Plasmonic Bragg Reflectors Research Articles

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 channeled phenomena in graphene-based plasmonic Bragg reflectors

Plasmonic Bragg reflectors based on graphene with multiple channeled phenomena are proposed and investigated numerically. As a mid-infrared waveguide, the monolayer graphene exhibits locally variable optical properties through the modulation of electric fields. The periodical change of the effective refractive index (ERI) on graphene can be determined by applying external gate voltage. When we introduce an unmatched configuration or gate voltage, periodicity is disrupted, and a defect resonance mode is generated. At this point, the structure can be regard as a Fabry-Perot cavity. Accordingly, multiple-channel effects can be achieved by introducing cascaded multiple defects or including double symmetrical Fabry-Perot structures. This design shows applications potential in the graphene-based optoelectronic devices, particularly in the development of low-cost hyperspectral imaging sensors in mid-infrared region.

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 Graphene-Based Bandwidth-Tunable Mid-Infrared Ultra-Broadband Plasmonic Filter

The closely spaced pair of parallel graphene sheets separated by uniform dielectric gratings is proposed and investigated numerically via the finite-difference time-domain (FDTD) method. This simple structure working as plasmonic Bragg reflectors can produce an original ultra-broadband band-stop filtering effect in the mid-infrared region. The transmission spectrum is tuned dynamically not only via varying the period of the grating but also by a small change in the chemical potential of graphene without re-fabricating new structures. In addition, the bandwidth of the stopband can also be engineered by changing the refractive index of the sandwiched dielectric grating. Simulation results are confirmed by theoretical calculations. As an application, a defect is introduced into the uniform dielectric grating, and the obvious Fabry-Perot-like resonant mode hence forms in the stopband. The proposed device without doubt can be used as highly tunable ultra-broadband band-stop filters and mid-infrared optical modulators. Our studies will play a significant role in the fabrication of ultra-compact versatile integrated circuits.

Characteristics of New Hybrid Plasmonic Bragg Reflectors Based on Sinusoidal and Triangular Gratings

In this paper, new structures for hybrid plasmonic Bragg reflectors (HPBRs), based on sinusoidal and triangular gratings, are proposed. The proposed structures exhibit better characteristics compared to previously studied rectangular HPBRs. Among these two structures, the triangular HPBR exhibits a better performance in terms of passband ripples and bandwidth than that of the sinusoidal one. Therefore, the structure of a microcavity as a defect in the proposed triangular HPBR is studied and its Q factor is compared with other types of plasmonic microcavities. It is also demonstrated that an apodization technique can reduce the size of the triangular HPBR-based microcavity and improve its Q factor.

Novel dielectric loaded plasmonic waveguide based Bragg reflector and high Q-factor micro-cavity

In this paper, a novel compact Bragg reflector and high-Q micro-cavity based on dielectric-loaded surface plasmon polariton waveguide (DLSPPW) are proposed. The Bragg reflector is formed by a finite array of periodic variations in the dielectric materials of the DLSPPW. The simulation results show that the proposed Bragg reflector has a much lower loss compared to conventional metal–insulator–metal (MIM) plasmonic Bragg reflectors at the expense of relatively loosen mode size. In addition micro-cavity formed by introducing a defect within the Bragg grating is also investigated, whose quality factor can reach as high as 774 at the wavelength of 1550nm.

CMOS-Compatible Plasmonic Bragg Reflectors Based on Cu-Dielectric-Si Structures

Cu-dielectric-Si hybrid plasmonic waveguide (HPW)-based plasmonic Bragg reflectors (PBRs) are fabricated on an SoI platform using standard CMOS technology and characterized in the 1515-1615-nm wavelength range. Optical stop-bands are experimentally observed, depending on the grating size and the number of grating periods. PBRs with 20 periods exhibit ~ -30-dB transmission within the stop-band, ~ -10-dB transmission outside the stop-band (both are normalized by the corresponding 2- μm-long straight HPW), steep band edges of ~ 0.92 dB/nm, and small ripples in the transmission spectra beyond the band edges, in agreement with those predicted from finite-difference-time-domain simulations. These favorable performances, together with ease fabrication and CMOS compatibility, make the proposed HPW-based PBRs useful for dense Si photonic integrated circuits.

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.

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.

Plasmonic Bragg Reflectors: Optimization and Application to Isolation

Plasmonic Bragg reflectors (PBRs) enhance extraordinary optical transmission (EOT) through arrays of subwavelength apertures. Here, we characterize PBRs with variable line depth, number of holes in the array, and number of lines. The experimentally determined optimum depth of 45 nm and the EOT enhancement saturation for only three PBR lines is discussed in terms of the surface plasmon polariton skin depth and scattering losses. We demonstrate the implementation of PBRs to effectively isolate two arrays of nanoholes, and to suppress the interaction between adjacent periodic array structures. As an application of this isolation, PBRs are used to isolate elements of multiple arrays in order to reduce interference, and thereby provide a more uniform spectral output from all regions of the superarray.

Plasmonic Bragg reflectors for enhanced extraordinary optical transmission through nano-hole arrays in a gold film

We demonstrate the use of plasmonic Bragg reflectors (PBRs) to enhance the extraordinary optical transmission (EOT) from an array of subwavelength apertures in a gold film. Arrays of partially milled lines and dimples are placed at the edges of an array of nano-holes in a gold film. These PBR structures, with half the pitch of the array, capture light scattered away from the array by Bragg reflection. By appropriate positioning of the PBR, the light is reflected in-phase with the EOT light and thereby doubles the EOT without shifting the wavelength of the resonant transmission peak. Furthermore, the PBR structures show strong polarization dependence that is also strongly dependent on the structure of the PBR, as explained in terms of scattering of the surface waves.