Download Torsional Aluminum electrostatic light modulators

NTHU ESS5841
F. G. Tseng
The Principles and Applications of Micro Transducers
Spring/2001, 6-2, p1
Lecture 6-2 Optical Transducers II—
Indirect Optical sensors, Optical actuators
3. Indirect Optical Sensors
a. Pyroelectric Detectors
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Non-centrosysmmetric
structure
exhibits
spontaneous electrical polarization, and can be
charged by expansion through illumination or
temperature change. No DC response.
Using optical chopper to produce an alternating
temperature change T, giving an alternating charge
on the electrodes, and cancel the temperature offset:
Q  pqAT
b. Bolometers

Two thermally sensitive resistors with one of them
shielded from incident radiation to serve as a
reference.
NTHU ESS5841
F. G. Tseng
The Principles and Applications of Micro Transducers
Spring/2001, 6-2, p2
c. Thermopiles
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Thermoelectric effect
NTHU ESS5841
F. G. Tseng
The Principles and Applications of Micro Transducers
Spring/2001, 6-2, p3
d. Golay Cells
Incident radiation heats a volume of trapped gas:
PV  nRT
NTHU ESS5841
F. G. Tseng
The Principles and Applications of Micro Transducers
Spring/2001, 6-2, p4
NTHU ESS5841
F. G. Tseng
The Principles and Applications of Micro Transducers
Spring/2001, 6-2, p5
 Optical actuators
1. Light emitting diodes
a. Light emitting diodes
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Principle: an electron makes a transition from the
conduction band to the valence band, and the energy
transfer into photon.
The emitted is incoherent (no temporal or spatial
relationship)
Direct band materials (GaAs, GaP), has quantum
efficency~1, while indirect band materials (Si, Ge,
SiC) has quantum efficiency <<1.
Emissions only occur from injected electrons on the
P+side.
Common red LED made from GaP, green LEDs
using nitrogen doped GaP, blue LEDs use SiC (very
week), or GaN (bright).
NTHU ESS5841
F. G. Tseng
The Principles and Applications of Micro Transducers
Spring/2001, 6-2, p6
b. Silicon light emitting diodes

Very low quantum efficiency, ~10-8-10-4. doping
with C, Er, or Y can increase to 10-3. Not practical
for commercial applications.
c. Organic light emitting diodes (OLED)
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Quantum efficience~5%~25%
Organic electroluminescent (EL): Hole transport
layer (Aromatic amine) in contact with electron
transport layer (host material doped a small
concentration of fluorescent molecules).
Anode
ITO(hole injection), electron injection electrode:
metal alloy.
Thin, flexible, and low power. Limitation: short life
time (~6000 hours)
NTHU ESS5841
F. G. Tseng
The Principles and Applications of Micro Transducers
Spring/2001, 6-2, p7
d. Gas and solid state lasers
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First was proposed by Einstein around 1916, but first
practiced by Theodore Maiman at Hughes Malibu
Research Lab. In 1`960.
Principle: atoms in high-energy states were hit by
photons at the wavelength of high-low energy
electron transition, the electrons would be stimulated
to make the transition to form more photons in that
wavelength.
The two reflective mirrors placed at the opposite
ends of the active luminescent region forms a
Fabry-Perot
resonator
(spacing
n/2)
for
constructive interference.
Different lasers: Ruby laser (Al2O3 and 0.03%Cr2O3,
694 nm, red), Neodymium laser (Nd-Yag, 1.06 nm, IR,
530 nm green), He-Ne laser (633 nm, red), Ion laser
(Ar-ion, 488 nm and 514.5 nm, and Kr ion), CO2
laser (IR at 10.6 m, most efficient, ~>1kW), Excimer
NTHU ESS5841
F. G. Tseng
The Principles and Applications of Micro Transducers
Spring/2001, 6-2, p8
laser (KrCl, 222nm, KrF: 248 nm, XeCl: 308 nm,
XeF: 351 nm), semiconductor laser (III-V compounds,
1-10 W)
e. Micromachined incandescent lamps
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Board output spectra covering long wavelength IR
through visible ranges.
from hot body radiation, peak emission can be
controlled by temperature.
f. Plasma light sources
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Ionization of gases at low pressure (noble gases, neon,
argon, etc…).
Phosphors are used to change the emitted wavelength
(from mercury plasma UV to visible light)
Need high voltage (> 60V), and electrode can be
sputtered away in 10 seconds for 500 nm.
g. Electroluminescent light sources
NTHU ESS5841
F. G. Tseng



The Principles and Applications of Micro Transducers
Spring/2001, 6-2, p9
Application of AC electric fields to phosphors
between conductive electrodes.
Dielectric layer cover phosphors both side for
reducing DC currents.
High voltage: 100-400 V p-p.
h. Field emission displays
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Utilize arrays of ultra-sharp field emitter tips as
cathods to provide electron flux to illuminate
phosphors (like television tube).
Potential replacement to liquid crystal displays
because no back light necessary, more energy
conservative.
Fields at 107V/cm, micromachined devices can
reduce to 20-60 V with a gap of 200 m.
To improve quality, individual pixels can be driven
by 2000 tips.
2 billion field emitter tips with 1 m minimum
features are desired in a single display.
i. Bioluminescence
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Chemiluminescence or bioluminescence, no heat and
called “cold light”, involves only oxidation/reduction
NTHU ESS5841
F. G. Tseng

The Principles and Applications of Micro Transducers
Spring/2001, 6-2, p10
reaction.
In many fireflies, with the chemiluminescent
compounds luciferins, activated by enzyme
luciferase.
2. Light Modulators
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Mechanical (dominating) or non-mechanical means.
b. Liquid crystal displays
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Simple, low power, inexpensive. To gate the passage
of light. Been known since 1800, but applied until
mid-1960 by RCA.
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Nematic liquid crystals: rod shaped molecules (2*0.5
nm in diameter), maintain a degree of parallel
alignment despite the disruption of thermal energy.
Fast enough for displays, but not enough for optical
signal processing
NTHU ESS5841
F. G. Tseng

The Principles and Applications of Micro Transducers
Spring/2001, 6-2, p11
Cholesteri liquid crystals: sensitive to temperature,
color change with temperature.
c. Reflective micromechanical light modulators
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Important issues: speed, sticking, fatigue, and
long-term reliability.
Advantages using MEMS: fast due to scale down, low
power, light modulator need to move only itself.
Most are electrostatic actuation, some are magnetic
and piezoelectric.
NTHU ESS5841
F. G. Tseng

The Principles and Applications of Micro Transducers
Spring/2001, 6-2, p12
Electrostatic reflective light modulators:
Westinghouse mirror matrix tube(1970’) :
Deflect up to 4, contrast to 10:1. two major issues:
epitaxial silicon port size control, not good enough
electron-beam focusing.
Silicon Cantilever light modulators:
Peterson (1977):
Electrostatic driven.
NTHU ESS5841
F. G. Tseng
The Principles and Applications of Micro Transducers
Spring/2001, 6-2, p13
Torsional silicon electrostatic light modulators
Peterson (1980, 1982):
Revealed fatigue mechanisms are not the same as
those found in macroscopic materials due to the
absence of grain boundaries. Demonstrated 1012
cycles.
NTHU ESS5841
F. G. Tseng
The Principles and Applications of Micro Transducers
Spring/2001, 6-2, p14
Torsional Aluminum electrostatic light modulators
TI DMDth (digital micromirror device):
No resonance frequency shifts for 40 billion cycles.
(lack of a significant grain structure which increase
lifetime)
NTHU ESS5841
F. G. Tseng
The Principles and Applications of Micro Transducers
Spring/2001, 6-2, p15
Deformable grating light modulators (DGM):
Solgaard et al. (1992), now silicon light machines,
Inc., San Jose, CA.
NTHU ESS5841
F. G. Tseng
The Principles and Applications of Micro Transducers
Spring/2001, 6-2, p16
Electrostatic membrane light modulator
NTHU ESS5841
F. G. Tseng
The Principles and Applications of Micro Transducers
Spring/2001, 6-2, p17
Magnetically deflected light modulators:
Caltech Tai’s group.
NTHU ESS5841
F. G. Tseng
The Principles and Applications of Micro Transducers
Spring/2001, 6-2, p18
Magnetic/electrostatic light modulators: