Lecture 21


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Lecture 21

  1. 1. Optical Properties <ul><li>Frequency, wavelength, and energy, trends in spectrum </li></ul><ul><li>Optical classifications </li></ul><ul><li>How does absorption, A, (or emission) relate to band structure (band gaps, donor/acceptor states)? </li></ul><ul><li>Reflection (R) </li></ul><ul><li>Refraction </li></ul><ul><li>Transmission (T) </li></ul><ul><li>Equations for R, A, T </li></ul><ul><li>Photoelasticity </li></ul><ul><li>Define Phosphorescence and Fluorescence. </li></ul><ul><li>Know the principles behind the ruby laser. </li></ul><ul><li>Know the principles behind optical data storage (DVDs). </li></ul>
  2. 2. Optical Properties
  3. 3. Electromagnetic Wave Propagation
  4. 4. Electromagnetic Radiation-Waves <ul><li>EM radiation travels in a vacuum at the speed of light (c=3*10 8 m/s). </li></ul><ul><li>The speed of light is related to dielectric permittivity and magnetic permeability of a vacuum. </li></ul><ul><li>The frequency and wavelength are also related to c. </li></ul><ul><li>The energy of light (photons) with a given frequency (or wavelength) is related to Planck’s constant (h=6.63*10 -34 J*sec). </li></ul><ul><li>Radiation can thus be defined in terms of energy, frequency, or wavelength. </li></ul>E ν λ ? ? ? ? ? ?
  5. 5. EM Radiation Spectrum <ul><li>Spans from gamma rays (radioactive materials) to x-rays, UV, visible, IR, microwave, radio/tv </li></ul>
  6. 6. Applications of various waves/photons
  7. 7. Optical classification <ul><li>There are three primary ways to describe the optical quality of a material (color comes later) </li></ul><ul><ul><li>Transparent: you can see through it (but color may change). </li></ul></ul><ul><ul><ul><li>Glass </li></ul></ul></ul><ul><ul><ul><li>Insulators </li></ul></ul></ul><ul><ul><ul><li>Some semiconductors </li></ul></ul></ul><ul><ul><li>Translucent: light is transmitted diffusely (internal scattering), usually related to defects such as grain boundaries or pores. </li></ul></ul><ul><ul><ul><li>Polycrystalline insulators </li></ul></ul></ul><ul><ul><li>Opaque: you can’t see through it. </li></ul></ul><ul><ul><ul><li>Bulk metals </li></ul></ul></ul><ul><ul><ul><li>Some semiconductors </li></ul></ul></ul>Adapted from Fig. 21.10, Callister 6e . (Fig. 21.10 is by J. Telford, with specimen preparation by P.A. Lessing.)
  8. 8. Optical Classification Intensity of the incident beam =Sum of the intensities of the transmitted , absorbed , and reflected beams. Materials with little absorption and reflection are transparent . You can see through them. Materials in which light is transmitted diffusely are translucent . Objects are not clearly distinguishable. Materials where light is absorbed and reflected are opaque .
  9. 9. Band structure of materials <ul><li>Recall back to discussions of band structure: </li></ul><ul><ul><li>Based on electron bonding and antibonding orbitals in individual atoms </li></ul></ul><ul><ul><li>Orbitals from adjacent atoms overlap in molecules </li></ul></ul><ul><ul><li>In a solid, there are so many orbitals that they form a continuous band. </li></ul></ul><ul><li>Transitions between bands only allowed for certain energies: </li></ul><ul><ul><li>Although a little bit of energy is available to every electron at room temperature (kT=25meV): </li></ul></ul><ul><ul><ul><li>they can only conduct current in a metal where there are vacant band states available. </li></ul></ul></ul><ul><ul><ul><li>For a semiconductor, a bandgap exists without any available band states within 25meV of the electron. Thus, no conductivity. </li></ul></ul></ul><ul><li>Note that photons can transfer their energy to electrons, or vice versa. </li></ul><ul><li>Optics can thus tell us about band structure, or band structure about the optical response. </li></ul>
  10. 10. <ul><li>If we can excite electrons in a material (or molecule or atom), as those electrons return to their ground state they may release their energy as light of discrete characteristic wavelengths: </li></ul><ul><ul><li>Light bulb </li></ul></ul><ul><ul><li>X-ray tube </li></ul></ul><ul><ul><li>Sun </li></ul></ul>Electromagnetic radiation generation http://www.chemistry.adelaide.edu.au/external/soc-rel/content/at-lvls.htm http://csep10.phys.utk.edu/astr162/lect/light/absorption.html
  11. 11. Electromagnetic radiation absorption <ul><li>Alternatively, only photons of certain energies can be absorbed by any particular atom (or crystal). </li></ul>http://csep10.phys.utk.edu/astr162/lect/light/absorption.html <ul><li>E 41 , E 31 , and E 21 are also conceivable. </li></ul><ul><li>So are E 32 and E 31 . </li></ul><ul><li>and E 21 . </li></ul><ul><li>etc. </li></ul>
  12. 12. Absorption in Metals • Absorption of photons by electron transition: • Absorption is usually very small (less than 5%) • Metals have a fine succession of energy states. • Near-surface electrons absorb visible light. Adapted from Fig. 21.4(a), Callister 6e .
  13. 13. Absorption in Metals Most of the absorbed radiation is re-emitted from the surface, less than 0.1 micron. Only very thin films of metals are transparent to visible light. Metals are only “transparent” to high frequency radiation ( x - and gamma -rays). A bright silvery color when exposed to light indicates that the metal is highly reflective: number & frequency of incoming photons is ~ equal in the incident and reflected beam (Al, Fe, Ti, Ag) . In some metals, short wavelength radiation ( green , blue , violet ) is not re-emitted. They appear red-orange or yellow (Cu, Au).
  14. 14. Absorption in Semiconductors/Insulators <ul><li>In a metal, just about any visible photon can be absorbed. </li></ul><ul><li>But for a semiconductor or insulator, photons must have at least an energy of E g to be absorbed (to successfully excite an electron into an available band state). </li></ul>
  15. 15. More semiconductors/insulators • Absorption by electron transition occurs if h  > E gap • If E gap < 1.7eV, full visible absorption, black or metallic • If E gap > 3.1eV, no visible absorption, transparent • If E gap in between, partial visible absorption, colors incident photon energy h  3.1 eV 1.7 eV
  16. 16. Absorption of colored light <ul><li>Which of the following is true? </li></ul><ul><ul><li>If my material absorbs blue , it must also absorb red . </li></ul></ul><ul><ul><li>If my material absorbs red , it must also absorb blue . </li></ul></ul><ul><ul><li>Think about this at home… </li></ul></ul>
  17. 17. Bandgap states <ul><li>Impurities/dopants cause energy levels in the bandgap (discrete sites and/or very narrow bands). </li></ul><ul><li>This allows excitation (or emission) via single or double photon interactions: </li></ul><ul><ul><li>Single photons from E v to E f , or from E f to E c . </li></ul></ul><ul><ul><li>Combination of photons, 1 st from E v to E f and the 2 nd from E f to E c . </li></ul></ul>
  18. 18. Radiation excitation by non-metals <ul><li>For a system with a band-gap, photons with energies greater than the band gap can be emitted! </li></ul><ul><ul><li>Light bulbs </li></ul></ul><ul><ul><li>Light Emitting Diodes (LED’s) </li></ul></ul><ul><ul><li>Lasers </li></ul></ul><ul><li>Generating low energy photons (IR, red) is easy. </li></ul><ul><li>Generating blue photons is much harder, as the bandgap must be larger and free of defects. </li></ul>
  19. 19. Methods of Photon Absorbtion
  20. 20. Refraction <ul><li>To understand reflection, we first must understand refraction. </li></ul><ul><li>The speed of any radiation in any medium (v) is related to the speed of light in a vacuum (c) over the index of refraction of the material. </li></ul><ul><li>The speed of radiation in a vacuum (c=v o ) is related to the dielectric permittivity and magnetic permeability of a vacuum. </li></ul><ul><li>The speed of radiation in any material can also be related to the material dependent dielectric permittivity and magnetic permeability. </li></ul><ul><li>The index of refraction (optical property) is thus easily determined if the electronic and magnetic properties are known (r means relative to a vacuum). </li></ul><ul><li>Since most materials are only weakly magnetic, the relative dielectric permittivity is often measured based on the optical index of refraction. </li></ul>
  21. 21. LIGHT INTERACTION WITH SOLIDS • Incident light is either reflected, absorbed, or transmitted: If photons of a certain color are absorbed, they obviously aren’t being transmitted (or reflected) to your eyes. This will affect the material color.
  22. 22. Refraction <ul><li>Fish use this concept to see you standing on the shore trying to catch them. </li></ul>
  23. 23. Reflection <ul><li>Remember the reflected intensity is proportional to the reflected fraction (R) and the initial intensity. </li></ul><ul><li>As ∆n between two bodies increases, so does reflection at the interface. </li></ul><ul><ul><li>This is regardless of the quality of the interface, an entirely different term that we don’t cover here. </li></ul></ul><ul><li>For light passing between a vacuum (or air) and a solid, the equation simplifies since n=1. </li></ul><ul><li>Note that this is strictly true only for photons perfectly normal to the interface. </li></ul><ul><ul><li>When there is an angle of incidence, this adds extra, complicating terms to the equations. </li></ul></ul>
  24. 24. Absorption <ul><li>Light traversing a material is absorbed exponentially, depending on the absorption coefficient ( β ) and the distance traveled (x or l). </li></ul><ul><ul><li>Transparent materials have a very small absorption coefficient, while strong absorbers obviously rapidly diminish the transmitted light intensity. </li></ul></ul><ul><ul><li>Note that β is technically wavelength dependent. </li></ul></ul>“non-absorbed beam” “Initial beam”
  25. 25. Light reflected and absorbed going through a material. “non-reflected beam:” “non-absorbed beam:” non-reflected beam 2:
  26. 26. COLOR OF NONMETALS in transmission • Color determined by sum of frequencies of --transmitted light, --re-emitted light from electron transitions. • Ex: Cadmium Sulfide (CdS) -- E gap = 2.4eV, -- absorbs higher energy visible light (blue, violet), -- Red/yellow/orange is transmitted and gives it color. • Ex: Ruby = Sapphire (Al 2 O 3 ) + (0.5 to 2) at% Cr 2 O 3 -- Pure sapphire is colorless (i.e., E gap > 3.1eV) -- adding Cr 2 O 3 : • alters the band gap • blue light is absorbed • yellow/green is absorbed • red is transmitted • Result: Ruby is deep red in color.
  27. 27. Transmission and Absorption <ul><li>The transmitted intensity is a function of the intensity not already reflected at the surface, the thickness of the material, and a measure of how good of an absorber it is (absorption coefficient, beta). </li></ul><ul><ul><li>Large beta = strong absorber. </li></ul></ul><ul><li>Remember these terms can vary with the wavelength of incident radiation. </li></ul>
  28. 28. Summary of basic optics <ul><li>Radiation striking an object will reflect, absorb, and/or transmit. </li></ul><ul><li>The color of the object depends on the energy dependence of each of these parameters, which depends on the bandstructure (for non-metals). </li></ul>
  29. 29. Photoelasticity <ul><li>The optical properties of some materials change with mechanical loading: photoelasticity . </li></ul>
  30. 30. LUMINESCENCE <ul><li>The radiated light may not have the same wavelength as the incident energy </li></ul><ul><ul><li>(but it must always have less E) </li></ul></ul><ul><ul><ul><li>UV -> blue </li></ul></ul></ul><ul><ul><ul><li>Blue -> red </li></ul></ul></ul><ul><ul><ul><li>Never red -> blue </li></ul></ul></ul>incident radiation <ul><li>After electrons are excited across the gap (usually with UV radiation), they may generate new photons by re-emission. </li></ul>Spontaneous emitted light
  31. 31. Luminescence types <ul><li>After the excitation source is turned off: </li></ul><ul><ul><li>If the luminescence ends rapidly, then the material is fluorescent (it will luminesce only as long as it is excited). </li></ul></ul><ul><ul><li>If the luminescence continues, the material is phosphorescent (light emission persists). </li></ul></ul><ul><li>Fluorescence: lasts << 1 second </li></ul><ul><li>Phosphorescence: 1 second or greater </li></ul>• Example: fluorescent lamps
  32. 32. LASER <ul><li>L ight A mplification by S timulated E mission of R adiation </li></ul><ul><li>Coherent (in phase) </li></ul><ul><ul><li>Most other light sources are incoherent—generated by independent, randomly timed optical events. </li></ul></ul><ul><li>The Ruby laser is a Al 2 O 3 crystal (sapphire) with .05% Cr 3+ . </li></ul><ul><li>Ruby prepared as a rod with ends polished flat and parallel. </li></ul><ul><li>One end is mirrored (silvered), the other end is perfectly parallel and is partially mirrored allowing most light to be internally reflected but some to spill past the mirror (transmission). </li></ul><ul><li>Ruby illuminated with ‘flash lamp.’ </li></ul>
  33. 33. Laser concept <ul><li>Before the ruby is illuminated with the ‘flash lamp,’ nearly all Cr 3+ dopant ions are in their ground states. </li></ul><ul><ul><li>Nearly all electrons are in their lowest energy levels. </li></ul></ul><ul><li>Photons with 560nm wavelength are at the right energy to excite electrons from the Cr 3+ into the conduction band where available energy levels are present. </li></ul><ul><li>Decay follows two paths. </li></ul><ul><ul><li>A) Fall back directly (like a metal). </li></ul></ul><ul><ul><li>B) Most decay into metastable intermediate state, staying there up to 3 ms before falling back to their ground state and emission of a photon in the visible range . </li></ul></ul><ul><li>If we can get a lot of these photons, we will have a nicely monochromatic beam (single wavelength, λ ). </li></ul>
  34. 34. Laser operation <ul><li>As the metastable intermediate states decay (up to 3 ms), the available time is long on optical scales and thus many Cr 3+ states get filled. </li></ul><ul><li>Initial spontaneous photon emission stimulates an avalanche of emissions from many metastable Cr 3+ ions. </li></ul><ul><li>Non-axial Photons decay rapidly. </li></ul><ul><li>Most photons generated along the axis of the rod reflect off the mirrored edges, ‘traversing’ from one end to the other and back. </li></ul><ul><li>Electrons continue to be excited and then emitted by the flash lamp. </li></ul><ul><li>Some electrons transmit through the partially silvered mirror on one side. </li></ul><ul><li>Eventually, a collimated (highly parallel), high intensity beam transmits through the laser edge. </li></ul>
  35. 35. What to do with a laser? <ul><li>Point </li></ul><ul><li>Marking </li></ul><ul><li>‘burn’ holes (machining), 1um features possible </li></ul><ul><li>Cutting (metal, cloth) </li></ul><ul><li>Medical applications (cutting and cauterizing=singe tissue after cutting it to seal off blood vessels and stop bleeding) </li></ul><ul><li>Heat treatments for very localized and rapid annealing </li></ul><ul><li>Localized sintering for 3d manufacture </li></ul><ul><li>welding (auto industry) </li></ul><ul><li>Distance measurement (to the moon, to your dorm wall) </li></ul><ul><li>Barcode scanners </li></ul><ul><li>CD’s/DVD’s/etc (data recording and reading) </li></ul><ul><li>Communications (via fiber optics) </li></ul>http://www.directedlight.com/
  36. 36. CD and DVD players <ul><li>Data is stored like a CD, in the form of small regions (bits) with different optical properties than the substrate (disk). </li></ul><ul><li>Composed of about 1.2 mm of multilayered, injection molded, clear polycarbonate plastic. </li></ul><ul><li>A continuous and very long track of data (7.5 miles), written from the center of the disk and spiraling outward. </li></ul>http://www.veeco.com/nanotheatre <ul><li>Tracks are only 740 nm apart, and data bits are usually < 400 nm on a side. </li></ul>
  37. 37. Optical recording <ul><li>Different methods for generating the bits: </li></ul><ul><ul><li>Stamping (create ‘craters’). </li></ul></ul><ul><ul><ul><li>Coat with metal to enhance reflectivity. </li></ul></ul></ul><ul><ul><li>Phase change (crystalline to amorphous transition depending on optical power and time). </li></ul></ul><ul><ul><ul><li>No topographic change but a change in the mechanical and optical properties since photoelastic. </li></ul></ul></ul><ul><li>Magneto-optical </li></ul><ul><ul><li>Domains oriented optically </li></ul></ul>http://www.veeco.com/nanotheatre
  38. 38. Double sided, double layer <ul><li>Both sides of the disk can be written on, and then read by turning over the disk or having 2 heads (one on top, the other on the bottom). </li></ul><ul><li>Double layer recording is also possible on each side (available in stores, 16 GB, 8 hours of movies). </li></ul><ul><li>Focusable, high intensity laser used to read/write at two depths. </li></ul><ul><li>Sometimes, 2 lasers with different wavelengths are used (one is reflected by semitransparent layer, the other is transmitted through it to the next layer). </li></ul>http://www6.tomshardware.com/storage/20040827/
  39. 39. SUMMARY <ul><li>Frequency, wavelength, and energy, trends in spectrum </li></ul><ul><li>Optical classifications </li></ul><ul><li>How does absorption, A, (or emission) relate to band structure (band gaps, donor/acceptor states, etc)? </li></ul><ul><li>Reflection (R) </li></ul><ul><li>Refraction </li></ul><ul><li>Transmission (T) </li></ul><ul><li>Equations for R, A, T </li></ul><ul><li>Photoelasticity. </li></ul><ul><li>Define Phosphorescence and Fluorescence. </li></ul><ul><li>Know the principles behind the ruby laser. </li></ul><ul><li>Know the principles behind optical data storage (DVDs). </li></ul>Next class: Review