Multi-Functional CVD Diamond Window for Infrared Imaging

Diamond is well known for its outstanding thermal, electrical and mechanical properties. However, it is also of great interest for optical applications. Indeed, it is for instance well suited for broadband applications of multi-band operation due to its good optical transmission over a very wide spectral range from visible to LongWave InfraRed (LWIR) 400nm-15µm. Besides, diamond structuration enables the realization of optical components such as lenses, gratings and so forth. Going further at a smaller scale in structuring diamond can also enable the realization of “artificial” materials, such as metasurfaces or metamaterials. By controlling the shape of the structures, one can control the wavefront of the light in order to achieve gratings [1], or antireflective coatings [2].<br/>In this paper, we report on technology developments to elaborate full diamond and diamond-based active meta-surfaces, leading to robust optical windows combining broadband antireflective properties and superhydrophobic/anti-rain/anti-mist behaviors obtained by nanostructuration. A final growth of boron-doped diamond electrodes on the top of the structures enables to add anti-fouling property through electrochemical principles, driven by application of a low electrical current.<br/>The technological process used for the fabrication of 2’’ diameter LWIR windows is based on nano-imprint lithography, which is a scalable well-known lithography technique compatible with curved surfaces. Then a plasma etching step leads to periodic conical diamond structures of ~2.5µm-period and ~8µm-height. The multi-functionality of the nanostructured window is demonstrated through several characterization means:<br/>1. The achieved double sided-transmission is measured by FTIR (Fast-Fourier Transform Infrared Spectroscopy) at ~85% in the [8µm-14µm] range, as compared to ~68% transmission in the case of flat polished diamond, showing a transmission increase up to 12% on one side at 0° of incidence,<br/>2. The mechanical hardness of the diamond nanostructures, fabricated on silicon for MWIR (MidWave InfraRed) multifunctional optics realization, is assessed by nano-indentation and nano-scratching experiments, showing quasi-indestructibility,<br/>3. The thermal dissipation in the diamond is observed in the MWIR range using a temperature mapping microscope, and we show an accelerated de-icing of the diamond-based window when compared to a germanium window, typically used in cryostat systems,<br/>4. The superhydrophobic feature is validated through drop contact angle measurement. The nanostructured diamond window is also integrated in front of a LWIR imager to characterize its benefits on imaging applications, especially during rainy conditions,<br/>5. Finally, a growth of boron doped diamond, with a doping level compatible with optical specifications and thus inducing a very low absorption in IR regions, leads to antifouling properties. We show the efficient electro-chemical cleaning and the resilience of a diamond window, previously contaminated with a mix of oil/gasoline.<br/>Those properties open the way to achieving the ultimate optical window that can operate in the harshest environments.<br/><br/>[1] P. Forsberg and M. Karlsson, "High aspect ratio optical gratings in diamond," Diam. Relat. Mater. 34, 19–24 (2013).<br/>[2] M. Karlsson and F. Nikolajeff, "Diamond micro-optics: microlenses and antireflection structured surfaces for the infrared spectral region," Opt. Express 11, 502 (2003).