Introduction to infrared detection and temperature sensing

B. Rami ReddyAAMU, Physics

Normal, AL 35762E-mail: rami.bommareddi@aamu.edu

NSF RISE Workshop/Short Course

July 12, 2007

Units of measurement

• Photon energy= hf = hc/λ (where f is the frequency)

• c= 3x108m/s; h =6.63x10-34J.s• If λ=500 nm • f =c/λ =3x108m/s/500x10-9 m=6x1014Hz• Wavelength (nm, μm, Å)• 1nm = 10-9 m; 1 μm=10-6m; 1 Å=10-10m• Wavenumber(cm-1)=1/ λ =1/500nm=20 000cm-1

• 1eV = 1.6x10-19J• 1 eV= 8066 cm-1

• 1 cm-1 = 30 GHz • Number of photons = power/photon energy

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 (0.4-0.7)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

Detectors• Photon detectors: respond to individual photons

– External/Photoemissive (photomultipliers)– Internal/Photoconductive&photovoltaic (semiconducors)

• Thermal detectors: respond to the heat content• Bolometers• Golay detectors• Calorimeters• Thermopyle detectors• Pyroelectric detectors

• PMT : >10% 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 @555nm• IRQC – No suitable materials

– No commercial device yet

photocathode

dynodes

PMT

V

C

semiconductor

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

●

●

Performance Parameters

• Spectral response = wavelength interval measured• Responsivity = electrical output/power• Noise Equivalent Power (NEP)• Detectivity (D*)= √area.Δf/NEP(λ, 1Hz, T) (cm √Hz/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 < 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.

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

detectorwindow

Relative response

• Unit power at all wavelengths

Wavelength

Rel

ativ

e ou

tput

(a.

u.)

Thermal detector

Photon detector

#Photons = Power/photon energy= P/hν = Pλ/hc

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 = √(4kTRΔf)

• Noise is eliminated at 0Kf

White noise

Noise power density

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 = √(ei/t) = √(2eiΔf)

Other noise sources

• 1/f noise– Source not known– Decreses at high

frequencies– Significant at <100Hz

• Interference noise

f

Noise Powerdensity

60 120 180 240

f

NoisePowerdensity

Note: acquire data above kHz to minimize

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 RT IR detector is needed

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)

Photon detectors

PMT

semiconductor

There is a need for alternate schemes for IR region

What are the limitations of existing detectors?

Note: Noise due to thermalization is large at RT

Why to cool a detector?

• To minimize thermal noise

Ev

Ec

Incident photon energy >bandgap energy hν >Eg

Bandgap energy ΔE=Eg=Ec-Ev

IR detectors: small bandgap noise (thermal contribution)

Nc = Nve-ΔE/kT

Ni population in the ith band (i= v or c)K Boltzmann constant=1.38x10-23J/KT absolute temperature

Ex: HgCdTe, ΔE=0.1eVSay there are 1000 electronsN= Nc+Nv=1000Nc/Nv = e-806.6/204=0.019=1.9% a large fraction (NEP is high)So cool it to minimize noise T=77KNc/Nv=e-806.6/53.5=3x10-7=3x10-5%Nc is negligible

N=1000 & Nc=0.019Nv

Nc+Nv=0.019Nv+Nv=1000Nv=1000/1.019=981Nc=19

Bandgaps and operating temperatures• Materialbandgap (eV) λcutoff(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

• λcutoff(μm) is the longest wavelength that can be detected

• For detection: hν≥Eg

• λcutoff(μm)= hc/Eg = 6.63x10-34x3x108 J.m/Eg

• =1.24/Eg (where Eg is in eV)

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 coeficient~1.4x10-5/K

• Thermal conductivity ~296 watt/meter.Kelvin

• SiN:small thermal expansion coefficient ~8x10-7/K

– Thermal conductivity ~3 watts/meter.Kelvin– Absorbs IR (8-14 μm)

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 detectors:use 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

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

Res

pons

e (a

.u.)

wavelength Pressure cellFlexible film

100Chop freq

D*

108

1010

1

light

Thermal detectors

• Thermocouple:junction 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

coldhot

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• Constant 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 ΔT/dt) where p is the pyroelectric coefficient

Calorimeter

• Some models are water cooled

• Difference in inlet and outlet temperature is used to estimate energy absorbed

• Power absorbed, P = dQ/dt = c ΔTdm/dtC = specific heat capacity

ΔT = change in temperature

dm/dt = rate of mass flow

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

Water out

Power absorbed P = dQ/dt = c ΔTdm/dtdm/dt mass flow rateΔT temperature changeC heat absorbed

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

MIcrobolometer

• Thermal imaging: 8 -12 microns or 3 -5 microns• At 25°C 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

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

Microbolometer (good for RT, but slow)• Materials: Si, barium strontium titanium oxide,

vanadium oxide

• Must have large temperature coefficient of resistance; TCR = (ΔR/R)/ΔT

• Detect temp. changes of 0.07K

• Response (10mV/K)

• Polycrystalline silicon-germanium

• Performance limit: heat is almost

lost by radiation

conductors

Bolometer material

Micro-optomechanical device

• Principle:absorption 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 691Au 296 14.2 129

IRQC

Materials

Asorption and emission

• absorption • emission

●

◦

● ●

hf

hf

◦

●

hf

Emitted wavelength depends on the energy gap

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

IRQC

Energy levels of Er3+ in LaF3 (J.A.P.)

Er3+ in fluoride fiber (OL)

ZBLAN fiber

ZrF4(53% )

BaF2(20% )

LaF3(3.9%)

AlF3(3%)

NaF(20%)

green

violet

blue

Infrared Quantum counter detection concept: Nobel Laureate Nicholas Bloembergen

• What is it? – Infrared-optical double resonance

IR

VIS

Uv-

vis

vis

IR

Uv-

vis

IR

IR

IR

vis

Uv-

vis

Ionic energy levels Ionic energy levels Ionic energy levels

3H4

1D2

3PJ

6H15/2

6H13/2

4F9/2

Pr3+ levels Dy3+ levels Er3+ levels Ho3+ levels

IRQC schemes in different systems

4I9/2

4F9/2

4S3/2

5F5

5S2

5I8

(a)

1G4

3H6

3P0(1I6)

(b)

3H6

3F4

1G4

VIS

IR

IR

Near-IR

VIS

IRQC scheme in Tm3+ doped system

uv

12345

7F6

5D0

5D1

5D2

5D3

5L10

5H3

5H7

5H4

5F5

0

703.6

856

5I5

407 / 418 / 429.9 / 445

0

10

20

30

35

ENERGY (

×10

3cm

-1)

5H5

591

-807

578

-589nm

578

-589

IRQC scheme in Eu3+ doped system

5I5

5H43

5D3

5D2

5D1

5D0

7F6

43

02

583nm597 709 490 414

791729583

IRQC studies in Eu3+ doped materials(JOSA-B)

7F6

5

0

5D4

5D3

5H6

5H5

5K5

5K8

488

-476.5

nm

488

-477

750

-804

490

-680 384

-437

1

Energy (10

3

cm-1

)

0

6

12

36

42

30

18

24

IRQC scheme in Tb3+ system

Fig. (a) Ar+ excitation (b) Ar+ and Ti:SapphireSample:LaF3:Tb3+

Why doped fiber?

Fiber vs. crystalDraw backs of crystal:

Small interaction length (~1 cm)Beam size is large 30 micronsMost of the incident light is wastedA small fraction of the light is collected small solid angle

Advantages of fiber:Small fiber core (~2 – 5 microns)long interaction lengthat least 50% of emitted light comes out

fiber

crystal

Light in output

Dichroic mirror for coupling/launching light into fiber

• Light coupling

DM

IR

VIS

FDoped fiber bundle

LUV

A typical fiber setup for IRQC measurements

PMT

IR infrared radiationVIS visible radiationUV ultravioletDM Dichroic mirrorL Matching lensF FilterPMT photomultiplier tubeFiber end

configuration

Blue

Yellow IRDM

VIS

IR Fused fiber couplerDoped fiber

UV

(a) Bidirectional pumping scheme

(b) Unidirectional pumping scheme

Doped fiber

FilterCoupling lens

F

Fiber pumping schemes

Temperature measurement

• Relaxation– Radiative – Non-radiative (exhibits temperature dependence)

• Multi-phonon relaxation• Ion-ion coupling/energy transfer interaction

• Non-radiative relaxation– Energy-gap between the excited level – Cut-off phonon frequency

• Reduced mass of the sample• Force constant• Ex: LaBr3 (175 cm-1), LaCl3 (260 cm-1), LaF3 (350 cm-1)

Why Temperature Dependence?

• Phonon-assisted radiative transitions• Single and multiphonon nonradiative decay• Thermal population of nearby levels which have

different decay rates• Increased or decreased radiation trapping caused by

changes in the absorption or emission line shapes• Similarly ion-ion coupling (energy transfer rate) is

also affected.

W(T) = W(0) [1+m]n

Where m is the phonon density

n is the number of phonons involved (n = energy gap/phonon energy)

W(0) = c exp(αΔE)

Relaxation pathways

[ ]Nnrnr mT ><+= 1)( ωω

Enr ce Δ−= αω

1

1/ −

>=< kTem ωh

nrijriji ωωτ Σ+Σ=

−)(

1

BLOCK DIAGRAM OF THE EXPERIMENTAL SETUP

SPEC. COMPUTER

SR430

DFIBER

LASER

CM Concave mirrorSA SampleSPEC SpectrometerD DetectorSR430 Multichannel scaler

CM

SA

Fluorescence spectrum of Sm-doped CaF2 laser: 458nm

0

5000

10000

15000

20000

25000

30000

500 600 700 800 900

4G5/2

6H5/2

4G5/2

6H7/2

4G5/2

6H9/2

4f55d 4f 6

Sm3+

Sm2+

Wavelength (nm)

Intensity (

a.u.)

0

5000

10000

15000

20000

25000

640 690 740 790 840 890

RT

100C

150C

200C

250C

300C

350C

400C

Temperature dependence of 726 nm emission

Wavelength (nm)

Inte

nsi

ty (

a.u.

)

726nmCaF2: Sm

Temporal evolution of 424 nm fluorescence

Lifetime versus temperature

Sm: CaF2

Peak :424nmSlope:6.1

Temperature (oC)

Lif

etim

e (μ

s)

0

500

1000

1500

2000

2500

680 700 720 740 760 780 800 820

700 broad peak

Temperature dependence of emission observed under 488 nm excitation

Wavelength (nm)

Inte

nsi

ty (

a.u.

)

567nm

599nm607nm

649nm

726nm

1.5

2

2.5

3

3.5

4

4.5

0 100 200 300 400 500 600

599nm

607nm

Sm3+ level lifetime: temperature dependence

Laser:488nm

Temperature (C)

Lif

etim

e(m

s)

100 200 300 400 500 600

450

500

550

600

650

700

750

Temperature dependence of lifetime

Temperature (0C)

Life

time

(μs)

LaF3: Er3+

fl= 547 nm