lithium niobateLiNbO3
PPLN Λ = 6.82 µm · 1064 → 532 nm · 2 px/µm
lithium niobate / 04 Quirks / 4.2

4.2 Doping the damage out: MgO, Zn, In, Sc, Hf, Zr

L1 Phenomenon

threshold doping

Adding about 5% magnesium oxide to congruent lithium niobate makes it withstand visible light that would scar the undoped crystal. Magnesium does not stop the light-driven current. It makes the crystal about a hundred times more photoconductive, so the separated charge leaks back before it builds a large field (Bryan et al. 1984 reported). Below a threshold concentration the effect is small; above it photorefraction collapses. A crystal closer to the ideal 50:50 Li:Nb ratio (near-stoichiometric) needs much less dopant.

Photorefractive damage threshold vs MgO (cw green, beam distortion after 10 min)■ CLN ○ NSLN
10⁻²10⁻¹10⁰10¹10²10³10⁴10⁵01234567GRIIRA, CLN ~5%GRIIRA, NSLN ~1.5%Bryan 1984: ≥ 4.5 at.%Furukawa 2000: > 0.78 mol%, NSLNMgO (% in the source's unit)damage threshold, kW/cm²dashed: guide, not a fit
CrystalkW/cm²
CLN1
CLN:3% MgO10
CLN:4.5% MgO75
CLN:6% MgO> 8,000
NSLN0.1
NSLN:0.6% MgO10
NSLN:1.8% MgO> 8,000
hover a point: crystal, value,
condition, locator, status
y axis
Series
7 points Volk & Wöhlecke 2009, Table 4.3, p. 97 (data Furukawa 2001)P-compmarkers: P-comp reported reportedunits as in each source; not converted

The MgO cliff

same green beam, same 10 minutes, rising MgO
10⁻²10⁻¹10⁰10¹10²10³10⁴10⁵01234567GRIIRA, CLN ~5%GRIIRA, NSLN ~1.5%Bryan 1984: ≥ 4.5 at.%Furukawa 2000: > 0.78 mol%, NSLNMgO (% in the source's unit)damage threshold, kW/cm²dashed: guide, not a fit Li sites, illustrative: Nb antisites (gold) replaced by Mg (green) up to the threshold3 of 6 antisite glyphs remain
Intensity axis
Crystal
 2.5 %
Hover a pointvalue, condition, locator 

Three decades between 3% and 6% MgO in congruent crystals, and 6% is a lower bound. Near-stoichiometric crystals exceed 8000 kW/cm² at 1.8%; both top values are lower bounds, not a measured plateau.

data Volk & Wöhlecke 2009, Table 4.3P-compthresholds Bryan et al. 1984reported Furukawa et al. 2000reportedlattice strip: drawing, not a model

L2 Thresholds and mechanism

Volk & Wöhlecke Table 4.3; Bryan 1984; Furukawa 2000
CrystalDamage threshold intensityConditionSource
CLN1 kW/cm2cw green, beam distortion after 10 minVolk & Wöhlecke 2009, Table 4.3, p. 97P-comp
CLN:3% MgO10 kW/cm2Volk & Wöhlecke 2009, Table 4.3P-comp
CLN:4.5% MgO75 kW/cm2Volk & Wöhlecke 2009, Table 4.3P-comp
CLN:6% MgO> 8000 kW/cm2Volk & Wöhlecke 2009, Table 4.3P-comp
NSLN0.1 kW/cm2Volk & Wöhlecke 2009, Table 4.3P-comp
NSLN:0.6% MgO10 kW/cm2Volk & Wöhlecke 2009, Table 4.3P-comp
NSLN:1.8% MgO> 8000 kW/cm2Volk & Wöhlecke 2009, Table 4.3P-comp

Table 4.3 data are from Furukawa et al. 2001.

QuantityValueSource
MgO threshold≥ 4.5 at.% MgOBryan et al. 1984reported
Mechanism at the thresholdphotoconductivity up about 100×; Glass current not reduced; Arrhenius activation 0.1 eV (resistant) vs 0.5 eV (undoped)Bryan et al. 1984reported
OH stretch line2.83 µm above threshold, 2.87 µm belowBryan et al. 1984reported
MgO threshold in compilations5 mol% (congruent melt); 4.5-4.9 mol%Volk & Wöhlecke 2009, p. 82P-comp Schwesyg 2011, p. 19P-comp
MgO threshold, near-stoichiometric ([Li]/[Nb] = 0.993)> 0.78 mol%: no measurable damage at 532 nm up to 2 MW/cm2Furukawa et al. 2000reported
Near-stoichiometric LN without Mglower damage resistance than CLNFurukawa et al. 2000reported
CLN vs NSLN crossoverNSLN shows more photorefraction at low and moderate intensity; CLN rises steeply above 100-200 W/cm2Volk & Wöhlecke 2009, p. 77P-comp
MgO:LN vs CLN transportphotoconductivity 10-100× higher; bulk photovoltaic current about 10× lower (green light)Schwesyg 2011, pp. 19-20P-comp
ZnO threshold, congruent meltabout 6-7 mol%Volk & Wöhlecke 2009, p. 82P-comp
In2O3, Sc2O31.7 mol% In2O3 or 1.5 mol% Sc2O3 act like 4.5-5 mol% MgOVolk & Wöhlecke 2009, p. 83P-comp
HfO2, ZrO2Hf about 2 at.% in the crystal; ZrO2 below 2 mol%Volk & Wöhlecke 2009, p. 83P-comp
LiNbO3:2% Zrno optical damage up to 20 MW/cm2Volk & Wöhlecke 2009, p. 83 (Kong et al.)P-comp
NSLN:1% Mg, VTE NSLN:Mgresistant to 2-8 MW/cm2; stable to 26 MW/cm2Volk & Wöhlecke 2009, p. 85P-comp
Space-charge field, NSLN:1% Mgoptical-damage δΔn about 10−8, more than 100× below CLN:5% MgVolk & Wöhlecke 2009, p. 85 (data Furukawa 2000)P-comp
Bulk laser damage, MgO:LNhighest 14 J/cm2 (1.053 µm, 1 ns) at 1 mol% MgO; lower above 3 mol% (MgO aggregates scatter)Furukawa et al. 1991reported
First reportsIQEC 1980 conference abstract (Zhong, Jin, Wu); journal report 1984Zhong et al. 1980S Bryan et al. 1984reported

Mechanism (Volk & Wöhlecke Sec. 2.5, 4.2.2; Schwesyg p. 20 P-comp): below the threshold, Mg2+ replaces Nb antisites on Li sites. At the threshold almost all NbLi is gone; further Mg goes onto Nb sites, Fe3+-like impurities are pushed onto Nb sites and stop acting as acceptors, and the OH band shifts. Trivalent and tetravalent dopants need less concentration, which the charge-compensation scheme explains qualitatively (p. 83).

L3 Definitions and other routes

units, operational thresholds
  • Threshold units differ by source: Bryan et al. state at.% MgO, the compilations mol% MgO in the melt or crystal. Each value is printed in the unit of its source and not converted.
  • The threshold intensity of Table 4.3 is an operational definition (beam distortion after 10 min of cw green light) and depends on beam size and observation time.
  • Other routes to a lower photovoltaic field: raise the dark conductivity (strong reduction, Ag doping, proton exchange) or suppress jpv (UV instead of blue-green, strong oxidation) (p. 75 P-comp). Heating CLN to about 200 °C also suppresses photorefractive damage (Schwesyg p. 19, citing its ref. 29 S).
  • Above-threshold MgO:LN still shows light-induced index changes when a pyroelectric field is present (4.6 L3).
  • The same threshold removes GRIIRA (4.4).

Sources

  1. Bryan et al. 1984D. A. Bryan, R. Gerson, H. E. Tomaschke, Increased optical damage resistance in lithium niobate. Applied Physics Letters 44 847-849 (1984), doi:10.1063/1.94946.Read: abstract
  2. Zhong et al. 1980G. G. Zhong, J. Jin, Z. K. Wu, Measurements of optically induced refractive-index damage of lithium niobate doped with different concentrations of MgO. 11th International Quantum Electronics Conference (IQEC), 1980; J. Opt. Soc. Am. 70, 631 (1980) abstract (1980).Read: not read
  3. Furukawa et al. 1991Y. Furukawa, A. Yokotani, T. Sasaki et al., Investigation of bulk laser damage threshold of lithium niobate single crystals by Q-switched pulse laser. Journal of Applied Physics 69 3372-3374 (1991), doi:10.1063/1.348537.Read: abstract
  4. Furukawa et al. 2000Y. Furukawa, K. Kitamura, S. Takekawa, A. Miyamoto, M. Terao, N. Suda, Photorefraction in LiNbO3 as a function of [Li]/[Nb] and MgO concentrations. Applied Physics Letters 77 2494-2496 (2000), doi:10.1063/1.1318721.Read: abstract
  5. Furukawa et al. 2001Y. Furukawa, K. Kitamura, A. Alexandrovski, R. K. Route, M. M. Fejer, G. Foulon, Green-induced infrared absorption in MgO doped LiNbO3. Applied Physics Letters 78 1970-1972 (2001), doi:10.1063/1.1359137.Read: abstract; data via Volk & Wöhlecke Table 4.3 and Fig. 4.14 (P-comp)
  6. 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)
  7. 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)