Lifetime and nonlinearity of modulated surface plasmon for black phosphorus sensing application

Anisotropic Sensing Performance in a High-Sensitivity Surface Plasmon Resonance Sensor Based on Few-Layer Black Phosphorus

Few-layer black phosphorus (FLBP) is a highly promising material for high sensitivity label-free surface plasmon resonance (SPR) sensors due to its exceptional electrical, optical, and mechanical properties. FLBP exhibits inherent anisotropy with different refractive indices along its two main crystal orientations, the zigzag and armchair axes. However, this anisotropic property is often overlooked in FLBP-based sensors. In this study, we conducted a comprehensive investigation of the SPR reflectivity and phase in a BK7-Ag-FLBP structure to understand the influence of the stacking sequence and the number of FLBP layers on the sensing performance. Clear resonant angle shifts caused by different stacking sequences of FLBP could be observed both theoretically and experimentally. In the theoretical study, the highest reflective and phase sensitivities were achieved with a 12-layer black phosphorus (BP) structure. The reflectivity sensitivity reached 287.9°/refractive index units (RIU) with the zz stacking 12-layer BP film exhibiting a sensitivity 76°/RIU higher than the ac stacking structure. Similarly, the phase sensitivity reached 1162°/RIU with the zz stacking 12-layer BP structure showing a sensitivity 276.9°/RIU higher than the ac stacking structure. The electric field distribution of the 12-layer BP structure with four different stacking sequences has also been analyzed. In the experiment study, the well-known Attenuated Total Reflection (ATR) θ-2θ SPR setup is utilized to detect the reflectivity and phase of BK7-Ag-FLBP structures. The FLBP samples with the same thickness but different stacking sequences show significant resonant angle shift (0.275°) and maximum phase difference variation (34.6°). The FLBP sample thickness and crystal orientations have been demonstrated using the angular-resolved polarized Raman spectroscopy (ARPRS). These theoretical and experimental results provide strong evidence that the stacking sequences of FLBP have a significant impact on the sensing performance of SPR sensors. By harnessing the anisotropic properties of materials like FLBP, novel structures of anisotropic-2D material-based SPR sensors could open up exciting possibilities for innovative applications.

Regular arrangement of dispersed 2D flakes detected by polarization of light

Regular arrangement of dispersed 2D flakes, as the "Wind-Chime" model, has been regarded as possible mechanism of spatial self-phase modulation. But this regular arrangement caused by the laser have not been confirmed, and the relation with the concentration of dispersed 2D flakes is still unclear. Here, the relationship between arrangement caused by electric field and polarized transmittance have been explored at first. Then, the model of flakes rotation to regular arrangement were established, which were proof by the response time by turning on/off electric field. On this basis, by building the polarization-related cross optical switch system, light-induced regular arrangement were observed and proven.

Tailoring linear and nonlinear plasmons of metal/MoS2/metal nanostructures

We investigated the linear and nonlinear response of the localized surface plasmons (LSPs) and surface plasmon polaritons (SPPs) in metal and MoS2 nanostructures. The results show that the response of LSPs and SPPs has an important influence on the energy exchange. SPPs with unique non-radiative characteristics can be used as energy recovery tanks to reuse the radiated energy of LSPs and promote the production of hot carriers. The energy exchange through plasmon modes can promote the transfer of hot electrons in the Au grating, the MoS2 layer, and the metal film. The fundamental field induces the increase of the second harmonic wave by introducing the second-order nonlinear source. In addition, the evolution of the lifetime of linear and nonlinear plasmonic modes is also investigated to study the underlying mechanism of the micro process in the plasmonic-photonic interaction. The plasmonic energy exchanging configuration overcomes the challenge by utilizing hot carriers. It is instructive in terms of improving the linear and nonlinear performance of plasmonic opto-electronic devices.

Multifrequency on-off modulation and slow light characterization of the patterned black phosphorus metamaterial based on dual plasmon-induced transparency

We present a monolayer patterned black phosphorus (BP) metamaterial for generating a tunable dual plasmon-induced transparency (PIT). We have derived the expression for the theoretical transmittance by introducing the coupled mode theory (CMT), and the calculated results of the expression highly overlap with the simulation results. The quarterly frequency synchronous switch with two different operating bands is designed by the carrier density and scattering rate on the dual PIT modulation effect. Two parameters were selected as important markers to show the performance of the optical switch: the modulation depth (MD) and the insertion loss (IL). The theoretical analysis of this structure shows that the higher modulation depth (5.45d B Collapse

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Polarization-sensitive switchable display through critical coupling between graphene and a quasi-BIC

Enhanced light-matter interaction of a local field is of prime importance in optics as it can improve the performance of nanophotonic devices. Such enhancement can be achieved by utilizing the optical bound states in the continuum (BICs). In this study, a dielectric metasurface is proposed that could enhance the light-matter interactions in graphene. A symmetry-protected BIC was observed in such a metasurface, which could transform into a quasi-BIC with a high quality (Q-) factor when the in-plane symmetry is broken. As the graphene monolayer was introduced into the system, its absorption was enhanced by the quasi-BIC resonance. By optimizing the graphene Fermi energy and the asymmetry parameter of the metasurface to satisfy the critical-coupling condition, a tunable absorber could be achieved. The absorbing intensity could be efficiently modulated by varying the polarization direction of the incident light, the maximum difference of which was up to 95.4%. Also, further investigation showed that such a feature indicates potential application in digital switches and image displays, which could be switched by incident polarization only, and therefore without dependence on an additional structural change.

High-performance polarization-independent black phosphorus refractive index sensors enabled by a single-layer pattern design

The in-plane orientation-dependent electrical and optical properties of two-dimensional (2D) anisotropic materials attract significant attention because of the intriguing underlying physics. However, this feature limits their further development in polarization-independent applications such as refractive index sensors and light absorbers. In this paper, polarization-independent optical properties of black phosphorous (BP) metadevices are achieved by the design of a single-layer pattern of 2D anisotropic material. Finite-difference time-domain (FDTD) simulation results indicate that the absorption spectrum remains unchanged as the polarization angle of the incident light varies from 0° to 360°. The performance of the BP metadevices when used as refractive index sensors is also studied. The results show that the polarization-independent BP sensors exhibit high sensitivity and figures of merit (FOMs). This work opens up the possibility of fabricating optically polarization-independent devices based on a single-layer pattern of 2D anisotropic material.

Tunable Lifetime and Nonlinearity in Two Dimensional Materials Plasmonic-Photonic Absorber

We investigate a framework of local field, quality factor and lifetime for tunable graphene nanoribbon plasmonic-photonic absorbers and study the second order and third order nonlinear optical response of surface plasmons. The energy exchange of plasmonic-photonic absorber occurs in two main ways: one way is the decay process of intrinsic loss for each resonant mode and another is the decay process of energy loss between graphene surface plasmon (GSP) mode and the external light field. The quality factor and lifetime of the plasmonic-photonic absorber can be obtained with using the coupled mode theory (CMT) and finite difference time domain (FDTD) method, which are effectively tunable with changing Fermi energy, carrier mobility and superstrate refractive index. The evolutions of total energy and lifetime of GSP are also shown, which are helpful for the study of micro processes in a two-dimensional material plasmonic-photonic absorber. The strongly localized fundamental field induces a desired increase of second harmonic (SH) wave and third harmonic (TH) wave. The manipulation of the quality factor and lifetime of the GSP makes graphene an excellent platform for tunable two-dimensional material plasmonic-photonic devices to realize the active control of the photoelectric/photothermal energy conversion process and higher harmonic generation.

Switching the Nonlinear Optical Absorption of Titanium Carbide MXene by Modulation of the Surface Terminations

Surface terminations of two-dimensional materials should have a strong influence on the nonlinear optical (NLO) properties, but the relationship between surface terminations and NLO properties has not yet been reported. In this work, switching the NLO properties of MXenes (Ti₃C₂T_x) via "surface terminations modulation" is explored. The surface terminations of Ti₃C₂T_x are modulated by electrochemical treatment, resulting in different states (viz., Ti₃C₂T_x(pristine), Ti₃C₂T_x(═O rich), and Ti₃C₂T_x(-OH rich)). The sign and magnitude of the effective NLO absorption coefficient (β_eff) change with the surface terminations. Ti₃C₂T_x(═O rich) shows a relatively large saturable absorption (SA) with laser excitation at 515 nm (β_eff = -1020 ± 136.2 cm GW^-1), while reverse saturable absorption (RSA) is found in Ti₃C₂T_x(pristine) and Ti₃C₂T_x(-OH rich). The RSA of Ti₃C₂T_x(pristine) and Ti₃C₂T_x(-OH rich) is attributed to excited-state absorption, while the SA of Ti₃C₂T_x(═O rich) is associated with Pauli blocking. With laser excitation at 800 nm, the β_eff of Ti₃C₂T_x(-OH rich) is 113 ± 3.2 cm GW^-1, 1.68 times that of Ti₃C₂T_x(pristine); the RSA is caused by photon-induced absorption. Our results reveal a correlation between surface terminations and NLO properties, highlighting the potential of MXenes in photoelectronics.

Dual-directional group delays during optical topological transitions in black phosphorus-based asymmetric hyperbolic metamaterials

We theoretically study the optical properties of TM waves when their magnetic field direction is perpendicular to the armchair and zigzag optical axes of black phosphorus, respectively. It is found that hyperbolic dispersion and elliptic dispersion coexist in periodically arranged black phosphorus multilayers. Interestingly, by tilting the symmetric multilayers to be asymmetric, the elliptical part of the original two dispersions disappears as the wavelength increases. As such only the hyperbolic dispersion remains, showing an optical topological transition. In the region of the topological transition, a large transmitted group delay (3ps) and a reflected group delay (0.2ps) of the TM waves occurs simultaneously. The corresponding group velocities are slowed down to approximately c/1000 and c/100 (c is the speed of light in a vacuum), respectively. This dual-directional group delays significantly increase the wave-matter interaction so that nonreciprocal perfect absorptions can be realized in the mid-infrared band. Such asymmetrical black phosphorus hyperbolic metamaterials can be applied to the directional, tunable, and nonreciprocal perfect absorbers and also to devices based on strong wave-matter interactions.

Black phosphorus for near-infrared ultrafast lasers in the spatial/temporal domain

Two-dimensional (2D) materials have attracted extensive interests due to their wide range of electronic and optical properties. After continuous and extensive research, black phosphorus (BP), a novel member of 2D layered semiconductor material, benefit for the unique in-plane anisotropic structure, controllable direct bandgap characteristic, and high charge carrier mobility, has attracted tremendous attention and successfully applied in ultrafast pulse generation. This article, which focuses on near-infrared ultrafast laser demonstration of BP, present discussion of preparation methods for high quality BP nanosheet, various BP based ultrafast lasers in the spatial/temporal domain, and the future research needs.

Germanium Nanosheets with Dirac Characteristics as a Saturable Absorber for Ultrafast Pulse Generation

Bulk germanium as a group-IV photonic material has been widely studied due to its relatively large refractive index and broadband and low propagation loss from near-infrared to mid-infrared. Inspired by the research of graphene, the 2D counterpart of bulk germanium, germanene, has been discovered and the characteristics of Dirac electrons have been observed. However, the optical properties of germanene still remain elusive. In this work, several layers of germanene are prepared with Dirac electronic characteristics and its morphology, band structure, carrier dynamics, and nonlinear optical properties are systematically investigated. It is surprisingly found that germanene has a fast carrier-relaxation time comparable to that of graphene and a relatively large nonlinear absorption coefficient, which is an order of magnitude higher than that of graphene in the near-infrared wavelength range. Based on these findings, germanene is applied as a new saturable absorber to construct an ultrafast mode-locked laser, and sub-picosecond pulse generation in the telecommunication band is realized. The results suggest that germanene can be used as a new type of group-IV material for various nonlinear optics and photonic applications.

Efficient HIE-FDTD method for designing a dual-band anisotropic terahertz absorption structure

The finite-difference time-domain (FDTD) method is considered to be one of the most accurate and common methods for the simulation of optical devices. However, the conventional FDTD method is subject to the Courant-Friedrich-Levy condition, resulting in extremely low efficiency for calculating two-dimensional materials (2DMs). Recent researches on the hybrid implicit-explicit FDTD (HIE-FDTD) method show that the method can efficiently simulate homogeneous and isotropic 2DMs such as graphene sheet; however, it is inapplicable to the anisotropic medium. In this paper, we propose an in-plane anisotropic HIE-FDTD method to simulate optical devices containing graphene and black phosphorus (BP) sheets. Numerical analysis shows that the proposed method is accurate and efficient. With this method, we present a novel multi-layer graphene-BP-based dual-band anisotropic terahertz absorption structure (GBP-DATAS) and analyze its optical characteristics. Combining the advantages of graphene and BP localized surface plasmons, the GBP-DATAS demonstrates strong anisotropic plasmonic resonance and high absorption rate in the terahertz band.

Photoelectric switch and triple-mode frequency modulator based on dual-PIT in the multilayer patterned graphene metamaterial

A multilayer patterned graphene metamaterial composed of rectangular graphene, square graphene, and X-shaped graphene is proposed to achieve dual plasmon-induced transparency (PIT) at terahertz frequency. The coupled mode theory calculations are highly consistent with the finite-difference time-domain numerical results. Interestingly, a photoelectric switch has been realized, whose extinction ratio and modulation degree of amplitude can be 7.77 dB and 83.3% with the insertion loss of 7.2%. In addition, any dips can be modulated by tuning the Fermi levels of three graphene layers with minor or ignorable changes of the other two dips. The modulation degrees of frequency are 8.0%, 7.4% and 11.7%, respectively, which can be used to design a triple-mode frequency modulator. Moreover, the group index of the multilayer structure can be as high as 150. Therefore, it is reasonable to believe that a multifunctional device can be realized by the proposed structure.

A Tunable Triple-Band Near-Infrared Metamaterial Absorber Based on Au Nano-Cuboids Array

In this article, we present a design for a triple-band tunable metamaterial absorber with an Au nano-cuboids array, and undertake numerical research about its optical properties and local electromagnetic field enhancement. The proposed structure is investigated by the finite-difference time domain (FDTD) method, and we find that it has triple-band tunable perfect absorption peaks in the near infrared band (1000–2500 nm). We investigate some of structure parameters that influence the fields of surface plasmons (SP) resonances of the nano array structure. By adjusting the relevant structural parameters, we can accomplish the regulation of the surface plasmons resonance (SPR) peaks. In addition, the triple-band resonant wavelength of the absorber has good operational angle-polarization-tolerance. We believe that the excellent properties of our designed absorber have promising applications in plasma-enhanced photovoltaic, optical absorption switching and infrared modulator optical communication.

Oxidation-Resistant Black Phosphorus Enable Highly Ambient-Stable Ultrafast Pulse Generation at a 2 μm Tm/Ho-Doped Fiber Laser

Black phosphorus (BP) ranks among the most promising saturable absorber materials for ultrafast pulse generations at 2 μm. However, the easy-to-degrade characteristic of BP seriously limits the long-term operation of ultrafast fiber lasers and hence becomes a bottleneck for its relevant practical applications. In this paper, a modified electrochemical delamination exfoliation process was explored to produce high-performance, large-size, and oxidation-resistant BP nanosheets, where BP nanosheets in high yield with evenly coated tetra-n-butyl-ammonium organics by precisely controlling the intercalation chemistry can be obtained. A mode-locked Tm/Ho co-doped fiber laser with high temporal stability and long-term operation capability was demonstrated based on the innovatively fabricated BP saturable absorber. The self-starting mode-locking operation featuring a high signal-to-noise ratio of 58 dB and long-term stability has been verified for at least 3 weeks, which indicates the successful passivation of the employed synthesis method. These results fully indicated that passivated BP is an efficient candidate in a 2 μm range ultrafast photonic field, which will promote the ultrafast optical application of BP and also other infrared photonic and photoelectronic devices.