Small Dielectric Objects Research Articles

Optomechanically induced optical trapping system based on photonic crystal cavities.

Optical trapping has proven to be a valuable experimental technique for precisely controlling small dielectric objects. However, due to their very nature, conventional optical traps are diffraction limited and require high intensities to confine the dielectric objects. In this work, we propose a novel optical trap based on dielectric photonic crystal nanobeam cavities, which overcomes the limitations of conventional optical traps by significant factors. This is achieved by exploiting an optomechanically induced backaction mechanism between a dielectric nanoparticle and the cavities. We perform numerical simulations to show that our trap can fully levitate a submicron-scale dielectric particle with a trap width as narrow as 56 nm. It allows for achieving a high trap stiffness, therefore, a high Q-frequency product for the particle's motion while reducing the optical absorption by a factor of 43 compared to the cases for conventional optical tweezers. Moreover, we show that multiple laser tones can be used further to create a complex, dynamic potential landscape with feature sizes well below the diffraction limit. The presented optical trapping system offers new opportunities for precision sensing and fundamental quantum experiments based on levitated particles.

Dynamic dielectrophoretic cell manipulation is enabled by an innovative electronics platform

We report on a portable dielectrophoresis manipulation platform, for the positioning and immobilization of small dielectric objects, such as cells or microbes, using a circular array of up to six symmetrically arranged and independently controllable micro-electrodes. The system’s micro-controller-instructed electronic drivers can be operated in three distinct modes. We demonstrate the system’s function by actuating bacterial cells to specific locations and orientations within a region of interest. Using a deep learning approach, we map voltage phase combinations of the six electrodes to geometrical trap position locations, thereby facilitating smooth trajectory planning.

Boundary layer method for solving full Maxwell equations in the presence of an electromagnetic inhomogeneity of small diameter

We consider solutions to the time-harmonic Maxwell's equations in two and three dimensions. For such solutions we derive high-order terms in the asymptotic expansions of the perturbations resulting from the presence of diametrically small electromagnetic inhomogeneity with parameters different from the background medium. Our study is rigorous and is founded on layer potential techniques. Our formulas may be awaited to head effective computational identification algorithms, aimed at reconstructing small dielectric object from electromagnetic boundary measurements.

Magnetoquasistatic resonances of small dielectric objects

A small dielectric object with positive permittivity may resonate when the free-space wavelength is large in comparison with the object dimensions if the permittivity is sufficiently high. We show that these resonances are described by the magnetoquasistatic approximation of the Maxwell's equations in which the normal component of the displacement current density field vanishes on the surface of the particle. They are associated to values of permittivities and frequencies for which source-free quasistatic magnetic fields exist, which are connected to the eigenvalues of a magnetostatic integral operator. We present the general physical properties of magnetoquasistatic resonances in dielectrics with arbitrary shape. They arise from the interplay between the polarization energy stored in the dielectric and the energy stored in the magnetic field. Our findings improve the understanding of resonances in high-permittivity dielectric objects and provide a powerful tool that greatly simplifies the analysis and design of high index resonators.

Reference-Free Material Characterisation of Objects Based on Terahertz Ellipsometry

Material characterization in the 0.1 - 10 THz range has been a major topic of research since its first demonstration 30 years ago. Advances in terahertz generation, detection, and data acquisition have contributed to improved bandwidth, signal power and signal-to-noise ratio. However, material characterization is still performed using conventional spectroscopic measurement schemes which require detailed information about the test object's shape, location, orientation relative to the measurement system, and a reference measurement. Here, we present a method for reference-free material characterization of dielectric objects using a terahertz time-domain spectroscopy system without a priori knowledge about the object and its position. The proposed method is based on ellipsometry combined with lensless imaging for the estimation of the refractive index. The method is a multistage procedure designed for small test objects in reflection mode. The diffracted terahertz radiation is separated from the specular reflected radiation in a post-processing step to enable a valid material parameter estimation. In this way, small dielectric objects can be located, imaged with a sub-mm resolution, and their material parameters extracted.

High-Resolution Long-Range THz Imaging for Tunable Continuous-Wave Systems

Imaging in the terahertz frequency range has attracted growing interests since the first image of a leaf more than 20 years ago, due to its countless applications in basic and applied research, medical imaging, and nondestructive testing. However, most terahertz imaging approaches rely on focusing optics which require knowledge about the imaging scene before the actual imaging takes place. Further, imaging is mostly restricted to short distances and high resolution is only achieved for systems with a high bandwidth. Here, we present a method that enables high-resolution imaging of small metallic and dielectric objects at distances up to 2 m based on a synthetic aperture. We derive a simple approximation for the resolution of partial circular synthetic apertures with limited bandwidth. The bandwidth limitation is encountered by replacing the measured signals with replica signals of high bandwidth and equal round-trip time so that the resolution is only limited by the carrier frequency and signal-to-noise ratio of the measurement system.

Compressive Sensing Holographic Microwave Random Array Imaging of Dielectric Inclusion

Holographic microwave imaging is an innovative method to image biological objects based on their dielectric properties, which has the advantages of high spatial resolution. However, the image reconstruction method is always a critical issue in holographic microwave imaging. This research aims to investigate the feasibility and effectiveness of applying compressive sensing (CS) technique to the holographic microwave imaging for small dielectric object detection. This paper presents a compressive sensing holographic microwave random array imaging (CS-HMRAI) method for imaging of dielectric objects. A numerical system consists of various dielectric models and imaging processing model are developed to evaluate the proposed approach. The split Bregman (SB) and orthogonal matching pursuit (OMP) algorithms are applied to HMRAI for evaluation of small inclusions embedded in dielectric objects. Various experiments are conducted to identify lesions using the proposed CS-HMRAI method and results are compared with HMRAI and HMRAI via OMP methods. Both simulation and experimental results demonstrate that CS-HMRAI via SB can produce high-quality images and detect arbitrarily shaped small inclusions with random sizes and locations by using significantly fewer sensors and scanning times than the HMRAI and CS-HMRAI via OMP approaches. The proposed approach has the potential for further investigation for breast tumor detection in a fast and cost-effective manner.

Electromagnetic induction holography imaging for stroke detection.

This paper presents the application of the electromagnetic induction holography (EMIH) approach to imaging the electromagnetic activity of the brain, with a particular focus on stroke detection. An integral equation formulation is presented to describe the scattered magnetic field from the distribution of optically small dielectric and magnetic objects of arbitrary shapes adsorbed onto a planar electromagnetic substrate. A numerical computer model was developed in a MATLAB environment to validate the theory. Several realistic human head models were developed to investigate the detectability of strokes with the multi-channel EMIH system. Small strokes can be clearly identified with the correct location and size in the reconstructed head images. The simulation results demonstrated the feasibility of detecting and imaging small strokes using the approach.

Microwave Measurement System for Detection of Dielectric Objects in Powders

Detection and removal of undesirable objects is an important issue in many material processing industries. This paper presents a microwave-based measurement technique for detection of dielectric objects in powder materials flowing through metal pipes. A nonintrusive microwave sensor is developed, which uses multiple resonant modes to obtain high sensitivity with respect to permittivity variations inside the pipe. Undesirable objects are detected based on the scattering parameters of the sensor, which are measured using a fast-sampling microwave transmitter and receiver unit. We present a detection algorithm derived from the likelihood-ratio test, which includes a parametric model of the local statistical distribution of the measured scattering parameter data. The model involves a set of Mobius transformations that map the measured scattering parameters to a domain where the data are Gaussian distributed. All unknown parameters in the model are estimated from data using maximum likelihood. Based on measurement results from a gravity-fall experimental setup, we conclude that small dielectric objects can be reliably detected in heterogeneous flows of dielectric powder.

Atomic force microscopy combined with optical tweezers (AFM/OT)

The role of mechanical properties is essential to understand molecular, biological materials, and nanostructures dynamics and interaction processes. Atomic force microscopy (AFM) is the most commonly used method of direct force evaluation, but due to its technical limitations this single probe technique is unable to detect forces with femtonewton resolution. In this paper we present the development of a combined atomic force microscopy and optical tweezers (AFM/OT) instrument. The focused laser beam, on which optical tweezers are based, provides us with the ability to manipulate small dielectric objects and to use it as a high spatial and temporal resolution displacement and force sensor in the same AFM scanning zone. We demonstrate the possibility to develop a combined instrument with high potential in nanomechanics, molecules manipulation and biological studies. AFM/OT equipment is described and characterized by studying the ability to trap dielectric objects and quantifying the detectable and applicable forces. Finally, optical tweezers calibration methods and instrument applications are given.

2.5D Quantitative Millimeter Wave Imaging of a Hidden Object on a Simplified Human Body Model Using Value Picking Regularization

In this contribution, the authors provide a proof of principle for quantitative imaging of concealed objects on the human body using millimeter waves. A two-and-a-half-dimensional (2.5D) quantitative millimeter wave imaging algorithm is applied to reconstruct a hidden dielectric object on a clothed simplified human body model. At millimeter wave frequencies, the incident field is typically a fully three-dimensional (3D) Gaussian beam, illuminating only a limited spot on the body. Due to the large dimensions of the human body in terms of wavelengths, a 3D discretization is hardly feasible. Therefore, it is assumed that the electromagnetic properties of the body do not significantly change within the illuminated spot, along the longitudinal direction of a person. Hence, only the cross-section of a human body model is discretized. This 2.5D assumption however is still not sufficient to reduce the forward problem to a feasible size. Therefore, a priori knowledge on the illumination and on the scattering properties of the clothed human body is used to deduce a simplified model to describe the cross-section of the clothed human abdomen. The complex permittivity profile of a small dielectric object, hidden underneath clothing and representing some type of explosive, is reconstructed. The complex permittivity profiles of all other scatterers are assumed to be known. The presented quantitative inverse scattering algorithm is based on a Newton-type optimization, combined with an approximate line search and regularized by applying Stepwise Relaxed Value Picking regularization. The input data of the quantitative inverse scattering problem are synthetic scattering data since the authors are not aware of any amplitude and phase measurement data for concealed weapon detection yet made available to the inversion community at these high frequencies.

Calculating optical forces using the boundary integral method

In this paper, we show that the boundary integral method is highly efficient for the calculation of optical forces on small dielectric and metallic objects. The boundary integral formulation for the Maxwell equations is stated, and an implementation of the equations is described, tested and used to derive new bistability results for two dielectric spheres in counterpropagating incoherent laser beams.

Semi‐analytical time‐domain low‐frequency scattering formulation for biological applications

AbstractIn this paper, an efficient time domain formulation is proposed for a low‐frequency scattering by a complicated inhomogeneous dielectric scatterer. The formulation is based on the Born approximation and a sampling theorem. To extend the valid region of the proposed solution, the Rayleigh scattering formulation for a small dielectric object is used to modify the Born current inside the scatterer. In order to verify the obtained formulation, scatterings by several canonical objects such as a cubic box and a sphere are compared, which are calculated by the proposed formulation and the known exact or approximate solutions of a Mie series and the Rayleigh scattering. In addition, scattering by a human head is investigated at a very low‐frequency band (< 100 Hz). For the simulation, two excitations are considered: plane wave incidence from outside the head and an artificial point source located inside the head. The head model is discretized by a 3 × 3 × 3 mm3 cubic box, which is comprised of eight types of tissue. These are blood, bone, fat, grey matter, white matter, muscle, skin and vitreous humor. Copyright © 2006 John Wiley & Sons, Ltd.

The topological asymptotic expansion for the Maxwell equations and some applications

The aim of the topological sensitivity analysis is to derive an asymptotic expansion of a design functional with respect to a topological perturbation of the domain. In this paper, such an expansion is obtained for the 3D Maxwell equations when we introduce a small dielectric object in the domain and when we insert a small metallic obstacle. Some numerical results are presented in the context of buried object detection and shape inversion of 3D objects in free space from time-domain scattered field data.

How mesoscopic objects localized in a layered structure affect the light-field distribution in near-field surface microscopy

This paper discusses the process by which images are formed in the near-field optical microscopy of nanoscopic particles localized inside or on the surface of a single-layer dielectric structure. A small dielectric object that scans the surface of the sample constitutes the near-field radiation source and the probe. The model is based on self-consistent integral equations for the electric fields in the system. The electric interaction in the near wave zone is described in terms of the quasi-static approximation. The electric-field distribution is numerically modelled in both the near and far wave zones of the system. It is shown how the layered dielectric environment and the substrate can affect the contrast and form of the images of mesoscopic particles, depending on their dielectric properties. It is demonstrated that the change of the resolution with increasing distance between the probe and the sample surface substantially depends on the method of recording the signal. © 2004 Optical Society of America

Simulated Output Images of Near-Field Optics by Volume Integral Equation: Object Placed on the Dielectric Substrate

We investigated near-field optical (NFO) imaging characteristics of a small object placed on a dielectric slab by a computer-code using a three-dimensional volume integral equation with an effective iteration technique called the generalized minimal residual method. A simplified three-dimensional NFO microscope that consists of a small dielectric object placed on the dielectric substrate and a small dielectric sphere as a scanning probe-tip was considered. Calculating two-dimensional output images obtained from scattered far fields, we studied the effect of the substrate on NFO output images, the comparison of NFO output images with electrostatic field around the small object, the dependence of output image characteristics on the wavelength and the difference of imaging characteristics between incident plane waves and incident evanescent waves.

Near-field distributions of resonant modes in small dielectric objects on flat surfaces

We report numerical simulations of the coupling of waves, either propagating or evanescent, with the eigenmodes of dielectric nanocylinders and nanospheres upon substrates. The multiple interaction of light between these objects and the dielectric surface at which the evanescent waves are created is taken into account. In this way, we present an accurate procedure for predicting and controlling the creation of large field enhancements concentrated both within and near the nanoparticle compared with the angle of incidence and the state of polarization.

Resonant Cavity Perturbation̵Some New Applications of an Old Measuring Technique

Microwave resonators have been used as sensors for determining material properties of dielectric objects, including permittivity, density, moisture content, and mass, from measurements of frequency shift and change in cavity transmission characteristics when a small dielectric object is inserted into a microwave resonant cavity. Fundamental principles of the shape-and size-independent measurements are discussed, and experimental results for moisture content and mass measurement of individual soybeans, peanuts and grain kernels, as well as for mass determinations on irregularly shaped objects of common plastic materials are presented. Measurements are fast, nondestructive, contactless, and have the potential for continuous operation.

Near-field imaging with a waveguide resonator probe in collection and reflection mode: II. Numerical simulation

The moment method for calculation of diffraction near fields behind small dielectric objects is extended to two-dimensional scanning near-field optical microscopy (SNOM) with a waveguide resonator probe by including the object-probe interaction and multiple reflections within the probe into the algorithm. Simple, approximate expressions are derived for excitation of the propagating mode and backscattering of a free space field by the waveguide probe. SNOM amplitude and phase images of small dielectric bars (cross section < lambda 2) are calculated for collection and reflection mode. Measurements with 3 m microwaves yield a good agreement.

Contactless Mass Determination of Arbitrarily Shaped Objects by Microwave Resonator Measurements

ABSTRACTThe basis is presented for using a microwave resonant cavity as an effective “contactless balance”, providing information about the mass of dielectric objects inserted into the cavity. An uncertainty analysis presented in the paper confirms that the mass of small dielectric objects can be determined by this technique with an accuracy better than 4%.