Hsing-Chih Liang

A concise, efficient continuous-wave eye-safe Nd:GdVO₄/KGW Raman laser at 1525 nm is here demonstrated. A Nd:GdVO₄ crystal was used to produce the fundamental field at 1341 nm and a KGW crystal generated the intracavity Stokes field at 1525 nm via wavelength conversion of stimulated Raman scattering. The output power of the Stokes field at 1525 nm could achieve 2.1 W under the pump power of 30 W. Furthermore, two different lithium triborate (LBO) crystals with critical phase matching were exploited to obtain deep-red emission at 714 nm via the intracavity sum frequency generation of 1341 and 1525 nm waves. One cutting angle was in the XY plane and the other was in the XZ plane. The empirical thermo-optical coefficients for the LBO crystal were exploited to systematically analyze the critical phase matching conditions. Numerical results revealed that the type-I phase matching angle in the XY plane was near θ = 90° and ϕ = 3.3° at room temperature, whereas the type-I phase matching angle in the XZ plane was near θ = 86.3° and ϕ = 0° at a temperature around 47 °C. The numerical values for the optimal temperatures for the two different cutting angles were found to be in good agreement with experimental results. At the pump power of 30 W, the output power at 714 nm was approximately 2.9 W by using the LBO crystal with the cutting angle in the XY plane. On the other hand, the maximum output power at 714 nm could be up to 3.2 W under the pump power of 30 W by using the cutting angle in the XZ plane. Furthermore, the linewidth of the SFG emission was confirmed to be nearly the same for the two different cutting angles. The overall linewidth could be narrower than 0.2 nm. The developed laser at 714 nm can be useful in the exploration of ionic and atomic radium isotopes with laser spectroscopy.

The buildup dynamics of diode-pumped passively mode-locked solid-state laser is thoroughly explored using the real-time measurement with temporal sampling rate of up to 40 GHz. A concise cavity is developed to ensure the transient dynamics purely arising from the gain medium and saturable absorber. Experimental results reveal that the laser output in the buildup process exhibits numerous passively Q-switched pulses followed with a damped relaxation oscillation prior to the stable mode locking. Furthermore, it is confirmed that the laser output has already displayed single clean mode-locked pulses inside the first several Q-switched envelopes before stepping into the stage of relaxation oscillation. The present real-time exploration is expected to provide important information for practical applications with temporal modulation of the pump intensity.

The buildup dynamics of diode-pumped passively mode-locked solid-state laser is thoroughly explored by the real-time measurement with temporal sampling rate up to 40 GHz. A concise cavity is developed to ensure the transient dynamics purely arising from the gain medium and saturable absorber. Experimental results reveal that the laser output in the buildup process exhibits numerous passively Q-switched pulses followed with a damped relaxation oscillation prior to the stable mode locking. Furthermore, it is confirmed that the laser output has already displayed single clean mode-locked pulses inside the first several Q-switched envelopes before stepping into the stage of relaxation oscillation. The present real-time exploration is expected to provide important information for practical applications with temporal modulation of the pump intensity.

A novel scheme was developed for a diode-pumped passively Q-switched Nd:YVO₄/KGW Raman laser at 589 nm with a diode-to-orange conversion efficiency reaching 11.4%. The compact near-concentric cavity was designed to achieve the criterion of good passive Q-switching and to contain a coupled resonator for intracavity stimulated Raman scattering (SRS) and second harmonic generation (SHG). The dependence of the output performance on the initial transmission of the saturable absorber was explored in detail. Furthermore, the output performance was studied by considering the influence of the pump-to-mode size ratio. By using an initial transmission of 50%, the highest pulse energy and peak power were 110 μJ and 118 kW, respectively, at a pump duration of 40 μs and a pump frequency of 10 kHz.

A compact high-power continuous-wave (CW) laser at 714 nm is originally developed via intracavity stimulated Raman scattering (SRS) and sum frequency generation (SFG). The fundamental wave at 1342 nm and the first-Stokes Raman wave at 1525 nm are generated by using a Nd:YVO₄ and a undoped YVO₄ crystals, respectively. Compared to the self-Raman laser, the separation of the gain media for generating the fundamental and Raman waves can effectively reduce the thermal lens effect in the Nd:YVO₄ crystal and efficiently enhance the SRS in the undoped YVO₄ crystal. Furthermore, the undoped YVO₄ crystal is coated to act as a high-reflection mirror for minimizing the cavity losses. At a pump power of 40 W, the output power at 714 nm can reach 1.8 W. The present compact design for CW laser source at 714 nm is believed to be practically useful for laser cooling and trapping of radium.

In this study, a simple method for generation of liquid crystal (LC) grating with crystal/quasicrystal/kaleidoscopic patterns via photoalignment is proposed. The crystal/quasicrystal/kaleidoscopic patterns are created using multiple-beam interference when the recording beam is divided into multiple beams by passing a multiple-aperture mask and focused by a lens. Two-dimensional (2D) patterns are recorded on LC cells based on the laser-induced adsorption of azo dyes that guide the orientation of the LC molecules. Measurements indicate that the 2D LC gratings with crystal/quasicrystal patterns are electro-optically tunable and possess polarisation-independent diffraction efficiency.

In this work, the spatial effects of the pumping and lasing were considered into the coupled rate equations of passively Q-switched lasers for the condition of no saturation in the gain medium. A transcendental equation of energy utilization was derived, and the solution to the equation was approximated by an analytic model developed in previous work. The reflection factor of output coupler with low reflectivity was modified for more accurate output energy evaluations. Experimentally, an end-pumped Nd:YVO₄/Cr⁴⁺:YAG laser in a concave-plano cavity with output coupling reflectivity ranging from 10% to 92% was demonstrated, and two different pumping spot sizes of laser diode were adopted for validating the developed spatial model. The experimental results showed good agreements with the theoretical calculations.

A truncated basis is employed to analyze the influence of the point interaction on the eigenvalues and eigenfunctions in quantum billiards. The point interaction is numerically confirmed to cause the shift in the eigenvalue that leads the original eigenfunctions to be superposed to form the new eigenfunction. The amplitude and nodal-line patterns of the eigenfunctions are found to vary significantly with the coupling strength. It is further verified that the point-driven Chladni plates can be employed to analogously manifest the noticeable dependence of the nodal-line pattern on the coupling strength. More importantly, the dispersion relation between the frequency and the wave number for the flexural wave can be precisely determined in the process of utilizing the point interaction to model the modern Chladni plates.

We theoretically demonstrate that a family of vortex-lattice structures can be flexibly generated using a multi-beam interference approach. Numerical calculation presents a variety of crystalline and kaleidoscopic patterns. Based on the numerical analysis, we experimentally realized these structure beams by combining an amplitude mask with multiple apertures and a spiral phase plate. The excellent agreement between the experimental and theoretical results not only validates the presented method, but also manifests the structure of vortex lattices.

This study demonstrates a simple approach for controlling the pretilt angles of liquid crystals (LCs) using gelator-doped cells fabricated with homogeneously aligned substrates. The competition between the gelator-induced spontaneous vertical alignment and planar alignment layer coated on the substrates results in various pretilt angles of LCs. The LC pretilt angle can thus be controlled from a homeotropic to planar state by adjusting the anchoring energy of the planar alignment layer and gelator concentration used for doping. A no-bias pi cell with multistable property is demonstrated by utilizing the proposed method. Graphical Abstract