Tunable 3D Photonic Crystals for Mid-IR Gas Sensing using Metamaterials

Photonic Crystals (PhCs) are periodically structured dielectric materials that have the potential to slow down the group velocity of light, known as the slow-light effect. Due to the complex architecture of PhCs, planar microfabrication techniques result in the fabrication of PhCs being complicated and costly. PhCs were fabricated by a direct laser writing (DLW) approach with 2-Photon Polymerization (2PP) using a negative-tone UV curable resin (IP-DIP). The capabilities of PhCs in two-dimensional (2D) and three-dimensional (3D) were modeled using finite element analysis (FEA). The photonic bandgap (PBG) analysis was used to tune the crystals to specific wavelengths making them tunable. The sensors were then infiltrated with Zinc Oxide (ZnO) to compensate for low refractive index contrast. PhCs were tested for physical and optical property changes by infiltration. The infiltrated PhCs showed enhanced IR gas sensing results compared to traditional methods. The PhCs were fabricated using a commercially available 2PP system (Nanoscribe Photonic Professional GT+). The response of fabricated structures was studied using Fourier Transform Infrared spectroscopy (FTIR). The accuracy of structures was confirmed using Scanning Electron Microscopy (SEM). The structures were infiltrated with ZnO using the Gemstar Atomic Layer Deposition (ALD) system with a controlled operation for coating the inner walls. The morphology of infiltrated PhCs was characterized by SEM and energy dispersive spectroscopy (EDS) followed by FTIR to observe changes to the absorption spectra. Defect engineering was also performed on PhCs and was experimentally observed. The infiltration was tested for physical parameters such as hardness, elasticity, thermogravimetric analysis (TGA) etc to observe any property change. The gas sensing setup was then integrated within the FTIR with gas flow controlled by mass flow controllers. The PhCs were tested for Carbon-dioxide (CO2) at room temperature with 1% CO2 in a dry nitrogen mixture created in-house using the manometric method. The chamber was constantly purged with dry Nitrogen (N2) before and after the tests to remove any moisture or trace gases. Infiltrated PhCs show improved sensing capabilities of PhC. In comparison without PhC, an enhancement of 12 was obtained when PhCs were used. The tests were repeated using three different PhCs and all three PhC sensors show enhancement confirming repeatability. A test with an increased height of PhC was performed to obtain even further enhancement confirming increased slow-light action with an increase in the PhC path.The results obtained may help develop sensors (a) with capabilities of on-chip IR sensing, (b) sensors selective to different gases, (c) use of 2PP in the fabrication of micro-optics sensing systems, and (d) use of SIS for enhancing the optical properties of 2PP fabricated structures for better applications. Hence, the sensors developed in this work have the potential to overcome challenges and play an important role in on-chip sensing platforms leading to advantages in climate change, self-monitoring etc.