Switchable and tunable terahertz metamaterial absorber with broadband and multi-band absorption.

In this paper, we propose and demonstrate a switchable terahertz metamaterial absorber with broadband and multi-band absorption based on a simple configuration of graphene and vanadium dioxide (VO2). The switchable functional characteristics of the absorber can be achieved by changing the phase transition property of VO2. When VO2 is insulating, the device acts as a broadband absorber with absorbance greater than 90% under normal incidence from 1.06 THz to 2.58 THz. The broadband absorber exhibits excellent absorption performance under a wide range of incident and polarization angles for TE and TM polarizations. Moreover, the absorption bandwidth and intensity of the absorber can be dynamically adjusted by changing the Fermi energy level of graphene. When VO2 is in the conducting state, the designed metamaterial device acts as a multi-band absorber with absorption frequencies at 1 THz, 2.45 THz, and 2.82 THz. The multi-band absorption is achieved owing to the fundamental resonant modes of the graphene ring sheet, VO2 hollow ring patch, and coupling interaction between them. Moreover, the multi-band absorber is insensitive to polarization and incident angles for TE and TM polarizations, and the three resonance frequencies can be reconfigured by changing the Fermi energy level of graphene. Our designed device exhibits the merits of bi-functionality and a simple configuration, which is very attractive for potential terahertz applications such as intelligent attenuators, reflectors, and spatial modulators.

Switchable broadband terahertz spatial modulators based on patterned graphene and vanadium dioxide.

We numerically demonstrate a switchable broadband terahertz spatial modulator composed of ginkgo-leaf-patterned graphene and transition material vanadium dioxide (VO2). The phase transition property of VO2 is used to switch the spatial modulator from absorption mode to transmission mode, and the graphene behaves as dynamically adjustable material for a large scale of absorption and transmittance modulation. When VO2 is in the metallic state and the Fermi energy of graphene is set as 0.8 eV, the proposed modulator behaves as a broadband absorber with the absorbance over 85% from 1.33 to 2.83 THz. By adjusting the graphene Fermi level from 0 to 0.8 eV, the peak absorbance can be continuously tuned from 24.3% to near 100% under the absorption mode, and the transmittance at 2.5 THz can be continuously tuned from 87% to 35.5% under the transmission mode. To further increase the bandwidth, a three-layer-patterned-graphene is introduced into a new modulator design, which achieves a wide bandwidth of 3.13 THz for the absorbance over 85%. By the combination of the tunability of graphene and VO2, the proposed modulators not only can flexibly switch between dual-functional modulation modes of absorption and transmission but also possess deep modulation depth. Benefitting from the excellent modulation performance, the proposed switchable dual-functional spatial modulators may offer significant potential applications in various terahertz smart optoelectronic devices.

Enhancing third-harmonic generation by mirror-induced electric quadrupole resonance in a metal-dielectric nanostructure.

Electric quadrupole resonance (EQR), a commonly available high-order Mie-type resonance in all-dielectric nanoparticles, suffers from weak field enhancement and thus inferior third-harmonic generation (THG). In this work, according to the intrinsic centrosymmetry of current distribution, mirror-induced EQR in a silicon disk is effectively generated by introducing a bottom metal film with the perfect electric conductor (PEC) mirror effect, manifesting preeminent capabilities of tailoring far-field scattering and enhancing near-field intensity. The beneficial THG by mirror-induced EQR is enhanced by more than 50-fold as compared to that of the typical EQR without the PEC mirror effect. Furthermore, the influence of the silicon Kerr effect on THG is investigated under increasing pump intensity, achieving maximal efficiency of 2.2×10-4 under pump intensity I0=3GW/cm2. This work opens possibilities of exploring new mirror-induced Mie-type resonances in hybrid nanostructures, finding important applications in frequency conversion, spectroscopy, and sensing at the nanoscale.

Tunable mid-infrared dual-band and broadband cross-polarization converters based on U-shaped graphene metamaterials.

We propose a tunable dual-band reflective cross-polarization converter composed of periodically arranged single layer U-shaped graphene nanostructures in mid-infrared region. The proposed dual-band reflective cross-polarization converter can convert the polarization state of an incident wave from the linear polarization state to its cross polarization state at the operating frequencies of 34.67 and 44.13 THz with the high-efficiency polarization conversion ratio (PCR) approaching 100%. Furthermore, as a complementary structure, a reflective cross-polarization converter with a hollow-carved U-shaped graphene sheet shows a broadband polarization conversion performance with a bandwidth of 1 THz and the PCR over 90%. The bandwidth of this broadband converter can be further extended to 2 THz after certain geometric parameter optimization. More importantly, both the dual-band and broadband cross-polarization converters not only can dynamically tune their PCR peak frequencies and magnitudes by adjusting the chemical potential and relaxation time of graphene without changing the geometric structure but also have good angular stability with high PCR in a wide range of incident angle up to 55°. These polarization converters may have great potential applications in mid-infrared spectroscopy, radiometer, sensor, and other photonic devices.

High-efficient and low-coupling spoof surface plasmon polaritons enabled by V-shaped microstrips.

We propose a novel variety of V-shaped microstrips for highly efficient and strongly confined spoof surface plasmon polaritons (SSPPs) propagation. We analyze the dispersion characteristics of the V-shaped SSPPs microstrip units and find that the asymptotic frequency of the dispersion curve can be significantly reduced by adding the folded stub without increasing the lateral dimension of the structure. The V-shaped microstrip possesses the advantage of being compatible with a conventional microstrip without the need for complicated and bulky mode conversion structures in other typical grooved SSPP waveguides. Then, broadband transitions with a tapered microstrip and an array of graded height V-shaped units with good impedance matching and high mode conversion efficiency are designed. The simulated and measured results demonstrate that the proposed V-shaped microstrip has excellent broadband lowpass filter characteristics with the reflection coefficient (S11) less than -10 dB and the transmission coefficient (S21) higher than -3 dB in the frequency range from 0 to 10.3 GHz. Furthermore, the coupling characteristics of the parallel and symmetrically arranged V-shaped microstrips are investigated. Compared to conventional parallel microstrips with a separation of 2.8 mm, the proposed parallel V-shaped microstrips with 2 mm inner-overlapping have significantly lower coupling effects in the frequency ranging from 0 to 10 GHz. The low coupling, strong field confinement, and flexible dispersion manipulation of the proposed microstrip make it possible to achieve device miniaturization and noise interference suppression, which may have great potentials in the development of various highly integrated microwave plasmonic circuits, devices, and systems.

Graphene-based hybrid plasmonic waveguide for highly efficient broadband mid-infrared propagation and modulation.

In this paper, a graphene-based hybrid plasmonic waveguide is proposed for highly efficient broadband surface plasmon polariton (SPP) propagation and modulation at mid-infrared (mid-IR) spectrum. The hybrid plasmonic waveguide is composed of a monolayer graphene sheet in the center, a polysilicon gating layer, and two inner dielectric buffer layers and two outer parabolic-ridged silicon substrates symmetrically placed on both sides of the graphene. Owing to the unique parabolic-ridged waveguide structure, the light-graphene interaction and subwavelength SPPs confinement of the fundamental SPP mode for the hybrid waveguide can be significantly increased. Under the graphene chemical potential of 1.0 eV, the proposed waveguide can achieve outstanding SPP propagation performance with long propagation length of 12.1-16.7 µm and small normalized mode area of ~10-4 in the frequency range of 10-20 THz, exhibiting more than one order smaller in the normalized mode area while remaining the propagation length almost the same level with respect to the hybrid plasmonic waveguide without parabolic ridges. By tuning the graphene chemical potential from 0.1 to 1.0 eV, we demonstrate the waveguide has a modulation depth greater than 51% for the frequency ranging from 10 to 20 THz and reaches a maximum of nearly 100% at the frequency higher than 18 THz. Benefitting from the excellent broadband mid-IR propagation and modulation performance, the graphene-based hybrid plasmonic waveguide may open up a new way for various mid-IR waveguides, modulators, interconnects and optoelectronic devices.

Plasmonic waveguide with folded stubs for highly confined terahertz propagation and concentration.

We proposed a novel planar terahertz (THz) plasmonic waveguide with folded stub arrays to achieve excellent terahertz propagation performance with tight field confinement and compact size based on the concept of spoof surface plasmon polaritons (spoof SPPs). It is found that the waveguide propagation characteristics can be directly manipulated by increasing the length of the folded stubs without increasing its lateral dimension, which exhibits much lower asymptotic frequency of the dispersion relation and even tighter terahertz field confinement than conventional plasmonic waveguides with rectangular stub arrays. Based on this waveguiding scheme, a terahertz concentrator with gradual step-length folded stubs is proposed to achieve high terahertz field enhancement, and an enhancement factor greater than 20 is demonstrated. This work offers a new perspective on very confined terahertz propagation and concentration, which may have promising potential applications in various integrated terahertz plasmonic circuits and devices, terahertz sensing and terahertz nonlinear optics.

Broadband absorber with periodically sinusoidally-patterned graphene layer in terahertz range.

We demonstrate that a broadband terahertz absorber with near-unity absorption can be realized using a net-shaped periodically sinusoidally-patterned graphene sheet, placed on a dielectric spacer supported on a metallic reflecting plate. Because of the gradient width modulation of the unit graphene sheet, continuous plasmon resonances can be excited, and therefore broadband terahertz absorption can be achieved. The results show that the absorber's normalized bandwidth of 90% terahertz absorbance is over 65% under normal incidence for both TE and TM polarizations when the graphene chemical potential is set as 0.7 eV. And the broadband absorption is insensitive to the incident angles and the polarizations. The peak absorbance remains more than 70% over a wide range of the incident angles up to 60° for both polarizations. Furthermore, this absorber also has the advantage of flexible tunability via electrostatic doping of graphene sheet, which peak absorbance can be continuously tuned from 14% to 100% by controlling the chemical potential from 0 eV to 0.8 eV. The design scheme is scalable to develop various graphene-based tunable broadband absorbers at other terahertz, infrared, and visible frequencies, which may have promising applications in sensing, detecting, and optoelectronic devices.

Independent tuning of double plasmonic waves in a free-standing graphene-spacer-grating-spacer-graphene hybrid slab.

The independent excitation and tuning of double plasmonic waves are realized in a free-standing graphene-spacer-grating-spacer-graphene (GSGSG) hybrid slab, which consists of two graphene field effect transistors placed back-to-back to each other. Resulted from the high transparency and the tight confinement of surface plasmonic mode for the graphene, double plasmonic waves can be independently excited by guided-mode resonances (GMRs). Theoretical and numerical investigations are performed in the mid-infrared band. Furthermore, the tuning of individual GMR resonant wavelengths with respect to the system parameters is studied. The results provide opportunities to engineer the proposed hybrid slab for wavelength selective and multiplexing applications.

Enhanced spatial near-infrared modulation of graphene-loaded perfect absorbers using plasmonic nanoslits.

Modulating spatial near-infrared light for ultra-compact electro-optic devices is a critical issue in optical communication and imaging applications. To date, spatial near-infrared modulators based on graphene have been reported, but they showed limited modulation effects due to the relatively weak light-graphene interaction. In combination with graphene and metallic nanoslits, we design a kind of ultrathin near-infrared perfect absorber with enhanced spatial modulation effects and independence on a wide range of incident angles. The modulated spectral shift of central wavelength is up to 258.2 nm in the near-infrared range, which is more promising in applications than state-of-the-art devices. The modulation enhancement is attributed to the plasmonic nanoslit mode, in which the optical electric field is highly concentrated in the deep subwavelength scale and the light-graphene interaction is significantly strengthened. The physical insight is deeply revealed by a combination of equivalent circuit and electromagnetic field analysis. The design principles are not only crucial for spatial near-infrared modulators, but also provide a key guide for developing active near-infrared patch nanoantennas based on graphene.

Dielectric Tetramer Nanoresonators Supporting Strong Superchiral Fields for Vibrational Circular Dichroism Spectroscopy.

Chirality (C) is a fundamental property of objects, in terms of symmetry. It is extremely important to sense and distinguish chiral molecules in the fields of biochemistry, science, and medicine. Vibrational circular dichroism (VCD) spectroscopy, obtained from the differential absorption of left- and right- circularly polarized light (CPL) in the infrared range, is a promising technique for enantiomeric detection and separation. However, VCD signals are typically very weak for most small molecules. Dielectric metasurfaces are an emerging platform to enhance the sensitivity of VCD spectroscopy of chiral molecules via superchiral field manipulation. Here, we demonstrate a dielectric metasurface consisting of achiral germanium (Ge) tetramer nanoresonators that provide a proper and accessible high C enhancement (CE). We realize a maximum C enhancement (CE_max) with respect to the incident CPL (CE_max = Cmax/CRCP) of more than 750. The volume-averaged C enhancement (CE_ave = Cave/CRCP) is 148 in the 50 nm thick region above the sample surface and 215 in the central region of the structure. Especially, the corresponding CE_ave values are more than 89 and 183 even when a 50 nm thick chiral lossy molecular layer is coated on the metasurface. The metasurface benefits from geometrically achiral nanostructure design to eliminate intrinsic background chiral-optical signal from the substrate, which is useful in chiral sensing, enantioselectivity, and VCD spectroscopy applications in the mid-infrared range.

Highly Stable and Efficient Formamidinium-Based 2D Ruddlesden-Popper Perovskite Solar Cells via Lattice Manipulation.

Formamidinium (FA)-based 2D perovskites have emerged as highly promising candidates in solar cells. However, the insertion of 2D spacer cations into the perovskite lattice concomitantly introduces microstrain and unfavorable orientations that hinder efficiency and stability. In this study, by finely tuning the FA-based 2D perovskite lattice through spacer cation engineering, a stable lattice structure with balanced distortion, microstrain relaxation, and reduced carrier-lattice interactions is achieved. These advancements effectively stabilize the inherently soft lattice against light and thermal-aging stress. To reduce the photocurrent loss induced by undesired crystal texture, a polarity-matched molecular-type selenourea (SENA) additive is further employed to modulate the crystallization kinetics. The introduction of the SENA significantly inhibits the disordered crystallization induced by spacer cations and drives the templated growth of the quantum well structure with a vertical orientation. This controlled crystallization process effectively reduces crystal defects and enhances charge separation. Ultimately, the optimized FA-based perovskite photovoltaic devices achieve a remarkable power conversion efficiency (PCE) of 20.03% (certified steady-state efficiency of 19.30%), setting a new record for low-n 2D perovskite solar cells. Furthermore, the devices exhibit less than 1% efficiency degradation after operating at maximum power point for 1000 h and maintain excellent stability after thermal aging and cycles of cold-warm shock, respectively.

A terahertz broadband 3 dB directional coupler based on bridged PPDW.

In this paper, a novel broadband 3 dB directional coupler with very flat coupling based on bridged parallel plate dielectric waveguide (PPDW) is proposed and demonstrated. In the uniform coupling section, a bridge structure between the two PPDWs is employed to obtain accurate coupling value and achieve a broadband coupling. It is found that this new type of coupling structure exhibits excellent performance at terahertz frequencies. In order to achieve strong isolation between the adjacent ports and reduce the power reflection in all ports, two quarter-circle bend arms are introduced as the curved transition sections to connect the uniform coupling section. For this bridged coupler, it only needs the value of the uniform coupling length as short as 400 µm to achieve a broadband 3 dB coupling. In this case, the coupler's average return loss is greater than 28 dB, average isolation is better than 27 dB and average coupler loss is only 0.9 dB, over a percentage bandwidth of 12.5% at 1 THz. Compared to the conventional PPDW coupler, the bridged PPDW coupler shows significantly greater bandwidth (about 4.2 times), compact and mechanically stable with a much shorter uniform coupling length (reduced about 61%), which may have potential applications for terahertz integrated circuits and systems.

Wideband and high-order microwave vortex-beam launcher based on spoof surface plasmon polaritons.

The electromagnetic vortex carrying orbital angular momentum (OAM), which is first studied at optical frequency, has begun to attract widespread attention in the field of radio-frequency/microwave. However, for the OAM mode generated by traditional single antennas, there are problems such as low order and narrow bandwidth, and complex structures such as dual-fed networks may be required. In this paper, based on spoof surface plasmon polariton (SSPP) mode leaky-wave antenna, a single-port traveling-wave ring is proposed to radiate high-order OAM modes working near the cut-off frequency of SSPP state. The achieved 12-order OAM mode within 9.1-10.1 GHz (relative bandwidth of 10.4%) has the main radiation direction close to the antenna surface, forming a plane spiral OAM (PSOAM) wave, which reduces the requirements for mode purity in practical applications. This SSPP ring using periodic units as radiating elements can be an effective radiator for broadband and large-capacity OAM multiplexing communications. The structural characteristics of single feed contribute to the integration of microwave circuits.

A novel broadband coaxial probe to parallel plate dielectric waveguide transition at THz frequency.

In this paper, transmission characteristics of the fundamental mode (TE(10(12))) of Parallel-Plate Dielectric Waveguide (PPDW) at 0.4-1.0 THz (1 THz = 10(12) Hz) are studied. The investigation results show PPDW with virtually low attenuation and remarkable simple structure is a promising candidate as THz transmission medium. Then, a novel broadband coaxial probe to PPDW transition is designed. Although coaxial probe excitation has been used in microstrip lines and rectangular waveguides in microwave, millimeter-wave frequency domain, the present study shows that it is also an effective method to excite the PPDW at THz frequency. As the investigation results show, the return loss of coax-PPDW transition is better than 20 dB from 0.45 THz to 0.75 THz, and the insertion loss is as low as 0.18 dB, which will have wide potential application in the terahertz regime.

Broadband optical waveguide modulators based on strongly coupled hybrid graphene and metal nanoribbons for near-infrared applications.

In this paper, we numerically demonstrate a variety of broadband optical waveguide modulators based on the hybrid surface plasmon polariton (HSPP) concept for near-infrared applications. The modulator is composed of strongly coupled double-layer graphene and double rectangle cross-sectional metal nanoribbons separated by three Al2O3 spacers, which are interpolated in a SiO2 waveguide. Owing to the unique strong coupling of HSPPs between metal nanoribbons, the subwavelength confinement, the in-plane electric field component, the light-graphene interaction, and the modulation effect of the modulator are significantly enhanced. The results show the proposed modulator achieves an outstanding performance with a modulation depth (MD) over 2.3 dB µm-1 and a small normalized mode area of ∼10-5 in a wide range of wavelength from 1.3 to 1.8 µm. By optimizing the separation of the double rectangle metal nanoribbons at the telecommunication wavelength of 1.55 µm, the modulator exhibits a high MD of 3.12 dB µm-1, a small footprint of 1.8 µm2, an ultra-wide 3 dB modulation bandwidth of 380.23 GHz, and an ultra-low energy consumption of 29.39 fJ per bit. Furthermore, we also demonstrate a modulator based on two properly apart semicircular (rhombus) metal nanoribbons with a drastically enhanced MD of 11.3 (6.32) dB µm-1 at 1.55 µm. Benefitting from the strong subwavelength confinement and excellent broadband modulation performance, the proposed optical waveguide modulators offer a significant potential to realize various long-wave near-infrared integrated modulators, interconnects and optoelectronic devices.

Electrically Tunable Broadband Terahertz Absorption with Hybrid-Patterned Graphene Metasurfaces.

We numerically demonstrate a broadband terahertz (THz) absorber that is based on a hybrid-patterned graphene metasurface with excellent properties of polarization insensitivity, wide-angle, and active tunability. Our design is made up of a single-layer graphene with periodically arranged hybrid square/disk/loop patterns on a multilayer structure. We find that broadband absorption with 90% terahertz absorbance and the fractional bandwidth of 84.5% from 1.38 THz to 3.4 THz can be achieved. Because of the axisymmetric configuration, the absorber demonstrates absolute polarization independence for both transverse electric (TE) and transverse magnetic (TM) polarized terahertz waves under normal incidence. We also show that a bandwidth of 60% absorbance still remains 2.7 THz, ranging from 1.3 THz to 4 THz, for a wide incident angle ranging from 0° to 60°. Finally, we find that by changing the graphene Fermi energy from 0.7 eV to 0 eV, the absorbance of the absorbers can be easily tuned from more than 90% to lower than 20%. The proposed absorber may have promising applications in terahertz sensing, detecting, imaging, and cloaking.

Frequency-Reconfigurable Wide-Angle Terahertz Absorbers Using Single- and Double-Layer Decussate Graphene Ribbon Arrays.

We propose and numerically demonstrate two novel terahertz absorbers made up of periodic single- and double-layer decussate graphene ribbon arrays. The simulated results show that the proposed absorbers have narrowband near-unity terahertz absorption with ultra-wide frequency reconfiguration and angular stability. By tuning the Fermi level of graphene ribbons, the over 90% absorbance peak frequency of the absorber with single-layer graphene structure can be flexibly adjusted from 6.85 to 9.85 THz for both the transverse magnetic (TM) and transverse electric (TE) polarizations. This absorber with single-layer graphene demonstrates excellent angular stability with the absorbance peaks of the reconfigurable absorption bands remaining over 99.8% in a wide angle of incidence ranging from 0 to 70°. The tuning frequency can be significantly enhanced by using the absorber with double-layer graphene structure from 5.50 to 11.28 THz and 5.62 to 10.65 THz, approaching two octaves under TM and TE polarizations, respectively. The absorbance peaks of the reconfigurable absorption band of this absorber for both polarizations maintain over 70%, even at a large angle of incidence up to 70°. Furthermore, an analytical fitting model is also proposed to accurately predict the absorbance peak frequencies for this variety of absorbers. Benefitting from these attractive properties, the proposed absorber may have great potential applications in tunable terahertz trapping, detecting, sensing, and various terahertz optoelectronic devices.

Strongly Confined Spoof Surface Plasmon Polaritons Waveguiding Enabled by Planar Staggered Plasmonic Waveguides.

We demonstrate a novel route to achieving highly efficient and strongly confined spoof surface plasmon polaritons (SPPs) waveguides at subwavelength scale enabled by planar staggered plasmonic waveguides (PSPWs). The structure of these new waveguides consists of an ultrathin metallic strip with periodic subwavelength staggered double groove arrays supported by a flexible dielectric substrate, leading to unique staggered EM coupling and waveguiding phenomenon. The spoof SPP propagation properties, including dispersion relations and near field distributions, are numerically investigated. Furthermore, broadband coplanar waveguide (CPW) to planar staggered plasmonic waveguide (PSPW) transitions are designed to achieve smooth momentum matching and highly efficient spoof SPP mode conversion. By applying these transitions, a CPW-PSPW-CPW structure is designed, fabricated and measured to verify the PSPW's propagation performance at microwave frequencies. The investigation results show the proposed PSPWs have excellent performance of deep subwavelength spoof SPPs confinement, long propagation length and low bend loss, as well as great design flexibility to engineer the propagation properties by adjusting their geometry dimensions and material parameters. Our work opens up a new avenue for development of various advanced planar integrated plasmonic devices and circuits in microwave and terahertz regimes.