Pyroelectric detector

A pyroelectric detector is a thin slice of a pyroelectric crystal or ceramic, such as lithium tantalate (LiTaO₃), deuterated triglycine sulfate (DTGS) or a lead zirconate titanate ceramic, with electrodes on both faces and a black absorber on the front. These materials have a built-in electric polarization that depends on temperature. When absorbed light warms the element, the polarization changes and charge flows in the external circuit, so the output current is proportional to the rate of temperature change:

$$i = p\,A\,\frac{dT}{dt},$$

where $p$ is the pyroelectric coefficient and $A$ the electrode area. For LiTaO₃, $p \approx 1.8 \times 10^{-4}$ C/(m²·K). A constant illumination produces a constant temperature and no current, so a pyroelectric detector cannot measure steady light without a chopper. In exchange it is broadband like other thermal detectors such as the bolometer, needs no cooling, and can be much faster than a thermopile, since its signal does not wait for the element to reach thermal equilibrium.

Responsivity and frequency response

For light modulated faster than the element's thermal time constant, the temperature rate is set by the absorbed power and the heat capacity of the element, $dT/dt = \eta P/(c' A d)$, where $c'$ is the volumetric heat capacity and $d$ the thickness. The current responsivity is then

$$R_i = \frac{\eta\,p}{c'\,d},$$

independent of area. For LiTaO₃, $c' \approx 3.2 \times 10^6$ J/(m³·K), so a 25 µm element with unit absorptance gives $R_i \approx 2.3$ µA/W. Thin elements give more signal, which is why detector chips are polished to tens of micrometres.

In voltage mode, with a high-value load resistor and a JFET or op-amp buffer, the response rises with frequency up to the lower of two corner frequencies, set by the thermal time constant (typically 0.1–1 s) and the electrical time constant $R_L C_e$, is flat between them, and falls as $1/f$ above the higher one. The usable band is therefore set by the load resistance: large loads give high voltage responsivity at a few hertz, small loads or a transimpedance amplifier in current mode give lower sensitivity with bandwidth up to the kilohertz range and beyond. Room-temperature specific detectivity is typically of order 10⁸–10⁹ Jones, with DTGS at the upper end, below the thermal-detector background limit of about 1.8 × 10¹⁰.

Where it is used

Pyroelectric energy meters measure single laser pulses from nanojoules to joules: the element integrates the pulse energy as heat, and the peak voltage is proportional to it. They work at repetition rates up to roughly the kilohertz range, depending on the head, and are the counterpart of the thermopile head for pulsed sources on an optical power meter. DTGS detectors are the standard room-temperature detector in Fourier-transform infrared spectrometers, where the moving mirror modulates the signal. Passive infrared motion sensors use a dual-element pyroelectric detector behind a segmented Fresnel lens, with an 8–14 µm window matching the 9.3 µm emission peak of a 310 K human body; the two elements are wired in opposition so uniform ambient changes cancel. Pyroelectric arrays are also used for beam profiling in the mid-infrared and terahertz, where silicon cameras do not respond.

Measurement practice

For CW sources the beam is chopped, and the signal recovered with a lock-in amplifier at the chopping frequency; calibration then applies only at that frequency and duty cycle. For pulse energy, the pulse must be much shorter than the element's electrical time constant, and the reading depends on the beam staying within the calibrated area.

Pitfalls

All pyroelectric materials are also piezoelectric, so vibration, acoustic noise and mechanical shock produce spurious signals, called microphonics; mounting and shielding matter. Each material loses its polarization above its Curie temperature, low for triglycine sulfate (about 49 °C) and much higher for LiTaO₃, so TGS-family detectors must be kept cool and protected from intense beams. Large ambient temperature swings produce drift and, in high-impedance circuits, saturation of the preamplifier. An unchopped CW beam gives no reading, so a beam intense enough to damage the absorber can go unnoticed.

Common questions

Why can a pyroelectric detector not measure CW light?

Its current is proportional to $dT/dt$, which is zero once the element reaches a steady temperature. Chopping converts steady power into a periodic temperature change.

Pyroelectric or thermopile for laser measurements?

Pyroelectric heads measure pulse energy and chopped signals at higher speed; thermopiles measure average power of CW and high-repetition-rate lasers.

How does a PIR motion sensor work?

A warm body moving across the zones of the Fresnel lens changes the infrared power on the two elements in turn, producing a signal of alternating sign that the circuit recognizes as motion.

References: E. L. Dereniak, G. D. Boreman, Infrared Detectors and Systems (Wiley, 1996); A. Rogalski, Infrared and Terahertz Detectors, 3rd ed. (CRC Press, 2019); S. B. Lang, "Pyroelectricity: from ancient curiosity to modern imaging tool," Physics Today 58(8), 31 (2005).