20 40 60 80 30 210 60 240 90 270 120 300 150 330 180 0 0 20 40 60 80 100 120 140 160 180 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 RIss
Diffracted angle φ (degrees)
R ela tiv e in te ns ity 450 nm 535 nm 632 nm 0 20 40 60 80 100 120 140 160 180 0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 RIps
Diffracted angle φ (degrees)
Re la tiv e in te ns ity 450 nm 535 nm 632 nm P-polarization
Birefringent Gratings to Be Used as Polarized
Colour-separator for Liquid Crystal Displays
100 % 80 % 40 % 80 % 90 % 30 % 3-6 % 0 % total backlight polarizer LC analyser colour filter bright state statedark Transmission 100 % 80 % 40 % 80 % 90 % 30 % 3-6 % 0 % total backlight polarizer LC analyser colour filter bright state statedark Transmission
Lightguide
Diffracted light Grating
Man Xu, H.P. Urbach and Dick K.G. de Boer
Problem
Alternative approach
Near field
• Grating was designed such that,
– 0thdiffraction order is evanescent,
– 1st diffraction order propagates normal to the surface,
– Colours are separated.
• Grooves are filled with birefringent material
to select polarizationsuch that,
– P-polarization is blocked, – S-polarization is diffracted. 0 1 2 E S-polarization, Ey 0 0 . 0 10 . 02 H y P-polarization, Hy 0 20 40 60 80 100 120 140 160 180 0 200 400 600 800 1000 1200 1400 1600 Contrast ratio s/p
Diffracted angle φ (degrees)
C on tra st rat io 450 nm 535 nm 632 nm
The diffracted transmitted intensity strongly depends on incident angles and has high contrast ratio for angles of interest.
• no= ng: p-polarization is totally reflected.
• ne> ng: s-polarization is diffracted.
Far field
0 20 40 60 80 100 120 140 160 180 0 0.5 1 1.5 2 2.5 3 3.5x 10 -3 RIspDiffracted angle φ (degrees)
R el ati ve in te ns ity 450 nm 535 nm 632 nm 3 10− 0 20 40 60 80 100 120 140 160 180 0 0.5 1 1.5 2 2.5 3 3.5x 10 -3 RIpp
Diffracted angle φ (degrees)
Re la tive in te ns ity 450 nm 535 nm 632 nm 3 10− LCD – an inefficient system
The light efficiency is below 10% is because of the use of absorption based polarizers and colour filters.
