4.6 Pyroelectricity: charge from heat

Lithium niobate carries a built-in electric polarization along its c axis (spontaneous polarization), and that polarization shrinks as the crystal warms. Any temperature change therefore leaves uncompensated charge on the two faces perpendicular to c (pyroelectric effect). A wafer taken off a hotplate can build kilovolts across its thickness until the charge finds a path: a spark, a cracked film, a fried electrode, or dust pulled onto the surface. In thin-film devices the same charge shifts the refractive index through the Pockels effect.

− − − − − − − − − − − − − − − − − − − − − − −               
+----------------------------------------------------------+
|                                                          |
|                      +c ↑  E ↑  z-cut                |
|                                                          |
+----------------------------------------------------------+
+ + + + + + + + + + + + + + + + + + + + + + +               

Unscreened z-cut plate, uniform temperature change ΔT, no free charge, no electrodes, thickness d. Sign convention: +c per IEEE Std 176-1987 (P), the face that becomes negative on compression and positive on cooling.

Derived numbers derived

• Savage reports P_s falling by 3.7 µC/cm² between 25 and 450 °C, an average of 8.7×10⁻⁵ C/(m² K) derived, so p3 grows with temperature and large heating steps are underestimated. No p3 value below room temperature was found; cryogenic cooldown charge is not computed.
• The per-kelvin index shift from an unscreened pyroelectric field (2.5 to 5.9×10⁻⁵) is the same size as the thermo-optic dn_e/dT (3.3 to 3.8×10⁻⁵ /K near 1550 nm, 4.8). Whether a device sees it depends on screening.
• The unscreened field after a 95 K step, 15 to 36 kV/mm, is of the order of the congruent coercive field, 21 kV/mm (Table 7.1 P-comp). Real wafers lose charge to the air, surfaces and fixtures; the sparks and cracks in the practitioner reports of 4.10 are consistent with that discharge.
• z-cut vs x-cut: in z-cut films the charge sits on the top and bottom faces and the field is vertical, overlapping the guided mode. In x-cut the charge sits on surfaces perpendicular to the in-plane c axis: sidewalls and chip edges (from geometry).
• Ti-indiffused z-cut interferometers show large pyroelectric output instabilities; z-cut directional couplers and x-cut interferometers are thermally stable; the decay of induced voltage after a temperature change suggests field-dependent LN resistivity (Bulmer et al. 1986 reported).

• Pyroelectric field as a photorefractive driver: the pyroelectric field acts like an applied field; photocarriers screen it in the lit region, which writes an index change even in above-threshold MgO:LN (Schwesyg Ch. 5; Volk & Wöhlecke p. 53 citing Schein et al. 1978). Measured: Δn up to 0.8×10⁻⁴ for ΔT = 3 °C against 1.3×10⁻⁴ predicted; screening time τ_di = 14 s, specific photoconductivity κ = 0.27 pS cm/W; no such damage in CLN in the same window because its κ is more than 30× smaller (pp. 99-100 P; journal version not read).
• Remedies tested or proposed there: periodic poling reduces E_pyro up to 140× for the stated parameters, strongly dependent on duty cycle; short-circuit the z faces; flood the crystal with blue light to raise conductivity (p. 101 P).
• In TFLN: light and temperature changes leave long-lived index changes in z-cut microresonators (Ren et al. 2025reported); an auxiliary light reduces pyroelectric effects in cryogenic waveguides (Lange et al. 2026reported).
• Demonstrations of scale: a pyroelectric crystal has driven nuclear fusion (Naranjo et al. 2005reported (title)); heated monodomain LN emits electrons from its polar faces (Bourim et al. 2006reported (title)).
• Measurement methods and how they separate primary, secondary (piezoelectric) and thermally stimulated currents: review Jachalke et al. 2017reported; background Lang 2005reported. The p3 spread is partly a clamping and method spread; the figure plots all four values.

1. Savage 1966A. Savage, Pyroelectricity and Spontaneous Polarization in LiNbO3. Journal of Applied Physics 37 3071-3072 (1966), doi:10.1063/1.1703164.Read: abstract
2. Bartholomäus et al. 1994T. Bartholomäus, K. Buse, C. Deuper, E. Krätzig, Pyroelectric coefficients of LiNbO3 crystals of different compositions. physica status solidi (a) 142 55 (1994).Read: not read
3. Crystal Technology 1999Crystal Technology Inc., LiNbO3 material properties, data sheet DC-12 8/99. (1999).Read: data sheet values via a secondary compilation
4. Popescu et al. 2013S. T. Popescu, A. Petris, V. I. Vlad, Interferometric measurement of the pyroelectric coefficient in lithium niobate. Journal of Applied Physics 113 043101 (2013), doi:10.1063/1.4788696.Read: abstract
5. Gopalan et al. 1998V. Gopalan, T. E. Mitchell, Y. Furukawa, K. Kitamura, The role of nonstoichiometry in 180 deg domain switching of LiNbO3 crystals. Applied Physics Letters 72 1981-1983 (1998), doi:10.1063/1.121491.Read: abstract
6. Weis & Gaylord 1985R. S. Weis, T. K. Gaylord, Lithium niobate: Summary of physical properties and crystal structure. Applied Physics A Solids and Surfaces 37 191-203 (1985), doi:10.1007/BF00614817.Read: full
7. IEEE 1988IEEE, IEEE Standard on Piezoelectricity, IEEE Std 176-1987. (1988), doi:10.1109/IEEESTD.1988.79638.Read: selected: Table 5, Sec. 3.2.5, p. 22
8. Volk & Wöhlecke 2009Tatyana Volk, Manfred Wöhlecke, Lithium Niobate: Defects, Photorefraction and Ferroelectric Switching. Springer Series in Materials Science 115 (2009), doi:10.1007/978-3-540-70766-0.Read: selected chapters: Ch. 2.2 (pp. 14-19), Ch. 3 (pp. 51-61), Ch. 4 (pp. 75-77, 82-85, 94-97), Sec. 7.1.3 (p. 168)
9. Schwesyg 2011Judith R. M.-L. Schwesyg, Interaction of light with impurities in lithium niobate crystals. Dr. rer. nat. dissertation, Universität Bonn (2011).Read: selected: Sec. 2.2.2-2.3.2 (pp. 10-21), Ch. 5 Sec. 5.1 (pp. 69-72), Sec. 5.4-5.5 (pp. 99-101)
10. Schwesyg et al. 2011J. R. Schwesyg, M. Falk, C. R. Phillips, D. H. Jundt, K. Buse, M. M. Fejer, Pyroelectrically induced photorefractive damage in magnesium-doped lithium niobate crystals. Journal of the Optical Society of America B 28 1973 (2011), doi:10.1364/JOSAB.28.001973.Read: not read (thesis Ch. 5 read instead)
11. Bulmer et al. 1986C. H. Bulmer, W. K. Burns, S. C. Hiser, Pyroelectric effects in LiNbO3 channel-waveguide devices. Applied Physics Letters 48 1036-1038 (1986), doi:10.1063/1.96640.Read: abstract
12. Ren et al. 2025Xinyi Ren, Chun-Ho Lee, Kaiwen Xue et al., Photorefractive and pyroelectric photonic memory and long-term stability in thin-film lithium niobate microresonators. npj Nanophotonics 2 1 (2025), doi:10.1038/s44310-024-00052-3.Read: arXiv abstract
13. Lange et al. 2026Nina A. Lange, René Pollmann, Michael Rüsing et al., Photorefraction management in lithium niobate waveguides: High-temperature vs cryogenic solutions. Journal of Applied Physics 139 133105 (2026), doi:10.1063/5.0324002.Read: abstract
14. Naranjo et al. 2005B. Naranjo, J. K. Gimzewski, S. Putterman, Observation of nuclear fusion driven by a pyroelectric crystal. Nature 434 1115-1117 (2005), doi:10.1038/nature03575.Read: title
15. Bourim et al. 2006El Mostafa Bourim, Chang-Wook Moon, Seung-Woon Lee, In Kyeong Yoo, Investigation of pyroelectric electron emission from monodomain lithium niobate single crystals. Physica B: Condensed Matter 383 171-182 (2006), doi:10.1016/j.physb.2006.02.034.Read: title
16. Jachalke et al. 2017S. Jachalke, E. Mehner, H. Stöcker et al., How to measure the pyroelectric coefficient?. Applied Physics Reviews 4 021303 (2017), doi:10.1063/1.4983118.Read: abstract
17. Lang 2005S. B. Lang, Pyroelectricity: From Ancient Curiosity to Modern Imaging Tool. Physics Today 58 31-36 (2005), doi:10.1063/1.2062916.Read: abstract