Chongfan Technology
News
12
2025
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03
Broadband perfect Littrow diffraction metasurface under large-angle incidence
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In the field of laser technology, Littrow diffraction devices are widely used in laser resonators and spectrometer equipment due to their high efficiency. However, traditional diffraction devices often struggle to achieve broadband, high-efficiency diffraction at large angles of incidence, limiting their application in high-integration and complex functional optical systems. Recently, a research article published inNanophotonics[1]proposed a novel metasurface structure that successfully achieved broadband perfect Littrow diffraction at large angles of incidence, providing a new approach to solving the above-mentioned problems.
Figure 1 Comparison of metasurface diffraction efficiency based on traditional periodic grating and metagrating schemes
The article proposes a Littrow diffraction metasurface composed of a metagrating and an aperiodic multilayer film, solving the energy flow mismatch problem at large angles of incidence through non-local response. It details the metasurface theory, analyzes the energy flow distribution under Littrow and non-Littrow conditions, and the conditions for perfect anomalous reflection. A broadband reflector based on a metagrating is designed. The results show that the metagrating can enhance the control of Bloch waves, achieving 99% efficiency broadband perfect anomalous reflection at 3.11μm - 3.52μm after PSO optimization. Due to its strong non-local effect, the bandwidth is increased by 46% compared to the reflector based on the periodic grating.
Based on the Huygens-Fresnel principle, the article indirectly affects the reflection by controlling the field distribution (energy flow distribution and Poynting vector) above the metasurface. The incident light excites Bloch waves within the grating, and the separation layer contains propagating waves and evanescent waves. The main lateral energy flow transfer of the system relies on multiple scattering between the separation layer and the grating interface. The evanescent waves are ignored in the research process. The overall system energy flow distribution is 。 At the bottom of the metasurface, the energy flow, through adjustment, indirectly achieves the required energy flow.
Figure 2 Broadband anomalous reflection metasurface
Energy flow analysis under Littrow condition: When the metasurface operates near the Littrow condition, the reflection angle is equal to the incident angle. At this time, it can be obtained from the electromagnetic field distribution above the metasurface. After simplification, the condition for achieving perfect anomalous reflection (-1 order, and ) is, that is,. The Bloch wave propagates in the upper structure, and its field distribution is affected by the structural parameters. By adjusting the structural height, width, spacing, and boundary conditions, the reflection phase can be changed, and the bottom layer film maintains the Bragg reflection condition to ensure 100% reflectivity.Energy flow analysis under non-Littrow condition: When the incident wavelength changes, the reflection angle deviates from the Littrow angle. At this time, the total energy flow expression is different. When achieving -1 order perfect reflection (and ), after simplification, the propagation energy flow can be obtained from the electromagnetic field distribution at. It is affected by the reflection phase provided by the separation layer and the aperiodic thin film, and both play a key role in the resonant interference conditions of different optical paths. However, the modulation range of the propagating wave is limited, and further adjustment is needed to enhance the ability of the Bloch wave to transfer lateral energy flow. By simultaneously adjusting the propagating wave and the Bloch wave, the total energy flow satisfies the perfect anomalous reflection condition, achieving perfect anomalous reflection at any angle.
Through the above analysis, in the design process, the researchers first fixed the multilayer film thickness, scanned the structural linewidth and height, and found the parameters matching the target energy flow as the initial solution. Then, the particle swarm optimization (PSO) algorithm was used to optimize the thickness of the aperiodic thin film and the structural linewidth. The multilayer film mainly affects the propagating wave energy flow, and the upper structure controls the Bloch wave energy flow. Under the Littrow condition, the propagating wave energy flow is zero, mainly avoiding the non-local effect of the Bloch wave; when deviating from the Littrow condition, the propagating wave energy flow increases but the control ability is limited, and the Bloch wave still plays a decisive role, and the non-local effect needs to be enhanced to meet the perfect diffraction requirements.
This research successfully designed a non-local metasurface, achieving broadband perfect Littrow diffraction at a 70° incidence angle. Compared with existing research, it has advantages in terms of incident angle, bandwidth, and efficiency. Through this novel metasurface structure, the research team not only broke through the bandwidth limitation of traditional diffraction devices at large angles of incidence but also provided new possibilities for the design of future optical systems. This metasurface structure has broad prospects in practical applications and is expected to promote the development of related technologies.
Zhu, J., Zhou, S., He, T., Feng, C., Wang, Z., Dong, S., & Cheng, X. (2025). Broadband perfect Littrow diffraction metasurface under large-angle incidence. Nanophotonics, (0).
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