Near-field Regime Research Articles

Analytical Expressions for the Mutual Coupling of Loop Antennas Valid From the RF to Optical Regimes

Arrays of circular loop antennas are commonly employed at radio frequencies for communications and geophysical sensing, while also holding enormous potential in the optical regime for applications such as solar energy harvesting. Exact analytical expressions exist for predicting the mutual coupling between a variety of antennas, including dipoles and slots. However, due to the complexity of the integrals involved, analytical expressions for evaluating the coupling between loop antennas have not been previously available. This paper presents straightforward analytical expressions for efficient calculation of the coupling between two circular loops at arbitrary locations. The theory is extended to the optical regime by taking into account the dispersion and loss of the material comprising the loop antenna. These analytical expressions provide insight into the physics underlying the mutual coupling phenomenon. Along with the approximate analytical expressions, a useful pseudo-analytical representation is developed which is more exact, especially in the near-field regime, and can be easily and efficiently evaluated in MATLAB via numerical integration. It is shown that full-wave simulations for a two-element array of nanoloops can take up to six hours, while the corresponding analytical and pseudo-analytical implementations derived here take less than a minute.

Near‐field and far‐field shared structure for NFC and CNSS applications

This study presents a shared structure for both near- and far-field operations using magnetic resonance coupling and electromagnetic radiation. The proposed structure is composed of a rectangular loop and two monopoles. A duplexer is designed to route the near- and far-field signals. For far-field operation, both dual-band resonance and dual-sense circular polarisation required by the compass navigation satellite system (CNSS) are achieved. For near-field operation, a wireless communication link operating at 13.56 MHz is established. The effect of matching distance on the transmission performances is studied and the optimal matching network is obtained. The proposed design is fabricated and verified experimentally. In the far-field regime, it can achieve 10 dB impedance bandwidths of 17.2 and 15.8% and 3 dB axial ratio bandwidths of 4.6 and 12.8% and the two CNSS bands are fully covered. In the near-field regime, an average transmission coefficient of 0.59 is achieved within the operation range of 5-30 mm by the proposed design. The obtained good performance reveals that the proposed shared structure is suitable to be applied in handheld devices for near-field communication and CNSS applications.

Wide-Band and High-Efficiency 90° Polarization Rotator Based on Tri-Layered Perforated Metal Films

Tri-layered metal films perforated with rectangle hole arrays of different orientations and separated by glass/air spacers have been studied theoretically and experimentally. The devices can rotate the polarization of the incident light by 90° with high efficiency and wide bandwidth in the near-infrared and microwave regions. The high-efficiency and wide bandwidth polarization rotation can be ascribed to the near-field coupling regime rather than the Fabry–Perot resonance according to the investigation of the difference between the two regimes.

High-performance near-field electroluminescent refrigeration device consisting of a GaAs light emitting diode and a Si photovoltaic cell

We consider a near-field electroluminescent refrigeration device. The device uses a GaAs light emitting diode as the cold side, and a Si photovoltaic cell as the hot side. The two sides are brought in close proximity to each other across a vacuum gap. The cooling is achieved by applying a positive bias on the GaAs light emitting diode. We show that the choice of GaAs and Si here can suppress the non-idealities for electroluminescent cooling purposes: GaAs has a wide bandgap with low Auger recombination, and Si is a non-polar semiconductor which leads to significantly reduced sub-bandgap heat transfer. We show that by using this configuration in the near-field regime, the cooling power density can reach 105 W/m2 even in the presence of realistic Auger recombination and Shockley-Read-Hall recombination. In addition, with photovoltaic power recovery from the Si cell, the efficiency of the device can be further improved. Our work points to the significant potential of combining near-field heat transfer with active semiconductor devices for the control of heat flow.

High-performance near-field thermophotovoltaics for waste heat recovery

The US industries reject nearly 20–50% of the consumed energy into the environment as waste heat. Harvesting this huge amount of heat can substantially improve the energy usage efficiency. For waste heat in the medium temperature range (~ 500–900K), traditional solid-state waste heat recovery techniques like thermoelectric generators and thermophotovoltaics (TPVs) are still suffering from relatively low efficiency or power density. In this work, we analyze a near-field TPV system consisting of a plasmonic emitter (indium tin oxide) and a narrow-bandgap photovoltaic cell (InAs) that are brought to deep sub-wavelength distances for high-efficiency and high-power-density waste heat recovery. We show that despite the inclusion of realistic nonradiative recombination rates and sub-bandgap heat transfer, such a near-field TPV system can convert heat to electricity with up to nearly 40% efficiency and 11W/cm2 power density at a 900K emitter temperature, because of the spectral reshaping and enhancement by the thermally excited surface plasmons and waveguide modes. Thus, we show that for waste heat recovery, near-field TPV systems can have performances that significantly exceed typical thermoelectric systems. We propose a modified system to further enhance the power density by using a thin metal film on the cell, achieving a counterintuitively “blocking-assisted” heat transfer and power generation in the near-field regime.

Laser control of giant optical absorption and gain in quantum plasmonic particles

A quantum particle strongly interacting with a nearby metallic particle is a fundamental system in nanoplasmonics. The dielectric function of the quantum system can manifest rich quantum nonlinear optics phenomena when the spatial dependence of the vectorial nature of the probing laser field and the dipole moment are considered. We find that the quantum system can exhibit giant gain and giant absorption when the dipole moment orientation and the direction, polarization, and relative phase of the probe field components are properly tuned, especially in the near-field regime. This can be explained as due to the strong interactions between the quantum system and the metallic particle in the near-field regime at surface plasmon resonance. Interestingly, these giant features can be suppressed by a large probe Rabi frequency. The study enables remarkable coherent control of optical properties in quantum nanoplasmonics, useful for enhancing absorption in solar cells and light amplification for lasing and sensing.

Assembly of Plasmonic Nanoparticles on Nanopatterns of Polymer Brushes Fabricated by Electrospin Nanolithography.

This paper presents electrospin nanolithography (ESPNL) for versatile and low-cost fabrication of nanoscale patterns of polymer brushes to serve as templates for assembly of metallic nanoparticles. Here electrospun nanofibers placed on top of a substrate grafted with polymer brushes serve as masks. The oxygen plasma etching of the substrate followed by removal of the fibers leads to linear patterns of polymer brushes. The line-widths as small as ∼50 nm can be achieved by precise tuning of the diameter of fibers, etching condition, and fiber-substrate interaction. Highly aligned and spatially defined patterns can be fabricated by operating in the near-field electrospinning regime. Patterns of polymer brushes with two different chemistries effectively directed the assembly of gold nanoparticles and silver nanocubes. Nanopatterned brushes imparted strong confinement effects on the assembly of plasmonic nanoparticles and resulted in strong localization of electromagnetic fields leading to intense signals in surface-enhanced Raman spectroscopy. The scalability and simplicity of ESPNL hold great promise in patterning of a broad range of polymer thin films for different applications.

Enhancing Near-Field Radiative Heat Transfer with Si-based Metasurfaces.

We demonstrate in this work that the use of metasurfaces provides a viable strategy to largely tune and enhance near-field radiative heat transfer between extended structures. In particular, using a rigorous coupled wave analysis, we predict that Si-based metasurfaces featuring two-dimensional periodic arrays of holes can exhibit a room-temperature near-field radiative heat conductance much larger than any unstructured material to date. We show that this enhancement, which takes place in a broad range of separations, relies on the possibility to largely tune the properties of the surface plasmon polaritons that dominate the radiative heat transfer in the near-field regime.

Finite dipole model for extreme near-field thermal radiation between a tip and planar SiC substrate

Recent experimental studies have measured the infrared (IR) spectrum of tip-scattered near-field thermal radiation for a SiC substrate and observed up to a 50cm−1 redshift of the surface phonon polariton (SPhP) resonance peak [1,2]. However, the observed spectral redshift cannot be explained by the conventional near-field thermal radiation model based on the point dipole approximation. In the present work, a heated tip is modeled as randomly fluctuating point charges (or fluctuating finite dipoles) aligned along the primary axis of a prolate spheroid, and quasistatic tip-substrate charge interactions are considered to formulate the effective polarizability and self-interaction Green's function. The finite dipole model (FDM), combined with fluctuational electrodynamics, allows the computation of tip-plane thermal radiation in the extreme near-field (i.e., H/R≲1, where H is the tip-substrate gap distance and R is the tip radius), which cannot be calculated with the point dipole approximation. The FDM provides the underlying physics on the spectral redshift of tip-scattered near-field thermal radiation as observed in experiments. In addition, the SPhP peak in the near-field thermal radiation spectrum may split into two peaks as the gap distance decreases into the extreme near-field regime. This observation suggests that scattering-type spectroscopic measurements may not convey the full spectral features of tip-plane extreme near-field thermal radiation.

An analysis of beamed wireless power transfer in the Fresnel zone using a dynamic, metasurface aperture

Wireless power transfer (WPT) has been an active topic of research, with a number of WPT schemes implemented in the near-field (coupling) and far-field (radiation) regimes. Here, we consider a beamed WPT scheme based on a dynamically reconfigurable source aperture transferring power to receiving devices within the Fresnel region. In this context, the dynamic aperture resembles a reconfigurable lens capable of focusing power to a well-defined spot, whose dimension can be related to a point spread function. The necessary amplitude and phase distribution of the field imposed over the aperture can be determined in a holographic sense, by interfering a hypothetical point source located at the receiver location with a plane wave at the aperture location. While conventional technologies, such as phased arrays, can achieve the required control over phase and amplitude, they typically do so at a high cost; alternatively, metasurface apertures can achieve dynamic focusing with potentially lower cost. We present an initial tradeoff analysis of the Fresnel region WPT concept assuming a metasurface aperture, relating the key parameters such as spot size, aperture size, wavelength, and focal distance, as well as reviewing system considerations such as the availability of sources and power transfer efficiency. We find that approximate design formulas derived from the Gaussian optics approximation provide useful estimates of system performance, including transfer efficiency and coverage volume. The accuracy of these formulas is confirmed through numerical studies.

Stability Estimates for Linearized Near-Field Phase Retrieval in X-ray Phase Contrast Imaging

Propagation-based X-ray phase contrast enables nanoscale imaging of biological tissue by probing not only the attenuation but also the real part of the refractive index of the sample. Since only intensities of diffracted waves can be measured, the main mathematical challenge consists in a phase-retrieval problem in the near-field regime. We treat an often used linearized version of this problem known as the contract transfer function model. Surprisingly, this inverse problem turns out to be well-posed assuming only a compact support of the imaged object. Moreover, we establish bounds on the Lipschitz stability constant. In general this constant grows exponentially with the Fresnel number of the imaging setup. However, both for homogeneous objects, characterized by a fixed ratio of the induced refractive phase shifts and attenuation, and in the case of measurements at two distances, a much more favorable algebraic dependence on the Fresnel number can be shown. In some cases we establish order optimality of our estimates.

Self-Magnetism of Skin Effect as a Function of Nanoparticle Diameter on Absorption Frequency

When plasmon electron gas of nanoparticles (gold or silver) is exposed to electromagnetic wave, they can absorb it and hastily convert it into heat. This phenomenon has many applications such as destructing tumor cancer (one method of thermal therapy) or the application in near-field regime. However, in any application, recognizing the limited area of absorption frequency is very important. The paper investigates the dependence of maximum absorption frequency of gold and silver nanorods on size and aspect ratio (length/radius). The principal role of this property is due to the skin effect and self-magnetism on plasmon oscillation. In the bulk system, the plasmon gas oscillation regions are in order to micrometer in metal. When the size of particles becomes smaller than a micrometer, the influence of particle size on plasmon gas oscillation starts (the plasmon electron gas restricted by size particle), and the self-magnetism of skin effect operated. Decreasing of nanorod size leads to increasing of effective density of electron current causing blue shift. The presented model predicts the blue shift of transversal mode of nanorods. The theoretical results acquired from this model are comparable to the experimental result of gold and silver.

Spectral Shift of the Plasmon Resonance Between the Optical Extinction and Absorption of Gold and Aluminum Nanodisks

Comparing the optical properties of plasmonic nanoparticles in terms of scattering, absorption and extinction is an active and delicate issue in nano-optics. Absorption cross sections are usually difficult to be reliably measured. In this work, the absorption of gold and aluminum nanodisks was measured using quantitative wavefront sensing. We evidence a spectral shift between the retrieved absorption and the extinction maxima. Discrete dipole approximation calculations enabled us to unravel the origins of the spectral shifts, which are different depending on the material. In the case of gold nanodisks, the mismatch between extinction and absorption (∼15 nm) is related to the frequency shift of the plasmonic field intensity between near-field and far-field regimes, respectively probed through the heat generated within the nanoparticles and the optical extinction. For aluminum nanodisks, displaying dipolar plasmon resonances in the wavelength range 700 to 850 nm, the spectral shift between extinction and ab...

Nonequilibrium Fluctuational Quantum Electrodynamics: Heat Radiation, Heat Transfer, and Force

Quantum-thermal fluctuations of electromagnetic waves are the cornerstone of quantum statistics and inherent to phenomena such as thermal radiation and van der Waals forces. Although the principles are found in elementary texts, recent experimental and technological advances make it necessary to come to terms with counterintuitive consequences at short scales—the so-called near-field regime. We focus on three manifestations: (a) The Stefan–Boltzmann law describes radiation from macroscopic bodies but fails for small objects. (b) The heat transfer between two bodies at close proximity is dominated by evanescent waves and can be orders of magnitude larger than the classical (propagating) contribution. (c) Casimir forces, dominant at submicron separation, are not sufficiently explored for objects at different temperatures (at least experimentally). We explore these phenomena using fluctuational quantum electrodynamics (QED), introduced by Rytov in the 1950s, combined with scattering formalisms. This enables investigation of different material properties, shapes, separations, and arrangements.

High-performance electroluminescent refrigeration enabled by photon tunneling

Electroluminescent refrigeration, though theoretically proposed half a century ago, is rarely reported due to the requirement of extremely low nonidealities. Here, we theoretically show that by operating the device in the near-field regime with a vacuum gap down to 10nm, photon tunneling through evanescent waves can increase the tolerance of non-intrinsic nonradiative recombination to 31.6%. More importantly, the refrigeration rate may be enhanced by 2000-fold over the far-field scenario. In addition, the lowest achievable cooling temperature against the ambient condition of 300K extends from 284.2K to 270.6K. A self-consisted model based on the fluctuation-dissipation theory combined with dyadic Green's function method is developed considering the effect of the chemical potential of photons on the energy of Planck's quantum oscillators. This work opens a route to greatly enhance electroluminescent refrigeration, while relieving the strict material's requirement, for solid-state noncontact thermal management.

Concept of contrast transfer function for edge illumination x-ray phase-contrast imaging and its comparison with the free-space propagation technique.

Previous studies on edge illumination (EI) X-ray phase-contrast imaging (XPCi) have investigated the nature and amplitude of the signal provided by this technique. However, the response of the imaging system to different object spatial frequencies was never explicitly considered and studied. This is required in order to predict the performance of a given EI setup for different classes of objects. To this scope, in the present work we derive analytical expressions for the contrast transfer function of an EI imaging system, using the approximation of near-field regime, and study its dependence upon the main experimental parameters. We then exploit these results to compare the frequency response of an EI system with respect of that of a free-space propagation XPCi one. The results achieved in this work can be useful for predicting the signals obtainable for different types of objects and also as a basis for new retrieval methods.

Revisiting Newton's rings with a plasmonic optical flat for high-accuracy surface inspection.

Two parallel optical surfaces often exhibit colorful fringes along the lines of equal thickness because of the interference of light. This simple phenomenon allows one to observe subwavelength corrugations on a reflective surface by simply placing on it a flat reference dielectric surface, a so-called optical flat, and inspecting the resultant interference pattern. In this work, we extend this principle from dielectric surfaces to two-dimensional plasmonic nanostructures. Optical couplings between an Au nanodisk array and an Au thin film were measured quantitatively using two different techniques, namely, the classical Newton’s rings method and a closed-loop nano-positioning system. Extremely high spectral sensitivity to the inter-surface distance was observed in the near-field coupling regime, where a 1-nm change in distance could alter the resonance wavelength by almost 10 nm, >40 times greater than the variation in the case without near-field coupling. With the help of a numerical fitting technique, the resonance wavelength could be determined with a precision of 0.03 nm, corresponding to a distance precision as high as 0.003 nm. Utilizing this effect, we demonstrated that a plasmonic nanodisk array can be utilized as a plasmonic optical flat, with which nanometer-deep grooves can be directly visualized using a low-cost microscope.

Regularized Newton methods for x-ray phase contrast and general imaging problems.

Like many other advanced imaging methods, x-ray phase contrast imaging and tomography require mathematical inversion of the observed data to obtain real-space information. While an accurate forward model describing the generally nonlinear image formation from a given object to the observations is often available, explicit inversion formulas are typically not known. Moreover, the measured data might be insufficient for stable image reconstruction, in which case it has to be complemented by suitable a priori information. In this work, regularized Newton methods are presented as a general framework for the solution of such ill-posed nonlinear imaging problems. For a proof of principle, the approach is applied to x-ray phase contrast imaging in the near-field propagation regime. Simultaneous recovery of the phase- and amplitude from a single near-field diffraction pattern without homogeneity constraints is demonstrated for the first time. The presented methods further permit all-at-once phase contrast tomography, i.e. simultaneous phase retrieval and tomographic inversion. We demonstrate the potential of this approach by three-dimensional imaging of a colloidal crystal at 95nm isotropic resolution.

A study of optical vortices with the Talbot effect

The optical Talbot effect has been used to explore the topological charges of optical vortices. We recorded the self-imaging of a diffraction grating in the near-field regime with the optical vortex of several topological charges. Our twisted light was generated by a spatial light modulator (SLM). Previous studies showed that interferometric methods can determine the particular orbital angular momentum (OAM) states, but a large number of OAM eigenvalues are difficult to distinguish from the interference patterns. Here, we show that the Talbot patterns can distinguish the charges as well as the OAM of the vortices with high orders. Owing to the high sensitivity and self-imaging of the Talbot effect, several OAM eigenvalues can be distinguished by direct measurement. We check the experimental results with our theory. The present results are useful for classical and quantum metrology as well as future implementations of quantum communications.

Mode Matching and Coupling of Lensed and Cleaved Fibers Employing Near-Field Technique

An experimental coupling mechanism is presented that demonstrates efficient coupling between a high-power 980-nm laser diode and a lensed fiber by employing mode (spot size and wavefront) matching in the near-field regime. The results show that the beam spot pattern of the lensed fiber of a horizontally elliptical shape is spot-size matched to the laser. The wavefront of the lensed fiber with a minor radius of curvature ranging from −9 to $10~\mu \text{m}$ exhibits a curve wave that is also phase matched to the laser. This indicates that the similarity in the mode patterns of both the spot size and the wavefront between lensed fiber and laser is responsible for mode-matched beam coupling, leading to a coupling efficiency of more than 80%. In contrast to the lensed fiber, the spot-size of the cleaved fiber is round, and the wavefront is a plane wave. The different mode distributions between the laser and the cleaved fiber lead to a mode mismatch, resulting in a coupling efficiency of lower than 40%. This experimental coupling mechanism investigation in the near-field regime provides useful information for practical micro-optic design with better mode matching to achieve efficient coupling from active to passive components for interconnect applications.