**3.3 Laser chip fabrication**

The laser chip consists of three layers, the substrate, the cladding, and the polymer matrix. PMMA was selected as the polymer matrix because of the solubility of the dye in PMMA, as well as its low optical absorption within the wavelength range for activating the dye molecules, and its excellent properties for nanoimprint lithography.

To construct the dye laser, a glass substrate (SiO2) was spin-coated with Cytop, a lowrefractive-index material (n=1.34) as the lower cladding to ensure the vertical optical confinement. After an oxygen plasma treatment to improve the adhesion of Cytop to the PMMA, dye-doped PMMA (n=1.49) was spun on top of the Cytop layer to serve as the gain medium.

The Cytop and PMMA preparation process for the nanoimprint process is summarized in Figure 4. We began the fabrication process by depositing a 5 µm thick layer of Cytop (CTL-809M, Asahi Glass) on a silicon dioxide substrate. The deposition of the Cytop was accomplished via a series of spinning and thermal curing steps to ensure flatness and uniformity over the wafer.

First, we spun the Cytop on the substrate at 1500 rpm (adhesion promoters were not necessary). Next, the Cytop was baked at 65 °C for 60 s, 95 °C for 60 s, and 180 °C for 20 min. The ramping of the bake temperature was critical in attaining flat and uniform surfaces. The spinning and baking steps were then repeated two more times, with a final bake at 180 °C for 3 hours. After the chip cooled down, an oxygen plasma treatment (Anatech SP100) of the Cytop was necessary for the adhesion of Cytop to PMMA. We exposed the oxygen plasma to Cytop at an RF power of 80 W and O2 pressure of 200 mTorr for 30 s.

Next, dye (Rhodamine 640, Exciton)-doped PMMA (30 mM) was spin-coated on top of the Cytop layer at 500 rpm for 15 s and then 5000 rpm for 1 min. This produced a dye-doped polymer thin film with 600 nm thickness as the gain medium. A prebake at 170 °C for 2 min before the nanoimprint process ensured solvents were evaporated and improved the adhesion between the Cytop and PMMA. Then the substrate was ready for the nanoimprint process to define the laser cavity structure.

Fig. 4. The schematic procedure of the Cytop and PMMA substrate preparation process
