In the last decades great interest has been devoted to photonic crystals aiming at the creation of novel devices which can control light propagation. In the present work, two-dimensional (2D) and three-dimensional (3D) devices based on nanostructured porous silicon have been fabricated. 2D devices consist of a square mesh of 2
The concept of photonic crystal was proposed and discussed theoretically several decades ago [
In the present work, a relatively simple fabrication process is presented to manufacture 2D and 3D photonic crystals based on nanostructured porous silicon (PSi). A typical 2D structure consists of a square mesh of 2
There are several reasons to choose PSi as the dielectric material in the photonic crystal. On the one hand, the refractive index of PSi can be controlled by simply changing the porosity of the layers [
On the other hand, porous silicon is a very suitable material for the fabrication of photonic devices thanks to the ease of integration in silicon technology [
The 2D periodic dielectric structures that have been fabricated consist of a square mesh of 2
To fabricate the photonic structure, several steps are needed. First, PSi layers (2D) or multilayers (3D) are grown on a silicon wafer. PSi is grown by electrochemical etching crystalline silicon wafers in hydrofluoric acid solutions. Several parameters are important in this process, which determines the properties of the PSi layer. The most significant parameters are the applied current density, HF concentration in the solution, and etching time. By changing these parameters it is possible to control the porosity and the thickness of the PSi layers. To fabricate a multilayer structure, the applied current density is varied during the etching process. Further details are given in [
Once the PSi layer (or multilayer for 3D) is formed, a Cu grid is placed on top of the device in contact with the surface of PSi. The Cu grids used were 2000 Gilder TEM Cu mesh, with a structure of square veins 2
Schematic representation of the photonic crystal structure fabrication process.
In Figure
Scanning electron microscope image of a 3D photonic structure made by the new fabrication process shown.
The photonic band structure of 2D and 3D PSi-based photonic crystals was computed. In order to determine the PBS, the MIT Photonic Bands (MPB) package [
Transmittance response in the high-symmetry directions was calculated using the Translight software [
The dielectric structure used to calculate 2D PBS was a square grid of dielectric veins in air, with a width of
2D calculated PBS for a structure based on a square mesh of dielectric veins with a width of
Figure
The position and shape of the gaps of these structures can be tuned by changing either the pattern of the dielectric veins or their dielectric constant (i.e., porosity of the PSi layers). The pattern design is limited, in this case, to the available grid designs. However, the dielectric constant of the PSi layers is simply adjustable by changing the main parameters in the PSi fabrication process.
Figure
Complete TE gap map as a function of the PSi layer porosity. In the right
Moreover, the lower the porosity, the larger the size of the gap. This is an expected behavior since the lower the porosity, the larger the refractive index associated to the PSi layer, and consequently, the greater the contrast between refractive indexes.
In Figure
Figures
(a) Partial Γ-X gap map as a function of the PSi layer porosity. On the right
In Figure
Next, transmittance in the high-symmetry directions was computed by using the Translight software, considering a square mesh of air columns in a dielectric medium, with an area of
On the left, TM bands along the Γ-X direction for a 2D structure with a PSi layer of 40% porosity (
As it can be observed in Figure
3D photonic structures are fabricated following the same procedure as the 2D structures, but fabricating PSi multilayers, consisting in alternating PSi layers with different porosities, as represented in the inset of Figure
3D calculated PBS for a 3D structure based on alternating Psi layers of 40 and 80% porosity and
Figure
Figure
Partial Γ-A gap map as a function of the thickness of the different PSi layers. The lattice parameter used was
A novel and simple fabrication process of 2D and 3D photonic crystal based on nanostructured porous silicon, to operate in the thermal infrared region, is presented. 2D slabs, based on a square mesh of 2
3D photonic structures are based on the 2D structures, but alternating PSi layers with different porosity. The 3D structures exhibit new partial gaps in the new high-symmetry directions Γ-A and Γ-L. These new partial gaps can be tuned by changing the porosity of the PSi layers used, in the same way as the 2D structures, but also it is possible to control these new gaps by modifying the thickness of the individual PSi layers. The theoretical results suggest that these structures could be very promising candidates to develop low-cost silicon-based photonic crystal devices to operate in the thermal infrared region.
The authors thank L. García-Pelayo for technical support in the realization of the present work and acknowledge Centro de Computación Científica (Universidad Autónoma de Madrid) for providing computational resources for numerical calculations. The authors also gratefully acknowledge funding from Comunidad de Madrid (Spain) under project “Microseres” and Ministerio de Economía y Competitividad (Spain) under Research Project MAT2011-28345-C02-01.