Thickness-Dependent Gilbert Damping and Soft Magnetism in Metal/Co-Fe-B/Metal Sandwich Structure.

The achievement of the low Gilbert damping parameter in spin dynamic modulation is attractive for spintronic devices with low energy consumption and high speed. Metallic ferromagnetic alloy Co-Fe-B is a possible candidate due to its high compatibility with spintronic technologies. Here, we report thickness-dependent damping and soft magnetism in Co-Fe-B films sandwiched between two non-magnetic layers with Co-Fe-B films up to 50 nm thick. A non-monotonic variation of Co-Fe-B film damping with thickness is observed, which is in contrast to previously reported monotonic trends. The minimum damping and the corresponding Co-Fe-B thickness vary significantly among the different non-magnetic layer series, indicating that the structure selection significantly alters the relative contributions of various damping mechanisms. Thus, we developed a quantitative method to distinguish intrinsic from extrinsic damping via ferromagnetic resonance measurements of thickness-dependent damping rather than the traditional numerical calculation method. By separating extrinsic and intrinsic damping, each mechanism affecting the total damping of Co-Fe-B films in sandwich structures is analyzed in detail. Our findings have revealed that the thickness-dependent damping measurement is an effective tool for quantitatively investigating different damping mechanisms. This investigation provides an understanding of underlying mechanisms and opens up avenues for achieving low damping in Co-Fe-B alloy film, which is beneficial for the applications in spintronic devices design and optimization.

Inverse-designed ultra-compact multi-channel and multi-mode waveguide crossings.

In this work, we use the inverse design method to design three-channel and four-channel dual-mode waveguide crossings with the design regions of 4.32 µm-wide regular hexagon and 6.68 µm-wide regular octagon, respectively. Based on the highly-symmetric structures, the fundamental transverse electric (TE0) and TE1 modes propagate through the waveguide crossings efficiently. Moreover, the devices are practically fabricated and experimentally characterized. The measured insertion losses and crosstalks of the three-channel and dual-mode waveguide crossing for both the TE0 and TE1 modes are less than 1.8 dB and lower than -18.4 dB from 1540 nm to 1560 nm, respectively. The measured insertion losses of the four-channel and dual-mode waveguide crossing for the TE0 and TE1 modes are less than 1.8 dB and 2.5 dB from 1540 nm to 1560 nm, respectively, and the measured crosstalks are lower than -17.0 dB. In principle, our proposed scheme can be extended to waveguide crossing with more channels and modes.

Different-mode power splitters based on a multi-dimension direct-binary-search algorithm.

In this work, we design, fabricate, and characterize a different-mode (waveguide-connected) power splitter ((W)PS) by what we believe to be a novel multi-dimension direct-binary-search algorithm that can significantly balance the device performance, time cost, and fabrication robustness by searching the state-dimension, rotation-dimension, shape-dimension, and size-dimension parameters. The (W)PS can simultaneously generate the fundamental transverse electric (TE0) and TE1 mode with the 1:1 output balance. Compared with the PS, the WPS can greatly shorten the adiabatic taper length between the single-mode waveguide and the grating coupler. The measured results of the different-mode (W)PS indicate that the insertion loss and crosstalk are less than 0.9 (1.3) dB and lower than -17.8 (-14.9) dB from 1540 nm to 1560 nm. In addition, based on the tunable tap couplers, the different-mode (W)PS can be extended to multiple output ports with different modes and different transmittances.

Three-dimensional mode-division multiplexing system.

Blindly increasing the channels of the mode (de)multiplexer on the single-layer chip can cause the device structure to be too complex to optimize. The three-dimensional (3D) mode division multiplexing (MDM) technology is a potential solution to extend the data capacity of the photonic integrated circuit by assembling the simple devices in the 3D space. In our work, we propose a 16 × 16 3D MDM system with a compact footprint of about 100 µm × 5.0 µm × 3.7 µm. It can realize 256 mode routes by converting the fundamental transverse electric (TE0) modes in arbitrary input waveguides into the expected modes in arbitrary output waveguides. To illustrate its mode-routing principle, the TE0 mode is launched in one of the sixteen input waveguides, and converted into corresponding modes in four output waveguides. The simulated results indicate that the ILs and CTs of the 16 × 16 3D MDM system are less than 3.5 dB and lower than -14.2 dB at 1550 nm, respectively. In principle, the 3D design architecture can be scaled to realize arbitrary network complexity levels.

Multifunctional Plasmon-Induced Transparency Devices Based on Hybrid Metamaterial-Waveguide Systems.

In this paper, we design a multifunctional micro-nano device with a hybrid metamaterial-waveguide system, which leads to a triple plasmon-induced transparency (PIT). The formation mechanisms of the three transparent peaks have their own unique characteristics. First, PIT-I can be switched into the BIC (Friedrich-Wintge bound state in continuum), and the quality factors (Q-factors) of the transparency window of PIT-I are increased during the process. Second, PIT-II comes from near-field coupling between two bright modes. Third, PIT-III is generated by the near-field coupling between a low-Q broadband bright mode and a high-Q narrowband guide mode, which also has a high-Q transparent window due to the guide mode. The triple-PIT described above can be dynamically tuned by the gate voltage of the graphene, particularly for the dynamic tuning of the Q values of PIT-I and PIT-III. Based on the high Q value of the transparent window, our proposed structure can be used for highly sensitive refractive index sensors or devices with prominent slow light effects.

Continuously tunable metasurfaces controlled by single electrode uniform bias-voltage based on nonuniform periodic rectangular graphene arrays.

Metasurfaces, the two-dimensional artificial metamaterials, have attracted intensive attention due to their abnormal ability to manipulate the electromagnetic wave. Although there have been considerable efforts to design and fabricate beam steering devices, continuously tunable devices with a uniform bias-voltage have not been achieved. Finding new ways to realize more convenient and simpler wavefront modulation of light still requires research efforts. In this article, a series of novel reflective metasurfaces are proposed to continuously modulate the wavefront of terahertz light by uniformly adjusting the bias-voltage. By introducing the innovation of nonuniform periodic structures, we realize the gradient distribution of the reflected light phase-changing-rate which is the velocity of phase changing with Fermi energy. Based on strict phase distribution design scheme, a beam scanner and a variable-focus reflective metalens are both demonstrated successfully. Furthermore, dynamic and continuous control of either the beam azimuth of beam scanner or the focal length of metalens can be achieved by uniformly tuning the Fermi energy of graphene. Our work provides a potentially efficient method for the development and simplification of the adjustable wavefront controlling devices.

Double Electromagnetically Induced Transparency and Its Slow Light Application Based On a Guided-Mode Resonance Grating Cascade Structure.

In recent years, the achievement of the electromagnetically induced transparency (EIT) effect based on the guided-mode resonance (GMR) effect has attracted extensive attention. However, few works have achieved a double EIT-like effect using this method. In this paper, we numerically achieve a double EIT-like effect in a GMR system with a three-layer silicon nitride waveguide grating structure (WGS), using the multi-level atomic system model for theoretical explanation. In terms of slow light performance, the corresponding two delay times reach 22.59 ps and 8.43 ps, respectively. We also investigate the influence of wavelength detuning of different GMR modes on the transparent window and slow light performance. Furthermore, a wide-band flat-top transparent window was also achieved by appropriately adjusting the wavelength detuning between GMR modes. These results indicate that the EIT-like effect in the WGS has potential application prospects in low-loss slow optical devices, optical sensing, and optical communications.

Asymmetric dielectric grating on metallic film enabled dual- and narrow-band absorbers.

We investigated a mid-infrared (mid-IR) dual-band absorber consisting of a continuous gold film coated with an asymmetric silicon grating. In each unit cell of the grating, there are three unequally spaced silicon strips. Numerical results reveal that the (+1, -1) planar surface plasmon polariton (SPP) waves excited by the transverse-magnetic (TM) incidence can be coupled with different Fabry-Pérot (FP) resonances and the resonant energy is dissipated to the ohmic loss. Under the normal incidence condition, the absorber provides two high-absorbance peaks at wavelengths of 3.856 µm and 4.29 µm, with the absorption bandwidths of ∼25.7 cm-1 and ∼21.5 cm-1. When changing the angle of the incidence, it is observed an interesting feature that either of the peaks does not split. The presented structure offers an approach to the design of optical components for multi-spectral control of mid-IR signals.

All-optical switch and logic gates based on hybrid silicon-Ge2Sb2Te5 metasurfaces.

We numerically propose a hybrid metasurface (MS) to realize all-optical switch and logic gates in the shortwave infrared (SWIR) band. Such MS consists of one silicon rod and one Ge2Sb2Te5 (GST) rod pair. Utilizing the transition from an amorphous state to a crystalline state of GST, such MS can produce an electromagnetically induced transparency (EIT) analogue with active control. Based on this, we realize all-optical switching at 1770 nm with a modulation depth of 84%. Besides, three different logic gates, NOT, NOR and OR, can also be achieved in this metadevice simultaneously. Thanks to the reversible and fast phase transition process of GST, this device possesses reconfigurable ability as well as fast response time, and has potential applications in future optical networks.

Suppressing the unwanted resonance mode in a metal-insulator-metal structure using fine-structured gratings.

We numerically present that suppressing the unwanted resonance mode in a metal-insulator-metal (MIM) structure can be achieved by using a fine-structured gold grating. In each period of the grating, a sub-grating consisting of multiple gold strips is used. Investigations on a high quality-factor (Q-factor) MIM structure with the grating and usual gratings are carried out. Comparisons show that the proposed grating supports different diffraction orders. Moreover, the unwanted mode which exists in the case of usual gratings can be significantly suppressed, and the desired mode can be kept almost unchanged. Thus, the MIM structure with our grating shows a single resonance at ~3.9 µm with a high Q-factor (~260) and an ultra-narrow linewidth (~10 cm-1) over a broad spectral region. This study provides a simple and effective approach to selective manipulating the resonance modes in MIM structures, which is useful for design of mid-infrared narrowband filters, emitters, and absorbers.

High quality factor electromagnetically induced transparency-like effect in coupled guided-mode resonant systems.

We numerically study a dielectric coupled guided-mode resonant (GMR) system, which includes two silicon (Si) grating waveguide layers (GWLs) stacked on CaF2 substrates. It is confirmed that the coupling between the top and bottom GMR modes starts once a Fabry-Perot (F-P) resonator is introduced, and electromagnetically induced transparency (EIT)-like spectral responses occur in the coupled GMR systems. A very narrow transparency window with a high-quality (Q) factor EIT-like effect of up to 288,892 was demonstrated. Furthermore, EIT-like response wavelengths can be flexibly designed in wide wavelength range by modifying either the GMR resonance frequencies or the space between two GWLs. Therefore, this EIT-like response in coupled GMR systems would pave the way towards novel sensors with extremely high sensitivity.

Control of resonance mode using a fine-structured grating: toward a high resolving power filter.

A band-stop filter with high resolving power is designed for mid-infrared spectroscopic sensing and imaging applications. By utilizing a truncated grating consisting of multiple gold nanowires in each unit cell of the periodic structure, compression of the stop bandwidth and improvement of the resonance extinction are realized simultaneously. It shows only a single stop band at ∼4.474 µm with 15 dB extinction and >80% background transmission over a wide spectral range. The obtained narrow linewidth (∼1.45 nm) enables a spectral resolving power higher than 3000. Flexible control of the filtering performance is investigated by adjusting the variety of geometrical parameters. Moreover, the structure may have potential applications in refractive index sensing.

Active Enhancement of Slow Light Based on Plasmon-Induced Transparency with Gain Materials.

As a plasmonic analogue of electromagnetically induced transparency (EIT), plasmon-induced transparency (PIT) has drawn more attention due to its potential of realizing on-chip sensing, slow light and nonlinear effect enhancement. However, the performance of a plasmonic system is always limited by the metal ohmic loss. Here, we numerically report a PIT system with gain materials based on plasmonic metal-insulator-metal waveguide. The corresponding phenomenon can be theoretically analyzed by coupled mode theory (CMT). After filling gain material into a disk cavity, the system intrinsic loss can be compensated by external pump beam, and the PIT can be greatly fueled to achieve a dramatic enhancement of slow light performance. Finally, a double-channel enhanced slow light is introduced by adding a second gain disk cavity. This work paves way for a potential new high-performance slow light device, which can have significant applications for high-compact plasmonic circuits and optical communication.

Plasmonic Refractive Index Sensor with High Figure of Merit Based on Concentric-Rings Resonator.

A plasmonic refractive index (RI) sensor based on metal-insulator-metal (MIM) waveguide coupled with concentric double rings resonator (CDRR) is proposed and investigated numerically. Utilizing the novel supermodes of the CDRR, the FWHM of the resonant wavelength can be modulated, and a sensitivity of 1060 nm/RIU with high figure of merit (FOM) 203.8 is realized in the near-infrared region. The unordinary modes, as well as the influence of structure parameters on the sensing performance, are also discussed. Such plasmonic sensor with simple framework and high optical resolution could be applied to on-chip sensing systems and integrated optical circuits. Besides, the special cases of bio-sensing and triple rings are also discussed.

Active control of broadband plasmon-induced transparency in a terahertz hybrid metal-graphene metamaterial.

A hybrid metal-graphene metamaterial (MM) is reported to achieve active control of broadband plasmon-induced transparency (PIT) in the THz region. The unit cell consists of one cut wire (CW), four U-shaped resonators (USRs) and monolayer graphene sheets under the USRs. Via near-field coupling, broadband PIT can be produced through the interference between different modes. Based on different arrangements of graphene positions, not only can we achieve electrical switching of the amplitude of broadband PIT, but can also realize modulation of the bandwidth of the transparent window. Simultaneously, both the capability and region of slow light can be dynamically tunable. This work provides schemes to manipulate PIT with more degrees of freedom, which will find significant applications in multifunctional THz modulation.

Ultra-compact beam splitter and filter based on a graphene plasmon waveguide.

This paper presents a sheet of graphene-ribbon waveguide as a simple and ultra-compact splitter and filter in the mid-infrared waveband. The central wavelength of the graphene surface plasmons (GSPs) and the coupling intensity of this splitter can be tuned by changing the physical parameters, such as the chemical potential, the width of the waveguide, the gap between neighboring graphene ribbons, the refractive index of the substrate, the carrier relaxation time, etc. The effects of these parameters on GSP waves and beam-splitter specifications are numerically depicted based on the finite-difference time-domain method. This proposed structure can be used to construct an ultra-compact fast-tunable beam splitter, filter, modulator, and switch in the mid-infrared range.

Rapid Focused Ion Beam Milling Based Fabrication of Plasmonic Nanoparticles and Assemblies via "Sketch and Peel" Strategy.

Focused ion beam (FIB) milling is a versatile maskless and resistless patterning technique and has been widely used for the fabrication of inverse plasmonic structures such as nanoholes and nanoslits for various applications. However, due to its subtractive milling nature, it is an impractical method to fabricate isolated plasmonic nanoparticles and assemblies which are more commonly adopted in applications. In this work, we propose and demonstrate an approach to reliably and rapidly define plasmonic nanoparticles and their assemblies using FIB milling via a simple "sketch and peel" strategy. Systematic experimental investigations and mechanism studies reveal that the high reliability of this fabrication approach is enabled by a conformally formed sidewall coating due to the ion-milling-induced redeposition. Particularly, we demonstrated that this strategy is also applicable to the state-of-the-art helium ion beam milling technology, with which high-fidelity plasmonic dimers with tiny gaps could be directly and rapidly prototyped. Because the proposed approach enables rapid and reliable patterning of arbitrary plasmonic nanostructures that are not feasible to fabricate via conventional FIB milling process, our work provides the FIB milling technology an additional nanopatterning capability and thus could greatly increase its popularity for utilization in fundamental research and device prototyping.