Light control at the micro scale
High-speed laser scanning has found applications in monitoring moving objects, capturing transient information of dynamic processes and observing biomolecules’ motility. Scanning a wide area in a short period of time is also essential in atmospheric studies, geological surveys and other similar activities. Various techniques enabling higher scan rates have been proposed. For example, galvanometric mirrors have been used for beam steering. The scanning speed of such mechanically scanning mirrors is limited to ~ 100 Hz in 2D. However, all-optical techniques based on acousto-optic deflectors have achieved micrometre resolution. In the EU-funded project REALTIMEIMAGING (Real time imaging with near field focusing plates), researchers turned to ‘Digital micro-mirror devices’ (DMDs). This technology is particularly suited for real-time imaging because it provides exceptional controllability over thousands of ‘microelectromechanical system’ (MEMS) micro-mirrors. By spatially switching the light through micro-mirror arrays, researchers were able to utilise DMDs as digital reflective light modulators. Such devices provided 2D scanning speeds up to 32.5 kHz for a wide range of wavelengths and twice as high diffraction efficiency compared to commonly used liquid crystal display technology. DMDs have also been used in dispersive imaging systems, but their relatively low power efficiency often poses limitations on performance. The REALTIMEIMAGING team sought an alternative for sub-wavelength phase manipulation for micro-scale light control in ‘Gap-plasmon metasurfaces’ (GPMs). Researchers fabricated a GPM-based grating operating at 1 550 nm to replace existing gratings in dispersive imaging systems. The power efficiency of this array of unit cells was measured at as high as 75.6 %, and the resolution achieved when incorporated into the dispersive imaging system as low as 300 μm. Sub-wavelength manipulation of the wavefront phase at the infrared wavelength opens the door to a wide range of telecommunication applications. This is notably the case since the proposed device is planar and can therefore be easily integrated with other components — a key property in future miniaturisation of complex systems. MEMS micro-mirror arrays for beam steering by individually tuning the mirror angles have in fact already been used to achieve an imaging area of 5 mm x 5 mm in a 50 ns timeframe. The details are described in a paper published in the Journal of Micro- and Nano-manufacturing.
Keywords
Subwavelength, imaging, laser scanning, REALTIMEIMAGING, digital micro-mirror devices, MEMS