# Single-Mode Polymer Ridge Waveguide Integration of Organic Thin-Film Laser
## 3 The Organic Thin-Film Laser  
We used the well-known guest-host active organic laser material containing Tris-(8-hyd-roxquinoline)aluminum (Alq\({}_{3}\)) as the host material and the laser dye 4-(Dicyanomethylene)-2 -methyl-6-judloidyl-9-enyl-4H-pyran (DCM2) as the guest material (see Figure 1b). Figure 3 shows the photoluminescence spectra (PL spectra) of Alq\({}_{3}\) and Alq\({}_{3}\):DCM2 with different DCM2 doping concentrations. The samples were evaporated and characterized on glass substrates.  
Figure 2: TE0 and TE1 mode distribution with: **(a,c)** air as upper cladding medium; and **(b,d)** EpoClad as upper cladding medium.  
Figure 3 shows a redshift in the emission spectra of the Alq\({}_{3}\):DCM2 samples with increasing DCM2 doping concentration. The reason for this is the Solid State Solvation Effect (SSSE). It describes the polarity influence of the host matrix on the emitter molecule [8]. Furthermore, Figure 3 also shows a decreased Alq\({}_{3}\) emission in the PL spectra with increasing doping concentration of DCM2. The fraction of Alq\({}_{3}\) emission is about 18% (see dashed line in Figure 3) with the DCM2 doping concentration of 1%. When the doping concentration increases to 3%, the fraction of Alq\({}_{3}\) emission drops to 5%. With further increasing of the doping concentration, as shown in Figure 3, the Alq\({}_{3}\) emission fraction in the PL spectra drops even more. This is due to the Forster resonant energy transfer [9; 10] where with decreasing donor/acceptor mean distance the probability of a resonant energy transfer increases. The great advantages of this material system are the broad optical laser tuning range of up to 115 nm, low lasing threshold down to approximately 3 J/cm\({}^{2}\), and high optical gain, which saturates at about 300 cm\({}^{-1}\)[11; 12; 13]. Moreover, the Alq\({}_{3}\):DCM2 thin-film is characterized by its absolutely smooth surface after the vapor deposition process. The surface roughness was measured to be R\({}_{\text{a}}=950\) pm (arithmetic mean value) and R\({}_{\text{q}}=1.2\) nm (root mean square value). However, surface roughness, morphological variations as well as the pump process itself can cause slightly stochastic fluctuations and noise in the signal. Therefore, in the spectral analysis, the Karhunen-Loeve Transformation can be applied [14]. Different kinds of resonating structures could be demonstrated to achieve lasing in organic thin-films, such as DFB resonators, micro disks, spheres, and distributed Bragg reflectors (DBR) [15; 16; 17; 18; 19]. The DFB resonator, however, stands out because of its ease of fabrication and its optical properties such as the extraordinary wavelength selectivity and high efficiency. The emission wavelength (\(\lambda_{\text{Bragg}}\)) of the distributed feedback laser can be varied by choosing the suitable grating periodicity \(\Lambda\). It follows the Bragg condition:  
\[\lambda_{\text{Bragg}}=\frac{2\cdot n_{\text{eff}}\cdot\Lambda}{m}, \tag{1}\]  
where \(n_{\text{eff}}\) is the effective refractive index of the allover waveguide structure, \(\Lambda\) is the grating period, and \(m=1,2,3,...\), is the diffraction order.