Nonlinear Loss Mechanisms Research Articles

Experimental investigation of supertransmission for an intrinsic localized mode in a cyclic nonlinear transmission line.

In this experimental study of the nonlinear loss mechanism between traveling localized excitation and the underlying extended normal mode spectrum for a 1D lattice, three types of cyclic, electric, nonlinear transmission lines (NLTLs) are used. They are nonlinear capacitive, inductive, and capacitive+inductive NLTLs. To maintain a robust, steady-state traveling intrinsic localized mode (ILM), a traveling wave driver is used. The ILM loses energy because of a resonance between it and the extended NLTL modes. A wake field excitation is detected directly from ILM velocity experiments by the decrease in ILM speed and by the observation of the wake. Its properties are quantified via a two-dimensional Fourier map in the frequency-wavenumber domain, determined from the measured spatial-time voltage pattern. Simulations support and extend these experimental findings. We find for the capacitive+inductive NLTL configuration, when the two nonlinear terms are theoretically balanced, the wake excitation is calculated to become very small, giving rise to supertransmission over an extended driving frequency range.

Atmospheric Erosion by Giant Impacts onto Terrestrial Planets

Abstract We examine the mechanisms by which the atmosphere can be eroded by giant impacts onto Earth-like planets with thin atmospheres, using 3D smoothed particle hydrodynamics simulations with sufficient resolution to directly model the fate of low-mass atmospheres. We present a simple scaling law to estimate the fraction lost for any impact angle and speed in this regime. In the canonical Moon-forming impact, only around 10% of the atmosphere would have been lost from the immediate effects of the collision. There is a gradual transition from removing almost none to almost all of the atmosphere for a grazing impact as it becomes more head-on or increases in speed, including complex, nonmonotonic behavior at low impact angles. In contrast, for head-on impacts, a slightly greater speed can suddenly remove much more atmosphere. Our results broadly agree with the application of 1D models of local atmosphere loss to the ground speeds measured directly from our simulations. However, previous analytical models of shock-wave propagation from an idealized point-mass impact significantly underestimate the ground speeds and hence the total erosion. The strong dependence on impact angle and the interplay of multiple nonlinear and asymmetrical loss mechanisms highlight the need for 3D simulations in order to make realistic predictions.

High efficiency laser action in mildly doped Yb:LuYAG ceramics

We report the first laser oscillations of 5 at%, 10 at% and 20 at% Yb3+ doped LuYAG ceramic samples and their spectroscopic characterization. The maximum output power, 8.7 W at 1029 nm, is achieved by 10 at% sample. A slope efficiency as high as 90.1% is obtained from the 10 at% doped sample. To our knowledge, these are the highest laser slope efficiencies obtained so far for Yb3+ doped LuYAG ceramics. With the 20 at% doped ceramic the maximum output power was 7.2 W at 1047.6 nm with a corresponding slope efficiency of 47.4%, whereas a decrease of the output power is observed for absorbed pump power higher than 15.6 W, probably due to the appearance of a non-linear loss mechanism. Finally, we explored the tunability of the ceramics finding the broadest continuous tuning range (from 992 nm to 1060 nm, i.e. 68 nm), with 5 at% and 10 at%, while it decreases to 54 nm with 20 at%.

A method to suppress harmonics in standing-wave thermoacoustic engines

Harmonic generation in thermoacoustic engines is regarded as a non-linear loss mechanism that extracts acoustic power from the fundamental wave into harmonics. It is shown that the use of specifically designed insert that limits the gas flow area over a very-limited part of the resonator can suppress this non-linear loss mechanism causing a significant increase in the generated acoustic power in the fundamental mode. In this experimental work, a standing-wave thermoacoustic engine is built and operated without inserts and with inserts of different shapes, porosities, and thicknesses. The self-generated dynamic pressure waves are captured under different operating conditions and then are decomposed into a fundamental component and harmonics. Results for different inserts are presented and discussed. All inserts caused lower harmonic content with respect to the no-insert case but inserts of low gas flow area cause severe flow blockage and thus a severe reduction in the produced acoustic powers. Results analyze the relationships between the generated acoustic power in the fundamental mode and the amplitudes of the dynamic pressures of the fundamental, first harmonic, and second harmonics. The blockage effects caused by the insert are discussed.

Stimulated Brillouin scattering in silicon/chalcogenide slot waveguides.

We study slot waveguide geometries, comprising a combination of soft glasses and high-index guiding structures, for enhancing stimulated Brillouin scattering (SBS). We show that strong optical and acoustic mode confinement in these waveguides can lead to a substantial increase in SBS gain, comparable to or greater than recently proposed suspended silicon nanowire structures. We compute the optimal parameters of the structure and examine the physics of optical and acoustic confinement within slot waveguides. Finally, we compute the effects of linear and nonlinear loss mechanisms on optimum pump/Stokes powers and waveguide lengths.

Octave-spanning mid-infrared supercontinuum generation in silicon nanowaveguides.

We report, to the best of our knowledge, the first demonstration of octave-spanning supercontinuum generation (SCG) on a silicon chip, spanning from the telecommunications c-band near 1.5μm to the mid-infrared region beyond 3.6μm. The SCG presented here is characterized by soliton fission and dispersive radiation across two zero group-velocity dispersion wavelengths. In addition, we numerically investigate the role of multiphoton absorption and free carriers, confirming that these nonlinear loss mechanisms are not detrimental to SCG in this regime.

Experimental evidence of a nonlinear loss mechanism in highly doped Yb:LuAG crystal

We report a rigorous study of the spectroscopic, laser and thermal properties of a 10at.% and a 15at.% Yb:LuAG crystals. A loss mechanism is observed in the medium with the highest doping, pumped at 936 nm and 968 nm, as a sharp and dramatic decrease of the laser output power is measured at higher excitation densities. The nonlinearity of the loss mechanism is confirmed by the fluorescence data and by the thermal lens. In particular, the dioptric power of the thermal lens acquired at different pumping levels shows a strong deviation of the expected linear trend. Here we report the influence of both the concentration and the ion excitation density of Yb3+ on the output powers, the slope efficiencies and the thresholds. Conversely excellent results are achieved with the 10at.%, which does not show any loss mechanism as at 1046 nm it delivers 11.8 W with a slope efficiency of η(s) = 82%, which is, to the best of our knowledge, the highest value reported in literature for this material.

Characterization of Nb SRF cavity materials by white light interferometry and replica techniques

Much work has shown that the topography of the interior surface is an important contributor to the performance of Nb superconducting radiofrequency (SRF) accelerator cavities. Micron-scale topography is implicated in non-linear loss mechanisms that limit the useful accelerating gradient range and impact cryogenic cost. Aggressive final chemical treatments in cavity production seek to reliably obtain “smoothest” surfaces with superior performance. Process development suffers because the cavity interior surface cannot be viewed directly without cutting out pieces, rendering the cavities unavailable for further study. Here we explore replica techniques as an alternative, providing imprints of cavity internal surface that can be readily examined. A second matter is the topography measurement technique used. Atomic force microscopy (AFM) has proven successful, but too time intensive for routine use in this application. We therefore introduce white light interferometry (WLI) as an alternative approach. We examined real surfaces and their replicas, using AFM and WLI. We find that the replica/WLI is promising to provide the large majority of the desired information, recognizing that a trade-off is being made between best lateral resolution (AFM) and the opportunity to examine much more surface area (WLI).

Integrated analysis of non-linear loss mechanisms in Yb:YAG ceramics for laser applications

Abstract Transparent 9.8 at% Yb:YAG ceramic samples were prepared by reactive sintering of commercial oxides and using 0 or 1 wt% polyethylene glycol (PEG) as dispersant. The optical quality of the samples turns out to be improved by using a dispersant and optical transmittance close to the commercial samples has been obtained. On the other hand the laser characterization evidenced the activation of a non-linear loss mechanism occurring only in the sample containing PEG and despite its better optical quality, at high excitation level. A SEM analysis of material microstructure could not explain this behavior. A state of the art TEM analysis at nanometric scale was performed providing high resolution chemical spectroscopic results that indicate the presence of amorphous and crystalline silicate phases playing different roles in the two samples.

Effects of the excitation density on the laser output of two differently doped Yb:YAG ceramics

We report the behavior of two Yb(3+) doped ceramics (i.e. 10% at. and 20% at.) under quasi-continuous wave laser pumping. Two different behaviors are found depending on the density of Yb(3+) in the excited level. Experimental results show that at low population inversion density, the maximum output power and the efficiency are almost independent on the doping concentration. In particular, an output power as high as 8.9 W with a corresponding slope efficiency of 52% with respect to the injected pump power was reached with the 20% at. sample. Conversely, at high population inversion densities, the 20% doped sample shows a sudden decrease of the laser output for increasing pump power, due to the onset of a nonlinear loss mechanism. Finally, we report a comparison of the experimental results with numerical simulations for the evaluation of the inversion density and of the temperature distribution.

Efficient simulation of non-linear effects in 2D optical nanostructures to TM waves

We develop a theory to study stationary TM-type waves propagating in a nanostructured layer of 2D non-linear optical metamaterial or plasmonic device. It is assumed that the layer is inhomogeneous and contains non-linear isotropic elemental materials with non-linearity and loss mechanisms, including both linear and non-linear losses. While modeling of the non-linear propagation of the TE-type scalar waves is straightforward, the TM-type waves within the standard E-field formulations of non-linear optics cannot be treated in a purely scalar H-field context since an implicit equation for the non-linear dielectric functions should be resolved otherwise. A new formulation, which is built on the solution of the implicit equation for the non-linear dielectric function, is proposed. We use a general cubic non-linearity to illustrate all of the important features of the proposed approach. The general solution for scalar H-field waves is validated versus our previously tested particular cases, and important differences are shown between those cases and the general solution. These details, for example, include the link between linear and non-linear loss mechanisms, and connection between the linear and non-linear dielectric functions. The proposed approach is used for modeling a non-linear focusing device with optically controlled isotropic Kerr-type non-linearity; the simulation results prove the predicted functioning of the device.

Self-phase-modulation induced spectral broadening in silicon waveguides

The prospect for generating supercontinuum pulses on a silicon chip is studied. Using ~4ps optical pulses with 2.2GW/cm(2) peak power, a 2 fold spectral broadening is obtained. Theoretical calculations, that include the effect of two-photon-absorption, indicate up to 5 times spectral broadening is achievable at 10x higher peak powers. Representing a nonlinear loss mechanism at high intensities, TPA limits the maximum optical bandwidth that can be generated.

Distinction of two-photon absorption from other nonlinear loss mechanisms by phase-conjugate interferometry

We show that phase-conjugate interferometry is a measuring technique for third-order optical nonlinearities that can distinguish two-photon absorption from other nonlinear loss mechanisms. For the case of 3,3′-diethyloxadicarbocyanine iodide at 1064 nm, the nonlinear loss is dominated by linear absorption from the two-photon excited state. We observe a highly nonlinear absorption process in solutions of 4-methoxy-4′-nitrostilbene at 532 nm and show that it is due to a more complicated loss mechanism. This mechanism could have applications in optical limiting.

Q-SWITCHED LASER WITH CONTROLLABLE PULSE LENGTH

Experimental results of a recently proposed technique for obtaining Q-switched laser pulses of controllable length are presented. A lithium iodate doubling crystal inside the cavity of a Nd: YAG laser running at 0.946 μ is used to provide both output coupling and an easily adjustable, nonlinear loss mechanism. Controllable pulse lengths in the range of 200 nsec to 1 μ at 0.473 μ have been achieved at nominally constant energy.