Fundamentals of Modern Optics Winter Term 2012/2013 Prof. Thomas Pertsch Abbe School of Photonics Friedrich-Schiller-Universität Jena

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1 Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx 1 Fundamentals of Modern Optics Winter Term 01/013 Prof. Thomas Pertsch bbe School of Photonics Friedrich-Schiller-Universität Jena This script is based on the lecture series Theoretische Optik by Prof. Falk Lederer at the FSU Jena and adapted to English by Prof. Stefan Skupin for the international education program of the bbe School of Photonics. Table of content 0. Introduction (Ray optics - geometrical optics) Introduction Postulates Simple rules for propagation of light Simple optical components Ray tracing in inhomogeneous media (graded-index - GRIN optics) Ray equation The eikonal equation Matrix optics The ray-transfer-matrix Matrices of optical elements Cascaded elements Optical fields in dispersive and isotropic media Maxwell s equations daption to optics Temporal dependence of the fields Maxwell s equations in Fourier domain From Maxwell s equations to the wave equation Decoupling of the vectorial wave equation Optical properties of matter asics Dielectric polarization and susceptibility Conductive current and conductivity The generalized complex dielectric function Material models in time domain The Poynting vector and energy balance Time averaged Poynting vector Time averaged energy balance The Kramers-Kronig relation Normal modes in homogeneous isotropic media Transversal waves Longitudinal waves Plane wave solutions in different frequency regimes Time averaged Poynting vector of plane waves eams and pulses - analogy of diffraction and dispersion Propagation of stationary beams in homogeneous isotropic media Propagation of Gaussian beams Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx.6.3 Gaussian optics Gaussian modes in a resonator Pulse propagation Diffraction theory Interaction with plane masks Propagation using different approximations The general case - small aperture Fresnel approximation (paraxial approximation) paraxial Fraunhofer approximation (far field approximation) Fraunhofer diffraction at plane masks (paraxial) Fraunhofer diffraction pattern Examples: Remarks on Fresnel diffraction Fourier optics - optical filtering Imaging of arbitrary optical field with thin lens Transfer function of a thin lens optical imaging Optical filtering and image processing The 4f-setup Examples: The polarization of electromagnetic waves Introduction Polarization of normal modes in isotropic media Polarization states Principles of Optics in Crystals Susceptibility and Dielectric Tensor The optical classification of crystals the index ellipsoid normal modes in anisotropic media Normal modes propagating in principal directions Normal modes for arbitrary propagation direction dispersion relation special case: uniaxial crystals Optical Fields in Isotropic, Dispersive and Piecewise Homogeneous Media asics definition of the problem Decoupling of the vectorial wave equation interfaces and symmetries transition conditions fields in a layer system matrix method fields in one homogeneous layer the fields in a system of layers Reflection Transmission Problem for Layer Systems general layer systems single interface Periodic multi-layer systems - ragg-mirrors - 1D photonic crystals Fabry-Perot-resonators Guided Waved in Layer Systems Field structure of guided waves dispersion relation for guided waves guided waves at interface - surface polariton guided waves in a layer film waveguide... 31

2 Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx 3 Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx how to excite guided waves Statistical optics - coherence theory asics Statistical properties of light Interference of partially coherent light Introduction 'optique' (Greek) lore of light 'what is light'? Is light a wave or a particle (photon)? D.J. Lovell, Optical necdotes Light is the origin and requirement for life photosynthesis 90% of information we get is visual ) Origin of light atomic system determines properties of light (e.g. statistics, frequency, line width) optical system other properties of light (e.g. intensity, duration, ) invention of laser in 1958 very important development Schawlow and Townes, Phys. Rev. (1958). laser artificial light source with new and unmatched properties (e.g. coherent, directed, focused, monochromatic) applications of laser: fiber-communication, DVD, surgery, microscopy, material processing,...

3 Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx 5 Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx 6 C) Light can modify matter light induces physical, chemical and biological processes used for lithography, material processing, or modification of biological objects (bio-photonics) Fiber laser: Limpert, Tünnermann, IP Jena, ~10kW CW (world record) ) Propagation of light through matter light-matter interaction dispersion diffraction absorption scattering frequency spatial center of wavelength spectrum frequency frequency spectrum Hole drilled with a fs laser at Institute of pplied Physics, FSU Jena. matter is the medium of propagation the properties of the medium (natural or artificial) determine the propagation of light light is the means to study the matter (spectroscopy) measurement methods (interferometer) design media with desired properties: glasses, polymers, semiconductors, compounded media (effective media, photonic crystals, meta-materials) Two-dimensional photonic crystal membrane.

4 Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx 7 D) Optics in our daily life Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx 8 E) Optics in telecommunications transmitting data (Terabit/s in one fiber) over transatlantic distances small story describing the importance of light for everyday life, where all things which rely on optics are marked in red m telecommunication fiber is installed every second.

5 Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx 9 F) Optics in medicine, life sciences Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx 10 G) Optical sensors and light sources new light sources to reduce energy consumption new projection techniques Deutscher Zukunftspreis IOF Jena + OSRM

6 Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx 11 H) Micro- and nano-optics ultra small camera Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx 1 I) Relativistic optics Insect inspired camera system develop at Fraunhofer Institute IOF Jena

7 Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx 13 Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx 14 K) What is light? electromagnetic wave (c= 3*10 8 m/s) amplitude and phase complex description polarization, coherence Region Spectrum of Electromagnetic Radiation Wavelength (nanometers) Wavelength (centimeters) Frequency (Hz) Energy (ev) Radio > > 10 < 3 x 10 9 < 10-5 Microwave x x Infrared x x x Visible x x x x Ultraviolet x x x X-Rays x x Gamma Rays < 0.01 < 10-9 > 3 x > 10 5 L) Schematic of optics quantum optics electromagnetic optics wave optics geometrical optics geometrical optics << size of objects daily experiences optical instruments, optical imaging intensity, direction, coherence, phase, polarization, photons wave optics size of objects interference, diffraction, dispersion, coherence laser, holography, resolution, pulse propagation intensity, direction, coherence, phase, polarization, photons electromagnetic optics reflection, transmission, guided waves, resonators laser, integrated optics, photonic crystals, ragg mirrors... intensity, direction, coherence, phase, polarization, photons quantum optics small number of photons, fluctuations, light-matter interaction intensity, direction, coherence, phase, polarization, photons in this lecture electromagnetic optics and wave optics no quantum optics advanced lecture

8 Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx 15 M) Literature Fundamental 1. Saleh, Teich, 'Fundamenals of Photonics', Wiley, 199. Mansuripur, 'Classical Optics and its pplications', Cambridge, Hecht, 'Optik', Oldenbourg, Menzel, 'Photonics', Springer, Lipson, Lipson, Tannhäuser, 'Optik'; Springer, orn, Wolf, 'Principles of Optics', Pergamon 7. Sommerfeld, 'Optik' dvanced 1. W. Silvast, 'Laser Fundamentals',. grawal, 'Fiber-Optic Communication Systems', Wiley 3. and, 'Light and Matter', Wiley, Karthe, Müller, 'Integrierte Optik', Teubner 5. Diels, Rudolph, 'Ultrashort Laser Pulse Phenomena', cademic 6. Yariv, 'Optical Electronics in modern Communications', Oxford 7. Snyder, Love, 'Optical Waveguide Theory', Chapman&Hall 8. Römer, 'Theoretical Optics', Wiley,005. Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx (Ray optics - geometrical optics) The topic of Ray optics geometrical optics is not covered in the course Fundamentals of modern optics. This topic will be covered rather by the course Introduction to optical modeling. The following part of the script which is devoted to this topic is just included in the script for consistency. 1.1 Introduction Ray optics or geometrical optics is the simplest theory for doing optics. In this theory, propagation of light in various optical media can be described by simple geometrical rules. Ray optics is based on a very rough approximation (0, no wave phenomena), but we can explain almost all daily life experiences involving light (shadows, mirrors, etc.). In particular, we can describe optical imaging with ray optics approach. In isotropic media, the direction of rays corresponds to the direction of energy flow. What is covered in this chapter? It gives fundamental postulates of the theory. It derives simple rules for propagation of light (rays). It introduces simple optical components. It introduces light propagation in inhomogeneous media (graded-index (GRIN) optics). It introduces paraxial matrix optics. 1. Postulates ) Light propagates as rays. Those rays are emitted by light-sources and are observable by optical detectors. ) The optical medium is characterized by a function n(r), the so-called refractive index (n(r) 1 - meta-materials n(r) <0) c n c n speed of light in the medium c n C) optical path length delay i) homogeneous media nl ii) inhomogeneous media n() r ds D) Fermat s principle

9 Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx 17 Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx 18 n() r ds0 Rays of light choose the optical path with the shortest delay. 1.3 Simple rules for propagation of light ) Homogeneous media n = const. minimum delay = minimum distance Rays of light propagate on straight lines. ) Reflection by a mirror (metal, dielectric coating) The reflected ray lies in the plane of incidence. The angle of reflection equals the angle of incidence. C) Reflection and refraction by an interface Incident ray reflected ray plus refracted ray The reflected ray obeys b). The refracted ray lies in the plane of incidence. ii) Parabolic mirror Parallel rays converge in the focal point (focal length f). pplications: Telescope, collimator iii) Elliptic mirror Rays originating from focal point P 1 converge in the second focal point P The angle of refraction depends on the angle of incidence 1 and is given by Snell s law: n sin n sin 1 1 no information about amplitude ratio. 1.4 Simple optical components ) Mirror i) Planar mirror Rays originating from P 1 are reflected and seem to originate from P. iv) Spherical mirror Neither imaging like elliptical mirror nor focusing like parabolic mirror parallel rays cross the optical axis at different points connecting line of intersections of rays caustic

10 Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx 19 Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx 0 parallel, paraxial rays converge to the focal point f = (-R)/ convention: R < 0 - concave mirror; R > 0 - convex mirror. for paraxial rays the spherical mirror acts as a focusing as well as an imaging optical element. paraxial rays emitted in point P1 are reflected and converge in point P C) Spherical interface (paraxial) paraxial imaging n1 n n1 y 1 (*) n n R 1 1 (imaging formula) z1 z ( R) paraxial imaging: imaging formula and magnification m = -z /z1 (proof given in exercises) ) Planar interface Snell s law: n1sin1 nsin for paraxial rays: nn 1 1 external reflection ( n 1 n ): ray refracted away from the interface internal reflection ( n 1 n ): ray refracted towards the interface total internal reflection (TIR) for: n sin 1 sin TIR n 1 n1 n n n1 (imaging formula) z1 z R n1 z m (magnification) n z 1 (Proof: exercise) if paraxiality is violated aberration rays coming from one point of the object do not intersect in one point of the image (caustic) D) Spherical thin lense (paraxial)

11 Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx 1 Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx two spherical interfaces (R 1, R, ) apply (*) two times and assume y=const ( small) variation of the path: r() s r () s L nds nds ngrad nr y with focal length: n 1 f f R1 R z (imaging formula) m (magnification) z1 z f z1 (compare to spherical mirror) 1.5 Ray tracing in inhomogeneous media (graded-index - GRIN optics) n() r - continuous function, fabricated by, e.g., doping curved trajectories graded-index layer can act as, e.g., a lens Ray equation Starting point: we minimize the optical path or the delay (Fermat) computation: n() r ds0 L nr s ds r r r ds d d d dr dr d r d r dr dr dr ds 1 ds ds ds dr dr ds1 ds ds ds dr dr ds ds ds dr dr Lgrad nr n ds ds ds integration by parts and, fix d dr grad n n ds ds ds r L 0 for arbitrary variation d dr grad n n ray equation ds ds Possible solutions: ) trajectory x(z), y(z) and ds dz 1dx dz dy dz solve for x(z), y(z) paraxial rays (ds dz )

12 Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx 3 d dx dn nx, y, z dz dz dx d dy dn nx, y, z dz dz dy ) homogeneous media straight lines C) graded-index layer n(y) - paraxial, SELFOC dy paraxial 1 and dz dz ds 1 n ( y) n0 1 y n( y) n 0 1 y for a 1 d dy d dy d y d y 1 dn( y) ny ny ny ds ds dz dz dz dz n y dy for n(y)-n 0 <<1: d y dz y 0 yz ( ) y0 cosz sinz dy ( z) y0sinz0cosz dz 1.5. The eikonal equation bridge between geometrical optics and wave eikonal S(r) = constant planes perpendicular to rays from S(r) we can determine direction of rays grad S(r) (like potential) grads r nr Remark: it is possible to derive Fermat s principle from eikonal equation geometrical optics: Fermat s or eikonal equation S r S r grad S r ds n r ds eikonal optical path length phase of the wave Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx Matrix optics technique for paraxial ray tracing through optical systems propagation in a single plane only rays are characterized by the distance to the optical axis (y) and their inclination () two algebraic equation x matrix dvantage: we can trace a ray through an optical system of many elements by multiplication of matrices The ray-transfer-matrix in paraxial approximation: y y 1 1 Cy D 1 1 =0: same 1 same y focusing D=0: same y1 same collimation y y C D M C D Matrices of optical elements ) free space 1 d M 0 1 ) refraction on planar interface

13 Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx 5 Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx M 0 n1 n C) refraction on spherical interface M 1 0 n n1 nr n1 n D) thin lens 1 0 M 1 f 1 E) reflection on planar mirror 1 0 M 0 1 F) reflection on spherical mirror (compare to lens) 1 0 M R Cascaded elements yn 1 y1 N 1 C D M 1 C D M=M N.M M. Optical fields in dispersive and isotropic media.1 Maxwell s equations Our general starting point is the set of Maxwell s equations. They are the basis of the electromagnetic approach to optics developed in this lecture..1.1 daption to optics Maxwell s equations (macroscopic) r rot Er div Dr ext(,) rt t Dr rot Hr jmakr(,) rt div r 0 t electric field Er [V/m] magnetic flux density r [Vs/m ] or [tesla] or magnetic induction dielectric flux density Dr [s/m ] magnetic field Hr [/m] external charge density ext (,) r t [s/m 3 ] macroscopic current density jmakr (,) r t [/m ] uxiliary fields Dr Er Pr 0 1 Hr r Mr 0 dielectric polarization Pr [s/m ], magnetic polarization Mr [Vs/m ] (magnetization) electric constant (vacuum permittivity) s/vm c 0 magnetic constant (vacuum permeability) Vs/m

14 Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx 7 Light matter interaction The effect of the medium gives rise to polarization Pr f E and magnetization Mr f. In order to solve Maxwell s equations we need material models describing those quantities. In optics, we generally deal with non-magnetizable media Mr 0 (exceptions are metamaterials with Mr 0). In Maxwell s equations we also have so-called source terms: free charge density ext (,) r t [s/m 3 ] macroscopic current density jmakr(,) r t jcond (,) r t jconv (,) r t [/m ] conductive current density j (,) r t f E cond convective current density j (,) r t (,) r t v(,) r t conv ext In optics, we generally have no free charges which change at speeds corresponding to the frequency of light: (,) r t 0 j (,) r t 0 ext conv With the above simplifications, we can formulate Maxwell s equations in the context of optics: Hr rot Er 0 0div Er divp(, r t) t Pr Er rot Hr jr (, t) 0 div H(,) r t 0 t t In optics, the medium (or more precisely the mathematical material model) determines the dependence of the polarization on the electric field, PE, ( ) and the dependence of the (conductive) current density on the electric field, j( E ). Once we have specified those relations, we can solve Maxwell s equations consistently. Example: In vacuum, both polarization and current density are zero, and we can solve Maxwell s equations directly (most simple material model). Remark: We can define a bound charge density Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx 8 (,) r t divp(,) r t b and a bound current density Pr jb (,) r t t This essentially means that we can describe the same physics in two different ways (see generalized complex dielectric function below). Complex field formalism: Maxwell s equations are also valid for complex fields and are easier to solve This fact can be exploited to simplify calculations, because it is easier to deal with complex exponential functions [exp(ix)] than with trigonometric functions [cos(x) and sin(x)]. convention in this lecture real physical field: E (,) r t r complex mathematical representation: Er E 1 (,) r t Er E (,) r t Re Er r This means in general: For calculation we use the complex fields [ Er ] and for physical results we go back to real fields by simply omitting the imaginary part. This works because Maxwell s equations are linear and no multiplications of fields occur. Therefore, be careful when multiplications of fields are required go back to real quantities before! (relevant for, e.g., calculation of Poynting vector, see Chapter below)..1. Temporal dependence of the fields When it comes to time dependence of the electromagnetic field, we can distinguish two different types of light: ) monochromatic light stationary fields harmonic dependence on temporal coordinate exp( i t) phase is fixed coherent, infinite wave train e.g.: Er Er ()exp( i t) Monochromatic light approximates very well the typical output of a continuous wave (CW) laser. Once we know the frequency we have to compute the spatial dependence of the (stationary) fields only. ) polychromatic light non-stationary fields finite wave train

15 Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx 9 With the help of Fourier transformation we can decompose the fields into infinite wave trains and use ) (see next section) Er Er (, )exp( it) d 1 Er (, ) (,)exp( t t) dt Er i.1.3 Maxwell s equations in Fourier domain We want to plug the Fourier decompositions of our fields into Maxwell s equations in order to get a more simple description. For this purpose, we need to know how a time derivative transforms into Fourier space. Here we used integration by parts: 1 1, exp, exp (, ) dt t i t i dt t i t i Er t Er Er FT rule: i t Now we can write Maxwell s equations in Fourier domain: rot Er (, ) i0hr (, ) 0div Er (, ) div Pr (, ) rot Hr (, ) jr ipr (, ) i Er (, ) div Hr (, ) From Maxwell s equations to the wave equation From Maxwell s equations it is straight forward to derive the wave equation by using the two curl equations. ) Time domain derivation We start from applying the curl a second time on rot E(,) r t Hr (,) (,) (,) t Pr 0 0 t E r t rotrot E r rot 0 t t j r t t nd find the wave equation for the electric field rotrot Er (,) c t 0 1 jr P(,) r t E r t 0 0 The blue terms require knowledge of the material model. dditionally, we have to make sure that all other Maxwell s equations are fulfilled, in particular: div0 E(,) r t P(,) r t 0 Once we know the electric field, we can easily compute the magnetic field: Hr 1 rot E(,) r t t 0 t t Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx 30 Remarks: analog procedure possible for H, i.e., we can derive a wave equation for the magnetic field generally, the wave equation for E is more convenient, because we have P (E) given from the material model however, for inhomogeneous media H can be the better choice for the numerical solution of the partial differential equation analog procedure possible for H E generally, wave equation for E is more convenient, because P (E) given for inhomogeneous media H can be better choice ) Frequency domain derivation We can do the same procedure in the Fourier domain and find rotrot Er c Er jr Pr and div 0 (, ) (, ) E r P r 0 magnetic field: i Hr (, ) rot Er (, ) (, ) (, ) i 0 (, ) 0 (, ) 0 transferring the results from the Fourier domain into the time domain -i t for stationary fields: take solution and multiply by e. for non-stationary fields and linear media inverse Fourier transformation Er Er (, )exp( i t) d.1.5 Decoupling of the vectorial wave equation For arbitrary isotropic media generally all 3 field components are coupled. For problems with translational invariance in at least one direction, as e.g. for homogeneous infinite media, layers or interfaces, there can be decoupling of the components. Let s assume invariance in the y-direction and propagation only in the x-z-plane. Then all spatial derivatives along the y-direction disappear and the operators in the wave equation simplify. generally all field components are coupled for translational invariance in e.g. the y-direction and propagation only in the x-z-plane / y 0

16 Script Fundamentals of Modern Optics, FSU Jena, Prof. T. Pertsch, FoMO_Script_01-10-b.docx 31 Ex Ez () x x z E x () 0 E y E x E z () E z x z z rot rot E grad div E E decomposition of electric field 0 E x EE E E E y, E 0 0 E z x () with Nabla operator 0 (), and Laplace z x z gives two uncoupled wave equations () E E (, r ) i0j (, r ) 0 P(, r ) c () () () E E (, r ) grad div E i0j (, r ) 0 P(, r ) c properties propagation of perpendicularly polarized fields treated separately alternative notations: s TE (transversal electric) p TM (transversal magnetic) E and E can be