Search

The ability to manipulate cavity resonant modes is of critical importance in laser physics and applications. By exploiting the parity time (PT) symmetry, we propose and experimentally realize a single-mode FP laser with improved output power and high-speed modulation have been demonstrated. The proposed PT symmetric laser consists of two coupled structurally identical FP resonators. The gain and loss in two FP resonators can be manipulated independently by changing the injection currents. In the PT symmetric FP laser, single-mode operation is accomplished by selectively breaking of PT symmetry depending solely on the relation between gain-loss and coupling. Single-mode lasing with output power of 1.7 dBm and a sidemode suppression ratio (SMSR) exceeding 24 dB is demonstrated. The 3 dB bandwidth of 7.9 GHz is achieved and clear eye-openings were obtained for 2.5 Gbps and 10Gbps NRZ operation over 10 km single-mode fibers. Furthermore, the PT symmetry breaking is experimentally confirmed with measured loss and coupling coefficient of two FP resonators. The influence of cavity length, facet reflectivity, and electrical isolation between two P-side electrodes on the side mode suppression ratio and output optical power is also been demonstrated, paving the way for further improvement of the PT symmetric FP laser., Comment: 8 pages, 6 figures

The dynamic quadrupole Love number of Titan measured by \Cassini is $k_\mathrm{2,obs}=0.616\pm 0.067$, strongly indicating a global subsurface ocean. However, the theoretical Love number due to equilibrium tides is at most $k_\mathrm{2,eq}^\mathrm{max}\approx 0.48$ in the absence of an ice shell on top of the ocean. In reality, there is an outer ice shell of thickness $ 100\,\mathrm{km}$, reducing the equilibrium-tide Love number to $k_\mathrm{2,eq}\approx 0.42$. Therefore, other types of tidal response, like dynamic tides, may be also present in Titan. We propose that the ocean is stably stratified. As a result, there exist standing ocean waves (gravity modes) with eigen-frequencies close to the tidal frequency. Such a gravity mode (g-mode) is resonantly excited. It bends the outer ice shell radially and thus enhances the dynamic Love number by $k_\mathrm{2,g}$. In order for $k_\mathrm{2,g}$ to account for the discrepancy between $k_\mathrm{2,eq}$ and $k_\mathrm{2,obs}$, the Brunt-Vaisala frequency in the ocean is required to be $3.3\times 10^{-4}\,\mathrm{rad\, s^{-1}}$. It is compatible with the volatile-rich model for Titan that was proposed to explain the methane-rich atmosphere. The three components of the tidal potential with azimuthal degrees, $m=-2,0,2$, correspond to the three components of the quadrupole Love number, $k_\mathrm{2,-2}$, $k_\mathrm{2,0}$ and $k_\mathrm{2,2}$. They can excite retrograde, axisymmetric and prograde g-modes equally in the absence of rotation. However, Coriolis force induced by Titan's rotation breaks the symmetry among these modes. Most likely, only one of the Love-number components is significantly enhanced by a g-mode, while the other two are still attributed to equilibrium tides. This prediction is testable by observation. If confirmed, the smaller components of the Love number can be used to constrain the thickness of the outer ice shell., Comment: Comments are welcome. Submit to Icarus

As established by ground based surveys, white dwarfs with hydrogen atmospheres pulsate as they cool across the temperature range, $12500\,\mathrm{K} \gtrsim T_{\mathrm{eff}} \gtrsim 10800\,\mathrm{K}$. Known as DAVs or ZZ Ceti stars, their oscillations are attributed to overstable g-modes excited by convective driving. The effective temperature at the blue edge of the instability strip is slightly lower than that at which a surface convection zone appears. The temperature at the red edge is a two-decade old puzzle. Recently, {\it Kepler} discovered a number of cool DAVs which pulsate at higher frequencies and with much smaller photometric amplitudes than expected based on trends extrapolated from DAVs found by ground based observations. Remarkably, some of them exhibit sporadic outbursts separated by days, each lasting several hours, and releasing $\sim 10^{33}-10^{34}\,\mathrm{erg}$. We provide quantitative explanations for both the red edge and the outbursts. The minimal frequency for overstable modes rises abruptly near the red edge. Although high frequency overstable modes exist below the red edge, their photometric amplitudes are generally too small to be detected by ground based observations. Nevertheless, these overstable parent modes can manifest themselves through nonlinear mode couplings to damped daughter modes which generate limit cycles giving rise to photometric outbursts., Comment: accepted to ApJ

Tidal dissipation inside giant planets is important for the orbital evolution of their natural satellites. It is conventionally treated by parameterized equilibrium tidal theory, in which the tidal torque declines rapidly with distance, and orbital expansion was faster in the past. However, Lainey et al. (2017) find that some Saturnian satellites are currently migrating outward faster than predicted by equilibrium tidal theory. Resonance locking between satellites and internal oscillations of Saturn, proposed by Fuller et al. (2016), naturally matches the observed migration rates. Here, we show that the resonance locking theory predicts dynamical tidal perturbations to Saturn's gravitational field in addition to those produced by equilibrium tidal bulges. We show that these perturbations can likely be detected during Cassini's proximal orbits if migration of satellites results from resonant gravity modes, but will likely be undetectable if migration results from inertial wave attractors or dissipation of the equilibrium tide. Additionally, we show that the detection of gravity modes would place constraints on the size of the hypothetical stably stratified region in Saturn., Comment: accepted to MNRAS

The inner moons of Jupiter and Saturn migrate outwards due to tidal energy dissipation within the planets, the details of which remain poorly understood. We demonstrate that resonance locking between moons and internal oscillation modes of the planet can produce rapid tidal migration. Resonance locking arises due to the internal structural evolution of the planet and typically produces an outward migration rate comparable to the age of the solar system. Resonance locking predicts a similar migration timescale but a different effective tidal quality factor $Q$ governing the migration of each moon. It also predicts nearly constant migration timescales a function of semi-major axis, such that effective $Q$ values were larger in the past. Recent measurements of Jupiter and Saturn's moon systems find effective $Q$ values that are smaller than expected (and are different between moons), and which correspond to migration timescales of $\sim$10 Gyr. If confirmed, the measurements are broadly consistent with resonance locking as the dominant source of tidal dissipation in Jupiter and Saturn. Resonance locking also provides solutions to several problems posed by current measurements: it naturally explains the exceptionally small $Q$ governing Rhea's migration, it allows the large heating rate of Enceladus to be achieved in an equilibrium eccentricity configuration, and it resolves evolutionary problems arising from present-day migration/heating rates., Comment: Published in MNRAS

Rubble piles are a common feature of solar system bodies. They are composed of monolithic elements of ice or rock bound by gravity. Voids occupy a significant fraction of the volume of a rubble pile. They can exist up to pressure $P\approx \epsy\mu$, where $\epsy$ is the monolithic material's yield strain and $\mu$ its rigidity. At low $P$, contacts between neighboring elements are confined to a small fraction of their surface areas. As a result, the effective thermal conductivity of a rubble pile, $\kcon\approx k(P/(\epsy\mu))^{1/2}$, can be orders of magnitude smaller than, $k$, the thermal conductivity of its monolithic elements. In a fluid-free environment, only radiation can transfer energy across voids. It contributes an additional component, $\krad=16\ell\sigma T^3/3$, to the total effective conductivity, $\keff=\kcon +\krad$. Here $\ell$, the inverse of the opacity per unit volume, is of order the size of the elements and voids. An important distinction between $\kcon$ and $\krad$ is that the former is independent of the size of the elements whereas the latter is proportional to it. Our expression for $\keff$ provides a good fit to the depth dependence of thermal conductivity in the top $140\,\mathrm{cm}$ of the lunar regolith. It also offers a good starting point for detailed modeling of thermal inertias for asteroids and satellites. Measurement of the response of surface temperature to variable insolation is a valuable diagnostic of a regolith. There is an opportunity for careful experiments under controlled laboratory conditions to test models of thermal conductivity such as the one we outline., Comment: Accepted to ApJ, 7 pages, 4 figures

Five pairs of large solar system satellites occupy first order mean-motion resonances (MMRs). Among these, the pairs of Mimas-Tethys and Titan-Hyperion are special. They are located much deeper in resonance than the others and their critical arguments librate with much greater amplitudes. These characteristics are traced to the insignificant damping, over $\Gyr$ timescales, of Mimas's orbital inclination and Hyperion's orbital eccentricity. Absent that, these resonances would not survive. Instead their librations would be overstable and escape from resonance would occur on the relevant damping time. Unlike the aforementioned MMRs, those involving Enceladus-Dione, Io-Europa, and Europa-Ganymede are limited by eccentricity damping. They must either remain at the shallow depths they currently occupy, or, if they venture deeper, retreat after a limited time. The latter seems almost certain for Enceladus-Dione and quite likely for the others, We examine the MMRs involving Mimas-Tethys and Titan-Hyperion under the assumption that they formed as a result of convergent migration. Capture probabilities are $\sim 6\%$ for the former and $100\%$ for the latter. The possibility of collisional excitation of their large librations is investigated but largely discounted., Comment: 7 pages, 4 figures, submitted to AJ

The pulsar triple system, J0337+1715, is remarkably regular and highly hierarchical. Secular dynamics controls its long term evolution with orbital commensurability having negligible effect. Secular interactions transfer angular momentum between inner and outer orbits unless their apsidal lines are parallel or anti-parallel. These choices correspond, respectively, to orthogonal eigenmodes p and a. Each is characterized by an eccentricity ratio set by the masses and semi-major axes, i.e., $e_{p, 1}/e_{p, 2}\sim a_1/a_2$ while $e_{a, 1}/e_{a, 2}\sim (a_1/a_2)^{-3/2}(m_2/m_1)$. Mode p dominates the system's current state so $e_1/e_2$ always remains close to $e_{p, 1}/e_{p, 2}$. A small contribution by Mode a causes $e_1$ and $e_2$ to oscillate with period $\sim 10^3\,\yr$. Orbital changes should be apparent in a few years. These will reveal the forcing of the apsidal precession of the inner orbit by general relativity (GR), and possibly also the smaller contribution due to the tidal and rotational distortion of the inner white dwarf (WD). Phinney (1992) proposes that the epicyclic energy of a WD-pulsar binary reaches equipartition with the kinetic energy of a single convective eddy when the WD's progenitor fills its Roche lobe. We extend Phinney's theory to apply to modes rather than individual orbits. Thus we predict that Mode p and Mode a achieved equipartition with eddies in the giant envelopes of the progenitors of the outer and inner WD, respectively. The most effective eddies are those with lifetimes closest to the orbit period. These were more energetic in the progenitor of the outer WD. This explains why Mode p overwhelms Mode a, and also why the inner binary's orbit is far more eccentric than other WD-pulsar binaries with similar orbit periods. Mode a's small but finite amplitude places a lower bound of $Q\sim 10^6$ on the tidal quality parameter of the inner WD., Comment: 8 pages, 4 figures, 1 table; accepted to ApJ

Fast Radio Bursts (FRBs) are isolated, $\ms$ radio pulses with dispersion measure (DM) of order $10^3\DMunit$. Galactic candidates for the DM of high latitude bursts detected at $\GHz$ frequencies are easily dismissed. DM from bursts emitted in stellar coronas are limited by free-free absorption and those from HII regions are bounded by the nondetection of associated free-free emission at radio wavelengths. Thus, if astronomical, FRBs are probably extra-galactic. FRB 110220 has a scattering tail of $\sim 5.6\pm 0.1 \ms$. If the electron density fluctuations arise from a turbulent cascade, the scattering is unlikely to be due to propagation through the diffuse intergalactic plasma. A more plausible explanation is that this burst sits in the central region of its host galaxy. Pulse durations of order $\ms$ constrain the sizes of FRB sources implying high brightness temperatures that indicates coherent emission. Electric fields near FRBs at cosmological distances would be so strong that they could accelerate free electrons from rest to relativistic energies in a single wave period., Comment: 5 pages, accepted by ApJL