arXiv subjects

W. Fang

Publications and source records attributed to W. Fang.

23 records · Page 2Linked to original sources

arXiv2006

Random lasing in weakly scattering systems

We present detailed experimental and numerical studies of random lasing in weakly scattering systems. The interference of scattered light, which is weak in the passive systems, is greatly enhanced in the presence of high gain, providing coherent and resonant feedback for lasing. The lasing modes are confined in the vicinity of the pumped volume due to absorption of emitted light outside it. In the ballistic regime where the size of gain volume is less than the scattering mean free path, lasing oscillation occurs along the direction in which the gain volume is most extended, producing directional laser output. The feedback for lasing originates mainly from backscattering of particles near the boundaries of pumped region. It results in nearly constant frequency spacing of lasing modes, which scales inversely with the maximum dimension of the gain volume.

physics.optics↗

arXiv2006

Coupled Quintessence and Phantom Based On a Dilaton

Based on dilatonic dark energy model, we consider two cases: dilaton field with positive kinetic energy(coupled quintessence) and with negative kinetic energy(phantom). In the two cases, we investigate the existence of attractor solutions which correspond to an equation of state parameter $ω=-1$ and a cosmic density parameter $Ω_σ=1$. We find that the coupled term between matter and dilaton can't affect the existence of attractor solutions. In the Mexican hat potential, the attractor behaviors, the evolution of state parameter $ω$ and cosmic density parameter $Ω$, are shown mathematically. Finally, we show the effect of coupling term on the evolution of $X(\fracσ{σ_0})$ and $Y(\frac{\dotσ}{σ^2_0})$ with respect to $N(lna)$ numerically.

hep-th↗

arXiv2006

Control of lasing in fully chaotic open microcavities by tailoring the shape factor

We demonstrate experimentally that lasing in a semiconductor microstadium can be optimized by controlling its shape. Under spatially uniform optical pumping, the first lasing mode in a GaAs microstadium with large major-to-minor-axis ratio usually corresponds to a high-quality scar mode consisting of several unstable periodic orbits. Interference of waves propagating along the constituent orbits may minimize light leakage at particular major-to-minor-axis ratio. By making stadium of the optimum shape, we are able to maximize the mode quality factor and align the mode frequency to the peak of the gain spectrum, thus minimizing the lasing threshold. This work opens the door to control chaotic microcavity lasers by tailoring the shape factor.

physics.optics↗

arXiv2005

Quantum Cosmology and Dark Energy Model of Born-Infeld Type Scalar Field

In this paper, we consider a quantum model of gravitation interacting with a Born-Infeld(B-I) type scalar field $ϕ$. The corresponding Wheeler-Dewitt equation can be solved analytically for both very large and small cosmological scale factor. In the condition that small cosmological scale factor tend to limit, the wave function of the universe can be obtained by applying the methods developed by Vilenkin, Hartle and Hawking. Both Vilenkin's and Hartle-Hawking's wave function predicts nonzero cosmological constant. The Vilenkin's wave function predicts a universe with a cosmological constant as large as possible, while the Hartle-Hawking's wave function predicts a universe with positive cosmological constant, which equals to $\frac{1}λ$. It is different from Coleman's result that cosmological constant is zero, and also different from Hawking's prediction of zero cosmological constant in quantum cosmology with linear scalar field. We suggest that dark energy in the universe might result from the B-I type scalar field with potential and the universe can undergo a phase of accelerating expansion. The equation of state parameter lies in the range of $-1<$w$<-{1/3}$. When the potential $V(ϕ)=\frac{1}λ$, our Lagrangian describes the Chaplygin gas. In order to give a explanation to the observational results of state parameter w$<-1$, we also investigate the phantom model that posses negative kinetic energy. We find that weak and strong energy conditions are violated for phantom B-I type scalar field. At last, we study a specific potential with the form $V_0(1+\fracϕ{ϕ_0})e^{-(\fracϕ{ϕ_0})}$ in phantom B-I scalar field in detail. The attractor property of the system is shown by numerical analysis.

hep-th↗

arXiv2004

Dark Energy and Dilaton Cosmology

We studied the dilaton cosmology based on Weyl-Scaled induced gravity. The potential of dilaton field is taken as exponential form. An analytical solution of Einstein equation is found. The dilaton can be a candidate for dark energy that can explain the accelerated universe. The structure formation is also considered. We find the the evolutive equation of density perturbation, and its growth is quicker than the one in standard model which is consistent with the constraint from CMBR measurements

hep-th↗