arXiv subjects

H. P. Urbach

Publications and source records attributed to H. P. Urbach.

13 recordsLinked to original sources

arXiv2021

High Finesse Cavity with Anapole-Assisted Resonant Subwavelength Particle Mirror

Strong light interaction with a subwavelength object has been a long pursuing goal with difficulties mainly arising from the diffraction limit. We propose a high finesse cavity with one mirror made of a subwavelength resonant particle as a platform to enhance this interaction. High-quality eigenmode solutions are obtained for such a highly non-paraxial cavity with a very high field concentration at the particle. The eigenmode solutions interact with the small particle in a more general way than by the electric dipole approximation. With the help of the anapole excitation in the dipole term, the particle is designed to scatter like a pure magnetic quadrupole, and in this way, it has anear-unity reflectivity when used as a mirror for the strongly focused field of the eigenmode. Light-matter interactions at the subwavelength scale can be greatly enhanced due to the small size of the particle and the high finesse of the cavity, which can be potentially interesting for applications in nano optics, quantum optomechanics, nonlinear optics, and subwavelength metrology beyond the electric dipole approximation.

physics.optics↗

arXiv2020

Fisher-Information Efficient Metagratings for Transverse Displacement Metrology at the Measurement Independent Shot-Noise Limit

We derive the measurement independent precision bound for transverse displacement metrology using a classical source. Using tools from quantum estimation theory and metamaterial designs, we prove that the bound is achieved efficiently using optimized metagratings by detecting only a very small amount of photons used to probe the displacement, and we also reveal the direct link between the resonant property in the unit cell and the conditions to achieve the bound: one with the 0th dipole resonance and the other one with the anapole condition of the 1st dipole.

physics.optics↗

arXiv2018

Lateral position correction in ptychography with sub-pixel accuracy

Ptychography, a form of Coherent Diffractive Imaging, is used with short wavelengths (e.g. X-rays, electron beams) to achieve high-resolution image reconstructions. One of the limiting factors for the reconstruction quality is the accurate knowledge of the illumination probe positions. Recently, many advances have been made to relax the requirement for the probe positions accuracy. Here, we analyze and demonstrate a straightforward approach that can be used to correct the probe positions with sub-pixel accuracy. Simulations and experimental results with visible light are presented in this work.

eess.IV↗

arXiv2018

Retrieving the Size of Deep-subwavelength Objects via Tunable Optical Spin-Orbit Coupling

We propose a scheme to retrieve the size parameters of a nano-particle on a glass substrate at a scale much smaller than the wavelength. This is achieved by illuminating the particle using two plane waves to create rich and non-trivial local polarization distributions, and observing the far-field scattering pattern into the substrate. A simple dipole model which exploits tunneling effect of evanescent field into regions beyond the critical angle, as well as directional scattering due to spin-orbit coupling is developed, to relate the particle's shape, size and position to the far-field scattering with remarkable sensitivity. Our method brings about a far-field super-resolution imaging scheme based on the interaction of vectorial light with nanoparticles.

physics.optics↗

arXiv2017

Non-negligible magnetic dipole scattering from metallic nanowire for ultrasensitive deflection sensing

It is generally believed that when a single metallic nanowire is sufficiently small, it scatters like a point electric dipole. We show theoretically when a metallic nanowire is placed inside specially designed beams, the non-negligible magnetic dipole contribution along with the electric dipole resonance can lead to unidirectional scattering in the far-field, fulfilling Kerker's condition. Remarkably, this far-field unidirectional scattering encodes information that is highly dependent on the nanowire's deflection at a scale much smaller than the wavelength. The special role of small but non-negligible magnetic response and plasmonic resonance are highlighted for this extreme sensitivity as compared with the dielectric counterpart. Effects such as scattering efficiency and shape of the nanowire's cross section are also discussed.

physics.optics↗

arXiv2017

Study of cost functionals for ptychographic phase retrieval to improve the robustness against noise, and a proposal for another noise-robust ptychographic phase retrieval scheme

Recently, efforts have been made to improve ptychography phase retrieval algorithms so that they are more robust against noise. Often the algorithm is adapted by changing the cost functional that needs to be minimized. In particular, it has been suggested that the cost functional should be obtained using a maximum-likelihood approach that takes the noise statistics into account. Here, we consider the different choices of cost functional, and to how they affect the reconstruction results. We find that seemingly the only consistently reliable way to improve reconstruction results in the presence of noise is to reduce the step size of the update function. In addition, a noise-robust ptychographic reconstruction method has been proposed that relies on adapting the intensity constraints

cs.IT↗

arXiv2016

Spatial Mode Selective Waveguide with Hyperbolic Cladding

Hyperbolic Meta-Materials~(HMMs) are anisotropic materials with permittivity tensor that has both positive and negative eigenvalues. Here we report that by using a type II HMM as cladding material, a waveguide which only supports higher order modes can be achieved, while the lower order modes become leaky and are absorbed in the HMM cladding. This counter intuitive property can lead to novel application in optical communication and photonic integrated circuit. The loss in our HMM-Insulator-HMM~(HIH) waveguide is smaller than that of similar guided mode in a Metal-Insulator-Metal~(MIM) waveguide.

physics.optics↗

arXiv2016

Accurate feeding of nano antenna by polarization singularities for lateral and rotational displacement sensing

Addressing subwavelength object and displacement is crucial in optical nanometrology. We show in this Letter that nano antennas with subwavelength structures can be addressed precisely by incident beams with singularity. This accurate feeding beyond the diffraction limit can lead to dynamic control of the unidirectional scattering in the far field. The combination of polarization discontinuity of the incoming singular beam, along with the rapid phase variation near the antenna leads to remarkable sensitivity of the far field scattering to displacement at deep subwavelength scale. This opens a far field deep subwavelength postion detection method based on the interaction of singular optics with nano antennas.

physics.optics↗

arXiv2016

Image formation properties and inverse imaging problem in aperture based scanning near field optical microscopy

Aperture based scanning near field optical microscopes are important instruments to study light at the nanoscale and to understand the optical functionality of photonic nanostructures. In general, a detected image is affected by both, the transverse electric and magnetic field components of light. The discrimination of the individual field components is challenging, as these four field components are contained within two signals in the case of a polarization-resolved measurement. Here, we develop a methodology to solve the inverse imaging problem and to retrieve the vectorial field components from polarization- and phase-resolved measurements. Our methodology relies on the discussion of the image formation process in aperture based scanning near field optical microscopes. On this basis, we are also able to explain how the relative contributions of the electric and magnetic field components within detected images depend on the probe geometry, its material composition, and the illumination wavelength. This allows to design probes that are dominantly sensitive either to the electric or magnetic field components of light.

physics.optics↗

arXiv2015

Broadband active tuning of unidirectional scattering from nanoantenna using combined radially and azimuthally polarized beams

We propose an approach to actively tune the scattering pattern of a Mie-type spherical antenna. The scheme is based on separate control over the induced electric dipole and induced magnetic dipole using two coherent focused beams of radial polarization and azimuthal polarization. By carefully tuning the amplitude and phase relation of the two beams, a broadband unidirectional scattering can be achieved, even at the wavelength where the antenna scatters most efficiently. By moving the focus of one beam, a drastic switch of the unidirectional scattering can be observed. Such scheme enables the design of ultra-compact optical switches and directional couplers based on nanoantennas.

physics.optics↗

arXiv2015

Extension of Marcatili's analytical approach for rectangular silicon optical waveguides

Marcatili's famous approximate analytical description of light propagating through rectangular dielectric waveguides, published in 1969, gives accurate results for low-index-contrast waveguides. However, photonic integrated circuit technology has advanced to high-index-contrast (HIC) waveguides. In this paper, we improve Marcatili's model by adjusting the amplitudes of the components of the electromagnetic fields in his description. We find that Marcatili's eigenvalue equation for the propagation constant is also valid for HIC waveguides. Our improved method shows much better agreement with rigorous numerical simulations, in particular for the case of HIC waveguides. We also derive explicit expressions for the effective group index and the effects of external forces on the propagation constant. Furthermore, with our method the phenomenon of avoided crossing of modes is observed and studied.

physics.optics↗

arXiv2015

Optimized 3D simulation method for modeling of out-of-plane radiation in silicon photonic integrated circuits

We present an accurate and fast 3D simulation scheme for out-of-plane grating couplers, based on two dimensional rigorous (finite difference time domain) grating simulations, the effective index method (EIM), and the Rayleigh-Sommerfeld diffraction formula. In comparison with full 3D FDTD simulations, the rms difference in electric field is below 5% and the difference in power flux is below 3%. A grating coupler for coupling from a silicon-on-insulator photonic integrated circuit to an optical fiber positioned 0.1 mm above the circuit is designed as example.

physics.optics↗

arXiv2008

Electromagnetic Spin-Orbit Interactions via Scattering

The longitudinal components of orthogonal-circularly polarized fields carry a phase singularity that changes sign depending on the polarization handedness. The addition of orbital angular momentum adds to or cancels this singularity and results in polarization-dependent scattering through round and square apertures, which we demonstrate analytically, numerically, and experimentally. By preparing the incident polarization and arranging the configuration of sub-wavelength apertures, we produce shadow-side scattered fields with arbitrary phase vorticity.

physics.optics↗