Optical Materials 27 (2004) 199–201 www.elsevier.com/locate/optmat

Multimode distributed feedback laser emission in a dye-doped optically pumped polymer thin-film F. Sobel a, D. Gindre a, J.-M. Nunzi a,*, C. Denis b, V. Dumarcher b, C. Fiorini-Debuisschert b, K.P. Kretsch b, L. Rocha b a

b

Laboratoire Propri
et
es Optiques des Mat
eriaux et Applications, UMR-CNRS 6136, Universit
e d’Angers, 2 Boulevard Lavoisier, 49045 Angers, France Laboratoire Composants Organiques, DRT-LIST, DECS-SE2M, Commissariat a l’Energie Atomique, Saclay, 91191 Gif-sur-Yvette, France Accepted 9 March 2004 Available online 8 June 2004

Abstract We report on particular features of thin film distributed feedback (DFB) lasers. Devices are optically pumped using a Lloydmirror interferometer. For a given DFB grating period, the number of lasing modes is film thickness dependent. Spectral content of the devices is analysed using planar waveguide theory. An excellent agreement between the theoretical transverse electric mode structure and the laser emission spectrum is found.
2004 Elsevier B.V. All rights reserved. Keywords: Organic device; Polymer laser; Distributed feedback; Slab waveguide; Transverse mode

1. Introduction Distributed feedback (DFB) lasers have been extensively studied for applications in laser diodes [1]. First experiments in a DFB scheme were realized in solutions of organic dyes [2]. Organic materials have focused renewed research interest in recent years, aiming toward the all-organic laser diode [3–7]. DFB structures can be obtained by several processes such as lithography [7], UV-photolithography [8], or photo-induced surface-relief grooving [9]. Here we use a dynamical photo-induced DFB scheme where the grating is formed by interferences from the direct and reflected pumping pulses. Kogelnik and Shank’s [10] theoretical treatment of DFB lasers using coupled wave theory showed that optical feedback is then obtained from a pump beam induced spatial modulation of both gain and refractive index. We are interested in a spectral analysis of the DFB laser action. Tuning is simply achieved by changing the angle between two interfering pump beams. *

Corresponding author. Tel.: +33-241-735-364; fax: +33-241-735216. E-mail address: [email protected] (J.-M. Nunzi). 0925-3467/$ - see front matter
2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.03.009

Broad band tuning range has been demonstrated in several polymer materials using this configuration [11– 13]. In this paper, we study and discuss by means of linear planar waveguide theory the modal content of laser emission in DCM-dye-doped PVK-polymer films.

2. Experimental Thin films were composed of a polymer matrix (polyvinyl-carbazole -PVK-) doped by DCM laser-dye at concentrations ranging between 0.003 and 0.03 M. Films were deposited by spin-coating from chlorobenzene solutions onto glass substrates and then heated at 100 C for drying. Film thickness can be chosen between 200 and 3000 nm. DCM absorption in a PVK matrix peaks around 480 nm. Experimental set-up is shown in Fig. 1. The pump source is a frequency-doubled Q-switched mode-locked Nd3þ :Yag laser delivering 35 ps pulses at k ¼ 532 nm with a 1 Hz repetition rate. The laser beam is focused to a narrow stripe of 20 · 2000 lm using a cylindrical lens to increase pump intensity and improve directivity of the laser emission induced light propagation into the layer.

200

F. Sobel et al. / Optical Materials 27 (2004) 199–201

Fig. 1. Setup for DFB laser experiments. The Lloyd-mirror interferometer is composed of a sample mount and a planar edge mirror at right angle.

The pump source is used by means of a Lloyd-mirror interferometer. It consists of a plane mirror orthogonal to the sample. The emitted beam, coming out through the edge of the film, is directly collected by a plastic optical fibre (core diameter: 0.9 mm). The light is then dispersed and analysed with a 600 lines/mm grating spectrometer coupled to a cooled CCD camera. The pump beam is incident right to the corner of the Lloyd mirror. One part reaches directly the film while the other part overlaps the first one after reflection on the mirror. An interference pattern is created, and gain and index of refraction are modulated inside the beam impact area. Bragg condition [2] strictly selects the propagation constant b of the waveguided modes undergoing DFB laser emission. b is related to the grating period K by the angle a: b¼

2mp sina neff kp

the spectrum of the DFB laser at fixed angles, that means with fixed grating periods. From one to three laser modes appear within the stimulated emission band, as the film thickness is increased from 215 to 1600 nm [11]. All laser lines peak obviously at different wavelengths. The nonperiodic spacing between laser lines cannot be attributed to the free spectral range (FSR) of a DFB cavity. Therefore, these laser modes are not longitudinal modes. We linearly varied the grating period with a 5 min step and made a cross section of the emission spectrum for each angle (Fig. 2). For a 2.9 lm film thickness, seven modes appear and each laser mode can be tuned

ð1Þ

kp ð2Þ 2 sin a where neff is effective index of refraction of the material, kp is the pump wavelength, a is the angle between the pump incident axis and the longitudinal mirror axis, and m is the grating diffraction order (m ¼ 2 in our configuration). The Bragg condition determines the laser emission wavelength and the b-induced wavelength must fall inside the stimulated emission bandwidth to be amplified. K¼

3. Results and theoretical analysis Influence of the film thickness was investigated using a DCM-doped PVK film on a glass substrate. Indices of PVK and glass are 1.68 and 1.5, respectively. We studied

Fig. 2. Plot of laser emission wavelengths as a function of the incident angle a for a 2.9 lm-thick DCM-doped PVK film. Seven waveguided modes are tuned over 30 nm around 620 nm. Theoretical dispersion equations for TE modes in a planar asymmetric waveguide (continuous lines) are superimposed to the experimental data (circles).

F. Sobel et al. / Optical Materials 27 (2004) 199–201

over a 30 nm range, from 605 to 635 nm. Spacing between each line decreases linearly from blue to red. We performed a theoretical analysis of the modal content in the waveguide structure. The effect can be interpreted using the linear asymmetric planar waveguide theory. The Helmholtz equation must be solved for each layer of the asymmetric waveguide: oEix þ ðk02 n2i
b2z ÞEix ¼ 0 oy 2

tanðkx eÞ ¼

þ pk2x

1
pk0 p2 2

justifies the appearance of several TE laser modes. So the spectral structure of thin film DFB laser emission is a signature of the transverse modes waveguided into the film. This could make mode analysis of DFB laser emission a straightforward technique for the study of optical constants of luminescent polymer thin films, as an alternative to ellipsometric or m-line studies for instance.

ð3Þ

where k0 ¼ 2p , and ni ði ¼ 0; 1; 2Þ is the refractive index of k each layer, Eix is the transverse component of the electric optical field (TE) propagating in the ith layer, with respect to the propagation direction into the slab and bz is the longitudinal component of each wave vector ðb2 ¼ b2z þ kx2 Þ. Solutions of these coupled equations are well established. Consideration of the boundary conditions at the interfaces gives rise to the transcendental mode equation related to the transverse electric guided modes [14]. They depend on the transverse wave vector component kx as p0 kx

201

ð4Þ

x

where pi2 ¼ b2z
k02 n2i , kx2 ¼ k02 n21
b2z . The dispersion equations can be obtained by a numerical determination of kx (Fig. 2). These curves determine the different effective refractive indices of each mode with regards to the incident monochromatic wave. The DFB laser emits waves inside the planar waveguide, and the interference grating selects different wavelengths. To compare theoretical and experimental results, we replaced b in the dispersion equations by the angle a using Eqs. (1) and (2), and superimposed both results (Fig. 2). There is a good agreement between drawings. This reveals the particular features of DFB laser emission in thin films.

4. Conclusion The spectral structure of a dynamic photo-induced organic DFB laser in a dye-doped polymer thin film was investigated and analysed. Those lasers are tunable over thirty nanometers and can emit multimode spectra. A theoretical study using planar waveguide properties

Acknowledgements We thank R. Chevalier and J.P. Lecoq for technical support. Part of the work performed at Saclay was supported by the European Union ESPRIT Project No. 28580, LUPO: ‘‘A novel approach to solid state short wavelength laser generation using luminescent polymers’’.

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