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Spectrometer Based on Multifocal Metalens

Recently, the team of Professor Chen Xianzhong from Heriot-Watt University proposed a new method based on the multi-focus metalens design scheme to control the dispersion of different wavelength beams. When the working distance is only 300 μm, the wavelength is 500 ~ 679 nm. In the visible light range, the spectral recognition with nanoscale resolution is realized, which provides a new idea for the development of on-chip spectrometers.

Research Background

Dispersion phenomenon widely exists in nature, usually caused by the change of the refractive index of the material with the wavelength of the incident light. Dispersion is an unavoidable consideration in various fundamental research and practical applications, such as state-of-the-art microscopes, metrology instruments, cameras, and pulse stretching in optical fibers. On the one hand, the existence of dispersion will cause polychromatic light to go away during transmission, causing crosstalk in communication to distort the signal, or introducing chromatic aberration in the imaging process to reduce the imaging quality; on the other hand, dispersion is also important in spectroscopy. There are important applications such as optical spectrum analyzers that require dispersion enhancement to improve their resolution. The elimination and enhancement of dispersion are very important in many basic researches and industrial applications, how to precisely control dispersion is a current hot issue.

Traditional dispersion control usually requires a combination of dispersion elements made of a variety of different materials (such as diffraction gratings, lenses, prisms, etc.), and each introduction of a dispersion element made of different materials can provide dispersion control for the entire system. More degrees of freedom. However, the more components are introduced in the dispersion control system to control the dispersion, the difficulty of precise alignment increases, which makes the dispersion control system bulky and complex, and it is difficult to meet the needs of the current field of on-chip photonic integration. In stark contrast to traditional dispersion control methods, optical metasurfaces composed of subwavelength nanostructures can introduce effective dispersion through the formulation of structural geometric parameters and arrangements to achieve dispersion control and management. In recent years, the rapid development of nano-manufacturing technology has solved the problem of metasurface device processing, and greatly improved the work efficiency of metasurface. The uniform height profile of metasurface also greatly reduces the optical alignment required to control dispersion. challenge.

Facing the urgent demand for high-resolution spectral analysis in the field of on-chip photonic integration, the dispersion characteristics and ultra-light and ultra-thin properties of metasurfaces provide a new idea for the development of on-chip spectrometers. On-chip spectrometers also have great application potential in the fields of medical care, food safety monitoring, and lab-on-a-chip (Lab-on-a-chip), especially for the development of miniaturized, portable, and wearable smart sensing devices. In view of this, Chen Xianzhong's team from Heriot-Watt University in the UK proposed a new method based on the multi-focus metalens design scheme to control the dispersion of different wavelength beams. In the case of a working distance of only 300 μm, at a wavelength of 500 ~ 679 nm Spectral identification with nanometer resolution is achieved in the visible range.

Research innovation

Based on the multi-focus metalens model, the author's team included the wavelength information into the phase profile of the metalens, designed a multi-focus metalens that can separate beams of different wavelengths and converge them at a preset position on the focal plane, and realized the spectrometer function. The metasurface spectrometer has a working distance of only 300 μm (the design focal length of the metalens), and achieves nanoscale spectral resolution in the visible range of 500–679 nm. Figure 1 shows a schematic diagram of a metasurface spectrometer, where the wavelength information of an incident polychromatic beam can be precisely mapped to different positions on a circular ring in the focal plane. The ring is composed of multiple focal points of different wavelengths, and the azimuth angle of each focal point corresponds to an incident wavelength. For the incident monochromatic light, the adjacent wavelength focus near the corresponding wavelength focus will also converge the beam due to insufficient dispersion, but the incident wavelength can be accurately identified by the location of the maximum intensity bright spot. Under the incidence of polychromatic light, by analyzing the distribution of normalized intensity on the ring, each central wavelength of the composed polychromatic beam can be obtained.

Fig.1 Schematic diagram of metasurface spectrometer based on multi-focus metalens

Figure 2 shows the intensity distribution and spectral analysis results on the focal plane of the metasurface spectrometer under the incidence of polychromatic light composed of wavelengths of 510 nm, 581 nm and 633 nm. The experimental results of the intensity distribution are basically consistent with the simulation results. By analyzing the intensity distribution on the focus ring after compensating the sight function of different wavelengths, the identification result of the central wavelength of polychromatic light with a relative error of less than 0.5% can be obtained. In addition, the metasurface spectrometer can achieve high-resolution wavelength detection at 1 nm under both monochromatic and polychromatic light incidents.

Fig. 2 Intensity distribution and spectral analysis results on the focal plane of the metasurface spectrometer under the incidence of polychromatic beams composed of 510 nm, 581 nm and 633 nm wavelengths

The proposed spectrometer can not only accurately detect the central wavelength of the incident light, but also has the possibility to identify the spectral linewidth. The author's team proposes to use the intensity distribution obtained by incident single-frequency beam with extremely narrow linewidth as a benchmark, and calibrate the intensity difference produced by different linewidth spectra to identify the linewidth of the spectrum to be measured. In addition, processing larger-sized metasurfaces can enable the proposed spectrometer to achieve more accurate linewidth detection and even identification of continuous spectra. Figure 3 shows the recognition of Gaussian spectral shapes with different linewidths by a larger-scale metasurface spectrometer. In theory, it is possible to accurately identify spectral linewidths. In addition, processing larger-sized metasurfaces can also help improve the resolution and working bandwidth of metasurface spectrometers.

Detection of Spectral Linewidth by Metasurface Spectrometer

Summary and Outlook

This study designs and experimentally demonstrates a metasurface spectrometer based on the intrinsic dispersion and multi-focus properties of the metalens. In this work, the independent design of each focal convergence wavelength provides a new degree of freedom for spectrometer dispersion control. The proposed metasurface spectrometer achieves nanoscale spectral resolution in the visible range. The design method is flexible and robust, and provides a new scheme for controlling the desired dispersion of polychromatic light incidence. The design is expected to facilitate the development of many application fields such as on-chip spectral analysis, information security and information processing.

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