Shenglai Wang

Study on nonlinear refractive properties of KDP and DKDP crystals

The nonlinear refractive index n2 is an important parameter for the nonlinear optical properties of a medium. This study primarily targets to measure the third-order nonlinear optical properties of potassium dihydrogen phosphate (KDP) and deuterated potassium dihydrogen phosphate (DKDP) with 70% deuteration content at λ = 532 nm using the Z-scan technique. The relationship between nonlinear refraction and direction has been studied in detail. Nonlinear refractive index n2, γ and third-order nonlinear susceptibilities xk3 (esu) were calculated through normalized transmittance curves and theoretical formulas. The results indicate that the nonlinear refractive indexes of KDP and DKDP are positive and possess obvious self-focusing effects in high-power laser systems.

Nonlinear optical effects in nitrogen-doped graphene

The band structure of graphene can be adjusted by incorporating other elements or functional groups, correspondingly, the response to an external light field will also be changed. In this study, we investigated the nonlinear optical (NLO) responses of N-doped graphene nanosheets (N-Gns) with an open aperture Z-scan method, under the excitation of picosecond laser pulses. To the best of our knowledge, this is the first time that two-photon absorption (2PA) of N-Gns has been reported. The normalized nonlinear transmission demonstrates that with the increasing excitation energy, the saturable absorption (SA) and the 2PA appear successively. Compared to the 532 nm excitation, the 1064 nm excitation exhibits smaller saturable intensity Is and larger 2PA coefficient β. At the same time, the N-Gns present superior NLO property compared to pure graphene. Consequently, N-Gns will be a promising candidate for SA or optical limiting (OL) applications.

Crystal growth and characterization of thulium calcium oxyborate high-temperature piezoelectric crystals

New rare-earth calcium oxyborate crystals TmCa4O(BO3)3 (TmCOB) were grown by using the Czochralski pulling method, and the mechanical and thermal properties were investigated. In addition, the dielectric, elastic and piezoelectric performance was evaluated by impedance measurement, where the piezoelectric charge coefficients were determined to be on the order of d11 = 1.7, d12 = 3.8, d13 = −4.2, d15 = −0.92, d24 = 4.9, d26 = 7.8, d31 = −0.75, d32 = −2.4, d33 = 2.2 and d35 = −5.2 pC N−1. The temperature-dependent electrical resistivity and electro-elastic properties were studied from room temperature to 900 °C, where a high electrical resistivity of ~6 × 107 Ω cm and a low dielectric loss of ~21% were achieved at 900 °C, with high thermal stability of electromechanical coupling factors and piezoelectric coefficients over the temperature range of 20–900 °C. All of these properties demonstrate that TmCOB crystals are an attractive candidate for high-temperature sensor fabrications.

Effect of Fe3+ on third-order optical nonlinearity of KDP single crystals

A series of trace Fe3+-doped KH2PO4 single crystals were grown using the conventional temperature cooling method. The Fe3+ ion content in the as-grown crystals and the corresponding transmission spectrum were measured, respectively. With an increase of Fe3+ ion concentration, the transmittance of the crystal gradually decreases within the wavelength range of 200 nm to 370 nm. Under picosecond pulse laser irradiation, third-order nonlinear optical characteristics were systematically measured at λ = 532 nm. The existence of nonlinear absorption and nonlinear refraction was demonstrated. The results show that the nonlinear refractive index n2 is positive, indicating the presence of a self-focusing effect for the as-grown crystals. Specially, nonlinear absorption and refraction have a heavy dependence on the Fe3+ ion content and crystal directions (II and z). With increasing Fe3+ ion concentration, a significant variation has been observed with respect to the nonlinear absorption coefficient β, nonlinear refractive index n2 and third-order nonlinear susceptibility χ(3). A similar pattern for II < z has been presented for nonlinear absorption and refraction. The investigations suggest that nonlinear absorption and refraction caused by distortion of the electron cloud might be associated with the H2PO4− and (FeO4)2− groups.

Characteristics of nonlinear optical absorption and refraction for KDP and DKDP crystals

High-quality KDP and DKDP crystals with 12%, 70% and 80%-deuterium content were grown by the conventional temperature cooling method. The nonlinear optical characteristics of K(DxH1-x)2PO4 crystals, including nonlinear absorption and refraction, were systematically investigated through the Z-scan method at λ = 532 nm. The results demonstrate the existence of nonlinear absorption and refraction, and a nonlinear refractive index with a positive sign indicates the generation of self-focusing effect. Moreover, the results indicate that nonlinear absorption and refraction were associated with crystal direction and structure, especially the distribution of H2PO4 - or D2PO4 - groups. For K(DxH1-x)2PO4 crystals, a similar rule of β was z > I > II, while n2 (or χR(3)) was z > II ≈I. These results will evoke more attention to KDP and DKDP applications, especially frequency conversion and electro-optic switch.

Optical Limiting of Carboxyl–Graphene Oxide

Using the Z-scan method, the nonlinear optical absorption property of carboxyl-functionalized graphene oxide (GO) was researched for the first time. The strong optical limiting (OL) effect was found in carboxyl-GO, which can be attributed to the two-photon absorption (2PA). At 532-nm excitation condition, the threshold of OL behavior is only 8% of that at 1064 nm excitation, and the saturable intensity Is as well as the 2PA coefficient β2 are 1.31 GW/cm2 and 0.17 cm/GW, respectively. It indicates that carboxyl-GO is an excellent nanomaterial for OL application, especially in short wavelength scope.

The relationship between the laser damaged threshold and step velocity in different supersaturation regions

The laser damaged threshold (LDT) of potassium dihydrogen phosphate (KDP) crystals grown at different supersaturation points were investigated using a Nd:YAG nanosecond laser. The growth rate was measured by a laser polarization interference system. Combining the step slopes, the dependence of v–(σ) was obtained. It is interesting that the dependence of LDT-(σ) can also be divided into three areas and shows a similar dependence to that of v–(σ). The LDT reduces rapidly at the transition supersaturation area, and has the lowest value at σ = 0.07. In different supersaturation regions, the micro morphology of the KDP crystals was observed using an atomic force microscope. The nature of the step movement and bunching together of steps may contribute to the dependence of LDT-(σ).

Study on the cracking of a KDP seed crystal caused by temperature nonuniformity

The phenomenon of the cracking of KDP z-cut seed crystals with a series of sizes due to the temperature difference of the growth solution was explored in detail. Thermal stress generated in the KDP seed crystal at the moment of cracking was calculated using the finite-element method. Temperature differences leading to cracking for heated samples were nearly twice those of cooled specimens. For the cooled samples, the cracks occurred mainly along the (100), (110) and (001) crystal planes and expanded from the sample periphery to the interior spaces, and the opposite crack propagation direction was displayed in the heated samples, with cracks observed mainly along the (110) plane. More importantly, the temperature differences were observed to decrease with an increase in the sample size. In addition, the calculated results indicated that the field distribution of tensile stress is coincident with the points of crack initiation. The maximum stresses calculated were found to be greater than the fracture strength of 6.67 MPa for the KDP crystal along the [100] direction, and the direction of large stresses was found to be normal to the fracture surface. It is interesting that the maximum stress in fracture crystals decreases with an increase in the crystal size as cracks are generated.

Effect of cyclohexane diamine tetraacetic acid on micro morphology of rapidly grown potassium dihydrogen phosphate crystals

Potassium dihydrogen phosphate (KDP) crystals were grown from aqueous solutions with different concentrations of cyclohexane diamine tetraacetic acid (CDTA) by the “point seed” rapid growth technique. The effects of CDTA on the surface micro morphology of KDP crystals were systematically studied by atomic force microscopy. The prismatic face growth rates of KDP crystals grown from various solutions were measured by a laser polarization interference system. The results shown that the steps became smooth after CDTA doping. When the doping concentration of CDTA is 100 ppm, the bunching of steps on the (100) face reduced dramatically and the step velocity increased obviously due to the complexation between CDTA and the metal impurities in the original solution. When the doping concentration of CDTA is 500 ppm, the excess CDTA adsorbed on the growth surface and impeded the step movement, leading to an increase in step bunching and reduction in step velocity compared with the situation of 100 ppm.

Influence of temperature on the growth and surface morphology of Fe3+ poisoned KDP crystals

A series of potassium dihydrogen phosphate (KDP) crystals and Fe3+ doped KDP crystals were grown at different temperatures and at the same supersaturation value of 0.02. The prismatic face growth rates of the KDP crystals were measured using a laser polarization interference system. The surface morphologies of the growth steps on the {100} face were comparatively studied using an atomic force microscope systematically. The results showed that the bunching of steps was related to the supersaturation interval in which it was located, and approached the supersaturation dead zone of σd in which the bunching and the slope of the steps were high. The supersaturation dead zone σd and linear zone σ* decreased with rising temperature. Varying the temperature caused considerable changes in the growth speed, step morphology and step motion speed. For the crystal grown in a doped solution, the bunching and slope of the steps reached a maximum value at around 45 °C. It was interesting that the crystal growth rate in the doped solution was higher than that in the undoped solution at around 65 °C and 75 °C, the bunching of the steps was reduced and the width of the steps widened significantly. At the same supersaturation, the growth rate increased with rising temperature and the steps became straight.