1. Thermal detectors for Far-Infrared
SP-ANR meeting, 15 March 2013

2. Main technologies of IR detection
Thermocouples
Thermal detectors
Resistive bolometers
Pyroelectric detectors
Dielectric detectors
Dielectric bolometers
Monocrystal
Photoconductor
Semi-conducting
Heterostructure
Multiple quantum wells
Photonic detectors
Photodiodes
Photocapacitance

3. Principle of thermal detectors
Probe temperature change
Physical parameter variation
Incident flux variation
Absorption
Volume change: Thermometers, Golay cells
Electric resistance: Bolometers
Potential difference: Thermocouple, thermopile
Electric charge: Pyroelectric detectors
Electric permittivity: Dielectric detectors
Sensitivity
Noise
Response time
Tension or current change

4. Pyroelectric detectors
Ferroelectric material: a spontaneous polarisation exists under a certain temperature (Curie point)
Dependence on the temperature: pyroelectricity
Types
of energy
in a crystal
Thermal
Kinetic
Electric
Pyroelectricity
Piezoelectricity

5. Pyroelectric detectors
Probe temperature change
Physical parameter variation
Incident flux variation
Absorption
Crystal dilatation
 Dipole moment variation
 Charge on the surface
 Current when coming back to equilibrium state
Tension or current change

6. Pyroelectric detectors
Properties:
Works at ambiant temperature, providing TC>Tamb LiTaO3 : TC = 618°C
Sensitive to temperature variations only
Sensitivity depends on the used crystal
Surface
Pyroelectric coefficient P in C/K : dq=PdT
Current source

7. Thermopiles
Based on thermocouples: 2 different conductors in contact, voltage production when there is a temperature difference

8. Thermopiles
Sensitivity = Tension variation depending on incident flux variation
S : thermal sensitivity (V/K) depending on Seebeck effect of the thermocouple materials
Z: thermal impedance (K/W)
Independent of the spectral domain

9. Thermopiles Properties:
Sensitive to temperature difference between the two junctions
Works at ambient temperature
Sensitivity depends on the materials
Voltage source