Optical phase conjugation: principles, techniques, and applications

Various single-crystalline nanostructures of potassium tantalite-niobate (KTa1−xNbxO3, KTN) were synthesized by a hydrothermal method at 200 °C. Interesting structures including KTaO3 nano-octahedrons, KTa0.5Nb0.5O3, KTa0.35Nb0.65O3, and KTa0.77Nb0.23O3 nanocubes, KTa0.25Nb0.75O3 tower-like nanostructures, and KNbO3 truncated octahedrons were obtained. The amount of KOH and polyethylene glycol (PEG) is crucial for the formation of pure phase KTa1−xNbxO3 with specific morphologies. The dissolution−precipitation growth mechanism should be accountable for the formation of perfect KTN cubes, and the competition between the supersaturation ratio and the surface energies of nanoclusters may react to the growth of octahedron and truncated octahedron KTN. The experimental results show that the formation of high-quality single-crystalline KTN with tower-like nanostructures may be ascribed to the oriented attachment mechanism.

Controllable Hydrothermal Synthesis of KTa1−xNbxO3 Nanostructures with Various Morphologies and Their Growth Mechanisms

We report what is to our knowledge the first observation of mechanism transformation with wavelength of self-pumped phase conjugation (SPPC) BaTiO3:Ce crystals in the near infrared. in It was found that the SPPC mechanism changed from a backscattering and four-wave-mixing mechanism to a cat mechanism when the pump wavelength was varied from 706 to 770 nm. In addition, SPPC was obtained when the pump beam entered the crystal by the +c face, and a variation with wavelength of optical beam paths in the crystal was also observed.

Mechanism transformation with wavelength of self-pumped phase conjugation in BaTiO(3):Ce.

Since quasi-phase-matching of nonlinear optics was proposed in 1962, nonlinear photonic crystals were rapidly developed by ferroelectric domain inversion induced by electric or light poling. The three-dimensional (3D) periodical rotation of ferroelectric domains may add feasible modulation to the nonlinear coefficients and break the rigid requirements for the incident light and polarization direction in traditional quasi-phase-matching media. However, 3D rotating ferroelectric domains are difficult to fabricate by the direct external poling technique. Here, we show a natural potassium–tantalate–niobate (KTN) perovskite nonlinear photonic crystal with spontaneous Rubik’s cube-like domain structures near the Curie temperature of 40 °C. The KTN crystal contains 3D ferroelectric polarization distributions corresponding to the reconfigured second-order susceptibilities, which can provide rich reciprocal vectors to compensate for the phase mismatch along an arbitrary direction and polarization of incident light. Bragg diffraction and broadband second-harmonic generation are also presented. This natural nonlinear photonic crystal directly meets the 3D quasi-phase-matching condition without external poling and establishes a promising platform for all-optical nonlinear beam shaping and enables new optoelectronic applications for perovskite ferroelectrics. A three-dimensional nonlinear photonic crystal that manipulates light incident from any direction could allow new types of beam shaping for optoelectronic devices. In certain crystals, atoms arrange in mesoscopic structures called supercells, which cause light to interfere in useful ways. Generally, such crystals can only affect light beams that are incident from particular directions, and with certain polarizations. Now, Haohai Yu at Shandong University in Jinan, China, and co-workers have grown crystals of potassium tantalate niobate, with the constituent atoms in a critical ratio that provides a Rubik’s-cube-like structure with supercells 3–10 µm in size. The researchers observed rotating ferroelectric polarization and second harmonic generation in laser beams incident from all three dimensions (x, y, and z directions). They hope that the crystal could provide a useful platform for applications such as terahertz radiation and holography.

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Three-dimensional nonlinear photonic crystal in naturally grown potassium–tantalate–niobate perovskite ferroelectrics

Self-pumped phase conjugation is generally believed to be attributed to degenerate four-wave mixing. Accordingly, the phenomenon requires the existence of three waves at certain interaction regions for the excitation of the fourth one. Although this condition is not satisfied by a train of femtosecond pulses with a repetition rate of 110 MHz, we have observed self-pumped conjugate pulses retroreflected by a BaTiO3 crystal. The conjugate character of the pulses is demonstrated with the distorter method.

Self-pumped phase conjugation with femtosecond pulses by use of batio3

A new model is proposed to explain the origin of the self‐pumped phase conjugation (SPPC) that we have observed in KTa1−xNbxO3:Fe crystals. In this model SPPC is generated by both four‐wave mixing and backscattering processes rather than by only one of them. The advantage of this kind of phase conjugator is also discussed.

Self‐pumped phase conjugation with a new mechanism in KTa1−xNbxO3:Fe crystals

This letter reports the two‐wave mixing experimental results of tetragonal Fe‐doped KTa1−xNbxO3. A 7.3 cm−1 gain coefficient and 42% diffraction efficiency have been obtained. For the first time internally self‐pumped phase conjugation is being observed in this crystal. At room temperature, the conjugate reflectivity measured is 21%.

Photorefractive properties and self‐pumped phase conjugation of tetragonal Fe‐doped KTa1−xNbxO3 crystal

The influences of iron doping on the photorefractive properties of KTa1−xNaxO3 (KTN) crystals were studied. The two‐wave‐mixing gain coefficients and the grating formation rates were measured in undoped and Fe‐doped KTN samples. Photorefractive parameters, such as the effective carrier number density, were determined for these samples. It was shown that the gain coefficient increases significantly with the doped Fe content, but the grating formation rate decreases with it, and that all the influences of iron doping originate mainly from the variation of the effective carrier number density.

Influences of iron doping on the photorefractive properties of KTa1−xNbxO3 crystals

The dynamics of photorefractive diffraction in paraelectric KTa1−xNbxO3 crystals during writing, reading, and erasing have been studied systematically. Experimental observations related not only to the diffraction from the first‐order photorefractive grating but also to that from the second‐order photorefractive grating, and where a writing beam with spots smaller than, as well as larger than, the entrance surface of the crystal was used. All the complicated behaviors observed have been explained qualitatively. An electron‐hole transport model and a nonequilibrium screening model have been applied, and the existence of an internal dc electric field induced during writing simultaneously by an optical field and an externally applied field has been suggested and demonstrated. The significant influence of this induced field is indicated.

Studies of photorefractive diffraction dynamic in paraelectric KTa1-xNbxO3 crystals

This letter reports the two-wave mixing experimental results of tetragonal Fe-doped KTa1-xNbxO3. A 7.3 cm-1 gain coefficient and 42% diffraction efficiency have been obtained. For the first time internally self-pumped phase conjugation is being observed in this crystal. At room temperature, the conjugate reflectivity measured is 21%.

Yingwu Lian

Papers

Self‐pumped phase conjugation with a new mechanism in KTa1−xNbxO3:Fe crystals

Photorefractive properties and self‐pumped phase conjugation of tetragonal Fe‐doped KTa1−xNbxO3 crystal

Influences of iron doping on the photorefractive properties of KTa1−xNbxO3 crystals

Studies of photorefractive diffraction dynamic in paraelectric KTa1-xNbxO3 crystals

Photorefractive properties and self-pumped phase conjugation of tetragonal fe-doped kta1-xnbxo3 crystal