X-cut Crystal Research Articles

The study of spontaneous domain nucleation in the interelectrode gap of phase modulator based on titanium indiffused waveguides in lithium niobate crystals

The paper presents the analysis of nucleating kinetics and growing of switched domains in the surface layer of monodomain lithium niobate X-cut crystal in the interelectrode gap of integrated optical phase modulators. The work proposes the morphology model of domains growing along the boundary of surface electrodes in X-cut phase modulators. The mechanism of spontaneous needle-like domain growing as a result of the electric field induced by the pyroelectric effect at temperature changing of the crystal was theoretically substantiated. The Comsol Multiphysics cross-platform was used for the numerical estimation of the pyroelectric field in the interelectrode gap. The needle-like domain structures were studied experimentally at industrial samples of integrated optical phase modulators based on Ti:LiNbO3 waveguides. The experimental research of the form and size of domains was performed with the anisotropic etching method by HF solution and followed by visual analysis. For non-destructive testing, the authors used scanning electron microscopy and piezo-response force microscopy. For the first time, the morphology of needle-like domains occurring in the interelectrode gap of phase modulators based on lithium niobate was experimentally studied. The results showed the theoretical and numerical model of domain growing that involves the pyroelectric nature of the electric field. It was demonstrated that along the electrode boundary, the needle-like domains grow up to 20 μm long at normal conditions and achieve 30 μm after the thermal shock by cooling at ∆T = – 125 °С. The discovered switched domains in the interelectrode gap can affect electro-optical characteristics of integrated optical phase modulators with the lithium niobate base and should be taken into account in the future design of electrode topology and modulator usage.

Pure mode axis and the electromechanical coupling coefficients of the 0.67Pb(Mg1/3Nb2/3) O3-0.33PbTiO3 single crystal

Pure mode axes of the 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 (PMN-0.33PT) single crystals are investigated by slowness surfaces method. It is found only the Z-axis is a pure mode axis. The piezoelectricically stiffened Christoffel equation of the PMN-0.33PT crystal is solved in the rotating coordinate system. All of the single acoustic modes of thickness vibration are obtained. For the pure longitudinal mode of Z-cut crystal, the electromechanical coupling coefficient is up to 0.82. However, it is only 0.34 for the pure shear modes of X-cut or Y-cut crystal. It is along the pure mode axis that the maximum electromechanical coupling coefficients can be achieved.

Low-threshold Sheet Optical Parametric Oscillator by Triply-resonant Cavity Phase Matching

Microcavity laser represents a novel type of integrated platform potentially allowing low-threshold and high-efficiency lasing behavior. Phase matching is a key parameter to achieve efficient lasing processes. Here we report a study on a triply-resonant phase-matching process in a sheet optical parametric oscillator. The oscillator contains an x-cut KTiOPO4 crystal, and the triply-resonant phase-matching was achieved by temperature tuning of the cavity. The measured oscillation threshold is as low as 13 μJ at 243.6 °C temperature along with a slope efficiency of 17.5% at 65.3 °C. Experimental results show that under some temperatures, triply-resonant sheet optical parametric oscillator can realize a limited number of longitudinal modes (n ≤ 3). Our results represent the first observation of triply-resonant phase-matching process in an on-chip optical parametric oscillator, opening up the possibilities on future applications including quantum chips, vertical-cavity surface-emitting lasers and narrow-band two-photon sources.

Anisotropic lasing properties in the 1059−1086 nm range of Yb:YCa4O(BO3)3 crystal

Efficient laser action was demonstrated in the long-wavelength emission sideband of Yb:YCa4O(BO3)3 crystal by effectively suppressing lasing in the main emission band with specific mirror coatings. With an X-cut crystal, a maximum output power of 13.0 W at wavelengths around 1065 nm with E//Z was generated with an optical-to-optical efficiency of 45.1%; with a Z-cut crystal, the highest output power around 1085 nm with E//X amounted to 15.5 W, the optical-to-optical efficiency being 44.4%. The most efficient laser oscillation realized with Y-cut crystal consisted of two orthogonal polarization components, E//Z and E//X, that occurred in different wavelength regions centered at about 1065 and 1084 nm, producing a total output power of 15.0 W with an optical-to-optical efficiency of 46.3%.

Tunable diffraction grating with transparent indium-tin oxide electrodes on a lithium niobate X-cut crystal

A tunable diffraction grating based on an electrooptic X-cut lithium niobate crystal has been manufactured and experimentally analyzed. The period of electrodes is 290 μm, the electrode width is 117.5 μm, and the thickness of an electrode is 150 – 160 nm. The electrodes are made of a transparent conducting indium-tin oxide that serves as an antireflection coating with the aim of increasing the optical transmission. In order to prevent crystal polarization switching and electrical breakdown an optimized electrode topology with end ellipticity 1:1 and increased interelectrode gap is used. The optical diagram of the tunable grating with alternating electrode potentials for various gap voltages is analyzed. The intensity of the zero order of diffraction is shown to decrease by 40 % at a voltage of 800 V. At the same time, the origination of new diffraction orders at angles ± λ / (2d) is noted. The measurement of the forward-bias and reverse-bias regions of the modulation characteristic reveals the absence of hysteresis, which confirms the correctness of the electrode topology design.

High-order Stokes generation in a KTP Raman laser pumped by a passively Q-switched ND:YLF laser

High-order Stokes wave was observed in an x-cut KTP crystal based on stimulated Raman scattering (SRS) pumped by a passively Q-switched Nd:YLF laser with a Cr4+:YAG saturable absorber. Output spectra including the fundamental wave at 1047nm and six Stokes wavelengths at 1077nm, 1110nm, 1130nm, 1143nm, 1164nm, 1180nm based on two Raman frequency shift at 267.4cm−1 and 693.0cm−1 were obtained simultaneously. We also detected green light generation with output power of 12mW from self frequency mixing in the KTP crystal. The maximum total output power reached 452mW at the repetition frequency of 8.1kHz, corresponding to the optical-to-optical conversion efficiency of 4.61% and pump-to-Raman conversion efficiency of 3.6%.

Comparative study of 0° X-cut and Y + 36°-cut lithium niobate high-voltage sensing.

A comparison study between Y + 36° and 0° X-cut lithium niobate (LiNbO3) was performed to evaluate the influence of crystal cut on the acoustic propagation to realize a piezoelectric high-voltage sensor. The acoustic time-of-flight for each crystal cut was measured when applying direct current (DC), alternating current (AC), and pulsed voltages. Results show that the voltage-induced shift in the acoustic wave propagation time scaled quadratically with voltage for DC and AC voltages applied to X-cut crystals. For the Y + 36° crystal, the voltage-induced shift scales linearly with DC voltages and quadratically with AC voltages. When applying 5 μs voltage pulses to both crystals, the voltage-induced shift scaled linearly with voltage. For the Y + 36° cut, the voltage-induced shift from applying DC voltages ranged from 10 to 54 ps and 35 to 778 ps for AC voltages at 640 V over the frequency range of 100 Hz-100 kHz. Using the same conditions as the Y + 36° cut, the 0° X-cut crystal sensed a shift of 10-273 ps for DC voltages and 189-813 ps for AC voltage application. For 5 μs voltage pulses, the 0° X-cut crystal sensed a voltage induced shift of 0.250-2 ns and the Y + 36°-cut crystal sensed a time shift of 0.115-1.6 ns. This suggests a frequency sensitive response to voltage where the influence of the crystal cut was not a significant contributor under DC, AC, or pulsed voltage conditions. The measured DC data were compared to a 1-D impedance matrix model where the predicted incremental length changed as a function of voltage. When the voltage source error was eliminated through physical modeling from the uncertainty budget, the combined uncertainty of the sensor (within a 95% confidence interval) decreased to 0.0033% using a Y + 36°-cut crystal and 0.0032% using an X-cut crystal for all the voltage conditions used in this experiment.

Dyakonov hybrid surface waves at the isotropic-biaxial media interface.

Dyakonov surface waves (DSWs) at the interface of an isotropic dielectric material and a biaxial dielectric crystal were considered in the case in which two principal axes of the biaxial crystal lie in the plane of the interface. Both x-cut and y-cut biaxial crystals were considered, and all permittivity scalars were assumed to be real and positive. A rather simple expression to relate the components of the wave vector of a DSW was derived, and analytical expressions for the limit angles at which the DSWs can propagate and the Poynting vector components of the DSWs were obtained. These expressions were numerically investigated for several biaxial materials. Different conditions for the relative permittivity scalars at which the DSWs can exist were identified. As an example, DSWs can exist not only for the positive, but also for the negative y-cut biaxial crystals.

Growth, polarized Raman and fluorescence properties of TbCa4O(BO3)3 single crystal

A single crystal of TbCa4O(BO3)3 (TbCOB) with diameter of 35mm and length of 80mm has been successfully grown using the Czochralski (Cz) method. The polarized Raman spectra were studied to analyze the vibrational structure of TbCOB, especially the modes from [BO3]3− triangles that are responsible for the nonlinear optical generation. The fluorescence properties of the X-cut, Y-cut, and Z-cut non-centrosymmetric TbCOB crystals were measured for the first time, and both f–d and f–f transitions were identified according to the energy level diagram. Based on the non-radiative energy transfer process, TbCOB crystal shows the characteristic emission of Tb3+ corresponding to 5D4→7F6,5,4,3 transitions with the green light of 542nm (5D4→7F5) being the most prominent group. Additionally, the lifetime decay indicates single exponential character for the excited wavelength at 378nm, following the result of τ=2.07ms at room temperature. The long fluorescence lifetime and good spectra properties of TbCOB single crystal indicate its potential optical prospects.

Continuous-Wave and Passive Q-Switching Laser Performance of Yb:YCa<inline-formula><tex-math>$_{\bf 4}$ </tex-math></inline-formula>O(BO<inline-formula><tex-math>$_{\bf 3}$</tex-math></inline-formula>)<inline-formula> <tex-math>$_{\bf 3}$</tex-math></inline-formula> Crystal

We have conducted an investigation into the continuous-wave (CW) and passive Q-switching laser performance of the Yb:YCa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> O(BO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (Yb:YCOB) crystal, aiming to evaluate its capability of power or energy scaling in compact end-pumped lasers. Efficient CW laser operation was realized with output coupling changed over a wide range from 0.4% to 60%. Operating in CW mode in the long-wavelength sideband around 1085 nm, the Z-cut Yb:YCOB laser produced an output power of 10.7 W with an optical-to-optical efficiency of 56%; in CW operation of the Y-cut Yb:YCOB laser oscillating in the main emission band, an output power of 17.0 W was reached, with optical-to-optical and slope efficiencies being respectively 62% and 78% (with respect to incident pump power). In passive Q-switching regime, the X-cut crystal proved to be the most advantageous in generating high average power and high pulse energy. With a Cr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup> :YAG crystal plate serving as saturable absorber whose initial transmission was T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> = 97.5%, and with an output coupling of T = 20%, an average output of 10.72 W was generated at a pulse repetition rate of 26.3 kHz, the resulting pulse duration was 22.3 ns. While under Q-switching operational conditions of T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> = 90.0% and T = 40%, a maximum average output power of 4.13 W could be reached at a rather low repetition rate of 3.23 kHz, yielding a pulse energy that was as high as 1.28 mJ, the resulting pulse duration and peak power being 5.0 ns and 256 kW, respectively.

Output-coupling-dependent laser operation of monoclinic Yb:Ca4LaO(BO3)3 crystal

We have studied the laser operation of Yb:LaCOB crystal for what we believe is the first time. Adopting a plane-concave cavity, the laser properties were measured for the X-cut, Y-cut, and Z-cut crystals. The best output power of 3.2 W was obtained with a slope efficiency 69.90% by employing the Y-cut crystal. A more efficient operation with slope efficiency of 82.31% was realized for the X-cut crystal, with the output power of 2.0 W. The laser spectra were collected under the absorbed pump power of 3.9 W for X cut and Z cut and 6.7 W for Y cut. The emission wavelengths were dependent of the transmittance of output couplers.

Contribution of the inverse flexoelectric effect to counterpropagating two-wave mixing of light beams in photorefractive crystals

This paper presents the results of a theoretical analysis of the contribution of the inverse flexoelectric and photoelastic effects to the photorefractive response that accompanies counterpropagating mixing of a steady-state reference wave with a phase-modulated signal wave on reflective holograms in samples of X-cut crystals of symmetry classes 23, 4¯3m, 4¯2m, 422, 622, 222, and 3m. Experimental studies of such mixing of waves with circular polarization of opposite signs in a Ba12TiO20:Ni crystal made it possible to estimate its flexoelectric coefficient.

The potential of Yb:YCa_4O(BO_3)_3 crystal in generating high-energy laser pulses

The passive Q-switching laser performance of Yb:YCa(4)O(BO(3))(3) is studied with crystals cut along the principal optical axes. Using a Cr(4+):YAG saturable absorber with initial transmission of 93.7% and an output coupler of transmission of 40%, efficient Q-switched laser operation is achieved with a X-cut crystal, generating an output power of 2.14 W at a pulse repetition rate of 4.5 kHz. The resulting laser pulse is 9.3 ns in duration, with the energy being as high as 476 μJ and the peak power amounting to 51.2 kW. The results demonstrated in this work reveal the great potential of this crystal in developing high-energy compact pulsed lasers.

Multiaxis electric field sensing using slab coupled optical sensors

This paper provides the details of a multiaxis electric field sensor. The sensing element consists of three slab coupled optical-fiber sensors that are combined to allow directional electric field sensing. The packaged three-axis sensor has a small cross-sectional area of 0.5 cm×0.5 cm by using an x-cut crystal. A method is described that uses a sensitivity-matrix approach to map the measurements to field components. The calibration and testing are described, resulting in an average error of 1.5°.

Optical planar waveguide in sodium-doped calcium barium niobate crystals by carbon ion implantation

There is great interest in niobate crystals which belong to the tetragonal tungsten bronze (TTB) families owing to their intriguing properties. As one representative of such crystals, CBN (calcium barium niobate) has attracted rapidly growing attention. Because it has a higher Curie temperature than SBN (strontium barium niobate), possesses outstanding ferroelectric and it possesses optical properties. In addition, doped with sodium, CBN will show a higher Curie temperature than pure CBN. We report on the fabrication and characterization of optical planar waveguide in x-cut sodium-doped calcium barium niobate crystal by using C ion implantation. The guided-mode properties at the wavelength of 633 and 1539nm are investigated through prism-coupling measurements, respectively. By applying direct end-face coupling arrangement, the near-field optical intensity distribution of waveguide modes is measured at 633nm. For comparison, the modal profile of the same guided mode is also numerically calculated by the finite difference beam-propagation method via computer software BeamPROP. The transmission spectra of the waveguide before and after ion implantation treatments were investigated also. Our experiment results reveal that the waveguide could propagate light with transverse magnetic polarized direction only and it is assumed that the polarization selectivity of CBN crystal may responsible for this phenomenon.

Locally Er-Doped Near-Stoichiometric LiNbO3Crystals Prepared by Standard Er Diffusion and Post-VTE Treatment

The feasibility of preparing locally Er-doped near-stoichiometric (NS) LiNbO3 crystals for integrated optics applications is demonstrated by a two-step process with standard diffusion (1130°C/154 h) of Er metal followed by vapor transport equilibration (VTE) treatment under three different conditions of 1135°C/22 h, 1115°C/50 h, and 1125°C/60 h. Detailed studies on the crystalline phase, Li composition, diffused surface roughness, and emission characteristics of Er3+ ions indicate that there is an upper limit on the initial Er metal film thickness: ∼20 nm for an X-cut crystal and ∼30 nm for a Z-cut crystal. When the initial Er film thickness is below this limit, the post-VTE does not induce formation of ErNbO4 precipitate and the diffused surface retains high quality with a root mean square roughness <3 nm. Depending on the VTE condition adopted, the VTE results in the increase of ([Li]+[Er])/[Nb] ratio in the diffused layer from congruent point (94.5%) to 97.4%–99.4%. Secondary ion mass spectrometry study shows that the post-VTE does not affect the Gauss nature of the Er profile, but leads to the increase of diffusion depth by as much as 1.6 μm. In comparison with the standard Er diffusion, the post-VTE results in the decrease of Er diffusivity by three to nine times. The higher the VTE temperature is, the lower the Er diffusivity is. In addition, the post-VTE also results in definite reduction of OH− content in crystal, slight lengthening of lifetime and slight narrowing of linewidth of Er3+ emission at 1530 nm.

Annealing behavior of lithium niobate irradiated with He-ions at 100 K

In order to investigate the formation and the annealing of defects, x- and z-cut lithium niobate (LiNbO3) crystals were irradiated with 200keV He-ions at a temperature of 100K. Subsequently, thermal treatment was performed at temperatures of 250°C and 300°C, respectively. The defect concentration was measured by means of Rutherford-backscattering spectroscopy (RBS) using 1.4MeV He-ions immediately after the irradiation at 100K, after warming to room temperature as well as after thermal treatment. After the irradiation the x-cut crystal is apparently more damaged compared to z-cut which is a consequence of the preferential arrangement of displaced Nb-atoms on vacant octahedral sites. Upon thermal treatment the samples show a completely different annealing behavior. At an ion fluence of 1×1016cm-2, the defect concentration decreases for x-cut material, but increases for z-cut samples with increasing annealing time. This is explained by the formation of aligned dislocation loops during the thermal treatment.

Off-Congruent LiNbO3 Crystals Prepared by Li-Poor Vapor Transport Equilibration for Integrated Optics

X- and Z-cut off-congruent LiNbO3 (LN) crystals for use for integrated optics were prepared by carrying out Li-poor vapor transport equilibration (VTE) treatment on 1-mm-thick congruent plates at 1100 °C for durations ranging from 65 to 170 h. The Li2O contents and crystalline phase in these Li-poor VTE-treated crystals were characterized by using Raman scattering, optical absorption, and powder X-ray diffraction spectroscopy. The studies of optical absorption in the ultraviolet region show that the Li-poor VTE treatments result in different extents of red-shifts of the optical absorption edges (OAEs), and the amount of the red-shift increases with the prolonged VTE duration. The mean Li2O contents in all VTE samples were evaluated from the measured OAEs. The results show that the VTE-induced reduction of the mean Li2O content can reach as much as ∼1 mol% for both cases of X-cut and Z-cut, and the mean Li2O content and the VTE duration follows the linear relations: cLi = (48.62 ± 0.06) − (0.0052 ± 0.0004)t for the X-cut crystals and cLi = (48.53 ± 0.08) − (0.0063 ± 0.0005)t for the Z-cut crystals. The out-diffusion of Li shows a slight anisotropy. The diffusivity in a Z-cut crystal is slightly larger. The results of visible absorption, Raman scattering, and powder X-ray diffraction show that all the VTE crystals still retain the LiNbO3 phase, consistent with the result predicted from the LiNbO3 phase diagram. The critical VTE duration, longer than which the VTE will induce LiNb3O8 phase in the crystal, is roughly evaluated to be about 577 and 476 h for an X-cut and a Z-cut plate, respectively.

Comparison of laser performance of Yb:YCa4O(BO3)3 crystals cut along the principal optical axes

Efficient high-power continuous-wave laser operation is demonstrated with Yb:YCa4O(BO3)3 crystals cut along the X, Y, and Z principal axes, end pumped by a high-power diode. Highest output powers of 6.3–7.3 W are generated, with slope efficiencies in the range of 73–83%. The standard behavior characterized by fixed linear polarization of the output and decreasing oscillation wavelength when increasing the output coupling was observed only for the X-cut crystal; the Y-cut crystal led to the polarization state changing and coexistence, while the Z-cut crystal resulted in dual-band laser operation with a fixed polarization state.

Laser damage resistance of RbTiOPO_4: evidence of polarization dependent anisotropy

Nanosecond-laser induced damage of RbTiOPO(4) crystals (RTP) has been studied at 1064 nm as a function of propagation direction and polarization orientation. A significant difference in the Laser Induced Damage Threshold (LIDT) was observed for x-cut and y-cut crystals in Pockels cell configuration, where the light propagation direction is along the x and y axes of the crystal respectively. In Pockels cell configuration the polarization is oriented at 45? with respect to the z-axis of the crystal. Experiments with the polarization oriented parallel to the principal axes of the crystal pointed out the importance of the polarization direction for the LIDT whereas the propagation direction did not significantly influence the LIDT. Comparison of the experimental data with a simple model reveals the influence of frequency doubling on the LIDT in Pockels cell configuration. In the case of the y-cut Pockels cell, the generation of frequency doubled light causes an LIDT below the LIDT of x and z-polarized light at the fundamental wavelength.