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Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 20, Chapter 21 - 1
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
activator level
Valence band
Conduction band
trapped states Eg
Eemission
• If residence time in trapped state is relatively long (> 10-8 s) -- phosphorescence
• For short residence times (< 10-8 s) -- fluorescence
Example: glow-in-the-dark watch
Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 20, Chapter 21 - 2
Photoluminescence
• 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, Ca10F2P6O24 with 20% of F - replaced by Cl
-
• Adjust color by doping with metal cations
Sb3+ blue
Mn2+ orange-red
Hg atom
UV light
electrode electrode
Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 20, Chapter 21 - 3
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-) blue
(Zn, Cd) S + (Cu++Al3+) green
Y2O2S + 3% Eu red
• Note: light emitted is random in phase & direction
– i.e., is noncoherent (out of phase)
Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 20, Chapter 21 - 4
Electroluminescence
• When a forward-biased potential of relatively high magnitude is
applied across a p-n junction diode, visible light can be emitted.
• The conversion of electrical energy into light energy is termed
electroluminescence, and the device is called a light-emitting
diode (LED).
• The wavelength of the light is related to the band gap of the
semiconductor.
Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 20, Chapter 21 - 5
Organic Light Emitting Diodes (OLEDs)
• 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)
Flexible OLED display (Samsung)
Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 20, Chapter 21 - 6
Photoconductivity
• The conductivity of semiconductors depends on the numbers of
free electrons and holes in the conduction band and valence
band, respectively.
• When light is radiated, the number of charge carriers is
increased.
• The attendant increase in conductivity by light is called
photoconductivity.
Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 20, Chapter 21 - 7
Solar Cells
• Incident photon of light produces
electron-hole pair.
• Typical potential of 0.5 V produced
across junction
• Current increases with light intensity.
• Ex) polycrystalline
silicon solar cell
• Operation of solar cell is the reverse of that for the LED.
Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 20, Chapter 21 - 8
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
Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 20, Chapter 21 - 9
Population Inversion (e.g. Ruby Laser)
• More electrons in excited energy states than in ground states
• In the metastable state, electrons may reside for up to 3 msec before spontaneous emission.
Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 20, Chapter 21 - 10
Operation of the Ruby Laser
• Xe flash lamp (incoherent light) “pumps” electrons in the
lasing material to excited states.
• Both ends of ruby rod are silvered such that one is totally
reflecting and the other partially transmitting.
Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 20, Chapter 21 - 11
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
a high intensity, coherent, and
highly collimated laser beam.
– The monochromatic red beam
has a wavelength of 694.3 nm.
• This is an example of a
pulsed laser
Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 20, Chapter 21 - 12
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., CO2), and yttrium-aluminum-garnet (YAG)
• Wavelengths for laser beams are within visible and infrared
regions of the spectrum
• Uses 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
Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 20, Chapter 21 - 13
• 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 + h
recombination ground state
photon of
light
Semiconductor Laser: Laser Diode (LD)
gE
hc
Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 20, Chapter 21 - 14
Semiconductor Laser
Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 20, Chapter 21 - 15
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
Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 20, Chapter 21 - 16
Optical-Fiber Communication
1 in the binary
format 0 in the binary
format
Encoding for
optical
communication
Semiconductor
Laser (IR)
Amplifying signal for long distance
(also called waveguide)
Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 20, Chapter 21 - 17
Total Internal Reflectance
n1
n2
sin 2
sin1
n2
n1
n2 < n1
1
c
2
• Fiber optic cables are clad in low n material so that light will
experience total internal reflectance and not escape from the optical
fiber.
1 = incident angle
2 = refracted angle
c = critical angle
c exists when 2 = 90°
For 1 > c light is internally
reflected
Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 20, Chapter 21 - 18
Example: Diamond in air • What is the critical angle c for light passing from diamond
(n1 = 2.41) into air (n2 = 1)?
n1
n2
sin 2
sin1
Rearranging the equation
• Solution: At the critical angle,
1 c
2 90and
sin1 sinc n2
n1
sin(90) n2
n1
Substitution gives
sinc 1
2.41c 24.5
Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 20, Chapter 21 - 19
Optical Fibers (cont.)
• High purity silica glass is used as the fiber material.
• Plastic coating is applied to fibers.
Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 20, Chapter 21 - 20
Optical Fiber Designs
Step-index Optical Fiber
Graded-index Optical Fiber
Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 20, Chapter 21 - 21
• 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
SUMMARY
Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 20, Chapter 21 - 22
Problem set (due to June 13):
21.8; 21.11; 21.14; 21.18
ANNOUNCEMENTS
Reading: pp. 452 ~ 463