4.3 Photorefraction in thin films: rings and waveguides

L1 Phenomenon

TFLN microrings

In a thin-film ring the light is confined to a sub-micron film and builds up by circulating, and photorefraction responds fast (Jiang et al. 2017 reported (title)). It shifts a ring's resonance to shorter wavelength as power rises, opposite to the heating shift. It can hold a written shift for hours, which is a nuisance for a frequency reference and a tool for trimming.

L2 Observations and ring model

reported results; template model

Observation	Value	Platform	Source
Fast photorefractive response	title-level claim	LN microresonators	Jiang et al. 2017reported (title)
Photorefractive tuning rate	0.64 GHz/aJ (84 MHz per photon); Q up to 10⁵; quenching of photorefraction observed	LN photonic-crystal nanobeam	Liang et al. 2017reported (arXiv abstract)
Cladding dependence	removing the dielectric cladding mitigates photorefraction	TFLN microrings	Xu et al. 2021reported
Cryogenic Bragg scattering	a strongly excited mode imprints a long-lived periodic space-charge field; reconfigurable mode splitting	TFLN rings, cryogenic	Xu et al. 2021reported
Memory	recovery time > 10 h after light excitation and temperature change; strong dependence on film orientation; used for optical trimming	z-cut TFLN microresonators	Ren et al. 2025reported (arXiv abstract)
Subwavelength grating	standing-wave pump writes a photorefractive grating; mode splitting; first-order QPM for backward SHG	TFLN microcavity	Hou et al. 2024reported
Sign	photorefractive index change opposite to Kerr and thermal; enables turnkey soliton generation (numerical, ordinary polarization)	TFLN rings	Lin et al. 2021reported
Self-starting soliton	photorefraction lets soliton mode locking self-start and switch bi-directionally	LN microresonator	He et al. 2019reported (arXiv abstract)
Doped thin films	in-diffused Zn (4.0 mol% surface) and Zr (1.1 mol%) raise photorefractive damage resistance up to 100× vs undoped TFLN; 600 nm z-cut planar guides	TFLN	Suntsov et al. 2026reported
Waveguide management	high-temperature operation raises the damage threshold; at cryogenic temperature an auxiliary co-propagating light restores phase-matching spectra and reduces pyroelectric effects	LN waveguides (SFG)	Lange et al. 2026reported
Power ceiling for SHG	LN damage threshold, even MgO-doped, keeps TFLN SH output well below 1 W; PPLT reaches > 1 W SH at 775 nm (4.5 W pump, 7 mm)	TFLN vs thin-film LiTaO₃	Kuznetsov et al. 2025reported (preprint)

Ring model: intracavity energy U from coupled-mode theory; detuning Δ(t) = Δ₀ + g_thT_c(t) + g_prE_sc(t). T_c relaxes with τ_th toward a value proportional to absorbed power; E_sc relaxes with τ_pr = εε₀/(σ_d + κI), I the intracavity intensity, toward a value of opposite sign. g_th uses dn/dT (4.8); g_pr, τ_th, κ and absorption are templates. The figure shows the qualitative competition, not a fit to any device.

Ring sweep: through-port T(λ), both directionssolid: λ increasing; dashed: decreasing

sweep: 1.00e+00 nm/s
P: 1.00e+00 mW
thermal shift, max: 1.01e+01 pm
PR shift, max: −5.35e+00 pm
T_min up / down: 0.002 / 0.383
Q_L, linewidth: 5.00e+05, 3.10 pm

Pump power 1.00e+00 mW

Sweep speed 1.00e+00 nm/s

PR strength 1.0 ×

Photorefraction

Cladding

dn_e/dT

Inset

dn_e/dT 1550 nm: Jundt 1997 Sellmeier, coefficients S, derivative derived or 1523 nm S r₃₃ 30.8 pm/V P-compn_e 2.1376 at 1550 nm P-db PR sign opposite to thermal reported cladding trend reported R 100 µm, Q_i 10⁶, n_g 2.3, Γ 0.8, R_th, τ_th, κ, σ_d, PR gain: model parameters, no data template

Heat pulls, charge pushes

TFLN ring, radius 100 µm, 1550 nm laser swept across one resonance

Pump power1.0e+00 mW

Sweep speed1.0e+00 nm/s

Photorefraction

Photorefraction strength 1.0 ×

Cladding

dn_e/dT, 10⁻⁵ /K

Direction shown by the laser

Laser position in the sweep −8.1 pm

Transmission: 0.870
Thermal shift, max: +10.1 pm
Photorefractive shift, max: −5.4 pm
Loaded Q, linewidth: 5.00e+05, 3.10 pm

dn/dT 1550 nm, Jundt coefficients S, derivative derived or 1523 nm Sr₃₃ 30.8 pm/V P-compn_e 2.1376 at 1550 nm P-dbcladding trend reportedR 100 µm, Q_i 10⁶, n_g 2.3, Γ 0.8opposite-sign photorefraction reportedring, heat sinking, photoconductivity: model parameters, no data templatelight: 1550 nm, false color, dashed

L3 Time scales and measurement

thin-film conductivities: no measurement found

Time scale: the photoconductive relaxation time scales as 1/(κI), so resonant build-up of intensity shortens it; cladding changes the effect (Xu et al. 2021 reported). No measured thin-film dark or photoconductivity was found; the model treats both as templates.
Measurement: power-dependent resonance position at slow and fast sweep, pump-probe on two modes, or time-resolved transmission after a power step. Thermal and photorefractive shifts separate by sign and time constant.
Error sources: films from ion slicing differ in hydrogen and defect content from bulk; anneal history matters (anneals change material loss, Shams-Ansari et al. 2022 reported); results do not transfer between foundries without measurement.

Sources

Jiang et al. 2017H. Jiang, R. Luo, H. Liang, X. Chen, Y. Chen, Q. Lin, Fast response of photorefraction in lithium niobate microresonators. Optics Letters 42 3267 (2017), doi:10.1364/OL.42.003267.Read: title
Liang et al. 2017H. Liang, R. Luo, Y. He, H. Jiang, Q. Lin, High-quality lithium niobate photonic crystal nanocavities. Optica 4 1251 (2017), doi:10.1364/OPTICA.4.001251.Read: arXiv abstract
Xu et al. 2021Yuntao Xu, Mohan Shen, Juanjuan Lu, Joshua B. Surya, Ayed Al Sayem, Hong X. Tang, Mitigating photorefractive effect in thin-film lithium niobate microring resonators. Optics Express 29 5497 (2021), doi:10.1364/OE.418877.Read: abstract
Xu et al. 2021Yuntao Xu, Ayed Al Sayem, Chang-Ling Zou, Linran Fan, Risheng Cheng, Hong X. Tang, Photorefraction-induced Bragg scattering in cryogenic lithium niobate ring resonators. Optics Letters 46 432 (2021), doi:10.1364/OL.414702.Read: abstract
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
Hou et al. 2024Jiankun Hou, Jiefu Zhu, Ruixin Ma et al., Subwavelength Photorefractive Grating in a Thin-Film Lithium Niobate Microcavity. Laser & Photonics Reviews 18 2301351 (2024), doi:10.1002/lpor.202301351.Read: abstract
Lin et al. 2021Z. Lin, Z. Kang, P. Xu, Y. Tian, S. He, Turnkey generation of Kerr soliton microcombs on thin-film lithium niobate on insulator microresonators powered by the photorefractive effect. Optics Express 29 42932 (2021), doi:10.1364/OE.446527.Read: abstract
He et al. 2019Yang He, Qi-Fan Yang, Jingwei Ling et al., Self-starting bi-chromatic LiNbO3 soliton microcomb. Optica 6 1138 (2019), doi:10.1364/OPTICA.6.001138.Read: arXiv abstract
Suntsov et al. 2026S. Suntsov, S. Schnippering, K. Hasse, D. Kip, Photorefractive damage thresholds in Zn- and Zr-doped thin-film lithium niobate. APL Photonics 11 086117 (2026), doi:10.1063/5.0336844.Read: abstract
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
Kuznetsov et al. 2025Nikolai Kuznetsov, Zihan Li, Tobias J. Kippenberg, Watt-level second harmonic generation in periodically poled thin-film lithium tantalate. arXiv (2025), arXiv:2512.07968.Read: arXiv abstract
Shams-Ansari et al. 2022Amirhassan Shams-Ansari, Guanhao Huang, Lingyan He et al., Reduced material loss in thin-film lithium niobate waveguides. APL Photonics 7 081301 (2022), doi:10.1063/5.0095146.Read: abstract