Introduction to infrared sensors

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Introduction to infrared sensors

• B. R. Reddy
• AAMU, Physics
• Normal, AL 35762
• E-mail brreddy_at_aamu.edu

June 08, 2004
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Units of measurement

• Photon energy h? hc/? (where ? is the
  frequency)
• c 3x108m/s h 6.63x10-34J.s
• If ?500 nm
• ? c/? 3x108m/s/500x10-9 m6x1014Hz
• Wavelength (nm, µm, Å)
• 1nm 10-9 m 1 µm10-6m 1 Å10-10m
• Wavenumber(cm-1)1/ ? 1/500nm20 000cm-1
• 1eV 1.6x10-19J
• 1 eV 8066 cm-1
• 1 cm-1 30 GHz
• Number of photons power/photon energy

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Electromagnetic waves

• wave type wavelength(m) frequency(Hz)
• Gamma rays 10-11-10-17 1019-1025
• X-rays 10-9-10-12 1017-1020
• VUV 10-8-10-9 1017
• UV 10-7-10-8 1016
• Visible (0.4-0.8)10-6 1014
• IR 10-6-10-3 1011-1014
• Microwave 10-3-0.1 109-1011
• TV 0.3-8 108
• Radiowaves 10-106 107-102
• AC power 60

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Detectors
PMT
photocathode

• Photon detectors respond to individual photons
• External/Photoemissive (photomultipliers)
• Internal/Photoconductivephotovoltaic
  (semiconducors)
• Thermal detectors respond to the heat content
• Bolometers
• Golay detectors
• Calorimeters
• Thermopyle detectors
• Pyroelectric detectors
• PMT gt10 quantum efficiency
• operate at room temperature
• Semiconductors (for IR) require cooling to
  cryogenic temp.
• Phosphors (IR can stimulate visible Radiation)
• Photographic film (UV/VIS)
• Human Eye (400-700nm) max _at_555nm
• IRQC No suitable materials
• No commercial device yet

dynodes
C
V
semiconductor
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Photon and phonon

• Photon is a quantum of light
• Absorption electron goes to a higher level
• Emission electron falls down to a lower level
• Obey certain selection rules
• Phonon is a quantum of lattice vibration
• Relaxation radiative-- light emission
• Nonradiative relaxation-- no light emission
• Nonradiative gases collisional relaxation
• Solids Multiphonon relaxation

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Performance Parameters

• Spectral response wavelength interval measured
• Responsivity electrical output/power
• Noise Equivalent Power (NEP)
• Detectivity (D) varea/NEP(?, 1Hz, T) (cm
  vHz/watt)
• D indicates the wavelength at which it was
  measured, the chopping frequency and the noise
  bandwidth.
• Signal/noise ratio ( 3 or higher)
• Response time How fast does it respond
• Quantum efficiency electrons/photons lt 1
• NEP is the incident light level impinging on a
  diode which produces photocurrent equal to the
  noise level.
• Function of detector responsivity, noise (of the
  detector circuitry) and frequency bandwidth
  over which the noise is measured.

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Transmission of detector windows

• Quartz 180 nm
• Glass 360nm 3 µm
• Fluorite 125nm 9 µm
• ZnSe 550nm 16 µm
• Si 1.1 9 µm
• 22 50 µm

detector
window
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Relative response
Photons Power/photon energy P/h? P?/hc

• Unit power at all wavelengths

Photon detector
Relative output (a.u.)
Thermal detector
Wavelength
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Johnson noise (Thermal noise)

• Due to random motion of electrons in resistive
  elements (thermal agitation)
• Increases with temperature
• Occurs at all frequencies (white noise)
• Noise voltage depends on the frequency bandwidth
  of the system
• Vrms v(4kTR?f)
• Noise is eliminated at 0K

Noise power density
White noise
f
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Shot noise

• Due to random movement of discrete charges across
  a junction (pn-junction)
• Electrons are released at random times (photo
  tube)
• broadband (expressed as noise per unit bandwidth)
  
• statistical noise associated with photocurrent
  and dark current
• Irms v(ei/t) v(2ei?f)

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Other noise sources

• 1/f noise
• Source not known
• Decreses at high frequencies
• Significant at lt100Hz

• Interference noise

Noise Power density
120
180
240
60
f
Noise Power density
Note acquire data above kHz to minimize
f
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Why infrared detectors?

• Infrared near-infrared (700 nm to 2 microns)
• Mid-infrared region (2 to 5 microns)
• LW infrared (above 6 microns)
• Clouds absorb visible light
• Atmospheric gases absorb certain wavelengths
• Atmospheric windows
• Mid-infrared region (2 to 5 microns)
• Longwave infrared region ( 10 microns)
• Useful for space communications
• So a detector is needed

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IR detector applications

• Military
• Industrial process control
• Security systems
• Medical applications
• Astronomy
• Thermal imaging and pollution control
• Cover a wide range 0.8 to 100 microns)

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Photon detectors
What is the limitation of existing detectors?
PMT
semiconductor
There is a need for alternate schemes for IR
region
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Why to cool a detector?

• To minimize thermal noise

Bandgap energy ?EEgEc-Ev
Incident photon energy gtbandgap energy h? gtEg
Ec
IR detectors small bandgap noise (thermal
contribution)
Ev
Ex HgCdTe, ?E0.1eV Say there are 1000
electrons N NcNv1000 Nc/Nv
e-806.6/2040.0191.9 a large fraction
(NEP is high) So cool it to minimize noise
T77K Nc/Nve-806.6/53.53x10-73x10-5 Nc is
negligible
Nc Nve-?E/kT Ni population in the ith band (i
v or c) K Boltzmann constant1.38x10-23J/K T
absolute temperature
N1000 Nc0.019Nv NcNv0.019NvNv1000 Nv1000/
1.019981 Nc19
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Bandgaps and operating temperatures

• Material bandgap (eV) ?cutoff(µm) temp.(K)
• Si 1.12 1.1 295
• Ge 0.67 1.8 295
• CdTe 1.5 0.83 295
• PbS 0.42 2.9 295
• InSb 0.23 5.4 77
• HgCdTe 0.1 12 77
• ?cutoff(µm) is the longest wavelength that can be
  detected
• For detection h?Eg
• ?cutoff(µm) hc/Eg 6.63x10-34x3x108 J.m/Eg
• 1.24/Eg (where Eg is in eV)

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Infrared detection

• Thermal detection
• Photon detectors
• IRQC
• MIRROR (uses bimaterial cantilevers)
• Microoptomechanical infrared receiver with
  optical readout (MIRROR)-optomechanical
• Ex SiN/Au
• Au large thermal expansion coeficient1.4x10-5/K
• Thermal conductivity 296 watt/meter.Kelvin
• SiNsmall thermal expansion coefficient 8x10-7/K
• Thermal conductivity 3 watts/meter.Kelvin
• Absorbs IR (8-14 µm)

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Thermal detectors

• Bolometer temperature changes when exposed to
  radiation, causing a proportionate change in
  resistance.
• Thermopile a number of thermocouples are
  connected in series.
• A minimum of two junctions (one at a higher temp.
  the other at a lower temp.).
• a junction is made of two different materials.
• Pyroelectric detectorsuse ferroelectric crystals
  (chopper)
• Possess permanent dipole moment below curie temp.
• Heat changes lattice distance hence
  polarization changes
• Polarization change also changes the capacitance
• So current or voltage changes

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Thermal detectors

• Golay detectors Heat causes a change in
  pressure. A thin film absorbs incident radiation
• the enclosed gas is heated.
• A tube connects heated cell to another cell that
  has a flexible film. This film is distorted by a
  pressure change in the other. This film acts as a
  (light) deflecting mirror.
• Very slow
• Slow response
• Useful to detect Visible to mm wavelengths

1010
light
Response (a.u.)
D
108
Flexible film
wavelength
Pressure cell
100
1
Chop freq
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Thermal detectors

• Thermocouplejunction of two different metals
• Work function is different. A current is
  generated when heated. Requires a reference.
  Produces (µV/C)
• Thermopile several thermocouples connected in
  series
• Pyroelectric detectors electric polarization
  changes with temperature, resulting in a
  detectable current

hot
cold
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Pyroelectric detectors

• Uses temperature sensitive ferroelectric crystals
  (TGS, SBN, LiNbO3, Lithium tantalate)
• Electrodes are attached to the crystals
• Spontaneous polarization can be measured as a
  voltage
• Contant T internal charge distribution is
  neutralized by free electrons and surface
  charges. So no voltage is detected.
• If the temperature changes, the lattice distance
  polarization changes, producing transient
  voltage
• Modulate the radiation detector temperature
  alternates
• Below curie temp. individual dipoles align (net
  internal field)
• Heat (radiation) disrupts the alignment and
  charge distribution on the faces and hence the
  stored charge on the electrodes
• Measure change in the stored charge (chopper is
  used)
• Current, I pA (d ?T/dt) where p is the
  pyroelectric coefficient

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Calorimeter

• Some models are water cooled
• Difference in inlet and outlet temperature is
  used to estimate energy absorbed
• Power absorbed, P dQ/dt c ?Tdm/dt
• C specific heat capacity
• ?T change in temperature
• dm/dt rate of mass flow

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Calorimeter

• Design
• Al/Cu alloy coated with black paint (embedded
  with thermocouples)
• Absorbed radiation increases temperature (sensed
  by embedded thermocouples)
• Thermal inertia is inherent (heat conduction is
  involved)
• Its response is very slow
• Useful for the detection of very high powers
• Useful from UV to far IR

Water in
Power absorbed P dQ/dt c ?Tdm/dt dm/dt mass
flow rate ?T temperature change C heat absorbed
Water out
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Phosphors

• IR stimulates emission of visible radiation
• These phosphors have been previously excited by
  UV radiation
• Ex ZnS stimulated by IR (1 to 3 microns)
• Rare-earth doped phosphors convert near-IR to
  visible

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MIcrobolometer

• Thermal imaging 8 -12 microns or 3 -5 microns
• At 25C an object emits 50x more radiation at 8-
  12 micron band than at 3 -5 micron band
• Thermal detectors (bolometers) measure the total
  energy absorbed by a change in the temperature of
  the detector elements
• Principle electrical resistance varies with
  temperature
• Note If an absorber is thermally isolated- any
  increase in absorbed radiation produces increase
  in emitted radiation

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Microbolometer

• Individual elements are suspended by electrical
  conductors
• Measure change in resistance determine the
  temperature change and IR input
• Intensity of 1mW/cm2 increases temperature by 1K.
• Slow response device has to absorb enough heat
  to reach equilibrium before an accurate
  measurement could be made
• Solution miniaturization (response time is
  proportional to thickness of the absorber)
• 0.5 microns, response time of 10 ms

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Microbolometer (good for RT, but slow)

• Materials Si, barium strontium titanium oxide,
  vanadium oxide
• Must have large temperature coefficient of
  resistance TCR (?R/R)/?T
• Detect temp. changes of 0.07K
• Response (10mV/K)
• Polycrystalline silicon-germanium
• Performance limit heat is almost
• lost by radiation

conductors
Bolometer material
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Absorption and emission
When the incident radiation energy equals
the energy level gap then it is absorbed,
otherwise it is transmitted through the
sample. Whenever an electron jumps to a lower
level the difference in energy comes out as
light Absorption and emission have to obey
selection rules Atoms are stable in the ground
state. Excited atoms relax to the lower
levels Allowed transitions lifetimes are
ns Forbidden transitions lifetimes ms to µs
t
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Micro-optomechanical device

• Principleabsorption of IR raises the temp. The
  material is distorted. Deflection of a VIS beam
  is monitored
• SiN/Au material

Visible reflector (Au)
IR absorber (SiN)
Mat. Ther.cond. Exp.coef. Heat cap. SiN 3 0.8x10-
6 691 Au 296 14.2 129
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Atomic energy levels

• Electron orbits

• Energy levels

e
?
N
Intensity (a.u.)
frequency
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Infrared Quantum counter detection concept Nobel
Laureate Nicholas Bloembergen

• What is it?
• Infrared-optical double resonance

IR
Uv-vis
IR
VIS
Uv-vis
Uv-vis
IR
vis
vis
IR
IR
Ionic energy levels
Ionic energy levels
Ionic energy levels
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4S3/2
5S2
4F9/2
5F5
4I9/2
5I8
IRQC schemes in different systems
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IRQC scheme in Tm3 doped system
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IRQC scheme in Eu3 doped system
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IRQC scheme in Tb3 system
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Fig. (a) Ar excitation (b) Ar and
TiSapphire SampleLaF3Tb3
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Cut-off phonon LaF3 350 cm-1 LaCl3
260cm-1 LaBr3 175cm-1
Note Material selection is important
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STEP ETU
AVALANCE
ABS.
t
t
t
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IR to Visible upconversion studies

• Sequential two/three photon excitation (single
  ion process)
• Energy transfer upconversion (two/three ions)
• Avalanche absorption (two ions)

IR
VIS
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4I9/2?4I15/2
4S3/2?4I15/2
Composition La2O3(2.3) PbO(13.7) TeO2(28.7) Mg
TiO3(6.4) SiO2(25.4) B2O3(21.9) Ba3Y3WO9(1.6)
4S3/2?4I13/2
Er3 -doped glass (A.P.L.)
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?
O
O
?
Ion(1)
Ion(2)
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Why doped fiber?

• Fiber vs. crystal

Draw backs of crystal Small interaction length
(1 cm) Beam size is large 30 microns Most of
the incident light is wasted A small fraction of
the light is collected small solid angle
crystal
Light in
output
fiber
Advantages of fiber Small fiber core (2 5
microns) long interaction length at least 50
of emitted light comes out
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Energy levels of Er3 in LaF3 (J.A.P.)
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ZBLAN fiber ZrF4(53 ) BaF2(20
) LaF3(3.9) AlF3(3) NaF(20)
violet
blue
green
Er3 in fluoride fiber (OL)
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Dichroic mirror for coupling and launching light
into fiber

• Light coupling

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Fiber pumping schemes
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IRQC studies in Eu3 doped materials (JOSA-B)
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