Chapter 22: Optical Properties

Transcription

1 Chapter 22: Optical Properties ISSUES TO ADDRESS... What phenomena occur when light is shined on a material? What determines the characteristic colors of materials? Why are some materials transparent and others are translucent or opaque? How does a laser operate? Chapter 22-1

2 Figun_22_p785c Solar cells: Convert Light to electricity Figun_22_p785b Figun_22_p785a Chapter 22-2

3 01 Introduction 02 Electromagnetic Radiation 03 Light Interactions with Solids 04 Atomic and Electronic Interactions 05 Refraction 06 Reflection 07 Absorption 08 Transmission 09 Color 10 Opacity and Translucency in Insulators 11 Luminescence 12 Photoconductivity 13 Lasers 14 Optical Fibers in Communications Chapter 22-3

4 1. Optical Properties Optical property refers to a material's response to exposure to electromagnetic radiation and, in particular, to visible light. Light has both particulate and wavelike characteristics Photon - a quantum unit of light Light, heat (or radiant energy), radar, radio waves, and x-rays are all forms of electromagnetic radiation. Chapter 22-4

5 2. Electromagnetic radiation is related to the electric permittivity and the magnetic permeability of a vacuum Fig_22_01 An electromagnetic wave showing electric field and magnetic field components and the wavelength and also direction of propagation. Chapter 22 -

6 Fig_22_02 Chapter 22 -

7 3. Light Interactions with Solids Incident light is reflected, absorbed, scattered, and/or transmitted: I 0 = I T + I A + I R + I S Reflected: I R Absorbed: I A Transmitted: I T Incident: I 0 Scattered: I S Optical classification of materials: Transparent Translucent Opaque single crystal polycrystalline dense polycrystalline porous Chapter 22-7

8 4. Atomic and electronic interactions Electronic polarization to shift the electron cloud and light diffracted no transmitted light + transmitted light + electron cloud distorts Electron transition Fig_22_03 Chapter 22-8

9 Optical Properties of Metals: Absorption Absorption of photons by electron transitions: Energy of electron unfilled states DE = hν required! Planck s constant (6.63 x J/s) freq. of incident light filled states Unfilled electron states are adjacent to filled states Near-surface electrons absorb visible light. Chapter 22-9

10 Reflection of Light for Metals Electron transition from an excited state produces a photon. I R photon emitted from metal surface Energy of electron unfilled states conducting electron Electron transition filled states Chapter 22-10

11 Reflection of Light for Metals (cont.) Reflectivity = I R /I 0 is between 0.90 and Metal surfaces appear shiny Most of absorbed light is reflected at the same wavelength Small fraction of light may be absorbed Color of reflected light depends on wavelength distribution Example: The metals copper and gold absorb light in blue and green => reflected light has gold color Chapter 22-11

12 5. Refraction Transmitted light distorts electron clouds. no transmitted light + transmitted light + electron cloud distorts The velocity of light in a material is lower than in a vacuum. n = index of refraction -- Adding large ions (e.g., lead) to glass decreases the speed of light in the glass. -- Light can be bent as it passes through a transparent prism Material n Typical glasses ca Plastics PbO (Litharge) 2.67 Diamond 2.41 Chapter 22-12

13 In ionic materials, the larger the size of the component ions the greater the degree of electronic polarization and the higher the n values. Chapter 22-13

14 6. Reflectivity of Nonmetals For normal incidence and light passing into a solid having an index of refraction n: R reflectivi ty Ir/Io n n Example: For Diamond n = 2.41 \ 17% of light is reflected Chapter 22-14

15 Example: Diamond in air What is the critical angle ϕ c for light passing from diamond (n 1 = 2.41) into air (n 2 = 1)? Solution: At the critical angle, and Rearranging the equation Substitution gives Chapter 22-15

16 Total Internal Reflectance: the Smell s law n 2 < n 1 n 2 n 1 ϕ 1 = incident angle ϕ 2 = refracted angle ϕ c = critical angle ϕ c exists when ϕ 2 = 90 For ϕ 1 > ϕ c light is internally reflected Fiber optic cables are clad in low n material so that light will experience total internal reflectance and not escape from the optical fiber. Chapter 22-16

17 Scattering of Light in Polymers For highly amorphous and pore-free polymers Little or no scattering These materials are transparent Semicrystalline polymers Different indices of refraction for amorphous and crystalline regions Scattering of light at boundaries Highly crystalline polymers may be opaque Examples: Polystyrene (amorphous) clear and transparent Low-density polyethylene milk cartons opaque Chapter 22-17

18 7. Light Absorption The amount of light absorbed by a material is calculated using Beer s Law β = absorption coefficient, cm -1 = sample thickness, cm I0 = incident (nonreflected) light intensity I = transmitted light intensity T Rearranging and taking the natural log of both sides of the equation leads to Chapter 22-18

19 Selected Light Absorption in Semiconductors Absorption of light of frequency ν by by electron transition occurs if hν > Egap Examples of photon energies: blue light: hν = 3.1 ev red light: hν = 1.8 ev Energy of electron unfilled states incident photon energy hν E gap filled states If Egap < 1.8 ev, all light absorbed; material is opaque (e.g., Si, GaAs) If Egap > 3.1 ev, no light absorption; material is transparent and colorless (e.g., diamond) If 1.8 ev < Egap < 3.1 ev, partial light absorption; material is colored Chapter 22-19

20 Computations of Minimum Wavelength Absorbed (a) What is the minimum wavelength absorbed by Ge, for which E g = 0.67 ev? Solution: (b) Redoing this computation for Si which has a band gap of 1.1 ev Note: the presence of donor and/or acceptor states allows for light absorption at other wavelengths. Chapter 22-20

21 8. Light transmission Fig_22_07 Chapter 22-21

22 Transmittance (%) 9. Color of Nonmetals Color determined by the distribution of wavelengths: -- transmitted light -- re-emitted light from electron transitions Example 1: Cadmium Sulfide (CdS), E g = 2.4 ev -- absorbs higher energy visible light (blue, violet) -- color results from red/orange/yellow light that is transmitted Example 2: Ruby = Sapphire (Al 2 O 3 ) + (0.5 to 2) at% Cr 2 O 3 -- Sapphire is transparent and colorless (E g > 3.1 ev) -- adding Cr 2 O 3 : alters the band gap blue and orange/yellow/green light is absorbed red light is transmitted Result: Ruby is deep red in color sapphire ruby 50 wavelength, λ (= c/ν)(μm) Chapter 22-22

23 10. Opacity and translucency in insulators depends to a great degree on their internal reflectance and transmittance characteristics. interior reflection and refraction. diffuse as a result of multiple scattering events. Opacity results from extensive scattering that virtually none of the incident beam is transmitted Transparent Translucent Opaque single crystal polycrystalline dense polycrystalline porous Chapter 22-23

24 11. Luminescence Luminescence reemission of light by a material Material absorbs light at one frequency and reemits it at another (lower) frequency. Trapped (donor/acceptor) states introduced by impurities/defects Conduction band If residence time in trapped state is relatively long (> 10-8 s) -- phosphorescence E g trapped states E emission For short residence times (< 10-8 s) -- fluorescence Valence band activator level Example: Toys that glow in the dark. Charge toys by exposing them to light. Reemission of light over time phosphorescence Chapter 22-24

25 Photoluminescence: Fluorescent lamps Hg atom electrode UV light electrode Arc between electrodes excites electrons in mercury atoms in the lamp to higher energy levels. As electron falls back into their ground states, UV light is emitted (e.g., suntan lamp). Inside surface of tube lined with material that absorbs UV and reemits visible light - For example, Ca 10 F 2 P 6 O 24 with 20% of F - replaced by Cl - Adjust color by doping with metal cations Sb 3+ blue Mn 2+ orange-red Chapter 22-25

26 Cathodoluminescence Used in cathode-ray tube devices (e.g., TVs, computer monitors) Inside of tube is coated with a phosphor material Phosphor material bombarded with electrons Electrons in phosphor atoms excited to higher state Photon (visible light) emitted as electrons drop back into ground states Color of emitted light (i.e., photon wavelength) depends on composition of phosphor ZnS (Ag + & Cl - ) (Zn, Cd) S + (Cu + +Al 3+ ) Y 2 O 2 S + 3% Eu blue green red Note: light emitted is random in phase & direction i.e., is noncoherent Chapter 22-26

27 Photoconductivity Additional charge carriers may be generated as a consequence of photon-induced electron transitions in which light is absorbed a photoconductive material is illuminated, the conductivity increases. CdS is commonly used in light meters. Sunlight may be directly converted into electrical energy in solar cells ( p-n semiconductors). This conversion of electrical energy into light energy is termed electroluminescence, and the device that produces it is termed a light-emitting diode (LED). Chapter 22-27

28 13. The LASER The laser generates light waves that are in phase (coherent) and that travel parallel to one another LASER Light Amplification by Stimulated Emission of Radiation Operation of laser involves a population inversion of energy states process Chapter 22-28

29 Population Inversion More electrons in excited energy states than in ground states Chapter 22-29

30 Operation of the Ruby Laser pump electrons in the lasing material to excited states e.g., by flash lamp (incoherent light). Direct electron decay transitions produce incoherent light Chapter 22-30

31 Operation of the Ruby Laser (cont.) Stimulated Emission The generation of one photon by the decay transition of an electron, induces the emission of other photons that are all in phase with one another. This cascading effect produces an intense burst of coherent light. This is an example of a pulsed laser Chapter 22-31

32 Continuous Wave Lasers Continuous wave (CW) lasers generate a continuous (rather than pulsed) beam Materials for CW lasers include semiconductors (e.g., GaAs), gases (e.g., CO 2 ), and yttrium-aluminum-garnet (YAG) Wavelengths for laser beams are within visible and infrared regions of the spectrum Users of CW lasers 1. Welding 2. Drilling 3. Cutting laser carved wood, eye surgery 4. Surface treatment 5. Scribing ceramics, etc. 6. Photolithography Excimer laser Chapter 22-32

33 Semiconductor Laser Applications Apply strong forward bias across semiconductor layers, metal, and heat sink. Electron-hole pairs generated by electrons that are excited across band gap. Recombination of an electron-hole pair generates a photon of laser light electron + hole neutral + hν recombination ground state photon of light Fig , Callister & Rethwisch 9e. Chapter 22-33

34 Semiconductor Laser Applications Compact disk (CD) player Use red light High resolution DVD players Use blue light Blue light is a shorter wavelength than red light so it produces higher storage density Communications using optical fibers Fibers often tuned to a specific frequency Banks of semiconductor lasers are used as flash lamps to pump other lasers Chapter 22-34

35 Other Applications of Optical Phenomena New materials must be developed to make new & improved optical devices. Organic Light Emitting Diodes (OLEDs) More than one color available from a single diode Also sources of white light (multicolor) Chapter 22-35

36 Other Applications - Solar Cells p-n junction: conductance electron n-type Si p-n junction p-type Si hole P-doped Si Si Si Si P Si Si B Si Si Operation: -- incident photon of light produces elec.-hole pair. -- typical potential of 0.5 V produced across junction -- current increases w/light intensity. n-type Si p-n junction p-type Si light creation of hole-electron pair Solar powered weather station: Si B-doped Si polycrystalline Si Los Alamos High School weather station (photo courtesy P.M. Anderson) Chapter 22-36

37 14. Other Applications - Optical Fibers Schematic diagram showing components of a fiber optic communications system Fig , Callister & Rethwisch 9e. Chapter 22-37

38 Optical Fibers (cont.) fibers have diameters of 125 μm or less plastic cladding 60 μm thick is applied to fibers Fig , Callister & Rethwisch 9e. Chapter 22-38

39 Step-index Optical Fiber Optical Fiber Designs Graded-index Optical Fiber Fig , Callister & Rethwisch 9e. Fig , Callister & Rethwisch 9e. Chapter 22-39

40 SUMMARY Light radiation impinging on a material may be reflected from, absorbed within, and/or transmitted through Light transmission characteristics: -- transparent, translucent, opaque Optical properties of metals: -- opaque and highly reflective due to electron energy band structure. Optical properties of non-metals: -- for Egap < 1.8 ev, absorption of all wavelengths of light radiation -- for Egap > 3.1 ev, no absorption of visible light radiation -- for 1.8 ev < Egap < 3.1 ev, absorption of some range of light radiation wavelengths -- color determined by wavelength distribution of transmitted light Other important optical applications/devices: -- luminescence, photoconductivity, light-emitting diodes, solar cells, lasers, and optical fibers Chapter 22-40