Nanoscale resolution on-chip spectrometer

Recently, the team of Professor Chen Xianzhong of Heriot-Watt University in the United Kingdom proposed a new method to control the dispersion of different wavelength beams based on multifocal superlens design, which achieved nanoscale resolution spectral recognition in the visible light range with a wavelength of 500 ~ 679 nm under the condition that the working distance was only 300 μm, which provided a new idea for the development of on-chip spectrometer.

The research results were published in Light: Science & Applications under the title “Compact multi-foci metalens spectrometer.” Dr. Wang Ruoxing, a young teacher of North China Electric Power University (Baoding), Dr. Ansari Muhammad Afnan, a postdoctoral fellow at Heriot-Watt University, UK, are the co-first authors of the paper, and Professor Chen Xianzhong of Heriot-Watt University is the corresponding author of the paper.

Dispersion is widespread in nature and is usually caused by the change in the refractive index of a material with the wavelength of the incident light. Dispersion is unavoidable in a variety of basic research and practical applications, such as state-of-the-art microscopy, metrology instruments, cameras, and pulse expansion in optical fibers. On the one hand, the presence of dispersion will cause the polychromatic light to drift away during transmission, causing crosstalk in communication to distort the signal, or introducing chromatic aberration during the imaging process and reducing the imaging quality; On the other hand, dispersion also has important applications in spectroscopy, such as optical spectrum analyzers that require dispersion enhancement to improve their resolution. The elimination and enhancement of dispersion is crucial in many basic research and industrial applications, and how to accurately control dispersion is a hot issue at present.

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

Facing the urgent need for high-resolution spectroscopy analysis in the field of on-chip photonic integration, the dispersion characteristics and ultra-light and ultra-thin characteristics of metasurfaces provide a new idea for the development of on-chip spectrometers. On-chip spectrometers also have great application potential in medical, food safety monitoring and lab-on-chip (Lab-on-a-chip), especially for the development of miniaturized, portable, wearable intelligent sensing devices. In view of this, Chen Xianzhong’s team of Heriot-Watt University in the United Kingdom proposed a new method to control the dispersion of different wavelength beams based on multifocal superlens design, and achieved spectral recognition with nanoscale resolution in the visible light range with a wavelength of 500 ~ 679 nm under the condition that the working distance is only 300 μm.

Based on the multifocal hyperlens model, the author’s team included wavelength information in the phase profile of the metalens, and designed a multifocal microlens that can separate different wavelength beams and converge them at a preset position on the focal plane, realizing the function of the spectrometer. 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 of a metasurface spectrometer where wavelength information from an incident complex beam can be accurately mapped to different locations on a ring in the focal plane. The ring consists of several focal points of different wavelengths, each with an azimuth corresponding to an incident wavelength. For monochromatic light incidence, the adjacent wavelength focus near the corresponding wavelength focus also converges 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 complex light, by analyzing the distribution of normalized intensity on the ring, the center wavelengths that make up the complex beam can be obtained.

Figure 1: Schematic diagram of a metasurface spectrometer based on multifocal metalenses

Figure 2 shows the intensity distribution and spectral analysis results on the focal plane of the metasurface spectrometer under the incidence of complex photocolor light consisting of 510 nm, 581 nm, and 633 nm wavelengths. The experimental results of the intensity distribution are basically consistent with the simulation results. By analyzing the intensity distribution after compensating for different wavelength visual functions on the focal ring, the identification result of the center wavelength of the complex color light with a relative error of less than 0.5% can be obtained. In addition, metasurface spectrometers can achieve high-resolution wavelength detection of 1 nm under both monochromatic and polychromatic light incidence.

Figure 2: Intensity distribution and spectral analysis results on the focal plane of the metasurface spectrometer under the incidence of a polychromatic beam consisting of 510 nm, 581 nm, and 633 nm wavelengths

The proposed spectrometer can not only accurately detect the central wavelength of incident light, but also have the possibility of identifying spectral linewidth. The author’s team proposes to identify the linewidth of the spectrum to be measured by calibrating the intensity difference generated by different linewidth spectra based on the intensity distribution obtained by the incidence of a single-frequency beam with a very narrow linewidth. In addition, machining larger metasurfaces enables the proposed spectrometer to achieve more accurate linewidth detection and even continuous spectral identification. Figure 3 shows the recognition of Gaussian spectra with different linewidths by a larger-sized metasurface spectrometer, which can theoretically accurately identify spectral linewidths. In addition, machining larger metasurfaces helps to improve the resolution and operating bandwidth of metasurface spectrometers.

Figure 3: Detection of spectral linewidths by a large-scale metasurface spectrometer

Summary and outlook

In this study, a metasurface spectrometer based on the intrinsic dispersion and multifocal characteristics of hyperlenses was designed and experimentally proved. In this work, the independent design of each focal convergence wavelength provides new degrees of freedom for spectrometer dispersion control. The proposed metasurface spectrometer achieves nanoscale spectral resolution in the visible light range. The flexible and robust design method provides a new scheme for controlling the desired dispersion of multicolor light incidence. The design is expected to facilitate the development of many application areas such as on-chip spectroscopy, information security, and information processing. (Source: China Optics WeChat public account)

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