Nan Ei Yu

Thermal inhibition of high-power second-harmonic generation in periodically poled LiNbO3 and LiTaO3 crystals

A coupled thermo-optical model shows strong thermal inhibition of second-harmonic generation (SHG) in periodically poled (PP) LiNbO3 (PPLN) and stoichiometric LiTaO3 (PPSLT) crystals. Three-dimensional simulations performed for a 1.064μm fundamental wavelength pulsed nanosecond laser beam 150μm in radius show the onset of significant temperature nonuniformities along and across the irradiated zone, and strong thermal dephasing and inhibition of SHG in these crystals for input powers >10W. PPSLT is found to have a significant advantage over PPLN due to the higher heat conductance, decreasing these temperature nonuniformities and allowing one to maintain the irradiated zone within the corresponding SHG temperature tolerance range (ΔT≈3K).

Stable High-Power Green Light Generation with Thermally Conductive Periodically Poled Stoichiometric Lithium Tantalate

Periodically poled stoichiometric lithium tantalate was used for an efficient green light generation based on frequency doubling of a pulsed Nd:YVO4 laser. The achieved average peak power density of 9.2 MW/cm2 was sufficiently stable without spatial distortion of green light at 532 nm at room temperature (33°C). A small phase matching temperature adjustment (0.5°C) was required at the maximum average output power of 4.4 W owing to its high thermal conductivity.

Efficient optical parametric oscillation based on periodically poled 1.0 mol % MgO-doped stoichiometric LiTaO3

Effective optical parametric oscillation (OPO) at room temperature was demonstrated by a periodically poled 1.0 mol% MgO-doped stoichiometric lithium tantalate (Mg:SLT) crystal. The slope conversion efficiency of total output power was ∼65% and the maximum output power was 655 mW at 1.3 W pumping without photorefraction. The difference of the OPO performance was investigated between 0.5 and 1.0 mol % Mg:SLT crystals, and a 1.0 mol % MgO-doped device exhibited better OPO performance and high resistance of photorefraction. The effective nonlinear coefficient of deff was found to be 10pm∕V by single-pass first-order quasiphase matched second-harmonic generation at 1064 nm.

Periodically poled near-stoichiometric lithium tantalate for optical parametric oscillation

A 35-mm-long periodically poled device was fabricated from a near-stoichiometric lithium tantalate, and optical parametric oscillation was demonstrated with a low oscillation threshold (106 mW). The emitted signal and idler wavelengths were 1.53–1.63 and 3.06–3.49 μm, respectively, and the highest slope conversion efficiency was 65% with a maximum output power of 628 mW at 1.1 W pumping. The influence of Curie temperature inhomogeneity on fabrication of the device was investigated to improve poling quality in long devices.

Optical pulse compression using cascaded quadratic nonlinearities in periodically poled lithium niobate

Efficient pulse compression was achieved by the use of cascaded quadratic nonlinearities in 10-mm-long periodically poled MgO-doped lithium niobate. An off-diagonal component of the nonlinear coefficient was utilized for group-velocity matched cascaded interactions at the fundamental wavelength of 1560 nm. Simultaneously compressed fundamental and second-harmonic pulses of ∼35 fs duration were obtained from the 110-fs-pump pulses.

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.

Nanosecond pulsed laser energy and thermal field evolution during second harmonic generation in periodically poled LiNbO3 crystals

Computational study of nanosecond pulse laser radiation in periodically poled LiNbO3 (PPLN) crystals reveals the complex spatio-temporal evolution of the 1.064μm fundamental harmonic (FH) and second harmonic (SH) energy fields with associated temperature fields, leading to the thermal dephasing and inhibition of second harmonic generation (SHG). The investigated range of the laser input power is W0=0.5–50W (with the pulse energy Q0=0.01–1mJ∕pulse and repetition rate of 50kHz). For input laser powers W0>10W the FH and SH energy fields are found to strongly couple with nonuniform temperature field, leading to significant thermal dephasing and SHG efficiency loss. Heat generation and temperature distributions also exhibit very significant nonuniformities along and across the laser beam, maximizing at the rear or inside the crystal, depending on the input power. However, conformal temperature tuning along the operating crystal inhibits these nonuniformities, and significantly enhances SHG efficiency under hig...

70-mm-long periodically poled Mg-doped stoichiometric LiNbO3 devices for nanosecond optical parametric generation

A low-threshold, nanosecond, optical parametric generator (OPG) was produced by using a 70-mm-long, periodically poled, Mg-doped, stoichiometric LiNbO3 (PPMgSLN). The periodically poled structure was obtained by applying an electric field of 1.8kV∕mm, which is lower than the coercive field of 3.9kV∕mm. The PPMgSLN OPG pumped with a 6ns, 6.8kHz, Nd:YAG microchip laser had a generation threshold of 12MW∕cm2 and a slope efficiency of 37% of signal at 1.61μm.

Morphological and SERS Properties of Silver Nanorod Array Films Fabricated by Oblique Thermal Evaporation at Various Substrate Temperatures

Aligned silver nanorod (AgNR) array films were fabricated by oblique thermal evaporation. The substrate temperature during evaporation was varied from 10 to 100 °C using a home-built water cooling system. Deposition angle and substrate temperature were found to be the most important parameters for the morphology of fabricated films. Especially, it was found that there exists a critical temperature at ~90 °C for the formation of the AgNR array. The highest enhancement factor of the surface-enhanced Raman scattering (SERS), observed in the Ag films coated with benzenethiol monolayer, was ~6 × 107. Hot spots, excited in narrow gaps between nanorods, were attributed to the huge enhancement factor by our finite-difference time-domain (FDTD) simulation reflecting the real morphology.

Backward Terahertz Generation in Periodically Poled Lithium Niobate Crystal via Difference Frequency Generation

We have investigated the terahertz radiation generated by femtosecond laser pulses in a periodically poled lithium niobate crystal. Two different types of terahertz radiation, forward and backward, could be generated in quasi-phase-matching structures. The generation of these two different types of radiation is due to difference frequency generation under the quasi-phase matching condition. A broadband femtosecond optical pulse contributes to wave mixing, which occurs between two components of an ultrashort laser pulse and a terahertz pulse, in a quasi-phase-matching crystal. We observed a backward terahertz radiation at approximately 1.3 THz and the frequencies of terahertz radiations are easily controlled by varying the domain structures.