Subwavelength Metallic Gratings Research Articles

Light-wave manipulation of subwavelength metallic gratings by electrically controlling the surface charge distribution based on surface-plasmon excitation

Light-wave manipulation of subwavelength metallic gratings by electrically controlling the surface charge distribution based on surface-plasmon excitation

Triple-broadband asymmetric transmission via cross-polarization conversion based on composite chiral metasurface structure

In this paper, a novel composite chiral metasurface (CCMS) was proposed and investigated, which is composed of a rectangle-patch-ring resonator (RPRR) structure sandwiched between two twisted sub-wavelength metal gratings separated by a dielectric substrate. Both simulation and experimental results demonstrate that the proposed CCMS achieves a significant asymmetric transmission (AT) effect and efficient cross-polarization conversion for the incident linear polarization wave in a triple-broadband range. Simulation results are in good agreement with the experiments. Specifically, the CCMS converts normal impinging y-/x-polarization wave along the forward/backward (-z/+z) direction into transmitted x-/y-polarization wave with the cross-polarization transmission coefficient over 0.7 at 4.15–5.5 GHz, 7.29–10.77 GHz and 12.49–16.59 GHz, respectively. In addition, the AT coefficient (Δlin) and the total transmittance (Tx) of the x-polarization wave propagation along the –z axis direction are both over 0.5 in the triple-broadband frequency range. The simulated polarization azimuth rotation and ellipticity angles, induced surface current, and electric field vector distributions further confirm the characteristics of the triple-broadband high-efficiency AT effect and cross-polarization conversion of the designed CCMS. The proposed CCMS design serves as an important reference for practical applications of microwave devices.

Excitation of Terahertz Plasmonic Resonance on a Subwavelength Metallic Grating in an Attenuated Total Reflection Scheme Using a Monochromatic Radiation Source

Excitation of Terahertz Plasmonic Resonance on a Subwavelength Metallic Grating in an Attenuated Total Reflection Scheme Using a Monochromatic Radiation Source

Polarization-sensitive UV detector based on a double-layer subwavelength grating of Al and ZnO

Polarization-sensitive detectors are of great importance in the fields of remote sensing and imaging, environmental monitoring, and medical diagnosis. The surface plasmon effect can the enable polarization sensitivity of photodetectors through metallic gratings. However, limited by the precision of the nano-fabrication process, it is difficult to fabricate an ultraviolet (UV) polarization-sensitive detector integrated with sub-wavelength metal gratings and the polarization extinction ratio is relatively low. In this paper, an Al–ZnO composite double-layer grating structure was designed. The ZnO active layer and the Al layer were both fabricated into same-sized grating structures. Through this design, the slit width could be enlarged to some degree, while the response to 90° polarized light remained low. It is beneficial to realize a high polarization ratio and to spare the need for rigid fabrication accuracy. In addition, the influence of the structural parameters of the grating on the performance of the detector was studied by simulation. It was found that the resonance wavelength can be adjusted by changing the slit width and grating height, respectively. This provides a useful means for polarization-sensitive detection in different wavelength ranges. The polarization extinction ratio of the detector with a double-layer composite grating can reach 52 in the UV band (365 nm). This provides a good alternative to replace the traditional framework relying on the combination of polarizers and detectors. Moreover, it is a promising structure for high-density integrated photodetectors and imaging chips in the future.

Metasurface with Directional‐Controlled Asymmetric Transmissions

The diverse design freedom and exotic electromagnetic properties of metasurfaces enable them to achieve unprecedented capability and superiority with regard to conventional optical elements. Herein, a bifunctional microwave metasurface with an asymmetric bidirection transmission phase that can be controlled is reported. The proposed metasurface is composed of bilayered double‐arrow‐shaped structures sandwiching a subwavelength metal grating. Such design can achieve not only linear polarization (LP) conversion for two oppositely directed wave incidences but also phase control independently by introducing a rotation and adjusting the geometrical parameters of the double‐arrow‐shaped structures. The metasurface breaks the symmetry and realizes different functions for opposite propagation directions, such as deflection and focusing, orbital angular momentum (OAM) mode generation, and holographic imaging. The experimental results suggest that the proposed Janus metasurface can be further exploited as an attractive platform for various applications ranging from microwave reconfigurable beam antenna and multifunctional polarization control to optical imaging, sensing, and information processing.

Linearly polarized light emission from GaN micro-LEDs for 3D display

A three-dimensional (3D) display provides an immersive user experience and represents the frontier of display technology. The 3D display based on polarizer films is relatively mature. However, the currently used polarizer films lead to bulk systems and low efficiency. Micro-light-emitting diodes (micro-LEDs) have the advantages of small size, high contrast, low power consumption, and high response frequency. The 3D micro-LED displays combine the characteristics of 3D display and the advantages of micro-LEDs simultaneously, which can be used in portable electronics or micro-LED projectors and provides users with 3D experience. Here, linearly polarized (LP) micro-LEDs with varied chip diameters are numerically and experimentally investigated. Subwavelength metal gratings are integrated directly on the p-GaN of micro-LEDs. A maximum extinction ratio of 14.17 dB and a corresponding energy loss of 50% are achieved directly from LP micro-LEDs under the injection current density of 200 A/cm2. The size-dependent effect of LP micro-LED is also investigated and it is found that smaller sized LP micro-LEDs have smaller extinction ratio due to sidewall light leakage. The reported GaN-based LP micro-LEDs have excellent potential to be utilized in 3D micro-LED displays.

Study of structural effects on the polarization characteristics of subwavelength metallic gratings in short infrared wavelengths.

Applications of subwavelength grating based-polarizers for polarimetric detections are being hindered due to the limited extinction ratio. In this work, the structural effect, including the line edge roughness (LER), of the gratings on the polarizing characteristics was studied by both numerical simulations using finite difference and time domain (FDTD) method and experiments, aiming to figure out the optimal grating profile for achieving high transmittance as well as high extinction ratio. Two different configurations of the gratings, one is dual layer Au lines and the other is parabolic shaped Al lines on structured spin-on-carbon (SOC) films were systematically studied and compared. Nanofabrication of the gratings by electron beam lithography without lift-off process were conducted and optical measurements of their polarization properties demonstrate superior performance of the developed polarizers. The origin of the structural effect was explained by the local surface plasmonic modes, existing in the nano-slits in metallic gratings, which is instructive for further enhancement of the polarization performance.

Highly efficient and tunable terahertz polarization converter based on double subwavelength metallic gratings infiltrated with liquid crystal.

In this work, a tunable cross-polarized transmission structure at the terahertz frequency was demonstrated, and the polarization state during modulation was investigated. The proposed structure can significantly enhance the polarization conversion performance of nematic liquid crystals by leveraging the formation of a Fabry-Perot-like resonant cavity that consists of two metal gratings. As a result, the incident waves are continuously reflected in the liquid crystal layer to complete the accumulation of polarization angle changes. From the experimental results acquired, it was concluded that the insertion loss of the cross-polarization transmission was less than 3 dB and the extinction ratio was larger than 28 dB in the frequency range of 388-426 GHz. Our work provides useful insights for improving the efficiency of cross-polarization conversion by enhancing the resonance process in a Fabry-Perot-like resonant cavity and, thus, significantly extending the equivalent optical path.

Understanding of Wood Anomalies in Metallic Gratings From Phase Gradient Metasurfaces

Phase-gradient metasurfaces (PGMs) have provided unprecedented opportunities for manipulating light. Here, we reexamine ordinary and well-studied subwavelength metallic gratings (OMGs) from the concept of PGMs to provide more insight into their diffraction properties. We will show that due to the existence of gauge invariance in PGMs, i.e., the diffraction law of PGMs is independent of the choice of initial value of abrupt phase shift that induces the phase gradient, the well-studied OMGs can be regarded as a PGM strictly, with its diffraction properties can be fully predicted by generalized diffraction law with phase gradient. In particular, the generalized diffraction law reveals that the phase gradient plays a significant role in the famous effect of Wood’s anomalies and Rayleigh’s conjecture.

A Novel Wideband and High-Efficiency Electronically Scanning Transmitarray Using Transmission Metasurface Polarizer

Inspired by the recent progress of metasurfaces, a novel wideband and high-efficiency electronically reconfigurable transmitarray (TA) is presented that can enable two-dimensional (2-D) beam steering and multiple beam mode switching in a digital manner. The proposed unit cell consists of a reconfigurable anisotropic metallic resonator, which locates between a pair of orthogonally oriented subwavelength metal gratings. One-bit phase quantization is obtained by controlling the ON or OFF state of two positive-intrinsic-negative (p-i-n) diodes integrated on the anisotropic metallic resonator layer. To choke the radio frequency (RF) signal effectively, a bias layer is connected by the metallic via hole to the center of the resonator, where the electric field is minimal. The simulated results show that the proposed unit cell can achieve a low insertion loss of 1 dB from 9.3 to 10.9 GHz for two phase states. Then, the radiation performance of a 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times\,\,10$ </tex-math></inline-formula> element array fed by a pyramidal horn antenna is theoretically calculated, simulated, and measured. The measured maximum gain is 19.1 dBi at 10.0 GHz with an aperture efficiency of 25.8% and a 3 dB gain bandwidth of 15.9%. The measured results demonstrate the 2-D beam scanning capability within the 50° range.

Intensity-tunable terahertz bandpass filters based on liquid crystal integrated metamaterials.

In this paper, we demonstrate an intensity-tunable THz bandpass filter by introducing liquid crystal (LC) integrated with asymmetric frequency selective surface (FSS) and subwavelength metal gratings. Here, the tunable THz filter is derived from the inner polarization state conversion in composited devices, and the incident linear polarization can be converted into 90° orthogonal components. By controlling the LC orientation under the applied electric field with the metamaterial electrodes, the polarization conversion process can be actively modulated; thus, the polarization-dependent and tunable THz bandpass filter is achieved. Based on the multilayer design and the inner Fabry-Perot-like resonance mechanism, the LC-integrated metamaterials filter presents better filtering performance than the single FSS filter, and the Q-value is improved from 7.7 to 13.8 at the working frequency. Our simulated work paves the way for the design of new and efficient THz filters.

Optical spatiotemporal differentiator using a bilayer plasmonic grating.

As a key element in wave-based analog computation, optical differentiators have been implemented to directly perform information processing, such as edge detection and pulse shaping, in both spatial and temporal domains. Here, we propose an optical spatiotemporal differentiator, which simultaneously performs first-order spatial and temporal differentiation in transmission by breaking the mirror symmetry of a subwavelength bilayer metal grating. The spatial and temporal performance of the plasmonic differentiator is evaluated numerically using the output field profiles of an optical beam and pulse envelope, showing resolutions of ∼2µm and ∼50fs, respectively. Moreover, the function of spatiotemporal differentiation is demonstrated with input flat-top pulse fields. The proposed optical differentiator has potential applications in ultra-compact real-time optical multifunctional computing systems and parallel signal processing.

Miniaturized spectroscopy with tunable and sensitive plasmonic structures.

A broad linewidth and a lack of spectral analysis limit the applications of plasmonic sensors. In this Letter, a plasmonic sensor with a large sensitivity in the terahertz (THz) range is proposed based on high-quality factor (>1000) surface lattice resonance in subwavelength near-flat metallic gratings. Moreover, such a highly selective spectral manipulating scheme, plus the greatly localized plasmonic resonance, enables miniaturized spectroscopy based on a single detector by integrating an electro-optical material with the gratings. A spectral resolution of 0.1 GHz at a center frequency of 1.1 THz is predicted showing a four times improvement of measuring efficiency. This technique shows promising potential in on-site matter inspection and point-of-care testing.

Refractive index sensor based on a Tamm Fabry-Perot hybrid resonance.

A highly sensitive refractive index sensor is proposed by using the resonant coupling of a Tamm state to a Fabry-Perot resonance in slits periodically pierced in a metal film. This new hybrid resonance exists at the interface between a dielectric Bragg mirror and a subwavelength metal grating. Contrary to a classic Tamm plasmon, it is sensitive to the ambient media refractive index due to its confinement inside the grating slits. The proposed sensor output can be either the reflected intensity or preferably the wavelength, and its sensitivity and figure of merit are numerically investigated with the help of rigorous coupled wave analysis. The sensitivity of the studied sensor is 87 nm/RIU for a refractive index range from 1.25 to 1.38, and, at the expense of the resolution, it can reach up to 160 nm/RIU for a grating duty cycle of 50%. The figure of merit is around ${{7.5}}\;{{\rm{RIU}}^{- 1}}$ with a large measuring range. The index sensor performances and operating resonance wavelength can be modified by adjusting the grating geometric parameters (height and duty cycle). The possibility to achieve wavelength modulation in any spectral range makes the proposed grating Tamm structure an attractive candidate to design refractive index sensors and photonic components in the infrared and terahertz domains.

Modified rigorous coupled-wave analysis for grating-based plasmonic structures with a delta-thin conductive channel: far- and near-field study.

The modified rigorous coupled-wave analysis technique is developed to describe the optical characteristics of the plasmonic structures with the grating-gated delta-thin conductive channel in the far- and near-field zones of electromagnetic waves. The technique was applied for analysis of the resonant properties of AlGaN/GaN heterostructures combined with a deeply subwavelength metallic grating, which facilitates the excitation of the two-dimensional plasmons in the terahertz (THz) frequency range. The convergence of the calculations at the frequencies near the plasmon resonances is discussed. The impact of the grating's parameters, including filling factor and thickness of the grating, on resonant absorption of the structure was investigated in detail. The spatial distributions of the electromagnetic field in a near-field zone were used for the evaluation of total absorption of the plasmonic structures separating contributions of the grating-gated two-dimensional electron gas and the grating coupler.

Liquid crystal integrated metamaterial for multi-band terahertz linear polarization conversion

We experimentally investigate the linear polarization conversion for terahertz (THz) waves in liquid crystal (LC) integrated metamaterials, which consist of an LC layer sandwiched by two orthogonally arranged sub-wavelength metal gratings. A Fabry–Perot-like cavity is well constructed by the front and rear gratings, and it shows a strong local resonance mechanism, which greatly enhances the polarization conversion efficiency. Most importantly, the Fabry–Perot-like resonance can be actively tuned by modulating the refractive index of the middle LC layer under the external field. As a result, the integrated metamaterial achieves multi-band tunable linear polarization conversion.

Enhanced infrared photoresponse induced by symmetry breaking in a hybrid structure of graphene and plasmonic nanocavities

The infrared photoresponse of a hybrid structure of graphene and plasmonic nanocavities is enhanced by plasmonic symmetry breaking. The hybrid structure overcomes two bottleneck problems of metal-graphene-metal photodetectors, i.e. photoresponse cancellation due to contact symmetry and limited light absorption of graphene. In the hybrid structure, the graphene with two contacts is supported by a metal plane with a dielectric spacer, and covered by metal patches. When the metal patches fuse with one contact only, plasmonic nanocavities are formed at this specific contact, giving rise to locally enhanced absorption of graphene and a prolonged junction borderline. At the other contact, the absorption of graphene is suppressed by the bottom metal plane. As a result, a contrast as high as 105 times between the photoresponses at the two contact-graphene junctions is achieved. Due to the superior capability to couple the incident light into a localized mode, the plasmonic nanocavity enhances the responsivity of graphene one order of magnitude more effectively than a subwavelength metal grating. Further characterizations reveal that the resonant behavior of the hybrid structure is well controllable, the photoresponse time is shorter than several microseconds, and the photoresponse mechanism is attributed to photothermoelectric effect of photoexcited hot carriers.

Terahertz single-pixel near-field imaging based on active tunable subwavelength metallic grating

Terahertz (THz) single-pixel near-field imaging, producing images by introducing a scene with a series of spatially resolved subwavelength patterns while recording the correlated intensity on a single-element detector, is a promising technology for imaging applications. Spatial light modulator (SLM) is one of the key devices in THz single-pixel near-field imaging. Combined with a Digital Micromirror Device to shape the optical pump beam, a photo-induced tunable high-efficiency, ultra-thin, and fast THz SLM is presented by integrating designed subwavelength metallic grating into silicon on sapphire. In the experiments, the SLM is demonstrated with an over 60% THz peak amplitude modulation depth and broad bandwidth under a relative low pump fluence (80 μJ/cm2). The measurements, simulations, and calculations agree well with each other. Meanwhile, sub-nanosecond decay time extracted by fitting the |ΔT| curve suggests a fast response device. A two-dimensional grating is also explored, and the polarization-independent feature makes it easier to use. Finally, an imaging demonstration is conducted to verify the usability of the designed device. These results demonstrate the feasibility of realizing super-resolution and even real-time imaging simultaneously.

Enabling selective absorption in perovskite solar cells for refractometric sensing of gases

Perovskite solar cells are currently considered a promising technology for solar energy harvesting. Their capability to deliver an electrical signal when illuminated can sense changes in environmental parameters. We have numerically analyzed the variation of the current delivered by a perovskite cell as a function of the index of refraction of air, that is in contact with the front surface of the cell. This calculation identifies which geometrical and material structures enhance this behavior. After replacing the top transparent electrode of a solar cell by an optimized subwavelength metallic grating, we find a large variation in the responsivity of the cell with respect to the change in the index of refraction of the surrounding medium. Such a refractometric sensor can be interrogated electronically, avoiding the cumbersome set-ups of spectral or angular interrogation methods. We present an adaptation of the performance parameters of refractometric sensors (sensitivity and figure of merit) to the case of opto-electronic interrogation methods. The values of sensitivity and Figure of Merit are promising for the development of refractometric perovskite-based sensors.

Resonance absorption of visible light by subwavelength gratings

Diffraction of light of a visible spectral range by subwavelength metal gratings is investigated theoretically and experimentally. It is demonstrated that under certain conditions an effect of plasmon resonance occurs, at which a complete absorption of the incident radiation takes place. It is shown that the influence of the incident beam width on the diffraction efficiency is significant for the incident angles, at which the plasmon resonance occurs.