Photonic technologies and applications often require light signals to be manipulated in three dimensions such as in optical computing, fiber imaging, data transport and quantum technologies. To be useful, the shape and direction of light must be controllable. Scientists have developed “aperiodic photonic volume elements” (APVEs), which are microscopic voxels. These voxels direct the flow of light in a controlled manner due to their specific refractive indices located at predefined positions. The advantage of APVEs is that they are highly precise due to the ability to their capabilities in three dimensions.
Researchers at the Medical University of Innsbruck (Austria), led by Alexander Jesacher, proposed a new way of fabricating APVEs. A study published in Advanced Photonics Nexus (APNexus) describes how they used direct laser writing to arrange the voxels in three dimensions with specific refractive indexes inside of borosilicate glass.
To determine the optimum placement of the voxels in order to achieve the necessary precision, the team designed an algorithm that stimulates the flow of light through a medium. They report that they were able to generate between 154,000 and 308,000 voxels, each occupying a volume of approximately 1.75 µm × 7.5 µm × 10 µm, within just 20 minutes.
Consistency of the voxel profiles is necessary for the precision desired, to the team used dynamic wavefront control to compensate for any spherical aberration (beam profile distortion) during the focusing of laser on the substrate. The researchers report that this ensured the consistency of each voxel profile at all depths within the medium.
To demonstrate the applicability of the method, the researchers developed three types of APVEs: an intensity shaper for controlling the intensity distribution of the input beam; an RGB multiplexer that manipulated the transmission of the red-green-blue (RGB) spectra of the input beam; and a Hermite–Gaussian (HG) mode sorter to enhance data transfer speeds.
According to the researchers, they used the intensity shaper to convert a Gaussian beam into a “microscopic smiley-shaped light distribution”. Then they used the multiplexer to add different colors to the smiley light distribution. Finally, they used the HG mode sorter to convert multiple Gaussian mode inputs delivered by the optical fibers into HG modes. They report that the devices were able to transmit the input signal in all cases without significant loss and that they achieved a record-high diffraction efficiency of up to 80 percent, setting a new benchmark for the standard of APVEs.
“The results reported in this paper greatly advance the field of ultrafast laser direct writing. The novel method could open doors to an ideal low-cost platform for a rapid prototyping of highly integrated 3D light shapers,” says Paulina Segovia-Olvera, APNexus Editorial Board Member of the Center for Scientific Research and Higher Education at Ensenada (CICESE). “The demonstration of a solid method for producing consistent, reproducible, and reliable APVEs not only adds to the current knowledge in the field but also enables new avenues in applied photonics.”
The researchers noted that the simple, low-cost and high-precision method can likely be extended to other substrates and including to nonlinear materials.
“The flexibility of our method could make it viable for designing a wide range of 3D devices for applications in information transport, optical computing, multimode fiber imaging, nonlinear photonics, and quantum optics,” concludes Jesacher.
Paper: “Direct laser written aperiodic photonic volume elements for complex light shaping with high efficiency: inverse design and fabrication,” Adv. Photon. Nexus 2(3) 036006 (2023), doi 10.1117/1.APN.2.3.036006.
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