Question 8

Question 1

q8a)

You see represented below a device meant at dispersing white (polychromatic) light and guide to the lower layer only light of pre‐selected wavelengths. The device would use optical phenomena such as diffraction to manipulate incident white light. Wavelengths possibly selected depending on the angle, would be collected by underlying waveguides, embedded in a layer with lower refractive index, acting as a cladding with tunable refractive index. The waveguides, of non‐cylindrical shape, are designed in order to possibly achieve also focussing during light transmission. Irrespective of the real chances this device has of actually functioning, let’s focus only on how to realize such complex structure.
It consists of a substrate A which can be either metallic (e.g. Al, Si, Ag, Au, Cu etc…) or it can also be a quartz slide covered by thin films of ITO or FTO, which are commercially available.
On top of substrate A, layer B is made of a glass ceramic nanocomposite material based on silica containing SnO2 nanocrystals. It also contains waveguide structures C with a funnel shape and thickness identical to B. The smaller circular opening of C is therefore in contact with the substrate A, while the larger section reaches the surface of layer B. Aiming at transparent waveguides, the funnels are made with high refractive index ceramic nanocrystals (TiO2, HfO2, etc…), assembled to minimize porosity/scattering and maximize transmittance in the visible range. Nevertheless, the funnels can be randomly located over the surface of A.
The top layer D is a photonic crystal made of spherical particles in a highly ordered monolayer, responsible for light diffraction/selection.

How would you realize layer A+B containing the funnel shaped guides C? Please describe the main steps and justify each of your choices (i.e. stating why you find your strategy advantageous, and where you expect to encounter issues, and why). In particular, propose an idea, how you would make the funnel shaped structures C, embedded in B.

Answer 1

A:
- Spin coating of ITO nanoparticles
- or by molecular approach (sol-gel) & dip coating into sol & anneal
+ its easy to do and fast
problems: uniformity hard to achieve with spin coating, dip coating more time consuming

B:
Cons:
- put glassceramic precursor solution on A
- Fabrication of mold (has the funnels in it) and put on A, press down
- Hydrolyse & Gel → Evaporate solvent → Xerogel → remove mold
- increase temperature to precipitate SnO2 nanocrystals & densification (control porosity by temperature and time)

Pros:
+ different shapes possible by mold
+ you can change the number and placement of the funnels very easily
Problems: densification could shrink glass-ceramic, height of mold to fill everything (enough for capillary effects, if liquid fills everything)

C (if he asks):
- sol gel & annealing 3D printing method:
- A same as above
- print polymer in funnel shape onto A with micro SLA
- Pour precursor solution on A
- heat & densification & burn polymer funnels away

Question 2

q8b)

How would you realize the upper layer D?
Please describe the synthesis of the photonic crystals and their assembly on top of B.

Answer 2

What are photonic balls: assemblage of a polycrystal which exhibits grating (surface periodicity), bragg diffraction (need several crystal planes) and scattering
Photonic balls synthesis:
- colloidal crystallization in emulsion droplets (monodispersed water droplets with polystyrene colloids in oil stream) water is removed by heating → self-assembly into monodispersed photonic balls

Assembly:
- Spin coating in a easy evaporatable liquid (e.g. chloroform)

photonic crystals - make polymer balls:
- use droplet flow (for 150nm - 2um): styrene → polymerization is light induces
- spin coat monolayer on substrate (can do Bragg reflection)

Critical thinking:
- Ball size: 0.5 - 30 um → photonic balls from paper have a size of: 5-50um → problem to do very small balls

Question 3

q8c)

List at least 2 ways with which you could further increase the wavelength selection by the photonic crystal layer D, so that the light reaching waveguides C would lack a bigger part of the spectrum.

Answer 3

• change size of polystyrene colloids → different crystal plane distance => Bragg reflection changes
• Introduce gold nanoparticles in the photonic ball, which lower the reflection of the light in the blue
  / green part of the spectrum (more red appearance)

for polymer balls:
- different size of photonic crystal
- multi-layer photonic crystal: use different sizes of crystal as multilayer
- composite materials: use spheres coated with different refractive index materials

Question 4

q8d)

Is “order” necessary, beneficial or detrimental in layer B and in funnels C?

Explain your answer

Answer 4

C has to be transparent → has to be disordered else there is grating & Bragg diffraction (not good)

B has to be ordered so that the light is reflected & only goes through the waveguides / funnels C to A