Y. T. Yu

Efficient continuous-wave self-Raman Yb:KGW laser with a shift of 89 cm −1

We demonstrated a continuous-wave (CW) self-Raman laser with high conversion efficiency by using Yb:KGW as the Raman crystal. The first Stokes line of wavelength centered at 1095.2 nm with spectral bandwidth of 8 nm and the cascaded Raman conversion wavelength at 1109.5 nm with spectral bandwidth of 3.4 nm were observed with a Raman shift of 89 cm⁻¹ with respect to the fundamental laser wavelength at 1085.0 nm with spectral bandwidth of 10 nm. The CW Raman output power of 1.7 W was attained under the diode pump power of 7.8 W which corresponds to the slope efficiency and the diode-to-Stokes optical conversion efficiency of 26.6% and 21.8%, respectively.

Exploration of water jet generated by Q-switched laser induced water breakdown with different depths beneath a flat free surface.

The dynamics of a water jet on a flat free surface are investigated using a nanosecond pulsed laser for creating an oscillating bubble with different depths beneath the free surface. A thin jet is shown to deform a crater surface resulted from surface depression and cause a circular ring-shaped crater on the connection surface between the crater of surface depression and the thin jet. The collapse of this circular ring-shaped crater is proposed to the crown-like formation around a thick jet. The evolution of the bubble depth suggests a classification of four distinctive ranges of the bubble depths: non-crown formation when the parameter of bubble depth over the maximum bubble radius γ ≤ 0.5, unstable crown formation when 0.5 ≤ γ ≤ 0.6, crown-like structure with a complete crown wall when 0.6 ≤ γ ≤ 1.1, and non-crown formation when 1.1 ≤ γ. Furthermore, the orientation of the crown wall gradually turns counterclockwise to vertical direction with increasing γ from 0.5 to 1.1, implying a high correlation between the orientation of the crown wall and the depth of the bubble. This correlation is explained and discussed by the directional change of the jet eruption from the collapse of circular ring-shaped crater.

Exploring the influence of boundary shapes on emission angular distributions and polarization states of broad-area vertical-cavity surface-emitting lasers

We design the stadium-shaped and rectangular vertical-cavity surface-emitting lasers (VCSELs) to investigate the influence of boundary shapes on the emission angular distributions and polarization states. For the stadium-shaped VCSELs, the emission angular distribution prefers to be almost omnidirectional because the lasing mode with purely scarred structure is seldom to be excited. On the contrary, the rectangular VCSELs usually generate dominant lasing modes with the morphology of quasi-periodic linear ridges, which can make emission angular distribution to be concentrated on the certain direction. From the polarization-resolved light-current curves, the stadium-shaped VCSEL is quite prone to exhibit numerous abrupt changes (kinks) associated with polarization switching with increasing current, whereas for rectangular VCSEL there is no conspicuous kink to be seen during a wide range of current changing from near to far above lasing threshold.

Extracting photon periodic orbits from spontaneous emission spectra in laterally confined vertically emitted cavities.

We report our observation of the signature of photon periodic orbits in the spontaneous emission spectra of large-aperture vertical-cavity surface-emitting lasers (VCSELs). The high-resolution measurement clearly demonstrates that over a thousand cavity modes with a narrow linewidth can be perfectly exhibited in the spontaneous emission spectrum just below the lasing threshold. The Fourier-transformed spectrum is analyzed to confirm that the spontaneous emission spectra of large-aperture VCSELs can be exploited to analogously investigate the energy spectra of the 2D quantum billiards.

Exploring morphological variations of a laser-induced water jet in temporal evolution: formation of an air bubble enclosing a water drop

We explore the spatio-temporal dynamics of a water jet that is generated by laser-induced water breakdown beneath a flat free surface. We find that morphological variations in the temporal evolution can be divided into three categories depending on the depth parameter ?, which is the ratio of the water-breakdown depth to the maximum bubble radius. For a depth parameter in the range 0.8???????1.03, we observe an intriguing pattern formation in which an air bubble perfectly encloses a water drop through the process of the Plateau?Rayleigh instability.

Extracting trajectory equations of classical periodic orbits from the quantum eigenmodes in two-dimensional integrable billiards

The trajectory equations for classical periodic orbits in the equilateral-triangular and circular billiards are systematically extracted from quantum stationary coherent states. The relationship between the phase factors of quantum stationary coherent states and the initial positions of classical periodic orbits is analytically derived. In addition, the stationary coherent states with noncoprime parametric numbers are shown to correspond to the multiple periodic orbits, which cannot be explicable in the one-particle picture. The stationary coherent states are further verified to be linked to the resonant modes that are generally observed in the experimental wave system excited by a localized and unidirectional source. The excellent agreement between the resonant modes and the stationary coherent states not only manifests the importance of classical features in experimental systems but also paves the way to manipulate the mesoscopic wave functions localized on the periodic orbits for applications.

Characterizing classical periodic orbits from quantum Green's functions in two-dimensional integrable systems: Harmonic oscillators and quantum billiards

A general method is developed to characterize the family of classical periodic orbits from the quantum Greens function for the two-dimensional (2D) integrable systems. A decomposing formula related to the beta function is derived to link the quantum Greens function with the individual classical periodic orbits. The practicality of the developed formula is demonstrated by numerically analyzing the 2D commensurate harmonic oscillators and integrable quantum billiards. Numerical analyses reveal that the emergence of the classical features in quantum Greens functions principally comes from the superposition of the degenerate states for 2D harmonic oscillators. On the other hand, the damping factor in quantum Greens functions plays a critical role to display the classical features in mesoscopic regime for integrable quantum billiards, where the physical function of the damping factor is to lead to the coherent superposition of the nearly degenerate eigenstates.

Exploiting broad-area surface emitting lasers to manifest the path-length distributions of finite-potential quantum billiards.

Broad-area vertical-cavity surface-emitting lasers (VCSELs) with different cavity sizes are experimentally exploited to manifest the influence of the finite confinement strength on the path-length distribution of quantum billiards. The subthreshold emission spectra of VCSELs are measured to obtain the path-length distributions by using the Fourier transform. It is verified that the number of the resonant peaks in the path-length distribution decreases with decreasing the confinement strength. Theoretical analyses for finite-potential quantum billiards are numerically performed to confirm that the mesoscopic phenomena of quantum billiards with finite confinement strength can be analogously revealed by using broad-area VCSELs.

The Influences of Boundary Shapes on Polarization Characteristics and Lasing Modes in Broad-Area Vertical-Cavity Surface-Emitting Lasers With Cryogenic Detuning: Regular Versus Chaotic Cavities

The broad-area rectangular-shaped and stadium-shaped VCSELs with nearly the same aspect ratio are designed to explore the influence of lateral boundary shapes on polarization states and lasing spectra with large detuning by cryogenic cooling. For the rectangular-shaped VCSEL, the lasing modes usually exhibit linearly polarized state and their far-field emissions mainly concentrate on four diagonal directions. On the other hand, the lasing modes of the stadium-shaped VCSEL generally display two orthogonally polarized states and their far-field emissions extensively spread on azimuthal directions. More intriguingly, the lasing spectra of the two devices clearly manifest the features of the energy-level distribution in regular and chaotic quantum billiards, respectively.

Investigation on polarization features of broad-area square-shaped VCSELs with different frequency detuning: High-order modes assisted in stable polarization emission

We experimentally confirm frequency detuning is the crucial factor for being capable of polarization switching in broad-area square-shaped VCSELs. Intriguingly, the VCSEL can exhibit stable polarization in light output for relatively large frequency detuning.