Shaping the color

1. Shaping the color Optical property of photonic crystals Shine

2. Where the nature’s colors from? • The absorption of light by pigments • Some biomaterials that are cast at the scale of light into bio-optical diffraction gratings • Natural gratings that diffract light are based on dielectric lattices with periodicity at optical wavelengths. • They can be 1D, 2D, and 3D • Studying of the Natural gratings suggests that 3D optical diffraction gratings can be synthesized with dielectric lattices that are geometrically complementary. i.e., one is the inverse of the other.

3. Photonic crystals • Photonic crystals are materials patterned with a periodicity in dielectric constant, which can create a range of 'forbidden' frequencies called a photonic bandgap. Photons with energies lying in the bandgap cannot propagate through the medium. • When made with the right materials and structure they can be considered to be the optical analogue of the semiconductor enable them to be used as active materials in optical transistors, diodes, and other devices.

4. Photonic semiconductors • The solution of Maxwell’s equations for the propagation of light in a dielectric lattice shows that discontinuities appear in frequency Vs wavevector photon dispersion due to optical Bragg diffraction. • Photonic band gap (PBG) is a frequency range in reciprocal wavevector space k, where the propagation of light is forbidden in all directions in the crystal. In this energy range light incident on the crystal is completely reflected and light created within the crystal is completely trapped. • The gap can be incomplete, called a stopgap, when there are certain directions in which the range of frequencies is allowed to propagate (like leak).

5. (a) This photonic waveguide formed from a thin silicon membrane contains a triangular lattice of air holes separated by 300 nm. (b) A plot of transmission versus wavelength for the device shows that it has a photonic band gap between 725 nm and 825 nm if the electric field associated with the electromagnetic wave is perpendicular to the direction in which the light wave propagates (red). In contrast, the light can pass freely through the material when the magnetic field is perpendicular to the direction of travel (blue). In this case, the photonic band gap is said to be incomplete. (c) A photonic crystal viewed from above. Laser radiation with a wavelength that lies inside the band-gap range enters the structure from the left and is completely blocked. (d) Light at longer wavelengths outside the band gap can pass through the structure.

6. Defects • It’s the imperfection of solid-state materials rather than their perfection that provides them with interesting properties and ultimately function and utility. • By introducing micron scale point, line or bend defects into a perfect photonic lattice, electromagnetic modes emerge within the PBG which can localize light at a vacancy and confine and guide light along and around tiny waveguides.

7. Color tunability • Maxwell’s equations for photonic crystals are scalable, in contrast to the electronic crystals. • One way to tune the photonic crystal properties is by manipulating the lattice structure and dimensions. • Another way is manipulating the dielectric or refractive index contrast of the materials that comprise the crystal (changing the composition of the crystal or exploiting the properties such as optical non-linearity, metal-nonmetal transition, liquid crystal, to effect the refractive index change.

8. Change color • The brilliant opalescence can be immediately observed when a colloidal crystal is illuminated with white light. So the self- assembled photonic crystals are a efficient means to achieve color, i.e., structural color through the interaction of light with a periodic dielectric microstructure rather than from a pigment. • With an almost infinite choice of construction materials, and the less demanding requirement for human eyes, it’s a simple and robust use for this material. • Also pls note that it can change both color and intensity by its refractive index, degree of filling, and filling homogeneity.