# Broadband Holography via Structured Black Silicon Nano-Antennas
## 3 Results  
Figure 1a,b shows the concept of BSi holograms. The fabrication process starts calculating the binary amplitude interference pattern (see Materials and Methods). This is then recorded in the resist layer, on top of the Si surface, through a photolithography process. Once the sample is developed, the subsequent dry etching process generates subwavelength Si nanoantennas in the resist-free areas of the Silicon wafer.  
To project the hologram image onto a screen, we illuminated the sample with a laser source of a specific wavelength. The image is formed by using the optical setup shown in Figure 1c, in which the hologram and the sample are at 90\({}^{\circ}\) with respect to each other.  
The broadband reflectivity of the flat Si and the broadband absorbance of the BSi allows the hologram to work at different wavelengths.  
Figure 1: (**a**) Holographic reconstruction procedure and image forming by illuminating the sample with a coherent laser source. \(\theta\) is the projection angle of the holographic image. (**b**) Schematic overview showing an incident laser beam reflected from the undeveloped silicon surface while the BSi region absorbs the incident light. (**c**) Optical setup used to reconstruct the hologram image.  
Figure 2 shows the role of Si nanotips as nanoantennas, which enable a change of the reflectivity of the substrate from 0.35 to 0.1. In particular, Figure 2a-d shows the effect of the dry-etching time on the interference pattern realised in the sample. Figure 2e-h shows the evolution of Si nanoantennas' growth by increasing the etching time: (e) 20 min, (f) 30 min, (g) 40 min, and (h) 50 min.  
Finally, Figure 2i-n plots the normalised reflectance of the structure for different etching times. The figure demonstrates the fact that the response of the structure is completely flat among all the visible wavelengths.  
It can be seen that in Figure 2a for \(t=20\) min, the sample appears to be brown. This originates from the fact that at a low etching time, the Si nanotips are not yet created and the system is still reflective.  
As the etching time increases, the height of pillars increases as well, reducing the reflectivity of the surface [26]. For an etching time comparable to \(t=50\) min (Figure 2h) we observe that the height of the Si nanotips starts to decrease. Another effect that appears beyond this etching time is also the etching of the protected layer, and how the sample starts to become completely black.  
Figure 3a shows the original digital image that we chose to record for our BSi holograms, where it is composed of a sketch of the Rubiks cube with the word PRIMALIGHT. To show the large-scale possibilities of this technique and the possibility to fabricate more than one hologram at the same time, Figure 3b shows four different fabricated samples on the same Si wafer. Although the patterns are designed to work at 532 nm, all of them are able to work for multiple wavelengths. The dimension of the fabricated sample are 3 cm by 3 cm and 2 cm by 2 cm.  
Figure 3c shows the numerically reconstructed image from computer simulations. Figure 3d-f shows the experimentally reconstructed images of the Rubiks cube with the word PRIMALIGHT,  
Figure 2: (**a–d**) Structural evolution of a BSi holographic pattern by ICP-RIE, inspected after 20 min, 30 min, 40 min, and 50 min. The pictures display the microscopic image of the fabricated holographic pattern. (**e–h**) Shows the cross-section SEM images of the Si nanoantennas for 20 min, 30 min, 40 min, and 50 min, respectively. (**i–n**) Normalised reflectance of fabricated BSi for different etching time: 20 min, 30 min, 40 min, and 50 min, respectively.  
using the hologram highlighted by a red circle in Figure 3b and illuminated with different wavelengths, being (d) 440 nm, (e) 532 nm, and (f) 632 nm, respectively. The reconstructed images show very good accuracy with respect to computer simulations. We calculated efficiencies for the three wavelengths of 10%, 14%, and 12%, respectively.