Plasmonic Band Gap Research Articles

Plasmonic band-gap filter by using tapered waveguide

Plasmonic band-gap filter by using tapered waveguide

Broad-spectrum light absorption and efficient charge transfer in plasmonic PbS/Cu2-xS/CdS polyhedral heterojunction for improved photothermal/photocatalytic performance

The multidimensional structure, abilities of light-harvesting and charge-transfer, photothermal conversion of semiconductor heterojunction are very important characters for high-performance photocatalysis. However, it remains a challenge to rationally design heterojunction photocatalyst that meets the demand. Herein, plasmonic PbS/Cu2-xS/CdS polyhedral heterojunctions, which Cu2-xS and CdS nanoparticles are deposited on the 24-faceted PbS, are prepared for efficient photothermal-assisted photocatalysis for the first time. Owing to the synergistic effect of plasmonic resonance and bandgap excitations, the ternary hybrids display excellent light absorption ranging from ultraviolet to near-infrared (NIR) region. The unique polyhedral structure and rough surface of the hybrids generate a large surface area, offering abundant active sites for catalytic reaction. Importantly, the different work functions and appropriate band arrangements of the three semiconductors provide a double Z-scheme band structure, which leads to efficient charge transfer and separation driven by built-in electric fields. Moreover, the fast hot electron injection from plasmonic Cu2-xS to PbS and CdS also provides energetic electrons for photocatalysis. As a result, the polyhedral hybrids display efficient photocatalytic H2 activity (24.3 times of pure CdS) under visible light irradiation and a high apparent quantum efficiency under NIR light irradiation. Furthermore, the ternary hybrids show excellent photothermal conversion and photothermal-assisted photocatalysis due to plasmonic heating and nonradiative relaxation. The temperature-dependent photocatalytic tests indicate that the photothermal effect has about 35% enhancement on photocatalysis. This work provides an inspiration to develop new functional materials for the field of solar-driven energy conversion.

Tunable On‐Chip Terahertz Isolator Based on Nonreciprocal Transverse Edge Spin State of Asymmetric Magneto‐Plasmonic Waveguide

AbstractTerahertz (THz) isolators are crucial to one‐way transmission in THz application systems. However, it is still challenging to implement THz isolators with integrated structure and tunable characteristics. Herein, a THz isolator based on a magneto‐plasmonic waveguide (MPWG) has been demonstrated. In this InSb‐air‐metal asymmetric waveguide, the plasmonic band gap is excited and thermally tuned by surface plasmons. Moreover, under weak external magnetic fields (EMFs), magneto‐plasmons excite nonreciprocal transverse edge spin states, which break the time‐reversal symmetry of the MPWG and lead to a pair of isolation bandgaps that only allow one‐way transmission. The experimental results of both broadband time‐domain spectrum system and single‐frequency system confirm this isolator can achieve more than 20 dB isolation and only 4 dB insertion loss. More importantly, its isolation can be actively modulated by EMFs from 0 to 0.23 T, and also realize broadband tuning of over 1 THz frequency range by controlling the temperature from 80 to 280 K. This topological edge band structure of transverse spin states and its active manipulation mechanism are helpful to deepen the understanding of nonreciprocal plasmon propagation, and this broadband tunable THz on‐chip isolator is significant to urgently apply in strong THz sources and ultra‐sensitive detectors.

Visible light driven robust photocatalytic activity in vanadium-doped ZnO/SnS core-shell nanocomposites for decolorization of MB dye towards wastewater treatment

Vanadium metal oxide (V2O5) doped ZnO/SnS core-shell nanocomposites are fabricated by hydrothermal method at 2200C for 10 hours. All synthesized Nanocomposites were optically, structurally, morphologically and magnetically examined by different techniques such as UV-DRS, XRD, TEM, electron paramagnetic resonance (EPR), XPS and PL techniques. The photocatalytic activity (PCA) was studied for methylene blue (MB) model pollutant dye under 300 W visible light illumination. XRD peaks confirmed the mixed phase abundance with hexagonal ZnO and orthorhombic SnS. TEM micrographs inferred uneven surface with more agglomeration. XPS conclude V4+ oxidation state for vanadyl atoms along with constitutes elements. The UV-DRS inferred red shift in bandgap. EPR argued that vanadyl ions perform tetragonal compression distorted octahedral site symmetry. PL study described quantum confinement effect. The achieved MB dye degradation under visible light illumination is more than 96% in 2 hours. Enhanced PCA argued owing to plasmonic effect and tunable energy bandgap nature of heterogeneous semiconducting materials.

Plasmonic effect and bandgap tailoring of Ag/Ag2S doped on ZnO nanocomposites for enhanced visible-light photocatalysis

Ag/Ag2S-ZnO nanocomposites were prepared via a simple hydrothermal process followed by a plasmonic Ag+ reduction through a photo-deposition method. Ag2S was introduced to narrow the overall composite bandgap and activate the surface plasmon resonance (SPR) effect of the Ag+ cation present. The physicochemical properties of the as-synthesised catalysts were characterised by X-ray diffraction (XRD), scanning and transmission electron microscopies (SEM and TEM), Brunauer-Emmett-Teller (BET) analysis. Fourier-transform infrared spectroscopy (FTIR), Ultraviolet diffuse reflectance spectroscopy (UV–vis DRS), photoluminescence emission spectra (PL) and X-ray photoelectron spectroscopy (XPS) was conducted to investigate the photo-absorption and emission spectra of the nanocomposites. The degradation efficiency of all the synthesised catalysts (ZnO, Ag2S, Ag/ZnO and Ag2S/ZnO) prior to the final product, Ag/Ag2S/ZnO was tested and compared. Results showed that the ternary Ag/Ag2S/ZnO achieved a 98 % phenol removal compared to 50 %, 11 %, 64 % and 93 % for ZnO, Ag2S, Ag/ZnO and binary Ag2S/ZnO, respectively. The degradation kinetics followed the Langmuir-Hinshelwood model, which typically describes heterogeneous photocatalytic surface reactions. The linear fits had R2 values higher than 0.97, which confirms the degree of accuracy or statistical fitness to the kinetic model. Degradation scavenger test confirmed the holes (h+) as the main inhibitor and identified the superoxide O2•¯ radical as the main active specie responsible for the degradation. Total organic carbon analysis using the ternary Ag/Ag2S-ZnO catalyst only achieved a 74% phenol mineralization after 24 h of photocatalysis. Recyclability tests showed good phenol removal stability of Ag/Ag2S-ZnO at 41 % after five recycle runs. Hence, a synergistic degradation mechanism responsible for the efficient photo-degradation performance was proposed.

Optical biosensors using plasmonic and photonic crystal band-gap structures for the detection of basal cell cancer

One of the most interesting topics in bio-optics is measuring the refractive index of tissues. Accordingly, two novel optical biosensor configurations for cancer cell detections have been proposed in this paper. These structures are composed of one-dimensional photonic crystal (PC) lattices coupled to two metal–insulator–metal (MIM) plasmonic waveguides. Also, the tapering method is used to improve the matching between the MIM plasmonic waveguides and PC structure in the second proposed topology. The PC lattices at the central part of the structures generate photonic bandgaps (PBGs) with sharp edges in the transmission spectra of the biosensors. These sharp edges are suitable candidates for sensing applications. On the other hand, the long distance between two PBG edges causes that when the low PBG edge is used for sensing mechanism, it does not have an overlapping with the high PBG edge by changing the refractive index of the analyte. Therefore, the proposed biosensors can be used for a wide wavelength range. The maximum obtained sensitivities and FOM values of the designed biosensors are equal to 718.6, 714.3 nm/RIU, and 156.217, 60.1 RIU−1, respectively. The metal and insulator materials which are used in the designed structures are silver, air, and GaAs, respectively. The finite-difference time-domain (FDTD) method is used for the numerical investigation of the proposed structures. Furthermore, the initial structure of the proposed biosensors is analyzed using the transmission line method to verify the FDTD simulations. The attractive and simple topologies of the proposed biosensors and their high sensitivities make them suitable candidates for biosensing applications.

Wavelength-dependent nonlinear absorption of gold nanobipyramids with large saturable modulation depth

The light–matter interaction in noble metal nanoparticles is intrinsically determined by the surface plasmon resonance (SPR) and bandgap. To determine the process of electronic transition and the limitation of energy bands on nonlinearity at different excitation wavelengths, we investigate the broadband nanosecond nonlinear absorption of gold nanobipyramids from 532 nm to 850 nm. The experimental results indicate that with the increase of the incident light intensity, gold nanobipyramids have the characteristic of transforming from saturable absorption (SA) to reverse saturable absorption (RSA) at 532 nm to 690 nm, and exhibit a single SA after 690 nm until 850 nm. Based on the wavelength-dependent response, the SA is attributed to the intraband transition of sp-band, whereas sequential two-photon absorption leads to RSA. Unlike other metal nanoparticles, gold nanobipyramids exhibited a significantly higher modulation depth up to 47%. Lastly, building on the excellent SA characteristic, we achieve pulse width compression of nanosecond laser pulses at multi-wavelengths.

Energy band structure of multistream quantum electron system

In this paper, using the quantum multistream model, we develop a method to study the electronic band structure of plasmonic excitations in streaming electron gas with arbitrary degree of degeneracy. The multifluid quantum hydrodynamic model is used to obtain N-coupled pseudoforce differential equation system from which the energy band structure of plasmonic excitations is calculated. It is shown that inevitable appearance of energy bands separated by gaps can be due to discrete velocity filaments and their electrostatic mode coupling in the electron gas. Current model also provides an alternative description of collisionless damping and phase mixing, i.e., collective scattering phenomenon within the energy band gaps due to mode coupling between wave-like and particle-like oscillations. The quantum multistream model is further generalized to include virtual streams which is used to calculate the electronic band structure of one-dimensional plasmonic crystals. It is remarked that, unlike the empty lattice approximation in free electron model, energy band gaps exist in plasmon excitations due to the collective electrostatic interactions between electrons. It is also shown that the plasmonic band gap size at first Brillouin zone boundary maximizes at the reciprocal lattice vector, G, close to metallic densities. Furthermore, the electron-lattice binding and electron-phonon coupling strength effects on the electronic band structure are discussed. It is remarked that inevitable formation of energy band structure is a general characteristics of various electromagnetically and gravitationally coupled quantum multistream systems.

Study of plasmonic bandgap by optimization of geometrical parameters of metallic grating devices

The optimization and comparison of different geometrical parameters of metallic nano-gratings that affect the plasmonic bandgap significantly is being reported in the present research work. The optimization and comparison of geometrical parameters of metallic nano-grating has been important subject to the development of plasmonic devices and systems. This study presents the effect of far field reflection spectra of 1D buried and exposed gratings designed in COMSOL on the plasmonic bandgap using the finite element analysis. In this work, we demonstrated various new trends of PBG energy in gold (Au) nano-gratings by controlling slit width, periodicity and film thickness. The variation of incidence angle of p-polarized light is also studied for the angular dependence, which shows corresponding opening of PBG in the dispersion curves.

Plasmonic TiO2/Al@ZnO nanocomposite-based novel dye-sensitized solar cell with 11.4% power conversion efficiency

We report the synthesis, characterization and effect of infusion of aluminium plasmons (Al3+) into titania/zinc oxide (TiO2/Al@ZnO or TAZ) nanocomposite photoanode on the efficiency of novel plasmonic Dye-Sensitized Solar Cells (DSSCs). On comparison with bare titania/zinc oxide (TiO2/ZnO or TZ) nanocomposite, plasmonic TAZ exhibits a negligible change in crystallographic and morphological properties, whereas the photoconduction and light harvesting capability are significantly enhanced. Prompted by surface plasmon modes, TAZ exhibits strong emissions in the blue-yellow region of visible spectrum, as observed with photoluminescence (PL) study. Owing to plasmon-induced PL, light enhancement takes place within the photoanode. Therefore, photoconductivity of TAZ is observed to be 4 folds higher than that of TZ. The narrowing of bandgap upon Al3+ infusion is confirmed by Kubelka-Munk plot and cyclic voltammetry. Specifically, we also focused on the architecture of novel DSSCs with silver counter electrode via a facile preparation method for the first time. Blending the advantages of nanocomposite, surface plasmon resonance and bandgap narrowing, the multifaceted TAZ features with short-circuit current density JSC ~ 33 mA cm−2, open-circuit voltage VOC ~ 0.41 V, Fill-factor FF ~ 0.81 and a remarkable efficiency of 11.4%, which opens up the opportunity to optimize and design a new class of next generation plasmonic DSSCs.

Surface modes in plasmonic stubbed structures

We present an analytical and numerical study about the existence of surface localized modes, known as Tamm states, in a one-dimensional (1D) comb-like plasmonic band gap structure. Surface plasmon polaritons (SPPs) waveguides with coupled resonators have been widely studied in recent years, because of their potential applications in highly integrated optical circuits. The system studied here is composed of an infinite 1D waveguide, along which stubs of length d1 are grafted periodically with spacing period d2. The analytical study has been performed by means of the Green’s function method which allows the calculation of the dispersion relations of the bulk, surface states of the plasmonic structure and the transmission coefficient. The band structure, as well as the transmission spectrum exhibit passbands separated by stopbands. The surface modes inside the gaps of the semi-infinite structure can be introduced by a defect at its surface. The analytical results are confirmed by numerical simulation using finite element method via Comsol Multiphysics software. These structures can be used to realize highly sensitive plasmonic sensors.

Effect of dynamic ions on band structure of plasmon excitations

In this paper, we develop a new method to study the plasmon energy band structure in multispecies plasmas. Using this method, we investigate a plasmon dispersion band structure of various quasineutral plasma systems with arbitrary degree of electron degeneracy. The linearized Schrödinger–Poisson model is used to derive an appropriate coupled pseudoforce system from which the energy dispersion structure is calculated. It is shown that the introduction of ion dynamics, as opposed to static ion assumption in the jellium model with a wide plasmon bandgap, can significantly modify the plasmon dispersion character leading to a new low-level energy band caused by the electron–ion interactions. The investigation on the effect of ion charge-state and chemical potential of electrons on the plasmonic band structure indicates some interesting features and reveals the fundamental role played by ions in the phonon assisted plasmon excitations in different kinds of plasma systems. Moreover, our study confirms that ion charge screening has a significant impact on plasmon excitations in multispecies plasmas. The plasmon band structure in pair-ion or electron–positron plasmas indicates the unique role of positive charges on collective excitations. Current research helps us to better understand the underlying mechanisms of collective interactions in charged environment and the important role played by heavy charged particles on elementary plasmon excitations, which have important applications in plasmonic devices. The method developed in this research may also be extended to study magnetized quantum plasmas as well as to investigate surface plasmon–polariton interactions in nanometallic structures.

Terahertz Spoof Surface Plasmonic Logic Gates.

SummaryLogic gates are important components in integrated photonic circuitry. Here, a series of logic gates to achieve fundamental logic operations based on linear interference in spoof surface plasmon polariton waveguides are demonstrated at terahertz frequencies. A metasurface-based plasmonic source is adopted to couple free-space terahertz radiation into surface waves, followed by a funnel-shaped metasurface to efficiently couple the surface waves to the waveguides built on a domino structure. A single Mach-Zehnder waveguide interferometer can work as logic gates for four logic functions: AND, NOT, OR, and XOR. By cascading two such interferometers, NAND and NOR operations can also be achieved. Experimental investigations are supported by numerical simulations, and good agreement is obtained. The logic gates have compact sizes and high intensity contrasts for the output “1” and “0” states. More complicated functions can be envisioned and will be of great value for future terahertz integrated computing.

Extreme field confinement in zigzag plasmonic crystals

We propose extreme field confinement in a zigzag plasmonic crystal that can produce a wide plasmonic bandgap near the visible frequency range. By applying a periodic zigzag structure to a metal–insulator–metal plasmonic waveguide, the lowest three plasmonic crystal bands are flattened, creating a high-quality broadband plasmonic mirror over a wavelength range of 526–909 nm. Utilizing zigzag plasmonic crystals in a three-dimensional tapered metal–insulator–metal plasmonic cavity, extreme field confinement with a modal volume of less than 0.00005 λ3 can be achieved even at resonances over a wide frequency range. In addition, by selecting the number of zigzag periods in the plasmonic crystal, critical coupling between the cavity and the waveguide can be achieved, thereby maximizing the field intensity with an enhancement factor of 105 or more. We believe that zigzag plasmonic crystals will provide a powerful platform for implementing broadband on-chip plasmonic devices.

Terahertz composite plasmonic slabs based on double-layer metallic gratings.

One composite plasmonic slab with a broad bandgap (40%) is experimentally and numerically demonstrated in the terahertz (THz) region. The composite slab consists of double-layer metallic gratings and a dielectric film, which supports two resonant modes. Electric field vectors and charge distributions proved that the low-frequency resonant mode originates from the symmetric plasmonic mode, while the high-frequency resonant mode is induced by the hybrid mode of plasmonic and dielectric modes. Compared with the double-layer metallic grating, the inserted dielectric film significantly enhances the transmission of the transverse magnetic (TM) waves and induces Fano resonances. The near-field coupling between metal gratings and dielectric film can be manipulated by changing the thickness and the refractive index of dielectric films. We further demonstrated that the plasmonic bandgap can be manipulated by tuning the grating width. These results suggest that this composite plasmonic slab is promising in terahertz integrated components development such as a filter, polarizer, or sensor.

Emergence of point defect states in a plasmonic crystal

Plasmonic crystals are well known to have band structure including a band gap, enabling the control of surface plasmon propagation and confinement. The band dispersion relation of bulk crystals has been generally measured by momentum-resolved spectroscopy using far field optical techniques while the defects introduced in the crystals have separately been investigated by near field imaging techniques so far. Particularly, defect related energy levels introduced in the plasmonic band gap have not been observed experimentally. In order to investigate such a localized mode, we performed electron energy-loss spectroscopy (EELS) on a point defect introduced in a plasmonic crystal made up of flat cylinders protruding out of a metal film and arranged on a triangular lattice. The energy level of the defect mode was observed to lie within the full band-gap energy range. This was confirmed by a momentum-resolved EELS measurement of the band gap performed on the same plasmonic crystal. Furthermore, we experimentally and theoretically investigated the emergence of the defect states by starting with a corral of flat cylinders protrusions and adding sequentially additional shells of those in order to eventually form a plasmonic band-gap crystal encompassing a single point defect. It is demonstrated that a defectlike state already forms with a crystal made up of only two shells.

Adjustable Plasmonic Bandgap in One-Dimensional Nanograting Based on Localized and Propagating Surface Plasmons

Adjustable Plasmonic Bandgap in One-Dimensional Nanograting Based on Localized and Propagating Surface Plasmons

Surface Plasmon Resonance Based Titanium Coated Biosensor for Cancer Cell Detection

A new optimized bowl-shaped mono-core surface plasmon resonance based cancer sensor is proposed for the rapid detection of different types of cancer affected cell. By considering the refractive index of each individual cancer contaminated cell with respect to their normal cell, some major optical parameters variation are observed. Moreover, the cancerous cell concentration is considered at 80% in liquid form and the detection method is finite element method with 2 100 390 mesh elements. The variation of spectrum shift is obtained by plasmonic band gap between the silica and cancer cell part which is separated by a thin (35 nm) titanium film coating. The proposed sensor depicts a high birefringence of 0.04 with a maximum coupling length of 66 $\mu$ m. However, the proposed structure provides an optimum wavelength sensitivity level between about 10 000 nm/RIU and 17 500 with a resolution of the sensor between 1.5 × 10−2 and 9.33 × 10−3 RIU. Also, the transmittance variance of the cancerous cell ranges from almost 3300 to 6100 dB/RIU and the amplitude sensitivity ranges nearly between −340 and −420 RIU $^{-1}$ for different cancer cells in major polarization mode with the maximum detection limit of 0.025. Besides, the overall sensitivity performance is measured with respect to their normal cells which can be better than any other prior structures that have already proposed.

Role of higher order plasmonic modes in one-dimensional nanogratings

By theoretically investigating the optical behavior of one-dimensional gold nanogratings using Fourier Modal Method, we have shown that both integer and non-integer multiples of surface plasmon polariton wavelengths should be taken into consideration in special optical contrast ratio for highly sensitive sensing. The emergence of higher modes is the key factor for the formation of observed plasmonic band gap. Through considering the significant role of grating period and thickness respectively in horizontal and vertical surface resonances, it was demonstrated that for gold thicknesses below 100 nm, the dominant phenomenon is horizontal surface resonances while for increased thicknesses both horizontal and vertical surface resonances mediate. The transmission minima are insensitive to the grating thickness, which confirms that their origins are not vertical surface resonances. This study can open an avenue towards designing highly sensitive sensors with focus not only on the plasmonic resonance wavelength but also on its integer and non-integer multiples whose origins should be investigated in both horizontal and vertical surface resonances.

Optical characteristics of one dimensional metal-dielectric photonic band gap material

This paper describes the optical transmittance and reflection of one dimensional metal-dielectric photonic-band gap material (1D M-D PBG), which is made of different thicknesses ITO and Ag layers. It is found that structures with a unit size below 80 nm and a smaller metal fraction leads to improvement of optical transmittance. For unit sizes larger than 80 nm, the reflection at the shorter and longer wavelengths increases. This is due to the generation of a structural and plasmonic band gap. In addition, the reflection in both ranges increases and broadens by increasing Ag films thicknesses. The reflection spectrum induced by structure shifts towards longer wavelength as a result of unit size increasing and the reflection due to plasmonic band gap piles beyond to optical range. The results are very useful for optical filter of 1D M-D PBG design.