys. 83 4236 [24] Delysse S, Nunzi J M and Scala-Valero C 1998 Appl. Phys.

B 66 439 [25] Nunzi J-M, Sobel F, Gindre D, Sahraoui B, Denis C, Dumarcher V, Fiorini C, Paci B and Rocha L 1999 Proc. SPIE 3796 [26] Kogelnik H and Shank C V 1972 J. Appl. Phys. 43 2327 [27] Gautier-Thianche E, Sentein C, Lorin A and Nunzi J M 1999 Opt. Mater. 12 295 [28] Kretsch K P, Blau W J, Dumarcher V, Rocha L, Fiorini C, Nunzi J-M, Pfeiffer S, Tillmann H and H¨orhold H-H 1999 Distributed feedback laser action from polymeric waveguides doped with oligo phenylene vinylene model compounds Appl. Phys. Lett. at press [29] Meier M, Mekis A, Dodabalapur A, Timko A, Slusher R E, Joannopoulos J D and Nalamasu O 1999 Appl. Phys. Lett. 74 7

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