‘Accurate modelling of light at the nanoscale for efficient solar cells’
In this thesis three main ways in which the symmetry of a 3D photonic band gap crystals is disrupted, are described. ‘By studying symmetry-disruptions, we learn a lot about nanophotonic media, as functioning in everyday life,’ D. Devashish says. Investigated are: (i) finite sized disruptions, (ii) material absorptions and (iii) point defects acting as a resonant cavity.
‘We validated our numerical models with respect to analytical models, and interpreted our numerical findings using fundamental theories of physics,’ Devashish says. ‘In studying the reflectivity of 3D photonic band gap crystals with finite support, we found a good balance using numerical computations - to model the light propagation - and the physical limitations as observed in experimental settings.’
Observed was that the stop band hardly changes with the incident light angle. This supports the experimental notion that strong reflectivity peaks measured with a large numerical aperture, gives a faithful signature of the 3D band gap.
Then, a 3D photonic band gap crystal with finite support as a back reflector to a thin silicon film in the visible regime, was studied. ‘Intense reflectivity peaks were predicted and actually observed,’ Devashish says. ‘Our numerical study reports a nearly 2.6 times enhanced frequency-, angle-, polarization-averaged absorption (between λ = 680 nm and λ = 890 nm) compared to a thin silicon film only. Within our field of research this observation was highly rated: that the maximum reflectivity inferred, is not limited by the finite size of the crystal.’
Devashish believes there are good reasons his results to be of practical use in the middle-term future in photovoltaics.
He concludes: ‘Our results indicate that 3D photonic band gap crystals with resonant cavities, are interesting candidates for the absorbing medium of a solar cell, in order to enhance the photovoltaic efficiency. Our analysis provides a numerical signature of cavity resonances, appearing due to the locally disrupted lattice symmetry in a 3D inverse woodpile photonic crystal.’
During the PhD project Devashish found himself learning physics. ‘The results we found in numerical analysis, posed from itself the logical question: What does it mean?’ he says.
‘We carefully constructed new theoretical insights, which were published, amongst others, in Physical Review Letters B. Within the Complex Photonic Systems group, in collaboration with the Mathematics of Computational Science Group, I enjoyed modelling real-world systems using physics based mathematical theories. It enriched my scientific expertise, and contributed to the expressiveness and practical importance of my work.’
Also, important fundamental mathematical results were obtained. In the last chapter, Devashish investigated and implemented a novel numerical method, to provide an accurate model of light propagation in nanophotonic media.
‘We developed a novel solver using the discontinuous Galerkin finite-element method (DGFEM) for the time-harmonic Maxwell equations with periodic dielectric materials,’ Devashish explains. ‘For accurate eigenvalue computations, we have explicitly implemented the divergence constraint, which is often neglected.’
Both as a scientist and as a person, Devashish mentions his growing skills on interaction in research, and on presenting his results to a broader audience.
‘I now can talk for hours about my research, for example to children and students,’ he says. ‘I know how to hold their attention. Within Mesa+ I enjoyed the Mesa+ Days and colloquium meetings – such as within the Advanced Nanophotonics program - in which I was allowed to present my work. I was given detailed and critical feedback in a very positive way.’
During international conferences Devashish met Development & Engineering (D&E) experts working within the ASML company, located at Veldhoven, The Netherlands.
‘I am happy to join their D&E department in the years to come,’ he says. ‘Surely I will miss the academic research to some extent. My main drive is to see back what I did, in devices used in everyday life. That really would mean a lot to me.'