Phys 322 Chapter 8 Lecture 22 Polarization
Reminder: Exam 2, October 22nd (see webpage) Dichroism = selective absorption of light of certain polarization
Linear dichroism - selective absorption of one of the two P-state (linear) orthogonal polarizations
Circular dichroism - selective absorption of L-state or R-state circular polarizations
Using dichroic materials one can build a polarizer Dichroic crystals
Anisotropic crystal structure: one polarization is absorbed more than the other
Example: tourmaline
Elastic constants for electrons may be different along two axes Polaroid
1928: dichroic sheet polarizer, J-sheet long tiny crystals of herapathite aligned in the plastic sheet
Edwin Land 1938: H-sheet 1909-1991
Attach Iodine molecules to polymer molecules - molecular size iodine wires Presently produced: HN-38, HN-32, HN-22 Birefringence
Elastic constants for electrons may be different along axes
Resonance frequencies will be different for light polarized Refraction index depends on along different axes polarization: birefringence
Dichroic crystal - absorbs one of the orthogonal P-states, transmits the other Optic axis of a crystal: the direction of linear polarization along which the resonance is different from the other two axes (assuming them equal) Calcite (CaCO3)
Ca C O Image doubles Ordinary rays (o-rays) - unbent Extraordinary rays (e-rays) - bend Calcite (CaCO3)
emerging rays are orthogonaly polarized
Principal plane - any plane that contains optical axis Principal section - principal plane that is normal to one of the cleavage surfaces Birefringence and Huygens’ principle Crystal structure and birefringence
NaCl - cubic crystal
Four 3-fold symmetry axes - optically isotropic Anisotropic, uniaxial birefringent crystals: hexagonal, tetragonal, trigonal
Uniaxial crystal: atoms are arranged symmetrically around optic axis
O-wave - E everywhere is perpendicular to the optic axis, no c/v
When E is parallel to optic axis: ne c/v||
Birefringence ne - no principal indices of refraction
Calcite: (ne - no) = -0.172 (negative uniaxial crystal) Crystal structure and birefringence
Negative uniaxial crystal Positive uniaxial crystal Biaxial crystals
Two optic axes and three principal indices of refraction Orthorhombic, monoclinic, triclinic
Example: mica, KH2Al3(SiO4)3 Birefringent polarizers
Nicol prism Glan-Thompson Wollaston prism polarizer
1828, William Nicol Polarization by scattering
Scattering can polarize light
Scattering can depolarize light Polarization by reflection
Reminder: Fresnel equations show r||=0 at B - Brewster angle
1 B tan n2 n1 At Brewster angle reflected and transmitted rays form right angle
B
Reflected light will be fully
polarized at B Fresnel equations tan2 2 R i t R r || 2 Fresnel equations tan i t 2 sin sin i t i t R r 2 sin i t sin i t
tani t r|| tan … i t
Reflectance of unpolaized light: R R R || 2 Fresnel equations
Multiple plate polarizer
Degree of polarization:
V I p Itot
analyzing polarizer I I V max min Imax Imin detector Glare is horizontally polarized
Puddle reflection viewed Puddle reflection viewed through polarizer that through polarizer that transmits only horizontally transmits only vertically polarized light polarized light
Light reflected into our eyes from the puddle reflects at about Brewster's Angle. So parallel (i.e., vertical) polarization sees zero reflection.
Polarizer sunglasses transmit only vertically polarized light. Polarizers are very useful in photography.
Without a polarizer With a polarizer
The effect of a polarizer is probably the one “filter” effect that you can’t reproduce later using Photoshop! Retarders
The two orthogonal P-states develop mutual phase shift as they pass through a retarder
After propagating through: E E|| E polarized light phase shift: E E|| E birefringence extraordinary 2 in-phase ordinary dne no 0 in vacuum direction of polarization: E|| E0|| coskx t fast axis - the one that propagates faster slow axis - the one that propagates slower E E0 cos kx t Full-wave plate 2 dn n e o = m.2, m=…,-1,0,+1,… 0 E|| E0|| coskx t Alternatively: dne no m0 E E0 cos kx t
full wave - no observable effect
Note: n=n(), and it is generally true only for one wavelength Half-wave plate = +m.2, m=…,-1,0,+1,… 2 dne no 0 dne no 0 / 2 m0 E|| E0|| coskx t E E0 cos kx t
Polarization is rotated around the optic axis
Special case: =45o Polarization is rotated by 90o Quarter-wave plate . 2 = /2 +m 2, m=…,-1,0,+1,… dne no 0 dne no 0 / 4 m0 E|| E0|| coskx t E E coskx t Linear polarization is converted into 0 circular/elliptical and vice versa Reflective retarders
Total internal reflection: phase shift between the two components. Glass - n=1.51, and 45o shift occurs at incidence angle 54.6o.
Fresnel rhomb
Unlike devices that use birefringent materials this is achromatic Circular polarizer
Can you make a polarizer that transmits right circularly polarized light but not the left circularly polarized light? Phys 322 Chapter 8 Lecture 23 Polarization
Reminder: Exam 2, October 22nd (see webpage) Optical activity
Discovered in 1811, Dominique F. J. Arago
Optically active material: any substance that cause a plane of polarization to appear to rotate
Dextrorotatory (d-rotatory) - polarization rotates right (CW) Levorotatory (l-rotatory) - polarization rotates left (CCW) Latin: dextro - right levo -left Optical activity
Depending on the structure, quartz can be d-or l-rotatory Optical activity
1895, Fresnel: linearly polarized light can be represented at the sum of R- and L-states that propagate at different speeds (circular birefringence): E E 0 ˆi cosk z t ˆjsin k z t R 2 R R E E 0 ˆi cosk z t ˆjsin k z t L 2 L L
kR kL ˆ kR kL ˆ kR kL E E0 cos z ti cos z jsin z 2 2 2 Optical activity
kR kL ˆ kR kL ˆ kR kL E E0 cos z ti cos z jsin z 2 2 2
ˆ z = 0: E E0i cost Optical activity
kR kL ˆ kR kL ˆ kR kL E E0 cos z ti cos z jsin z 2 2 2 ˆ ˆ z =d (kR > kL): E E0i cos jsin cost
d nL nR 0
nL > nR : d-rotatory nL < nR : l-rotatory Nature of optical activity Optical activity and biology
-helix DNA (protein) Induced optical effects:
Photoelasticity = stress birefringence
Faraday effect = magnetic field induced birefringence Faraday isolator
/4 /4 Induced optical effects:
Kerr effect = birefringence induced by electric field (n~ E2)
Pockels effect = birefringence induced by electric field (n~ E) Pockels cell
Laser amplifier Liquid crystals
- possesses properties of liquid and solid Nematic liquid crystal: molecules spontaneously arrange in parallel manner
Optical modulator: Prearrange crystals using windows with microgroves
http://www.cs.sandia.gov/~sjplimp/lammps/movies.html
Modeling a nematic liquid crystal, S. S. Patnaik, S. J. Plimpton, R. Pachter, W. W. Adams, Liquid Crystals, 19, 213-220 (1995). Liquid crystal display
Screen Liquid crystal display Liquid crystal modulators Mathematical description of polarization
1852: Stokes parameters Assume we have a set of 4 filters, each changes the intensity of unpolarized incident light by 1/2: George G. Stokes 1819-1903 1. Isotropic
2. Linear polarizer at 0o
3. Linear polarizer at +45o
4. Circular polarizer (transmits R) Stokes parameters
Position each of the filters one by one in front of I0 any beam and record transmitted irradiance
I1 Stokes parameters: I
S0 2I0 incident radiation I2
S1 2I1 2I0 I S2 2I2 2I0 state of polarization 3
S3 2I3 2I0
S0 Typically is normalized to set S0=1 S1 2 2 2 2 Stokes vector: S0 S1 S2 S3 S2 S3 Stokes parameters
I0 unpolarized polarized polarized polarized light: at 0o at +45o at -45o I1 1 1 1 1 I
0 1 0 0 I2 0 0 1 -1 0 0 0 0 I3 polarized S 2I at 90o R -state L -state 0 0 S 2I 2I 1 1 1 1 1 0 S 2I 2I -1 0 0 2 2 0
0 0 0 S3 2I3 2I0 0 1 -1 Stokes parameters
I0 Mixing two noncoherent beams: ' '' S0 S0 S0
... I1 I
I2
I unpolarized polarized 3 light: at 90o S0 2I0 2 1 3 S 2I 2I 0 -1 -1 1 1 0 += S 2I 2I 0 0 0 2 2 0
0 0 0 S3 2I3 2I0 Jones vectors applicable only to polarized waves i x Ex t E e E Jones vector: ~ 0x E i Ey t y E0 ye Normalize: E ei x 1 Horizontal polarization: ~ 0x ~ Eh Eh 0 0 ~ 0 ~ 0 Vertical polarization: E v i y Ev E0 ye 1 ~ E ei x 1 ~ 1 1 Sum of two coherent 0x i x E E0xe E45 i x beams: E0xe 1 2 1 R-state: E ei x ~ 0x i 1 1 ~ 1 1 x i x ER E0xe E e ER i y / 2 ei / 2 0x i E0xe i 2 Jones vectors: superposition ~ 1 Eh 0 ~ 0 Ev 1 twice the amplitude Mixing waves: ~ 1 1 E45 2 1 ~ ~ 1 1 1 1 2 1 ER EL 2 i 2 i 2 0 ~ 1 1 ER 2 i horizontal ~ 1 1 EL 2 i Jones vectors: optical elements ~ 1 Eh A 0 ~ ~ ~ a11 a12 Ei E A E A ~ 0 t i E a21 a22 v 1 1 0 1 Example: horizontal polarizer A ~ 1 0 0 E45 2 1 ~ ~ 1 01 1 ~ 1. E ~ 1 1 Et A Eh h 0 0 0 0 ER 2 i ~ ~ 1 00 0 2. ~ 1 1 Et A Ev 0 0 1 0 EL 2 i ~ ~ 1 0 1 1 1 1 3. E A E 1 ~ t R Eh 0 0 2 i 2 0 2 Jones vectors: multiple optical elements
~ E ~ ~ ~ i Et A 3A 2A 1Ei A Ei
A1 A2 A2
See table 8.6 (page 378) for matrices