Jung Hoon Ro

Broadband quasi-phase-matched second-harmonic generation in MgO-doped periodically poled LiNbO 3 at the communications band

A novel scheme for broadband second-harmonic generation was developed with type I quasi phase matching by use of an off-digital nonlinear optical coefficient d(31) . This was achieved by matching of the group velocities of the second harmonic and the fundamental, while the phase velocities were quasi phase matched. The desired group-velocity dispersion could be obtained with LiNbO(3) doped with 5 mol. % of MgO. Efficient second-harmonic generation with a bandwidth of 52 nm was obtained for the fundamental wavelength centered at 1566 nm in MgO-doped periodically poled LiNbO(3) , compared with a bandwidth of 1.3 nm for the conventional scheme using d(33) . Efficient frequency doubling of an ultrafast pulse is expected without group-velocity walk-off or pulse distortion.

Efficient frequency doubling of a femtosecond pulse with simultaneous group-velocity matching and quasi phase matching in periodically poled, MgO-doped lithium niobate

We demonstrate efficient frequency doubling of 95-fs pulses with a small temporal broadening in a 10-mm-long, periodically poled, 5-mol % MgO-doped lithium niobate crystal. Simultaneous group-velocity matching and quasi phase matching were achieved by the off-diagonal nonlinear optical coefficient d32 at telecommunications range.

Subsecond relaxation of internal field after polarization reversal in congruent LiNbO3 and LiTaO3 crystals

We developed an experimental method for measuring subsecond temporal evolution of the internal field after polarization reversal in ferroelectric crystals, congruent LiNbO3 and LiTaO3 crystals. In each crystal the internal field exhibited two distinct relaxations, suggesting that they originate from two different types of defects. The fast one decays in subsecond range while the slow one lasts over days at room temperature. The subsecond relaxation could be fit to a stretched exponential. This method can be applied to other ferroelectric crystals to investigate the properties of defects.

Periodic Poling in 3-mm-Thick MgO:LiNbO3 Crystals

We have demonstrated periodical poling in 3-mm-thick 5 mol% MgO-doped LiNbO3(MgO:LN) crystals. Pulse-to-pulse poling processes in MgO:LN are characterized by an in situ optical monitoring method. Structures with a 30 µm period were fabricated in MgO:LN under an elevated temperature condition of 80°C for the first time.

Electron paramagnetic resonance study of the Eu2+ ion in a PbWO4 single crystal

The electron paramagnetic resonance of the Eu2+ ion in a PbWO4 single crystal, grown by the Czochralski method, has been investigated using an X-band spectrometer. The rotation patterns in the crystallographic planes together with spin-Hamiltonian parameters of Eu2+ show that the local site symmetry of the Eu2+ ion is tetragonal. The spectroscopic splitting tensor g, zero-field splitting parameters Bkq, and hyperfine tensor A are determined with the effective spin Hamiltonian. Energy levels of the ground state for Eu2+ embedded in the PbWO4 crystal are calculated. It turns out that the Eu2+ ion substitutes for the Pb2+ ion without any charge compensation.

Raman Spectrum in Monoclinic BiVO4

A boule of BiVO4 single crystal was grown by Czochralski technique and annealed for 24 hours at 500°C. The Laue and conoscopic image techniques were used to determine the crystallographic axes of BiVO4 single crystal. The Raman spectrum of c(bb) geometry of monoclinic BiVO4 was obtained and the Ag symmetry mode appearing at 56 cm-1 at room temperature was softened as the temperature approached to the ferroelastic-paraelastic phase trasition point (255°C). The disappearing of soft Ag mode at phase transition temperature indicates that it is closely related to the ferroelastic strain which controlles the phase transition, i.e., ferroelastic-paraelastic phase transition in BivO4.

In-situ Visualization Method of Domain Boundary for Stoichiometric Lithium Niobate Crystal

In-situ visualization method of anti-parallel domain boundary in stoichiometric lithium niobate crystal by an optical contrast enhancement technique was demonstrated. Using a collimated plain white light as a probing beam, clear diffraction images of the domain boundary could be obtained due to the phase difference of the incident beam across the boundary induced by the electro-optic effect. The phase shift is proportional to the voltage applied on the sample, and as a consequence the polarity of poled domain was easily distinguished. The experimental results are in a good agreement with the simulation based on Fresnel diffraction theory. This method could be an effective tool for understanding the kinetics of polarization reversal and domain patterning especially in stoichiometric lithium niobate crystals which have low optical contrast at the domain boundary due to the low remnant internal field compared to the congruent one.

Sidewise Domain Wall Velocity of MgO Doped Stoichiometric Lithium Niobate by Real-Time Visualization

A precise measurement of sidewise domain wall velocity of 1 mol% MgO-doped stoichiometric lithium niobate was performed using real-time optical observation in low-field range. Sidewise wall velocity of a single hexagonal shaped domain varied from 0.015 to 4.58 μm/s with the applied electric field ranging from 0.6 to 3.9 kV/mm. An empirical equation, which has an analogy to the current-voltage relation of a diode resistor circuit, was proposed. It is in the good agreement with the measured domain wall velocity data from low to high field range. The coercive field of 0.3 kV/mm was estimated from the suggested equation.

Group-velocity-matched cascaded quadratic nonlinearities of femtosecond pulses in periodically poled MgO:LiNbO3.

Large nonlinear phase shifts were generated with femtosecond pulses at 1560 nm through cascaded quadratic interactions in periodically poled MgO-doped LiNbO3. The off-diagonal component of the nonlinear coefficient was utilized for simultaneous quasi phase matching and group-velocity matching. The effective nonlinear refractive index was varied from -2.9 x 10(-14) to +3.3 x 10(-14) cm2/W by tuning the phase-mismatch conditions.

Domain wall kinetics of lithium niobate single crystals near the hexagonal corner

A mesospheric approach based on a simple microscopic 2D Ising model in a hexagonal lattice plane is proposed to explain macroscopic “asymmetric in-out domain wall motion” observation in the (0001) plane of MgO-doped stoichiometric lithium niobate. Under application of an electric field that was higher than the conventional coercive field (Ec) to the ferroelectric crystal, a natural hexagonal domain was obtained with walls that were parallel to the Y-axis of the crystal. When a fraction of the coercive field of around 0.1Ec is applied in the reverse direction, this hexagonal domain is shrunk (moved inward) from the corner site into a shape with a corner angle of around 150° and 15° wall slopes to the Y-axis. A flipped electric field of 0.15Ec is then applied to recover the natural hexagonal shape, and the 150° corner shape changes into a flat wall with 30° slope (moved outward). The differences in corner domain shapes between inward and outward domain motion were analyzed theoretically in terms of corner a...