# Single-Mode Polymer Ridge Waveguide Integration of Organic Thin-Film Laser
## 4 Device Fabrication  
First, a Polydimethylsiloxane (PDMS) stamp was made for replication of the DFB gratings. The grating area contains five DFB gratings arranged in parallel, with the dimension of 2 mm in width and 10 mm in length. The periodicity of the gratings was 195 nm to 215 nm with a period spacing of 5 nm. After the fabrication of the PDMS stamp, the bottom cladding with the grating structures was fabricated by a UV nanoimprint process. In this step, a mixture of EpoClad and \(\gamma\)-Butyrolactone (GBL) with a mixing ratio of 2:1 was used. The GBL diluted EpoClad was spin-coated onto a silicon wafer with 4000 rpm for 30 s and then soft baked at 120 \({}^{\circ}\)C for 2 min on a hot plate. After the soft bake, the PDMS stamp was brought into contact with the still sticky EpoClad. A subsequent flood-exposure  
Figure 3: PL spectra of Alq\({}_{3}\) and Alq\({}_{3}\):DCM2 at different doping concentrations.  
starts the UV-induced cross-linking of the polymer. After that, the substrate was baked again on a hot plate at 120 \({}^{\circ}\)C for 3 min. After the EpoCLad was completely polymerized, the silicon-PDMS stamp set was taken from the hot plate and the PDMS stamp was removed without any residue. The layer thickness of EpoCLad was about 1 m. Figure 4a shows the profile of one of the gratings with \(\Lambda=195\) nm and Figure 4b shows a good replication quality. This image was taken with an atomic force microscope. The measured average periodicity is 194.97 nm, which is extremely close to the desired value.  
The next fabrication step is structuring the waveguides onto the DFB gratings. In this case, a mixture of EpoCore and \(\gamma\)-Butyrolactone (GBL) in a mixing ratio of 2:1 was used. The mixture was spin-coated with 4600 rpm for 30 s onto the substrates with the pre-structured cladding layer. The substrates were first baked at 50 \({}^{\circ}\)C and then at 90 \({}^{\circ}\)C for 2 min each followed by a UV-lithography step using a chromium mask with the waveguide structures. After that, the substrates were put on the hot plate again for 2 min at 50 \({}^{\circ}\)C and for 3 min at 85 \({}^{\circ}\)C for a post-exposure bake. The non-cross-linked EpoCore was removed by dipping it for 10 s into the mr-Dev 600 developer. Finally, the substrates were rinsed in isopropyl alcohol to stop the development process. Figure 5 shows schematic sketch of the final device.  
Before starting the measurements, the final device was carved at the break points with a diamond scriber so that the silicon wafer split apart along its crystal lattice. Figure 6 presents the SEM images of the EpoCore waveguide on top of the DFB gratings.  
Figure 4: **(a)** Grating profile; and (b) 3D AFM image of the imprinted DFB grating with \(\Lambda=195\) nm into EpoCLad.  
Figure 5: Schematic sketch of the final device after the lithography process containing DFB grating and waveguide structures.  
The figures show a good waveguide integration on top of the DFB gratings as well as a satisfying waveguide end facet for the optical analysis of the integrated waveguide laser. Figure 6a also gives a detailed look at the waveguide and shows some irregularities and an uneven surface. This can probably be addressed to the grating structure below. The waveguides outside this grating area show good optical quality and a smooth surface (cf. Figure 6b). After the fabrication of the DFB gratings and the waveguides, 200 nm of the guest-host laser active material system Alq\({}_{3}\):DCM2 with a DCM2 doping concentration of 6% was co-evaporated in an ultrahigh vacuum chamber with \(p<10^{-8}\) mbar by organic molecular beam deposition method.