Optical Properties of Hydrogenated Amorphous Carbon (a-C:H) - a Hard Coating for IR-Optical Elements

Hydrogenated amorphous carbon films a-C:H were deposited on glass and germanium substrates. The films are transparent in the IR and are extremely hard (Mohs's hardness of about 8). The a-C:H coatings were prepared according to the method of Holland in an RF excited discharge sustained by various hydrocarbon gases. Thickness, density, refractive index (at 0.3 μm and 2 – 10 μm) and relative hydrogen content were determined. Variations in IR-refractive index and relative hydrogen content could be correlated with deposition conditions. With a refractive index of approximately 2 a-C:H is an ideal AR-coating for germanium (n = 4). Laser calorimetric measurements of optical absorption at 10.6 μm give a loss as low as 4% for a 1.3 μm thick coating on germanium (λ/4 for n = 2 at 10.6 μm). Preliminary damage tests with a CO2 laser (600 W, cw) were performed.