Browsing by Author "Lai, Y. H."

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Characterization and generation of high-power multi-axis vortex beams by using off-axis pumped degenerate cavities with external astigmatic mode converter
(2018-08-06)
Tuan, P. H.
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Hsieh, Y. H.
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Lai, Y. H.
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Huang, K. F.
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Chen, Y. F.
The generalized geometric mode with several high-order Hermite-Gaussian (HG) beams localized on ray periodic orbits in the degenerate resonator is generated by an off-axis pumped Nd: YVO4 laser, by performing beam transformation via an astigmatic mode converter, the generalized geometric modes are found that can be converted into the multi-axis vortex beams with the bundled-rings structures. Experimental results reveal that the generated multi-axis vortex beams can preserve quite stable beam structures even under high-power operation. Moreover, the radius of the bundled rings for the multi-axis vortex beams can be flexibly adjusted by the off-axis pumping to lead to vortex structures with easily controlled orbital angular momentum distribution. The good agreement between the experimental and theoretical results of propagation evolution for the astigmatic transformation of generalized geometric modes further verify the feasibility of using the proposed system to realize various high-powered, multi-center vortex beams with good reliability and predictability. (c) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Characterizing the topological charges distribution of the elliptical beams with vortex lattices
(2020-01-01)
Hsieh, Y. H.
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Lai, Y. H.
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Hsieh, M. X.
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Chen, Y. F.
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電子物理學系
A new numerical approach is proposed to analyze the topological charges distribution of elliptical beams with vortex lattices generated from the astigmatic transformations of Hermite-Gaussian beams. The birth and death of the topological charges for elliptical beams are thoroughly verified by continuously changing the astigmatic strength. The theoretical explorations are exploited to make a complete comparison with experimental measurements which are obtained from an off-axis pumped solid-state laser with an external astigmatic mode converter and a Mach-Zehnder interferometer. The good agreement between the theoretical and experimental results provides the feasibility to flexibly manipulate the topological charges of vortex lattices in further applications. (C) 2019 Optical Society of America
Electronic structure of a two-dimensional graphene monolayer in a spatially modulated magnetic field: Peierls tight-binding model
(2008-01-01)
Chiu, Y. H.
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Lai, Y. H.
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Ho, J. H.
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Chuu, D. S.
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Lin, M. F.
Magnetoelectronic properties of a two-dimensional (2D) monolayer graphene are investigated by the Peierls tight-binding model. They are dominated by the period, strength, and direction of a spatially modulated magnetic field. Such a field could induce the reduction in dimensionality, change of energy dispersions, anisotropy at low energy, composite behavior in state degeneracy, extra band-edge states, and asymmetry of energy bands. There are partial flatbands at the Fermi level and one-dimensional parabolic bands at others. These make density of states exhibit delta-function-like structure and asymmetric prominent peaks, respectively. Energies of the extra band-edge states strongly depend on the period, while those of the original band-edge states exhibit little dependence. Both of them grow as the strength increases. The modulated and uniform magnetic fields differ from each other in energy dispersion, state degeneracy, and dimensionality. Important differences between a monolayer graphene and a 2D electron gas also exist.
Experimental and theoretical explorations for optimizing high-power geometric modes in diode-pumped solid-state lasers
(2018-07-01)
Hsieh, Y. H.
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Lai, Y. H.
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Tsou, C. H.
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Liang, H. C.
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Huang, K. F.
The variation of transverse patterns on the cavity length is experimentally explored by using an off-axis pumped Nd:YVO4 laser to identify the range of the length for generating geometric modes (GMs). Using the inhomogeneous Helmholtz equation, it is theoretically confirmed that the stable range of the cavity length for generating GMs can be significantly enlarged by decreasing the output reflectance. Furthermore, experimental results reveal that the cavity length for scaling up the output power of GMs must be shortened due to the thermal lensing effect in the gain medium. Considering the balance between the output efficiency and the stable range of the cavity length, the optimal output reflectance is verified to be approximately 80%. With the optimal output reflectance, the average output power of GMs can be up to 6.33 W at a pump power of 12 W.
Exploring elliptical vortex beams with the spatial damping effect
(2019-02-15)
Chen, Y. F.
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Hsieh, M. X.
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Lai, Y. H.
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Hsieh, Y. H.
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Tu, C. W.
A general integral formula is analytically derived to represent the wave function of elliptical vortex modes. The derived formula can be straightforwardly employed to take account of spatial damping to explore vortex structure. Spatial damping is found to cause the central degenerate singularity of the circular vortex mode to be split into several singularities with different topological charges. For the non-circular vortex beam, the influence of spatial damping on the distribution of singularities is not significant. Theoretical analyses are confirmed to agree very well with the experimental measurements. (C) 2019 Optical Society of America
Generating high-power asymmetrical Laguerre-Gaussian modes and exploring topological charges distribution
(2018-11-26)
Hsieh, Y. H.
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Lai, Y. H.
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Hsieh, M. X.
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Huang, K. F.
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Chen, Y. F.
We employ an off-axis pumped Nd:YVO4 laser and control the reflectance of output coupler to directly generate asymmetrical Hermite-Gaussian (HG) modes with various transverse orders. By using an astigmatic mode converter, the generated asymmetrical HG modes are straightforwardly transformed into asymmetrical Laguerre-Gaussian (LG) modes with a crescent-like shape. The average output power of all the crescent-shaped LG modes can exceed 1W at the pump power of 4W. Furthermore, experimental results are theoretically verified by resonant modes derived from the inhomogeneous Helmholtz equation with the localized source distribution. Theoretical resonant modes are also used to explore the dependence of the phase structures of LG modes on the system loss. As the loss increases, it is found that the singularities at the origin will be rearranged and new singularities are formed in the outside region with average orbital angular momentum remaining unchanged. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Integral-based parallel algorithm for the fast generation of the Zernike polynomials
(2020-01-20)
Hsieh, Y. H.
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Yu, Y. T.
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Lai, Y. H.
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Hsieh, M. X.
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Chen, Y. F.
The integral representation of the Zernike radial functions is well approximated by applying the Riemann sums with a surprisingly rapid convergence. The errors of the Riemann sums are found to averagely be not exceed 3 x 10(-14), 3.3 x 10(-14), and 1.8 x 10(-13) for the radial order up to 30, 50, and 100, respectively. Moreover, a parallel algorithm based on the Riemann sums is proposed to directly generate a set of radial functions. With the aid of the graphics processing units (GPUs), the algorithm shows an acceleration ratio up to 200-fold over the traditional CPU computation. The fast generation for a set of Zernike radial polynomials is expected to be valuable in further applications, such as the aberration analysis and the pattern recognition. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Magnetic energy bands of a 2D graphite layer in the spatially modulated magnetic field
(2008-04-01)
Chiu, Y. H.
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Lai, Y. H.
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Ho, J. H.
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Chuu, D. S.
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Lin, M. F.
A 2D graphite layer exists in a spatially modulated magnetic field. The low magnetic energy bands, which are calculated from the Peierls tight-binding model, strongly depend on the period and strength of a modulated magnetic field. There are a lot of I D parabolic bands. Such bands exhibit the double degeneracy, strong energy dispersions, and extra band-edge states. Density of states owns many asymmetric square-root divergent structures, mainly owing to the band-edge states in 1D parabolic bands. The band-edge state energies decrease with the increasing field period, while the opposite is true for the dependence on the field strength. That a modulated magnetic field reduces dimensionality and thus drastically changes electronic properties could be examined by the experimental measurements. (c) 2007 Elsevier B.V. All rights reserved.
Mechanically controllable nonlinear dielectrics
(2020-03-01)
Ko, D. L.
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Tsai, M. F.
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Chen, J. W.
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Shao, P. W.
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Tan, Y. Z.
Strain-sensitive BaxSr1-xTiO3 perovskite systems are widely used because of their superior nonlinear dielectric behaviors. In this research, new heterostructures including paraelectric Ba0.5Sr0.5TiO3 (BSTO) and ferroelectric BaTiO3 (BTO) materials were epitaxially fabricated on flexible muscovite substrate. Through simple bending, the application of mechanical force can regulate the dielectric constant of BSTO from -77 to 36% and the channel current of BTO-based ferroelectric field effect transistor by two orders. The detailed mechanism was studied through the exploration of phase transition and determination of band structure. In addition, the phase-field simulations were implemented to provide theoretical support. This research opens a new avenue for mechanically controllable components based on high-quality oxide heteroepitaxy.
Point-driven modern Chladni figures with symmetry breaking
(2018-07-18)
Tuan, P. H.
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Lai, Y. H.
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Wen, C. P.
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Huang, K. F.
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Chen, Y. F.
Point-driven modern Chladni figures subject to the symmetry breaking are systematically unveiled by developing a theoretical model and making experimental confirmation in the orthotropic brass. The plates with square shape are employed in the exploration based on the property that the orientation-dependent elastic anisotropy can be controlled by cutting the sides with a rotation angle with respect to the characteristic axes of the brass. Experimental results reveal that the orientation symmetry breaking not only causes the redistribution of resonant frequencies but also induces more resonant modes. More intriguingly, the driving position in some of new resonant modes can turn into the nodal point, whereas this position is always the anti-node in the isotropic case. The theoretical model is analytically developed by including a dimensionless parameter to consider the orientation symmetry-breaking effect in a generalized way. It is numerically verified that all experimental resonant frequencies and Chladni patterns can be well reconstructed with the developed model. The good agreement between theoretical calculations and experimental observations confirms the feasibility of using the developed model to analyze the modern Chladni experiment with orientation symmetry breaking. The developed model is believed to offer a powerful tool to build important database of plate resonant modes for the applications of controlling collective motions of micro objects.
Wave representation for asymmetric elliptic vortex beams generated from the astigmatic mode converter
(2019-04-15)
Chen, Y. F.
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Lai, Y. H.
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Hsieh, M. X.
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Hsieh, Y. H.
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Tu, C. W.
An analytical wave representation is derived to express the asymmetric elliptic vortex beams generated by using an astigmatic mode converter to transform the asymmetric Hermite-Gaussian modes. The effect of the spatial asymmetry on the elliptic vortex beam can be straight forwardly considered in the analytical wave representation. A detailed comparison with experimental intensity patterns and phase structures is made. The good agreement between experimental results and theoretical analyses confirms the characterization and generation of the asymmetric elliptic vortex beams. (C) 2019 Optical Society of America