Researchers have developed an inkjet printing technique that can be used to print optical components such as waveguides. Because the printing approach can also fabricate electronics and microfluidics, it could advance a variety of devices such as optical sensors used for health monitoring and lab-on-a-chip devices that integrate and automate multiple laboratory functions onto a chip.
Inkjet printing is an additive manufacturing technique that uses tiny nozzles like the ones found in desktop inkjet printers to deposit a computer-generated pattern of drops (the “ink”) onto a substrate to build a structure. The researchers discovered that depositing the ink in two steps, rather than the traditional single step, enabled printing of lines with a specific height and with much smoother features than would otherwise be possible. The printed structures are considered to have 2.5 dimensions because although they are not flat, their complexity is limited compared to structures created with traditional 3D printing.
The researchers show that their technique can be used to print 2.5D optical waveguides and tapers made of acrylic polymer. The printing concept can also be used with other materials such as metallic inks to make electronics or sucrose mixtures for biodegradable applications.
Although printing of electronics is already used commercially, printing micro-fluidics is more challenging and prone to the same problems as waveguides. The fact that this approach could allow components with multiple functionalities to be fabricated with a single printer paves the way toward additive manufacturing of entire integrated circuits on chips. This means that optical components could be added to flexible hybrid electronics and that optoelectronic components such as light sources or detectors could be integrated into printed optical circuits.
Because of surface tension, inks deposited on a substrate tend to bulge or split. Depositing the ink in two steps allowed the researchers to turn the surface tension of the liquid into an advantage. After depositing a series of droplets, the ink printed in the second step seeks to minimize its surface energy by self-aligning between the droplets from the first print. Unlike previous inkjet printing approaches, the researchers did not have to prepattern the substrate, which increases the available design space and simplifies fabrication.
The new technique offers several advantages over classical photolithography, which is typically used to make tiny components on chips. Inkjet printing doesn't require a physical mask as with photolithography, it's easier to connect components and if you just want to quickly test an idea or vary a parameter, additive manufacturing methods such as inkjet printing simplify the process.
To evaluate the new printing method, the researchers created a polymer waveguide that was 120 microns wide and 31 microns tall with a taper that allowed light from an external laser source to enter the waveguide. They measured the optical loss within the waveguide to be 0.19 dB/cm, only an order of magnitude higher than state of the art waveguides created using photolithography. According to the researchers, the smallest possible waveguides consist of a single droplet of ink, the size of which is limited by the nozzle of the inkjet printer. For the printer used in the study, the narrowest waveguides would be in the 40-micron range with a height of around 10 micrometers. Typical industrial inkjet printers have similar limits.