Feifei Chen

Crystal growth and characterization of CTGS and Nd:CTGS for self-frequency-doubling applications

Crystal materials with integrated functionalities are desirable for the miniaturization of future optical devices. In this paper, nonlinear optical crystals of Ca3TaGa3Si2O14 (CTGS) and Nd3+ doped CTGS (Nd:CTGS) were grown using the Czochralski pulling method and explored for self-frequency-doubling applications. Based on the refractive indexes, the optimum phase matching directions for CTGS crystals were determined for type I (38.7°, 30.0°) and type II (61.1°, 0°) orientations, with respective effective nonlinear optical coefficients on the order of 0.44 pm V−1 and 0.34 pm V−1. Self-frequency-doubling was realized in Nd:CTGS crystals [(38.7°, 30.0°) (type I)] for the first time, and 18.8 mW laser power at 533 nm was achieved.

Photoacoustic trace detection of gases at the parts-per-quadrillion level with a moving optical grating

Significance The photoacoustic effect refers to the generation of sound through a process of optical heat deposition followed by thermal expansion, resulting in a local pressure increase that produces outgoing acoustic waves. In the linear acoustic regime, a unique property of the photoacoustic effect in a geometry with symmetry in one dimension is that when the optical source moves at the speed of sound, the amplitude of the acoustic wave increases linearly in time without bound. Here, the application of this effect to trace gas detection is described, using an optical grating that moves at the sound speed inside of a resonator equipped with a resonant piezoelectric crystal detector, yielding detection limits in the parts-per-quadrillion range. The amplitude of the photoacoustic effect for an optical source moving at the sound speed in a one-dimensional geometry increases linearly in time without bound in the linear acoustic regime. Here, use of this principle is described for trace detection of gases, using two frequency-shifted beams from a CO2 laser directed at an angle to each other to give optical fringes that move at the sound speed in a cavity with a longitudinal resonance. The photoacoustic signal is detected with a high-Q, piezoelectric crystal with a resonance on the order of 443 kHz. The photoacoustic cell has a design analogous to a hemispherical laser resonator and can be adjusted to have a longitudinal resonance to match that of the detector crystal. The grating frequency, the length of the resonator, and the crystal must all have matched frequencies; thus, three resonances are used to advantage to produce sensitivity that extends to the parts-per-quadrillion level.

Investigation of the crystal growth, thickness and radial modes of α-BiB3O6 piezoelectric crystals

Monoclinic α-BiB3O6 (α-BIBO) crystals have been actively studied for various applications. In this paper, large size α-BIBO single crystals were successfully grown by using the top-seeded solution growth method. The thickness extensional (TE) coupling factor kt and radial extensional (RE) coupling factor kp for α-BIBO crystals were studied. The equation to evaluate the kp value for piezoelectric crystals with monoclinic symmetry was deduced based on the internal energy and piezoelectric equations. For α-BIBO crystals, the coupling factor kt was found to be on the order of 44.0% at room temperature, while the experimental kp value was determined to be 32.0%, slightly lower than the theoretical value of 33.7%. In addition, the temperature dependence of TE and RE electromechanical properties were studied over the temperature range of 20–600 °C, where kt was found to show minimal temperature variation, while kp shifted from 32.0% at room temperature to 27.5% at 600 °C, with a negative variation of −14%. The above electromechanical properties indicate the potential usage of α-BIBO crystals for high temperature piezoelectric devices.

Thermal properties and CW laser performances of pure and Nd doped Bi2ZnB2O7 single crystals

Centimeter-sized Bi2ZnB2O7 (BZBO) and Nd:Bi2ZnB2O7 (Nd:BZBO) single crystals are grown using the Kyropoulos method. The thermal properties and continuous wave (CW) laser performance are investigated. Results show that the BZBO crystal possesses moderate specific heat (0.62 J g−1 K−1 at 20 °C), small thermal expansion (α11 = 6.9 × 10−6 K−1, α22 = 11.2 × 10−6 K−1 and α33 = 1.9 × 10−6 K−1 at 20 °C) and large thermal conductivities (4.3, 3.2 and 4.0 W m−1 K−1 along the crystallographic a-, b- and c-axes at 25 °C, respectively). Moreover, the CW laser tests are realized at a wavelength of 1.06 μm for the Nd:BZBO crystal (0.5 mol% Nd3+), and the output powers are found to be around 256 mW, 235 mW and 312 mW along the optical X-, Y- and Z-orientations, respectively. Nd:BZBO might be another kind of important laser and nonlinear optical crystal in the oxyborate family.

Insights into the polymorphism of Bi2W2O9: single crystal growth and a complete survey of the variable-temperature thermal and dielectric properties

Bismuth layered Aurivillius oxides have attracted longstanding research interest due to their unique structural characteristics and outstanding performance in ferroelectric dielectrics. The complex phase evolution process as a function of temperature and pressure in this family also provides fascinating platforms for understanding the chemical origin of their versatility. Herein, high-quality single crystals of a bismuth layered Aurivillius oxide, Bi2W2O9, with dimensions up to 29 × 23 × 11 mm3 were successfully grown via the top-seeded solution growth (TSSG) method using a mixture of WO3 and Li2B4O7 as a flux. The material crystallizes in an unexpected centrosymmetric orthorhombic Pbcn (no. 60) space group with unit cell parameters a = 23.705(2), b = 5.4148(6), and c = 5.4345(6) A. A complete survey of the anisotropic and variable-temperature thermal and dielectric properties was carried out with the aim of gaining detailed insights into their polymorphism evolution process. The results demonstrate that, with increasing temperature, the compound undergoes complicated phase transitions and, by analogy, signatures of the transitions are summarized and compared with those in Bi2WO6 and Bi2MoO6. The phase sequences and transition mechanisms of Bi2W2O9 are proposed by careful inspection and estimation.

Structural stability and electro-elastic property of YCOB crystal annealed in harsh environment

The YCa4O(BO3)3 (YCOB) piezoelectric crystal has been actively studied for high temperature sensor applications in the last few years. In this paper, the structure stability and electro-elastic properties of the YCOB crystal annealed in a harsh environment (high temperatures of 600–1100 °C and a low atmospheric pressure of 2 × 10−5 atm for 24 h) were studied. The chemical bonding energy of the annealed YCOB crystal was studied, with variations being less than 0.2 eV, showing the high stability of the electronic structure in the YCOB crystal. The energies of vacancy formation (EVF) for Y, Ca, O, and B atoms were analyzed via first principles calculation. The O atoms were found to possess the lowest EVF value, being easier to escape (annealing in critical conditions) and compensate (thermal treatment at elevated temperatures in air) when compared to other atoms, thus leading to oxygen vacancy defects and a decrease in the chemical bonding strength after the annealing process. This is deemed to be the main factor dominating the electro-elastic property changes and their recovery behaviours.The YCa4O(BO3)3 (YCOB) piezoelectric crystal has been actively studied for high temperature sensor applications in the last few years. In this paper, the structure stability and electro-elastic properties of the YCOB crystal annealed in a harsh environment (high temperatures of 600–1100 °C and a low atmospheric pressure of 2 × 10−5 atm for 24 h) were studied. The chemical bonding energy of the annealed YCOB crystal was studied, with variations being less than 0.2 eV, showing the high stability of the electronic structure in the YCOB crystal. The energies of vacancy formation (EVF) for Y, Ca, O, and B atoms were analyzed via first principles calculation. The O atoms were found to possess the lowest EVF value, being easier to escape (annealing in critical conditions) and compensate (thermal treatment at elevated temperatures in air) when compared to other atoms, thus leading to oxygen vacancy defects and a decrease in the chemical bonding strength after the annealing process. This is deemed to be the main f...

High temperature piezoelectric single crystals: Recent developments

High temperature piezoelectric materials serving at harsh environment without failure are desired for structural health monitoring and/or non destructive evaluation of the modern aerospace structures at elevated temperatures (above 600°C). In this report, different high temperature piezoelectric crystals, including the langasite (LGS) type crystals and GaPO4 in trigonal system, melilite type and fresnoite type crystals in tetragonal system, and oxyborate crystals in monoclinic system are surveyed, and their properties are compared. Among the LGS type piezoelectric crystals, the structural ordered trigonal Ca3TaGa3Si2O14 (CTGS) crystals as well as the Al substituted CTGS crystals shew relative low dielectric loss (<5%@600°C), high electromechanical coupling factor (12%∼18%@room temperature) and good temperature stability of piezoelectric properties up to 900°C, potential for usages in middle-high temperature range. Interestingly, the monoclinic RECa4O(BO3)3 (RECOB, Re: rare-earth elements) type crystal were found to own attractive dielectric, piezoelectric and electromechanical properties up to 1000°C, particularly, the YGdCOB crystals were determined to possess temperature independent behavior of piezoelectric properties (variations of piezoelectric d.26 <−5% over the tested temperature range), promising for ultrahigh temperature piezoelectric applications.

Hydrostatic piezoelectric properties of bismuth triborate crystals

In this report, the dielectric and hydrostatic piezoelectric properties of the monoclinic bismuth triborate crystals (BiB₃O₆, BIBO) were investigated at ambient temperature for potential hydrophone application. The pressure dependence of the hydrostatic charge and voltage coefficients dielectric properties (d_h and g_h), and hydrostatic figure of merit (FOM) d_h×g_h, were also studied. Results indicated that the BIBO crystals possess low dielectric loss being on the order of <;0.1% at room temperature. The hydrostatic piezoelectric coefficient d_h for BIBO crystals was experimentally obtained to be d_h =3.3 pC/N, while the piezoelectric hydrostatic coefficient g_h was obtained to be on the order of g_h =44.4×10^-3 V·m/N. Moreover, the FOM value was found to be on the order of 0.15 pm²/N, comparable to that of PMN-PT single crystals (0.12-0.78 pm²/N). In addition, the pressure coefficient a₁/d_h (1.0×10^-4 ppm/Pa) was calculated and found much lower than the reported PMN-PT single crystals (~0.001 ppm/Pa).

Single crystal growth and piezoelectric characterization of the bismuth triborate crystals

The bismuth triborate crystals BiB3O6 (BIBO) are multifunctional crystal materials with noncentrosymetric characteristics. In this paper, single crystal growth of BIBO crystals by using the top seeded solution method in a short period (within one month) was reported and the determination of the dielectric, elastic and piezoelectric constants were discussed for potential piezoelectric applications. For BIBO crystals with monoclinic symmetry, the full set of electro-elastic constants were evaluated taking advantage of the impedance method. Particularly, some parameters were obtained by designing and measuring the habitual facets of the BIBO crystals. It is interesting to find that the BIBO crystals possess high longitudinal piezoelectric coefficient d22 and shear piezoelectric coefficient d34, being on the order of 40.0pC/N and 18.7pC/N, respectively, 3-7 times that of langasite crystals, showing the advantages for applications in piezoelectric field.

Growth and piezoelectric properties of TmCOB single crystals

New rare-earth oxyborate crystals TmCOB were grown by using the Czochralski method. The piezoelectric coefficients for TmCOB 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. The temperature dependence of electro-elastic propertieswereinvestigated from 20 to 900°C, where the temperature coefficients of the elastic compliances (s^E₂₂, s^E₃₃ and s^E₆₆) were foundto be ranged from 146 to223 ppm/°C for TmCOB crystals. Particularly, the electromechanical factor k₁₂ was evaluated to be on the order of 15.4% at room temperature, with a small variation <;5.4% in the temperature range of 20-900°C. Moreover, the TmCOB crystals were obtained to possess high piezoelectric coefficientd₂₆ being 7.8 pC/N, 3-4 times that of quartz, and showing a minimal variation of -12.9% over the test temperature range, potential for high temperature piezoelectric applications.