Introduction to infrared sensors

B. R. Reddy AAMU, Physics Normal, AL 35762 E-mail: [email protected]

June 08, 2004

Units of measurement

• Photon energy= h ν = hc/ λ (where ν is the frequency) • c= 3x10 8 m/s; h =6.63x10

-34 J.s

• If λ=500 nm • ν =c/λ =3x10 8 m/s/500x10 -9 m=6x10 14 Hz • Wavelength (nm, μm, Å) • 1nm = 10 -9 m; 1 μm=10 -6 m; 1 Å=10 -10 m • Wavenumber(cm -1 )=1/ λ =1/500nm=20 000cm -1 • 1eV = 1.6x10

-19 J • 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 • X-rays • VUV • UV • Visible • IR • Microwave • TV • Radiowaves • AC power 10 -11 -10 -17 10 -9 -10 -12 10 -8 -10 -9 10 -7 -10 -8 (0.4-0.8)10 -6 10 -6 -10 -3 10 -3 -0.1

0.3-8 10-10 6 10 19 -10 25 10 17 -10 20 ~10 17 ~10 16 ~10 14 10 11 -10 14 10 9 -10 11 ~10 8 10 7 -10 2 60

Detectors

• Photon detectors: respond to individual photons – External/Photoemissive (photomultipliers) – Internal/Photoconductive&photovoltaic (semiconducors) PMT photocathode • Thermal detectors: respond to the heat content • Bolometers • Golay detectors • Calorimeters • Thermopyle detectors • Pyroelectric detectors • PMT : >10% quantum efficiency – operate at room temperature V semiconductor C • Semiconductors (for IR): require cooling to cryogenic temp.

dynodes • Phosphors (IR can stimulate visible Radiation) • Photographic film (UV/VIS) • Human Eye: (400-700nm) max @555nm • IRQC – No suitable materials – No commercial device yet

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/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 window detector

Relative response

• Unit power at all wavelengths Photon detector #Photons = Power/photon energy = P/h ν = Pλ/hc Thermal detector Wavelength

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 • V rms = √(4kTRΔf) • Noise is eliminated at 0K Noise power density White noise f

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 – I rms = √(ei/t) = √(2eiΔf)

Other noise sources

• 1/f noise – Source not known – Decreses at high frequencies – Significant at <100Hz • Interference noise Noise Power density 60 120 180 240 f Noise Power density Note: acquire data above kHz to minimize f

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

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)

What is the limitation of existing detectors?

Photon detectors PMT There is a need for alternate schemes for IR region semiconductor

Why to cool a detector?

• To minimize thermal noise E c Bandgap energy ΔE=E g =E c -E v Incident photon energy >bandgap energy h ν >E g IR detectors: small bandgap noise (thermal contribution) E v N N i c = N v e ΔE/kT population in the i th band (i= v or c) K Boltzmann constant=1.38x10

-23 J/K T absolute temperature N=1000 & N c =0.019N

v N c +N v =0.019N

v +N v =1000 N v =1000/1.019=981 N c =19 Ex: HgCdTe, ΔE=0.1eV

Say there are 1000 electrons N= N c +N v =1000 N c /N v = e -806.6/204 =0.019=1.9% a large fraction (NEP is high) So cool it to minimize noise T=77K N c /N v =e -806.6/53.5

=3x10 -7= 3x10 -5 % N c is negligible

Bandgaps and operating temperatures

• Material • Si • Ge • CdTe • PbS • InSb • HgCdTe bandgap (eV) 1.12

0.67

1.5

0.42

0.23

0.1

λ cutoff ( μm) 1.1

1.8

0.83

2.9

5.4

12 temp.(K) 295 295 295 295 77 77 • • λ cutoff ( μm) is the longest wavelength that can be detected • For detection: hν≥E g • λ cutoff ( μm)= hc/E g = 6.63x10

-34 x3x10 8 J.m/E g =1.24/E g (where E g 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 10 10 light D* 10 8 1 Chop freq 100 wavelength Pressure cell Flexible film

Thermal detectors

• Thermocouple:junction of two different metals hot cold • 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

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 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/dt C = 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 Power absorbed P = dQ/dt = c ΔTdm/dt dm/dt mass flow rate ΔT temperature change C heat absorbed Water out

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/cm 2 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) conductors • Polycrystalline silicon-germanium • Performance limit: heat is almost Bolometer material lost by radiation

Absorption and emission

τ 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

Micro-optomechanical device

• Principle:absorption of IR raises the temp. The material is distorted. Deflection of a VIS beam Visible reflector is monitored (Au) • SiN/Au material IR absorber (SiN) Mat.

SiN Au Ther.cond.

3 296 Exp.coef.

0.8x10-6 14.2

Heat cap.

691 129

Atomic energy levels

• Electron orbits • Energy levels e  N frequency

Infrared Quantum counter detection

concept: Nobel Laureate Nicholas Bloembergen • What is it? – Infrared-optical double resonance IR VIS IR Ionic energy levels vis Ionic energy levels IR IR IR vis Ionic energy levels

3PJ 1D2 4F9/2 4 4 S 3/2 F 9/2 5 S 2 5 F 5 3H4 Pr 3+ levels 6H13/2 Dy 3+ levels 6H15/2 4 I 9/2 Er 3+ levels Ho 3+ levels 5 I 8 IRQC schemes in different systems

3P0(1I6) IR 1G4 1G4 Near-IR VIS VIS 3F4 IR 3H6 (a) (b) IRQC scheme in Tm 3+ doped system 3H6

35 30 20 10 0

IRQC scheme in Eu 3+ doped system

7 F 6 5 4 0 5 D 3 5 D 2 5 D 1 5 D 0 5 I 5 5 F 5 5 H 5 5 H 4 5 H 7 5 H 3 5 L 10

42 36 30 24 488 -477 18 12 6 490 -680 384 -437 0 IRQC scheme in Tb 3+ system 5 K 5 5 K 8 5 H 5 5 H 6 5 D 3 5 D 4 0 1 5 7 F 6

Fig. (a) Ar + excitation (b) Ar + Sample:LaF 3 :Tb 3+ and Ti:Sapphire

Note: Material selection is important Cut-off phonon LaF 3 : ~350 cm -1 LaCl 3 : ~260cm -1 LaBr 3 : ~175cm -1

STEP ETU AVALANCE t t t

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

4 S 3/2 4 I  9/2 4 I  13/2 4 I 15/2 Er 3+ -doped glass (A.P.L.) 4 S 3/2  4 I 15/2 Composition La 2 O 3 (2.3%) PbO(13.7%) TeO 2 (28.7%) MgTiO 3 (6.4%) SiO 2 (25.4%) B 2 O 3 (21.9%) Ba 3 Y 3 WO 9 (1.6%)

 O Ion(1) O  Ion(2)

Why doped fiber?

Fiber vs. crystal 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 Light in fiber output Advantages of fiber: Small fiber core (~2 – 5 microns) long interaction length at least 50% of emitted light comes out

Energy levels of Er 3+ in LaF 3 (J.A.P.)

green blue violet ZBLAN fiber ZrF 4 (53% ) BaF 2 (20% ) LaF 3 (3.9%) AlF 3 (3%) NaF(20%) Er 3+ in fluoride fiber (OL)

Dichroic mirror for coupling and launching light into fiber • Light coupling IR DM L Doped fiber bundle F UV PMT VIS Fiber end configuration IR infrared radiation VIS visible radiation UV ultraviolet DM Dichroic mirror L Matching lens F Filter PMT photomultiplier tube A typical fiber setup for IRQC measurements

Yellow F Blue DM Doped fiber IR (a) Bidirectional pumping scheme VIS Coupling lens IR Doped fiber Fused fiber coupler (b) Unidirectional pumping scheme Filter UV Fiber pumping schemes

729 791 490 414 IRQC studies in Eu3+ doped materials (JOSA-B) 7 F 6 4 3 2 0 5 I 5 5 H 4 3 5 D 3 5 D 2 5 D 1 5 D 0