Researchers have developed a novel computational approach for designing advanced Terahertz (THz) optical elements, overcoming significant challenges in manipulating these waves for broadband applications. The team’s method centers on a sophisticated understanding of scalar diffraction, meticulously accounting for how THz waves change as they pass through specially structured surfaces and air. This precise modelling is crucial for creating devices that work effectively across a range of THz frequencies.
Designing broadband THz diffractive optical elements is inherently complex because it requires multiple frequencies to be accurately steered and constructively interfere at a target point. Theoretically, achieving perfect efficiency is impossible due to the nature of wave interference. However, this new research successfully integrates these physical limitations into a computational design process.
To tackle the intricate optimization required, the researchers implemented a refined algorithm called Gradient Descent Assisted Binary Search (GDABS). This algorithm, built upon existing binary search techniques, operates under the principle that effective solutions are abundant within the design possibilities. The algorithm iteratively tweaks the height of individual pixels in the optical element’s structure, assessing the impact of each change on the device’s performance using a Figure of Merit. By incorporating a gradient descent approach, the GDABS algorithm efficiently navigates the design space, converging rapidly to an optimal solution. The use of symmetry in the design further accelerated the process. Notably, this new GDABS method significantly reduces computation time compared to traditional techniques, while ensuring the optimized design represents a highly effective solution.
The capability of this algorithm was demonstrated through the design of three distinct THz optical components. First, a high-performance broadband spherical lens was created, capable of focusing a wide range of THz frequencies with minimal aberrations. This lens was physically fabricated using common 3D printing techniques and its performance was experimentally validated, showing good agreement with computational predictions. Furthermore, simulations confirmed the lens’s polarization insensitivity, as expected from the scalar diffraction model.
Second, the researchers designed spectral splitters, analogous to optical gratings, which separate different THz frequencies into spatially distinct beams. Different splitter designs were created to achieve both regular and arbitrarily spaced frequency separation, showcasing the versatility of the design approach. Interestingly, designs for arbitrary frequency separation exhibited enhanced spectral resolution.
Finally, a complex on-axis broadband transmissive hologram was designed to project a THz image of Mickey Mouse. This hologram demonstrated excellent image fidelity across the targeted broadband frequencies, with high transmission efficiency.
Further investigation into the robustness of these designs revealed a strong tolerance to fabrication imperfections, specifically random variations in pixel height expected from standard 3D printing processes. This indicates the practical viability of the designed THz elements for real-world applications. Despite limitations in measurement facilities which introduced some experimental inaccuracies, the overall results qualitatively validated the design methodology. This advancement paves the way for more sophisticated and efficient THz technologies across various fields, from imaging and sensing to communications.
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