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Optical Properties

Refraction & Dispersion

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An electromagnetic wave Electric field and magnetic field components, and the

wavelength .

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Figure 19.2 The spectrum of electromagnetic radiation,

Watt/m2

Dividing by Io

Transparent, translucent and opaque

Electronic Polarization: absorption & refraction (retarded waves)

Electron Transitions: Quantum behavior, excited and ground states

• Incident light is either reflected, absorbed, or transmitted:

Incident: Io

Reflected: IR Absorbed: IA

Transmitted: IT

Io IT IA IR

LIGHT INTERACTION WITH SOLIDS

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Example: Isolated atom

Figure 19.4 (a) Schematic representation of the mechanism of photon absorption for metallic materials in which an electron is excited into a higher-energy unoccupied state. The change in energy of the electron E is equal to the energy of the photon. (b) Reemission of a photon of light by the direct transition of an electron from a high to a low energy state.

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• Transmitted light distorts electron clouds.

+no

transmitted light

transmitted light +

electron cloud distorts

• Result 1: Light is slower in a material vs vacuum.

Index of refraction (n) = speed of light in a vacuum speed of light in a material

MaterialLead glassSilica glassSoda-lime glassQuartzPlexiglasPolypropylene

n2.11.461.511.551.491.49

--Adding large, heavy ions (e.g., lead can decrease the speed of light.--Light can be "bent"

• Result 2: Intensity of transmitted light decreases with distance traveled (thick pieces less transparent!)

Selected values from Table 21.1,Callister 6e.

TRANSMITTED LIGHT: REFRACTION

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Figure 19.5 Non Metallic Materials—Materials with a bandgap

an electron is excited across the band gap leaving behind a hole in the valence band. absorbed photon energy: E = Eg

Emission of a photon of light by a direct electron transition across the band gap.

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Figure 19.6

Photon absorption via a valence band-conduction band electron excitation for a material that has an impurity level that lies within the band gap

Emission of two photons involving electron decay first into an impurity state, and finally into the ground state

Generation of both a phonon and a photon as an excited electron falls first into an impurity level and finally back to its ground state

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• Absorption by electron transition occurs if h > Egap

• If Egap < 1.8eV, full absorption; color is black (Si, GaAs)• If Egap > 3.1eV, no absorption; colorless (diamond)• If Egap in between, partial absorption; material has a color.

Adapted from Fig. 21.5(a), Callister 6e.

SELECTED ABSORPTION: NONMETALSEnergy of electron

filled states

unfilled states

Egap

Io

blue light: h= 3.1eV

red light: h= 1.7eV

incident photon energy h

c19f07 Figure 19.7 The transmission of light through a transparentmedium for which there is reflection at front and back faces, aswell as absorption within the medium.

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19.9 COLOR

• Color determined by sum of frequencies of --transmitted light, --re-emitted light from electron transitions.

• Ex: Cadmium Sulfide (CdS)

-- Egap = 2.4eV, -- absorbs higher energy visible light (blue, violet), -- Red/yellow/orange is transmitted and gives it color.

• Ex: Ruby = Sapphire (Al2O3) + (0.5 to 2) at% Cr2O3

-- Sapphire is colorless (i.e., Egap > 3.1eV) -- adding Cr2O3 : • alters the band gap • blue light is absorbed • yellow/green is absorbed • red is transmitted • Result: Ruby is deep red in color. 40

60

70

80

50

0.3 0.5 0.7 0.9

Transm

itta

nce

(%

)

Ruby

sapphire

wavelength, c/)(m)

Adapted from Fig. 21.9, Callister 6e. (Fig. 21.9 adapted from "The Optical Properties of Materials" by A. Javan, Scientific American, 1967.)

COLOR OF NONMETALS

the light transmittance of three aluminum oxide specimens. From left to right: single-crystal material (sapphire), which is transparent; a polycrystalline and fully dense (nonporous) material, which is translucent; and a polycrystalline material that contains approximately 5% porosity, which is opaque.

19.10 OPACITY AND TRANSLUCENCY IN INSULATORS

Energy of electron

filled states

unfilled states

Egap

re-emission occurs

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• Process:

• Ex: fluorescent lamps

UV radiation

coating e.g., -alumina doped w/Europium

“white” lightglass

Adapted from Fig. 21.5(a), Callister 6e. Adapted from Fig. 21.5(a), Callister 6e.

19.11 APPLICATION: LUMINESCENCE

electron transition occurs

Energy of electron

filled states

unfilled states

Egapincidentradiation emitted

light

Fluorescence & phosphorescence

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• Description:

• Ex: Photodetector (Cadmium sulfide)

19.12 APPLICATION: PHOTOCONDUCTIVITY

Incident radiation

semi conductor:

Energy of electron

filled states

unfilled states

Egap

+

-A. No incident radiation: little current flow

Energy of electron

filled states

unfilled states

Egapconducting electron

+

-B. Incident radiation: increased current flow

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• p-n junction: • Operation: --incident photon produces hole-elec. pair. --typically 0.5V potential. --current increases w/light intensity.

n-type Si

p-type Sip-n junction

B-doped Si

Si

Si

Si SiB

hole

P

Si

Si

Si Si

conductance electron

P-doped Si

n-type Si

p-type Sip-n junction

light

+-

++ +

---

creation of hole-electron pair

• Solar powered weather station:

polycrystalline SiLos Alamos High School weatherstation (photo courtesyP.M. Anderson)

APPLICATION: SOLAR CELL

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Light-Emitting Diodes

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OLED

the ruby laser and xenon flash lamp.

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The ruby laser

(a)The chromium ions before excitation

(b)Electrons in some chromium atoms are excited into higher energy states by the xenon light flash.

(c)Emission from metastable electron states is initiated or stimulated by photons that are spontaneously emitted.

(d)Upon reflection from the silvered ends, the photons continue to stimulate emissions as they traverse the rod length.

(e)The coherent and intense beam is finally emitted through the partially silvered end.

The stim

ulated emission and light

amplification for a ruby laser.

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semiconductor laser

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Figure 19.17 layered cross section of a GaAs semiconducting laser. Holes, excited electrons, and the laser beam are confined to the GaAs layer by theadjacent n- and p-type GaAlAs layers.

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• When light (radiation) shines on a material, it may be: --reflected, absorbed and/or transmitted.• Optical classification: --transparent, translucent, opaque• Metals: --fine succession of energy states causes absorption and reflection.• Non-Metals: --may have full (Egap < 1.8eV) , no (Egap > 3.1eV), or partial absorption (1.8eV < Egap = 3.1eV). --color is determined by light wavelengths that are transmitted or re-emitted from electron transitions. --color may be changed by adding impurities which change the band gap magnitude (e.g., Ruby)• Refraction: --speed of transmitted light varies among materials.

SUMMARY