Chapter 21: Optical Properties

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Chapter 21 Optical Properties
ISSUES TO ADDRESS...
What happens when light shines on a material?
Why do materials have characteristic colors?
Why are some materials transparent and others
not?
Optical applications -- luminescence --
photoconductivity -- solar cell -- optical
communications fibers
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Optical Properties

• Light has both particulate and wavelike
  properties
• Photons - with mass

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Refractive Index, n
Transmitted light distorts electron clouds.
Light is slower in a material vs vacuum.
n refractive index

• Note n f (?)
• Typical glasses ca. 1.5 -1.7
• Plastics 1.3 -1.6
• PbO (Litharge) 2.67
• Diamond 2.41

Selected values from Table 21.1, Callister 7e.
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Total Internal Reflectance
n gt n
n(low)
n (high)
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Example Diamond in air

• Fiber optic cables are dressed in low n material
  for this reason.

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Light Interaction with Solids
Incident light is either reflected, absorbed,
or transmitted
Optical classification of materials
Adapted from Fig. 21.10, Callister 6e. (Fig.
21.10 is by J. Telford, with specimen preparation
by P.A. Lessing.)
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OPTICAL PROPERTIES

• An optical property describes the way a material
  reacts to exposure to light.
• Visible light is a form of electromagnetic
  radiation with
• wavelengths in the range of 400 to 700 nm
• corresponding to an energy range of 3.1 to 1.8
  electron volts (eV)

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Optical Properties of Metals Absorption
Absorption of photons by electron transition
Adapted from Fig. 21.4(a), Callister 7e.
Metals have a fine succession of energy
states. Near-surface electrons absorb visible
light.
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Light Absorption
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Optical Properties of Metals Reflection
Adapted from Fig. 21.4(b), Callister 7e.
Reflectivity IR/Io is between 0.90 and
0.95. Reflected light is same frequency as
incident. Metals appear reflective (shiny)!
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Reflectivity, R

• Reflection
• Metals reflect almost all light
• Copper gold absorb in blue green gt gold color

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Scattering

• In semicrystalline or polycrystalline materials
• Semicrystalline
• density of crystals higher than amorphous
  materials ? speed of light is lower - causes
  light to scatter - can cause significant loss of
  light
• Common in polymers
• Ex LDPE milk cartons cloudy
• Polystyrene clear essentially no crystals

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Selected Absorption Semiconductors
Absorption by electron transition occurs if hn
gt Egap
incident photon energy hn
Adapted from Fig. 21.5(a), Callister 7e.
If Egap lt 1.8 eV, full absorption color is
black (Si, GaAs)
If Egap gt 3.1 eV, no absorption colorless
(diamond)
If Egap in between, partial absorption
material has a color.
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Wavelength vs. Band Gap
Example What is the minimum wavelength absorbed
by Ge?
Eg 0.67 eV
If donor (or acceptor) states also available this
provides other absorption frequencies
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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 2.4
eV, -- absorbs higher energy visible light
(blue, violet), -- Red/yellow/orange is
transmitted and gives it color.
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Luminescence
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Luminescence

• Luminescence emission of light by a material
• material absorbs light at one frequency emits
  at another (lower) frequency.

• How stable is the trapped state?
• If very stable (long-lived gt10-8 s)
  phosphorescence
• If less stable (lt10-8 s) fluorescence
• Example glow in the dark toys. Charge them up
  by exposing them to the light. Reemit over time.
  -- phosphorescence

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Photoluminescence

• Arc between electrodes excites mercury in lamp to
  higher energy level.
• electron falls back emitting UV light (i.e.,
  suntan lamp).
• Line inner surface with material that absorbs UV,
  emits visible
• Ca10F2P6O24 with 20 of F - replaced by Cl -
• Adjust color by doping with metal cations
• Sb3 blue
• Mn2 orange-red

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Cathodoluminescence

• Used in T.V. set
• Bombard phosphor with electrons
• Excite phosphor to high state
• Relaxed by emitting photon (visible)
• ZnS (Ag Cl-) blue
• (Zn, Cd) S (CuAl3) green
• Y2O2S 3 Eu red
• Note light emitted is random in phase
  direction
• i.e., noncoherent

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Photoconductivity
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Photoconductivity
Description
Ex Photodetector (Cadmium sulfide)
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LASER Light

• Is non-coherent light a problem? diverges
• cant keep tightly columnated
• How could we get all the light in phase?
  (coherent)
• LASERS
• Light
• Amplification by
• Stimulated
• Emission of
• Radiation
• Involves a process called population inversion of
  energy states

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LASER Light Production

• pump the lasing material to the excited state
• e.g., by flash lamp (non-coherent lamp).

Fig. 21.13, Callister 7e.

• If we let this just decay we get no coherence.

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LASER Cavity

• Tuned cavity
• Stimulated Emission
• One photon induces the emission of another
  photon, in phase with the first.
• cascades producing very intense burst of coherent
  radiation.
• i.e., Pulsed laser

Fig. 21.15, Callister 7e.
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Continuous Wave LASER

• Can also use materials such as CO2 or yttrium-
  aluminum-garret (YAG) for LASERS
• Set up standing wave in laser cavity
• tune frequency by adjusting mirror spacing.
• Uses of CW lasers
• Welding
• Drilling
• Cutting laser carved wood, eye surgery
• Surface treatment
• Scribing ceramics, etc.
• Photolithography Excimer laser

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Semiconductor LASER

• Apply strong forward bias to junction. Creates
  excited state by pumping electrons across the
  gap- creating electron-hole pairs.

electron hole ? neutral h?
ground state
excited state
photon of light
Adapted from Fig. 21.17, Callister 7e.
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Uses of Semiconductor LASERs

• 1 use compact disk player
• Color? - red
• Banks of these semiconductor lasers are used as
  flash lamps to pump other lasers
• Communications
• Fibers often turned to a specific frequency
  (typically in the blue)
• only recently was this a attainable

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Applications of Materials Science

• New materials must be developed to make new
  improved optical devices.
• Organic Light Emitting Diodes (OLEDs)
• White light semiconductor sources
• New semiconductors
• Materials scientists
• ( many others) use lasers as tools.
• Solar cells

Fig. 21.12, Callister 7e. Reproduced
by arrangement with Silicon Chip magazine.)
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Solar Cells
p-n junction
Operation -- incident photon produces
hole-elec. pair. -- typically 0.5 V
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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Optical Fibers

• prepare preform as indicated in Chapter 13
• preform drawn to 125 ?m or less capillary fibers
• plastic cladding applied 60 ?m

Fig. 21.20, Callister 7e.
Fig. 21.18, Callister 7e.
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Optical Fiber Profiles
Step-index Optical Fiber
Graded-index Optical Fiber
Fig. 21.21, Callister 7e.
Fig. 21.22, Callister 7e.
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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 lt 1.8eV) ,
no (Egap gt 3.1eV), or partial absorption
(1.8eV lt 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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Core Problem

• Visible light having a wavelength of 5x10-7 m
  appears green.Compute the frequency and energy of
  a photon of this light.

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Solution

• In order to compute the frequency of a photon of
  green light,

Now, for the energy computation, we employ
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Core Problem

• Zinc selenide has a gap of 2.58 eV. Over what
  range of wavelengths of visible light is it
  transparent?

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Solution

• This problem asks us to determine the range of
  visible light wavelengths over which ZnSe (Eg
  2.58 eV) is transparent. Only photons having
  energies of 2.58 eV or greater are absorbed by
  valence-band-to-conduction-band electron
  transitions. Thus, photons having energies less
  than 2.58 eV are not absorbed the minimum photon
  energy for visible light is 1.8 eV (Equation
  21.16b), which corresponds to a wavelength of 0.7
  µm. From Equation 21.3, the wavelength of a
  photon having an energy of 2.58 eV (i.e., the
  band-gap energy) is just

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• Thus, pure ZnSe is transparent to visible light
  having
• wavelengths between 0.48 and 0.7 µm.

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Core Problem

• The fraction of nonreflected radiation that is
  transmitted through a 5 mm thickness of a
  transparentmaterial is 0.95. If the thickness is
  increased to 12 mm, what fraction what fraction
  of light will be transmitted?

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Solution

• In this problem we are asked to calculate the
  fraction of nonreflected light transmitted
  through a 12-mm thickness of transparent
  material, given that the fraction transmitted
  through a 5-mm thickness is 0.95. From Equation
  21.18, the fraction of nonreflected light
  transmitted is just IT/I'0

Using this expression we must first determine the
value of ß this is possible by algebraic
manipulation of Equation 21.18. Dividing both
sides of the equation by I'0 and then taking
natural logarithms leads to
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• Now solving for ß and also incorporating values
  for

and x provided in the problem statement gives
when x 12 mm
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ANNOUNCEMENTS
Chapter 21 Questions 1, 14, 16 Handover your
homework next week, before the lesson begins .