Optical imaging devices in the mid-infrared play an important role for a wide range of technological applications in areas such as thermography, automotive safety, and astronomy. The operation of such devices is enabled by the fact that the earth’s atmosphere shows two distinct “windows” of high transmission in the 3-5 μm and 8-12 μm spectral ranges, which can be leveraged to avoid disturbances from water vapor, dust, or other atmospheric influences. In order to realize advanced optical devices that go beyond traditional reflective and refractive systems, it is important to develop methods for controlling both the amplitude and the phase of incoming light at different spatial positions of an optical element. However, so far, such spatial light modulators (SLMs) are either not readily available in the mid-IR or require bulky machinery based on moving mirror approaches.
Here, we utilize ultra-thin metasurfaces consisting of subwavelength resonant elements to develop a novel platform for mid-infrared phase control. Specifically, we focus on reconfigurable metasurfaces, where the nanoscale resonators are combined with an active material layer to provide an external handle for tuning the phase of the incident light. To obtain SLM functionality, the metasurface can be subdivided into an array of individual metapixels, enabling spatial control over the light transmission.
As an active material, we incorporate germanium antimony telluride (GeSbTe, GST), which is ideally suited for use in reconfigurable metasurfaces due to its two non-volatile phases (amorphous and crystalline) at room temperature. The transition between these two phases produces a strong refractive index contrast and can be triggered using all-optical laser-based switching methods, enabling ultrafast control over the metasurface’s optical properties.
The development of such nanophotonics-based devices with external tunability will enable the technological realization of compact amplitude and phase modulators with a multitude of applications ranging from adaptive optics for infrared astronomy and night vision systems to improved free-space optical communication systems.
Active plasmonic metasurfaces for mid-infrared phase control and adaptive optics
This project has received funding from the European Union’s Horizon 2020 Framework Programme for Research and Innovation under grant agreement 665667 (call 2015).