Electromagnetic Local Density Of States Research Articles

Suppressing electromagnetic local density of states via slow light in lossy quasi-one-dimensional gratings

Suppressing electromagnetic local density of states via slow light in lossy quasi-one-dimensional gratings

Isotope engineering of near-field radiative thermal diodes

Guided by a rational design approach centering on the electromagnetic local density of states (LDOS), we explore the potential of isotopically engineering radiative thermal diodes for enhanced rectification with a focus on the near field. Based on fluctuational electrodynamics, we theoretically demonstrate that for thermal diodes pairing thin films of intrinsic silicon (i-Si) and lithium hydride (LiX), the rectification ratio can increase by over six times with varying isotopic compositions. This is because by leveraging the isotope-induced shift and broadening of the surface phonon polaritons in LiX, more LDOS contrast provided by i-Si can be effectively converted into thermal rectification. Moreover, we show that such improvement is fairly robust, as evidenced by the prediction of over 20% rectification enhancement across a wide physical and geometric parameter space. Finally, inspired by insights from the i-Si-based thermal diodes, we propose general guidelines for implementing isotope engineering in the design of practical devices, which are further illustrated via representative diodes employing vanadium dioxide and silicon carbide as the active materials. Our work highlights the efficacy of isotopes in boosting the performance of radiative thermal diodes, which also holds promise for broader applications such as thermal transistors, thermal switches, and thermophotovoltaics.

Selectively Exciting and Probing Radiative Plasmon Modes on Short Gold Nanorods by Scanning Tunneling Microscope-Induced Light Emission

We study the plasmon modes of gold nanorods (as short as ∼100 nm) on a nonmetallic conductive substrate using scanning tunneling microscope-induced light emission (STM-LE) with a nonplasmonic tungsten tip at room temperature in high vacuum (10–7 mbar). The far-field light is identified as the radiative decay of plasmon modes on the nanorods excited by inelastic electron tunneling. The spatial intensity distributions of the first three longitudinal multipolar modes on nanorods are spatially resolved on the order of 10–20 nm. These intensity distributions are related to the radiative electromagnetic local density of states and agree very well with numerical simulations. We discover that the presence of the tungsten tip with a high-dielectric constant influences the line shapes of the plasmon spectra and enhances the strength of the plasmon peaks.

Maximum electromagnetic local density of states via material structuring

Abstract The electromagnetic local density of states (LDOS) is crucial to many aspects of photonics engineering, from enhancing emission of photon sources to radiative heat transfer and photovoltaics. We present a framework for evaluating upper bounds on the LDOS in structured media that can handle arbitrary bandwidths and accounts for critical wave scattering effects. The bounds are solely determined by the bandwidth, material susceptibility, and device footprint, with no assumptions on geometry. We derive an analytical expression for the maximum LDOS consistent with the conservation of energy across the entire design domain, which upon benchmarking with topology-optimized structures is shown to be nearly tight for large devices. Novel scaling laws for maximum LDOS enhancement are found: the bounds saturate to a finite value with increasing susceptibility and scale as the quartic root of the bandwidth for semi-infinite structures made of lossy materials, with direct implications on material selection and design applications.

Molecular Plasmonics with Metamaterials.

Molecular plasmonics, the area which deals with the interactions between surface plasmons and molecules, has received enormous interest in fundamental research and found numerous technological applications. Plasmonic metamaterials, which offer rich opportunities to control the light intensity, field polarization, and local density of electromagnetic states on subwavelength scales, provide a versatile platform to enhance and tune light-molecule interactions. A variety of applications, including spontaneous emission enhancement, optical modulation, optical sensing, and photoactuated nanochemistry, have been reported by exploiting molecular interactions with plasmonic metamaterials. In this paper, we provide a comprehensive overview of the developments of molecular plasmonics with metamaterials. After a brief introduction to the optical properties of plasmonic metamaterials and relevant fabrication approaches, we discuss light-molecule interactions in plasmonic metamaterials in both weak and strong coupling regimes. We then highlight the exploitation of molecules in metamaterials for applications ranging from emission control and optical modulation to optical sensing. The role of hot carriers generated in metamaterials for nanochemistry is also discussed. Perspectives on the future development of molecular plasmonics with metamaterials conclude the review. The use of molecules in combination with designer metamaterials provides a rich playground both to actively control metamaterials using molecular interactions and, in turn, to use metamaterials to control molecular processes.

The performance analysis of near-field thermophotovoltaics considering temperature dependence of indium tin oxide emitter

Near-field thermophotovoltaics (NFTPV) is of great interest because it can achieve both high power density and high efficiency at medium to high temperature. One key to efficient photovoltaic conversion is an emitter that can selectively emit spectrum. Indium tin oxide (ITO) is considered as the emitter for NFTPV because its plasma frequency is tunable by simple technical means. However, as a typical degenerate semiconductor, the temperature dependence of its dielectric constant is often neglected in studying the performance of NFTPV. Therefore, we propose a NFTPV model considering the temperature-dependent property of ITO. By changing the applied voltage V, the vacuum gap d, and the emitter temperature T1, the performance of NFTPV with and without temperature dependence is compared and shows a significant difference. We find that the heat flux is always underestimated as well as the optimal efficiency is overestimated when the temperature dependence is not considered. At d = 10 nm and T1 = 1000 K, the optimal efficiency is even overestimated by about 270%. Without considering the temperature dependence, the output power is overestimated at d < 20 nm, while the opposite is true at d ≥ 20 nm. We clarify the main reasons for these misjudgments at a small gap originate from the underestimation of the intensity of surface phonon polaritons (SPhPs) below the bandgap and the overestimation of the intensity of surface plasmon polaritons (SPPs) above the bandgap. The main factor affecting the two kinds of surface modes in near-field regime is that high temperatures lead to low electromagnetic local density of states (LDOS) for ITO emitter near the SPPs resonant frequency, but high LDOS in the ultralow frequencies. In a larger gap, it is attributed to the underestimation of the waveguide modes above the bandgap and the nearly-full-band propagation modes. Our work illustrates the necessity of considering temperature dependence, enriches the understanding of NFTPV, and facilitates the development of related applications.

Fundamental limit to the rectification of near-field heat flow: The potential of intrinsic semiconductor films

We derive the fundamental limit to near-field radiative thermal rectification mediated by an intrinsic semiconductor film within the framework of fluctuational electrodynamics. By leveraging the electromagnetic local density of states, we identify {\epsilon}"_H/{\epsilon}"_L as an upper bound on the rectification magnitude, where {\epsilon}"_H and {\epsilon}"_L are respectively the imaginary parts of the film permittivity at high and low temperatures. This bound is tight and can be approached regardless of whether the film is suspended or supported. For intrinsic silicon the limit can in principle exceed 10^9. Our work highlights the possibility of controlling heat flow as effectively as electric current, and offers guidelines to potentially achieve this goal.

Giant radiative thermal rectification using an intrinsic semiconductor film

Rectification of heat flow via a thermal diode is not only of fundamental interest, but can also enable a range of novel applications in thermal management and energy conversion. However, despite decades of extensive research, large rectification ratios of practical importance have yet to be demonstrated. Here, we theoretically achieve giant rectification ratios (3 to almost 5 orders of magnitude) by leveraging near-field radiative thermal transport between two parallel planes. Guided by a rational design approach centering on the electromagnetic local density of states (LDOS), we employ a thin film of an intrinsic semiconductor—such as silicon—as one terminal of our radiative thermal diodes, which provides the necessary nonlinearity and a substantial LDOS contrast as the temperature bias is flipped. For the other terminal, we explore two kinds of materials which either serve as a narrowband or a broadband filter, both capable of converting the large LDOS contrast into giant thermal rectification. We further consider representative multilayer configurations in order to block backside radiation and improve mechanical stability. All the designs are expected to perform well over a wide range of film thicknesses, gap sizes, and temperatures. Our work offers an opportunity for realizing thermal diodes with unprecedented rectification ratios.

Spontaneous Emission Rate and the Density of States Inside a One Dimensional Photonic Crystal

Different densities of electromagnetic states inside a one dimensional photonic crystal (1D PC) are studied. Hertz vector formalism is used to calculate Green’s tensor inside a layered structure, semi-analytically. Based on the obtained Green’s tensor, the local density of electromagnetic states (LDOS) and the density of states (DOS) inside a 1D PC are calculated and discussed. The Green’s tensor is also used to approximate the density of Bloch states inside the 1D PC and is compared with its exact calculation based on the 1D PC dispersion relations. Using a practical 1D PC parameters in the visible range, the aforementioned quantities are calculated and verified with a full-wave solver based on finite element method (FEM). The formulations and the results are aimed to be helpful in thermal and spontaneous radiation studies.

Passive near-field imaging via grating-based spectroscopy.

Passive scattering-type scanning near-field optical microscopy (s-SNOM) has recently been developed for studying long-wavelength infrared (LWIR) waves. It detects surface-localized waves without any external illumination or heating and enables the imaging of hot-electron energy dissipation and nanoscale Joule heating. However, the lack of a wavelength selection mechanism in the passive LWIR s-SNOM makes it difficult to perform a thorough analysis of the surface-localized waves. Here, we develop a novel passive scanning near-field optical spectroscopy with a diffraction grating. The spectroscopic optics are designed to exhibit a high signal efficiency and mechanical performance at the temperature of liquid helium (4.2K). Using the developed passive LWIR near-field spectroscopy, the spectral information of thermally excited evanescent waves can be directly obtained without any influence from the external environment factors, including environmental heat. We have detected the thermally excited evanescent waves on a SiC/Au micropatterned sample at room temperature with a spatial resolution of 200nm and a wavelength resolution of 500nm at several wavelengths in the range of 14-15 µm. The obtained spectra are consistent with the electromagnetic local density of states calculated based on the fluctuation-dissipation theorem. The developed passive LWIR near-field spectroscopy enables the spectral analysis of ultrasmall surface-localized waves, making it a high-performance surface analysis tool.

Radiative Thermal Diode via Hyperbolic Metamaterials

Hyperbolic metamaterials (HMMs) support propagating waves with arbitrarily large wavevectors over broad spectral ranges, and are uniquely valuable for engineering radiative thermal transport in the near field. Here, by employing a rational design approach based on the electromagnetic local density of states, we demonstrate the ability of HMMs to substantially rectify radiative heat flow. Our idea is to establish a forward-biased scenario where the two HMM-based terminals of a thermal diode feature overlapped hyperbolic bands which result in a large heat current, and suppress the reverse heat flow by creating spectrally mismatched density of states as the temperature bias is flipped. As an example, we present a few high-performance thermal diodes by pairing HMMs made of polar dielectrics and metal-to-insulator transition (MIT) materials in the form of periodic nanowire arrays, and considering three representative kinds of substrates. Upon optimization, we theoretically achieve a rectification ratio of 324 at a 100 nm gap, which remains greater than 148 for larger gap sizes up to 1 um over a wide temperature range. The maximum rectification represents an almost 1000-fold increase compared to a bulk diode using the same materials, and is twice that of state-of-the-art designs. Our work highlights the potential of HMMs for rectifying radiative heat flow, and may find applications in advanced thermal management and energy conversion systems.

Super-Planckian thermal radiation between 2D phononic hBN monolayers

Near-field radiation heat transfer (NFRHT) in two-dimensional (2D) hexagonal boron nitride (hBN) monolayer and its composite symmetric-based structure was investigated. Contrary to three-dimensional (3D) materials, the splitting of modes (in 2D materials) into the longitudinal optical (LO) and transverse optical (TO) phononic modes disappears at the Γ-point even in some polar materials. The conductivity of atomic thin hBN monolayer depends on the LO phonon frequency under the long wavelength limit. Exploiting such concept, the electromagnetic local density of states (EM-LDOS) accounting for the near-field radiation spectrum was evaluated in free-space close to an interface with the hBN monolayer as well as composite structures. An intense narrow peak of LDOS was found in the case of hBN monolayer caused by the surface phonon polariton (SPhP) resonance. This peak was modified in the case of hBN-graphene composite due to hybridization, which strongly depends on two factors – the distance of observer (in vacuum) from interface and graphene Fermi energy. It was found that the flux density between monolayer hBN gets 2.8-fold enhanced as compared to the monolayer graphene configuration under 0.11 eV Fermi energy applied at a vacuum gap of 15 nm. The possibility of tuning Fermi energy has great potentials in designing thermally controlled devices.

Thin-Film Radiative Thermal Diode with Large Rectification

We propose a mechanism to substantially rectify radiative heat flow by having one side of the thermal diode offering a large contrast in the local density of electromagnetic states, while the opposite side serving as a narrow bandpass filter that limits the transport to a desired spectral range. To demonstrate, we realize such a scenario by matching thin films of metal-to-insulator transition materials and polar dielectrics in the near field. By leveraging the distinct scaling behaviors of the local density of states with film thickness for metals and insulators, we theoretically achieve rectification ratios over 140, a 10-fold improvement over the state of the art, with nanofilms of vanadium dioxide and cubic boron nitride in the parallel-plane geometry at experimentally feasible gap sizes (\ensuremath{\sim}100 nm). Our rational design offers relative ease of fabrication, flexible choice of materials, and robustness against deviations from optimal film thicknesses. We expect this work to facilitate the application of thermal diodes in solid-state thermal circuits and energy-conversion devices.

Harnessing the photonic local density of states in graphene moiré superlattices

In this work we investigate the electromagnetic local density of states (LDOS) near a twisted bilayer graphene (TBG) deposited over a general isotropic substrate. The band structure of the TBG is calculated within a tight-binding framework, and then used to determine the TBG's conductivity. The latter presents a nontrivial dependence upon the angle of twist, which shows up in the LDOS, allowing for a moir\'e pattern-dependent quantum emission. For some specific twist angles we show that it is possible to either enhance or decrease the LDOS by an order of magnitude at selected frequencies when compared to the monolayer. This impressive variation is explained in terms of the presence/absence of well defined surface plasmon polaritons (SPPs). Altogether our findings demonstrate that TBG is an alternative, versatile material platform for controlling spontaneous emission and enhancing light-matter interactions, and pave the way for further studies and applications of quantum emission in the emerging class of two-dimensional moir\'e materials.

Optical polarization analogue in free electron beams

Fast electrons spectromicroscopies enable to measure quantitatively the optical response of excitations with unrivaled spatial resolution. However, due to their inherently scalar nature, electron waves cannot access to polarization-related quantities. In spite of promising attempts based on the conversion of concepts originating from singular optics (such as vortex beams), the definition of an optical polarization analogue for fast electrons has remained a dead letter. Here, we establish such an analogue as the dipole transition vector of the electron between two well-chosen singular wave states. We show that electron energy-loss spectroscopy (EELS) allows a direct measurement of the \textit{polarized} electromagnetic local density of states. In particular, in the case of circular polarization, it measures directly the local optical spin density. This work establishes EELS as a quantitative technique to tackle fundamental issues in nano-optics, such as super-chirality, the local polarization of dark excitations or polarization singularities at the nanoscale.

Probing the Radiative Electromagnetic Local Density of States in Nanostructures with a Scanning Tunneling Microscope

A novel technique for the investigation of the radiative contribution to the electromagnetic local density of states is presented. The inelastic tunneling current from a scanning tunneling microscope (STM) is used to locally and electrically excite the plasmonic modes of a triangular gold platelet. The radiative decay of these modes is detected through the transparent substrate in the far field. Emission spectra, which depend on the position of the STM excitation, as well as energy-filtered emission maps for particular spectral windows are acquired using this technique. The STM-nanosource spectroscopy and microscopy results are compared to those obtained from spatially resolved electron energy loss spectroscopy (EELS) maps on similar platelets. While EELS is known to be related to the total projected electromagnetic local density of states, the light emission from the STM-nanosource is shown here to select the radiative contribution. Full electromagnetic calculations are carried out to explain the experimental STM data, and provide valuable insight into the radiative nature of the different contributions of the breathing and edge plasmon modes of the nanoparticles. Our results introduce the STM-nanosource as a tool to investigate and engineer light emission at the nanoscale.

Field localization of hexagonal and short-range ordered plasmonic nanoholes investigated by cathodoluminescence

Plasmonic nanoholes have attracted significant attention among nanoplasmonic devices, especially as biosensing platforms, where nanohole arrays can efficiently enhance and confine the electromagnetic field through surface plasmon polaritons, providing a sensitive detection. In nanohole arrays, the optical resonances are typically determined by the inter-hole distance or periodicity with respect to the surface plasmon wavelength. However, for short-range ordered (SRO) arrays, the inter-hole distance varies locally, so the plasmon resonance changes. In this study, we investigate the local resonance of SRO nanoholes using a cathodoluminescence technique and compare it with hexagonally ordered nanoholes. The cathodoluminescence photon maps and resonance peak analysis reveal that the electric fields are confined at the edges of holes and that their resonances are determined by inter-hole distances as well as by their distributions. This demonstrates the Anderson localization of the electromagnetic waves showing locally enhanced electromagnetic local density of states in SRO nanoholes.

Diffusion-inspired time-varying phosphorescent decay in a nanostructured environment

Structured environment controls dynamics of light-matter interaction processes via modified local density of electromagnetic states. In typical scenarios, where nanosecond-scale fluorescent processes are involved, mechanical conformational changes of the environment during the interaction processes can be safely neglected. However, slow decaying phosphorescent complexes (e.g. lanthanides) can efficiently sense micro- and millisecond scale motion via near-field interactions. As the result, lifetime statistics can inherit information about nano-scale mechanical motion. Here we study light-matter interaction dynamics of phosphorescent dyes, diffusing in a proximity of a plasmonic nanoantenna. The interplay between the time-varying Purcell enhancement and stochastic motion of molecules is considered via a modified diffusion equation, and collective decay phenomena is analysed. Fluid properties, such as local temperature and diffusion coefficient are mapped on phosphorescent lifetime distribution extracted with the help of inverse Laplace transform. We present rather simple photonic platform enabling contactless all-optical thermometry and diffusion measurement and paving a way for a plethora of possible applications. Among them, the proposed analysis can be used for detailed studies of nanofluidic processes in lab-on-a-chip devices, which are extremely hard or even impossible to analyse with other optical methods.

Titanium Nitride Modified Photoluminescence from Single Semiconductor Nanoplatelets

AbstractTitanium nitride (TiN) is an alternative plasmonic material that has the potential for visible and near‐infrared optical applications due to its distinct properties. Here, coupling effects between TiN nanohole array films and nearby excitonic emitters, semiconductor nanoplatelets (NPLs), are investigated using single particle spectroscopy. At the emission wavelength of the NPLs, the local field enhancement close to the surface of the TiN nanohole array films induces an increase in the radiative decay rates of the emitters by a factor of up to 2. This effect diminishes quickly as the distance between the TiN nanohole array films and emitters increases. At short wavelengths where the NPLs are excited, the TiN nanohole array films exhibit lossy dielectric characteristics. Local field modification at these wavelengths leads to a reduced local density of electromagnetic states, and hence the photoluminescence intensity of the emitters. This study shows the potential of TiN as an alternative plasmonic material for optoelectronic and photonic applications, especially in the long wavelength ranges.

Thermal radiation control from hot graphene electrons coupled to a photonic crystal nanocavity

Controlling thermal radiation is central in a range of applications including sensing, energy harvesting, and lighting. The thermal emission spectrum can be strongly modified through the electromagnetic local density of states (EM LDOS) in nanoscale-patterned metals and semiconductors. However, these materials become unstable at high temperature, preventing improvements in radiative efficiency and applications such as thermophotovoltaics. Here, we report stable high-temperature thermal emission based on hot electrons (>2000 K) in graphene coupled to a photonic crystal nanocavity, which strongly modifies the EM LDOS. The electron bath in graphene is highly decoupled from lattice phonons, allowing a comparatively cool temperature (700 K) of the photonic crystal nanocavity. This thermal decoupling of hot electrons from the LDOS-engineered substrate opens a broad design space for thermal emission control that would be challenging or impossible with heated nanoscale-patterned metals or semiconductor materials.