image: Figure 1(a). The schematic of THG imaging based on Si membrane metasurfaces. (bi and ci) The images of a Sector Star Target and the metasurfaces under white light source illumination. (bii and cii) Transformed visible images (at 504 nm) of the target via membrane metasurfaces under NIR light illumination (at 1512nm) view more
Credit: OEA
A new publication from Opto-Electronic Advances, 10.29026/oea.2023.220174 discusses third-harmonic generation and imaging with resonant Si membrane metasurface.
Near-Infrared (NIR) vision detectors and cameras play an essential role in today's high-tech tools for imaging, sensing and display technologies. Goggle- or binocular-based NIR cameras are particularly important for night vision, as well as medical and agriculture imaging. In conventional NIR cameras, the NIR light (700−2500 nm) is absorbed via a photocathode, leading to the discharge of electrons, which consequently strike an integrated flat screen, viewed by the eye or by an imaging sensor. While such devices have proven effective in the abovementioned applications, they are bulky, heavy, monochrome and limited to certain wavelength bands. The latter is a major technological limitation because photocathodes are functional only in one of the following ranges: 400-1000nm, 1000-2500nm or >2500 nm. However, Z. Zheng et al., in this paper, have demonstrated a thin film capable of covering all of these frequency bands without needing to convert the light to electrons and vice-versa.
In this research work, researchers have employed the concept of nonlinear metasurfaces. Metasurfaces are arrays of nanoscale resonators that can manipulate light properties, including the light's propagation direction, intensity, and wavelength (/colour). Metasurfaces, capable of converting the wavelength of light, are known as nonlinear metasurfaces. In this paper, a nonlinear metasurface composed of a thin silicon film is exploited. The film accommodates carefully designed and fabricated nanoscale holes, i.e. membrane geometry, that strongly resonate with the incoming light. After illuminating the designed metasurface with a NIR light, it generates a new colour at 1/3 of the original wavelength via a nonlinear process, so-called third harmonic generation (THG). By controlling the symmetry of the array of nano-holes, the researchers have demonstrated a versatile tool for tunning the light wavelengths and intensities, which is ultimately used for NIR imaging. Figure 1a illustrates the concept of NIR imaging for arbitrary objects. As a demonstration, the NIR light around telecommunication wavelength (1512 nm) passes through a sector star target and gets converted into the visible signal (504nm) via the metasurfaces. The images shown in 1(bii and cii) are formed on the CCD camera.
Such an innovative approach for NIR imaging is widely expandable to large frequency bands and multi-colour processes. Worth noting that the exploited material, i.e. silicon, is being heavily used in the CMOS industry today. Therefore, mass production of silicon metasurfaces does not need heavy investments. Moreover, silicon does not absorb NIR light in wavelengths >1000 nm, so heating is not a concern. Last but not least, silicon is a centrosymmetric material. Therefore nonlinear silicon metasurfaces can be used for other third-order nonlinear interactions beyond THG. For example, by using a process called four-wave mixing, one can involve several wavelengths in the NIR and visible ranges, enabling the generation of colourful images. In other words, the demonstrated platform in this paper is a building block for the next generation of thin, inexpensive, broadband, and colourful NIR cameras and detectors.
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The Advanced Optics and Photonics (AOP) group at Nottingham Trent University has been recently (2020) formed by Prof. Mohsen Rahmani, Dr Lei Xu and Dr Cuifeng Ying. The group's research agenda is concentrated on designing, fabricating and engineering subwavelength nanostructures that can resonantly couple to the incident light and manipulate the light's behaviour on demand. The main research streams of the group are: 1. Linear photonics, where metasurfaces are designed to reproduce the functions of bulk optics and, on occasion, offering functionalities that are not possible with conventional diffractive optics. 2. Nonlinear photonics, where all-optical conversation of light frequencies (colours) via engineered metasurfaces are designed and fabricated for NIR imaging, night-vision, etc. 3. Bio-photonics, where ultrasensitive nanoscale sensors are designed and developed for detection of low concentration substances/biomarkers down to the single protein level.
Nottingham Trent University (NTU), in the UK, is creating the University of the future. A university that opens its arms to all, challenges conventions, and enriches the world around us. NTU collaborates with the world's brightest minds to push boundaries, disrupt the status quo, and transform lives. Through NTU’s Research Peaks, Centres and Themes, NTU is delivering ground-breaking research that has a profound real-world impact on individuals, communities, businesses and policies. From social media addiction to sustainable farming, NTU inspires the brightest minds to rise up and work together to solve some of society's most significant global challenges. NTU has the third largest number of postgraduate students in the UK studying professional qualifications, and we have over 320 experts working across 52 distinct areas of research and analysis. In the most recent Research Excellence Framework results (2014), 90% of NTU's research was judged to be 'world-leading', 'internationally excellent' or 'internationally recognised'. We were also recently awarded the Queen's Anniversary Prize for Higher and Further Education in 2021 for our Cultural Heritage research. This is the second time that NTU has been bestowed the honour of receiving this award for our research.
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Zheng Z, Xu L, Huang LJ, Smirnova D, Kamali KZ et al. Third-harmonic generation and imaging with resonant Si membrane metasurface. Opto-Electron Adv 6, 220174 (2023). doi: 10.29026/oea.2023.220174
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Opto-Electronic Advances