Relaxation oscillations, stability, and cavity feedback in a superradiant Raman laser

We experimentally study the relaxation oscillations and amplitude stability properties of an optical laser operating deep into the bad-cavity regime using a laser-cooled $^{87}$Rb Raman laser. By combining measurements of the laser light field with nondemolition measurements of the atomic populations, we infer the response of the gain medium represented by a collective atomic Bloch vector. The results are qualitatively explained with a simple model. Measurements and theory are extended to include the effect of intermediate repumping states on the closed-loop stability of the oscillator and the role of cavity feedback on stabilizing or enhancing relaxation oscillations. This experimental study of the stability of an optical laser operating deep into the bad-cavity regime will guide future development of superradiant lasers with ultranarrow linewidths.Comment: 9 pages, 6 figure

Pressure Effects in Supercooled Water: Comparison between a 2D Model of Water and Experiments for Surface Water on a Protein

Experiments in bulk water confirm the existence of two local arrangements of water molecules with different densities, but, because of inevitable freezing at low temperature $T$, can not ascertain whether the two arrangements separate in two phases. To avoid the freezing, new experiments measure the dynamics of water at low $T$ on the surface of proteins, finding a crossover from a non-Arrhenius regime at high $T$ to a regime that is approximately Arrhenius at low $T$. Motivated by these experiments, Kumar et al. [Phys. Rev. Lett. 100, 105701 (2008)] investigated, by Monte Carlo simulations and mean field calculations, the relation of the dynamic crossover with the coexistence of two liquid phases in a cell model for water and predict that: (i) the dynamic crossover is isochronic, i.e. the value of the crossover time $\tau_{\rm L}$ is approximately independent of pressure $P$; (ii) the Arrhenius activation energy $E_{\rm A}(P)$ of the low-$T$ regime decreases upon increasing $P$; (iii) the temperature $T^*(P)$ at which $\tau$ reaches a fixed macroscopic time $\tau^*\geq \tau_{\rm L}$ decreases upon increasing $P$; in particular, this is true also for the crossover temperature $T_{\rm L}(P)$ at which $\tau=\tau_{\rm L}$. Here, we compare these predictions with recent quasi elastic neutron scattering (QENS) experiments performed by X.-Q. Chu {\it et al.} on hydrated proteins at different values of $P$. We find that the experiments are consistent with these three predictions.Comment: 18 pages, 5 figures, to appear on J. Phys.: Cond. Ma

Modules in the photoreceptor RGS9-1•Gβ5L GTPase-accelerating protein complex control effector coupling, GTPase acceleration, protein folding, and stability

RGS (regulators of G protein signaling proteins regulate G protein signaling by accelerating GTP hydrolysis, but little is known about regulation of GTPase-accelerating protein (GAP) activities or roles of domains and subunits outside the catalytic cores. RGS9-1 is the GAP required for rapid recovery of light responses in vertebrate photoreceptors and the only mammalian RGS protein with a defined physiological function. It belongs to an RGS subfamily whose members have multiple domains, including G gamma -like domains that bind G(beta5) proteins. Members of this subfamily play important roles in neuronal signaling, Within the GAP complex organized around the RGS domain of RGS9-1, we have identified a functional role for the G gamma -like-G(beta 5L) complex in regulation of GAP activity by an effector subunit, cGMP phosphodiesterase gamma and in protein folding and stability of RGS9-1, The C-terminal domain of RGS9-1 also plays a major role in conferring effector stimulation. The sequence of the RGS domain determines whether the sign of the effector effect will be positive or negative. These roles were observed in, vitro using full-length proteins or fragments for RGS9-1, RGS7, G(beta 5S), and G(beta 5s), The dependence of RGS9-1 on Gp, co-expression for folding, stability, and function has been confirmed in vivo using transgenic Xenopus laevis, These results reveal how multiple domains and regulatory polypeptides work together to fine tune G(t alpha) inactivation