Subwavelength Aperture Research Articles

A GaAs-based plasmonic source employing a nanoscale vertical cavity

In this paper, a new structure of a vertical cavity plasmonic source is presented. The structure, which is adopted from a metal–semiconductor–metal plasmonic photodetector, contains a gold substrate, n-type GaAs layer, active GaAs layer and p-type GaAs layer, respectively from bottom to up. A subwavelength aperture and metal gratings on both sides of the aperture are placed on the top of the device and act both as a gold clad and as the contact for applying DC bias. The metal gratings are improved by adding SiO2 between metal grooves and semiconductor. Applying voltage bias to p-and n-regions leads to photons generation in the active region. The generated photons in the active region excite a resonant surface plasmonic mode in the nanoscale cavity just below the aperture. One can use light outflowing from the nanoscale aperture, with wavelength of 882 nm, as a source to excite an arbitrary plasmonic guiding structure. The output beam has a suitable concentration so that in a 1000 nm distance from the aperture, still 37% of the power is concentrated in a width of 50 nm which is almost a unique specification among plasmonic sources, so far. The function of the device is validated using finite-difference time-domain numerical algorithm.

Plasmonic Color Filter Array with High Color Purity for CMOS Image Sensors.

We demonstrate the multiband color filtering of a standard RGB color and a complementary CMY color by a plasmonic color filter, composed of concentric corrugated metallic thin film rings. The surface plasmon resonance is excited by the periodic corrugation, and the coupled light is transmitted through the central subwavelength aperture. Color selectivity is achieved not only in the visible but also in the near-infrared (NIR) region. Therefore, simultaneous imaging with visible and NIR can be realized by the integration of plasmonic color filters with sensors. We investigate the angle of incidence dependence of the transmission color selectivity and the color purity of the fabricated plasmonic color filter array.

Evolution dynamics of optical angular momentum and torque through a birefringent metallic subwavelength aperture

The interaction of light and matter results in the transfer of linear momentum and angular momentum (AM) which induces optical force and torque, respectively. It is revealed that the linear momentum and AM are conserved for a system of matter and electromagnetic field. In this paper, we present a waveguide-mode analysis to show the dynamic evolution rules for both longitudinal and transverse angular momenta in a metallic subwavelength aperture with different symmetries. In addition, we demonstrate the relationship between AM and optical torque that is based on the AM conservation of light and the metallic subwavelength aperture. Accompanying with the change of AM is the existence of non-zero optical torque, which is promising for dynamic optical manipulation in nanoscale. We believe that unveiling such phenomenon is conducive to the further development of the angular-momentum-based optical mechanics in subwavelength aperture.

Continuously Frequency-Tuneable Plasmonic Structures for Terahertz Bio-sensing and Spectroscopy

Plasmon-based devices are powerful for use in highly sensitive evanescent-field detection and analysis, but they exhibit the problem of limited frequency tunability for fixed structures. This feature causes problems in the multi-frequency investigations required for materials characterization, bio-related research, etc. Here, we propose and fabricate a spiral-shaped plasmonic structure that enables a continuous frequency-tuneable evanescent-field concentration in the terahertz (THz) region with simple operation. The device also increases the electric field intensity at the subwavelength aperture, thus significantly amplifying the transmission. Highly tuneable transmission bands are observed by simply rotating the spiral plasmonic structure, which are in good agreement with the behaviour expected from electromagnetic simulation. Medical examinations are performed by measuring the interactions between the frequency-tuneable plasmons and bio-samples, which enables observing distinct tissue-dependent transmission spectra and images. The developed device simultaneously offers the advantages of both plasmonic devices and frequency-tuneable devices, which can increase the availability and versatility of evanescent-field THz sensing and analysis. The mechanism presented will shed light on THz plasmonics and motivate the implementation of a variety of applications based on plasmon-mediated THz technologies.

Rayleigh-Sommerfeld diffraction on a subwavelength scale: Theories and a resolution criterion

Classical resolution criteria such as Rayleigh's and Sparrow's are based on the fringe structure produced by Fresnel diffraction or Fraunhofer diffraction. However, these two diffraction theories are based on the assumption of large-aperture scale and are thus incapable of describing the diffraction of subwavelength structure. We find a singularity near the incident wave vector ${k}_{0}$ in a subwavelength aperture by considering Rayleigh-Sommerfeld diffraction. The diffraction fringe structure disappears once the aperture scale is smaller than a threshold value. A two-point resolution criterion unrelated to the fringe structure is proposed based on the second-order derivative of the field structure. Numerical results indicate that, for the illumination wavelength of $\ensuremath{\lambda}=500\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$, resolution of two 100-nm rectangular holes under near-field Rayleigh-Sommerfeld diffraction is improved by about 300 and 20% in the $x$ and $z$ directions, respectively, compared with the Sparrow criterion, while the lateral resolution limit is reduced to 35 nm.

Plasmon Enhancement of the Electric Field in Mid-Infrared Ge/Si Quantum-Dot Photodetectors with Different Thicknesses of the Active Region

The spatial distribution of the electric field in Ge/Si photodetector heterostructures coated with a gold film containing a regular two-dimensional array of subwavelength apertures is calculated by the finite-element method. The array period and aperture diameter are 1.2 and 0.7 μm, respectively. The efficiency of field enhancement is determined for different thicknesses of the active region occupied by quantum dots. It is demonstrated that the field-enhancement factor for an electromagnetic wave incident on the structure from the side of the substrate is ~3.5 times larger than that for a wave incident from the opposite side. In the first case, the field-enhancement factor varies nonmonotonically with the thickness of the active region.

Extraordinary Transmission of Light Through Freestanding Au Films with Subwavelength Apertures and Double Slits

We demonstrated the extraordinary transmission of light through a freestanding Au thin film with a 7×7 subwavelength aperture array and double slits. Firstly, we observed the dependence of the change in the light transmission through a plasmonic structure on the pitch size of the aperture array. Also, we fabricated a freestanding Au thin film to avoid the effects of absorption and reflection of the substrate. We perforated the 7×7 subwavelength hole array on the Au film for extraordinary optical transmission caused by the electric field enhancement from the structures. Additionally, we designed a hybrid structure composed of 7×7 subwavelength apertures and double slits to enhance the transmission of the light. The normalized-to-area transmission of the hybrid structures was approximately five times greater than that of the structures without double slits.

A Dual-Band Microwave Filter Design for Modern Wireless Communication Systems

Nowadays, modern communication system relies on the designs of high-performance devices to enhance communication effect for a high quality of life and smart city system. As a crucial signal processing step, microwave filter removes unwanted frequency components away from the received signal and enhances the useful ones. However, large loss, bulky size, and single-band greatly limit the practical applications in urban computing. Therefore, the filters with dual-band characteristic are highly desirable for modern wireless communication, such as device-to-device communication, environment monitoring, and automatic driving. In this paper, a dual-band microwave filter is designed and fabricated based on the theory of Mie-resonance extraordinary transmission. An electromagnetic wave cannot propagate through a subwavelength aperture drilled in a metallic film. By adding two dielectric cuboids of different sizes into the two apertures, two passbands appear in the frequency range of 10.0–12.0 GHz. In this range, the insertion loss is less than 0.4 dB, and 3-dB bandwidth is more than 48 MHz. Particularly, the two passband frequencies can be tuned by adjusting the size of the dielectric cuboids. This approach opens a way for designing tunable dual-band microwave bandpass filter, which is benefit for enhancing spectrum resource utilization.

Плазмонное усиление поля в фотоприемниках среднего ИК-диапазона на базе квантовых точек Ge/Si с различной толщиной активной зоны

AbstractThe spatial distribution of the electric field in Ge/Si photodetector heterostructures coated with a gold film containing a regular two-dimensional array of subwavelength apertures is calculated by the finite-element method. The array period and aperture diameter are 1.2 and 0.7 μm, respectively. The efficiency of field enhancement is determined for different thicknesses of the active region occupied by quantum dots. It is demonstrated that the field-enhancement factor for an electromagnetic wave incident on the structure from the side of the substrate is ~3.5 times larger than that for a wave incident from the opposite side. In the first case, the field-enhancement factor varies nonmonotonically with the thickness of the active region.

Multi-spectral frequency selective mid-infrared microbolometers.

Frequency selective detection of low energy photons is a scientific challenge using natural materials. A hypothetical surface which functions like a light funnel with very low thermal mass in order to enhance photon collection and suppress background thermal noise is the ideal solution to address both low temperature and frequency selective detection limitations of present detection systems. Here, we present a cavity-coupled quasi-three dimensional plasmonic crystal which induces impedance matching to the free space giving rise to extraordinary transmission through the sub-wavelength aperture array like a "light funnel" in coupling low energy incident photons resulting in frequency selective perfect (~100%) absorption of the incident radiation and zero back reflection. The peak wavelength of absorption of the incident light is almost independent of the angle of incidence and remains within 20% of its maximum (100%) up to θi≤45˚. This perfect absorption results from the incident light-driven localized edge "micro-plasma" currents on the lossy metallic surfaces. The wide-angle light funneling is validated with experimental measurements. Further, a super-lattice based electronic biasing circuit converts the absorbed narrow linewidth (Δλ/λ0< 0.075) photon energy inside the sub-wavelength thick film (< λ/100) to voltage output with high signal to noise ratio close to the theoretical limit. Such artificial plasmonic surfaces enable flexible scaling of light funneling response to any wavelength range by simple dimensional changes paving the path towards room temperature frequency selective low energy photon detection.

Enhanced Transmission Through Single Subwavelength Apertures Using Miniaturized Planar Resonators

An in-plane two-layer meandered wire resonator structure is utilized to enable transmission through a subwavelength aperture. The structure exhibits effectively 100% transmission at approximately 1.94 GHz through a λ 0/33 square aperture. The extremely high Q resonance presents a transmission greater than –10 dB over a bandwidth of 0.007%. A complementary subwavelength resonator is also proposed, which exhibits a narrow band transmission response operating at 1.92 GHz with a fractional bandwidth of 0.04% at an overall electrical size of λ0/43. A good agreement between measured and simulated results is achieved for both designs. The subwavelength structures show a slight sensitivity to fabrication tolerances, but are realizable with modern printed circuit fabrication techniques due to their planar nature.

Lamb Wave Focusing Transducer for Efficient Coupling to Wavelength-Scale Structures in Thin Piezoelectric Films

This paper describes the theoretical and experimental investigation of interdigitated transducers capable of producing focused acoustical beams in thin film piezoelectric materials. A mathematical formalism describing focused acoustical beams, lamb beams, is presented and related to their optical counterparts in two- and three-dimensions. A novel Fourier domain transducer design methodology is developed and utilized to produce near diffraction limited focused beams within a thin film AlN membrane. The properties of the acoustic beam formed by the transducer were studied by means of Doppler vibrometry implemented with a scanning confocal balanced homodyne interferometer. The Fourier domain modal analysis confirmed that 83% of the acoustical power was delivered to the targeted focused beam which was constituted from the lowest order symmetric mode, while 1% was delivered unintentionally to the beam formed from the anti-symmetric mode, and the remaining power was isotropically scattered. The transmission properties of the acoustic beams as they interact with devices with wavelength scale features were also studied, demonstrating minimal insertion loss for devices in which a subwavelength and pinhole apertures were included. [2018-0059]

Efficient terahertz transmission modulation in plasmonic metallic slits by a graphene ribbon array.

Extraordinary optical transmission is the widely known phenomenon of enhanced transmission of waves through subwavelength periodic metallic apertures. Here, we propose efficient terahertz transmission modulation in subwavelength metallic slits by a graphene plasmonic ribbon array. The extraordinary optical transmission through the subwavelength metallic slits can be tuned by coupling with the plasmonic resonance of graphene ribbon array, resulting in a deep transmittance depression. Numerical simulations show that maximal transmission modulation depth of 98.99% can be obtained at 1.03THz via this mechanism.

Flat metasurfaces to collimate electromagnetic waves with high efficiency.

Directional beaming of electromagnetic waves passing through a subwavelength aperture has attracted considerable interests in photonics, but the traditional approach of utilizing gratings to directionally couple surface waves (SWs) to a desired far-field direction faces the low-efficiency issue owing to high-order diffractions. Here we experimentally demonstrate that directional beaming of light can be realized with very high efficiencies, in which two specifically designed metasurfaces (MTSs) are placed at two sides of the aperture to serve as SW to propagating-wave meta-couplers. Different from the grating couplers, the well-designed phase-gradient meta-couplers can freely select the desired diffraction orders by suppressing the undesired diffraction orders. We design and fabricate MTSs with different phase gradients, and perform both far-field and near-field measurements to verify the predicted high-efficiency on/off-axis directional beaming effects. Experimental results are in good agreement with full wave simulations and theoretical analyses.

Optical Response of Nanohole Arrays Filled with Chalcogenide Low‐Epsilon Media

AbstractWhen an array of subwavelength apertures in a metal film is filled with a chalcogenide semiconductor acting as a low‐epsilon medium at optical frequencies, an increase in transmission is observed over a broad range of plasmonic frequencies, and found to be enabled by “laminar” flow of energy through the chalcogenide inclusions. As losses decrease in such a low‐epsilon medium, with causality‐bound dispersion of complex relative permittivity, a peak in transmission emerges at a wavelength tending toward that of unitary refractive index, while counterintuitively the absorption of the composite structure increases.

Multi-band plasmonic color filters for visible-to-near-infrared image sensors.

We propose a plasmonic color filter consisting of a single aperture surrounded by concentric periodic corrugations for simultaneous imaging of a spectral range from the visible to the near-infrared. The incident light coupled with surface plasmons propagates through the sub-wavelength aperture as beaming light. The beaming light transmission is able to suppress the spatial color cross-talk between the pixels in an image sensor. We analyzed the transmission characteristics of a plasmonic color filter with periodic corrugations in a silver thin film by using the finite-difference time-domain algorithm. We demonstrated a multi-band transmission wavelength selectivity, of about 100 nm, for the spectral bandwidth ranging from visible to near-infrared. The simultaneous discrimination of visible and near-infrared light with a high color purity by the plasmonic color filter achieves both improved image recognition and smaller system-size compared with conventional systems, which is particularly important for applications such as vehicle-mounted cameras, security, and biological tissue engineering.

Connection of Collimation, Asymmetric Beaming, and Independent Transmission-Reflection Processes in Concentric-Groove Gratings Supporting Spoof Surface Plasmons

Transmission through subwavelength apertures enables separation of the incidence half-space and the exit half-space, which leads to that the spatial distribution of the field in the latter is not affected by the distribution in the former. The distribution in the exit half-space is mainly determined by the properties of surface plasmons (SPs) at the exit-side interface. In this paper, for the microwave structures with one-side concentric corrugations around a single annular hole, we demonstrate the possible connections between asymmetric transmission in the beaming regime and collimation of the waves incident at different angles, which can be considered as two sides of the same phenomenon occurring due to the common effect of such a separation and the radiation shaping effect being possible due to the spoof SPs at the corrugated exit interface. Collimation manifests itself in that the waves incident at different angles from a wide range contribute to the single outgoing beam so that a far-zone observer cannot distinguish between the contributions of different angles of arrival. Asymmetry in transmission manifests itself in that the spatial shaping of radiation (beaming) in the exit half-space appears only for one of the two opposite incidence directions. Moreover, even in the structures with the same corrugations on both sides, i.e., without asymmetric transmission, spatial separation of two wave processes, e.g., two symmetric or asymmetric collimation processes, can be obtained for a wide range of nonzero angles of incidence.

Generation of vortex beams with multi topological charges, high purity and operating on broadband using a simple silver metasurface

We propose a simple approach to generate vortex beam based on silver metasurface under the incidence of circularly polarized light. The topological charge and purity of result beam is from -4 to 4 and more than 95% respectively, furthermore, it can operate on broadband (800∼1800 nm) including telecom wavelengths. The designed metasurface is composed of ten concentric rings, where each ring is composed of sub-wavelength rectangular apertures with circular array distribution. These rectangular apertures act as localized space-variant linear polarizers and space-variant polarization manipulation can introduce Pancharatnam–Berry (PB) phase. Simulation results show that this design is logically coincidence. The designed metasurface has the advantage of high fabrication tolerance and may be a potential candidate in future integrated optical communication system.

Localization of Surface Plasmon Waves in Hybrid Photodetectors with Subwavelength Metallic Arrays

The spectral characteristics of the hole photocurrent in plasmon photodetectors based on Ge/Si heterostructures with Ge quantum dots combined with regular arrays of subwavelength apertures of various shapes in a gold film on the semiconductor surface are investigated. Dispersion relations characterizing the propagation of surface plasmon waves along the metal–semiconductor interface are determined from the dependences of the photocurrent on the angle of incidence of light. It is established that the plasmonic enhancement of the photocurrent in rectangular aperture array is suppressed as compared to that in circular and square aperture arrays. It is found that, in hybrid structures with rectangular apertures, there exists a range of wave vectors where the energy of surface plasmons is independent of the wave vector of incident radiation. The results are explained by the excitation of dipole modes localized at rectangular apertures with a large aspect ratio by light waves.

Enhancing Efficiency of Electromagnetic Simulation in Time Domain with Transformation Optics

With sub-wavelength scaled structures in a large system, the conventional finite-difference time-domain method can consume much computational resources since it includes both the spatial and temporal dimension in the scheme. In order to reduce the computational cost, we combine the novel methodology “transformation optics” in the simulation to map a physical coordinate with designated non-uniform grids to a uniform numerical coordinate. For a demonstration, the transmission spectrum through a sub-wavelength metallic aperture with one-dimensional and two-dimensional coordinate transformation is simulated, and compared with uniform-grid cases. We show that the proposed method is accurate, and the computational cost can be reduced remarkably to at most 5.31%, in comparison with the simulation of the finest uniform grids demonstrated. We are confident that it should be helpful to the simulation study in sub-wavelength optics due to its verified accuracy and efficiency.