Absorption Of Graphene Research Articles

TPP-assisted multi-band absorption enhancement in graphene based on Fibonacci quasiperiodic photonic crystal

The multi-channel enhanced absorption properties in graphene monolayer are theoretically studied. It is realized by inserting a graphene monolayer in a dielectric film, which is sandwiched between a metallic film and a Fibonacci quasiperiodic structure. It is shown that three peaks with absorbance above 75% can be achieved, which is attributed to the Tamm plasmon polaritons and multiple photonic stopbands of the Fibonacci dielectric multilayers. Moreover, the distribution of the normalized electric field intensity is simulated to reveal the physical origin of such multichannel absorption effect. The designed graphene absorber possesses ultra-narrow absorption profiles and performs similar to an antenna. Furthermore, the multichannel absorption performances could be flexibly tuned by changing the angle of the incident and the geometric dimensions. Our results may find potential applications in the design of novel photoelectronic devices.

Towards Perfect Absorption of Single Layer CVD Graphene in an Optical Resonant Cavity: Challenges and Experimental Achievements.

Graphene is emerging as a promising material for the integration in the most common Si platform, capable to convey some of its unique properties to fabricate novel photonic and optoelectronic devices. For many real functions and devices however, graphene absorption is too low and must be enhanced. Among strategies, the use of an optical resonant cavity was recently proposed, and graphene absorption enhancement was demonstrated, both, by theoretical and experimental studies. This paper summarizes our recent progress in graphene absorption enhancement by means of Si/SiO2-based Fabry–Perot filters fabricated by radiofrequency sputtering. Simulations and experimental achievements carried out during more than two years of investigations are reported here, detailing the technical expedients that were necessary to increase the single layer CVD graphene absorption first to 39% and then up to 84%. Graphene absorption increased when an asymmetric Fabry–Perot filter was applied rather than a symmetric one, and a further absorption increase was obtained when graphene was embedded in a reflective rather than a transmissive Fabry–Perot filter. Moreover, the effect of the incident angle of the electromagnetic radiation and of the polarization of the light was investigated in the case of the optimized reflective Fabry–Perot filter. Experimental challenges and precautions to avoid evaporation or sputtering induced damage on the graphene layers are described as well, disclosing some experimental procedures that may help other researchers to embed graphene inside PVD grown materials with minimal alterations.

Enhanced absorption and electrical modulation of graphene based on the parity-time symmetry optical structure

In order to realize the ultrastrong absorption of graphene with electrical modulation properties, we designed a composite structure of graphene and parity-time (PT) symmetry photonic crystal, which is achieved by placing the graphene layer on the top layer of the PT symmetry photonic crystal. In this paper, the absorption properties of graphene and the electrical modulating properties of the structure were theoretically analyzed based on the transfer matrix method. The result shows that the proposed structure can achieve the absorption of 31.5 dB for the communication wavelength of 1550 nm; meanwhile, by setting the electric field intensity to ±0.02 V/nm, the absorption of graphene can be largely modulated to realize an electrically switchable effect, the modulation depth of graphene absorption can reach nearly 100%, and the operation speed is also close to 8.171 GHz. This investigation provides a novel approach to design graphene-based optoelectronic devices and optical communication devices.

Graphene Nanofilms/Silicon Near-Infrared Avalanche Photodetectors

Graphene/silicon Schottky junction diodes have been extensively studied and widely used in photodetection. However, limited to the low light absorption of single-layer graphene and bandgap of silicon, graphene/silicon devices exhibit low quantum efficiency in the infrared region. Here, we demonstrate a high-performance near-infrared Schottky diode by integrating silicon with the macroscopic assembled graphene nanofilms (nMAG). Because of the intense light absorption, low work function, and long carrier lifetime of nMAG, the device shows a detection spectra range up to 1870 nm with the responsivity of 0.4 mA/W and response speed of 371 ns. By driving the device into avalanche multiplication, 10⁴-10⁵ of avalanche gain has been obtained, and the responsivity at 1870 nm increases to 10 A/W. The device structure in this report could be compatible with the semiconductor process so that silicon-based infrared photodetectors with high performance and low cost could be potentially realized.

Theoretical studies on optical absorption in twisted bilayer graphene under vertical electric field

The interlayer twist angle is an important parameter that can tune the physical properties of graphene in a wide wavelength range. In this paper, we employ an effective continuum model to calculate the band structure of twisted bilayer graphene with different twist angles in the presence and absence of vertical electric field. Based on the transition rate of the electron-photon interaction, we calculate and simulate the optical absorption spectra caused by the interband and intraband transitions around the van Hove singularities. The calculation results show that the optical absorption caused by the interband transitions occurs in the wavelength range from visible light to near-infrared while it appears in far-infrared for intraband transitions. The optical absorption coefficient of the intra-band transitions is almost two orders of magnitude larger than that of inter-band transitions. In the absence of an external electric field, as the twist angle increases, the absorption peak of the inter band transition moves from the infrared light band to the visible light band, but the resonant peak position of its intra-band transition does not change. At the same time, the absorption coefficient values corresponding to the above two transitions will increase. When an electric field is applied perpendicular to the twisted bilayer graphene, the symmetry of the initial band structure of bilayer graphene is destroyed, which results in the splitting of the absorption peaks associated the with interband transitions, and the distance between the two splitting peaks increases with the electric field intensity increasing; while the position and amplitude of the absorption peak associated with the intraband transition are completely unaffected by the applied electric field. The theoretical calculation results in this paper can provide the theoretical guidance for further applying twisted graphene to optoelectronic devices such as tunable dual-band filters.

Graphene: A Pathway to Advanced Photonics

In this paper, we discuss the potential of graphene with its unique properties to create a new pathway for advancement in photonic devices. Modulators and photodetectors are discussed in detail as these are the foundation of advanced photonics. We begin with an outline of how graphene is discovered and how its production processes sped up its marketing value. In the next section, basic physics of modulator are discussed and how graphene modulators helped surpass the limitations. The challenges and problems are addressed along with possible solutions. Third section revolves around graphene photodetectors and its evolution. The low light absorption of graphene was the major setback to its progression. We discuss various mechanism and structures to counterfeit this issue. Recent publications around each are cited properly to show how far we are in the journey to advanced photonics. In the final two sections, other graphene based photonic and optoelectronic devices are mentioned along with a conclusion remark for the future and present of this material.

Light Enhanced Absorption of Graphene Based on Parity-time Symmetry Structure

Light Enhanced Absorption of Graphene Based on Parity-time Symmetry Structure

Environmentally-friendly graphene conductive ink using graphene powder, polystyrene, and waste oil

Waste oil and polystyrene are main sources of pollution that endanger our health. This project proposes an effective, environmentally-friendly method of producing conductive ink using expired waste oil, polystyrene, and graphene. We compared three types of differently-sized graphene powder, two of which are ball-milled. We hypothesized that the ink made with the graphene with the longest milling time will have the best conductivity and the lowest viscosity, thus the easiest to spread. Furthermore, we hypothesized that the film-forming properties would increase with the addition of more polystyrene, regardless of the type of powder. We also determined the microscopic lamellar pattern of the graphene powder. Increased ball-milling time resulted in more polarized powder distribution; smaller pieces of graphene were stacked together as well as larger flakes. We assessed the correlation between the conductivity of graphene powder and its free volume, highlighting how the graphene and waste oil bounded together. We later explored a combination of waste oil with graphene and evaluated the oil absorption of graphene. An ink with a conductive coating film resistance below 100 Ohm was made by altering the proportions of the composition of graphene, polystyrene, and oil. We determined that the best ink recipe consists of mineral oil (baby oil), graphene milled for 2.5 hours, and a polystyrene-to-graphene ratio of 0.5 to 1 because it compromises between low resistance, moderate viscosity, good spreadability, and good film-forming properties. This work has important implications on developing a novel way to recycle waste into applicable conductive ink.

Tunable and Polarization Sensitive Three-Band Terahertz Graphene Absorber

Tunable and Polarization Sensitive Three-Band Terahertz Graphene Absorber

Graphene photodetector employing double slot structure with enhanced responsivity and large bandwidth

Silicon photonics integrated with graphene provides a promising solution to realize integrated photodetectors operating at the communication window thanks to graphene’s ultrafast response and compatibility with CMOS fabrication process. However, current hybrid graphene/silicon photodetectors suffer from low responsivity due to the weak light-graphene interaction. Plasmonic structures have been explored to enhance the responsivity, but the intrinsic metallic Ohmic absorption of the plasmonic mode limits its performance. In this work, by combining the silicon slot and the plasmonic slot waveguide, we demonstrate a novel double slot structure supporting high-performance photodetection, taking advantages of both silicon photonics and plasmonics. With the optimized structural parameters, the double slot structure significantly promotes graphene absorption while maintaining low metallic absorption within the double slot waveguide. Based on the double slot structure, the demonstrated photodetector holds a high responsivity of 603.92 mA/W and a large bandwidth of 78 GHz. The high-performance photodetector provides a competitive solution for the silicon photodetector. Moreover, the double slot structure could be beneficial to a broader range of hybrid two-dimensional material/silicon devices to achieve stronger light-matter interaction with lower metallic absorption.

Gold Enhanced Graphene-Based Photodetector on Optical Fiber with Ultrasensitivity over Near-Infrared Bands

Graphene has been widely used in photodetectors; however its photoresponsivity is limited due to the intrinsic low absorption of graphene. To enhance the graphene absorption, a waveguide structure with an extended interaction length and plasmonic resonance with light field enhancement are often employed. However, the operation bandwidth is narrowed when this happens. Here, a novel graphene-based all-fiber photodetector (AFPD) was demonstrated with ultrahigh responsivity over a full near-infrared band. The AFPD benefits from the gold-enhanced absorption when an interdigitated Au electrode is fabricated onto a Graphene-PMMA film covered over a side-polished fiber (SFP). Interestingly, the AFPD shows a photoresponsivity of >1 × 104 A/W and an external quantum efficiency of >4.6 × 106% over a broadband region of 980–1620 nm. The proposed device provides a simple, low-cost, efficient, and robust way to detect optical fiber signals with intriguing capabilities in terms of distributed photodetection and on-line power monitoring, which is highly desirable for a fiber-optic communication system.

Graphene-coupled silica microsphere polarizer

Polarization-selective devices can control the polarization of the optical system. We develop a compact hybrid structure based on silica microsphere and monolayer graphene to tune the polarization of the optical system, which the working principle is polarization-dependent absorption of graphene combined with silica microsphere through whispering-gallery (WG) TE mode and TM mode. Because of different field distributions and polarization states of resonant modes in microsphere, the gap distance between microsphere and graphene can modify the quality factor and resonant wavelength of microsphere to realize the different variations for TE and TM WG modes, respectively. As gap distance is reduced from 2.2 µm to 0.3 µm, 11 dB polarization extinction ratio is realized between resonance polarized mode in the 90° polarization direction and that in the 0° polarization direction. Furthermore, for different original gap distance between microsphere and graphene, corresponding to different preconditions of quality factors of microsphere, one can obtain a polarization-selective device with the adjustable bandwidth. This results offer an attractive and effective photonic platform to achieve high-performance polarization-selective devices.

Photothermally stabilized Fabry-Perot cavity with patterned nanofilm for photoacoustic trace gas sensing

A photothermally stabilized Fabry-Perot cavity with patterned nanofilm was developed for sensitive photoacoustic gas sensing. The Fabry-Perot cavity with a 100 nm-thick and 2.5 mm-diameter multilayer graphene (MLG) demonstrated a minimum detectable acoustic pressure level as low as ~ 2.2 μPa/Hz1/2. The MLG film was patterned by laser manufacture to reduce the signal fluctuation caused by the environmental pressure. Furthermore, taking advantages of the broadband absorption of graphene, a low-cost single-wavelength heating laser was used as the heating source to photothermally stabilize the operation point of the sensor, which further reduced the sensor signal fluctuation to ~ 1 dB. The stabilized Fabry-Perot cavity demonstrated a minimum detectable concentration of ~ 380 ppb over 100 s average time for C2H2 detection. This photothermally stabilized Fabry-Perot cavity with advantages of high sensitivity, compact size and low cost provides a potential solution to high-performance acoustic detection and photoacoustic gas sensing.

Resonant nanocavity-enhanced graphene photodetectors on reflecting silicon-on-insulator wafers

The weak light absorption and zero-bandgap properties of two-dimensional graphene negatively affect electron–hole recombination and quantum yield, restricting its usefulness in practical optoelectronic applications. In this work, plasma-assisted chemical vapor deposition is used to synthesize three-dimensional graphene (3D-graphene) in situ on the surface of silicon-on-insulator (SOI) wafers, thereby creating high-performance broadband photodetectors. The nanocavity structure of the 3D-graphene integrates with the optical cavity structure of the SOI to enhance the interaction that occurs between the 3D-graphene and incident light. The resulting device has excellent performance in the near-infrared (NIR). The mechanism by which the light absorption of the photodetector is enhanced is explored in detail via experimental analysis and theoretical calculation. Photodetectors based on the 3D-graphene/SOI Schottky heterojunction exhibit a broad detection range (from 440 to 1550 nm), ultrahigh responsivity (27.4 A/W), and excellent detectivity (1.37 × 1011 Jones) at a wavelength of 1550 nm. The Schottky heterojunctions combine two structures (nanocavity and optical cavity) that enhance light absorption. They are also compatible with complementary metal-oxide-semiconductor technology, providing a strategy for manufacturing high-performance NIR photodetectors.

Simulation Study of In-Phase and Out-Phase Enhanced Absorption of Graphene Based on Parity–Time Symmetry One-Dimensional Photonic Crystal Structure

In the field of modern optical communication systems and photoelectric detection, new components with complex functions and excellent performance are urgently needed. In this paper, a graphene-based parity–time (PT) symmetry structure is proposed, which is achieved by preparing the graphene layer on the top of a PT-symmetry photonic crystal. The transfer matrix method was used to calculate the absorptance of graphene, and a unique amplified absorption effect was found. Meanwhile, the peak value and wavelength position of the absorption can be modulated via the applied electric field. The results show that by adjusting the negative square-wave electric field from −3.5 × 10−5 to −13.5 × 10−5 V/nm (or the positive square-wave electric field from 2 × 10−5 to 11 × 10−5 V/nm), the proposed structure can achieve in-phase (or out-phase) enhanced absorption for the communication wavelength 1550 nm, with the absorption of graphene from 17 to 28 dB (or 30 to 15 dB) corresponding to the square-wave modulation electric field change. The modulable absorption properties of graphene in the structure have potential in optoelectronic devices and optical communication systems.

Bandwidth-tunable near-infrared perfect absorption of graphene in a compound grating waveguide structure supporting quasi-bound states in the continuum

Recently, based on the selective excitation of the guided mode, researchers realized quasi-bound states in the continuum (quasi-BICs) in all-dielectric compound grating waveguide structures. In this paper, we introduce a graphene layer into an all-dielectric compound grating waveguide layer supporting quasi-BIC to achieve near-infrared perfect absorption of graphene. The underlying physical mechanism of perfect absorption can be clearly explained by the critical coupling theory derived from temporal coupled-mode theory in a single-mode, one-port system. By changing the Fermi level and the layer number of the graphene, the absorption rate of the system can be flexibly tuned. In addition, by changing the geometric parameter of the compound grating waveguide structure, the radiation coupling rate of the quasi-BIC can also be flexibly tuned. Therefore, the critical coupling condition can be maintained in a broad range of the Fermi level and the layer number of the graphene. The full width at half maximum of the near-infrared perfect absorption peak can be flexibly tuned from 5.7 to 187.1 nm. This bandwidth-tunable perfect absorber would possess potential applications in the design of 2D material-based optical sensors, electrical switchers, and solar thermophotovoltaic devices.

High resolution graphene angle sensor based on ultra-narrowband optical perfect absorption

We propose and experimentally demonstrate high resolution angle sensors based on ultra-narrowband graphene perfect absorbers. Perfect absorption at wavelength of 1452.8 nm with absorption bandwidth of 0.8 nm is numerically demonstrated for a designed angle sensor based on single graphene absorber at normal incidence, and the angular width of the resonant absorption is only 0.05°. In the experiment, peak absorption over 95% with bandwidth about 2.8 nm is measured at normal incidence for a fabricated graphene sensor, and the device has a wavelength-angle sensitivity over 17 nm per degree which agrees well with the simulation result. Meanwhile, an optoelectronic angle sensor with high resolution and fast response by using an array of graphene absorbers is proposed. The demonstrated graphene angle sensors with ultra compact size and high resolution could be of valuable applications in many fields.

Graphene perfect absorber with loss adaptive Q-factor control function enabled by quasi-bound states in the continuum

Numerous device structures have been proposed for perfect absorption in monolayer graphene under single-sided illumination, all of which requires the critical coupling condition, i.e., the balance between the loss of graphene and the leakage rate of the device. However, due to the difficulty of the precise control of the quality of synthesized graphene and unwanted doping in graphene transferred to the substrate, the loss of graphene is rather unpredictable, so that the perfect absorption is quite difficult to achieve in practice. To solve this problem, we designed a novel perfect absorber structure with a loss adaptive leakage rate control function enabled by the quasi-bound states in the continuum (BIC) and numerically demonstrated its performance. Our designed device is based on a slab-waveguide grating supporting both the quasi-BIC and the guided-mode resonance (GMR); the quasi-BIC with an adjustable leakage rate controlled by an incident angle is responsible for absorption, while the GMR works as an internal mirror. Since the proposed device scheme can have an arbitrarily small leakage rate, it can be used to implement a perfect absorber for any kind of ultrathin absorbing media. Due to the simple structure avoiding an external reflector, the device is easy to fabricate.

Room temperature sub-terahertz PIN-photodiode photodetector based on multiple graphene layer absorber

Given the importance of the 0.1- to 1-THz range in the security detection and diagnosis of cancer, we propose a structure for graphene-based terahertz (THz) photodetectors. We use the finite-difference time-domain and mathematical methods to calculate the light absorption in the graphene layers (GLs) and the detector’s figure of merits, including responsivity and detectivity. The proposed photodetector is based on single-layer and multilayer graphene absorbers within a p-type-intrinsic-n-type (PIN) photodiode structure. This detector is designed to achieve maximum absorption at the THz frequencies by creating adjustable Fabry–Perot resonances. Results show that it has two distinct and adjustable detection peaks in THz. So we set the operating frequency to 0.35 and 0.83 THz by adjusting the geometric parameters of the device, which also fit the atmospheric windows. Despite the low number of GLs, high absorption is provided at room temperature. It has high responsivity peaks of ∼1220 AW − 1 at the frequency of 0.35 THz and 420 AW − 1 at 0.83 THz, respectively, while we have achieved these characteristics with fewer GLs than other previous detectors.

Correlation between the optical absorption and twisted angle of bilayer graphene observed by high-resolution reflectance confocal laser microscopy.

We report a systematic study of the optical absorption of twisted bilayer graphene (tBLG) across a large range of twist angles from 0° to 30° using a high-resolution reflectance confocal laser microscopy (RCLM) system. The high-quality single crystalline tBLG was synthesized via the efficient plasma enhanced chemical vapor deposition techniques without the need of active heating. The sensitivity of acquired images from the RCLM were better than conventional optical microscopes. Although the highest spatial resolution of RCLM is still lower than scanning electron microscopes, it possesses the advantages of beam-damage and vacuum free. Moreover, the high intensity-resolution (sensitivity) images firstly allowed us to distinguish the slight absorption differences and analyze the correlation between the optical absorption and twisted angle of tBLG after data processing procedures. A maximum absorption (minimum transmission) was observed at the stacking angle of tBLG from 10° to 20°, indicating the interplay between the laser and the electron/hole van-Hove singularities when tBLG oriented around the critical angle (θc∼13°). The twisted angle correlated optical absorption paves an alternative way not only to visibly identify the interlayer orientation of tBLG but also to reflect the characterization of the interlayer coupling via its band structure.