## 3 Optical Characterization Measurement Results  
### Photonic Crystal Transmission Spectra  
Optical characterization of the a-Si PhC cavities was performed using an end-fire measurement set up; see Figure 3 for schematic. To measure the optical transmission spectra of the PhC cavities a broadband Amplified Spontaneous Emission (ASE) source (Amonics ALS-CL-15-B-FA, [[https://www.amonics.com/product/39](https://www.amonics.com/product/39)]([https://www.amonics.com/product/39](https://www.amonics.com/product/39))). was coupled to the polymer waveguide over the a-Si PhC cavity via a circulator, free-space optics and polarizing beam splitter. Light transmitted through the PhC cavity was collected from the polymer waveguide after propagation through the second lens system. Optical power was measured on a power meter and optical spectra were measured on an optical spectrum analyzer with a spectral resolution of 50 pm. Temperature of the a-Si PhC cavity was maintained at \(T\,=\,25\,^{\circ}\)C by an external temperature controller.  
Measurements of the transmission spectra of the PhC cavities show that resonances occur in the C and L bands relevant for optical telecommunications, see Figure 4. Spectrum of the PhC cavity with lattice spacing of 386 nm shows resonant reflection taking place at \(\lambda\,=\,1567.14,1558.65\,\)nm, 1548.43 nm, and 1536.60 nm with measured Q-factors of 15,862,4607, 3091, and 1176 respectively.  
Measurement of the DA PhC cavity transmission spectra show that multiple resonances can be obtained and that the position of these resonances varies with the PhC lattice constant; see Figure 5. Shift in resonance wavelengths with PhC lattice constants show that it is possible to have fine control over optical characteristics of the a-Si PhC cavity. Given the level of control that is available it should be possible to deploy a-Si PhC cavity in a wavelength-division multiplexing (WDM) application. Linear fits of the resonant wavelength versus PhC lattice constant data for each mode show that the average slope of the fits is computed to be \(\Delta\,\lambda/\Delta a\,=\,-1.4\,\pm\,0.6\). The average measured Q-factor for the  
Figure 3: (a) End-fire set up used to characterize PhC cavity transmission and reflection spectra. (b) End-fire set up used to characterize operation of hybrid external cavity laser.  
Figure 2: Schematic of the fabricated PhC cavity showing polymer waveguide sitting on PhC cavity.  
resonances shown in Figure 5 is \(Q\,=\,4294\), but measured Q-factors as high as \(Q\,=\,\)15,862 can be obtained; see Figure 4. In [15] it was reported that a similar DA PhC with lattice constant of 388 nm fabricated on c-Si had resonant wavelengths at \(\lambda\,=\,1539\) nm and \(\lambda\,=\,1547\) nm, it can be seen from Figure 5 that for the same DA PhC with the same lattice constant, but different hole radius, the a-Si PhC has resonant wavelengths at \(\lambda\,=\,1535\) nm and \(\lambda\,=\,1543\) nm, the variation in resonant wavelengths for the DA PhC can be attributed to different hole radius values, \(r\,=\,108\) nm in the case of c-Si and \(r\,=\,95\) nm in the case of a-Si, and differences in refractive index between a-Si and c-Si.