Materials for the recording of thin holographic grating couplers

Two types of recording media are investigated for the recording of thin and highly asymmetric holographic grating couplers: (1) commercial photoresists for thin surface relief gratings, and (2) dichromated gelatins (DCG). As a result of the investigations on the photoresists (Shipley S1805), we found that very high diffraction efficiencies (DE) of over 80% are obtained into the first order, when reconstructed at optimal geometrical conditions diffracting the output beam into free space with visible light at 514 nm. However, when readout is at normal incidence and the output beam is diffracted into the substrate, two significant changes occur. Namely, the incident light is now diffracted into both diffracted first orders, with a ratio varying between about 2 and 6 between the two first orders, depending on the exposure energy. The angular bandwidths of these thin blazed gratings were very large (>30°); yet in a double grating substrate-mode configuration, the bandwidth was significantly reduced to about 10°, indicating that the resultant DE is the product of the DEs of the two gratings. The DCG layers and processing procedures, which were optimized for recording at 514 nm in thin layers of 2–3 μm yielded the highest DEs, approaching 90%, indicating that relatively large refractive index modulations, reaching at least 0.05 are stored in these materials. Under substrate-mode conditions, we obtain DEs of over 70% into only one first order. As expected, these thin holographic volume (Bragg) grating couplers exhibited large angular bandwidths. As the bandwidths did not exhibit any significant exposure dependence, this indicates that in the investigated exposure ranges (50–100 mJ/cm²) the gratings are recorded within the entire depth of the DCG layer.