Hongde Liu

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

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.

Growth and properties of a noncentrosymmetric polyphosphate CsLa(PO3)4 crystal with deep-ultraviolet transparency

To explore new deep-ultraviolet (deep-UV) nonlinear optical crystals, the CsLa(PO3)4 crystal with noncentrosymmetric crystallographic structure was grown by spontaneous nucleation using the flux method and slow-cooling technique. A single crystal with a size of 28 mm × 14 mm × 5 mm was easily obtained. Two perfect cleavage behaviours of the CsLa(PO3)4 crystal were found and the reason was discussed from the point of view of bond length and symmetry. The thermal stability and specific heat capacity of CsLa(PO3)4 were investigated. The CsLa(PO3)4 crystal remarkably exhibits a deep-UV cutoff edge of 167 nm, which is one of the shortest values among phosphates to date. Polycrystalline powder of CsLa(PO3)4 presents a moderate second harmonic generation (SHG) response, which is about one half of that of KH2PO4 (KDP). Analysis of the dipole moments for the polyhedra and theoretical calculations by density functional theory (DFT) were performed for understanding the structure–function relationships in the CsLa(PO3)4 crystal.

Effect of oxygen stoichiometry on the insulator-metal phase transition in vanadium oxide thin films studied using optical pump-terahertz probe spectroscopy

Using optical pump-terahertz probe spectroscopy, we studied the dynamic behaviors of photoinduced insulator-metal phase transition in vanadium oxide thin films with different oxygen stoichiometry. We found that the insulator-metal phase transition of vanadium dioxide is very sensitive to oxygen stoichiometry: the increased oxygen content in vanadium oxide will reduce the magnitude of phase transition and change the dynamics of the phase transition. The transient complex photoconductivity of vanadium oxide thin films is investigated and analyzed with Drude-Smith model, supplying insight of the dynamic process of phase transition in vanadium oxide thin films.

High resistance against ultraviolet photorefraction in zirconium-doped lithium niobate crystals

The UV photorefraction of Zr-doped lithium niobate (LN:Zr) was investigated. The experimental results show that LN:Zr crystals have high resistance against photorefraction in the UV region as well as in the visible range and can withstand a UV light intensity of above 10(5) W/cm(2). According to the fitting results of erasing curves with UV and green light, a two-center O(2-/-)-defect model was suggested. Our results indicate that LN:Zr is an excellent candidate for optical damage resistance from the UV to the visible spectrum.

Light-induced domain reversal in doped lithium niobate crystals

Light-induced domain reversals of Mg-doped, Zr-doped, and nominally pure LiNbO3 crystals were investigated with focused 514.5 nm laser beams. It was found the light-reduced values of electric field for domain reversal are almost the same, about 3 kV/mm. Inverted domains always first nucleate on the −c surface within the illuminated region but appear at the edge of the spot on the +c surface. The light-induced inverted domains are hard to be reversed by application of a reverse electric field or by heating to temperatures as high as 600 °C. According to these experimental results, we present a qualitative model on the light-induced domain reversal process in LiNbO3.

Crystal growth and optical characteristics of beryllium-free polyphosphate, KLa(PO3)4, a possible deep-ultraviolet nonlinear optical crystal

Deep-ultraviolet nonlinear optical crystals are of great importance as key materials in generating coherent light with wavelength below 200 nm through cascaded frequency conversion of solid-state lasers. However, the solely usable crystal in practice, KBe2BO3F2 (KBBF), is still commercially unavailable because of the high toxicity of beryllium-containing and the extreme difficulty of crystal growth. Here, we report the crystal growth and characteristics of an beryllium-free polyphosphate, KLa(PO3)4. Centimeter-sized single crystals have been easily obtained by the flux method and slow-cooling technique. The second-harmonic generation efficiency of KLa(PO3)4 powder is 0.7 times that of KH2PO4; moreover, the KLa(PO3)4 crystal is phase-matchable. Remarkably, the KLa(PO3)4 crystal exhibits an absorption edge of 162 nm, which is the shortest among phase-matchable phosphates so far. These attributes make KLa(PO3)4 a possible deep-ultraviolet nonlinear optical crystal. An analysis of the dipole moments of the polyhedra and theoretical calculations by density functional theory were made to elucidate the structure-properties relationships of KLa(PO3)4.

Influence of composition on the photorefractive centers in pure LiNbO3 at low light intensity.

The influence of composition on the photorefractive effect in pure LiNbO3 crystals at low light intensity was investigated. The experimental results indicate that different defects dominate the photorefractive centers of pure LiNbO3 with various compositions. Bipolarons are considered to be responsible for the enhanced photovoltaic field in reduced near-stoichiometric LiNbO3, and their bulk photovoltaic constant kappa is estimated to be approximately 6.95 x 10(-32) m3/V. Q polarons (composed of two bipolarons) are introduced to explain the photorefractive effect of congruent LiNbO3 at both low and high light intensities.

The relationship between the switching field and the intrinsic defects in near-stoichiometric lithium niobate crystals

Several near-stoichiometric lithium niobate (LiNbO3) plates were prepared by the vapour transport equilibrium technique. Domain reversal was carried out on these samples by electric field poling. A nonlinear dependence of switching field on the concentration of anti-site niobium ( ) ions was observed. This nonlinear relationship was explained by a two-dimensional model based on the dynamics of the domain wall. A parameter of θc, denoting the critical position of the domain wall round ions, was introduced to describe domain reversal. The domain wall energy per unit area for near-stoichiometric lithium niobate was also roughly calculated to be larger than 0.27 J m−2.

Light-induced superlow electric field for domain reversal in near-stoichiometric magnesium-doped lithium niobate

Light-induced domain reversal of near-stoichiometric Mg-doped LiNbO3 crystal was investigated with a focused 532 nm continuous laser beam. The lowest electric field applied to accomplish domain nucleation is only 30 V/mm and 1/80 of the coercive field, which is safe and convenient for us to fabricate domain structures. Under this superlow applied field, the pinning effect of domain wall is so obvious that the inverted domain reveals a gear shape contrary to the hexagon in a higher applied field. Then two-dimensional domain patterns with the smallest domain size of 4 μm have been fabricated.

Improved sensitivity of nonvolatile holographic storage in triply doped LiNbO 3 :Zr,Cu,Ce

We have designed and grown triply doped LiNbO(3):Zr,Cu,Ce crystal and investigated its characteristics of nonvolatile holographic storage. Its observed that the photorefractive sensitivity of LiNbO(3):Zr,Cu,Ce has improved to 0.099 cm/J, which is about one order of magnitude larger than that of congruent LiNbO(3):Cu,Ce. And LiNbO(3):Zr,Cu,Ce also has high suppression to light-induced scattering. Our results indicated that triply doped LiNbO(3):Zr,Cu,Ce is an excellent candidate for nonvolatile holographic data storage.