Evanescent Regime Research Articles

Giant nonreciprocal effect under conditions of mutual influence of ferromagnetic and chiral resonances

A more than hundredfold increase in the nonreciprocity of electromagnetic wave transmission in a waveguide containing a transversely magnetized ferrite plate has been observed upon the introduction of a grating of bianisotropic elements. This effect takes place at a certain magnitude of a constant magnetic field, under the conditions of mutual influence of ferromagnetic and chiral resonances. In an evanescent regime, the non-reciprocity exhibits a severalfold increase. The effect depends on the mode of excitation of the chiral resonance, being observed in the case of excitation by a magnetic component of the microwave field.

Air-hole collapse and mode transitions in microstructured fiber photonic wires

We demonstrate robust, low bend loss photonic wires made from air-clad microstructured "grapefruit" fiber. By tapering the fiber and collapsing the air-holes, the guided mode evolves from being fully embedded within the fiber to a spatially-localized evanescent regime a few millimeters in length, where the mode is strongly influenced by the external environment. We show that in the embedded regime there is negligible loss when the taper is immersed in index-matching fluid, while in the evanescent regime the attenuation increases by over 35 dB. Furthermore, we show that an 11 microm wire in the embedded regime can be bent to a radius as small as 95 microm with bend-loss of 0.03 dB in a 500 nm band. The combination of spatial localization, strong dependence on the external environment andsmall bend radius make the device ideally suited for bio-photonic sensing.

Self-demodulation of elastic waves in a one-dimensional granular chain

The self-demodulation process in a nonlinear granular chain of identical beads is studied analytically and numerically. In such a medium, in accordance with the dispersion relation, longitudinal waves that have a frequency higher than the so-called cutoff frequency of the chain are evanescent. Here, we study the influence on the self-demodulation process of the transition from the propagative to the evanescent regime in pump wave propagation that takes place when the pump frequency increases. An analytical solution in discrete coordinates is derived for the case of two primary frequencies mixing into a single difference frequency. This solution is then numerically integrated in order to analyze the demodulation of the acoustic wave packet (i.e., of the harmonic acoustic wave modulated in a pulse mode). Temporal demodulated profiles can be strongly sensitive to the regime (propagative or evanescent) of primary wave transport. This model allows us to detect the cutoff frequency of longitudinal elastic waves in the chain, without receiving the primary waves, but receiving the low frequency nonlinearly radiated signal. The roles of frequency dependent attenuation, velocity dispersion, and observation distance are analyzed.

Microstructured optical fiber photonic wires with subwavelength core diameter

We demonstrate fabrication of robust, low-loss silica photonic wires using tapered microstructured silica optical fiber. The fiber is tapered by a factor of fifty while retaining the internal structure and leaving the air holes completely open. The air holes isolate the core mode from the surrounding environment, making it insensitive to surface contamination and contact leakage, suggesting applications as nanowires for photonic circuits . We describe a transition between two different operation regimes of our photonic wire from the embedded regime, where the mode is isolated from the environment, to the evanescent regime, where more than 70% of the mode intensity can propagate outside of the fiber. Interesting dispersion and nonlinear properties are identified.

Stratified Flow along a Corrugated Slope: Separation Drag and Wave Drag

Abstract Lee wave generation and horizontal flow separation in stratified flow along a slope, with corrugations or a ridge running directly downslope, are explored using analytical and numerical methods. Both of these processes are important to the drag on alongslope currents. The analytical solution for steady wave generation by stratified flow along a corrugated slope is extended to the evanescent flow regimes. There are two evanescent regimes, having intrinsic frequencies either above the buoyancy frequency N (fast flow), or below N sin(a) (slow flow), for nonrotating fluid and slope angle, a. Streamlines of the low speed evanescent solution tend to follow isobaths, while those of wave solutions tend to flow up over ridges and down in canyons. An analytical expression is developed for the wave drag felt by an isolated ridge on a slope. For a Gaussian ridge of alongslope length L, the drag becomes small when U/LN > 1 (the fast flow regime), or when U/(LN sin a) < 1/2 (the slow flow regime). Numerical ex...

Effects of seabed properties on acoustic wave fields in a seismo-acoustic ocean waveguide

Acoustic wave fields in an ocean waveguide with a sediment layer having continuously varying density and sound speed overlying an elastic subbottom are considered in this analysis. The objective of this study is to investigate the effects of seabed acoustic properties, including the density and sound speed of the sediment layer and subbottom, on the characteristics of the wave fields. Examination of the reflection coefficient, wavenumber spectrum, and noise intensity of the sound field through numerical analysis has shown that the variation in the acoustic properties in the sediment layer is an important factor in determining the reflected or noise sound fields. In particular, the sediment thickness-to-wavelength ratio and the types of variation of acoustic properties inside the layer give rise to many characteristics that potentially allow for acoustic inversion of the seabed properties. With regard to the wave-field components in a shallow-water environment, the various types of waves existing in a seismo-acoustic waveguide have been illustrated. The results indicate that the effects of the sediment properties on the wavenumber spectrum are primarily on the continuous and evanescent regimes of the wave field. The noise intensity generated by distributive random monopoles at various depths, together with the effect of refractive sound-speed distribution in the water column, has been obtained and analyzed.

Analytic study of gratings patterned by evanescent near field optical lithography

Simulation results which investigate the near field of conducting gratings are presented to show some of the major issues affecting evanescent near field optical lithography (ENFOL), namely ultimate resolution, depth of field, exposure variations due to edge enhancements, and resonant diffraction. Ultimate resolution down to 10 nm for grating structures is predicted, independent of illumination wavelength. The depth of field reduces approximately linearly as the feature size reduces in the evanescent regime. Variations in mask profile were investigated by modeling the radii of curvature of mask conductors from 1–10 nm. Strict mask profile control is shown to be important to avoid exposure variations due to the increasing zeroth transmitted order with increasing radii. A diffraction resonance occurs when the grating pitch matches the wavelength for a transverse magnetic excited grating. The cut off of the ±1 diffracted orders coincides with a plasmon resonance and a strong, frequency doubled interference pattern is produced. To avoid such resonant conditions, standard ENFOL requires a low coherence source and/or strongly absorbing resists. However, this near field interference offers the possibility of frequency-doubled interferometric replication of quasiperiodic structures, with strong intensity enhancement at the expense of reduced depth of field. Overall, the key to successful evanescent lithography is restricting the lithography to a depth in which high contrast is available with good process latitude due to the presence of sufficient numbers of diffracted orders of sufficient strength.

Frequency-domain Green's function for a planar periodic semi-infinite phased array. II. Diffracted wave phenomenology

For pt.I see ibid., vol.48, no.1, p.67-74, 2000. This second part of a two-paper sequence deals with the physical interpretation of the rigorously derived high-frequency asymptotic wave-field solution in part I, pertaining to a semi-infinite phased array of parallel dipole radiators. The asymptotic solution contains two parts that represent contributions due to truncated Floquet waves (FWs) and to the corresponding edge diffractions, respectively. The phenomenology of the FW-excited diffracted fields is discussed in detail. All possible combinations of propagating (radiating) and evanescent (nonradiating) FW and diffracted contributions are considered. The format is a generalization of the conventional geometrical theory of diffraction (GTD) for smooth truncated aperture distributions to the truncated periodicity-induced FW distributions with their corresponding FW-modulated edge diffractions. Ray paths for propagating diffracted waves are defined according to a generalized Fermat principle, which is also valid by analytic continuation for evanescent diffracted ray fields. The mechanism of uniform compensation for the FW-field discontinuities (across their truncation shadow boundaries) by the diffracted waves is explored for propagating ad evanescent FWs, including the cutoff transition from the propagating to the evanescent regime for both the FW and diffracted constituents. Illustrative examples demonstrate: (1) the accuracy and efficiency of the high-frequency algorithm under conditions that involve the various wave processes outlined above and (2) the cogent interpretation of the results in terms of the uniform FW-modulated GTD.

Relation between spatial confinement of light and optical tunneling

With the aim of obtaining a better insight in the optical tunneling process the near-field electrodynamics of a current-density (equivalently polarization) sheet is investigated, taking as a starting point the near-field optics of a single atom, and afterwards the tunneling field of a macroscopic medium is determined integrating over a distribution of sheets. The total electric field hitherto used to study tunneling times and effective tunneling velocities is divided into a nonretarded (matter attached) longitudinal part of standing-wave character, and a retarded (detached) transverse part propagating away from the matter-vacuum interface with the vacuum speed of light. For a current-density distribution phase shifted with a wave number ${q}_{\ensuremath{\Vert}}$ along the interface, the transverse part is nonzero in the vacuum and decays exponentially with a decay constant ${q}_{\ensuremath{\Vert}}^{\ensuremath{-}1}$ as a function of the distance from the interface. Since the source domain of photons is precisely the domain of the transverse current density, the optical tunneling process attains an important contribution associated with the lack of spatial localizability of a photon in the evanescent regime. It is shown that in an observationally equivalent electromagnetic propagator description of the space-time dynamics, where the source domain of the photons is identified with the domain of the total electron current density, the retarded transverse dynamics necessarily must include spacelike couplings in the evanescent regime. Since these are destroyed with the vacuum speed of light as the light-cone coupling moves away from the matter-vacuum interface, the Einstein causality is always obeyed. The link to previous studies of the optical tunneling process is established by investigating the transverse and longitudinal dynamics in the frequency domain. Finally, it is shown that surface currents may play an important role in the optical tunneling process, in particular in cases where the incident electromagnetic field generates divergence-free currents in the bulk of the source medium.

Thermal magnetic fluctuations of whistlers in a Maxwellian plasma

Thermal fluctuations have been measured with a magnetic loop antenna inside a large afterglow plasma in the regime of whistler waves (f≲fce≂30 MHz≪fpe≂3000 MHz; Ar, 2×10−4 Torr, 1 m diam×2.5 m length). The magnetic fluctuations B̃(ω) exhibit a 1/f-like spectrum for whistlers (f&amp;lt;fce), no resonant enhancement at the electron cyclotron frequency fce, and a flat spectrum in the evanescent regime (fce&amp;lt;f≪fpe). Thus the observed fluctuations are neither described by blackbody radiation laws (B̃∝ω) nor by cyclotron emission (lines at nfce), but resemble the decaying Alfvénic fluctuation spectrum calculated by Cable and Tajima [Phys. Rev. A 46, 3413 (1992)].

Perturbation of the Plane-Wave Reflectivity of A Depth-Dependent Elastic Medium By Weak Inhomogeneities

SUMMARY The first-order effects produced by plane inhomogeneities on the plane-wave response of a vertically heterogeneous elastic medium are derived. the derivation is based on a perturbation analysis of the wave equations which yields the Frechet derivatives of the displacement fields in terms of the Green's functions for the unperturbed medium. the accuracy and limitations of the first-order solutions are checked numerically by comparing τ-p seismograms calculated from the analytical expressions of the Frechet derivatives with a complete solution obtained by introducing discrete perturbations in the medium properties. Calculations performed with a realistic marine model show that the first-order perturbation theory is remarkably accurate at all angles of incidence, and even in the evanescent regime. Furthermore, it is shown that the formulae obtained allow us to model density or velocity perturbations of up to 10 per cent. the expressions of the Frechet derivatives are directly applicable to linear and non-linear inversion algorithms of plane-wave seismograms using gradient techniques. In addition, they may be useful in examining the sensitivity of the plane-wave response of complex geological sequences to various combinations of elastic parameters.

High-frequency noise on antennas in plasmas

Fluctuations in plasmas at frequencies near the electron plasma frequency (ωp ) have been measured with in situ wire antennas. Such observations are important to the understanding of basic plasma properties (discrete versus collective effects) and the use of antennas in plasmas. The experiments are performed in a large (1 m diameter ×2 m) pulsed dc discharge and afterglow plasma (ne&amp;lt;1012 cm−3, kTe&amp;lt;5 eV) with a weak axial magnetic field (B0=5 G). The fluctuations are detected from wire antennas (length L≫radius a≳Debye length λD) with a low-noise microwave receiver. The observations reveal three different physical processes that determine the noise spectra: (i) single particle shot noise, (ii) collective oscillations by bounded sheath–plasma resonances, and (iii) noise enhancements by longitudinal plasma waves. The first phenomenon (shot noise) gives rise to broadband noise (ωωp) on both electric and magnetic antennas. In the evanescent regime (ω&amp;lt;ωp) , the shot noise is induced by random electron motions through distances of, at most, a collisionless skin layer (c/ωp) around the antenna. Recalling that electron transit-time effects cause absorption of waves in a collisionless skin layer (known as ‘‘anomalous’’ skin absorption) the present observation of a collisionless skin emission effect can also be understood by the equivalence of blackbody absorption and emission coefficients. The second phenomenon (sheath–plasma resonance) is observed as a narrow resonant enhancement in the shot noise below cutoff (ω&amp;lt;ωp) on electric antennas surrounded by sheaths or dielectrics. The series sheath–plasma resonance, usually identified from reflection/absorption measurements with incident waves, is established here, for the first time, as a feature of thermal emission spectra from antennas. The third phenomenon (plasma dielectric ε(ω,k)→0) produces a broad noise enhancement at ω≳ωp on electric antennas. It can only be observed in the open-loop voltage of dipoles measured with a high-impedance transformer between antenna and 50Ω transmission line. The enhanced noise is electrostatic, randomly polarized, but not significantly enhanced by the presence of energetic electron tails. Existing theories can describe the new observations qualitatively but not quantitatively.

A dynamical‐chemical model of wave‐driven fluctuations in the OH nightglow

Wave‐induced temporal fluctuations in the intensity of the OH nightglow are related to the temperature oscillations of the wave field by a model that incorporates a five‐reaction photochemical scheme and the complete dynamics of linearized acoustic‐gravity waves in an isothermal, motionless atmosphere. The intensity I and rotational temperature T oscillations, δI and δT, are conveniently related by the ratio, where the overbar refers to time‐averaged quantities. The ratio η is a complex quantity that depends on the properties of the basic state atmosphere (temperature, thermodynamic parameters, major constituent O2, N2 and minor constituent O, O3, OH, H, HO2 concentrations, and scale heights), chemical reaction rate constants, wave period, horizontal wavelength, and direction of wave energy propagation (upward or downward). The intensity‐temperature oscillation ratio η is evaluated for a nominal case corresponding to an altitude of about 83 km in a nightside model atmosphere with an atomic oxygen scale height of −2.8 km; horizontal wavelength λx is 100 km, and wave energy propagation is upward. Over a broad range of acoustic periods |η| varies between 7 and 8, and η is approximately in phase with the temperature fluctuations. At gravity wave periods, |η| decreases with increasing period from a maximum value of about 7.0; at a period of about 3 hours, |η| is about 1.8. The phase of η and δT are within 45° in the gravity wave regime. The main effect of order of magnitude changes in λx is the modification of the location and width (in period) of evanescent regions. At hour periods, |η| increases as the magnitude of atomic oxygen scale height decreases; at periods of several hours, |η| is about 1/3 greater for an atomic oxygen scale height of −2 km than for the nominal scale height. The amplitude of η is essentially independent of the direction of wave energy propagation, but the phase of η relative to that of δT depends on the upward or downward sense of energy propagation at periods in close proximity to the evanescent regime. The magnitude of η at gravity wave periods can depend sensitively on the altitude of the OH emission layer; higher OH emission heights give smaller values of |η| at 10‐min periods, providing the O3 scale height is not too great. Neglect of minor constituent photochemistry in computing η is a tolerable approximation at acoustic wave periods, but it is entirely inadequate at gravity wave periods. Inclusion of dynamical effects is absolutely essential for a valid assessment of η at any period.

Seismic waves in a stratified half space

The response of a stratified elastic half space to a general source may be represented in terms of the reflection and transmission properties of the regions above and below the source. For P-SV and SH waves and both buried sources and receivers, convenient forms of the response may be found in which no loss of precision problems arise from growing exponential terms in the evanescent regime. These expressions have a ready physical interpretation and enable useful approximations to the response to be developed. The reflection representation leads to efficient computational procedures for models composed of uniform layers, which may be extended in an asymptotic development to piecewise smooth models.

Investigation of the microwave magnetoplasma matching effect in indium antimonide

Reflection and transmission of microwaves normally incident on a magnetized semiconductor plasma are investigated in the presence of a dielectric quarter-wave layer. Faraday geometry is specifically considered, although conclusions of this work can be generalized to other geometries. Experiments are performed on n-type InSb at 35 GHz between 80 and 300 K. Four characteristic effects caused by the presence of the quarter-wave plate are reported. The first of these, referred to as the ’’magnetoplasma matching effect’’, is the nonresonant minimum of reflectivity observed when the applied dc magnetic field is varied. This effect is shown to be observable over a broad range of values of the effective dielectric constant κeff (including extremely large values, e.g., the helicon wave regime), as long as κeff≳0. Observation of this effect provides a direct measure of κeff, independent of particular microscopic models of this function. The second effect is the enhancement of reflected phase shift. The phase as a function of dc magnetic field is found to have a well-defined minimum between cyclotron resonance and magnetoplasma matching conditions. This effect provides a new tool for investigation of electronic parameters in narrow-gap semiconductors. The enhanced phase change is observed even in the evanescent wave regime (κeff&amp;lt;0) —an especially important feature, since the region of evanescence is notoriously inaccessible to measurements. Third, a great enhancement of the power absorbed by the semiconductor half-space is shown to occur over a broad range of parameters. This is important when κeff is very large, so that half-space absorptivity (without the quarter-wave plate) is too small to be measured accurately. Fourth, extremely large enhancement of transmission through a slab (by several orders of magnitude) is observed over a wide range of dc magnetic fields. Finally, limitations of magnetoplasma matching caused by high-order mode generation are discussed.