Lanthanum Titanate LaTiO3 Optical Coating Material for Visible to Mid-wave IR Coatings

Applications

Lanthanum Titanate is a high-index, low absorption material usable for coatings in the near-UV (350 nm) to IR (>4 μm) regions. LaTiO3 deposited by IAD on substrates heated to at least 250°C has very low absorption (k ~0.001) to at least 5.5 μm in the mid-wave IR. In thin film layers it is useable to ~ 8 μm. No water absorption bands are present when deposited using a densifying process such as IAD. The index remains near 2 ± 0.1 with low dispersion in the 1 to 7 um region, making LaTiO3 useful as the high-index component in combination with SiO2 or as the low-n component with Ge. Dense, hard layers can be deposited by electron-beam evaporation for antireflection and multilayer filter coatings. Lanthanum

Titanate can be used in place of materials of similar index, namely, Tantalum Pentoxide or Zirconium Oxide in combination with Silicon Dioxide layers to form high indexcontrast multilayer structures. Advantages over Titanium Dioxide layers are significantly reduced absorption above 900 nm and

greater ease of evaporation. Hard, scratchresistant, low stress and adherent coatings can be deposited on glass and metal substrates.

Film Properties

Completely oxidized LaTiO3 films are absorption-free over the wavelength range below 400 nm (k<0.002 at 325 nm) to at least 4 μm. Evaporation causes a small oxygen loss and requires a partial pressure

of oxygen during reactive deposition. The use of IAD during deposition results in absorption-free layers at low substrate temperatures. Stress-free layers at least 1 μm thick have shown excellent adhesion to

Silicon, fused Silica, glass, and other oxide compounds. These coatings are hard, scratch resistant, and insoluble in boiling water. LaTiO3 produces an index ~2 at substrate temperatures less than that required for classical TiO2 processes. Additionally, the influence of the La on the TixOy system results in transparency higher than that of TiO2 below 400nm. Finally, whereas one fights high extinction coefficients with TiO2 (even without IAD) and stressed films for high layer counts (with Ta2O5), LaTiO3 offers more stable index, IAD feasibility and simplified reloading sensitivity than the classical TiO2 processes. The transmission curve presented below for a 543 nm thick layer on fused silica shows

behavior that indicates excellent index homogeneity. Inhomogeneity, evidenced by inconsistent half-wave (Tmax) values, is often indicative of mixed crystal phases in coatings of refractory oxide compounds

such as ZrO2, Nb2O5, and HfO2.

Advantages

• High-Index, low-absorption material

• Produces dense, hard layers

• Greater ease of evaporation over TiO2

Refractive Index

Refractive indices are dependent on the degree of oxidation and the film density achieved. Deposition processes for oxide compounds typically include IAD to increase the refractive index and to discourage crystalline growth, thus producing higher packing density. With Lanthanum Titanate, the film growth is highly dense and only a small index gain is achieved with IAD. Similarly, stability to moisture is high without IAD. Typical index values are plotted at right for films deposited with- and without-IAD. The addition of IAD increases the index by ~0.06.

Evaporation Parameters

Physical Properties of Solid Material

Forms and Sizes Available

LaTiO3 is available in pre-melted form, in shapes such as cones, rods or pieces for e-beam pockets.