Low-loss Regime Research Articles

Tailoring radiative and non-radiative losses of thin nanostructured plasmonic waveguides

Thin nanostructured metal films allow to control radiative and non-radiative losses of surface plasmon polariton modes without changing their group velocities. This effect is studied in plasmonic waveguides made of thin gold films drilled with very narrow slits and deposited on a GaAs substrate. The analysis is supported by high-resolution angle-resolved transmission measurements and rigorous electromagnetic calculations. We show that the excitation of air/gold and gold/GaAs surface waves leads to Fano-type resonances with specific light localization into the slits. As a result, gold/GaAs surface waves induce a modulation of radiative and non-radiative losses of air/gold surface waves. The minimum and maximum of the Fano-type resonance introduce two propagation regimes. In the radiative propagation regime, the losses due to the absorption are negligible, whereas an efficient inhibition of free-space coupling is demonstrated in low-loss propagation regime.

The influence of traditional steep land agricultural practices on runoff and soil loss

The erosion of soil from subsistence agricultures on lands with steep slopes and shallow soil is poorly documented, particularly in Brazil. This paper details the hydrological and soil loss responses to traditional agriculture (shifting agriculture) of a steeply sloped sector (32° incline) classified by FAO has having Regosol soil. This sector is in Guarapuava, Brazil and is covered by a shallow, clayed soil that originates from basalt rock. All measurements were carried out between May 2003 and April 2004. Three replicate plots were established at three randomly selected land sites, which were under different phases of the rotational cycle: burned, fallow or shrubs. A total of 76 natural rainfall events were measured throughout the study period and the accumulated precipitation was 1538.9 mm. Rainstorms ≥30 mm were important for runoff and soil loss. The average runoff (expressed as a proportion of the total rainfall) over the 12-month investigation period for each of the three sites was determined to be 2.4%, 1.0%, and 1.4% for burned, fallow, and shrubs, respectively. The response to rainfall during the measurement period differed completely at each site. The average soil loss was as follows: for shrubs, fallow, and burned sites, average loss was 1.2 t/(ha year), 5.1 t/(ha year), and 6.1 t/(ha year), respectively. The results suggest that fire and traditional agriculture tillage practices affected the topsoil. Shifting cultivation patterns in the steeply sloped sectors cause a succession of high, intermediate, and low soil loss regimes, which are important for understanding contemporary landscape dynamics and land degradation in the Guarapuava region.

Loss-induced transition of the Goos-Hänchen effect for metals and dielectrics

We report a unifying approach to the Goos-Hänchen (GH) shifts on external optical reflection for metals and dielectrics in particular for the case of high losses, that is for a large imaginary part of the dielectric constant. In this regime metals and dielectrics have a similar GH shift which is in contrast to the low-loss regime where the metallic and dielectric forms of the GH shift are very different. When going from the low-loss to the high-loss regime we find that metals show a much more prominent transition; we present a condition on the dielectric constant which characterizes this transition. We illustrate our theoretical analysis with a realistic example of seven lossy materials.

Analytical description of free-electron-laser oscillations in a perfectly synchronized optical cavity

We present an analytical description of free-electron-laser (FEL) oscillations in a perfectly synchronized optical cavity by solving the one-dimensional FEL equations. It is shown that the radiation stored in the cavity eventually evolves into an intense few-cycle optical pulse in the high-gain and low-loss regime despite the lethargy effect. The evolution of the leading slope of the optical pulse, which is defined from the front edge toward the primary peak, is found to play an important role in generating the intense few-cycle optical pulse. The phase space evolution of electrons which interact with the leading slope is solved analytically in a perturbation method, leading to an analytical solution for the optical pulse evolution. The peak amplitude and the pulse length at saturation are found to scale with the electron beam density and optical cavity loss. Those scalings agree well with the intense few-cycle pulses recently observed in a high-power FEL.

Joint density of states at extended defects in CVD diamond, observed via highly spatially resolved electron energy loss spectroscopy

AbstractResults are presented of highly spatially resolved electron energy loss spectroscopy conducted on extended defects in CVD diamond. Analysis of the low energy loss regime shows that the joint density of states in the vicinity of dislocation nodes and secondary stacking faults, formed by incomplete dissociation of partial dislocations, can be different from that in perfect bulk material. Extra intensity in the joint density of states below the conduction band edge and in the lower conduction band was observed, with the most marked increase at around 6 eV. This is an indicator for sp2‐bond character. Spatially these states are too widely dispersed to be innate to the dislocation cores, and they are thought to arise from point defect segregation to extended defects. Kramers–Kronig and valence band plasmon analysis suggest an increase in the effective electron density and mass at the dislocation nodes. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High-efficiency ultrashort pulse generation in a high-gain FEL oscillator near the perfect synchronism

It has been verified experimentally in JAERI-FEL that a high-gain FEL oscillator has the maximum extraction efficiency at the perfect synchronism of optical-cavity length. The simultaneous measurement of FEL efficiency and absolute cavity length has clearly shown that a sharp peak of detuning curve at the perfect synchronism appears in high-gain and low-loss regime and the FEL extraction efficiency exceeds the scaling law of short-bunch FEL oscillators. A numerical analysis indicates that lasing at the perfect synchronism is quasi-stationary superradiance with random fluctuations, which is analogous to a SASE FEL. Second-order autocorrelation measurements show that FEL pulses shorter than four optical cycles are generated successively for a number of round trips at the perfect synchronism, which is consistent with numerical results.

Gain characteristics of quantum-dot injection lasers

The current dependence of the optical gain in lasers based on self-organized InGaAs quantum dots in a AlGaAs/GaAs matrix is investigated experimentally. A transition from lasing via the ground state of quantum dots to lasing via an excited state is observed. The saturated gain in the latter case is approximately four times greater than for the ground state. This result is attributable to the fourfold degeneracy of the excited level of quantum dots. The effect of the density of the quantum-dot array on the threshold characteristics is investigated. A lower-density array of dots is characterized by a lower threshold current density in the low-loss regime, because the transmission current is lower, while dense quantum-dot arrays characterized by a high saturated gain are preferable at high threshold gains.

Electrical noise generated during capillary flow of aqueous poly(ethylene oxide) solutions

The relation between the electrical noise generated during flow of aqueous poly(ethylene oxide) PEO solutions in three capillaries of different lengths and entry shape but the same diameter and the flow behaviour was investigated. Flow into the square cut off capillary entry was characterised by a low pressure loss regime followed at higher shear rates by a high pressure loss regime while flow into the trumpet shaped capillary end exhibited only the low pressure loss regime. The high pressure loss flow was associated with strong electrical noise generation. The results suggest that the noise generation mechanism includes a current source supplied by the flow through the capillary volume with an associated streaming potential and a modulation of the current by mixing and turbulence at the capillary entrance. The noise measuring technique can potentionally be used to study disturbances in the entry flow to capillaries.

High-sensitivity dielectric polarization noise measurements

Techniques for measurement of low-level dielectric polarization noise spectra over a broad dynamic range are described. The method provides a powerful probe of 1/f noise and near-equilibrium dynamics in nonconducting materials of interest such as glass formers, liquid crystals, polymers, or ferroelectrics. Dielectric polarization noise was measured via voltage fluctuations in a capacitance cell filled with sample material. Measurements were carried out in the temperature range 80–400 K on glycerol and poly-vinyl-chloride near their respective glass transitions. To maximize the dynamic range and sensitivity and limit effects of stray capacitance an ultralow-noise junction field-effect transistor based preamplifier was operated adjacent to the sample in vacuum within the low-temperature cryostat. The technique offers potentially greater accuracy than susceptibility measurements in low-loss regimes, and may be useful in single-electron-transistor applications.

An analysis of new and existing FDTD methods for isotropic cold plasma and a method for improving their accuracy

Over the past few years, a number of different finite-difference time-domain (FDTD) methods for modeling electromagnetic propagation in an isotropic cold plasma have been published. We have analyzed the accuracy and stability of these methods to determine which method provides the greatest accuracy for a given computation time. For completeness, two new FDTD methods for cold plasma, one of which is based on the concept of exponential fitting, are introduced and evaluated along with the existing methods. We also introduce the concept of cutoff modification which can be easily applied to most of the FDTD methods, and which we show can improve the accuracy of these methods with no additional computational cost. Von Neumann's stability analysis is used to evaluate the stability of the various methods, and their accuracy is determined from a straightforward time-and-space harmonic analysis of the dispersion and dissipation errors. Results of numerical experiments to verify the accuracy analysis are presented. It is found that for low-loss plasma, the piecewise linear recursive convolution method (PLRC) method is the most accurate, but the method of Young (see Radio Sci., vol.29, p.1513-22, 1994) can use less memory and is nearly as accurate. In this low-loss plasma regime, cutoff modification can significantly reduce the error near cutoff at the expense of slightly greater error at lower frequencies. For strongly collisional plasmas, the PLRC method also provides the most accurate solution.

1/f Dielectric Fluctuations at the Glass Transition

AbstractThe temperature and frequency dependence of thermal dielectric polarization noise was investigated in glycerol and PVC in the vicinity of their glass transitions. Polarization fluctuations were observed via voltage noise which was produced within a sample capacitance cell. The high sensitivity and accuracy of the technique provides a powerful probe of dynamics in low-loss regimes of various materials. Noise spectra had 1/fβ form at lower temperatures just below the glass transition. In glycerol, the spectral exponent decreased smoothly with temperature, extrapolating to 1, i.e. 1/f noise, near T° where relaxation times diverge. The results support the idea that a glassy phase transition occurs in the static limit.

Study of form birefringence in waveguide devices using the semivectorial beam propagation method

Waveguide devices based on nonbirefringent substrates may still have form birefringence if index discontinuities exist. It is demonstrated that the semivectorial beam propagation method can be used to study the polarization dependence in waveguide devices. It is shown that the calculated coupling lengths of planar directional couplers match within 3.5% of analytical solutions. Results on polarization dependencies of coupling lengths and bending losses of rib waveguide devices are presented. Results of rib waveguide directional couplers indicate that the form birefringence can be significant. Losses of S-shaped bends are weakly polarization dependent in the low-loss regime.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>