Effect of boundary conditions in modeling of microsphere-assisted imaging.

Exploring the performance of label-free imaging relies heavily on adequate physical models and accurate numerical simulations. A particularly challenging situation is imaging through contact microspheres, which have demonstrated resolution values exceeding the diffraction limit. Here an ab initio modeling of microsphere-assisted imaging is reported and its results are analyzed. The key part of modeling is solving the light scattering problem, which requires handling a rather large computational domain and broad angle illumination made up of multiple mutually incoherent plane waves. To account for plane wave incidence, two simulation approaches are developed that differ only by boundary conditions-quasiperiodic and absorbing. The algorithms to find images in both approaches are discussed and the simulation results are compared for free space and microsphere-assisted imaging. It is shown that while the super-resolution in microsphere-assisted imaging can be demonstrated using both approaches, the latter allows a large reduction in the computational resources. This significantly extends the capability of the simulations, enabling a rigorous exploration of novel imaging regimes.

Ab initio simulation of imaging of wavelength-sized objects and estimation of resolution.

Image characterization in microscopy, in particular, the estimation of its resolution, requires detailed knowledge of its relation to the object. For objects with sizes comparable to or smaller than the operating wavelength, such a relation can be obtained only by considering electromagnetic scattering described by the Maxwell equations. Here we follow precisely the steps involved in the image formation in microscopy with broad angle illumination-starting from the Maxwell equations to find the scattered far fields for each plane wave, projecting them into a sensor array, and finally assembling the incoherent image by adding all coherent contributions. We consider a classical object-a narrow slit in an absorbing screen, which is taken as a very thin chromium film deposited on a glass substrate. The inapplicability of the Kirchhoff approximation for such a slit is addressed, and the calculated image is subsequently analyzed to evaluate its intrinsic resolution using a point spread function. The difference in image intensities defined using the Poynting vector and the electric field intensity is also discussed.

Wave optics of imaging with contact ball lenses.

Recent progress in microspherical superlens nanoscopy raises a fundamental question about the transition from super-resolution properties of mesoscale microspheres, which can provide a subwavelength resolution [Formula: see text], to macroscale ball lenses, for which the imaging quality degrades because of aberrations. To address this question, this work develops a theory describing the imaging by contact ball lenses with diameters [Formula: see text] covering this transition range and for a broad range of refractive indices [Formula: see text]. Starting from geometrical optics we subsequently proceed to an exact numerical solution of the Maxwell equations explaining virtual and real image formation as well as magnification M and resolution near the critical index [Formula: see text] which is of interest for applications demanding the highest M such as cellphone microscopy. The wave effects manifest themselves in a strong dependence of the image plane position and magnification on [Formula: see text], for which a simple analytical formula is derived. It is demonstrated that a subwavelength resolution is achievable at [Formula: see text]. The theory explains the results of experimental contact-ball imaging. The understanding of the physical mechanisms of image formation revealed in this study creates a basis for developing applications of contact ball lenses in cellphone-based microscopy.

Light scattering at a temporal boundary in a Lorentz medium.

Temporal discontinuity in the permittivity of a nondispersive dielectric (temporal boundary) is a conventional model for considering electromagnetic phenomena in dynamic materials and metamaterials. Here we apply a more general model of a Lorentz medium with the rapidly changing density of its structural elements (oscillators) or their resonant frequency to determine the realms of applicability of the conventional temporal boundary model. We demonstrate the dependence of the continuity conditions and the energy relations at a temporal boundary on the nonstationarity mechanism and the ratio between the rate of nonstationarity and the characteristic frequencies in the system.

Electromagnetic wave transformation in an oscillator medium with growing density.

The model of a medium made of oscillators can describe various materials, including artificial ones made of meta-atoms. Here the transformation of an electromagnetic wave in such a medium with rapidly growing oscillator density is studied. The initial polariton is shown to be transformed not only into new polariton modes but also into natural oscillations of the existing and created oscillators. The oscillations produce zero net polarization but consume significant amount of the initial polariton energy. Although the boundary conditions at the temporal jump are sufficient to find the new polaritons, the description of the polarization oscillations requires integrating the material equations because the oscillations are uncoupled from the fields. Under some conditions, the spatial dependence of the amplitude of the oscillations can provide a snapshot of the electromagnetic field distribution at the moment of rapid density growth. Some subtle issues related to the continuity conditions at the density jump are also discussed.

Levitation and propulsion of a Mie-resonance particle by a surface plasmon.

It is predicted that the optical force induced by a surface plasmon can form a stable equilibrium position for a resonant particle at a finite distance from the surface. The levitated particle can be efficiently propelled along the surface without touching it. The levitation originates from the strong interaction of the particle with the surface.

Resonant optical propulsion of a particle inside a hollow-core photonic crystal fiber.

Resonant propulsion of small nonresonant particles inside metal waveguides due to the formation of resonant states by the guided modes below their cutoffs has been predicted in the past. Here it is shown that stable resonant propulsion exists in hollow-core photonic crystal fibers, which are all-dielectric structures and are a major platform for various photonic applications. Specific features of the resonant propulsion are discussed together with the fiber design issues. The results may enable power-efficient transport of particles over long distances, particle sorting, and sensitive detection.

Ultimate propulsion of wavelength-sized dielectric particles.

It is shown that a wavelength-sized dielectric particle can form a resonant state with the below-cutoff mode of a waveguide even for rather small values of the refractive index contrast between the particle and the background. The excitation of the resonant state creates the propelling force twice the momentum flow of the incident mode, and the particle is reliably trapped at the waveguide center. Since neither the particle nor the waveguide possesses individually any resonant properties, this operating regime can be applicable for the manipulation of particles of various materials, sizes, and shapes.

Reflection and transmission of electromagnetic waves at a temporal boundary: comment.

Recently, Xiao et al. [Opt. Lett. 39, 574 (2014)] compared two sets of boundary conditions and the resulting transformation coefficients for an electromagnetic wave at a temporal boundary. They claimed to identify a correct set and to resolve the existing discrepancy in the literature. We point out that the boundary conditions discarded by Xiao et al. as incorrect have been used in the literature for rapidly growing plasma, for which the material model of Xiao et al. is not appropriate. We show that Xiao et al. misinterpreted the results from the literature by opposing two sets of boundary conditions that are related to different material models of the temporal boundary.

Rigorous calculation of the nonlinear Kerr coefficient for a waveguide using power-dependent dispersion modification.

It is proposed that the direct calculation of the dispersion for a waveguide using the effective power-dependent permittivity is a rigorous way to determine its nonlinear Kerr coefficient. The tensor permittivity accounts fully for the vectorial nature of the electric field. The calculated Kerr coefficients for the lowest transverse-magnetic modes of a nonlinear slab and wire are compared with the results of several formulas existing in the literature. The proposed approach can conveniently be implemented using standard mode solvers.

Resonant optical gun.

We propose a concept of a structure-a resonant optical gun-to realize an efficient propulsion of dielectric microparticles by light forces. The structure is based on a waveguide in which a reversal of the electromagnetic momentum flow of the incident mode is realized by exciting a whispering gallery resonance in the microparticle. The propelling force can reach the value up to the theoretical maximum of twice the momentum flow of the initial wave. The force density oscillates along the particle periphery and has very large amplitude.

Resonant pulling of a microparticle using a backward surface wave.

It is predicted that the optical force experienced by a dielectric particle excited resonantly by a surface wave can be directed opposite to the incident power flow when the exciting wave is a backward one. This is consistent with the electromagnetic momentum flow of the backward wave being directed opposite to the power flow. The magnitude of the force can be comparable to the momentum flow of the surface wave. Such forces bring a deeper understanding of the electromagnetics of backward surface waves and can be used in integrated photonic circuits and optofluidic devices.

Normal dispersion femtosecond fiber optical parametric oscillator.

We propose and demonstrate a synchronously pumped fiber optical parametric oscillator (FOPO) operating in the normal dispersion regime. The FOPO generates chirped pulses at the output, allowing significant pulse energy scaling potential without pulse breaking. The output average power of the FOPO at 1600 nm was ∼60 mW (corresponding to 1.45 nJ pulse energy and ∼55% slope power conversion efficiency). The output pulses directly from the FOPO were highly chirped (∼3 ps duration), and they could be compressed outside of the cavity to 180 fs by using a standard optical fiber compressor. Detailed numerical simulation was also performed to understand the pulse evolution dynamics around the laser cavity. We believe that the proposed design concept is useful for scaling up the pulse energy in the FOPO using different pumping wavelengths.

Eddy currents and magnetic moments of planar rings of arbitrary width.

We analyze the response of a metallic ring to an applied time-harmonic and spatially uniform magnetic field. The self-consistent current distribution and magnetic moment are compared with that from an equivalent LR circuit model and the range of parameters (size, frequency and conductivity) for the model applicability is analyzed. The circuit model does not perform quantitatively well in the parameter region with high magnetic moment and low losses that is important for metamaterial design. We show that scaling of the magnetic response is conveniently described by a single parameter which combines size, conductivity, and frequency.

Fresnel formulas for the forced electromagnetic pulses and their application for optical-to-terahertz conversion in nonlinear crystals.

We show that the usual Fresnel formulas for a free-propagating pulse are not applicable for a forced terahertz electromagnetic pulse supported by an optical pulse at the end of a nonlinear crystal. The correct linear reflection and transmission coefficients that we derive show that such pulses can experience a gain or loss at the boundary. This energy change depends on linear dielectric constants only. We also predict a regime where a complete disappearance of the forced pulse under oblique incidence occurs, an effect that has no counterpart for free-propagating pulses.

Quantum coherence in an optical modulator.

Semiconductor quantum well electroabsorption modulators are widely used to modulate near-infrared (NIR) radiation at frequencies below 0.1 terahertz (THz). Here, the NIR absorption of undoped quantum wells was modulated by strong electric fields with frequencies between 1.5 and 3.9 THz. The THz field coupled two excited states (excitons) of the quantum wells, as manifested by a new THz frequency- and power-dependent NIR absorption line. Nonperturbative theory and experiment indicate that the THz field generated a coherent quantum superposition of an absorbing and a nonabsorbing exciton. This quantum coherence may yield new applications for quantum well modulators in optical communications.

Two-dimensional theory of Cherenkov radiation from short laser pulses in a magnetized plasma.

The Cherenkov wakes excited by intense laser drivers in a perpendicularly magnetized plasma are a potential source of high-power terahertz radiation. We present a two-dimensional (2D) theory of the emission of magnetized wakes excited by a short laser pulse. The 2D model reveals the important role of the transverse size of the laser pulse missed in previous simple one-dimensional estimations of the radiation. We derived expressions for the radiated fields and for the angular/frequency distribution of the radiated energy. Beats in the radiation pattern behind the moving pulse are predicted and explained. For the interpretation of existing experimental results, the time dependence of the energy flux parallel and perpendicular to the laser path is examined.

Far-field emission of a semiconductor nanowire laser.

The polarization properties and angular distribution of intensity of the far fields from a nanowire laser are investigated. The far-field emission depends strongly on the mode type (HE11, TE01, TM01) and the radius of the nanowire. The emission is weakly directional, and a large part of it can be emitted in the backward direction. Our results can be applied for experimental determination of a lasing mode by its far fields as well as for optimization of laser emission.

Interaction of an electromagnetic wave with a suddenly stopped ionization front.

The theory of the interaction of an electromagnetic wave with a uniformly moving ionization front in a gas is extended to include the case when the front suddenly stops. This nonstationary character of the wave/front interaction, which is typical for experiments carried out in a finite-size gas tube, gives rise to fresh physical effects. First, currents induced near the plasma boundary after the front stops produce a static magnetic field not only in the plasma behind the front but also in the vacuum ahead of the front. Second, in the regime where the transmitted wave falls off behind the front, the skinning field leaks through the stopped front and produces a burst of highly frequency up-shifted radiation.