Instrumental optics
lecture 3
13 March 2020 Lecture 2 – summary
1. Optical materials (UV, VIS, IR) 2. Anisotropic media (ctd) • Double refraction
2. Anisotropic media • Refractive index: no, ne() • Permittivity tensor • Birefringent materials – calcite, • Optical axis YVO4, quartz
• Ordinary wave Polarization perpendicular to o.a.-푘 plane • Extraordinary wave Polarization in o.a.-푘 plane
• Birefringent walk-off 3. Polarization of light • Wave vector ellipsoid • states of polarization
Polarization of light
linear circular elliptical 휋 휋 Δ휙 = 0, sin Θ = 퐸 /퐸 Δ휙 = , 퐸 = 퐸 Δ휙 ∈ (0, ) 푦 푥 2 푥 푦 2 Plane wave, 푘||푧, 푧 = const:
• Fully polarized light 푖(휔푡+휙푥) 퐸푥(푡) 퐸0푥푒 퐸0푥 • Partially polarized light 푖(휔푡+휙 ) i(휔푡+휙푥) iΔ휙 퐸 푡 = 퐸푦(푡) = 퐸 푒 푦 =e 퐸 e • Fully depolarized light 0푦 0푦 0 0 0
Δ휙 = 휙푦 − 휙푥 Note: electric polarization 푃(퐸) vs. polarization of a wave! Linear polarizers
Key parameter: Other parameters: 퐼 extinction coefficient, 푦 • Spectral range, 퐼푥 • damage threshold, (assumption: incident light is • angle of incidence, unpolarized!) • beam geometry For polarizing beamsplitters: 2 extinction coefficients, reflection typically worse • clear aperture
Polarizer types: • Dichroic • Crystalline • Thin-film Dichroic polarizers
Dichroic polarizer: • Made from material with anisotropic absorption (or reflection) coefficient
long, „conductive” molecules/structures
• Polaroid: polyvinyl alcohol + iodine – one polarization component is absorbed. • Cheap, large aperture, broadband, poor extinction, low damage threshold
• Wire grid / nanowire: sub-wavelength spaced metallic wires reflect/absorb one polarization • Good extinction (> 103), large aperture, broadband, better damage threshold • Variation: elongated Ag nanoparticles Crystalline (birefringent) polarizers
High extinction (104 ÷ 105) Some designs have very high damage threshold Small apertures Complicated geometry Restricted spectral range
Poor transmission (Fresnel reflection), Wollaston Glan-Foucault Beam deflection reflected beam not glue polarized
Good transmission, Good transmission reflected beam (~Brewster angle), Glan-Thompson polarized , but low Glan-Taylor/ reflected beam damage threshold Glan laser (air gap) polarized Dielectric (thin-film) polarizers
Thin dielectric layers. Polarization-dependent reflectivity based on polarization-dependent interference effects in thin (sub-휆) layers of varying refractive index .
Polarizing beam splitter Extinction: 102 ÷ 103 Very high damage threshold possible Spectral range – UV-VIS / VIS-NIR / NIR-IR Acceptance angle: ~10 degrees
Wave plates o.a.
→
Half-wave plate: 횪 = 흅 + 2푚휋 Phase shift: Quarter-wave plate: 흅 휞 = + 2푚휋 ퟐ Wave plate order: 푚 Phase shift is wavelength-dependent! fast axis Half-wave plate acting on linear polarization
퐸in
퐸out
Quarter-wave plate action
퐸 e푖휔푡 0 1 Linear polarization at 45 deg. to optical axis of wave plate 퐸in = 2 1
푖휔푡 1 푖휔푡 퐸0e 퐸0e 1 퐸0 푖휋 cos 휔푡 퐸out = = Re 퐸out = 2 1 ⋅ 푒 2 2 푖 2 −sin 휔푡 Circular polarization! Construction of a wave plate
Quartz waveplate 2휋 푛푒 휆 −푛표 휆 푑 Phase shift: 훤 휆 = (birefringence: 0.009): 휆 푑 = 28 μm for zero-order HWP at 500 nm Quarter waveplate for 휆 = 550 nm In practice two thicker quartz plates with perpendicular SiO2 5th order + MgF2 5th order optical axes
True-zero-order wavplate: single thin quartz plate. SiO2, 0-order achromatic
SiO2, 5th order Jones calculus
2 푉푥 ∗ ∗ Jones vector: 푉 = , normalization: 푉 = 푉푥푉푥 + 푉푦푉푦 = 1. 푉푦
Examples: linear polarization
Circular polarization
Eliptical polarization, with ellipse axes aligned to reference frame:
cos휗 General eliptical polarization = sin휗ei휙
Jones vector is not unique: 푉 & 푒푖휑푉 correspond to the same state of polarization! (but remember about the global phase in interferometers etc.!) Partially polarized light – coherence matrix
Example: circularily polarized light: 휋 퐸0 퐸0 i(휔푡+ ) 퐸 = ei휔푡, 퐸 = e 2 푥 2 푦 2 퐸2 퐸 퐸∗ = 퐸 퐸∗ = 0 Unpolarized light 푥 푥 푦 푦 2 2 휋 2 퐸0 −i 퐸0 퐸 퐸∗ = e 2 = − 푖 푥 푦 2 2 2 휋 2 ∗ 퐸0 i 퐸0 퐸푦퐸푥 = e 2 = 푖 Linearly polarized light 2 2 (for fully polarized light in complex notation: 퐸푖퐸푗 = 퐸푖퐸푗) 2 퐸0 1 −푖 퐽 = 2 푖 1 Intensity Example 2: unpolarized light: i휔푡 i휔푡+휙random 퐸푥 = 퐸0e , 퐸푦 = 퐸0e Degree of polarization
∗ ∗ 2 퐸푥퐸푥 = 퐸푦퐸푦 = 퐸0
∗ ∗ 퐸푥퐸푦 = 퐸푦퐸푥 = 0 Jones matrices
Change of Jones vector due to polarization elements is described by the Jones matrix 푊
푉out = 푊푉in
Constant intensity -> Jones matrix is unitary: 푊†푊 = ퟏ
푉in 푉out Jones matrices for basic polarization elements
Free space propagation (distance 푑):
1 0 푊 = (same for isotropic dielectric) empty−space 0 1
Wave plate with optical axis aligned to the reference frame
1 0 푊 = waveplate 0 eiΓ
Polarizer along 푥 axis 1 0 푊 = polarizer 0 0
Jones matrices can also be used to transfrom coherence matices 퐽: 퐽′ = 푊†퐽푊 Change of basis Linear (퐿) – circular (퐾)
Reference frame rotation
Example: wave plate
푊plate Half-wave plate, /2
Γ = 푊plate
푊plate
Fast axis of waveplate
퐸ou푡 퐸in Quarter-wave plate, /4
Γ = /2 푊plate
For α = /4
Optically active material Circular birefringence: 푛푃 ≠ 푛퐿 (푃 – right, 퐿 – left )
Rotation angle independent of input! 푇 Stokes vector 푆0, 푆1, 푆2, 푆3 Measuring the state of polarization Natężenie
NPBS1 (2:1) NPBS2 (1:1) 푇 Stokes vector 푆0, 푆1, 푆2, 푆3 Poincaré sphere
Normalized Stokes vector
For fully polarized light:
In general ≤
Fully polarized light:
→ angle 2휗 around 푠3, angle 휑 wrt. Waveplates: rotation by angle 훤 wrt. axis equatorial plane defined by eigenpolarizations