Wavelength Scale Research Articles

Wavelength scaling of photoionization circular dichroism of atomic electron ring currents and its effect on photoelectron spin polarization

Circularly polarized orbitals in atoms are referred to as electron ring currents with a helical angular phase and thus manifest circular dichroism in photoionization by circularly polarized light fields. Opposite circular dichroism has been revealed in single-photon and nonadiabatic tunneling ionization regimes. The transition of the circular dichroism and its underlying mechanism are still not clear. Here, we systematically study the wavelength scaling of photoionization circular dichroism of electron ring currents. We present a universal description of the dichroic photoelectron energy spectra and the spin polarization from the infrared to the extreme ultraviolet regions. We reveal that the channel interference and competition via the intermediate states with different angular quantum numbers in a few-photon ionization regime give rise to the transition of the circular dichroism and the spin polarization of photoelectrons.

Directional light output enhancement of scintillators by mixed-scale microstructures based on soft-X-ray interference lithography and self-assembly

In this paper, we have successfully demonstrated that the directional control of the light output of the scintillator can be achieved by mixed-scale microstructures which integrate photonic crystals on the wavelength scale and microlens arrays on the micrometer scale. Compared to the reference sample, the mixed-scale microstructures can enhance the light output of the scintillator by 3.07-fold at the emergence angle of 35°. This enhancement in the light output exhibits both azimuthal angle and wavelength dependence. Our hybrid simulation and experimental characterization explain the physical mechanism behind the enhanced directional light output. This method holds potential for application in radiation detection systems and other light-emitting devices requiring directional light output.

Global and local analyses of the Görtler instability in hypersonic flow

The stationary Görtler instability in hypersonic flow over a concave wall is systematically investigated across a range of geometric and flow parameters using resolvent analysis, which seeks for the forcing and response pair that maximizes the energy amplification. The optimal forcing takes the form of streamwise vortices, while the optimal response is streamwise streaks. The growth of the optimal disturbance is contributed by both the lift-up and centrifugal mechanisms. The latter becomes dominant as the boundary layer develops, and its growth rate agrees well with that predicted by local stability analysis. In terms of changes in geometric parameters, an increase in curvature destabilizes the Görtler instability, as expected, while the effect of the angle subtended by the concave wall (the turning angle) is shown to be negligible. With respect to changes in flow parameters, the Görtler instability is stabilized at low Reynolds numbers, destabilized under the cold-wall effect, and insensitive to the change in Mach number. The most amplified spanwise wavelength scales with the boundary-layer thickness, which remains mostly unchanged when the freestream Mach number is varied from 3 to 10. A new dimensionless wavelength parameter is proposed to predict the wavelength of the most dangerous Görtler vortices in the compressible flow regime. The resolvent analysis results are confirmed by a three-dimensional numerical simulation, where the hypersonic flow is perturbed by a spatial white noise.

Gravitational wave memory in conformally flat spacetimes

We study the gravitational wave memory effect in spacetimes related to flat space by a conformal transformation. The discussion is general but the gravitational wave length scale is assumed to be small compared with the background curvature radius. The general formulas are applied to Friedmann-Robertson-Walker metrics of all spatial curvatures. The effect of new terms stemming from spatial curvature is potentially detectable in future gravitational wave measurements.

Squirmer hydrodynamics near a periodic surface topography.

The behaviour of microscopic swimmers has previously been explored near large-scale confining geometries and in the presence of very small-scale surface roughness. Here, we consider an intermediate case of how a simple microswimmer, the tangential spherical squirmer, behaves adjacent to singly and doubly periodic sinusoidal surface topographies that spatially oscillate with an amplitude that is an order of magnitude less than the swimmer size and wavelengths that are also within an order of magnitude of this scale. The nearest neighbour regularised Stokeslet method is used for numerical explorations after validating its accuracy for a spherical tangential squirmer that swims stably near a flat surface. The same squirmer is then introduced to different surface topographies. The key governing factor in the resulting swimming behaviour is the size of the squirmer relative to the surface topography wavelength. For instance, directional guidance is not observed when the squirmer is much larger, or much smaller, than the surface topography wavelength. In contrast, once the squirmer size is on the scale of the topography wavelength, limited guidance is possible, often with local capture in the topography troughs. However, complex dynamics can also emerge, especially when the initial configuration is not close to alignment along topography troughs or above topography crests. In contrast to sensitivity in alignment and topography wavelength, reductions in the amplitude of the surface topography or variations in the shape of the periodic surface topography do not have extensive impacts on the squirmer behaviour. Our findings more generally highlight that the numerical framework provides an essential basis to elucidate how swimmers may be guided by surface topography.

Aluminum nitride photonic integrated circuits: from piezo-optomechanics to nonlinear optics

The commercial success of radio-frequency acoustic filters in wireless communication systems has launched aluminum nitride (AlN) as one of the most widely used semiconductors across the globe. Over recent years, AlN has also been investigated as an attractive photonic integrated platform due to its excellent characteristics, such as enormous bandgaps (∼6.2 eV), quadratic and cubic optical nonlinearities, Pockels electro-optic effects, and compatibility with the complementary metal-oxide semiconductor technology. In parallel, AlN possesses outstanding piezoelectric and mechanical performances, which can provide new aspects for controlling phonons and photons at the wavelength scale using nanophotonic architectures. These characteristics pose AlN as a promising candidate to address the drawbacks in conventional silicon and silicon nitride platforms. In this review, we aim to present recent advances achieved in AlN photonic integrated circuits ranging from material processing and passive optical routing to active functionality implementation such as electro-optics, piezo-optomechanics, and all-optical nonlinear frequency conversion. Finally, we highlight the challenges and future prospects existing in AlN nanophotonic chips.

Analyzing of the Evolution and the Scaling Properties of a Sinusoidal Mound

The evolution of an initial surface (below its roughening temperature) bounded by a sinusoidal function and consisting of concentric circular steps in two dimensions has been investigated in the Diffusion Limited (DL) regime. Assuming that there were entropic interactions between steps and the local mass transfer took place due to the surface diffusion, the solution of the diffusion equation has been obtained by using polar coordinates in two dimensions. The results obtained in this investigation with analyzing the surface height’s evolution as a function of time are as follows: The surface’s height approximately decreases as τ^α (α≈0.35) and α is independent of the amplitude and the wavelength of the initial surface. The variations in the heights of the surfaces which have different amplitudes (A_01,A_02 ) and wavelengths (λ_1,λ_2 ) scale as (A_01⁄A_02 ) (λ_1⁄λ_2 )^3.

Dynamic modulation of thermal emission—A Tutorial

Thermal emission is typically associated with a blackbody at a temperature above absolute zero, which exchanges energy with its environment in the form of radiation. Blackbody thermal emission is largely incoherent both spatially and temporally. Using principles in nanophotonics, thermal emission with characteristics that differ considerably from those of a blackbody has been demonstrated. In particular, by leveraging intrinsic properties of emerging materials or via nanostructuring at the wavelength or sub-wavelength scale, one can gain control over the directionality, temporal coherence, and other more exotic properties of thermal radiation. Typically, however, these are fixed at the time of fabrication. Gaining dynamic control of thermal emission requires exploiting external mechanisms that actively modulate radiative properties. Numerous applications can benefit from such thermal emission control, for example, in solar energy harvesting, thermo-photovoltaic energy conversion, radiative cooling, sensing, spectroscopy, imaging, and thermal camouflage. In this Tutorial, we introduce thermal emission in two domains: the far-field and the near-field, and we outline experimental approaches for probing thermal radiation in both ranges. We discuss ways for tailoring the spatial and temporal coherence of thermal emission and present available mechanisms to actively tune these characteristics.

Anisotropic plasma sheath turbulence effects on the mode states of Whittaker–Gaussian beams

In this paper detection probability and crosstalk probability variance of the orbital angular momentum states of the diffraction-free Whittaker–Gaussian (WG) beams passing through an anisotropic plasma sheath turbulence (PST) are theoretically formulated. In this regard, using modified von Karman spectrum in the frame of Rytov theory the spiral spectrum of the WG beams is derived, and the role of the anisotropy parameters, refractive index fluctuation variance, initial topological charge, receiving aperture radius, the outer scale of turbulence, beam wavelength and waist in the mitigation of the turbulence-induced disturbance is numerically explored. The remarkable findings indicate that WG beams in anisotropic PST are less influenced than isotropic one, and increasing the anisotropy feature of plasma medium leads to alleviate modulated distortion. Furthermore, it is found out that the propagation performance of WG vortex beams with high-order initial topological charge is significantly better than those with lower order. We expect that our results will be helpful for designing optimal free-space communication systems.

Deep Sub–Wavelength Focusing Metalens at Terahertz Frequency

With the benefits of non–invasive and low radiation, terahertz radiation has shown great potential in biomedical imaging applications. However, the low spatial resolution of the imaging system significantly affects its application in these fields. Although immersion techniques and super–oscillation theory have achieved considerable success in improving the resolution of imaging systems, there are still problems with large focal spot sizes or large sidebands. Herein, a solid immersion lens based on super–oscillation is proposed to reduce the focal spot size when illuminated with circularly polarized light at a wavelength of 118.8 μm. The simulation results show that the lens can compress the full widths at half–maxima down to deep sub–wavelength scales, as small as 0.232 λ. At the same time, the maximum side–lobe ratio was 16.8%, which ensured that the device had a large field of view. The proposed method reveals new ideas in the field of super–resolution imaging.

Extreme focusing of high power x-ray lasers to relativistic intensity with a concave plasma lens

The intensity of present x-ray lasers is still far from relativistic due to equipment damage by the powerful radiation involved in the generation process. We propose to use a microscopic concave plasma lens to tightly focus terawatt x-ray pulses to relativistic intensity by avoiding the destructive instabilities arising from the laser-plasma interaction in the focusing process. Three-dimensional particle-in-cell simulations show that an intense x-ray laser pulse can be focused to a tiny spot of the wavelength scale at ultrahigh intensity while well preserving its original spatiotemporal profile. The resulting pulse can exceed the relativistic intensity threshold [$(1.37\ifmmode\times\else\texttimes\fi{}{10}^{24}/{\ensuremath{\lambda}}_{\mathrm{nm}}^{2})\phantom{\rule{3.33333pt}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$, with ${\ensuremath{\lambda}}_{\mathrm{nm}}$ being the wavelength normalized by nanometers] by two orders of magnitude. Such intense nano-sized x-ray pulses can be useful for exploring nonlinear phenomena in the strong-field quantum electrodynamics regime, as well as in high-energy-density science and astrophysics.

A Practice‐Oriented Benchmark Strategy to Predict the UV‐Vis Spectra of Organic Photocatalysts**

AbstractWith this work, we wish to facilitate further developments in photocatalysis by proposing reliable methods for the computational pre‐screening of potential photocatalysts. To this end, we have developed a new benchmark strategy, and we have applied it to evaluate the predictions given by two wavefunction and several density functional theory (DFT) methods for the UV‐vis absorption spectra of recently developed organic photocatalyst molecules. The novelty in our benchmark framework is that it focuses on evaluating the real‐world applicability of computational methods and does not penalize errors that do not contribute to spectral shapes. We employ a spectral fitting process where the calculated excitations are convoluted with Gaussians using two parameters for broadening and wavelength scaling. This way, most methods can sufficiently reproduce the experimental spectra, but they differ in how much adjustment they require from the parameters. Overall, the double hybrids (with the notable exception of DSD‐BLYP) are the best functionals that offer the highest predictive power as they require practically no scaling. They are exceptionally good in estimating the excitation energies with almost 90 % of the fitted spectra falling into the ±10 % scaling window. This is the same level of accuracy as provided by the STEOM‐DLPNO‐CCSD correlated wavefunction method. In terms of cost efficiency, M06 emerges as the best functional. It compensates a slightly less consistent performance with lower computational demand and availability in nearly all computational codes. Therefore, we recommend the use of double‐hybrid and M06 functionals for UV‐vis spectrum prediction of novel organic photocatalysts, and we also highlight that M06 can be used as a black‐box method even by those who are non‐experts in computational chemistry. The developed protocol and a user‐friendly notebook to assist the analysis are available on GitHub.

Phase-preserving theory and its linearization approximation for forward scattering field of scalar acoustic wave equation

The conventional wave-equation linearization methods, such as the first-order Born or Rytov approximation, always implicitly imply a weak-scattering assumption, making it valid only for weak perturbation models. To extend the wave-equation linearization theory to strong perturbation models, we consider a scenario that the reference model is smooth within the scale of the incident wave length, and propose a phase-preserving method which can predict the phase perturbation of forward scattering wave field. First, we introduce the WKBJ approximation to the scattered- and incident wave fields so that the integral of the unknown solution (i.e. the scattered field) in the nonlinear Ricatti integral equation can be replaced by the integral of scattering-angle and model perturbation, yielding an explicit expression of the scattered field. Theoretical derivation shows that the proposed phase-preserving method can accurately predict the phase-perturbation of forward scattered wave field regardless of the strength of velocity perturbations for one-dimensional wave propagation problem. To apply the phase-preserving approximation to the inverse problem, we further consider a scenario of small-angle forward propagation. In this case, the phase-preserving approximation can be linearized by neglecting the influence of scattering angles, leading to a linear relation between the scattered field and the model perturbation, which we refer to as the generalized Rytov approximation. Numerical experiments demonstrate that the generalized Rytov approximation can predict the phase perturbation of the scattered field with higher accuracy for small-angle forward propagation, and is suitable for strong model perturbations. The generalized Rytov approximation extends the validity and the scope of application of the traditional Rytov approximation. In specific application fields such as the seismic traveltime tomography or medical ultrasonic transmission imaging, a new traveltime/phase sensitivity kernel can be derived by replacing the conventional Rytov approximation with the proposed method, which can increase the inversion accuracy and speed up the convergence.

Hysteretic wavelength selection in isometric, unsupported radial wrinkling.

In [A. S. Pal, L. Pocivavsek and T. A. Witten, arXiv, DOI: 10.48550/arXiv.2206.03552], the authors discuss how an unsupported flat annulus contracted at its inner boundary by fraction Δ, buckles into a radial wrinkling pattern that is asymptotically isometric and tension-free. What selects the wavelength in such a pure-bending configuration, in the absence of any competing sources of work? In this paper, with the support of numerical simulations, we argue that competition between stretching and bending energies at local, mesoscopic scales leads to the selection of a wavelength scale λ* sensitive to both the width w and thickness t of the sheet: λ* ∼ w2/3t1/3Δ-1/6. This scale λ* corresponds to a kinetic arrest criterion for wrinkle coarsening starting from any finer wavelength λ ≲ λ*. However, the sheet can support coarser wavelengths: λ ≳ λ*, since there is no penalty to their existence. Since this wavelength selection mechanism depends on the initial value of λ, it is path-dependent or hysteretic.

The importance of laser wavelength for driving inertial confinement fusion targets. I. Basic physics

We reinvestigate the role that laser wavelength plays in driving inertial confinement fusion (ICF) targets. Different assumptions underlie previous analytic frameworks that provide predictions for wavelength scaling of many important target parameters. These are explored and compared to radiation-hydrodynamics simulations of laser-driven targets. We are particularly interested here in lasers with wavelengths between 0.193 μm [wavelength of the Argon Flouride (ArF) excimer laser] and 0.527 μm (the frequency-doubled glass Nd:glass laser). Short-wavelength drivers have significant advantages for directly driven ICF targets, which are summarized here. We show that constraints such as providing a certain pressure or avoiding laser-plasma instability thresholds allow shorter laser wavelengths to provide energy savings, pressure enhancements, and/or higher hydrodynamic efficiencies. We also consider potential disadvantages, such as increased laser imprint or exposure to the Landau–Darrieus instability. These are shown to be either minor and/or can be easily remediated.

Wavelength scaling of strong-field Rydberg-state excitation: Toward an effective S -matrix theory

$S$-matrix theory and the Born expansion, the lowest-order term of which is known as the ``strong-field approximation'' (SFA), play an indispensable role in our understanding of atomic and molecular processes in intense laser fields. Most phenomena in this field are reproduced by the first two terms of the Born series. However, for a long-range potential such as the Coulomb potential, the second-order term may be larger than the SFA term, raising the problem of the convergence of the Born series. By simultaneously measuring and simulating ionization and Rydberg-state excitation of an argon atom subject to a strong laser field for various wavelengths, we demonstrate that the wavelength scaling law of the measured ratio of ${\mathrm{Ar}}^{*}$ over ${\mathrm{Ar}}^{+}$ and the period of its oscillation with respect to the laser intensity can be well reproduced by the second-order term of the $S$-matrix expansion in terms of the Coulomb potential, but not by the lowest-order (SFA) term. We conjecture that the second-order term of the $S$-matrix expansion may provide an effective theory for the intense-laser--atom interaction.

Scaling Law of THz Yield from Two-Color Femtosecond Filament for Fixed Pump Power

In 3D + time numerical simulations, we study the wavelength scaling law for the energy of terahertz (THz) radiation emitted from a two-color femtosecond filament, which forms during cofocusing into air the fundamental and second harmonics of the laser pulse. In our simulations, the central wavelength of the fundamental harmonic varied from 0.8 to 8 μm and the numerical aperture varied from 0.006 to 0.03. While the harmonics and supercontinuum development are not extreme, so the harmonics spectra are clearly separated, the energy of the generated THz radiation is proportional to the oscillation energy of the electrons, which grows as the squared pump wavelength, and the total number of free electrons in the filament, which decreases quasi-exponentially with the pump wavelength. As a result, the scaling law for the THz energy on the pump wavelength is nonmonotonic with the maximum at 1.6–4 μm depending on the focusing conditions.

Theory of optical axion electrodynamics and application to the Kerr effect in topological antiferromagnets

Emergent axion electrodynamics in magneto-electric media is expected to provide novel ways to detect and control material properties with electromagnetic fields. However, despite being studied intensively for over a decade, its theoretical understanding remains mostly confined to the static limit. Here, we introduce a theory of axion electrodynamics at general frequencies. We define a proper optical axion magneto-electric coupling through its relation to optical surface Hall conductivity and provide ways to calculate it in lattice systems. By employing our formulas, we show that axion electrodynamics can lead to a significant Kerr effect in thin-film antiferromagnets at wavelengths that are seemingly too long to resolve the spatial modulation of magnetism. We identify the wavelength scale above which the Kerr effect is suppressed. Our theory is particularly relevant to materials like MnBi2Te4, a topological antiferromagnet whose magneto-electric response is shown here to be dominated by the axion contribution even at optical frequencies.

Rayleigh-Taylor instability of radiation pressure driven foils: 2D effects

Rayleigh–Taylor instability of radiation pressure accelerated ultra-thin foils of different thickness profiles and initial curvature is investigated in two dimensions using numerical simulations. The convex curvature of the foil (when viewed from the rear side of the foil) provides radially inward motion to the off-axis ions countering the radial divergence due to the Gaussian intensity distribution of the laser. Nonuniform foils, having maximum thickness on laser axis have similar effect. When a small ripple is superimposed on the foil on the scale of laser wavelength, the radiation pressure acts nonuniformly on the microscopic scale and the perturbation grows as the foil moves. After a certain distance of travel, the foil crests turn into cusps and the phase of quasi mono-energy ion acceleration terminates. The planar foil with uniform laser has a strong growth of Rayleigh–Taylor instability (RTI). The foils with curvature and nonuniform thickness have marginally lower growth rate. However, under all circumstances, RTI is found to induce additional energy spread, in radiation pressure acceleration (RPA) of thin foils.

Resist reflow methodology development to investigate interfacial interactions

BackgroundTo enable printing of smaller feature sizes, lithography has progressed into the extreme ultraviolet lithography (EUVL) regime. Alongside the wavelength scaling, a reduction in resist film thickness (FT) is observed to avoid large aspect ratios that would lead to pattern collapse. The further progression to high numerical aperture (NA) EUVL will require a further reduction in resist FT moving toward an ultra-thin film regime (<30-nm resist FT). This reduction in resist FT will make the interfacial interactions between resist and underlayer more dominant, potentially influencing material behavior and making resist design challenging.AimWe assess a reflow process as a means to investigate interfacial effects and in this way deconvolute the correlation between resist line volume, interfacial effects, and the reflow temperature (TR), defined as the temperature at which the resist line starts broadening, which is indicative of the glass transition temperature (Tg).ApproachWe pattern a model EUV chemically amplified resist at different nominal resist FTs and different critical dimensions (CDs) and half-pitch (HP) combinations to quantify changes in the TR.ResultsThe TR increases with the inverse of the CD, as well as the inverse of the resist FT. Moreover, the TR also scales with the area ratio (the ratio of the area in contact with the ambient to the area in contact with the underlayer).ConclusionsA linear relationship between TR and its volume factor (CD × FT) normalized for the area ratio (area in contact with the ambient to the area in contact with the underlayer) is found, revealing a combined dependency on line volume and interfacial interactions. This opens the potential for the use of the reflow methodology in investigating interfacial interactions.