Investigation of acoustic plasmons in vertically stacked metal/dielectric/graphene heterostructures for multiband coherent perfect absorption

Tunable mid-infrared photodetector based on graphene plasmons controlled by ferroelectric polarization for micro-spectrometer

Surface plasmonic detectors have the potential to be key components of miniaturized chip-scale spectrometers. Graphene plasmons, which are highly confined and gate-tunable, are suitable forin situlight detection. However, the tuning of graphene plasmonic photodetectors typically relies on the complex and high operating voltage based on traditional dielectric gating technique, which hinders the goal of miniaturized and low-power consumption spectrometers. In this work, we report a tunable mid-infrared (MIR) photodetector by integrating of patterned graphene with non-volatile ferroelectric polarization. The polarized ferroelectric thin film provides an ultra-high surface electric field, allowing the Fermi energy of the graphene to be manipulated to the desired level, thereby exciting the surface plasmon polaritons effect, which is highly dependent on the free carrier density of the material. By exciting intrinsic graphene plasmons, the light transmittance of graphene is greatly enhanced, which improves the photoelectric conversion efficiency of the device. Additionally, the electric field on the surface of graphene enhanced by the graphene plasmons accelerates the carrier transfer efficiency. Therefore, the responsivity of the device is greatly improved. Our simulations show that the detectors have a tunable resonant spectral response of 9-14μm by reconstructing the ferroelectric domain and exhibit a high responsivity to 5.67 × 105A W-1at room temperature. Furthermore, we also demonstrate the conceptual design of photodetector could be used for MIR micro-spectrometer application.

Transparent grating-based metamaterials for dynamic infrared radiative regulation smart windows

Dynamic infrared radiation regulation has been widely explored for smart windows because of its vital importance for comfortable and energy-efficient buildings. However, it remains a great challenge to synchronously achieve high visible transmittance and pronounced infrared tunability. Here, we propose a dynamic infrared tunable metamaterial composed of indium tin oxide (ITO) gratings, an air insulator, and an ITO reflector. The ITO grating-based infrared radiation regulator exhibits a high emissivity tunability of 0.73 at 8-13 μm while maintaining a high visible transmittance of 0.65 and 0.72 before and after actuation, respectively. By adjusting the geometric parameters, the tunable bandwidth can be further extended to 3-30 μm and the ultra-broadband tunability reaches 0.62. The excellent infrared tunable performance arises from the insulator thickness-dependent effect of Fabry-Pérot and propagating surface plasmon resonance coupling and decoupling, which lead to perfect and low absorption, respectively. This work provides potential for the advancement of smart window technology and makes a significant contribution to sustainable buildings.

Coherent perfect absorption from asymmetry transmissive Helmholtz resonator metamaterials

We propose an asymmetry transmissive (AT) Helmholtz resonator metamaterial to interact with dual incidences of different polarizations from opposite directions to achieve coherent perfect absorption at a terahertz regime. More specifically, the proposed design will solely allow the x-polarized incidence tunnel into the Helmholtz resonator cavity array with roughly half the energy reflected. Meanwhile, the transmitted fields will be converted into the y-polarized counterparts so as to go out through the metamaterial and finally coherent canceled with the partially reflected fields of the other incidence. Our design, introducing the Helmholtz resonator array in the electromagnetics with an extension of the principle of polarization conversion under the coherent perfect absorption, should pave the way for the quest of building up more advanced wave trapping meta-devices for various applications in different disciplines.

Miniaturized infrared spectrometer based on the tunable graphene plasmonic filter

Miniaturization of a conventional spectrometer is challenging because of the tradeoffs of size, cost, signal-to-noise ratio, and spectral resolution, etc. Here, a new type of miniaturized infrared spectrometer based on the integration of tunable graphene plasmonic filters and infrared detectors is proposed. The transmittance spectrum of a graphene plasmonic filter can be tuned by varying the Fermi energy of the graphene, allowing light incident on the graphene plasmonic filter to be dynamically modulated in a way that encodes its spectral information in the receiving infrared detector. The incident spectrum can then be reconstructed by using decoding algorithms such as ridge regression and neural networks. The factors that influence spectrometer performance are investigated in detail. It is found that the graphene carrier mobility and the signal-to-noise ratio are two key parameters in determining the resolution and precision of the spectrum reconstruction. The mechanism behind our observations can be well understood in the framework of the Wiener deconvolution theory. Moreover, a hybrid decoding (or recovery) algorithm that combines ridge regression and a neural network is proposed that demonstrates a better spectral recovery performance than either the ridge regression or a deep neural network alone, being able to achieve a sub-hundred nanometer spectral resolution across the 8∼14 µm wavelength range. The size of the proposed spectrometer is comparable to a microchip and has the potential to be integrated within portable devices for infrared spectral imaging applications.

Broadband coherent perfect absorption employing an inverse-designed metasurface via genetic algorithm

Coherent perfect absorption (CPA) possesses the unique characteristics of flexibly and actively molding the flow of light. However, restricted by the low design efficiency and limited geometry variety of metamaterial structures, the common CPA metamaterial absorbers based on artificial design show poor performance in bandwidth operation. Here, we proposed a tungsten-based metamaterial absorber to achieve broadband CPA via employing genetic algorithm inverse design. Under the irradiation of two coherent beams, the high coherent absorption (>90%) can be achieved within a wide range from 1.32 to 3.28 µm. By simply adjusting the relative intensity or phase difference of the two coherent beams, the absorption intensity can be continuously modulated to realize the transition between coherent perfect absorption and coherent perfect transparency. Moreover, the coherent absorption maintains greater than 90% over a broad range of incident angles for both TM and TE polarizations. The scattering matrix theorem is applied to explain the physical mechanism of CPA, and the analytical results exhibit good consistency with the numerical calculations. Such a tungsten-based CPA metamaterial absorber with broadband tunability and exceptional angular stability is expected to be utilized in optical signal processing chips, all-optical modulators, and optical switchers.

Ultra-broadband and completely modulated absorption enhancement of monolayer graphene in a near-infrared region

Achieving ultra-broadband and completely modulated absorption enhancement of monolayer graphene in near-infrared region is practically important to design graphene-based optoelectronic devices, however, which remains a challenge. In this work, by spectrally designing multiple magnetic plasmon resonance modes in metamaterials to be adjacent to each other, near-infrared light absorption in monolayer graphene is greatly improved to have an averaged absorption efficiency exceeding 50% in a very broad absorption bandwidth of about 800 nm. Moreover, by exerting an external bias voltage on graphene to change Fermi energy of graphene, the ultra-broadband absorption enhancement of monolayer graphene exhibits an excellent tunability, which has a nearly 100% modulation depth and an electrical switching property. This work is promising for applications in near-infrared photodetectors, amplitude modulators of electromagnetic waves, etc.

Omnidirectional, thin metasurface exhibiting selective absorption for un-polarized broadband incidence

Thin devices with large areas have strong and omnidirectional absorption over a wide bandwidth and are in demand for applications such as energy harvesting, structural color, and vehicle LiDAR (laser detection and ranging). Despite persistent efforts in the design and fabrication of such devices, the simultaneous realization of all these desired properties remains a challenge. In this study, a 190-nm-thick metasurface with an area of 3 cm2, incorporating dielectric cylinder arrays, a chromium layer, a silicon nitride (SiNx) layer, and an aluminum layer is theoretically and experimentally demonstrated. The developed device achieves an average absorptivity of ∼99% (97% in the experiment) in the entire visible spectrum 400-700 nm. Moreover, it exhibits strong absorption over a wide range of incident angles (∼91% and 90% at 60° in the calculation and experiment, respectively). Importantly, the feasibility of applying the developed metasurface absorber to solar thermophotovoltaics and vehicle LiDAR (laser detection and ranging) has been explored. Moreover, the photoresist can be replaced by other glues and easily scaled up to a large area using the roll-to-roll nanoimprinting process. With the excellent spectral properties and performance, this device is promising for large-area applications.

Resolving the Mechanism of Acoustic Plasmon Instability in Graphene Doped by Alkali Metals

Graphene doped by alkali atoms (ACx) supports two heavily populated bands (π and σ) crossing the Fermi level, which enables the formation of two intense two-dimensional plasmons: the Dirac plasmon (DP) and the acoustic plasmon (AP). Although the mechanism of the formation of these plasmons in electrostatically biased graphene or at noble metal surfaces is well known, the mechanism of their formation in alkali-doped graphenes is still not completely understood. We shall demonstrate that two isoelectronic systems, KC8 and CsC8, support substantially different plasmonic spectra: the KC8 supports a sharp DP and a well-defined AP, while the CsC8 supports a broad DP and does not support an AP at all. We shall demonstrate that the AP in an ACx is not, as previously believed, just a consequence of the interplay of the π and σ intraband transitions, but a very subtle interplay between these transitions and the background screening, caused by the out-of-plane interband C(π)→A(σ) transitions.

Coherent perfect absorption in Tavis-Cummings models

We theoretically study the conditions under which two laser fields can undergo Coherent Perfect Absorption (CPA) when shined on a single-mode bi-directional optical cavity coupled with two two-level quantum emitters (natural atoms, artificial atoms, quantum dots, qubits, etc.). In addition to being indirectly coupled through the cavity-mediated field, in our Tavis-Cummings model, the two quantum emitters (QEs) are allowed to interact directly via the dipole-dipole interaction (DDI). Under the mean-field approximation and low-excitation assumption, in this work, we particularly focus on the impact of DDI on the existence of CPA in the presence of decoherence mechanisms (spontaneous emission from the QEs and the leakage of photons from the cavity walls). We also present a dressed-state analysis of the problem to discuss the underlying physics related to the allowed polariton state transitions in the Jaynes-Tavis-Cummings ladder. As a key result, we find that in the strong-coupling regime of cavity quantum electrodynamics, the strong DDI and the emitter-cavity detuning can act together to achieve the CPA at two laser frequencies tunable by the inter-atomic separation which are not possible to attain with a single QE in the presence of detuning. Our CPA results are potentially applicable in building quantum memories that are an essential component in long-distance quantum networking.

Extremely broadband light absorption by bismuth-based metamaterials involving hybrid resonances

An ultra-broadband polarization-insensitive perfect absorber for the 400-4000 nm spectral range is proposed and studied. The absorber is composed of a dielectric film and a phase change material film sandwiched...

Acoustic graphene plasmon resonator based on gold nanowire arrays

An acoustic graphene plasmon (AGP) resonator based on gold nanowire arrays structure is proposed in this paper. The resonator is designed by continuous graphene layer/gold nanowire arrays/optical resonant cavity. The infrared light excites the AGP in the graphene/gold arrays structure and propagates along the graphene surface. The coupling efficiency can be improved by the optical resonator cavity. The finite-difference time-domain method is used to simulate and optimize the property of the resonator. The results show that the resonator has a stronger optical limiting effect and higher coupling efficiency. The AGPs are a prospective platform that enhances light-matter interactions, reduces spread loss, and exhibits a double resonance absorption phenomenon in the studied mid-infrared wavelength range. The research results provide a basis for the design of optoelectronic devices and more.

Single-layer graphene optical modulator based on arrayed hybrid plasmonic nanowires

Surface plasmon-polaritons (SPPs)-based waveguides, especially hybrid plasmonic nanowires, which have attracted extensive interests due to easy fabrication, high transmittance, subwavelength mode confinement and long propagation distance, are appropriate platforms for enhancing the interaction with graphene. Considering that graphene is a two-dimensional (2D) material with surface conductivity, it is important to enhance the in-plane electrical components parallel to graphene. Here, we propose a tunable graphene optical modulator based on arrayed hybrid plasmonic nanowires, utilizing strong subwavelength confinement of gap-surface plasmonic modes (GSPMs) and near-field coupling in the periodic metasurface structure to enhance effective light-matter interactions. The modulator has a typical modulation depth (MD) of 4.7 dB/μm, insertion loss (IL) of 0.045 dB/μm, and a broadband response. The modulation performance can be further optimized, achieving MD of 16.7 dB/μm and IL of 0.17 dB/μm. Moreover, with the optimized modulator, the 3 dB bandwidth can reach 200 GHz. The energy consumption of modulator is about 0.86 fJ/bit. Our design exhibits fascinating modulation performance, fabrication compatibility and integration potential. It may inspire the schematic designs of graphene-based plasmonic modulator and pave a way to the application of 2D materials-involved optoelectronic devices.

Dynamically tunable coherent perfect absorption in topological insulators at oblique incidence

The effective engineering of light absorption has been the focus of intensive research to realize the novel optoelectronic devices based on a topological insulator, a unique topologically protected surface Dirac-state quantum material with excellent prospects in electronics and photonics. Here, we theoretically proposed a versatile platform for manipulating the light-matter interaction employing the dynamically tunable coherent perfect absorption (CPA) in the topological insulator Bi_1.5Sb_0.5Te_1.8Se_1.2(BSTS). By simply varying the phase difference between two coherent counter-propagating beams, the BSTS-based CPA device can be continuously switched from the high transparency state to the strong absorption state, leading to the modulation of absorption ranging from 0.2% to 99.998%. Under the illumination of TE-polarized wave, the high absorption (>90%) can be implemented within a broad range from 0.47 to 1.51 μm through a proper incident angle alteration. In addition, the quasi-CPA wavelength can be flexibly selected by tuning the bulk thickness of BSTS film while maintaining high modulation depth of 10⁴. Such BSTS-based CPA device with flexible tunability, wide absorption modulation range, and high modulation depth is expected to be utilized in a wide range of potential applications such as in next-generation coherent detectors, coherent modulators, all-optical switches, and signal processors.