Tailorable Resonant Emissivity in the Mid-Infrared Range Between 10 μm and 25 μm on Highly Doped Pristine Silicon Gratings

We investigate the radiative properties at room temperature in the mid infrared spectral range from 2 μ m to 25 μ m, of one-dimensional surface gratings made of silicon. We use heavily doped silicon to take advantage of increased carrier concentration, which translates into plasma frequencies lying in the mid-infrared. Further control of the spectral and directional emissivity is enabled by structuring the pristine silicon surface in the form of one-dimensional grating. The resulting radiating plasmonic modes are engineered by changing the geometrical parameters of the grating including the etching depth, duty cycle and periodicity, leading to tailorable resonant spectral response. Our simulation results predict that variation of the incidence angle has an impact not only on shifting the resonance wavelength, but it also appears to have an impact on the peak absorptance level as well as the quality factor. To experimentally validate the theoretical and simulation predictions, three grating samples are fabricated with etching depth of 0.5 μ m and periodicity of 7 μ m, 8 μ m and 12 μ m. Reflectance and transmittance spectra are measured under different incidence angles from 30 to 80 degrees. The experimental findings fit well with numerical predictions. The resonant wavelengths are red-shifting in the range between 11 μ m and 25 μ m and the quality factor is increases from 18 to 38 while maximum emissivity increases from 0.4 to 0.82 with increasing incidence angle. The proposed metasurfaces are good candidates for selective thermal infrared emitters in applications involving the control over spectral and directional emission and conversion of thermal radiation.