Reverse design on-chip metalenses for high-fidelity mode field scaling

The advantage of high integration is widely used in communications, chip-scale optical signal processing and biomedical sensing and other fields. Since the photonic integrated devices that make up the photonic integration loop, such as raster couplers, wavelength beam splitters, mode converters, etc., usually have different input/output interface sizes, optical interconnection between different devices needs to be achieved with the help of tapered waveguides with gradient width. When the width scaling of the two connected devices is large (≥10:1), tapered waveguides typically require hundreds of microns in length to achieve high efficiency and mode purity transmission of the guided wave. Therefore, it limits the further development of on-chip integrated photonic devices to high integration, and hinders the practical application of integrated photonic circuits.

As a typical sub-wavelength photonic device, metalenses have been introduced into photonic integrated circuits in recent years to realize on-chip photonic interconnection, which provides an opportunity to solve the problem of low optical interconnection efficiency between devices with relatively large scale. However, the element structure design of traditional metalenses is based on periodic boundary conditions, ignoring the coupling between element structures, resulting in errors between the actual phase distribution and the ideal phase distribution of the designed metalens, especially when the NA is large (large width scaling ratio), the efficiency loss caused by this error is more obvious, which limits the device performance under high integrated density interconnect.

As shown in Figure 1, the research team from the Institute of Optoelectronic Technology of the Chinese Academy of Sciences proposed an on-chip metalens design method based on topology optimization, and the free-form metalens obtained can achieve efficient optical interconnection between waveguide devices with high width scaling ratio, and realize the dual functions of beam focusing and collimation in different guided wave transmission directions.

Figure 1: How an on-chip integrated metalens works. Source: Light: Advanced Manufacturing 4, 20 (2023)

The results were published in Light: Advanced Manufacturing under the title “High-fidelity mode scaling via topological-optimized on-chip metalens for compact photonic interconnection”. This work is supported by funds/projects such as the National Key Research and Development Program of China.

In this work, a metalens designed by traditional methods is used as the optimization initial structure, and the output port phase distribution is used as the optimization goal. By continuously approaching the phase distribution of the ideal high-NA metalens, efficient TE0 mode transmission between an 11μm wide waveguide and a 1μm narrow waveguide is successfully realized.

Efficient focusing in TE0 mode is achieved by forward transmission (from wide waveguide to narrow waveguide), and its relative transmission efficiency can reach 96%. Figure 2 shows the comparison of the electric field distribution of the metalens before and after optimization, and the optimized metalens significantly reduces stray light when light enters the narrow waveguide, and a large amount of energy is concentrated at the focal point, thus forming a more ideal focal spot, which has better focusing effect at the ideal focal length, deeper focus depth, and higher focusing efficiency.

Figure 2: Comparison of electric field distribution before and after optimization of metalens structure in focusing mode. Source: Light: Advanced Manufacturing 4, 20 (2023)

For collimation mode under backward transmission (narrow waveguide to wide waveguide), the optimized metalens can achieve collimation in TE0 mode, and its transmission efficiency can reach 69%, which is much higher than that of traditional taper waveguide (<10%). As shown in Figure 3, the device is designed to ensure equal phase plane generation in the directional emitter, and the introduction of metalenses results in higher radiant beam quality and less noise than traditional tapered waveguides.

Figure 3: Application of collimation mode in directional radiators. Source: Light: Advanced Manufacturing 4, 20 (2023)

This work provides new ideas for optical interconnect design and wavefront shaping of highly integrated PIC, and the proposed design method has potential application prospects in the fields of directional emitter, LiDAR, on-chip optical information processing, and optical computing. (Source: Advanced Manufacturing WeChat public account)

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