KTiOPO4 Crystals Research Articles

Singly resonant optical parametric oscillator based on adhesive-free-bonded periodically inverted KTiOPO_4 plates: terahertz generation by mixing a pair of idler waves

We implemented a singly resonant optical parametric oscillator based on adhesive-free-bonded periodically inverted KTiOPO4 plates. It has major advantages such as walk-off compensation and oscillation at four wavelengths. The threshold of the oscillation was measured to be 8 MW/cm2, which is about a factor of 4 lower than that based in two separate KTiOPO4 crystals. By frequency-mixing the dual-wavelength output in GaP stacks, we generated the terahertz radiation at 2.54 THz. The tuning range of the terahertz output was demonstrated to be 2.19-2.77 THz.

Continuous-wave intracavity frequency-doubled Nd:Yag-KTiOPO4 red laser

Efficient and compact red laser output at 669 nm is generated by intracavity frequency doubling of a continuous-wave (CW) diode-pumped Nd:YAG laser at 1338 nm. With 16.9 W of diode pump power and the frequency-doubling crystal KTiOPO4 (KTP), a maximum output power of 582 mW in the red spectral range at 669 nm has been achieved, corresponding to an optical-to-optical conversion efficiency of 3.4%; the output power stability over 4 h is better than 3.6%. To the best of our knowledge, this is first work on intracavity frequency doubling of a diode pumped Nd:YAG laser at 669 nm.

Bulk PPKTP by crystal growth from high temperature solution

Periodically-poled ferroelectric crystals show unprecedented efficiency and properties otherwise impossible to obtain. Unfortunately, the sample thickness obtainable today limits their use to low and moderate power application. With the aim of increasing the size of periodically domain-structured crystals with a controlled and regular grating period, we proposed an epitaxial growth process using seeds made of thin plates domain engineered by electric field poling. We demonstrated this process with the ferroelectric crystal KTiOPO4 (KTP) which is one of the most promising candidate materials for that purpose. The poling step requires a sample exhibiting (001) and (001̄) faces, so that the growth step has to be performed onto these faces. This constraint is a difficulty to circumvent as these faces are not present in the standard equilibrium morphology. It is then necessary to find the growth conditions enabling to work below the roughening temperature of these faces. By using a high temperature solution method, the so-called “flux method”, and by choosing an appropriate chemical composition of the flux solution, we obtained periodically domain-structured KTP layers with thicknesses up to 800μm and regular periodicity onto (001) and (001̄) faces of the initial PPKTP seeds.

Nanosecond-laser-induced damage in potassium titanyl phosphate: pure 532 nm pumping and frequency conversion situations

Nanosecond-laser-induced damage measurements in the bulk of KTiOPO4 (KTP) crystals are reported using incident 532 nm light or using incident 1064 nm light, which pumps more or less efficient second harmonic generation. No damage threshold fatigue effect is observed with pure 532 nm irradiation. The damage threshold of Z-polarized light is higher than the one for X- or Y-polarized light. During frequency doubling, the damage threshold was found to be lower than for pure 1064 or 532 nm irradiation. More data to quantify the cooperative damage mechanism were generated by performing fluence ramp experiments with varying conditions and monitoring the conversion efficiency. All damage thresholds plotted against the conversion efficiency align close to a characteristic curve.

Second harmonic and Raman imaging of He^+ implanted KTiOPO_4 waveguides

We report on the micro second harmonic (µ-SH) and micro Raman (µ-Raman) images of ion implanted channel and planar waveguides in KTiOPO4 (KTP) crystals. The μ-SH images reveal that the nonlinear properties in the waveguides have not been deteriorated during the implantation process. This is consistent with the μ-Raman images that evidences that lattice distortions are minimal at waveguide's volume. Both the structural and nonlinear properties of the KTP lattice have been only modified at the end of ions' trajectory, which is in good agreement with the positions with maximum refractive index changes.

Generation of squeezed TEM01 modes with periodically poled KTiOPO4 crystal

Spatial quantum optics and quantum information based on the high order transverse mode are of importance for the super-resolution measurement beyond the quantum noise level. We demonstrated experimentally the transverse plane TEM01 Hermite—Gauss quantum squeezing. The squeezed TEM01 mode is generated in a degenerate optical parametric amplifier with the nonlinear crystal of periodically poled KTiOPO4. The level of 2.2-dB squeezing is measured using a spatial balance homodyne detection system.

Angular spectrum characters of high gain non-critical phase match optical parametric oscillators

The angular spectrum gain characters and the power magnification characters of high gain non-walk-off colinear optical parametric oscillators have been studied using the non-colinear phase match method for the first time. The experimental results of the KTiOAsO4 and the KTiOPO4 crystals are discussed in detail. At the high energy single resonant condition, low reflective ratio of the output mirror for the signal and long non-linear crystal are beneficial for small divergence angles. This method can also be used for other high gain non-walk-off phase match optical parametric processes.

Second-harmonic conical refraction: observation of free and forced harmonic waves

Observation of second-harmonic conical refraction in KTiOPO4 crystal is reported. Two second-harmonic conical patterns, namely forced and free harmonic waves, are identified and registered with exceptional clarity and level of details. The forced second-harmonic wave evolves into the characteristic ring intensity distribution while the free-harmonic wave gives a single ray direction, resulting in a spot.

Efficient 18 μmKTiOPO_4 optical parametric oscillator pumped within an Nd:YAG/SrWO_4 Raman laser

A 1.8 μm optical parametric oscillator (OPO) based on a noncritically phase-matched KTiOPO4 crystal is demonstrated. OPO and stimulated Raman scattering techniques are successfully combined in an acousto-optically Q-switched Nd:YAG/SrWO4 Raman laser. The device efficiently realizes three steps of conversion: from a laser diode wavelength of 808 nm to the fundamental wavelength of 1064 nm; next, to the Stokes wavelength of 1180 nm; and finally to the OPO signal wavelength of 1810 nm. With an incident diode power of 7.2 W and a pulse repetition rate of 15 kHz, an average signal power of 485 mW is obtained with a diode-to-signal conversion efficiency of 6.75%. The beam quality factors (M2) of the signal wave in both horizontal and vertical directions are measured to be 1.7±0.2. The numerical output power results of the system, the thermal lensing, and the stability parameter of the cavity are also discussed.

Self-established noncollinear oscillation and angular tuning in a quasi-phase-matched mirrorless optical parametric oscillator

We experimentally demonstrate self-established noncollinear interactions in a quasi-phase-matched mirrorless optical parametric oscillator using a periodically-poled KTiOPO4 crystal with submicrometer periodicity. It is shown that the oscillation is established in a very specific geometric configuration where the two possible solutions of the energy and momentum conservation conditions become spatially degenerate and contribute coherently to the gain. In this configuration, the signal and the idler remain mutually counter-propagating. The tuning capabilities of a noncollinear mirrorless optical parametric oscillator are investigated.

Dispersing performance of KTP(011) crystal

According to the requirements for the performance of analyzers in soft x-ray regime of synchrotron radiation, the qualities of various crystals often used in soft x-ray range are investigated. We find that crystals KTP (KTiOPO4)(011) crystal is an ideal one among large d-spacing crystals. We propose an experimental method to measure diffraction efficiencies of crystals with synchrotron radiation source. In particular, we find that the diffraction efficiency is closely related to the source emittance. The lattice constant and diffraction efficiencies of KTP(011)crystal are also presented in this paper.

All-solid-state CW frequency-doubling Nd:LuVO4/KTP laser with 2.3 W output power at 545 nm

Efficient and compact green-yellow laser output at 545 nm is generated by intracavity frequency doubling of a continuous-wave (CW) diode-pumped Nd:LuVO4 laser at 1089 nm under the condition of suppressing the higher gain transition near 1066 nm. With 18.7 W of diode pump power and the frequency-doubling crystal KTiOPO4(KTP), as high as 2.3 W of CW output power at 545 nm is achieved, corresponding to an optical-to-optical conversion efficiency of 12.3%; the output power stability over 4 h is better than 5.4%.

Investigation of the Linear and Nonlinear Optical Susceptibilities of KTiOPO4 Single Crystals: Theory and Experiment

Experimental and theoretical studies of the linear and nonlinear optical susceptibilities for single crystals of potassium titanyl phosphate KTiOPO₄ are reported. The state-of-the-art full potential linear augmented plane wave method, based on the density functional theory, was applied for the theoretical investigation. The calculated direct energy band gap at Γ, using the Engel-Vosko exchange correlation functional, is found to be 3.1 eV. This is in excellent agreement with the band gap obtained from the experimental optical absorption spectra (3.2 eV). We have calculated the complex dielectric susceptibility ε(ω)dispersion, its zero-frequency limit ε₁(0) and the birefringence of KTiOPO₄. The calculated birefringence at the zero-frequency limit Δn(0) is equal to about 0.07 and Δn(ω) at 1.165 eV (λ = 1064 nm) is 0.074. We also report calculations of the complex second-order optical susceptibility dispersions for the principal tensor components: χ₁₁₃((2))(ω), χ₂₃₂((2))(ω), χ₃₁₁((2))(ω), χ₃₂₂((2))(ω), and χ₃₃₃((2))(ω). The intra- and interband contributions to these susceptibilities are evaluated. The calculated total second order susceptibility tensor components |χ(ijk)((2))(ω)| at λ = 1064 nm for all the five tensor components are compared with those obtained from our measurements performed by nanosecond Nd:YAG laser at the fundamental wavelength (λ = 1064 nm). Our calculations show reasonably good agreement with our experimental nonlinear optical data and the results obtained by other authors. The calculated the microscopic second order hyperpolarizability, β₃₃₃, vector component along the dipole moment direction for the dominant component χ₃₃₃((2))(ω) is found to be 31.6 × 10⁻³⁰ esu, at λ = 1064 nm.

Generation of two-color continuous variable quantum entanglement at 0.8 and 1.5 μm

Two-color continuous variable quantum entanglement at 0.8 and 1.5 μm was experimentally demonstrated by using an above-threshold optical parametric oscillator based on a periodically poled KTiOPO4 crystal. The system presented here has potential applications in future quantum information networks, e.g., it can be used to made a connection between a quantum memory device based on alkaline atoms and a quantum communication device based on telecommunication optical fibers.

Growth of KTiOPO4 crystals doped with zinc and studies of their physical properties and specific structural features

A series of potassium titanyl phosphate single crystals doped with zinc (KTP: Zn) is grown by spontaneous flux crystallization. Their properties and the way the additive is implanted in the crystal lattice are studied. The inclusion of zinc atoms in the KTP structure is evidenced by the data of chemical analysis and the results of studies of electrophysical properties (the growth of conductivity and increase of relaxation anomalies). Precision X-ray diffraction studies of KTP: Zn single crystals reveal changes in the channel of the structure which correlate with the physical properties of this crystal series. No substitution of zinc atoms for titanium, phosphorus, or potassium atoms is found in the structure. Zinc atoms can be located at structural defects, domain walls, and the crystal surface.

Near-infrared properties of periodically poled KTiOPO4 and stoichiometric MgO-doped LiTaO3 crystals for high power optical parametric oscillation with femtosecond pulses

A thorough theoretical analysis of the near-infrared properties of periodically poled KTiOPO4 (PPKTP) and stoichiometric MgO-doped LiTaO3 (MgO:PPSLT) crystals for optical parametric oscillation with excitation at 1 μm is presented. To the best of our knowledge, the optical, phasematching, wavelength tuning, and dispersive properties of these crystals for parametric interactions are discussed in detail for the first time. In addition, a new design for high power parametric devices based on PPKTP or MgO:PPSLT crystals with ultrafast Yb-ion laser pump is proposed. The results form a useful reference for the selection of materials and operating conditions for the practical design of high power femtosecond optical parametric oscillators with excitation at 1 μm.

Structure of KTiOAsO4 single crystals at 293 and 30 K

The unit cell parameters of KTiOPO4 and KTiOAsO4 single crystals are measured in the temperature range from room temperature to 20 K. It is found that the unit cell volume of the single crystals changes smoothly. With a decrease in temperature, the c parameter remains almost unchanged. In a certain temperature range, the linear temperature dependence of the a and b parameters is violated. An X-ray diffraction study of KTiOAsO4 single crystals is performed at T = 293 and 30 K. With a decrease in temperature, the electron density in the channels of the structure undergoes a redistribution, suggesting that at room temperature the state of the potassium ions is characterized by the dynamic and static disordering. The nonuniformity of the distribution of the electron density at the junctions of TiO6 octahedra and AsO4 tetrahedra is significantly enhanced in relation to that at the corresponding junctions in the KTiOPO4 structure. It has been experimentally established that the geometry of the tetrahedral anions makes a decisive contribution to the nonlinearity of KTiOAsO4 single crystals.

Collinear phase matching for second harmonic generation using conoscopic interferometry

The problem of finding phase-matching directions in noncentrosymmetric biaxial crystals is simplified here by the use of Conoscopic interferometry. Based on vector relations for wave propagation in birefringent media and solutions to phase-matching equations, we show that phase matching directions can be located on the conoscopic interferograms and that fringe numbers for dark-isochromes can be used as a guide to find phase-matching directions for a biaxial crystal. This technique can be generalized and extended to any anisotropic crystal. We have demonstrated this method for the particular case of a biaxial KTiOPO4 crystal, where it is found to be particularly suitable for finding the optimum-phase-matching directions.

Observation of two-color continuous variable quantum correlation at 08 and 15 μm

Quantum correlation between signal and idler beams at 0.8 and 1.5 μm was experimentally observed by using an above-threshold optical parametric oscillator based on a periodically poled KTiOPO4 crystal. The measured quantum noise reduction in the intensity difference of the twin beams was 2.5 dB. The system presented here is of interest for long-distance quantum information processing.

Monomode channel waveguide in KTiOPO4 crystal produced by Rb+ ion exchange combined with Si+ ion implantation

A novel ion exchanged channel KTiOPO4 waveguide formation technique is introduced, which can avoid a metal mask being dissolved in an ion exchanged molten salt. Rb+ ion exchange (340°C, 30 min) was first applied to a KTP sample to produce a planar waveguide substrate, and then Si+ ion implantation (3.0 MeV and 1.5 MeV with doses of 1×1015 ions/cm2 and 6×1014 ions/cm2, respectively) was carried out to construct channel stripes on the planar waveguide surface that has been deposited by a specially designed photoresist mask. The two-dimensional cross sectional refractive index profile of the channel waveguide was reconstructed by considering the shape of the channel waveguide as well as the index distribution of the planar waveguide.