Subwavelength Cavity Research Articles

Harmonic Engineering in Sub-Wavelength Cavities & Design of Ultrabroadband Microwave Absorbers by Reflection Phase Manipulation

Harmonic Engineering in Sub-Wavelength Cavities & Design of Ultrabroadband Microwave Absorbers by Reflection Phase Manipulation

Highly tunable quasi-bound states in the continuum responses in subwavelength plasmonic-dielectric hybrid nanostructures

Here, we study the quasi-bound states in the continuum (Q-BIC) response in subwavelength hybrid nanostructures consisting of plasmonic antennas and subwavelength dielectric cavities. Among them, the plasmonic antennas are metal nanorods with electric dipole resonances, and the dielectric cavity is a nanodisk with an anapole mode. By reasonably adjusting the phase difference between the two modes, we can excite Q-BIC mode in the hybrid cavity. As the two modes involved in the construction of Q-BIC are on two separate structures, the Q-BIC of this hybrid nanostructure has great adjustability, and the quality factor of resonant mode can be increased several times. Our research enriches the generations of Q-BIC mode in subwavelength cavities, which may help to enhance light-matter interaction.

Amperean superconductivity cannot be induced by deep subwavelength cavities in a two-dimensional material

Amperean superconductivity cannot be induced by deep subwavelength cavities in a two-dimensional material

Room-temperature-grown tungsten oxide hybridized dipole cavities to realize thermally tunable terahertz surface plasmons

Recent developments in the field of terahertz (THz) subwavelength structures have inspired novel discoveries and innovations involving the use of multidimensional materials in fields such as spectroscopy, photonics, biomedical imaging, nonlinearity, and communication technology. In this context, metal oxide materials with adaptable electronic and optical properties have emerged as promising candidates to realize dynamically reconfigurable THz subwavelength devices. Therefore, we experimentally demonstrated temperature-tunable surface plasmon resonances (SPRs) in an array of tungsten oxide (WO3) hybridized subwavelength cavities. By using THz time-domain spectroscopy, we examined the thermally tunable dielectric properties of a WO3 thin film over a temperature range of 25 °C–110 °C. On the basis of the tunable optoelectronic properties of WO3, we implement a THz subwavelength dipole cavity that can exhibit a temperature-dependent dynamic modification in SPR characteristics. Such tunability could be used to fabricate novel active SPR-based plasmonic devices like temperature sensors, spatial modulators, filters, and integrated THz optoelectronic elements.

Terahertz sensing of reduced graphene oxide nanosheets using sub-wavelength dipole cavities

Because of extraordinary optoelectronic properties, two-dimensional (2D) materials are the subject of intense study in recent times. Hence, we investigate sub-wavelength dipole cavities (hole array) as a sensing platform for the detection of 2D reduced graphene oxide (r-GO) using terahertz time-domain spectroscopy (THz-TDS). The r-GO is obtained by reducing graphene oxide (GO) via Hummer's method. Its structural characteristics are verified using X-ray diffraction (XRD) and Raman spectroscopy. We also assessed the morphology and chemistry of r-GO nanosheets by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDAX), and Fourier Transformed Infrared (FTIR) spectroscopy. Further, we studied the surface plasmon resonance (SPR) characteristics of r-GO nanosheets hybridized dipole cavities using THz-TDS by varying the r-GO thickness on top of the dipole cavities, since these cavities are well known for sustaining strong SPRs. Based on these, we experimentally obtained a sensitivity of 12 GHz/µm for the porous r-GO film. Thus, a modification in SPR characteristics can be employed towards the identification and quantification of r-GO by suitably embedding it on an array of dipole cavities. Moreover, we have adopted a generic approach that can be expanded to sense other 2D materials like Boron Nitride (BN), phosphorene, MoS2, etc., leading to the development of novel THz nanophotonic sensing devices.

Thermodynamically limited uncooled infrared detector using an ultra-low mass perforated subwavelength absorber

An uncooled detector has reached the thermodynamic temperature fluctuation limit, such that 98% of its total noise consisted of phonon and photon fluctuations of the detector body. The device has performed with a detectivity of 3.8×109cmHz/W, which is the highest reported for any room temperature device operating in the long-wave infrared (λ∼8−12µm). The device has shown a noise-equivalent temperature difference of 4.5 mK and a time constant of 7.4 ms. The detector contains a subwavelength perforated absorber with an absorption-per-unit-thermal mass-per-area of 1.54×1022kg−1m−2, which is approximately 1.6–32.1 times greater than the state-of-the-art absorbers reported for any infrared application. The perforated absorber membrane is mostly open space, and the solid portion consists of Ti, SiN x , and Ni layers with an overall fill factor of ∼28%, where subwavelength interference, cavity coupling, and evanescent field absorption among units induce the high absorption-per-unit-thermal mass-per-area. Readout of the detector occurs via infrared-absorption-induced deformation using a Mach–Zehnder interferometry technique (at λ=633nm), chosen for its long-term compatibility with array reads using a single integrated transceiver.

Sub-wavelength effects in a free electron laser oscillator.

Previous simulation studies of cavity based free electron lasers (FELs) have utilised models which average the optical field in the FEL interaction over an integer number of radiation wavelengths. In this paper, two unaveraged simulation codes, OPC and Puffin, are combined to enable modelling, for the first time, of a cavity based FEL at the sub-wavelength scale. This enables modelling of effects such as coherent spontaneous emission from the electron beam and sub-wavelength cavity length detuning. A cavity FEL operating in the mid-infrared is modelled and it is shown that, for small sub-wavelength cavity detunings, the FEL can preferentially lase at the third harmonic of the fundamental FEL wavelength. This novel result suggests other modes of operation may be possible and opens up cavity-based FEL operation to investigation of further, potentially useful, modes of operation.

Circularly polarized global navigation satellite systems metasurface antennas in sub-wavelength metallic cavities

AbstractTwo metasurface-inspired antennas embedded in a metallic cavity are introduced here. They are expected to be integrated on fast moving platforms enduring harsh accelerations and shocks. The metasurface allows enlarging the antenna bandwidth that is intrinsically reduced for small antennas embedded in sub-wavelength metallic cavities. The first one is only 60 × 60 × 20 mm3 (0.23λ1 × 0.23λ1 × 0.08λ1 at the frequency of 1164 MHz) and presents a dual-band behavior, covering both the lower and upper global navigation satellite systems (GNSS) bands (all GNSS bands are covered). It is fed by four probes and a dedicated circuit, ensuring the phase quadrature between adjacent feeds to achieve circular polarization over these two bands. For the second proposed antenna, circular polarization is achieved using two feed points connected to the radiating aperture of size 50 × 50 × 20 mm3 (0.26λ0 × 0.26λ0 × 0.10λ0 at the frequency of 1559 MHz). It covers the E1, L1, B1, and G1 GNSS bands. The numerical results are successfully validated by measurements.

Enhancing the sensitivity of photoacoustic spectrum system for liquid detection by coupling with acoustic metasurfaces

Photoacoustic spectroscopy (PAS) has been proven to have great potential in health monitoring applications, such as continuous blood glucose or oxygen monitoring. However, due to the significant mismatch between different acoustic media, 99.9% of acoustic energy cannot propagate across the gas–liquid interface, which greatly limits the liquid detection sensitivity of PAS. In this study, a Helmholtz-type acoustic metasurface (HAM) consisting of a perforated plate and a subwavelength cavity filled with air is proposed to enhance the acoustic transmission across the gas–liquid interface. The behavior of the air layer within the HAM is analogous to the transverse vibration of a membrane, which would drive the vibration of the water surface and provide an effective path for sound transmission from water to air. Experimental results confirmed that HAM can increase the power transmission coefficient of sound from water to air by two orders of magnitude while maintaining optical transparency for laser. Furthermore, the performance of the PAS system coupled with HAM is investigated. It is demonstrated that the liquid detection sensitivity of PAS systems can be enhanced remarkably, with a 409% magnification of the signal intensity. The working frequency of HAM can also be flexibly adjusted from 0.01 to 20 kHz through parameter configuration, enabling compatibility with the operating frequency of PAS systems. It is expected to be applied in the field of liquid ingredient analysis and health monitoring.

Accurate measurements of the ferromagnetic resonance linewidth of single crystal BaM hexaferrite spheres employing magnetic plasmon resonance theory

Accurate measurements of the intrinsic ferromagnetic linewidth of a single-crystal Sc-doped hexagonal M-type barium ferrite (BaM) 308-µm-diameter sphere at millimeter-wave frequencies employing a subwavelength cavity are described. The magnetic hysteresis of the resonance frequency and Q-factor of the dominant resonance (which has a plasmonic character) in a hexaferrite sphere are reported. Measurements performed in a fixture open to free space also show that the ferromagnetic linewidth significantly increases as compared to measurements in a closed cavity. This effect can be attributed to radiation losses, as revealed by an electrodynamic magnetic plasmon resonance model, and is relevant for measurements in open structures, such as microstrip lines or coplanar waveguides. The impact of finite resistivity on the linewidth was also analyzed.

Spectral properties of intermediate to high refractive index nanocubes.

Plasmonic resonances in sub-wavelength cavities, created by metallic nanocubes separated from a metallic surface by a dielectric gap, lead to strong light confinement and strong Purcell effect, with many applications in spectroscopy, enhanced light emission and optomechanics. However, the limited choice of metals, and the constraints on the sizes of the nanocubes, restrict the optical wavelength range of applications. We show that dielectric nanocubes made of intermediate to high refractive index materials exhibit similar but significantly blue shifted and enriched optical responses due to the interaction between gap plasmonic modes and internal modes. This result is explained, and the efficiency of dielectric nanocubes for light absorption and spontaneous emission is quantified by comparing the optical response and induced fluorescence enhancement of nanocubes made of barium titanate, tungsten trioxide, gallium phosphide, silicon, silver and rhodium.

Strong superradiance of coherently coupled magnetic dipole emitters mediated by whispering gallery modes of a subwavelength all-dielectric cavity

The interaction of magnetic dipole (MD) emitters and common photonic cavities is usually weak, which is partially due to the low magnetic near field enhancements of the cavities. Here, we show that whispering gallery modes (WGMs) of a subwavelength dielectric cavity can not only greatly boost the emission rate of a MD emitter but also bring efficient couplings between coherent MD emitters. In a WGM cavity, the maximal emission rate ({\gamma}max) of a single emitter occurs at an antinode of the field pattern. The emission of the MD emitter can also be greatly affected by another coherent one depending on the magnetic field response of the WGM. The maximal contribution can also reach {\gamma}max. Notably, the cooperative emission rate of the coherent MD emitters does not decay with distance in the considered range due to the high-quality feature of a WGM. In contrast to the emission, the absorption of an emitter is hardly affected by the coherent couplings between emitters mediated by a WGM. The difference between the performances of emission and absorption is highly related to the excitation behaviors of WGMs. Our results are important for enhanced magnetic light-matter interactions.

Experimental Investigation of Large Area Subwavelength Cavity Antennas With Planar Metamaterials

We experimentally investigated the large area subwavelength cavity antenna with artificial permeability-negative metamaterials in the GHz region. It is demonstrated that this new type of planar metamaterials has better directivity and higher gain with the radiation source using the large non-uniform distributed patch array than using a uniform distributed patch array, where the current distribution of the radiation source satisfies the Chebyshev distribution. The experimental values agreed well with simulated results. This new metamaterial antenna has potential applications in weak microwave signal detection and radio observation fields.

Hybrid resonant cavities: A route towards phase engineered THz metasurfaces

SummaryCoupled resonant cavities can enable strong photon energy confinement to facilitate the miniaturization of functional photonic devices for applications in designs of sensors, modulators, couplers, waveguides, color filters etc. Typically, the resonances in subwavelength plasmonic cavities rely on the excitation of surface plasmons at specific phase-matching conditions, usually determined by the lattice parameters and constituent material properties. Contrary to this notion, we experimentally demonstrate the control and manipulation of cavity resonances via suitably modifying the split ring resonator geometry in hybrid plasmonic-metasurface (dipole cavity-SRR) configuration without altering the lattice parameters. This results to the excitation of dual resonance peaks. Such dual channel characteristics demonstrate high quality (Q) factor, multi-band resonances, not permissible with typical (unhybridized) plasmonic dipole cavities. We envisage such hybrid meta-cavity designs can become important ingredients for futuristic terahertz devices that can hold the key for sixth generation (6G) communications, designer filters, dual channel sensors etc.

Electromagnetic Characterization of Shielded Spherical Gyromagnetic Resonators

The electromagnetic (EM) properties of shielded gyromagnetic spheres are characterized with the aid of an electrodynamic model formulated for the transverse electric (TE) modes of an isotropic sphere, in a similar way as dielectric resonators are commonly analyzed. Broadband characteristics of geometric and energy filling factors are presented. A new measure of the magnetic loss tangent is introduced which is positive despite the intrinsic permeability of the gyromagnetic medium being negative, where existing definitions yield a non-physical negative value. The sample size dependence of those quantities is also assessed. The conclusions from the analysis are applied to investigate and de-embed ferromagnetic linewidth errors which can occur in broadband measurements of magnetic garnet spheres. Magnetic anisotropy fields and the gyromagnetic <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$g$ </tex-math></inline-formula> -factor are also estimated from the measurement data. Experiments were performed in a two-port subwavelength cavity, covering a frequency range of up to 28 GHz. Additionally, two-port rectangular resonant cavity linewidth measurements at a few modes have been conducted. Good agreement between the two methods is found if the systematic error is accounted for.

Dual channel plasmonic hybrid system as potential multi-analyte and multi-parameter sensor

Confinement of field across subwavelength cavity through localized surface plasmon resonance (LSPR) is proposed for the better performance of plasmonic refractive index sensor. Transmission of light through dual nano cavities enables the simultaneous quantification of multiple analytes through a label free detection. Performance analysis of a highly tunable dual channel based nano SPR sensor that can detect and quantify different analyte molecules simultaneously is carried out. With the aid of finite element method (FEM), the sensor performance is optimized for concentration analysis of 0.07–0.12 g/ml of each glucose-lactose solution simultaneously via respective index of refraction. Both channels exhibit independent refractive index (RI) sensing with dual absorption maxima arising from field induced in either cavity resulting from localized surface plasmon resonance. Proposed structure exhibits a super ultra-broad RI sensing range of nearly 0.4 to 1.8 RIU with resonant peaks across near-IR region. Furthermore, when the channel RI separation comes too closer or become same, i.e., in case of single analyte analysis for subsequent variations, the merging of two plasmonic modes into one with an equal field distribution at both cavities is also observed. Thus, the proposed nano-structure may find application as a multipurpose and/or multi-channel-based analyte analyser.

A Dual-Band High-Gain Subwavelength Cavity Antenna with Artificial Magnetic Conductor Metamaterial Microstructures.

This paper presents dual-band high-gain subwavelength cavity antennas with artificial magnetic conductor (AMC) metamaterial microstructures. We developed an AMC metamaterial plate that can be equivalent to mu-negative metamaterials (MNMs) at two frequencies using periodic microstructure unit cells. A cavity antenna was constructed using the dual-band AMC metamaterial plate as the covering layer and utilizing a feed patch antenna with slot loading as the radiation source. The antenna was fabricated with a printed circuit board (PCB) process and measured in an anechoic chamber. The |S11| of the antenna was −26.8 dB and −23.2 dB at 3.75 GHz and 5.66 GHz, respectively, and the realized gain was 15.2 dBi and 18.8 dBi at two resonant frequencies. The thickness of the cavity, a sub-wavelength thickness cavity, was 15 mm, less than one fifth of the long resonant wavelength and less than one third of the short resonant wavelength. This new antenna has the advantages of low profile, light weight, dual-frequency capability, high gain, and easy processing.

Strong Purcell effect in deep subwavelength coaxial cavity with GeSn active medium.

We propose a deep subwavelength plasmonic cavity based on a metal-coated coaxial structure with Ge0.9Sn0.1 as the active medium. A fundamental surface plasmon polariton mode is strongly confined on the sidewall of the metal core, with the quality factor up to 5×103 at 10 K. By reducing the cavity dimension to a few nanometers, this cavity mode shows a strong plasmon binding with the mode volume down to 8×10-10 (λ/n)3, and significant size-dependent damping caused by the non-local optical response. The Purcell factor is achieved as high as 2×109 at 10 K and 7×108 at 300 K. This cavity design provides a systematic guideline of scaling down the cavity size and enhancing the Purcell factor. Our theoretical demonstration and understanding of the subwavelength plasmonic cavity represent a significant step toward the large-scale integration of on-chip lasers with a low threshold.

Near-field coupling of absorbing material to subwavelength cavities

In conjunction with our previous work on spectral transmission dependence on geometric parameters for subwavelength slits, we have investigated the optical behavior with the inclusion of an absorber on the extraordinary optical transmission of sliver slits resonant in the mid-wave infrared (MWIR). The placement of an absorbing layer causes a dramatic change to the dielectric environment of the subwavelength slit causing the cavity to become energetically leaky. We have found this broadens the spectral response of the cavity by increasing the imaginary component of the effective cavity index, reducing the cavity quality by a factor of more than seven. To mitigate this undesired effect, we have found that partially decoupling the absorber with a thin isolating layer helps restore the slit’s narrow spectral response and we explore the dependence of optical properties on the isolating layer’s depth. The optimum thickness of a silicon dioxide isolation layer for best quantum efficiency (Q.E.) was found to be 100 nm with a maximum Q.E. of 37.5%. This is more than double the Q.E. of the directly coupled absorber and the cavity Q increase is of the same order, with a corresponding narrowing of the resonance bandwidth. In addition, we explore the effect on the cavity of changing the dielectric environment at the input of the slit to improve the resonance properties in the propagation direction improving the Q by 20% and improving the Q.E. to 40.2%.

Space- and time-resolved second harmonic spectroscopy of coupled plasmonic nanocavities

Abstract Localized surface plasmon resonances of individual sub-wavelength cavities milled in metallic films can couple to each other to form a collective behavior. This coupling leads to a delocalization of the plasmon field at the film surface and drastically alters both the linear and nonlinear optical properties of the sample. In periodic arrays of nanocavities, the coupling results in the formation of propagating surface plasmon polaritons (SPP), eigenmodes extending across the array. When artificially introducing dislocations, defects and imperfections, multiple scattering of these SPP modes can lead to hot-spot formation, intense and spatially confined fluctuations of the local plasmonic field within the array. Here, we study the underlying coupling effects by probing plasmonic modes in well-defined individual triangular dimer cavities and in arrays of triangular cavities with and without artificial defects. Nonlinear confocal spectro-microscopy is employed to map the second harmonic (SH) radiation from these systems. Pronounced spatial localization of the SPP field and significant enhancements of the SH intensity in certain, randomly distributed hot spots by more than an order of magnitude are observed from the triangular arrays as compared to a bare silver film by introducing a finite degree of disorder into the array structure. Hot-spot formation and the resulting enhancement of the nonlinear efficiency are correlated with an increase in the lifetime of the localized SPP modes. By using interferometric SH autocorrelation measurements, we reveal lifetimes of hot-spot resonances in disordered arrays that are much longer than the few-femtosecond lifetimes of the localized surface plasmon resonances of individual nanocavity dimers. This suggests that hot spot lifetime engineering provides a path for manipulating the linear and nonlinear optical properties of nanosystems by jointly exploiting coherent couplings and tailored disorder.