Yi-qiang Qin

Two-dimensional structures of ferroelectric domain inversion in LiNbO3 by direct electron beam lithography

We report on the fabrication of domain-reversed structures in LiNbO3 by means of direct electron beam lithography at room temperature without any static bias. The LiNbO3 crystals were chemically etched after exposure of the electron beam, and then the patterns of domain inversion were characterized by atomic force microscopy (AFM). In our experiment, an interesting phenomenon occurred when the electron beam wrote a one-dimensional grating on the negative c face: a two-dimensional (2D) dotted array was observed on the positive c face, which is significant for its potential to produce 2D and three-dimensional photonic crystals. Furthermore, we also obtained 2D ferroelectric domain inversion in the whole LiNbO3 crystal by writing the 2D square pattern on the negative c face. Such a structure may be utilized to fabricate 2D nonlinear photonic crystal. AFM demonstrates that a 2D domain-reversed structure has been achieved not only on the negative c face of the crystal, but also across the whole thickness of th...

Wave-Front Engineering by Huygens-Fresnel Principle for Nonlinear Optical Interactions in Domain Engineered Structures

Wave-front engineering for nonlinear optical interactions was discussed. Using Huygens-Fresnel principle we developed a general theory and technique for domain engineering with conventional quasi-phase-matching (QPM) structures being the special cases. We put forward the concept of local QPM, which suggests that the QPM is fulfilled only locally not globally. Experiments agreed well with the theoretical prediction. The proposed scheme integrates three optical functions: generating, focusing, and beam splitting of second-harmonic wave, thus making the device more compact.

Crucial effects of coupling coefficients on quasi-phase-matched harmonic generation in an optical superlattice.

Coupling of optical parametric processes in an optical superlattice through quadratic nonlinearity was analyzed theoretically. Solving the coupled equations, we found that efficient quasi-phase-matched third-harmonic (TH) generation depends not only on the magnitude of the coupling coefficients but also on their ratio. Theoretically, all the fundamental energy can be transferred to the TH at a particular ratio. In other cases, there exists an optimum condition that corresponds to a maximum TH conversion efficiency. The result is of practical importance for the design of TH devices.

Quasi-phase-matched harmonic generation through coupled parametric processes in a quasiperiodic optical superlattice

We have investigated and fabricated an optical superlattice of LiTaO3, in which two building blocks, each containing a pair of antiparallel 180° ferroelectric domains, were arranged as a Fibonacci sequence. The second- and third-harmonic generation in this structure have been studied and measured. The results verifies that the second-harmonic spectrum and the third harmonic may be generated in a quadric nonlinear medium by a series of quasi-phase-matching processes. The experimental results are in good agreement with theory.

Engineering of a dual-periodic optical superlattice used in a coupled optical parametric interaction

Quasi-phase matching facilitates the phase matching of any parametric interaction by setting an appropriately modulated period. We propose a dual-periodic structure that is used in the construction of a frequency-conversion device. For example, we have designed and fabricated such a dual-periodic structure in a LiTaO3 crystal. Using a fundamental source at 1.064 µm, we obtained ultraviolet radiation at 355 nm and green radiation at 532 nm simultaneously by frequency tripling and frequency doubling the fundamental source. Theoretically, this idea can be further extended to the design of a multiperiodic structure for achievement of more quasi-phase-matched processes in a single optical superlattice.

FURTHER STUDIES ON ULTRASONIC EXCITATION IN AN ACOUSTIC SUPERLATTICE

This article analyzes theoretically the ‘‘in‐line field’’ scheme and the ‘‘crossed‐field’’ scheme for acoustic excitation of an acoustic superlattice for both resonators and transducers. The electrical impedance and the admittance are deduced and the electromechanical coupling coefficients are calculated.

Generation of coherent terahertz radiation with multifrequency modes in a Fibonacci optical superlattice

We investigate theoretically the generation of coherent narrow-band terahertz (THz) radiation with multifrequency modes in an optical superlattice produced by quasiperiodically poled ferroelectric crystal. The superlattice consists of two building blocks A and B, in which each containing a pair of antiparallel 180° ferroelectric domains, arranged as a Fibonacci sequence. Second-order nonlinear optical rectification of femtosecond pulses gives rise to a THz wave form whose multifrequency modes are determined by the quasiperiodic structure properties. The relationship between multifrequency modes terahertz generation and Fibonacci structure parameters is also analyzed and studied.

Second-harmonic and third-harmonic generation in a three-component fibonacci optical superlattice

Harmonic generation in a three-component Fibonacci optical superlattice (3CFOS) is analysed theoretically. The Fourier spectrum of the structural function of the 3CFOS is numerically calculated. The positions of reciprocal vectors are in good agreement with the theoretical prediction. The intensities of the second harmonic (SH) and third-harmonic (TH) are calculated relative to the depletion of the fundamental. The dependence of the SH and TH intensity on structural parameters is discussed. Numerical calculation shows that, compared with two-component Fibonacci optical superlattice (2CFOS), there are more plentiful harmonic spectrum and adjustable structure parameters in 3CFOS. By chosen the reciprocal vectors and adjusted the structure parameters, the higher transform efficiency of THG is obtained.

Nonlinear optical characterization of a generalized Fibonacci optical superlattice

Nonlinear optical characterization in a generalized Fibonacci-type optical superlattice, which can be generated by the concurrent inflation rule A→AmB and B→A (where m=positive integer), has been studied both theoretically and experimentally. Quasi-phase-matched second-harmonicgeneration spectrum and direct third-harmonic-generation with high efficiency have been measured in a LiTaO3 superlattice structure with m=2. Two different structures in real space have shown a similarity in reciprocal space.

Theoretical investigations of efficient cascaded third-harmonic generation in quasi- phase-matched and -mismatched configurations

Theoretical and numerical solutions of the nonlinear coupling-wave equations of second-harmonic (SH) and third-harmonic (TH) converters are investigated for both phase-matched and phase-mismatched configurations. For phase-mismatched TH generation, several kinds of schemes [the phase mismatch in either SH generation or sum-frequency generation (SFG) as well as the phase mismatch in both SH generation and SFG] are considered and analyzed. The optimal TH converting properties for generalized phase-mismatching operation are discussed and studied. It is found that efficient TH generation depends not only on the magnitude of the coupling coefficients but also on their ratio. A 100% TH output can be obtained at a particular ratio. The physical nature of the different ratios of the coupling coefficients is discussed.