Integrated photonics, optical computing, and digital holography are modern-day technologies that require light signals to be manipulated in three dimensions.
For this to be achieved, it is essential to be able to shape and guide the flow of light as per its preferred application. Provided that the refractive index has controlled light flow within a medium, particular refractive index tailoring is required to identify control of the light path inside the medium.
To this end, researchers have developed “aperiodic photonic volume elements” (APVEs), microscopic voxels with particular refractive indices situated at predefined positions, to direct the flow of light in a controlled way.
But sculpting such elements requires a high degree of accuracy and most light-shaping materials are restricted to 2D configurations or end up demeaning the output light beam profile.
In a recent study reported in the Advanced Photonics Nexus (APNexus), scientists headed by Alexander Jesacher from the Medical University of Innsbruck in Austria suggested an easy method for fabricating highly accurate APVEs for a range of applications.
The technique uses a method known as “direct laser writing” for the 3D arrangement of voxels of particular refractive indices within borosilicate glass.
Scientists developed an algorithm that excites the flow of light via a medium to identify the utmost placement of voxels for achieving the necessary level of accuracy. Based upon this, they were able to produce between 154,000 and 308,000 voxels, each absorbing a volume of around 1.75 µm × 7.5 µm × 10 µm, within just 20 minutes.
In addition, the team used dynamic wavefront control to compensate for any spherical aberration (beam profile distortion) during the focusing of the laser on the substrate. This guaranteed the consistency of each voxel profile at all depths within the medium.
The research group developed three kinds of APVEs to illustrate the applicability of the method: an intensity shaper for regulating the intensity distribution of the input beam, an RGB multiplexer that handled the transmission of the red-green-blue (RGB) spectra of the input beam, and a Hermite–Gaussian (HG) mode sorter to improve data transfer speeds.
The intensity shaper was utilized to convert a Gaussian beam into a microscopic smiley-shaped light distribution, tracked by the multiplexer to constitute various parts of the smiley distribution in various colors, and eventually, the HG mode sorter to convert several Gaussian mode inputs that have been delivered by the optical fibers into HG modes.
In all cases, the devices were able to transfer the input signal without considerable loss. They obtained a record-high diffraction efficiency of up to 80%, thereby 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.
Paulina Segovia-Olvera, APNexus Editorial Board Member, Center for Scientific Research and Higher Education at Ensenada (CICESE)
Segovia-Olvera added, “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.”
Besides its simplicity, minimal cost, and high accuracy, the method also has the potential to be extended to other substrates, such as 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.
Alexander Jesacher, Medical University of Innsbruck
Barre, N., et al. (2023) Direct laser-written aperiodic photonic volume elements for complex light shaping with high efficiency: inverse design and fabrication. Advanced Photonics Nexus. doi.org/10.1117/1.APN.2.3.036006.