Lithium niobate (LN) is one of the most important synthetic crystals. In the past two decades, many breakthroughs have been made in material technology, theoretical understanding, and application of LN crystals. Recent progress in optical damage, defect simulation, and on-chip devices of LN are explored. Optical damage is one of the main obstacles for the practical usage of LN crystals. Recent results reveal that doping with ZrO2 not only leads to better optical damage resistance in the visible but also improves resistance in the ultraviolet region. It is still awkward to extract defect characteristics and their relationship with the physical properties of LN crystals directly from experimental investigations. Recent simulations provide detailed descriptions of intrinsic defect models, the site occupation of dopants and the variation of energy levels due to extrinsic defects. LN is considered to be one of the most promising platforms for integrated photonics. Benefiting from advances in smart-cut, direct wafer bonding and layer transfer techniques, great progress has been made in the past decade for LNs on insulators. Recent progress on on-chip LN micro-photonic devices and nonlinear optical effects, in particular photorefractive effects, are briefly reviewed.

Recent Progress in Lithium Niobate: Optical Damage, Defect Simulation, and On‐Chip Devices

Usage of lithium niobate in nonlinear optics is seriously hampered by optical damage, in particular, where high intensity is needed. Doping with magnesium can improve its resistance against optical damage. However, since a rather large dopant concentration is required (more than 4.6mol% MgO) and since the distribution coefficient is unfavorable, it is difficult to grow crystals of high optical quality. The authors show that by doping with zirconium, one can obtain at the same time a higher resistance against optical damage, a lower doping threshold (only 2.0mol% ZrO2), a distribution coefficient near 1.0, and a low coercive field that is only one-third of that of congruent LiNbO3. These properties suggest that zirconium-doped lithium niobate is an excellent choice for nonlinear optical applications.

Highly optical damage resistant crystal: Zirconium-oxide-doped lithium niobate

The recent advances in the photorefraction of doped lithium niobate crystals are reviewed. Materials have always been the main obstacle for commercial applications of photorefractive holographic storage. Though iron-doped LiNbO3 is the mainstay of holographic data storage efforts, several shortcomings, especially the low response speed, impede it from becoming a commercial recording medium. This paper reviews the photorefractive characteristics of different dopants, especially tetravalent ions, doped and co-doped LiNbO3 crystals, including Hf, Zr and Sn monodoped LiNbO3, Hf and Fe, Zr and Fe doubly doped LiNbO3, Zr, Fe and Mn, Zr, Cu and Ce triply doped LiNbO3, Ru doped LiNbO3, and V and Mo monodoped LiNbO3. Among them, Zr, Fe and Mn triply doped LiNbO3 shows excellent nonvolatile holographic storage properties, and V and Mo monodoped LiNbO3 has fast response and multi-wavelength storage characteristics.

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Recent Advances in the Photorefraction of Doped Lithium Niobate Crystals

Photorefractive properties of congruent lithium niobate crystals codoped with HfO2 and Fe2O3 were investigated and it was found that Fe ions are still located at Li sites as photorefractive centers when the doping concentration of HfO2 goes above the threshold value. As a result, their photorefractive response speed and sensitivity are significantly enhanced. Meanwhile, the high saturation diffraction efficiency is still maintained. Experimental results definitely show that Hf is now the most effective doping element for LiNbO3:Fe crystal to improve its photorefractive properties.

Enhanced photorefractive properties of LiNbO3:Fe crystals by HfO2 codoping

Applications of lithium niobate in nonlinear optics at high light intensities are seriously hampered by optical damage. Recent investigations have shown that Hf(4+) and Zr(4+) ions have some advantages in suppressing optical damage of LiNbO(3) with respect to Mg(2+). Here we present Sn-doped LiNbO(3) (Sn:LN). Experimental results indicate that Sn:LN has similar optical damage resistance to Mg-doped LiNbO(3), but the doping threshold of Sn is only 2.5 mol.%, where its distribution coefficient is 0.98. Hence Sn(4+) ion turns out to be another good choice for increasing optical damage resistance of LiNbO(3).

Increased optical-damage resistance in tin-doped lithium niobate

Highly HfO2 doped lithium niobate crystals have been grown. The experimental results indicate that LiNbO3:Hf (up to 4?mol%) can withstand the same light intensity, of 5 ? 105?W?cm?2, as LiNbO3:Mg (6.5?mol%). And the OH? absorption bands of these LiNbO3:Hf crystals shift to 3487?cm?1 from the 3484?cm?1 for congruent pure LiNbO3. The difference spectra and fitting treatments show that the OH? absorption peak corresponding to (HfNb4+)??OH? is located at 3500?cm?1.

The optical damage resistance and absorption spectra of LinBO3:Hf crystals

We show through simulation that the improved quantitative rescattering model (QRS) can successfully predict the nonsequential double ionization (NSDI) process by intense elliptically polarized laser pulses. Using the QRS model, we calculate the correlated two-electron and ion momentum distributions of NSDI in Ne exposed to intense elliptically polarized laser pulses with a wavelength of 788 nm at a peak intensity of $5.0\ifmmode\times\else\texttimes\fi{}{10}^{14}$ W/${\mathrm{cm}}^{2}$. We analyze the asymmetry in the doubly charged ion momentum spectra observed by Kang et al. [Phys. Rev. Lett. 120, 223204 (2018)] in going from linearly to elliptically polarized laser pulses. Our model reproduces the experimental data well. Furthermore, we find that the ellipticity-dependent asymmetry arises from the drift velocity along the minor axis of the elliptic polarization. We explain how the correlated electron momentum distributions along the minor axis provide access to the subcycle dynamics of recollision. From these findings, we expect that we can extend the QRS model for NSDI toward more complicated laser fields in the future. 

Shuqi Li

Papers

The optical damage resistance and absorption spectra of LinBO3:Hf crystals

Enhanced photorefractive properties of LiNbO3:Fe crystals by HfO2 codoping

Nonsequential double ionization of Ne with elliptically polarized laser pulses