Newer technologies such as optical computing, integrated photonics, and digital holography require optical signal processing in three dimensions. To achieve this, it is necessary to be able to shape and direct the light flux according to the desired application. Given that the light flux within the medium is subject to the refractive index, specific assignment of the refractive index is necessary to achieve control of the light path within the medium.
To this end, scientists have developed so-called “non-periodic optical volume elements” (APVEs), microscopic pixels with specific refractive indices located at predetermined locations to direct the light flux in a controlled manner. However, sculpting these elements requires a high degree of precision, and most light-shaping materials are limited to 2D configurations or end up weakening the resulting light beam profile.
In a recent study published in Advanced Photonics Nexus (APNexus), researchers led by Alexander Gescher of the Medical University of Innsbruck in Austria have proposed a simple approach to fabricate high-resolution APVEs for a range of applications. The method uses a technique called “direct laser writing” to three-dimensionally arrange pixels of specific refractive indices within borosilicate glass.
In their study, the researchers designed an algorithm that stimulates the flow of light through a medium to determine the optimal position of voxels to achieve the necessary resolution. Accordingly, they were able to produce between 154,000 and 308,000 voxels, each occupying a size of approximately 1.75 μm x 7.5 μm x 10 μm, in just 20 minutes. In addition, they used dynamic wavefront control to compensate for any spherical aberration (beam profile distortion) while focusing the laser on the substrate. This ensures consistency of each voxel profile at all depths within the medium.
The team developed three types of APVEs to demonstrate the applicability of the method: an intensity modulator to control the intensity distribution of an input beam, an RGB multiplier that manipulates the transmission of the input beam’s red, green, and blue (RGB) spectra, and a Hermite-Gaussian (HG) mode sorter to improve data throughput speeds.
The team used an intensity modulator to convert a Gaussian beam into a microscopic smiley-shaped optical distribution, followed by a multiplexer to represent different parts of the smiley distribution with different colours, and finally an HG mode sorter to convert the multiple Gaussian mode inputs delivered by the optical fiber into HG modes. In all cases, the devices were able to transmit the input signal without significant loss and achieved record diffraction efficiency as high as 80 percent, setting a new benchmark for the APVE standard.
“The results reported in this paper significantly advance the field of ultrafast laser direct writing. The new method could open the doors to an ideal low-cost platform for rapid prototyping for highly integrated 3D light designers,” says Polina Segovia, member of the APNexus Editorial Board. Olvera is from the Center for Scientific Research and Higher Education at Ensenada (CICESE). “The demonstration of a solid method for producing a consistent, repeatable, and reliable APVE not only adds to existing knowledge in the field but also enables new avenues in applied photonics,” she adds.
This method, in addition to its simplicity, low cost, and high accuracy, can also be extended to other substrates, including nonlinear materials. Jesacher concludes, “The flexibility of our method could make it applicable to design a wide range of 3D devices for applications in information transmission, optical computing, multimodal fiber imaging, nonlinear photonics, and quantum optics.”
Nicholas Barry et al. Directly Laser-Scribbled Non-periodic Photonic Volume Elements for High-Efficiency Complex Light Shaping: Inverse Design and Fabrication, Advanced Photonics Nexus (2023). DOI: 10.1117/1.APN.2.3.036006
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