2.5D Inkjet Printing Technique for Fabricating Precision Optical Components

There have been many breakthroughs with optical 3D printing, and now researchers in Switzerland are introducing a new one. The team, from the Swiss Center for Electronics and Microtechnology (CSEM), developed an inkjet printing technique for manufacturing precision optical components, like waveguides, which allow a signal to propagate without a lot of energy loss by restricting the expansion of the wave.

But here’s a twist – the method is actually considered to be 2.5D printing, and not 3D printing, because the printed structures, while not flat, have limited complexity when compared to 3D printed ones.

“Inkjet printing is a very attractive method for fabricating optical components because the positions and sizes of features can easily be modified and there is virtually no material waste. However, the surface tension of the inks makes it difficult to print lines with a specific height, which is necessary to create a waveguide,” said Fabian Lütolf, a member of the CSEM team.

A series of droplets called pinning caps are printed (white). Liquid bridges between the caps are then formed by ink deposited in the second print. The caps immobilize the ink and prevent the formation of bulges in the line. This can also connect three or more junctions to make corners or sharp edges.

Inkjet printing, typically a one-step process, deposits a pattern of drops onto a substrate through tiny nozzles. But the CSEM team discovered that lines could be printed with smoother features, and at a specific height, by depositing the ink in two steps. Their technique can print 2.5D optical waveguides and tapers made of acrylic polymer, and other materials like metallic inks.

Pius M. Theiler, who’s now at ETH Zurich, Lütolf, and team leader Rolando Ferrini published a paper on their work, titled “Non-contact printing of optical waveguides using capillary bridges,” in the journal Optics Express.

Inks deposited on a substrate can split or bulge due to the liquid’s surface tension. But this two-step process can turn this issue into an advantage – the ink printed in step #2 self-aligns between the droplets from the first print, in order to lower its surface tension. This also means that the researchers did not have to pre-pattern the substrate, which is necessary with other inkjet printing techniques; this makes fabrication simpler and increases available design space.

The new technique first prints a series of spherical droplets, called pinning caps, which then pin down liquid bridges made from the ink of the second print. This configuration of droplets stops the ink from moving, which prevents bulges from forming in the printed line. This method can also connect several junctions to form sharp edges and corners, and also has several advantages over classical photolithography, often used to make tiny components on chips.

The researchers used an inkjet printing method to create this waveguide. Sending laser light through the waveguide (red) allowed them to measure the waveguide’s optical properties.

Lütolf explained, “Inkjet printing doesn’t require a physical mask like photolithography and it is easier to connect components. Also, if you just want to quickly test an idea or vary a parameter, additive manufacturing methods such as inkjet printing only require adaption of the digital design.”

A comparison of printed features with standard, one-step inkjet printing (a-g), as theoretically calculated for the new two-step inkjet printing method (h-n) and the actual print thereof (o-u). Scale bar = 200 micron.

The team created a polymer waveguide, measuring 20 microns wide and 31 microns tall, with a taper to allow light from from an external laser to enter the waveguide, in order to evaluate the new method. Optical loss within the waveguide was measured at  0.19 dB/cm – just an order of magnitude higher than photolithography-created waveguides.

“In the paper, we report the first inkjet-printed waveguides with loss characterization. For the applications we envision, the waveguides would carry light for short distances, and not across entire networks,” said Lütolf. “The current level of losses can be tolerated for such applications.”

The researchers say that the smallest waveguides consists of a solitary ink droplet, and that its size is limited by the printer’s nozzle. But the narrowest waveguides possible from the printer in their study were in the 40-micron range, with a height of about 10 micrometers – limits that are similar to what’s possible using industrial inkjet printers on the market.

Lütolf said, “With our current combination of materials and hardware, it’s not possible to make waveguides below 10 micrometers, as typically required for single mode operation. But we are close. There is, however, no fundamental physical limit that would prevent us from printing single mode waveguides.”

He said that multiple groups have been able to demonstrate capabilities, using methods like electrohydrodynamic printing (E-jet), in the submicron range. So, in the future, the researchers could combine these systems with their inkjet printing technique in order to make single mode waveguides.

Their new method can also be used to print electronics and microfluidics, which could help advance multiple applications, like lab-on-a-chip devices and optical sensors. According to Lütolf, while we already see commercial printing of electronics, it’s more difficult to print microfluidics.

“The fact that our 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,” Lütolf explained.

Right now, the team is working to optimize the new method, and the ink, in order to reduce how much light is lost by the waveguide. In addition, they hope to commercialize the process in the future, as well as make it more applicable for large-scale printing.

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