Near-field Energy Transfer Research Articles

FDTD Simulation of Near-Field Radiative Heat Transfer between Thin Films Supporting Surface Phonon Polaritons: Lessons Learned

Abstract We present the numerical simulation, using the finite difference time domain (FDTD) method, of the radiative heat transfer between two thin SiC slabs. We aim to explore the ability of the FDTD method to reproduce the analytical results for the Surface Phonon Polariton (SPhP) assisted near field radiative energy transfer between two SiC slabs separated by a nano/micro-metric vacuum gap. In this regard, we describe the key challenges that must be addressed for simulating general near-field radiative energy transfer problems using the FDTD method. FDTD is a powerful technique for simulating the near-field radiative energy transfer because it allows simulating arbitrary shaped nano-structured bodies, like photonic crystals, for which an analytical solution is not readily obtained. Key words : Near-Field Radiative Heat Transfer, Fluctuation Electrodynamics, Thermal Radiation, Surface Phonon Polariton, FDTD 1. Introduction When two bodies are located at short distance (in the nano/micro scale) the evanescent modes can tunnel between each other and contribute to the radiative energy transfer, which can exceed the black body radiation by several orders of magnitude [1–3]. Besides, at nano-metric distance, the excitation of surface plasmon polaritons (SPP) in metals or surface phonon polaritons (SPhP) in polar materials, produce even further enhancement of the radiative energy transfer at the resonance frequencies [4–7], allowing for quasi-monochromatic near-field radiative heat transfer. These phenomena have got rising attention in the recent years due to the strong development of nano/micro-technologies, where near-field effects have to be considered in the thermal analysis. Some of the key applications that has been proposed so far are nano-lithography [8], near-field imaging [9], thermophotonic heat pumping [10], and thermophotovoltaic (TPV) energy conversion [3], [11–16] . Most of the theoretical studies on near-field radiative heat transfer presented so far are based on fluctuation electrodynamics theory [17–19]. This theoretical framework assumes that the origins of thermal radiation are the fluctuating currents originated by the random motion of charges within the material, and the problem can be solved analytically by means of the Green’s functions [20], which provides the spatial transfer function between current sources and the corresponding electro-magnetic fields in a given location of the space. However, the complexity of Green’s function formalism limits the theoretical development on near-field radiative heat transfer to relatively simple geometries; mainly infinite extended multilayered structures [19], for which a closed analytical solution is available.

Modified light emission from emitters coupled to long-range guided modes in strongly absorbing layers

We demonstrate the near-field coupling and energy transfer between photoexcited dye molecules and guided modes in layers of strongly absorbing dielectrics. The dye molecules decay by exciting long-range guided modes (LRGMs) in a thin layer of chalcogenide glass. These modes can exist in spite of the very large absorption of the material forming the layer. The LRGMs are detected by coupling then out to free space radiation through a prism in the Krestschmann configuration. By calculating the dissipated power of a dipole, representing a dye molecule, in the vicinity of the absorbing thin film, we show that there is a large probability of decay exciting LRGMs. This probability can reach 35% for perpendicularly oriented dipoles. The demonstration of the excitation of LRGMs in thin films of absorbing dielectrics by near-field coupling of excited molecules opens the possibility to compensate for the losses in the propagation of these modes.

Control Theory of Near-Field Optical Energy Transfer in One-Dimensional Waveguide Consisting of Metallic Nanoparticles by Vibrating External Field

Control Theory of Near-Field Optical Energy Transfer in One-Dimensional Waveguide Consisting of Metallic Nanoparticles by Vibrating External Field

Control Theory of Near-Field Optical Energy Transfer in One-Dimensional Waveguide Consisting of Metallic Nanoparticles by Vibrating External Field

We have theoretically investigated the properties of optical energy transfer (OET) along an array of metallic nanoparticles (MNPs) on a one-dimensional chain consisting of organic materials under the local excitation by near-field light in these states. Remarkably, under the telescopic motion of organic chain induced by an external field resonant with its vibration, we have clarified that the efficiency of OET can be drastically modulated for particular wavelength region depending on the size of MNPs. Obtained results would provide an unconventional way to wavelength-tunable OET and signal processing.

Near-field radiative heat transfer between closely spaced graphene and amorphous SiO2

We study the near-field radiative energy transfer between graphene and an amorphous SiO${}_{2}$ substrate. In comparison with the existing theories of near-field radiative heat transfer our theory takes into account that the free carriers in graphene are moving relative to the substrate with a drift velocity $v$. In this case the heat flux is determined by both thermal and quantum fluctuations. We find that quantum fluctuations give an important contribution to the radiative energy transfer for low temperatures and high electric field (large drift velocities). For nonsuspended graphene the near-field radiative energy transfer gives a significant contribution to the heat transfer in addition to the contribution from phononic coupling. For suspended graphene (large separation) the corresponding radiative energy transfer coefficient at a nanoscale gap is $~$3 orders of magnitude larger than radiative heat transfer coefficient of the blackbody radiation limit.

Localized radiative energy transfer from a plasmonic bow-tie nano-antenna to a magnetic thin film stack

Localized radiative energy transfer from a near-field emitter to a magnetic thin film structure is investigated. A magnetic thin film stack is placed in the near-field of the plasmonic nano-antenna to utilize the evanescent mode coupling between the nano-antenna and magnetic thin film stack. A bow-tie nano-optical antenna is excited with a tightly focused beam of light to improve near-field radiative energy transfer from the antenna to the magnetic thin film structure. A tightly focused incident optical beam with a wide angular spectrum is formulated using Richards–Wolf vector field equations. Radiative energy transfer is investigated using a frequency domain 3D finite element method solution of Maxwell’s equations. Localized radiative energy transfer between the near-field emitter and the magnetic thin film structure is quantified for a given optical laser power at various distances between the near-field emitter and magnetic thin film.

Temperature dependence of optical near-field energy transfer rate between two quantum dots in nanophotonic devices

We have obtained a simple numerical model that explains the temperature behavior of multi-quantum-dot (QD) nanophotonic devices whose operations are based on optical near-field (ONF) interactions between any two resonant QDs that are in thermal equilibrium. This model involves a set of coupled rate equations that govern the temporal behavior of the QDs' energy level occupancies. Under a certain operating condition, this simple model can substitute for the more complex density matrix (DM) approach in modeling the temperature dependence of the ONF energy transfer rate (R(ONF)) between any two resonant QDs in thermal equilibrium. The same applies for modeling the system state-filling time (tau(S)). Applying our simple model to a two-QD system, we have derived analytical formulas for the interdot and the intradot transfer rates at finite temperatures (T > or = 0). Furthermore, by assuming a unidirectional energy transport operating condition, we have also derived an analytic formula for calculating tau(S) for a two-QD system. To the best of our knowledge, this work is the first instance of reporting such analytic equations. Approximated values of tau(S) obtained from our simple analytic equation are in reasonable agreements with those calculated by the DM approach.

Review of near-field thermal radiation and its application to energy conversion

Understanding energy transfer via near-field thermal radiation is critical for the future advances of nanotechnology. Evanescent waves and photon tunneling are responsible for the near-field energy transfer being several orders of magnitude greater than that between two blackbodies. The enhanced energy transfer may be used for improving the performance of energy conversion devices, developing novel nanofabrication techniques, and imaging nanostructures with higher spatial resolution. Near-field heat transfer can be analyzed using fluctuational electrodynamics. This article reviews the fundamentals of near-field radiation and outlines the recent advances in this field. Important results are presented for near-field energy transfer between parallel plates and between multilayered structures. Application of near-field thermal radiation in near-field thermophotovoltaic devices is also discussed. Copyright © 2009 John Wiley & Sons, Ltd.

Laser conferral of a directed character to near-field energy transfer

With sufficient intensity, an off-resonant laser beam can modify the character of electromagnetic near-field radiation produced by a nanoscale point source, narrowing its spatial distribution. The mechanism, for which the laser frequency is significantly off-resonant from the emission radiation, is detailed and analyzed through a quantum electrodynamical analysis. Results are calculated for various positions of a polarization-sensitive probe relative to the point source, sited within the throughput beam, and contour maps exhibit variations in the registered signal over a range of input intensities. A key feature is the clear exhibition of directed propagation features, usually emergent only in the wave zone, within the near-field region of the source.

High-Efficiency Organic Solar Concentrators for Photovoltaics

The cost of photovoltaic power can be reduced with organic solar concentrators. These are planar waveguides with a thin-film organic coating on the face and inorganic solar cells attached to the edges. Light is absorbed by the coating and reemitted into waveguide modes for collection by the solar cells. We report single- and tandem-waveguide organic solar concentrators with quantum efficiencies exceeding 50% and projected power conversion efficiencies as high as 6.8%. The exploitation of near-field energy transfer, solid-state solvation, and phosphorescence enables 10-fold increases in the power obtained from photovoltaic cells, without the need for solar tracking.

Efficient optical near-field energy transfer along an Au nanodot coupler with size-dependent resonance

We confirmed that the light intensity (λ=785 nm) scattered from an isolated hemispherical Au nanoparticle was resonantly enhanced at a diameter of 200 nm and a height of 50 nm, as observed experimentally using a collection-mode near-field optical microscope. The experimental results agreed with the calculated results using Mie’s theory. Furthermore, we observed resonant energy transfer of the optical near-field energy along a chain of Au nanoparticles. The magnitude of the transferred energy increased resonantly at the size of resonant light scattering for an isolated Au nanoparticle (200 nm diameter with 240 nm center-to-center separation).

Nanodot couplers provide efficient near-field energy transfer

Nanodot couplers provide efficient near-field energy transfer

Demonstration of a nanophotonic switching operation by optical near-field energy transfer

We have demonstrated the operation of a nanophotonic switch that uses three CuCl quantum cubes with a size ratio of 1:√:2. The switching mechanism is based on resonant optical near-field energy transfer between the resonant quantized excitonic energy levels of the quantum cubes. Using near-field optical spectroscopy, we observed a switching rise time of less than 100 ps and a repetition rate of 80 MHz. These results suggest the possibility of making a nanophotonic switching device smaller than 20 nm with a figure of merit 10–100 times higher than that of a conventional photonic switch.

Demonstrating nanophotonic switching using near-field pump-probe photoluminescence spectroscopy of CuCl quantum cubes.

We demonstrated a novel optical switching operation using three CuCl quantum cubes with a size ratio of. Their quantized excitonic energy levels resonate with one another, and the switching mechanism was based on the resonant near-field energy transfer between the quantum cubes. Using near-field pump-probe photoluminescence spectroscopy, we succeeded in controlling the near-field energy transfer and obtained a controlled (i.e. switched) signal in a differential photoluminescence spectrum with and without a pump beam. The internal quantum efficiency of the switching operation was close to 1. These results suggest the possibility of making a nanophotonic switching device smaller than 30 nm.

Single-molecule near-field optical energy transfer microscopy

The nano-optical interaction between a sharp tip and a single dipolar emitter is investigated. Changes of the excited state lifetime and the fluorescence rate of single molecules are recorded simultaneously as a function of the tip position relative to the molecule. A subdiffraction-limited area of decreased fluorescence and shortened lifetime is observed for gold-coated Si3N4 tips. The results are discussed in terms of molecular fluorescence in a system of stratified media. The outlined methodology holds promise for applications in ultrahigh-resolution near-field optical imaging at the level of single fluorophores.

Nanophotonics: design, fabrication, and operation of nanometric devices using optical near fields

This paper reviews progress in nanophotonics, a novel optical nanotechnology, utilizing local electromagnetic interactions between a few nanometric elements and an optical near field. A prototype of a nanophotonic integrated circuit (IC) is presented, in which the optical near field is used as a carrier to transmit a signal from one nanometric dot to another. Each section of this paper reviews theoretical and experimental studies carried out to assess the possibility of designing, fabricating, and operating each nanophotonic IC device. A key device, the nanophotonic switch, is proposed based on optical near-field energy transfer between quantum dots (QDs). The optical near-field interaction is expressed as the sum of the Yukawa function, and the oscillation period of the nutation of cubic CuCl QDs is estimated to be less than 100 ps. To guarantee one-directional (i.e., irreversible) energy transfer between two resonant levels of QDs, intrasublevel transitions due to phonon coupling are examined by considering a simple two-QD plus phonon heat bath system. As a result, the state-filling time is estimated as 22 ps for CuCl QDs. This time is almost independent of the temperature in the Born-Markov approximation. Using cubic CuCl QDs in a NaCl matrix as a test sample, the optical near-field energy transfer was experimentally verified by near-field optical spectroscopy with a spatial resolution smaller than 50 nm in the near-UV region at 15 K. This transfer occurs from the lowest state of excitons in 4.6-nm QDs to the first dipole-forbidden excited state of excitons in 6.3-nm QDs. To fabricate nanophotonic devices and ICs, chemical vapor deposition using an optical near field is proposed; this is sufficiently precise in controlling the size and position of the deposited material. A novel deposition scheme under nonresonant conditions is also demonstrated and its origin is discussed. In order to confirm the possibility of using a nanometric ZnO dot as a light emitter in a nanophotonic IC, spatially and spectrally resolved photoluminescence imaging of individual ZnO nanocrystallites was carried out with a spatial resolution as high as 55 nm, using a UV fiber probe, and the spectral shift due to the quantum size effect was found. To connect the nanophotonic IC to external photonic devices, a nanometer-scale waveguide was developed using a metal-coated silicon wedge structure. Illumination (wavelength: 830 nm) of the metal-coated silicon wedge (width: 150 nm) excites a TM plasmon mode with a beam width of 150 nm and propagation length of 2.5 /spl mu/m. A key device for nanophotonics, an optical near-field probe with an extremely high throughput, was developed by introducing a pyramidal silicon structure with localized surface plasmon resonance at the metallized probe tip. A throughput as high as 2.3% was achieved. Finally, as an application of nanophotonics to, a high-density, high-speed optical memory system, a novel contact slider with a pyramidal silicon probe array was developed. This slider was used for phase-change recording and reading, and a mark length as short as 110 nm was demonstrated.

Direct observation of optically forbidden energy transfer between CuCl quantum cubes via near-field optical spectroscopy.

We report, for the first time, evidence of near-field energy transfer among CuCl quantum cubes using an ultrahigh-resolution near-field optical microscopy and spectroscopy in the near UV region at 15 K. The sample was high-density CuCl quantum cubes embedded in a NaCl matrix. Measured spatial distributions of the luminescence intensities from 4.6-nm and 6.3-nm quantum cubes clearly established anticorrelation features. This is thought to be a manifestation of the energy transfer from the lowest state of exciton in 4.6-nm quantum cubes to the first dipole-forbidden excited state of exciton in 6.3-nm quantum cubes, which is attributed to the resonant optical near-field interaction.

Scanning Near-Field Fluorescence Resonance Energy Transfer Microscopy

A new microscopic technique is demonstrated that combines attributes from both near-field scanning optical microscopy (NSOM) and fluorescence resonance energy transfer (FRET). The method relies on attaching the acceptor dye of a FRET pair to the end of a near-field fiber optic probe. Light exiting the NSOM probe, which is nonresonant with the acceptor dye, excites the donor dye introduced into a sample. As the tip approaches the sample containing the donor dye, energy transfer from the excited donor to the tip-bound acceptor produces a red-shifted fluorescence. By monitoring this red-shifted acceptor emission, a dramatic reduction in the sample volume probed by the uncoated NSOM tip is observed. This technique is demonstrated by imaging the fluorescence from a multilayer film created using the Langmuir–Blodgett (LB) technique. The film consists of l- α-dipalmitoylphosphatidylcholine (DPPC) monolayers containing the donor dye, fluorescein, separated by a spacer group of three arachidic acid layers. A DPPC monolayer containing the acceptor dye, rhodamine, was also transferred onto an NSOM tip using the LB technique. Using this modified probe, fluorescence images of the multilayer film reveal distinct differences between images collected monitoring either the donor or acceptor emission. The latter results from energy transfer from the sample to the NSOM probe. This method is shown to provide enhanced depth sensitivity in fluorescence measurements, which may be particularly informative in studies on thick specimens such as cells. The technique also provides a mechanism for obtaining high spatial resolution without the need for a metal coating around the NSOM probe and should work equally well with nonwaveguide probes such as atomic force microscopy tips. This may lead to dramatically improved spatial resolution in fluorescence imaging.

Subwavelength spectoscopy, exciton supertips and mesoscopic light-matter interactions

Subwavelength optical light sources and probes are leading to new devices, technologies and techniques such as near-field microscopy, spectroscopy and biochemical sensing. We have prepared nanometer light sources as small as 40 nm in diameter and exciton sources that are even smaller by adding various inorganic and organic crystals and molecularly doped polymers to micropipettes and nanofabricated fiber-optic tips. We demonstrate subwavelength spectroscopy with highly spatially resolved flourescence spectral of nanocrystals (perylene) in polymeric glasses (PMMA). We also demontrate an interfacial Kasha (external heavy-atom) effect on a perylene nanocrystal surface. We give results on near-field optics and energy transfer at interfaces of nanocrystals (anthracene or diphenyl-anthracene) and dyes (rhodamine B) or dyes embedded in Langmuir-Blodgett films. We discuss the rise of new phenomena when the light source and the sample form a combined mesoscopic system, with subsequent quantum optics effects.

Author index of volume 28

The elimination of physical conductors as media for power transmission is an important step towards reducing the bulk of waste material generated when electronic gadgets are disposed of. In addition, the increasing deployment of low-power autonomous electronics in less accessible environments has provided an impetus for the development of wireless energy transfer alternatives to wired power delivery. This paper presents a review of near-field magnetic wireless energy transfer link architectures, and design approaches for realizing performance objectives specifically suited to low-power deployments. First, the paper provides a brief history of low-power magnetic wireless energy transfer development. This is followed by a fundamental description of the spatial regions surrounding an electromagnetic field source. Then, the paper presents a summary of basic topologies of magnetic wireless energy transfer link implementations, while emphasizing their distinctive features. Design approaches, which enable the realization of various link performance criteria, are also discussed. Finally, this paper highlights emergent wireless energy transfer link design trends inspired by communication network paradigms.