Transcript Shaping the color - University of Guelph

Shaping the color
Optical property of photonic crystals
Shine
Where the nature’s colors from?
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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.
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.
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).
(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.
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.
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.
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.