lithium niobate / 05 Making / 5.10
5.10 Periodic poling of thin films
L1 Phenomenon
To convert colours efficiently, the crystal's polarization is flipped in stripes a few micrometres wide. A comb electrode on the film carries a short high-voltage pulse; under each tooth the domains reverse.
Periodic poling of x-cut thin film: field and thresholdgap 10 µm (template), film 0.6 µm
cross-section along Z, film depth down; # |E| > E_th, shading |E|/E_th
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.:+##################################+:. top view, 3 periods along Y: A comb and counter electrode, - reversed, + original
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Period
| Λ | 4.38 µm | reported |
| E_peak in the film | 43.3 kV/mm | derived |
| Depth fraction reversed (gap centre) | 1.00 | derived |
| Reach across the gap | 1.00 | derived |
| Duty cycle D | 0.53 | template |
| d_eff (d33 34 pm/V, older-scale compilation) | 21.55 pm/V | template |
Marks: 21 kV/mm bulk CLN reported; 28.2 kV/mm thin film S; peak above 3 Ec = 63 kV/mm in practice P.
Field solved once per volt (finite volume, LN free permittivities, SiO2 and Si template permittivities, air cover) and scaled by V/g. Domain growth kinetics are template; only the threshold rule and the field are physics here.
Period
- Peak field in the film
- 43.3 kV/mm
- Duty cycle
- 0.53
- Reach across the gap
- 100%
- d_eff
- 21.55 pm/V
- Depth reversed
- 100%
Gap 10 µm (template), film 0.6 µm. Field solved once per volt (finite volume; LN free permittivities; SiO2 and Si permittivities and the air cover are template) and scaled by V/g derived. Reference fields: 21 kV/mm bulk CLN reported, 28.2 kV/mm thin film S, peak above 3 Ec = 63 kV/mm in practice P. d_eff uses d33 = 34 pm/V (older-scale compilation) template.
L2 Model and data
| Quantity | Value | Source; status |
|---|---|---|
| Bulk CLN poling field | 21 kV/mm | P, P-comp |
| First-order QPM by external-field poling (1993) | room temperature; 20.7 mW blue; 600 %/W cm2 | P(abs) |
| Wafer-scale TFLN poling | 4-inch, 600 nm x-cut on 4.7 µm SiO2 on 500 µm Si; periods 4.38-5.51 µm; reversal lengths 0.5-10.17 mm; about 1 cm2 per operation; 100% success, 98% high-quality on average; 50% duty cycle | P |
| Poling pulse | peak field above 3 Ec; silicone oil against air breakdown; current monitored | P |
| Minimum field for domain growth in thin film | 28.2 kV/mm (quoted from their ref. 20) | S |
| Shortest x-cut TFLN period | 600 nm | P(abs) |
| Bipolar preconditioning, MgO:TFLN | submicrometre periods (747 nm) | P(abs) |
| First nanophotonic PPLN waveguides (2018) | 2600 %/W/cm2 at a 4 µm period | P(abs) |
| Poling with in-situ SHG monitoring | efficiency value not verified | P(title) |
L3 Details and open points
- Thin-film coercive fields exceed bulk; candidate reasons given are bonding to SiO2 and Li+ out-diffusion during LNOI annealing P.
- Domain quality is checked by confocal SH microscopy and PFM P, P(abs); in bulk, by HF etching of the -z face for 20 min P.