Indoor air quality monitoring is crucial for the health of the families especially when the children are involved as they are in a higher probabilities of getting disorders directly from pollutants. Out of these many pollutants inside the home, Voltaile organic compounds (VOC's) dominates the others as they were found to be 2 to 5 times higher in indoors compared to outdoor environment. In order to know the precise concentrations of these pollutants, one needs a monitoring technology which obeys the following three aspects.
1) Sensitivity
2) Selectivity
3) Longevity
The current available monitoring devices are based on metal oxide and optical based sensors. The problem with the metal oxide sensors are they require heating element for sensing ranges from 100 degrees to 300 degrees or even more based on the pollutants. These metal oxide based sensors provides higher sensitivities but they lack selectivity issues due to the presence of heating element. Also, the longevity is also the issue with these metal oxide sensors as they frequently need maintenance. Also, the gradient change of temperatures effects the sensitivity of these sensors. Since, optical based sensors offers higher sensitivities and longevity, they suffer from surrounding light and near infra red scattering. Major disadvantage of the above mentioned technologies are they don't have the ability to self-clean. It requires the manual cleaning of sensors which is time consuming and requires maintenance. By considering the above challenges, we are adopting a unique technology which is gaining momentum these days which is "Silicon Photonics or Nanophotonics". This technology offers sensitivity, selectivity and longevity at the nano scales. By confining the light into to the waveguides, and by increasing the time of interaction of light, the higher sensitivities can be obtained. We are using a unique material known as "Polydimethylsiloxane (PDMS)" as a coating which selectivity allows the VOC's into the sensing area and ejects them out after few seconds thus achieving the self-cleaning of VOC's. By monitoring VOC's accurately, our solution helps in installing the device in buildings, homes and by few modifications, can also be installed in space crafts. We are also trying to make the device so small so that it can also be used as personal werable.
Due to lock down, people restricted to stay indoors and we've heard that the people are actively talking about the benefits of lock down on environment as the pollution levels reduced drastically. This is true for the outdoor environment but there is no lock down inside the homes and people are spending huge amounts of time inside their homes like never before thus posing a chances of increasing indoor air pollution due to various activities. This inspired us to choose this challenge. Our work is also inspired from the work of "Recardo Janeiro" and his colleagues from department of electrical and computer science, khalifa university of science and technology, UAE.
We are building the sensor using silicon photonics by choosing appropriate materials required for guiding the light. Silicon waveguide is chosen with the dimensions of 450x220 nm of width and height which is capable of guiding the light of wavelength 1550 nm which is buried in Sio2 layer (cladding). The PDMS is taken as a coating on the cladding material which selectively allows the VOC compounds into it.
We are using Lumerical mode software in order to find the modes propagating inside the waveguide and lumerical FDTD software for simulating the entire structure.
We are using ATom: L2 Volatile Organic Compounds (VOCs) from the Trace Organic Gas Analyzer (TOGA) data which is available in NASA's Open Data Portal. This dataset is helpful for us to find out what kind of VOC's are invloved, their sources and concentrations.
The previous version of our research has been presented and published in IEEE Sensors 2018 conference and received positive feedback with some modifications and we did the same. Link to the paper is given below.
https://ieeexplore.ieee.org/document/8589932
The Presentation slides of the proposed solution is found in the given link below:
Initially, the light source of 1556 nanometer wavelength is chosen and guided in the Silicon waveguide (450x220nm) buried in silicon dioxide cladding of thickness 3 micrometer. The waveguide basically supports two modes namely, TE and TM modes. For sensing applications, TM mode gives the better results as it involves evanescent fields. We have chosen PDMS as a selective material for VOC's as it allows only weakly polar solvents. The PDMS has a property of swelling after absorbing the VOC's and then get it's original shape back. The PDMS is only coated in the sensing part which in this case on four microring resonators. The reason for choosing four microring resonators because, the interaction of VOC's with the light increases if the light takes longer path possible. α and β are coupling and propagation coefficients and can be expresses as general solutions of entire system. They form the matrix N whose diagonal elements are occupied by coupling coefficients and upper and lower diagonals are occupied by propagation constants.
They form the eigen states of the proposed system with known eigen solutions. If no VOC compounds are present, the eigen state and eigen solutions of the system remains same as before and if any VOC's gets interacted with the system, the PDMS swells and these VOC's changes the refractive indices of the resonators thus effecting the coupling and propagation coefficients. Thus eigen states changes so does the eigen solutions. By solving these eigen solutions, we can find how much VOC concentrations on the sensor.
We are using ATom: L2 Volatile Organic Compounds (VOCs) from the Trace Organic Gas Analyzer (TOGA) data which is available in NASA's Open Data Portal. Link to the data is given below:
https://data.nasa.gov/dataset/ATom-L2-Volatile-Organic-Compounds-VOCs-from-the-T/rfvs-hq7z