Broadband absorption of sunlight is key for solar cell technologies, so metasurface-based structure has emerged as a promising technique for their efficiency improvement. [15][16][17] It is widely used in photothermal energy generation, [18,19] thermal emitters, [20] and detectors [21] to combine the metasurface-based PAs like a blackbody with the broadband absorption of sunlight. To realize the broadband absorption of sunlight, carbon-based surfaces, [22][23][24] Si-based surfaces, [25][26][27][28] and thin-film metallic structures [29,30] are demonstrated with low-surface absorptivity over the whole solar spectrum. For the carbon-based surfaces, they generally have broadband absorption, nonpolarized selection, and wide-angle insensitivity for solar spectrum. However, most carbon-based absorbers have large thickness of tens to hundreds of micrometers which is a challenge for device integration. Silicon-based surfaces have good performance photovoltaic characteristics. However, some Sibased light-trapping schemes [16,27,28] indicated that limitation of Lambertian expression [31] brings the absorption of silicon cells only in the range from 400 to 1100 nm. In addition, it still has challenge to use patterned metal materials that are unstable at the high temperature to achieve solar spectral absorption. It is promising to design the ultrathin absorbers that have broadband solar absorption while save enabling material saving and shorter deposition times. [32] Due to the high-temperature stability and metal-like optical properties in the visible and nearinfrared spectral regions, titanium nitride (TiN) is an ideal candidate for solar absorbing materials. [33,34] In some reports, [35][36][37] it has shown that Mie resonance by combining ultrathin TiN gratings with different refractive index materials achieves perfect absorption in the near-infrared spectral region. However, inverse design toward this kind of Mie resonance is rarely reported, which may provide novel approach to the study of TiN structure with multimode coupling for broadband absorption.In order to obtain a TiN structure with excellent absorption characteristics, it is necessary to introduce inverse design. As shown in Figure 1a, in the traditional design process, most researches obtain the initial structure through a priori method, including but not limited patterned structure (topological structure), multilayer structure, and stage structure, Broadband absorption of sunlight plays a crucial role in applying solar energy. However, despite being a decade-old technology, there are only a handful of simple metasurfaces designed by conventional methods. This work theoretically combines inverse design with broadband absorption of sunlight to optimize a metasurface that exhibits triple coupling mode resonance for maximizing solar spectral absorption. The metasurface consists of dual-layer titanium nitride (TiN) cylinder grating arrays, TiN dielectric layers, and silicon nitride layers. The simulation results reveal the high absorptivity of 93% in the range o...

In this paper, we innovatively demonstrate a rotatable direct-binary-search algorithm. Based on this unique inverse design method, the coupling region of nanophotonic device can be realized with multi-shape and multi-rotation pixels. In addition, the novel 1× 2 mode converters with multipurpose design goals on a 220 nm-thick top silicon-on-insulator platform are proposed by utilizing this enhanced algorithm, which can simultaneously achieve power splitting and mode conversion. By 3D fine difference time domain solutions, the 1 × 2 mode converter that converts TE0 mode into TE1, with a footprint of 2.7 µm × 2.4 µm, exhibits the excess loss of 0.1 - 0.2 dB (TE1 mode), crosstalk of lower than -20.6 dB (TE0 mode) and reflection loss of lower than -19.5 dB (TE0 mode) from 1500 nm to 1600 nm. The 1 × 2 mode converter that transforms TE0 into TE2 occupies the footprint of 3.6 µm × 3 µm. The excess loss is 0.3 - 0.4 dB (TE2 mode) in the wavelength range of 1500 - 1600 nm. The crosstalks are lower than -17.5 dB (TE1 mode) and -25.1 dB (TE0 mode), and the reflection loss is lower than -18.3 dB (TE0 mode). Besides, the fabrication tolerances caused by both expansion or contraction of etched pattern contour and round corner effect are also investigated.

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Based on high symmetric structure, we propose the arbitrary-input and ultra-compact 1 × 2 and 1 × 3 power splitters by utilizing inverse design method. These devices can realize the functionality of power splitting, when the optical field is launched from arbitrary port. The shapes of their structures are 3.8 μm-wide regular hexagon and 4.0 μm-wide regular octagon, respectively. By utilizing 3D fine difference time domain solutions, the simulated results indicate that the excess loss of the 1 × 2 power splitter is less than 1.5 dB from 1,500 to 1,600 nm, and the excess loss and crosstalk of the 1 × 3 power splitter are less than 1.9 dB and lower than − 15.5 dB over 100 nm bandwidth at the centered wavelength of 1,550 nm respectively. In addition, the tolerances to fabrication errors are also investigated.