Small Waveguides for Wearable Displays

Researchers optimized a laser writing process to create extremely narrow waveguides in a type of silicone (PDMS). The new waveguides could be used to transport light in a variety of sensors, lab-on-a-chip systems and wearable devices. (Source: Y. Pu, EPFL)

For the first time, researchers have fabri­cated waveguides just over one micron wide in a clear silicone commonly used for bio­medical appli­cations. The tiny, flexible wave­guides can be used to make light-based devices such as bio­medical sensors and endo­scopes that are smaller and more complex than currently possible. “To the best of our knowledge, these are the smallest optical wave­guides ever created in poly­dimethyl­siloxane, or PDMS,” said research team member Ye Pu of École Fédérale de Lausanne in Switzer­land. “Our flexible waveguides could be integrated into micro­fluidic lab-on-a-chip systems to eli­minate bulky external optics needed to perform blood tests, for example. They might also deliver light for wearable devices such as a shirt featuring a display.”

The new optical wave­guides are not only thinner than a piece of dust, they also exhibit very low light loss when used with certain wave­lengths of light. A light-based signal can travel through the new wave­guides for 10 centimeters or more before an unac­ceptable degra­dation of the signal will occur. The researchers made the new waveguides by opti­mizing laser direct writing, a micro­fabrication approach that creates detailed 3-D structures by poly­merizing a light sensitive chemical with a precisely positioned focused laser. Polymeri­zation converts relatively small molecules called monomers into large, chainlike polymers.

The new approach does not require a photo­initiator, which is typically used to effi­ciently absorb the laser light and convert it into chemical energy that initiates polymeri­zation. “By not using a photo­initiator, we simplified the fabri­cation process and also enhanced the compa­tibility of the final device with living tissue,” Pu said. “This enhanced biocom­patibility could allow the approach to be used to make implan­table sensors and devices.” The new flexible wave­guides could also serve as building blocks for photonic printed circuit boards that use high-speed optical signals rather than electrical links to transmit data in computers and other electronic devices.

To achieve a small optical waveguide that effi­ciently confines light, there must be a large dif­ference between the refractive index of the material making up the waveguides and the surrounding PDMS. The researchers used phenyl­acetylene for the wave­guides because, compared to tradi­tionally used materials, it has a higher refractive index once poly­merized. As an added benefit, it also can be easily loaded into PDMS simply by soaking the PDMS in liquid phenyl­acetylene. After soaking the PDMS in phenyl­acetylene, the researchers used focused ultrafast laser pulses to induce a multi­photon absorption in which multiple photons are absorbed at once.

Multi­photon laser direct writing produces much finer structures than one-photon processes because the volume of polymeri­zation at each writing spot is much smaller. Using multi­photon laser direct writing also allowed the researchers to directly initiate phenyl­acetylene polymeri­zation without a photo­initiator. They then evaporated any unpoly­merized phenyl­acetylene by heating the PDMS. The researchers showed that this new approach could make flexible wave­guides in PDMS that are just 1.3 microns wide. For the 650 to 700 nano­meter spectral band, only 0.07 percent of the light trans­mitted through the waveguides is lost every centi­meter. Opti­mizing the setup would likely allow fabrication of waveguides that are smaller than 1 micron, according to the researchers.

The researchers are now working to improve the yield of the fabri­cation process by deve­loping a control system that would help avoid material damage during laser writing. They also plan to create an array of narrow wave­guides in PDMS that could be used to construct a very flexible endoscope with a diameter of less than one milli­meter. “Such a small, mechani­cally flexible endo­scope would allow a number of hard-to-reach places in the body to be imaged for diagnosis in the clinic, or for monitoring in a mini­mally invasive surgery,” said Pu. (Source: OSA)

Reference: G. Panusa et al.: Photoinitiator-free multi-photon fabrication of compact optical waveguides in polydimethylsiloxane, Opt. Mat. Exp. 9, 128 (2018); DOI: 10.1364/OME.9.000128

Link: Optics Laboratory (LO), École Polytechnique Fédérale de Lausanne EPFL, Lausanne, Switzerland

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