Foundations of Optics
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Lecture 1: Foundations of Optics Outline 1 Electromagnetic Waves 2 Material Properties 3 Electromagnetic Waves Across Interfaces 4 Fresnel Equations Christoph U. Keller, Leiden University, [email protected] Lecture 1: Foundations of Optics 1 Electromagnetic Waves Electromagnetic Waves in Matter Maxwell’s equations ) electromagnetic waves optics: interaction of electromagnetic waves with matter as described by material equations polarization of electromagnetic waves are integral part of optics Maxwell’s Equations in Matter Symbols D~ electric displacement r · D~ = 4πρ ρ electric charge density ~ 1 @D~ 4π H magnetic field r × H~ − = ~j c @t c c speed of light in vacuum ~j electric current density 1 @B~ r × E~ + = 0 E~ electric field c @t B~ magnetic induction r · B~ = 0 t time Christoph U. Keller, Leiden University, [email protected] Lecture 1: Foundations of Optics 2 Linear Material Equations Symbols dielectric constant D~ = E~ µ magnetic permeability B~ = µH~ σ electrical conductivity ~j = σE~ Isotropic and Anisotropic Media isotropic media: and µ are scalars anisotropic media: and µ are tensors of rank 2 isotropy of medium broken by anisotropy of material itself (e.g. crystals) external fields (e.g. Kerr effect) mechanical deformation (e.g. stress birefringence) assumption of isotropy may depend on wavelength (e.g. CaF2 for semiconductor manufacturing) Christoph U. Keller, Leiden University, [email protected] Lecture 1: Foundations of Optics 3 Wave Equation in Matter static, homogeneous medium with no net charges: ρ = 0 for most materials: µ = 1 combine Maxwell, material equations ) differential equations for damped (vector) wave µ @2E~ 4πµσ @E~ r2E~ − − = 0 c2 @t2 c2 @t µ @2H~ 4πµσ @H~ r2H~ − − = 0 c2 @t2 c2 @t damping controlled by conductivity σ E~ and H~ are equivalent ) sufficient to consider E~ interaction with matter almost always through E~ but: at interfaces, boundary conditions for H~ are crucial Christoph U. Keller, Leiden University, [email protected] Lecture 1: Foundations of Optics 4 Plane-Wave Solutions ~ ~ i(~k·~x−!t) Plane Vector Wave ansatz E = E0e ~k spatially and temporally constant wave vector ~k normal to surfaces of constant phase j~kj wave number ~x spatial location ! angular frequency (2π× frequency ν) t time ~ E0 (generally complex) vector independent of time and space ~ ~ −i(~k·~x−!t) could also use E = E0e damping if ~k is complex ~ E0 describes the polarization and the absolute phase real electric field vector given by real part of E~ sum of solutions is also a solution Christoph U. Keller, Leiden University, [email protected] Lecture 1: Foundations of Optics 5 Complex Index of Refraction temporal derivatives ) Helmholtz equation !2µ 4πσ r2E~ + + i E~ = 0 c2 ! spatial derivatives ) dispersion relation between ~k and ! !2µ 4πσ ~k · ~k = + i c2 ! complex index of refraction 4πσ !2 n~2 = µ + i ; ~k · ~k = n~2 ! c2 split into real (n: index of refraction) and imaginary parts (k: extinction coefficient) n~ = n + ik Christoph U. Keller, Leiden University, [email protected] Lecture 1: Foundations of Optics 6 Transverse Waves plane-wave solution must also fulfill Maxwell’s equations ~ ~ ~ ~ ~ ~ n~ k ~ E0 · k = 0; H0 · k = 0; H0 = × E0 µ j~kj isotropic media: electric, magnetic field vectors normal to wave vector ) transverse waves ~ ~ ~ E0, H0, and k orthogonal to each other, right-handed vector-triple ~ ~ conductive medium ) complex n~, E0 and H0 out of phase ~ ~ ~ E0 and H0 have constant relationship ) consider only E Christoph U. Keller, Leiden University, [email protected] Lecture 1: Foundations of Optics 7 Energy Propagation in Isotropic Media time-averaged Poynting vector D E c S~ = Re E~ × H~ ∗ 8π 0 0 Re real part of complex expression ∗ complex conjugate h:i time average energy flow parallel to wave vector in isotropic media ~ D~ E c jn~j 2 k S = jE0j 8π µ j~kj energy flow is proprtional to index of refraction! in anisotropic materials (e.g. crystals), energy propagation and wave vector are not parallel! Christoph U. Keller, Leiden University, [email protected] Lecture 1: Foundations of Optics 8 Quasi-Monochromatic Light monochromatic light: purely theoretical concept monochromatic light wave always fully polarized real life: light includes range of wavelengths ) quasi-monochromatic light quasi-monochromatic: superposition of mutually incoherent monochromatic light beams whose wavelengths vary in narrow range δλ around central wavelength λ0 δλ 1 λ measurement of quasi-monochromatic light: integral over measurement time tm amplitude, phase (slow) functions of time for given spatial location slow: variations occur on time scales much longer than the mean period of the wave Christoph U. Keller, Leiden University, [email protected] Lecture 1: Foundations of Optics 9 Polychromatic Light or White Light δλ wavelength range comparable wavelength ( λ ∼ 1) incoherent sum of quasi-monochromatic beams that have large variations in wavelength cannot write electric field vector in a plane-wave form must take into account frequency-dependent material characteristics intensity of polychromatic light is given by sum of intensities of constituting quasi-monochromatic beams Christoph U. Keller, Leiden University, [email protected] Lecture 1: Foundations of Optics 10 Material Properties Index of Refraction complex index of refraction 4πσ !2 n~2 = µ + i ; ~k · ~k = n~2 ! c2 no electrical conductivity ) real index of refraction transparent, dielectric materials: real index of refraction conducting materials (metal): complex index of refraction also transparent, conducting materials, e.g. ITO index of refraction depends on wavelength (dispersion) index of refraction depends on temperature index of refraction roughly proportional to density Christoph U. Keller, Leiden University, [email protected] Lecture 1: Foundations of Optics 11 Glass Dispersion en.wikipedia.org/wiki/File:Dispersion-curve.png Christoph U. Keller, Leiden University, [email protected] Lecture 1: Foundations of Optics 12 Wavelength Dependence of Index of Refraction tabulated by glass manufacturer various approximations to express wavelength dependence with a few parameters typically index increases with decreasing wavelength Abbé number: nd − 1 νd = nF − nC nd : index of refraction at Fraunhofer d line (587.6 nm) nF : index of refraction at Fraunhofer F line (486.1 nm) nC: index of refraction at Fraunhofer C line (656.3 nm) low dispersion materials have high values of νd Abbe diagram: νd vs nd Christoph U. Keller, Leiden University, [email protected] Lecture 1: Foundations of Optics 13 Glasses ν d 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 2.05 2.05 68 nd 2.00 2.00 nd ν Abbe-Diagram nd – d Description of Symbols 1.95 1.95 N- or P-glass Lead containing glass 66 46A 67 1.90 N-glass or lead containing glass 1.90 Glass suitable for Precision Molding 31A LASF Fused Silica 1.85 9 57 1.85 41 40 SF 51 50 44 47 45 6 1.80 43 1.80 21 33 56A 11 34 14 33A 37 LAF 4 1.75 7 1.75 35 2 10 34 69 10 KZFS 8 1 8 LAK 64 1.70 14 15 1.70 35 9 BASF 8 12 10 5 2 7 51 KZFS5 1.65 22 5 2 1.65 21 SSK KZFS11 BAF 2 2 16 60 8 KZFS4 53 A 4 2 52 14 4 5 1.60 1.60 5 SK F 58A 57 4 5 PSK 1 4 11 BALF KZFS2 LF 3 1.55 2 BAK 5 1 1.55 LLF 51 KF 5 PK 7 7 9 K 10 BK ZK7 1.50 52 A 10 1.50 51A 5 FK FS 1.45 1.45 August 2010 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 ν d Christoph U. Keller, Leiden University, [email protected] Lecture 1: Foundations of Optics 14 Empirical Models of Index of Refraction tabulated, measured index as function of wavelength approximations to express smooth wavelength dependence most common: Sellmeier equation and coefficients: B λ2 B λ2 B λ2 2(λ) = + 1 + 2 + 3 n 1 2 2 2 λ − C1 λ − C2 λ − C3 reflects resonances that drive index variation with wavelength only applicable over a limited wavelength range BK7: B1 =1.03961212, C1 =6.00069867e-3, B2 =2.31792344e-1, C2 =2.00179144e-2,B3 =1.01046945, C3 =1.03560653e2 Christoph U. Keller, Leiden University, [email protected] Lecture 1: Foundations of Optics 15 Transparent Materials Christoph U. Keller, Leiden University, [email protected] Lecture 1: Foundations of Optics 16 Internal Transmission internal transmission per cm typically strong absorption in the blue and UV almost all glass absorbs above 2 µm Christoph U. Keller, Leiden University, [email protected] Lecture 1: Foundations of Optics 17 Metal Reflectivity Christoph U. Keller, Leiden University, [email protected] Lecture 1: Foundations of Optics 18 Electromagnetic Waves Across Interfaces Introduction classical optics due to interfaces between 2 different media from Maxwell’s equations in integral form at interface from medium 1 to medium 2 ~ ~ D2 − D1 · ~n = 4πΣ ~ ~ B2 − B1 · ~n = 0 ~ ~ E2 − E1 × ~n = 0 4π H~ − H~ × ~n = − K~ 2 1 c ~n normal on interface, points from medium 1 to medium 2 Σ surface charge density on interface K~ surface current density on interface Christoph U. Keller, Leiden University, [email protected] Lecture 1: Foundations of Optics 19 Fields at Interfaces Σ = 0 in general, K~ = 0 for dielectrics complex index of refraction includes effects of currents ) K~ = 0 requirements at interface between media 1 and 2 ~ ~ D2 − D1 · ~n = 0 ~ ~ B2 − B1 · ~n = 0 ~ ~ E2 − E1 × ~n = 0 ~ ~ H2 − H1 × ~n = 0 normal components of D~ and B~ are continuous across interface tangential components of E~ and H~ are continuous across interface Christoph U.