Yashaswini,

You can measure reflectance using FTIR spectrometer and placing the sample on a rotation stage to probe certain directions corresponding to a reciprocal lattice. Then you compare it with band structure calculations. Take a look at our publication.

https://www.researchgate.net/publication/233809471_Large-scale_synthesis_of_a_siliconphotonic_crystal_with_a_completethree-dimensional_bandgapnear_1.5_micrometres?ev=prf_pub

Hope it helps.

Article Large-Scale Synthesis of a Silicon Photonic Crystal with a C...

I agree with Serge.

Only measurements in comparison with theory can indicate a complete photonic bandgap.

We have used angle-resolved transmission spectroscopy (as suggested by Serge) and compared it to numerical predictions of the angle-resolved transmission. With the same structural and materials parameters that we had used for the numerical transmission calculations (Scattering matrix method) we have then done band structure calculations (Photonic Bands Package, MPB).

At the end, experimental and theoretical transmission agrees very well, and MPB suggests a complete photonic band gap.

https://www.researchgate.net/publication/236908905_Titania_Woodpiles_with_Complete_Three-Dimensional_Photonic_Bandgaps_in_the_Visible

Article Titania Woodpiles with Complete Three-Dimensional Photonic B...

I just want to say I wish it would be simpler, similar to probing the a band gap of a semiconductor with photoconductivity or luminescence as function of wavelength. I mean that when you do such studies on semiconductors, you get a pretty good idea what the band gap is. Sure, you can confirm it with modeling but these techniques can go to a battle first and bring you what you need. With PBG, good luck doing measurements and NOT having any model to compare with! Those "spaghetti lines" give you all kinds of stop bands ( don't confuse them with band gaps!) plus controlling the sample angle - how would you know what direction you're probing...

It's like going fishing with SEM - one can always find something cool looking which may turn out to be an artefact.

thnx Grabtchak and Fischer. I will see the references......many thnx for the materials.....

Yes, reflection or transmission spectra for photonic crystals give exactly the opposite result as they do have not absorption. However, unlike the case of semiconducors, the measurements strongly depend on direction. So, you have to put the simple on a rotational table.

Thnx popescu.....