Transcript Optical properties

Optical Properties
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Refraction & Dispersion
An electromagnetic wave
Electric field and magnetic field components, and the wavelength
.
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Figure 19.2 The spectrum of electromagnetic radiation,
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Watt/m2
Dividing by Io
Transparent, translucent and opaque
Electronic Polarization: absorption & refraction (retarded waves)
Electron Transitions: Quantum behavior, excited and ground states
LIGHT INTERACTION WITH SOLIDS
• Incident light is either reflected, absorbed, or
transmitted:
Io  IT  IA  IR
Example: Isolated atom
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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: REFRACTION
• Transmitted light distorts electron clouds.
no
transmitted
light
• 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
--Adding large, heavy ions (e.g., lead
can decrease the speed of light.
--Light can be
"bent"
Material
Lead glass
Silica glass
Soda-lime glass
Quartz
Plexiglas
Polypropylene
n
2.1
1.46
1.51
1.55
1.49
1.49
Selected values from Table 21.1,
Intensity of transmitted light decreases
with distance traveled (thick pieces less transparent!)
• Result 2:
Callister 6e.
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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
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Emission of a photon of light by a
direct electron transition across the
band gap.
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
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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
SELECTED ABSORPTION: NONMETALS
• Absorption by electron transition occurs if hn > Egap
incident photon
energy hn
Adapted from Fig. 21.5(a), Callister 6e.
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.
• If Egap < 1.8eV,
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Figure 19.7 The transmission of light
through a transparent
medium for which there is reflection at front
and back faces, as
well as absorption within the medium.
19.9 COLOR
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COLOR OF NONMETALS
• 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.
Ruby = Sapphire (Al2O3) + (0.5 to 2) at% Cr2O3
-- Sapphire is colorless
• Ex:
(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.
Adapted from Fig. 21.9, Callister 6e. (Fig. 21.9 adapted from "The
Optical Properties of Materials" by A. Javan, Scientific American, 1967.)
19.10 OPACITY AND TRANSLUCENCY IN INSULATORS
the light transmittance of three aluminum oxide specimens. From left to right: singlecrystal 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.
19.11 APPLICATION: LUMINESCENCE
• Process:
Energy of el ectron
unfilled states
incident
radiation
Egap
emitted
light
filled s tates
electron
transition occurs
Adapted from Fig. 21.5(a), Callister 6e.
Adapted from Fig. 21.5(a), Callister 6e.
• Ex: fluorescent lamps
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Fluorescence & phosphorescence
19.12 APPLICATION: PHOTOCONDUCTIVITY
• Description:
• Ex: Photodetector (Cadmium sulfide)
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APPLICATION: SOLAR CELL
• p-n junction:
• Operation:
--incident photon produces hole-elec. pair.
--typically 0.5V potential.
--current increases w/light intensity.
• Solar powered weather station:
polycrystalline Si
Los Alamos High School weather
station (photo courtesy
P.M. Anderson)
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Light-Emitting Diodes
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OLED
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the ruby laser and xenon flash lamp.
The ruby laser
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(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 stimulated emission and light
amplification for a ruby laser.
(a) The chromium ions before excitation
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 the
adjacent n- and ptype GaAlAs layers.
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SUMMARY
• 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.
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