Showing papers by "Tsing-Hua Her published in 2015"

Femtosecond laser ablation of polymethylmethacrylate via dual-color synthesized waveform

Abstract: We have demonstrated the laser ablation of PMMA using dual-color waveform synthesis of the fundamental (ω) and its second-harmonic (2ω) of a femtosecond Ti:Sapphire laser. A modest and yet clear modulation (∼22%) in ablated area versus relative phase between the 2ω and ω beams with a power-ratio of 15% (28/183 mW) is revealed. This is explained qualitatively by the dependence of ablation on multiphoton ionization of which the rate is related to the relative phase of the synthesized waveform. At higher peak power ratios, the modulation decreases rapidly, as the two-photon-ionization rate of the 2ω dominates over that of the three- to four- photon ionization of the ω beam. This technique demonstrates the feasibility of phase-controlled laser processing of materials.

Transverse mode competition in index-antiguided waveguide lasers

Abstract: Transverse mode competition in large-mode-area index-antiguided planar waveguide lasers is investigated. We show that, with very large core width and/or index difference between the core and cladding, high-order modes can oscillate and contribute to output lasing power. We have developed a theoretical model for transverse mode competition that takes into account transverse spatial hole burning. Experiments conducted on index-antiguided waveguide lasers with core width of 220 μm and 400 μm are compared to theory with good agreement.

Continuous-wave hybrid index-antiguided and thermal- guided planar waveguide laser with large mode area

Abstract: We present lasing experiments in index-antiguided (IAG) planar waveguides made of Nd:YAG (refractive index = 1.820) sandwiched by Terbium Gallium Garnet (refractive index = 1.954). For a core width of 220 μm, we observe fundamental mode oscillation with maximal 2.62 W output power and 0.109 slope efficiency. An analytical model is developed taking into account simultaneous index-antiguiding, gain guiding and thermal induced refractive focusing. For a core width of 400 μm, fundamental mode oscillation is only observed near the lasing threshold, suggesting that the maximally core width in IAG waveguides for fundamental mode operation depends on the strength of index antiguiding.

Transverse mode competition in index-antiguided planar waveguide lasers with large mode area

Abstract: We study transverse mode competition in index-antiguided planar lasers theoretically and experimentally. Robust fundamental mode operating 10 times above lasing threshold is predicted and confirmed experimentally.

Overcoming Losses in Metamaterials

Abstract: Periodic loading of waveguides was introduced in the 50's and Man-made Superlattices in the 70's, which have metamorphosed into metamaterials. Now, we have metamaterials, combining the two features into something truly phenomenal mainly because the advent of nanotechnology allowing the concept of circuit elements to be applied where resonances may be designed in terms of shapes and sizes. However, since the present emphasis involves applications in IR and visible light, the principal new features of metamaterials require forms and symmetry in sizes needing techniques developed in nanotechnology, i.e. involving e-beam and atomic beam lithography. There are fundamental limitations imposed by finite electronic mean-free-paths on conductivity, and short mean free paths exist for phonons from nonlinear potentials. Not much can be done with the phonon scattering, but electron scattering may be reduced. In addition to the use of surface plasmons, the interaction volume may be reduced to the nanometer domain with nanotechnology, inspired by the realization that a single atom may be involved in efficient interactions with light. To deal with phase de-coherence, it is emphasized that the size of the individual elements forming the metamaterials should be below the phase coherence length to avoid de-coherence effects in the metamaterials excited by the incident light.

Control of absorption in femtosecond laser-dielectric interaction with the polarization of a seeding pulse

Abstract: We investigate femtosecond pulse transmission through BK7 in the presence of its second harmonic as a seeding pulse. Enhanced absorption is observed when the pulses are co-polarized. Such phenomenon can be used to control absorption.