1. Introduction

After the first report of the organic semiconductor lasers [1], optically pumped polymer lasers have been studied extensively [2–10]. Compared with conventional lasers, the thin-film polymer lasers possess the advantages of low pump threshold, high compactness, and easy design for the optical integration. Realization of the electrically pumped organic lasers attracts broad research interests while faces many physical and technical challenges [11]. The distributed feedback (DFB) structure is considered as the most important configuration for the thin-film polymer lasers [11–13]. Two kinds of configurations of the DFB resonators are commonly employed: (1) The Bragg grating structures are written into the polymer, so that the polymer material acts both as the DFB resonator and as the gain medium [14–19]. (2) The polymer is spin-coated onto the top of the grating structures [7, 20-21]. In both cases, the active material is discontinuous or strongly modulated in thickness. In particular, if this kind of DFB configuration is used in the electrically pumped laser device, some unavoidable problems may be encountered due to the thickness modulation of the active medium, including inhomogeneous injection current density, short-circuit problems, charge trapping or scattering at the dramatically spatial modulation at the polymer-grating interfaces, and the inefficient utilization of the gain volume. In this paper, we propose a DFB configuration of active waveguide grating structures (AWGS) for the polymer lasers, where a typical light-emitting conjugated polymer poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(1,4-benzo-{2,1’,3}-thiadiazole)] (F8BT from Sigma Aldrich) is used as the active medium and is spin-coated onto the silica substrate to form the waveguide, whereas, a photoresist grating is fabricated on top of the active waveguide using interference lithography. Based on the homogeneous and continuous thin film properties of the gain material, laser emission in the green is obtained with very narrow linewidth. This kind of configuration is proposed to be of particular importance for the design of electrically pumped thin-film polymer lasers.

2. The AWGS for the design of polymer lasers

Figure 1(a) shows the design of the AWGS for the realization of DFB surface-emitting polymer lasers. The solution of F8BT in chloroform with a concentration of 20 mg/ml is spin-coated onto the glass substrate with an area of 20×20 mm² and a thickness of 1 mm, forming high-quality thin film with a thickness of about 200 nm. The grating structures on top of this active polymer waveguide layer is fabricated using interference lithography, which are written directly into the photoresist (PR) (S1805 from Rohm & Haas) with a modulation depth of about 200 nm and a period of 350 nm, as shown in the atomic force microscopic image (measured by Agilent 5500) in Fig. 1(b). A diode-pumped frequency-tripled (355 nm) solid-state laser with a pulse length of 500 ps and a repetition rate of 6.25 kHz has been employed to perform the interference lithography process and to pump the resultant polymer laser.

 

Fig. 1 (a) The design of the AWGS. (b) The AFM image of the grating structures.

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To understand the advantages of our proposed laser configuration, we can make a comparison between the AWGS with the conventional design with the active medium spin-coated on top of the DFB grating. It is understandable that as the polymer layer is spin-coated directly onto the flat substrate, instead of onto the grating structures, the AWGS provides high-quality active waveguide which favors homogeneous, stable, and strong confinement of the oscillation modes in the microcavity formed by the DFB-waveguide mechanism. Moreover, the structural parameters of both the active waveguide and the grating can be controlled and characterized precisely and independently for the AWGS, enabling more quantitative evaluation and understanding of the laser actions. However, in the conventional configuration with the active polymer spin-coated onto the grating structures, the active layer of polymer is modulated inevitably by the grating structures, which depends strongly on the thickness of the polymer layer and modulation depth of the grating, as has been demonstrated in Ref [6]. and Ref [10]. Furthermore, theoretical analysis in section 3 indicates that the AWGS configuration facilitates stronger confinement and much better oscillating modes in the active medium than the conventional one, and the AWGS is less sensitive to the defects or inhomogeneity in the grating structures.

The AWGS configuration potentially provides a promising design especially suitable for the electrically pumped polymer lasers. This can be understood by considering the following mechanisms. The AWGS configuration may reduce significantly the probability of the short-circuit problems and facilitate homogeneous injection current density, as well as homogeneous excitation of the active medium, as compared with the conventional design with the active polymer coated on top the grating structures. However, the electric excitation may be modulated strongly due to the strong modulation in the thickness of the polymer layer and in the grating depth in the conventional design. The interfacial area between the grating structures and the polymer may be reduced significantly in the AWGS as compared with the conventional design. This may undoubtedly reduce significantly the charge trapping and fluorescence quenching effects induced by the grating structures.

3. Theoretical modeling

In the reported DFB polymer lasers, the light-emitting polymer is generally coated on top of the grating structures, as shown in Fig. 2(a) . According to the simulation, the eigen modes are distributed partially in the active waveguide and partially in the grating structures, as shown in Fig. 2(a). However, for the AWGS configuration, the field distribution of the eigen mode is confined almost completely in the active waveguide layer, as shown in Fig. 2(b). This implies much more efficient utilization of the active volume and more efficient extraction of the pump energy, as well as much better oscillation modes, in the AWGS configuration.

 

Fig. 2 The eigen modes (Electric field energy) of the DFB design with (a) the active layer spin-coated on top of the grating structures and (b) the grating fabricated on top of the continuous active. The color spots denote the.

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In the above simulations, the polymer layer has a thickness of 300 nm in Fig. 2(a) and 200 nm in Fig. 2(b), and the PR grating has a period of 350 nm and a modulation depth of 200 nm. Thus, the active polymer layer is about 100 nm thicker than the modulation depth of the grating in Fig. 2(a). The substrate is assumed to be made of silica and the medium on top of the laser device is air.

It is known that the surface roughness and the thickness of the spin-coated active layer above the grating structures are not easy to control. However, for the AWGS configuration, the surface roughness and the thickness of the active material layer can be easily characterized and precisely controlled, as shown in Fig. 2(b). This explains why the polymer laser based on AWGS is potentially more advantageous over the conventional configurations.

Furthermore, the DFB configuration that the conjugated polymer is spin-coated onto the grating structures is more sensitive to the defects or additional modulations in the grating structures. Figure 3 compares the sensitivity of the conventional configuration in (a) and the AWGS configuration in (b) to the identical defects in the grating structures. It can be seen clearly that any defects or inhomogeneity of the grating structures may destroy the mechanisms for DFB in the conventional design shown in Fig. 3(a). However, for the AWGS, the DFB mechanisms are almost not disturbed by defects or additional modulations. Thus, the simulation results in Fig. 3 show convincingly the advantages of the AWGS configuration over that shown in Fig. 2(a) or Fig. 3(a).

 

Fig. 3 The response to the defects in the grating structures of (a) the configuration with the grating underneath the active polymer layer and (b) the AWGS. The color spots denote the TE electric energy distribution.

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Full-wave simulations using software COMSOL Multiphysics based on the finite element method were employed to demonstrate the energy distribution of the optical electric field in Fig. 2 and Fig. 3. The calculation was done for solving the eigen modes using “RF(radio frequency) Module/In-Plane Waves/TE Waves”, which means investigation of the interaction of the electromagnetic fields with matters (periodical structures of dielectric materials in this work) in the plane of incidence with the light wave polarized perpendicular to the plane of incidence (TE polarization). The solver was chosen as “Eigenfrequency” to find out the eigen modes of the AWGS. The periodic boundary condition was used to both sides of the truncate computation domain, and the scattering boundary condition was used to the top and bottom of the computation domain. All of the eigen modes in Fig. 3 and Fig. 4 have been characterized at 563.8 nm according to the laser emission. The TE polarization has been employed with the light wave polarized parallel to the extending direction of the DFB grating, which has been based on the polarization direction of the pump laser at 355 nm.

 

Fig. 4 (a) The molecular structure of the active polymer semiconductor F8BT. (b) The absorption and photoluminescence spectra of F8BT.

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4. Polymer laser based on AWGS

The 355 nm pulsed laser used for interference lithography, as described in section 2, is used as the pump source of the polymer laser, which is incident at an angle of about 20° and is focused into a spot as large as 400 μm in diameter on the F8BT film. The pump pulse energy is adjusted by an attenuator wheel. Figure 4 shows that F8BT has an absorption spectrum peaked at about 470 nm and a photoluminescence spectrum centered around 560 nm.

Figure 5 shows the performance of the surface-emitting polymer laser under the excitation of the 355-nm pulsed laser with the specifications listed above. Figure 5(a) shows the photograph of the green-emitting polymer laser based on AWGS, where the radiation energy is confined almost completely in the center of the vertical rainbows, as can be observed, implying much improved transverse mode as compared with that in the conventional design. This can be confirmed by the experimental data in Fig. 5(b), where a control laser device is fabricated using the conventional design and the same polymer, and a comparison is made for the transverse modes measured at the same distance from the laser device between the lasers based on the AWGS (left panel) and the conventional design (right panel). The half divergence angle of the beam from the AWGS laser was measured to be about 0.44°, whereas, that of the conventionally designed laser device on the right panel was measured to be as large as 2°, implying significantly improved transverse mode by the AWGS configuration.

 

Fig. 5 (a) Photograph of the surface-emitting polymer laser in the green. (b) Comparison of the transverse modes between the lasers based on the AWGS (left panel) and the conventional configuration with the active polymer spin-coated on top of the grating structures (right panel).

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Figure 6(a) shows the spectra of the polymer laser emission at different pump fluences measured using an Ocean optics Maya 2000 PRO spectrometer, and Fig. 6(b) shows the laser emission intensity as a function of the pump fluence, which implies a pump threshold of about 115 μJ/cm². The laser emission is centered at about 563.8 nm with a linewidth of about 0.5 nm at full width at half maximum, as shown in detail in the inset of Fig. 6(a). However, the spectrometer has a resolution of 0.2 nm. Therefore, the practical spectral linewidth of the laser radiation is smaller than 0.5 nm. The narrow linewidth implies excellent oscillation modes in the DFB cavity.

 

Fig. 6 (a) Measured spectra of the lasing emission at different pump fluences. (b) Output intensity of the polymer laser as a function of the pump fluence, indicating a pump threshold of about 115 μJ/cm².

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It should be noted that the pump threshold of our laser is still slightly higher than that of the previously reported results using the same polymer while different DFB configurations [16,19]. However, our fabrication of the AWGS using interference lithography into S1805 photoresist is apparently not the best technique for realizing the AWGS laser. Interference lithography process introduces impurities into the laser medium during the fabrication process, including any substance in the photoresist and in the developer solutions. The water washing process may introduce dirty and surface problems. All of these may reduce the emission efficiency of the polymer. Thus, optimization of the fabrication technique is crucial and is still a challenge for the successful realization of the AWGS configuration. Furthermore, the 355 nm excitation is not within the correct absorption band of F8BT (peaked at about 470 nm) due to our lack of correct pump laser source. Furthermore, the absorption of F8BT at 355 nm is only about 30% the peak absorption at about 470 nm. Thus, we believe that our AWGS laser device can be improved to have similar or even better performance than the reported. However, the potentially more suitable configuration for the design of electrically pumped laser device implies particular importance of this laser design.

5. Summary

In summary, the AWGS are demonstrated as a DFB configuration for polymer lasers, where the grating structures are fabricated on top of the polymeric active laser medium. This kind of configuration is potentially more suitable for the design of electrically pumped organic DFB lasers. This is based on the homogeneous polymer film without thickness-modulation by the grating structures, the reduced probability of short-circuit problems, the small and flat contacts between the grating and the polymer. Theoretical simulations also show strong confinement of the eigen modes of the AWGS in the active polymer layer. Although the performance of the laser is currently not as excellent as previously reported, there is large space for the improvement if the fabrication technique and the structural parameters are optimized further.