Thin-film polyetherimides with controlled refractive indices

A series of soluble, fully aromatic polyetherimides were prepared as candidate materials for optical coating applications. Most of the new polymer coatings possessed high transparency in the optical and near-infrared spectral regions at thicknesses ranging from 1 to 10 microns. The refractive indices obtained ranged from 1.60 to 1.80 at visible wavelengths, with the highest values generally being obtained near 400 nm followed by a gentle decline as wavelength increased to 700 nm and beyond. The refractive index values could be controlled by varying the dianhydride and diamine composition. All of the polyimides showed good thermal stability to 400°C and displayed glass transition temperatures above 220°C, making them excellent candidates for device applications where increased refractive index and high optical clarity are desired. The paper will discuss the preparation and physical and optical properties of the polymers and compare them to other high index coating systems. Polymers having high indices of refraction have immense applications in optics and photonics due to their ability to reduce reflection losses at interfaces and, hence, increase light output. The use of semiconductor materials in photonics sources, detectors, and devices has increased the requirements for high index polymers in applications ranging from light-emitting diodes (LEDs) to planar light wave circuits. Most semiconductor materials have refractive indices in the range of 2.5 to 3.5. Optically clear polymers or epoxies that could significantly reduce reflection losses at the semiconductor-air interface do not exist because the index of refraction of most optical polymers or epoxies are in the range of 1.5 to 1.55. High-brightness LEDs have garnered much interest lately for myriad applications and potentially as replacement white light sources. Gallium nitride blue LEDs are typically grown on sapphire or silicon carbide substrates and as a result need some high refractive index medium between the chip and the lens to increase the light extraction efficiency. A polymeric material with an index of refraction in the range of 1.7 to 1.8 could definitely increase the light output from such packaged LED sources. Further, for most integrated optics or planar photonic circuits, the material of choice on semiconductor materials such as silicon, indium phosphide, and gallium arsenide is a high index polymer with high index contrast because of optimized losses and the capability of tuning. In addition, the mechanical characteristics of polymers allow them to be processed by unconventional techniques such as molding, casting, stamping, embossing, etc., which lead to rapid low-cost shaping for waveguide formation. High index polymers can also be used for beam splitting applications in diffraction gratings. The high index of refraction would decrease the optical thickness of the gratings, thus facilitating thinner grating structures. Because these materials can be spun in layer thicknesses ranging from a few hundred nanometers to as high as 10 microns, these high index polymers can be used as antiglare coatings for eyeglasses and other ophthalmic applications. We have previously reported a new class of amorphous aromatic polyetherimides that possess high refractive indices (1.62 to 1.78) at visible wavelengths. These solvent-soluble polyetherimides are eminently suitable for optical applications due to their excellent optical clarity at wavelengths greater than 400 nm. Also, the polymers are already imidized during the synthesis process, which provides additional advantages for optical device applications in that the high refractive indices are already "built-in" and the coatings only require complete solvent removal and in that the formation of voids that contribute to light scattering is avoided. Sensitivity to moisture and variations in temperature, as is typical in polyimide precursor solutions, are also precluded. These polymers also possess the usual characteristics of aromatic polyimides, such has high glass transition temperatures (>225°C), thermal stability (<5% weight loss at 400°C) and good mechanical strength. Thicknesses ranging from 1 to 10 microns were obtained with these polyetherimide systems. They were also easily coated on to substrates such as glass, quartz, and silicon with good adhesion.