Srs 99 020

The photonic structures were fabricated by electron beam deposition at Optiforms, on 500-um-thick, 100-mm-diameter, p-doped single-side-polished <100> crystalline silicon wafers, with resistivity 1–10 Ω cm (UniversityWafer 783). To block transmission in the wavelength range below silicon bandgap, the unpolished side of the wafers were coated with silver paste. In the wavelength range of 0.3−1.8 μm, the reflection spectra of the samples were characterized using a spectrophotometer (Agilent Cary 6000i) with an unpolarized light source and a high Lambertian reflectance standard (Labsphere SRS-99-020). A diffuse reflectance accessory (DRA 1800) with a 150-mm-diameter integrating sphere was used to collect both specular and diffuse components of reflection at an 8° angle of incidence. In the wavelength range of 4−25 μm, a Fourier transform infrared spectrometer (Thermo Scientific Nicolet 6700) with a diffuse gold integrating sphere (PIKE Technologies) was used to characterize the samples. The absorptivity/emissivity spectra were obtained by subtracting the reflectance from unity. A scanning electron microscope (Nova FEI 450) was used to image the cross sections of the stacks in Fig. 3a, b.

Figure 7 shows a schematic diagram of the experimental system used in the present study. A white-light-emitting diode (LA-HDF5010; Hayashi Watch Works, Tokyo, Japan) illuminated the surface of the exposed cortex via a light guide. The angle of illumination was approximately 45° with respect to the cortical surface, to exclude specular reflection. Diffusely reflected light from the brain was captured with a 24-bit RGB charge coupled device camera (DFK-31BF03.H; Imaging Source LLC, Charlotte, NC, USA) with a zoom lens to acquire an RGB image. A standard white diffuser with 99% reflectance (SRS-99-020; Labsphere, North Sutton, NH, USA) was used as a reference material to calibrate the white balance of the camera. RGB images were acquired with a temporal resolution of eight frames per second (fps) and stored in a personal computer. An RGB image was acquired at an exposure time of 65 ms, for a temporal resolution of 15 fps. The field of view of the system was 5.45 × 4.09 mm² with 1024 × 768 pixels. Lateral resolution of the images was estimated as 5.3 μm.

The specular reflection was measured by multiple angle reflectometry (Film Tek 4000, scientific computing international, Carlsbad, USA) at normal and 70° incidence. The total reflectance was measured by optical spectrometer (OE65000, Ocean Optics spectrometer, USA) with an integrating sphere of 8° incidence (AvaSphere-50, Avantes, UK). The total reflectance measurement was calibrated by a white Lambertian scatter (SRS-99-020, LabSphere, USA). The scattering distribution of the samples was measured with an angular scatterometer, where a photodetector (New Focus Model 2032, Newport, USA) was rotated in a circular arc around the sample and the scattering intensity was evaluated in steps of 0.1°. A 40 mW argon-ion laser of 488 nm (60X, American Laser Corporation, USA) was used as the incident light source. The detailed description of the setup can be found in a previous publication33 (link). Canon EOS 5D Mark II camera was used to shoot the photos of the white DTU logo on the black silicon wafer. Except for the auto white balance function, no further post processing was used on the photos. The RGB profile of the photo was extracted by ImageJ.
The silicon surface texturing, SEM characterizations, and the specular reflection measurements were carried out in a class 10–100 cleanroom (Danchip, DTU, Denmark).