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

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

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

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

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

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

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

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

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