Scientists at Université Laval, including our very own Simon Duval, recently published an article about a "novel fabrication technique for the fabrication of on-surface photonic devices in polymers" using mid-IR femtosecond lasers. In this article we will explore the origins and implications of this breakthrough.

The Basics

Light inside an optical fiber

Optical waveguides allow light transmission from A to B via guiding structures; the most famous example are fiber optics, now vastly used in telecommunication networks. This is possible because of the total internal reflection phenomenon, where light can be reflected at the boundary of two medias depending on its angle and the difference between the materials' refraction indexes. In optical fibers, this means light signals cannot escape their glass core (high refractive index) as it bounces off the polymeric clad (low refractive index).

Another mean to create waveguides is to locally increase the refraction index of a material. This can be done by striking the material with a highly absorbent laser beam, which is a very fast, precise and repeatable process. As depicted on the image, the processed grey volume has a lower refractive index than what is in the show in white. When entering the low index grey material, the light bounces off the higher index areas and is then guided through the channel. It will also be reflected at the top surface, as air has a lower refraction index than any area of the material – which is why it is named an optical surface waveguide.

Laser scribbing a surface waveguide
Schematics of a surface waveguide sensing device
Principle of operation for surface waveguide sensing device

Surface waveguides can be used for different applications: temperature or pressure monitoring, substance identification, concentration levels, etc. For such sensing, a light is guided through the surface waveguide and the intensity is measured at the other end. If the refractive index of the substance in contact with the outer surface of the waveguide changes, then the intensity of the light on the receiving end will also change accordingly. By characterizing the relationship between refractive indexes and other parameters, those can be sensed via the surface waveguide.

A Scalable Process

What the team unveiled is a repeatable manufacturing process for low-cost custom sensors at scale, democratizing this sensing technology to a much larger audience. Polymers are cheap, bio-compatible materials and now there is a method to turn almost any plastic into a high-tech sensing device. This process requires a match between a laser wavelength and a material's absorption band in order to change the refractive index at the surface of the material, without causing additional deformation or burning of the matrix.

Femtum's UltraTune 3400 is establishing itself as the laser of choice for the processing of polymeric bio-sensors. With a tunable source between 2.8 and 3.4 µm, manufacturers are able to create waveguides in almost any polymers at high speed.

The article validates the "unparalleled sensitivity and precision" for PMMA and polycarbonate based sensors. "The waveguide’s morphology allows light confinement up to the very interface of the polymer with the external medium, and thus, opens novel avenues for enhanced sensing and probing applications."