Plasmonic Subwavelength Research Articles

(Invited) Quantitative Photonic Characterization and Modelling of Light Emission, Confinement, and Enhancement in Nanostructured Materials

Optical spectroscopy and imaging provide non-destructive, contactless, and fast strategies to study the nature and technological aspects of thin films and nanostructured materials for photonic and optoelectronic applications. Furthermore, the design, engineering, and optimization of such devices requires quantitative information with high-precision standards in measuring and modelling as is the case, for example, in the determination of the optical critical dimension (OCD) in the semiconductor industry. Our application of these strategies to the study of tunable self-assembled nanolasers, hybrid photonic-plasmonic sensors, and photovoltaic devices, among many other materials and devices has led us to the development of i) new modelling strategies based on the Finite-Difference Time-Domain (FDTD) method, and ii) home-made customizable characterization strategies based on spectroscopic ellipsometry (SE). The advantages of the FDTD method include its ability to i) obtain a wide spectral response from one time-domain simulation, ii) model single non-periodic structures, and iii) flexibility and generality to include, for example, non-linear and thermal effects. On its part, SE stands out from other optical techniques because, as a self-referenced and phase sensitive strategy, it can deliver extreme precision and sensitivity and has demonstrated flexibility that has been implemented in multiple spectral ranges and operation conditions. With this in mind, our group has systematically demonstrated the synergic use of FDTD-SE strategy, with complementary strengths and close-matched precision, in a range of materials and devices. We will report the fundamentals, status of our on-going efforts, and a critical review of the perceived limitations and expected outcomes of the FDTD-SE strategy as a quantitative tool for photonic and plasmonic subwavelength structures of interest in optoelectronic, sensing, and energy applications.We gratefully acknowledge the financial support from the RGC (Projects CityU - 11210218, 11219919, 11215121 and 11310122) and ITC (Project ITS/461/18) of HKSAR, China.

Spoof surface plasmons spawning pencil-beam antennas of subwavelength profile.

Slot-array antennas based on metallic waveguides have been widely used to generate pencil-beams, attracting attention due to their design simplicity and compact size. However, current slot-array antennas possess wavelength-scale profiles, which do not align optimally with the low-profile requisites of contemporary integrated communication and radar systems. Here, we propose a low-profile slot-array antenna designed specifically for the pencil-beam generation. Constructed with the two-dimensional-array (2D-array) slots situated on a sub-wavelength domino plasmon waveguide, the pencil-beam is generated with a peak gain of up to 21.6 dBi. Moreover, the generated pencil-beam allows for a wide scanning range of over 73.6° by adjusting the operating frequency from 45 to 65 GHz. Our research shows great potential for enhancing millimeter-wave radar capabilities and advancing communication systems.

A subwavelength graphene surface plasmon polariton-based decoder

A subwavelength graphene surface plasmon polariton-based decoder

Design and simulation of subwavelength plasmonic D flip-flop with state remaining feature

Design and simulation of subwavelength plasmonic D flip-flop with state remaining feature

Comprehensive field pattern analysis for tailoring of reflectance in a hybrid subwavelength plasmonic grating refractive index sensor and its potential for noninvasive salivary glucose monitoring.

A compact hybrid two-dimensional plasmonic subwavelength grating composed of gold and semiconductor ZnS is proposed. By implementing the finite-difference time-domain numerical technique, detailed field pattern analysis and reflectance characteristics of the grating structure are comprehensively investigated, tailored, and improved. An unfamiliar phenomenon of exponential decrease in resonance wavelength with an increase in groove width is observed, validated, and empirically modeled. This confirms that the reflectance resonance dip is because of the surface plasmon resonance in the grating structure, unlike the resonance dip obtained in the diffraction grating because of the Fabry-Perot resonance. A rigorous sensitivity analysis is performed for both generalized bulk and surface analyte detection. The surface sensitivity is observed to be 100.5nm/RIU at dip 1 for 10-nm-surface analyte thickness. The bulk sensitivity for dip 1 and dip 2 was 104.3nm/RIU and 800nm/RIU, respectively. The refractive index range variation of dip 1 for the surface analyte is correlated with the refractive index of the blood by using the linear refractive index model and Gladstone-Dale law for blood. A linear regression analysis correlating blood glucose and salivary glucose with a surface analyte is used. The proposed sensor is observed to be promising for noninvasive salivary glucose monitoring with high surface sensitivity of 1.104nm/mg/dl with a compact footprint of about 0.5µm×0.2µm in x-z dimensions.

Plasmon emission by interlayer excitons in twisted bilayers: Effect of large plasmonic wavevector components

While the emission of localized plasmons is often treated in the real-space picture, working in wavevector space makes it possible to resolve the contributions of various plasmonic Fourier components. By calculating the rate of plasmon emission by interlayer excitons in twisted ${\mathrm{MoSe}}_{2}/{\mathrm{WSe}}_{2}$ van der Waals heterostructures as a function of plasmon localization, twist angle, and temperature, we show that a much wider range of exciton states can radiate into subwavelength plasmon modes than into free space. Most of the emission in this case takes place via plasmonic Fourier components that exceed the maximum photon wavevector in free space. We demonstrate that this provides an avenue for continued Purcell scaling of the emission rate with mode size. Furthermore, the scaling is well in excess of the Purcell trend for sufficiently large twist angles and strong plasmonic localization.

A low loss platform for subwavelength terahertz graphene plasmon propagation

Terahertz (THz) waves are assumed to be the candidate for next generation wireless communication owing to the broad frequency band and high channel capability. In order to accommodate the high data transfer rate, THz communication devices with high efficiency are in urgent need. Besides, the seamless connections between optical fibers and THz devices are also of great importance for on-chip THz propagating and manipulation. Here we propose a semiconductor nanowire (GaAs) based hybrid graphene plasmon waveguide, and realize a platform supporting graphene plasmon modes with low loss and strong optical energy confinement. Investigation on the influence of modal properties on nanowire geometrical shapes and orientations, key geometric parameter, Fermi energy level and carrier mobility of graphene revealed an achievement of a good balance between mode confinement and propagation loss. Higher than 50% modulation in the propagation length is reported. The proposed structure is capable of realizing future THz communication components and tunable devices.

Theoretical Challenges in Polaritonic Chemistry.

Polaritonic chemistry exploits strong light–matter coupling between molecules and confined electromagnetic field modes to enable new chemical reactivities. In systems displaying this functionality, the choice of the cavity determines both the confinement of the electromagnetic field and the number of molecules that are involved in the process. While in wavelength-scale optical cavities the light–matter interaction is ruled by collective effects, plasmonic subwavelength nanocavities allow even single molecules to reach strong coupling. Due to these very distinct situations, a multiscale theoretical toolbox is then required to explore the rich phenomenology of polaritonic chemistry. Within this framework, each component of the system (molecules and electromagnetic modes) needs to be treated in sufficient detail to obtain reliable results. Starting from the very general aspects of light–molecule interactions in typical experimental setups, we underline the basic concepts that should be taken into account when operating in this new area of research. Building on these considerations, we then provide a map of the theoretical tools already available to tackle chemical applications of molecular polaritons at different scales. Throughout the discussion, we draw attention to both the successes and the challenges still ahead in the theoretical description of polaritonic chemistry.

Spatial mapping of optical modes in plasmonic nanoantenna by scanning tunneling microscopy

Using of inelastic electron tunnelling is very promising approach to study of subwavelength photons and plasmons sources. Such sources are very important for improving of on-chip data processing. One of the ways for development of efficient and compact optical electrically-driven sources is using of nanoantenna placed into the tunnel junction. In this work, singe optical nanoantenna was investigated under ultra-high vacuum and ambient conditions. Photon maps of nanoantenna excited under scanning tunnel microscope tip was observed and the obtained results was compared with the theoretical predictions of electromagnetic near-field distribution.

Structured plasmonic beam: in-plane manipulation of light at the nanoscale

The brief review on recent approaches on the formation of a new class of subwavelength scale localized structured surface plasmon polaritons (SPP) beams is discussed. For the Janus-like particle (including the geometrically symmetric particles with different dielectrics) the morphology of the field localization area and its properties depends on the particle shape and material. Plasmonic hook (PH) beam does not propagate along straight line but instead follow curved self-bending trajectory. Wavefront analysis behind of such symmetric and asymmetric mesoscale rectangle structure reveals that the unequal phase of the transmitted plane wave results in the irregularly concave deformation of the wavefront inside the dielectric which later leads to creation of the PH. Such dielectric structures placed on metal film enable the realization of new ultracompact wavelength-selective and wavelength-scaled in-plane nanophotonic components. SPP have potential to overcome the constrains on the speed of modern digital integrated devices limitation due to the metallic interconnects and increase the operating speed of future digital circuits.

Direct comparison with terahertz metamaterials and surface-enhanced Raman scattering in a molecular-specific sensing performance.

Signal enhancement of spectroscopies including terahertz time-domain spectroscopy (THz-TDS) and surface-enhanced Raman scattering (SERS) is a critical issue for effective molecular detection and identification. In this study, the sensing performance between THz-TDS and SERS individually accompanied by the proper plasmonic subwavelength structures was compared. For the precisely quantitative study on the optical properties of rhodamine 6G (R6G) dyes, SERS incorporates with the non-linearly enhanced Raman emissions at the molecular characteristic peaks while THz-TDS refers to the transmittance change and the shift of the spectral resonance. The local molecular density-dependent trade-off relationship between limit-of-detection and quenching was observed from both measurements. The specificity for two samples, R6G and methylene blue, is determined by the discriminations in spectral features such as the intensity ratio of assigned peaks in SERS and transmittance difference in THz-TDS. The comprehension of field enhancement by the specific nanostructures was supported by the finite-element method-based numerical computations. As a result, both spectroscopic techniques with the well-tailored nanostructures show great potential for highly sensitive, reproducible, label-free, and cost-effective diagnosis tools in the biomedical fields.

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.

A Review of Plasmonic Photonic Crystal Fiber

"Nanophotonics technology is the study of the confinement of electromagnetic fields on a subwavelength scale and surface plasmons plays a major role in advancing this arising field. They can take various forms, ranging from freely propagating electron density waves along metal surfaces to localized electron oscillations on metal nanoparticles. The interaction between the free electrons' oscillations and electromagnetic waves of light gives the surface plasmons their appealing characteristics. Their ability of light confinement and propagation through subwavelength structures grants the construction of photonic devices with minimum size, hence the characterize and design of plasmonic devices can be resolved using numerical simulation. This review starts with general introduction about Plasmonic photonic crystal fiber. This is followed by a historical background and literature review of Plasmonic PCF. Some types of designs are illustrated. Finally, the applications of Plasmonic PCF are mentioned dependent on the types of design including the circular plasmonic photonic crystal fiber technique.

Plasmonic ultrathin metal grid electrode induced optical outcoupling enhancement in flexible organic light-emitting device

We propose a plasmonic ultrathin metal grid electrode as a promising light outcoupling strategy in a flexible organic light-emitting device (OLED). The plasmonic ultrathin metal grid electrode consists of an ultrathin Au grid film incorporating with plasmonic subwavelength corrugations, which was fabricated by lithography and nanoimprint technologies, demonstrates excellent optical and electrical properties as the substitute of ITO transparent electrode in flexible OLED. The transmittance of the plasmonic ultrathin Au grid electrode is 88.5% at 550 nm and the sheet resistance is about 36 Ω/sq. The periodic subwavelength corrugations of the plasmonic ultrathin Au grid electrode excite the surface plasmon-polariton (SPP) resonance which is an adverse optical mode with power loss in OLED, and the photons trapped in SPP mode are effectively outcoupled and extracted by the plasmonic corrugations. Compared to the ITO-based device, OLED based on plasmonic ultrathin Au grid electrode realizes a desirable enhancement of 60.3% in current efficiency. In addition, the flexible OLED based on the proposed electrode also exhibits decent mechanical robustness and flexibility with stable electroluminescence performances.

Finite-size effects in wave transmission through plasmonic crystals: A tale of two scales

We study optical coefficients that characterize wave propagation through layered structures called plasmonic crystals. These consist of a finite number of stacked metallic sheets embedded in dielectric hosts with a subwavelength spacing. By adjustment of the frequency, spacing, number as well as geometry of the layers, these structures may exhibit appealing transmission properties in a range of frequencies from the terahertz to the mid-infrared regime. Our approach uses a blend of analytical and numerical methods for the distinct geometries with infinite, translation invariant, flat sheets and nanoribbons. We describe the transmission of plane waves through a plasmonic crystal in comparison to an effective dielectric slab of equal total thickness that emerges from homogenization, in the limit of zero interlayer spacing. We demonstrate numerically that the replacement of the discrete plasmonic crystal by its homogenized counterpart can accurately capture a transmission coefficient akin to the extinction spectrum, even for a relatively small number of layers. We point out the role of a geometry-dependent corrector field, which expresses the effect of subwavelength surface plasmons. In particular, by use of the corrector we describe lateral resonances inherent to the nanoribbon geometry.

Unveiling breathing plasmon modes in aluminum metal–insulator–metal cavities by cathodoluminescence

Aluminum (Al) processes excellent plasmon response from the ultraviolet (UV) to visible spectrum. Understanding of the deep sub-wavelength plasmon response of Al nanostructures is essential for the Al-based plasmon device design, such as UV surface-enhanced resonance Raman scattering and emission control of emitters. In this work, by using cathodoluminescence, the plasmonic properties of Al metal–insulator–metal (MIM) disk nanocavities are investigated. The resonant breathing modes rather than edge modes are resolved by the CL spectra and real-space mode patterns, which are in good agreement with the electromagnetic calculations. Moreover, the dispersion behavior of plasmon modes of the MIM cavity can be traced back to the propagating plasmon modes in an Al MIM slab, which shows that the electromagnetic fields are strongly confined in the cavities. Furthermore, a mode volume as small as 1.1 × 105 nm3 is obtained for the 240 nm diameter cavity, demonstrating these MIM resonators to be ideal candidates for studies of strong plasmon-matter interactions.

Design and analysis of an electrically pumped GaAs quantum dot plasmonic nanolaser

Recently, there is increasing attention to electrically pumped room temperature sub-wavelength plasmon sources because of their various potential applications mainly in the integrated plasmonic field. In this paper, a GaAs/AlGaAs quantum-dot based waveguide integrated plasmonic nanolaser is introduced and theoretically investigated. Using a semi-classical rate equation model, the performance of our nanolaser is studied and its characteristics are presented in detail. The proposed nanolaser has a tiny footprint of 0.068 μm2, room temperature operating condition and monolithic process while having remarkable output performance. The new structure generates 1 mW output power with a 23.6 mA injection current. The threshold pump current of this device is calculated to be about 4.7 mA and at this current level output power is about 198 μW. Also, the proposed device provides a wide spectral bandwidth of about 541 GHz in the threshold pumping rate. The compact cavity design provides significant mode confinement and considerable overlap between the lasing mode and the gain medium. This structure provides a Q factor about 30 and Purcell factor about 66.

Low-loss and broadband silicon mode filter using cascaded plasmonic BSWGs for on-chip mode division multiplexing.

A mode splitter is a key device to eliminate undesired modes but allow desired modes go through for an on-chip mode-division multiplexing (MDM) system. Here, we propose a silicon high-order mode (HOM) pass filter based on the cascaded plasmonic bridged subwavelength gratings (BSWGs). A metal bridge is introduced to generate a plasmonic hybrid mode, which has a significant influence on the fundamental mode but a neglected impact on the first-order mode. A silicon HOM-pass filter for handling the TM0 and TM1 modes is optimized by using the 3D full-vectorial finite difference time domain (3D-FV-FDTD) method. The numerically simulated results indicate that the optimized mode filter is with a low loss of 0.63 dB and a mode extinction ratio (ER) of 26.4 dB based on 4-cascaded plasmonic BSWGs. The 3 dB bandwidth is over 493 nm from 1222 nm to 1715 nm. With the mode ER > 15.0 dB, a broad bandwidth of 150 nm can be achieved. The performance of the proposed mode filter is tolerant to the width error of ± 50 nm. The proposed silicon HOM-pass filter can be utilized in on-chip MDM systems for mode controlling.

Ultralow-threshold, continuous-wave upconverting lasing from subwavelength plasmons.

Miniaturized lasers are an emerging platform for generating coherent light for quantum photonics, in vivo cellular imaging, solid-state lighting and fast three-dimensional sensing in smartphones1-3. Continuous-wave lasing at room temperature is critical for integration with opto-electronic devices and optimal modulation of optical interactions4,5. Plasmonic nanocavities integrated with gain can generate coherent light at subwavelength scales6-9, beyond the diffraction limit that constrains mode volumes in dielectric cavities such as semiconducting nanowires10,11. However, insufficient gain with respect to losses and thermal instabilities in nanocavities has limited all nanoscale lasers to pulsed pump sources and/or low-temperature operation6-9,12-15. Here, we show continuous-wave upconverting lasing at room temperature with record-low thresholds and high photostability from subwavelength plasmons. We achieve selective, single-mode lasing from Yb3+/Er3+-co-doped upconverting nanoparticles conformally coated on Ag nanopillar arrays that support a single, sharp lattice plasmon cavity mode and greater than wavelength λ/20 field confinement in the vertical dimension. The intense electromagnetic near-fields localized in the vicinity of the nanopillars result in a threshold of 70 W cm-2, orders of magnitude lower than other small lasers. Our plasmon-nanoarray upconverting lasers provide directional, ultra-stable output at visible frequencies under near-infrared pumping, even after six hours of constant operation, which offers prospects in previously unrealizable applications of coherent nanoscale light.

Combination of Surface Plasmon Polaritons and Subwavelength Grating for Polarization Beam Splitting

A polarization beam splitter (PBS) based on the plasmonic subwavelength grating (PSWG) is proposed and investigated. The PBS is composed by a directional coupler with a PSWG as the coupling region, which offers additional freedom for index tailoring. The mode properties are strongly modified by the unique structure of the PSWG resulting in selective coupling with the two polarization states in the neighbor waveguides. The calculations show that the insertion loss of PBS is less than 1 dB, and the extinction ratios of transverse electric and transverse magnetic polarizations are as high as 27 dB and 30 dB respectively with nearly 4-μm coupling region length. In addition, the fabrication tolerance of the device is investigated in detail.