In a study published in Sensors, researchers experimentally, numerically, and theoretically investigated an optical microsensor based on high-aperture metalens in an amorphous silicon film.
How are Metalenses Revolutionizing Optics?
Manufacturing high-performance compact optical sensors for controlling the polarization of light is a challenging task. Conventional methods of polarization control, such as polarization modulators, waveplates, and linear polarizers, are bulky and need several observations.
There are fast and compact optical polarization sensors, such as polarization-imaging devices, plasmonic and optical on-chip polarimeters, and gyroid photonic crystals-based chiral beam splitters. However, metasurface-based polarization sensors are the more efficient, compact, and cost-effective option.
Metalenses are a flat lens technology created by optical parts concentrating light using metasurfaces. They offer a new range of lightweight lens designs, and their flat surface and reduced thickness help avoid optical distortions common in classic curved refractive lenses.
Advantages of Metalenses
In advanced optics, there has been increased interest in metasurface technology. As a result, extensive research has been conducted on the conceptual frameworks, design, manufacturing, and applications of metasurfaces.
Metasurface antenna arrays permit simultaneous modification of optical properties, such as phase, amplitude, and polarization of electromagnetic waves. In addition to their exceptional control capabilities, metasurfaces are flat, ultrathin, light, and compact.
Metalens has already been used in a variety of advanced technologies, including light-field optical imaging, endoscopy, augmented and virtual reality, and a quantum entanglement optical chip, all of which point to its enormous potential for future applications like machine vision for mini-drones, autonomous and AI robots in agriculture, healthcare, and quantum information technology.
Limitations of Current Metalens Technology
Most metalenses work on the principle of physically isolating individual polarization states. However, these polarization sensors have various limitations, such as the absence of focusing metalens, a high aspect ratio of nanopillars, and registering an optical signal requiring photosensors to be separated in space.
Investigation of Metalens-Based Polarization Microsensor in a Thin Silicon Film
Researchers investigated a metalens of the polarization state of laser light in an amorphous silicon film.
A low aspect ratio, large numerical aperture, and short focal distance metalens with an extremely small diameter (in order of microns) was fabricated on a thin amorphous silicon layer with a 120 m thickness. The metalens was purposefully designed microscopic to speed up the calculations.
The photosensors were clustered around the metalens’ focal point, which focused photons at a distance of 633 nm. The metalens comprised 110 nm wide binary subwavelength diffraction gratings that were sector-shaped and featured grooves and ridges. A circular aperture of radius 4 m generated the incident light field.
The simulation was run with the finite difference time domain approach in the Fullwave software, which used the Maxwell equations’ difference solution to perform the calculation.
Placing one or two intensity photodiodes in the focus plane enabled beam polarization detection. For example, monitoring the central intensity of the focus point allowed for the unequivocal identification of three forms of polarization.
Significant Findings of the Study
Current metasurfaces-based polarization sensors based on metalens lacked a focusing lens and deflected light with varying polarizations at various angles. This resulted in the need for a photosensor matrix and larger sensor sizes. They deflect distinct forms of polarization at varying angles to the optical axis
The polarization sensor investigated in this study realized several polarization states by producing patterns in the metalens focus: left circular polarization, right circular polarization, and linear polarization generated a light ring in focus, a circular focal spot, and an elliptic spot with two sidelobes, respectively.
In the experiment, the values of the focus diameter, the ring diameter, and the distance between the sidelobes were 20%greater than the simulation results. The metalens was positioned in the diverging region of the Gaussian optical vortex, which explained why the focusing spots were larger in the experiment than in the simulation.
The polarization sensor investigated in this study was smaller than 10 microns, focused light at one wavelength distance, and operated on an entirely new concept. The sensor created diffraction patterns in focus along the optical axis rather than deflecting light at varying angles to produce distinct polarizations. This allowed the photosensors matrix to be replaced by one or two photosensors with hundreds of nanometers of the sensing area.
The reported metalens sensor has potential applications in the life sciences, microscopy, and medicine, where precise control of polarization states is needed.
Kotlyar, V., Nalimov, A., Kovalev, A., & Stafeev, S. (2022) Optical Polarization Sensor Based on a Metalens. Sensors. https://www.mdpi.com/1424-8220/22/20/7870/htm