My Research

Hi there!

Welcome to My Research. On this page you will find information about some of my spirals in plantscurrent research. I work on many other topics as well, though this page features one big research area only at this moment.

Spiral curves and Photonic Crystal Fibers

Spirals curves are well known mathematically and have entranced us for centuries. We see them in nature everywhere,   in snails, sunflowers, daisies, the arrangement of leaves in plants and trees, and even in massive structures such as galaxies. Spirals are self similar curves and have some very interesting properties.

petals of a rose in follow spiral growthThere is more than one type of spiral and each kind has its own special properties: equiangular or logarithmic spiral, archimedean spiral, golden spiral, fermat spiral and others…

spiral galaxyThe fact that these curves manifest themselves in the natural world at many scales and in diverse fields indicates that they have properties worth harnessing. This led me to investigate how to use spiral curves in Photonics and especially in Photonic Crystal Fibers.

What are spiral PCF?

A Spiral PCFSpiral PCF have lots of similarities to standard PCF, the main difference is that in spiral PCF the air holes are arranged in a spiral pattern (if you look at the transvere cross section of the PCF). There is no twist, helix or spiral of the air holes along the length of the fiber. The air holes are uniform. The image of a spiral PCF illustrates my point.

Why spiral designs are so cool (for PCF)

standard hexagonal PCF These PCF have some fantastic properties that are exploited in any number of applications. So my view is NOT that hex PCF are uncool, far from it. My view is simply that rather than asking the hex design to ‘be everything to everyone’, we ought to explore other designs that are more suitable for a particular application.

 Is it really so easy?

stack and draw process for hex PCF fabricationIn one word- no! There are some good reasons why the hex design dominates- the most important being that the ‘stack and draw’ technique is an extremely well understood and mature technique globally.

Other designs may not be easy to fabricate with it. So how do we overcome this challenge, if at all.

How?

Recent progress in materials and fab has given us techniques like extrusion extrusion for PCF

and drilling that can be quite useful for unconventional designs. Other materials such as soft glass, chalcogenides, polymers are also increasingly being used in many areas. Thus, in some material systems, at least, it may well be sensible to explore and exploit non-hex designs.

 Why?

Usually the answer to this question would be how an article begins! But here we go, if you’ve stuck around till now, you have some interest (yay!). So let me list some of the reasons we should explore

– there is any number of designs out there to choose from, it makes sense to pick the best one for what you want

– it may be more cost effective in the long term by offering better performance

– diversity in design and thought is important for innovation

– curves like spirals are well understood mathematically

– some of that stuff is fun to play with

My  piece on spirals now!!

So for all those reasons listed above, I started playing around with spiral curves which form a family. There are many kinds and their properties are different.

The first one I looked at was the fermat or golden spiral (see my papers Golden spiral photonic crystal fiber: polarization and dispersion propertiesPolarization and dispersion properties of elliptical hole golden spiral photonic crystal fiber). It gives the most even distribution of points in space.Sunflower and golden spiral

Sunflower seeds grow with this beautiful pattern.

There is also something quite seductive about the idea, that if the ‘golden ratio’ applies to much of nature’s perfect designs, why cant it give something good to photonics? If you remove the central point in a golden spiral distribution, the core area becomes asymmetric- hence can give large birefringence in a PCF. Though there is no periodicity, this kind of structure has a band gap. Its not even similar to quasi-crystals which were all the rage for some time and got the Nobel for chemistry in 2012!

The Equiangular spiral was the next spiral that attracted me. Its a self similar equiangular spiral in naturecurve- as it grows it never loses its shape, which remains same. shells often have this shape.  if we draw several such archiemedean spirals and place holes (for a PCF) along these, the distance between the second and first ring is only 0.49r as compared to 0.89r for a hex. This implies tighter field confinement. by being able to arrange the holes at angles other than 60degrees, we can make this confinement tighter for non-linear applications along with great dispersion control.

At the present I am working on adapting the Archimedean Spiral as well as on exploring bandgap properties of Spiral PCF.

For some links to read more about spirals see

http://mathworld.wolfram.com/Spiral.html

http://www.ics.uci.edu/~eppstein/junkyard/spiral.html

Design and optimization of solar cells

hut pillars image2

SEM image of hut like micro pillar array

Tremendous effort is being spent on developing high efficiency Photovoltaic (PV) cells or solar cells. Organic, inorganic, hybrid, single and multijunction cells are all part of a vast array of cells. Different materials (established materials like Si)  including more recent entrants like perovskites and others are being studied.

A key consideration is lowering or eliminating the Fresnel reflection that inevitably occurs when sunlight is incident on the cell surface (due to index difference between air and the cell material). Anti reflection coatings (ARCs), plasmonic nanostructures, other texturing patterns that concentrate light by multiple reflections are all research areas to improve light absorption.

My work is in modeling the cell response to incident light and designing structures that reflect less. Currently I am working on “hut like” micro pillar pillar arrays on Si cells that trap light more effectively even than nanowires and nanopyramids. This work in collaboration with University of Bath and London South Bank University  follows from the micro pillar study we did earlier.  Another project is looking at GaAs solar cells and designing ZnS and MgF2 ARCs for these with Univeristy of Northumbria, Newcastle.

An idea that I am currently thinking about is modeling porous Si nanowires on cell surfaces ( which include an upconverter layer to harvest photons that are not absorbed as their energy is below the bandgap).

Other areas:

Development of time domain Finite Element techniques

Modelling of Graphene waveguides

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3 comments on “My Research

  1. Pingback: The Solar Cell Diary – part 1 | artiagrawal

  2. Pingback: Sunday at Laser World of Photonics | artiagrawal

  3. Light without heat. It can be achieved by an elliptical Archimedean spiral photonic crystal fibers. It reduces photon spin and therefore reduces the heat associated with light. This will reduce the Fresnel reflection. Sunlight should be filtered of its heat in this fashion to improve greatly the efficiency of the solar cell. Worth a try.

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