Plasmonic Bragg Reflector Research Articles

Quasi-TPPs/Fano resonance systems based on an MDM waveguide structure and its sensing application.

In this paper, quasi-Tamm plasmon polaritons (TPPs)/Fano resonance systems based on metal-dielectric-metal (MDM) waveguides are proposed. TPPs are surface electromagnetic modes formed at the interface between a metal and a one-dimensional dielectric photonic crystal (PhC). A metal plasmonic Bragg reflector (PBR) in a MDM waveguide is equivalent to a dielectric PhC, which is realized by periodic MDM waveguide width modulation and leads to the photonic bandgap. By introducing a thin Ag baffle and a PBR in MDM waveguide core, the quasi-TPPs are excited at the interface between the Ag baffle and the PBR, when the phase-matching condition is met. The proposed structure can be fabricated with focused ion beam or electron beam direct-writing lithography, avoiding complex fabrication procedures of manufacturing dielectric PhC by filling the MDM waveguide core with different dielectric materials. Furthermore, an MDM waveguide side-coupled resonator system is constructed to generate Fano resonance by placing a PBR on the side of the MDM waveguide and an Ag baffle in the waveguide core. The Fano resonance originates from the interference between a broad continuum state provided by the Ag baffle and a discrete state provided by quasi-TPPs. The sensing performance of the Fano resonance system is investigated. In this design, the open PBR structure replaces the traditional closed resonant cavity, which makes it more convenient to contact with analytes. The numerical simulations demonstrate that a high sensitivity of 1500nm/RIU and figure of merit value of 4.08×105 are achieved.

CsPbBr3 nanocrystals plasmonic distributed Bragg reflector waveguide laser

The recent development of perovskite-based lasers showcased the outstanding optical properties of the material such as high absorption coefficient and high quantum yield. The lasers were demonstrated in the form of nanowires and nanoplates, which are difficult to be integrated on a chip in the form of high-density arrays due to the difficulties in positioning them on the chip. The solution to this problem should be to use the well-known lithography process in the fabrication process of the lasers. In this work, we demonstrate several perovskite-based plasmonic lasers that were fabricated by using the lithographic in-mold patterning method that relies on the electron beam lithography process. The lasers utilized CsPbBr3 perovskite nanocrystals as the gain material and plasmonic distributed Bragg reflector grating structure as the optical feedback provider to achieve a low lasing threshold of 42.5 μJ/cm2 with a linewidth of 0.6 nm (FWHM) at room temperature. The use of the lithographic process in the fabrication of the lasers makes it possible to fabricate and integrate them on a chip in a relatively high-density manner, so that they can be used extensively in quantum optics and on-chip integrated photonics applications.

Unidirectional plasmonic Bragg reflector based on longitudinally asymmetric nanostructures**Project supported by the Shenzhen Research Foundation, China (Grant Nos. JCYJ20160608153308846, JSGG20170822093953679, and JCYJ20180507182444250), the National Key Research and Development Program of China (Grant No. 2017YFC0803506), the National Natural Science Foundation of China (Grant Nos. 61261033 and 61162007), and the Youth Innovation Promotion Association of Chinese Academy of Sciences (Grant No. 20160320).

Plasmonic Bragg reflectors are essential components in plasmonic circuits. Here we propose a novel type of plasmonic Bragg reflector, which has very high reflectance for the right-side incidence and meanwhile has extremely large absorption for the left-side incidence. This device is composed of longitudinally asymmetric nanostructures in a metal–insulator–metal waveguide. In order to efficiently analyze, design, and optimize the reflection and transmission characteristics of the proposed device, we develop a semi-analytic coupled-mode model. Results show that the reflectance extinction ratio between plasmonic modes incident from the right-side and the left-side reaches 11 dB. We expect this device with such striking unidirectional reflection performance can be used as insulators in nanoplasmonic circuits.

Tunable Multiple-Step Plasmonic Bragg Reflectors with Graphene-Based Modulated Grating.

We propose a novel plasmonic Bragg reflector (PBR) based on graphene with multiple-step silicon structure. The monolayer graphene bears locally variable optical properties by modulation of electric fields, and the periodical change of effective refractive index on graphene can be obtained by external bias voltage in the mid-infrared region. Through patterning the PBR units into multiple-step structures, we can decrease the insertion loss and suppress the rippling in transmission spectra. By introducing the defect into the multiple-step PBRs, the multiple resonance modes are formed inside the stopband by increasing the step number. This work may pave the ways for the further development of ultra-compact low-cost hyperspectral sensors in the mid-infrared region.

Characteristics of Plasmonic Bragg Reflectors with Graphene-Based Silicon Grating.

We propose a plasmonic Bragg reflector (PBR) composed of a single-layer graphene-based silicon grating and numerically study its performance. An external voltage gating has been applied to graphene to tune its optical conductivity. It is demonstrated that SPP modes on graphene exhibit a stopband around the Bragg wavelengths. By introducing a nano-cavity into the PBR, a defect resonance mode is formed inside the stopband. We further design multi-defect PBR to adjust the characteristics of transmission spectrum. In addition, through patterning the PBR unit into multi-step structure, we lower the insertion loss and suppress the rippling in transmission spectra. The finite element method (FEM) has been utilized to perform the simulation work.

A Dielectric-Thickness-Adjusting Method for Manipulating Graphene Surface Plasmon Polariton

We propose and numerically investigate a dielectric-thickness-adjusting method to manipulate the graphene surface plasmon polariton (SPP). The dispersion relationships of graphene SPP at different dielectric thickness are derived by solving the analytic equations. In addition, the SPP effective index at cutoff dielectric thickness is obtained according to different dielectric permittivity and working frequencies. As a typical application, a plasmonic Bragg reflector is designed by alternately depositing dielectric gratings along the transverse direction of the SPP propagation. The performance of the Bragg reflector is analyzed at different grating thickness, and the effective index at cutoff thickness is verified by numerical simulation. The proposed method will have important potential prospects in designing graphene-based wave trapping and slow wave devices in future.

Tunable plasmonic Bragg reflector with different graphene nanoribbon widths

We propose and numerically analyze a Bragg reflector composed of periodically arranged graphene nanoribbon waveguides with different widths. Because of the unique property of the graphene edge mode, the effective index contrast used for the reflector can be obtained by designing graphene nanoribbons with different widths without changing the dielectric substrate structure. Good band stop filtering characteristics are shown at the band gap of the transmission spectrum by numerical simulation. The performance of the proposed Bragg reflector is analyzed in terms of different parameters, such as the chemical potential, the number of periods, and the size of the unit cell. The proposed Bragg reflector will be expected to have important potential applications in the highly integrated SPP-based photonic devices.

Nonlinear Manipulation of Surface Plasmon Polaritons in Graphene-Based Bragg Reflector

Here, we take the advantage of the strong electromagnetic field of surface plasmon polaritons (SPPs) on graphene to design a tunable terahertz plasmonic Bragg reflector by the intensity of propagating SPP wave. A periodic variation in the effective mode index of SPPs in the direction of their propagation on a graphene layer, required for Bragg scattering, is created by a properly designed silicon\silicon dioxide substrate. A small change in the applied voltage between the graphene sheet and the substrate leads to a noticeable tuning of the stopband region of the reflector. Deposition of a Kerr nonlinear medium on the substrate and in the immediate vicinity of the graphene layer is employed to control the propagation characteristics of SPPs by their intensity. Our numerical simulation results via a developed nonlinear finite difference time domain method reveal that by increasing the SPP wave intensity up to 0.9 ${\bf MW}/{\bf cm}^2 $ which is below the damage threshold of graphene, a red shift in the Bragg frequency up to 220 GHz is achievable. Valuable potential applications can be envisioned for this Bragg reflector as filters and switches due to its fast and high level of tunability with applied gate voltage and the intensity of propagating SPP wave.

Actively controlled plasmonic Bragg reflector based on a graphene parallel-plate waveguide

We investigate theoretically and numerically a graphene parallel-plate waveguide structure with two alternate chemical potentials (which can be realized by alternately applying two biased voltages to graphene). A plasmonic Bragg reflector can be formed in infrared range because of the alternate effective refractive indexes of SPPs propagating along graphene sheets. By introducing a defect into the Bragg reflector, and then the defect resonance mode can be formed. Thanks to the tunable permittivity of graphene by bias voltages, the central wavelength and bandwidth of SPPs stop band, and the wavelength of the defect mode can be tuned.

Graphene-based tunable plasmonic Bragg reflector with a broad bandwidth

We propose and numerically analyze a plasmonic Bragg reflector formed in a graphene waveguide. The results show that the graphene plasmonic Bragg reflector can produce a broadband stopband that can be tuned over a wide wavelength range by a small change in the Fermi energy level of graphene. By introducing a defect into the Bragg reflector, we can achieve a Fabry-Perot-like microcavity with a quality factor of 50 for the defect resonance mode formed in the stopband. The proposed Bragg reflector could be used as a broadband ultrafast tunable integrated filter and a broadband modulator. In addition, the defect microcavity may find applications in graphene-based resonators.

Transmission characteristics of a plasmonic Bragg reflector based on a metal-embedded slot structure

A 3D plasmonic Bragg grating formed by a periodic arrangement of metal in a slot waveguide is proposed and investigated. The metal-embedded slot structure supports two different modes: the symmetric mode and the asymmetric mode. The symmetric mode is chosen to design a Bragg grating for long range propagation with compact modal size, and the Bragg grating demonstrates a wide stop-band (1520–1595 nm) with a 1‰ transmission of only 40 periods. Moreover, a defect is introduced into the Bragg grating. Simulated results show that with the tapered metal, this defect mode exhibits a narrow-band filtering effect of a relatively high Q-factor (Q = 454.7). The performance of the proposed structures is simulated by the finite-difference-time-domain method. In addition, by changing the polymer refractive index, this defect mode filtering effect can be reconfigured, which shows good potential for sensors and filters.

Plasmonic Bragg reflectors based on metal-embedded MIM structure

We propose and investigate a metalembedded metal–insulator–metal (MIM) structure plasmonic Bragg reflector (PBR) using the finite-difference time-domain (FDTD) method with PMLs (perfectly matched layers) boundary conditions. It improves the performance of conventional step profile MIM PBRs to some extent. Our numerical study reveals that the metal-embedded PBRs exhibit lower insertion loss, narrower bandgap, and reduced rippling in the transmission spectrum when compared with the step PBRs at the same normalized index contrast and transmission levels. The defect mode of the metal-embedded PBRs also exhibits higher transmission. To suppress the sidelobes in the transmission spectrum, we further smooth the end of the embedded metal, which demonstrates a better performance. Then, we find with respect to the Bragg wavelength, the longer wavelengths have a tendency to spread in the wider regions of the insulator layer; however, the shorter wavelengths have a tendency to spread in the embedded metal regions. The apodized PBRs with the embedded metal length decreasing (increasing) efficaciously suppress the ripples at the right (left) band edges. Then, we use the impedance theoretical model to explain this phenomenon. Finally, we realize a flat-top transmission band filter by connecting two apodized PBRs, and the band and center wavelength can be adjusted.

Near-field nanofocusing through a combination of plasmonic Bragg reflector and converging lens

We report the near-field nanofocusing through a plasmonic lens containing a Bragg reflector and a converging lens, which consist of semitransparent annular grooves milled into a gold film with different periods along the radial direction. By illuminating the structure with a linearly polarized light, two tightly focal spots were detected by scanning near-field optical microscope. This plasmonic lens has considerably reduced direct light transmission, making the focal spots obvious. By raising the radius of half of every groove, one single spot was obtained. Furthermore, theoretical simulations prove that the light intensity of the focal spots can be doubled through adding the Bragg reflector surrounding the converging lens.

Characteristics of plasmonic Bragg reflectors with insulator width modulated in sawtooth profiles

We present a new metal-insulator-metal (MIM)-based plasmonic Bragg reflector (PBR) design that solves the technical problems of conventional step profile MIM PBRs through the use of sawtooth profiles. Our numerical study revealed that the sawtooth PBRs exhibit lower insertion loss, narrower bandgap, and reduced rippling in the transmission spectrum when compared with the step PBRs. The defect mode of the sawtooth PBR also exhibits a higher transmission, narrower linewidth, and higher Q-factor.

Design and analysis of a surface plasmon polariton modulator using the electro-optic effect

A metal-insulator-metal (MIM) waveguide with a periodic change of dielectric materials in its insulator layer is proposed to create a voltage-controlled one-dimensional plasmonic Bragg reflector. KNbO(3) is used as one portion of the dielectric material so that the refractive index of the insulator layer can be varied by an external applied voltage. In our analysis, the transfer matrix method is employed to design and optimize the transmission spectra of the MIM waveguide Bragg reflector. The reflectivity at the on (bias voltage of 65 V) and off (bias voltage of -65 V) states is shown to be >74% and <7%, respectively, for the operating wavelength of 1.5 mum. The results are also verified by the finite-difference time-domain method. Furthermore, the modulation response of the MIM waveguide is studied. For the device area and average insulator layer thickness to be 10 mumx40 mum and 615 nm, respectively, it is found that the turn-on reflectivity of the waveguide is only slightly reduced to 69% at the -3 dB point in comparison with 75% for its steady-state performance.

A low-loss metal-insulator-metal plasmonic bragg reflector

In this paper, we present a low-loss plasmonic Bragg reflector structure with high light-confinement. We show that periodic changes in the dielectric materials of the metal-insulator-metal waveguides can be utilized to design efficient subwavelength Bragg reflectors and micro-cavities. FDTD simulation results of the designed Bragg reflector using realistic material parameters justify that the transfer matrix calculations are adequate for the design purposes.