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.

Bloch surface waves assisted active modulation of graphene electro-absorption in a wide near-infrared region.

Light modulation has been recognized as one of the most fundamental operations in photonics. In this paper, we theoretically designed a Bloch surface wave assisted modulator for the active modulation of graphene electro-absorption. Simulations show that the strong localized electrical field generated by Bloch surface waves can significantly enhance the graphene electro-absorption up to 99.64%. Then by gate-tuning the graphene Fermi energy to transform graphene between a lossy and a lossless material, electrically switched absorption of graphene with maximum modulation depth of 97.91% can be achieved. Meanwhile, by further adjusting the incident angle to tune the resonant wavelength of Bloch surface waves, the center wavelength of the modulator can be actively controlled. This allows us to realize the active modulation of graphene electro-absorption within a wide near-infrared region, including the commercially important telecommunication wavelength of 1550 nm, indicating the excellent performance of the designed modulator via such mechanism. Such Bloch surface waves assisted wavelength-tunable graphene electro-absorption modulation strategy opens up a new avenue to design graphene-based selective multichannel modulators, which is unavailable in previous reported strategies that can be only realized by passively changing the structural parameters.

Active tuning of longitudinal strong coupling between anisotropic borophene plasmons and Bloch surface waves.

Strong coupling between the resonant modes can give rise to many resonant states, enabling the manipulation of light-matter interactions with more flexibility. Here, we theoretically propose a coupled resonant system where an anisotropic borophene localized plasmonic (BLP) and Bloch surface wave (BSW) can be simultaneously excited. This allows us to manipulate the spectral response of the strong BLP-BSW coupling with exceptional flexibility in the near infrared region. Specifically, the strong longitudinal BLP-BSW coupling occurs when the system is driven into the strong coupling regime, which produces two hybrid modes with a large Rabi splitting up to 124 meV for borophene along both x- and y-directions. A coupled oscillator model is employed to quantitatively describe the observed BSW-BLP coupling by calculating the dispersion of the hybrid modes, which shows excellent agreement with the simulation results. Furthermore, benefited from the angle-dependent BSW mode, the BSW-BLP coupling can be flexibly tuned by actively adjusting the incident angle. Such active tunable BLP-SBW coupling with extreme flexibility offered by this simple layered system makes it promising for the development of diverse borophene-based active photonic and optoelectronic devices in the near infrared region.

Controllable hybridization between localized and delocalized anisotropic borophene plasmons in the near-infrared region.

In this Letter, we theoretically propose a coupled borophene plasmonic system, where an anisotropic localized plasmonic (LP) mode and a delocalized guided plasmonic (DGP) mode can be simultaneously excited. This allows us to manipulate the optical response of the strong LP-DGP coupling with exceptional flexibility in the near-infrared region, which is not possible with the conventional metallic plasmonic structures, and overcomes some shortcomings of coupled structures based on the other 2D materials. Specifically, the spatially LP-DGP coupling can arise when the system is driven into the strong coupling regime; this gives rise to a transparency window which can be well described by a coupled oscillation model. The bandwidth of the window is governed by the coupling strength which can be passively adjusted by the spacer thickness, while the center wavelength and the number of windows can be actively modulated by tuning the borophene electron density and the incident angle.

Electrically tunable graphene metamaterial with strong broadband absorption.

The coupling system with dynamic manipulation characteristics is of great importance for the field of active plasmonics and tunable metamaterials. However, the traditional metal-based architectures suffer from a lack of electrical tunability. In this study, a metamaterial composed of perpendicular or parallel graphene-Al2O3-graphene stacks is proposed and demonstrated, which allows for the electric modulation of both graphene layers simultaneously. The resultant absorption of hybridized modes can be modulated to more than 50% by applying an external voltage, and the absorption bandwidth can reach 3.55 µm, which is 1.7 times enhanced than the counterpart of single-layer graphene. The modeling results demonstrate that the small relaxation time of graphene is of great importance to realize the broadband absorption. Moreover, the optical behaviors of the tunable metamaterial can be influenced by the incident polarization, the dielectric thickness, and especially by the Fermi energy of graphene. This work is of a crucial role in the design and fabrication of graphene-based broadband optical and optoelectronic devices.

Resolved Infrared Spectroscopy of Aqueous Molecules Employing Tunable Graphene Plasmons in an Otto Prism.

Real-time and in situ detection of aqueous solution is essential for bioanalysis and chemical reactions. However, it is extremely challenging for infrared microscopic measurement because of the large background of water absorption. Here, we proposed a wideband-tunable graphene plasmonic infrared biosensor to detect biomolecules in an aqueous environment, employing attenuated total reflection in an Otto prism configuration and tightly confined plasmons in graphene nanoribbons. Benefiting from the graphene plasmonic electric field enhancement, such a biosensor is able to identify the molecular chemical fingerprints without the interference of water absorption. As a proof of concept, the recombinant protein AG and goat anti-mouse immunoglobulin G (IgG) are used as the sensing analytes, of which the vibrational modes (1669 and 1532 cm-1) are very close to the OH-bending mode of water (1640 cm-1). Simulation results show that the fingerprints of protein molecules in the water environment can be selectively enhanced. Therefore, the water absorption is successfully suppressed so that two protein modes can be resolved by sweeping graphene Fermi energy in a wide waveband. By further optimizing the incident angle and graphene mobility to improve the mode energy of graphene plasmons, maximum enhancement factors of 112 and 130 can be achieved for amide I and II bands. Our work provides an effective approach for the highly sensitive and selective in situ identification of aqueous-phase molecular fingerprints in fields of healthcare, food safety, and biochemical sensing.

Highly stable all-in-one photoelectrochemical electrodes based on carbon nanowalls.

Photoelectrochemical (PEC) cells offer a promising approach for developing low-cost solar energy conversion systems. However, the lack of stable and cost-effective electrodes remains a bottleneck that hampers their practical applications. Here, we propose a kind of integrated all-in-one three-dimensional (3D) carbon nanowall (CNW) electrode without sensitized semiconductors for stable all-carbon PEC cells. The all-in-one CNW electrodes were fabricated by directly growing CNW on both sides of the SiO2/Si/SiO2 wafer employing the radio frequency plasmon enhanced chemical vapor deposition method. Benefitting from the interconnected 3D textured structure, the CNW can effectively absorb the incident light and provide a large electrochemical reaction interface at the CNW surface that promotes the separation of photogenerated charge carriers, which makes it a superior electrode material. Experimental results show that the all-in-one CNW electrodes possess excellent PEC performance with a photocurrent density of 830 µA cm-2. Moreover, the CNW electrodes exhibit excellent photoresponses over a wide waveband and superior stability with a maintained photocurrent response, even after 60 d, which outperforms the electrodes using the other two-dimensional layered materials or semiconductor sensitized electrodes. Such an all-in-one electrode with impressive photovoltaic properties provides a promising platform for PEC applications that is eco-friendly with high efficiency, excellent stability and low cost.

Single-layer graphene-coated gold chip for electrochemical surface plasmon resonance study.

Surface plasmon resonance (SPR) employs a gold (Au) thin film (ca. 50 nm in thickness) chip to generate a surface plasmonic wave (SPW) for in situ monitoring of the interface/surface, which makes it intrinsically compatible with electrochemistry for combined electrochemical surface plasmon resonance (EC-SPR) investigations. However, conventional SPR Au chips suffers from a high background signal, narrow electrochemical window, and limited electrochemical stability. Presented in this work is a novel SPR chip composed of the Au/long-chain alkane thiol self-assembled monolayer/single-layer graphene (Au/SAM/G) sandwich architecture to address these problems. On this chip, the single-layer graphene serves as a working electrode for electrochemical measurement, and the underlying Au film serves as the SPW support for SPR monitoring; the sandwiched thiol monolayer enables the electrical separation of the graphene and Au film to protect the Au film from electrochemical polarization. Our experiment indicates that the electrochemical window of such a chip extends beyond the hydrogen/oxygen evolution reaction potential on Au with significantly improved electrochemical stability and suppressed background signal. Moreover, its intrinsic SPR sensitivity is completely reserved even compared to that of the conventional SPR Au chip. This Au/SAM/G chip may offer a valuable solution to the EC-SPR investigations in harsh conditions. Graphical abstract ᅟ.

Graphene-assisted multilayer structure employing hybrid surface plasmon and magnetic plasmon for surface-enhanced vibrational spectroscopy.

A graphene-assisted vertical multilayer structure is proposed for high performance surface-enhanced Raman scattering (SERS) and surface-enhanced infrared absorption (SEIRA) spectroscopies on a single substrate, employing simultaneous localized surface plasmon in the visible region and magnetic plasmon resonance in the mid-infrared region. Such multilayer structure consists of a monolayer graphene sandwiched between Ag nanoparticles (NPs) and a metal-insulator-metal (MIM) microstructure, which can be easily fabricated by a standard surface micromachining process. Benefiting from the large near field enhancement by the hybrid plasmons in both visible and mid-infrared regions, a high enhancement factor of up to 107 for SERS and 105 for SEIRA can be achieved. Additionally, the strong magnetic resonance of the MIM microstructure can be tuned in broadband to selectively enhance the desired vibration modes of molecules. The strong SERS and SEIRA enhancement together with easy fabrication provides new opportunities for developing integrated plasmonic devices for multispectral detection of molecules on the same substrate.

Mode energy of graphene plasmons and its role in determining the local field magnitudes.

We theoretically study the mode energy of graphene plasmons and its fundamental role in determining the local field magnitudes. While neglecting the magnetic field energy of the mode, we derive a concise expression for the total mode energy, which is independent on the details of the mode field distributions and valid for both propagating and localized modes. We find that the mean square of the local electric fields of a graphene plasmonic mode scales linearly with the light absorption rate of the mode and the electron relaxation time of graphene. The possible strategies for improving the local field magnitudes of graphene plasmons are also discussed. Our theoretical analysis presented here may benefit the design of various graphene-based optical and optoelectronic devices for light-harvesting or energy conversion.

Mechanism of propagating graphene plasmons excitation for tunable infrared photonic devices.

The mechanism of propagating graphene plasmons excitation using a nano-grating and a Fabry-Pérot cavity as the optical coupling components is studied. It is demonstrated that the system could be well described within the temporal coupled mode theory using two phenomenological parameters, namely, the intrinsic loss rate and the coupling rate of a graphene plasmonic mode, and their analytical expressions are derived. It is found that the intrinsic loss rate is solely determined by the electron relaxation time of graphene, while independent of the field distributions of the modes. Such result originates from the negligible magnetic field energy of the graphene plasmonic mode. The coupling rate is governed by the optical coupling components parameters, and varies periodically with the Fabry-Pérot cavity length. By modulating the two rates, quality factors and absorption rates can be adjusted. Furthermore, it is revealed that low refractive index of the Fabry-Pérot cavity material is vital to the enlargement of tunable band, and the underlying physics is discussed. Such plasmon excitation configuration is insensitive to light incident angle and could serve as a platform for many tunable infrared photonic device, such as surface-enhanced infrared absorption spectroscopies, infrared detectors and modulators.

Strong coherent coupling between graphene surface plasmons and anisotropic black phosphorus localized surface plasmons.

The anisotropic plasmons properties of black phosphorus allow for realizing direction-dependent plasmonics devices. Here, we theoretically investigated the hybridization between graphene surface plasmons (GSP) and anisotropic black phosphorus localized surface plasmons (BPLSP) in the strong coupling regime. By dynamically adjusting the Fermi level of graphene, we show that the strong coherent GSP-BPLSP coupling can be achieved in both armchair and zigzag directions, which is attributed to the anisotropic black phosphorus with different in-plane effective electron masses along the two crystal axes. The strong coupling is quantitatively described by calculating the dispersion of the hybrid modes using a coupled oscillator model. Mode splitting energy of 26.5 meV and 19 meV are determined for the GSP-BPLSP hybridization along armchair and zigzag direction, respectively. We also find that the coupling strength can be strongly affected by the distance between graphene sheet and black phosphorus nanoribbons. Our work may provide the building blocks to construct future highly compact anisotropic plasmonics devices based on two-dimensional materials at infrared and terahertz frequencies.

Graphene-Based Long-Period Fiber Grating Surface Plasmon Resonance Sensor for High-Sensitivity Gas Sensing.

A graphene-based long-period fiber grating (LPFG) surface plasmon resonance (SPR) sensor is proposed. A monolayer of graphene is coated onto the Ag film surface of the LPFG SPR sensor, which increases the intensity of the evanescent field on the surface of the fiber and thereby enhances the interaction between the SPR wave and molecules. Such features significantly improve the sensitivity of the sensor. The experimental results demonstrate that the sensitivity of the graphene-based LPFG SPR sensor can reach 0.344 nm%-1 for methane, which is improved 2.96 and 1.31 times with respect to the traditional LPFG sensor and Ag-coated LPFG SPR sensor, respectively. Meanwhile, the graphene-based LPFG SPR sensor exhibits excellent response characteristics and repeatability. Such a SPR sensing scheme offers a promising platform to achieve high sensitivity for gas-sensing applications.

Conformal Graphene-Decorated Nanofluidic Sensors Based on Surface Plasmons at Infrared Frequencies.

An all-in-one prism-free infrared sensor based on graphene surface plasmons is proposed for nanofluidic analysis. A conformal graphene-decorated nanofluidic sensor is employed to mimic the functions of a prism, sensing plate, and fluidic channel in the tradition setup. Simulation results show that the redshift of the resonant wavelength results in the improvement of sensitivity up to 4525 nm/RIU. To reshape the broadened spectral lines induced by the redshift of the resonant wavelength to be narrower and deeper, a reflection-type configuration is further introduced. By tuning the distance between the graphene and reflective layers, the figure of merit (FOM) of the device can be significantly improved and reaches a maximum value of 37.69 RIU(-1), which is 2.6 times that of the former transmission-type configuration. Furthermore, the optimized sensor exhibits superior angle-insensitive property. Such a conformal graphene-decorated nanofluidic sensor offers a novel approach for graphene-based on-chip fluidic biosensing.

[Study on the absorption spectrum properties of flexible black silicon doped with sulfur and fluorine based on first-principles].

It is quite urgent to need a flexible photodetector in the ultraviolet-visible-near infrared region for building a miniaturization broadband spectrometer. In the present paper, one kind of flexible black silicon doped with sulfur and fluorine was proposed and the optical absorption spectrum was investigated in broadband region. Firstly, the electronic structure, band structure and the optical absorption properties of the flexible black silicon doped with sulfur and fluoride were calculated using the first-principles pseudo potential calculations based on density-functional theory. Then, the absorption spectrum model of the flexible black silicon was built based on both the first-principles and finite domain time difference method. The results show that the cut-off wavelength has a red shift as the band gap of doped material becomes narrower. The higher the doping concentration is, the higher the optical absorption coefficient is obtained. The absorption coefficient of flexible black silicon doped with 50% sulfur is 8.3 times higher than that of 1.5% sulfur doping sample at the wavelength of 1 500 nm while the ratio turns to be 3 times when doped with 50% and 1.5% fluoride. The black silicon with small-size surface microstructure has the highest absorptance in the near-infrared region at the same doping concentration of 50%. Finally, a sample of flexible black silicon was fabricated by the femtosecond laser auto scanning system. The test results indicate that the absorptance of the sample is higher than 95% both in the ultraviolet and visible region and is fluctuated from 70% to 80% in the near-infrared region. It shows that as a novel light-absorbing material in broadband region the flexible black silicon doped with Sulfur and Fluorine has an potential application in exploring miniaturization broadband spectroscopy.