Visible-light-driven photochemistry has continued to attract heightened interest due to its capacity to efficiently harvest solar energy and its potential to solve the global energy crisis. Plasmonic nanostructures boast broadly tunable optical properties coupled with catalytically active surfaces that offer a unique opportunity for solar photochemistry. Resonant optical excitation of surface plasmons produces energetic hot electrons that can be collected to facilitate chemical reactions. This review sums up recent theoretical and experimental approaches for understanding the underlying photophysical processes in hot electron generation and discusses various electron-transfer models on both plasmonic metal nanostructures and plasmonic metal/semiconductor heterostructures. Following that are highlights of recent examples of plasmon-driven hot electron photochemical reactions within the context of both cases. The review concludes with a discussion about the remaining challenges in the field and future oppor...

Surface-Plasmon-Driven Hot Electron Photochemistry

Trapped ions are among the most promising systems for practical quantum computing (QC). The basic requirements for universal QC have all been demonstrated with ions, and quantum algorithms using few-ion-qubit systems have been implemented. We review the state of the field, covering the basics of how trapped ions are used for QC and their strengths and limitations as qubits. In addition, we discuss what is being done, and what may be required, to increase the scale of trapped ion quantum computers while mitigating decoherence and control errors. Finally, we explore the outlook for trapped-ion QC. In particular, we discuss near-term applications, considerations impacting the design of future systems of trapped ions, and experiments and demonstrations that may further inform these considerations.

Trapped-ion quantum computing: Progress and challenges

The authors demonstrate a 70 GHz modulator in a 10-μm-long two-dimensionally localized gap-plasmon waveguide system.

All-plasmonic Mach–Zehnder modulator enabling optical high-speed communication at the microscale

We investigate theoretically the effects of generation and injection of plasmonic carriers from an optically excited metal nanocrystal to a semiconductor contact or to surface molecules. The energy distributions of optically excited hot carriers are dramatically different in metal nanocrystals with large and small sizes. In large nanocrystals, the majority of hot carriers has very small excitation energies, and the excited-carrier distribution resembles the case of a plasmon wave in bulk. For nanocrystal sizes smaller than 20 nm, the carrier distribution extends to larger energies and occupies the whole region EF < e < EF + ℏω. The physical reason for the above behaviors is nonconservation of momentum in a nanocrystal. Because of the above properties, nanocrystals of small sizes are most suitable for designing opto-electronic and photosynthetic devices based on injection of plasmonic electrons and holes. For gold nanocrystals, the optimal sizes for efficient generation of hot carriers with overbarrier ene...

Theory of Photoinjection of Hot Plasmonic Carriers from Metal Nanostructures into Semiconductors and Surface Molecules

Control over the dispersion of the refractive index is essential to the performance of most modern optical systems. These range from laboratory microscopes to optical fibres and even consumer products, such as photography cameras. Conventional methods of engineering optical dispersion are based on altering material composition, but this process is time-consuming and difficult, and the resulting optical performance is often limited to a certain bandwidth. Recent advances in nanofabrication have led to high-quality metasurfaces with the potential to perform at a level comparable to their state-of-the-art refractive counterparts. In this Review, we introduce the underlying physical principles of metasurface optical elements (with a focus on metalenses) and, drawing on various works in the literature, discuss how their constituent nanostructures can be designed with a highly customizable effective index of refraction that incorporates both phase and dispersion engineering. These metasurfaces can serve as an essential component for achromatic optics with unprecedented levels of performance across a broad bandwidth or provide highly customized, engineered chromatic behaviour in instruments such as miniature aberration-corrected spectrometers. We identify some key areas in which these achromatic or dispersion-engineered metasurface optical elements could be useful and highlight some future challenges, as well as promising ways to overcome them. Flat metasurface optics provides an emerging platform for combining semiconductor foundry methods of manufacturing and assembling with nanophotonics to produce high-end and multifunctional optical elements. This Review highlights the design of metasurfaces, recent advances in the field and initial promising applications.

Flat optics with dispersion-engineered metasurfaces

We design and experimentally demonstrate an ultrashort integrated polarization splitter on silicon-on-insulator (SOI) platform. Our polarization splitter uses a hybrid plasmonic waveguide as the middle waveguide in a three-core arrangement to achieve large birefringence, allowing only transverse-magnetic (TM) polarized light to directionally couple to the cross port of the directional coupler. Finite-difference time-domain (FDTD) and eigenmode expansive (EME) calculations show that the splitter can achieve an extinction ratio of greater than 15 dB with less than 0.5 dB insertion losses. The polarization splitter was fabricated on SOI platform using standard complementary metal-oxide-semiconductor (CMOS) technology and measured at telecommunications wavelengths. Extinction ratios of 12.3 dB and 13.9 dB for the transverse-electric (TE) and TM polarizations were obtained, together with insertion losses of 2.8 dB and 6.0 dB.

CMOS compatible polarization splitter using hybrid plasmonic waveguide.

Horizontal metal/insulator/Si/insulator/metal nanoplasmonic slot waveguide (PWG), which is inserted in a conventional Si wire waveguide, is fabricated using the standard Si-CMOS technology. A thin insulator between the metal and the Si core plays a key role: it not only increases the propagation distance as the theoretical prediction, but also prevents metal diffusion and/or metal-Si reaction. Cu-PWGs with the Si core width of ~134–21 nm and ~12-nm-thick SiO2 on each side exhibit a relatively low propagation loss of ~0.37–0.63 dB/µm around the telecommunication wavelength of 1550 nm, which is ~2.6 times smaller than the Al-counterparts. A simple tapered coupler can provide an effective coupling between the PWG and the conventional Si wire waveguide. The coupling efficiency as high as ~0.1–0.4 dB per facet is measured. The PWG allows a sharp bending. The pure bending loss of a Cu-PWG direct 90° bend is measured to be ~0.6–1.0 dB. These results indicate the potential for seamless integration of various functional nanoplasmonic devices in existing Si electronic photonic integrated circuits (Si-EPICs).

Silicon-based horizontal nanoplasmonic slot waveguides for on-chip integration

An ultra-compact broadband TE-pass polarizer was proposed and demonstrated on the silicon-on-insulator (SOI) platform, using the horizontal nanoplasmonic slot waveguide (HNSW). Detailed design principle was presented, taking advantage of the distinct confinement region of the TE and TM modes in the HNSW. TM mode cut-off could be achieved when waveguide width was below 210 nm. Proof-of-concept devices were subsequently fabricated in a CMOS-compatible process. The optimized device had an active region length of 1 μm, three orders of magnitude smaller than similar device previously demonstrated on the SOI platform. More than 16 dB polarization extinction ratio was achieved across 80 nm wavelength range, with a relatively low insertion loss of 2.2dB. The compact device size and excellent broadband performance could provide a simple yet satisfactory solution to the polarization dependent performance drawback of the silicon photonics devices on the SOI platform.

CMOS compatible horizontal nanoplasmonic slot waveguides TE-pass polarizer on silicon-on-insulator platform

Integrated silicon-on-insulator waveguide-based silicide Schottky-barrier photodetectors were fabricated using low-cost standard Si complementary metal-oxide-semiconductor processing technology. The thin epitaxial NiSi2 layer formed by solid-state Ti-interlayer mediated epitaxy on the top of Si-waveguide absorbs light propagating through the waveguide effectively and exhibits excellent rectifying property on both p-Si and n-Si. NiSi2∕p-Si detectors with tapered geometry demonstrate dark current of ∼3.0nA at room temperature, responsivity of ∼4.6mA∕W at wavelengths ranging from 1520to1620nm, and 3dB bandwidth of ∼2.0GHz. The approaches for further improvement in responsivity are addressed.

Near-infrared waveguide-based nickel silicide Schottky-barrier photodetector for optical communications

In this paper, a Mach-Zehnder silicon nanoplasmonic electro-optic modulator is proposed and theoretically analyzed. It is composed of horizontal metal-SiO2-Si-metal plasmonic slot waveguides for phase shifting and ultracompact V-shape splitter/combiner to link the plasmonic slot waveguides and the conventional Si dielectric waveguides. The proposed modulator can be directly integrated into existing Si electronic photonic integrated circuits (EPICs) and be fabricated using standard Si complementary metal-oxide-semiconductor (CMOS) technology. The modulator's parameters are optimized through systematic 2-dimensional numerical simulations. For a modulator with 3-µm-long Ag-SiO2(2 nm)-Si(50 nm)-Ag phase shifter and 0.35-µm-long splitter/combiner operating at 1.55-µm wavelength, simulation shows an insertion loss of ~-8 dB, an extinction ratio of ~7.3 dB - with a switching voltage of ~5.6 V, and a bandwidth of ~500 GHz. A possible approach to reduce the switching voltage is addressed.

Shiyang Zhu

Papers

CMOS compatible polarization splitter using hybrid plasmonic waveguide.

Silicon-based horizontal nanoplasmonic slot waveguides for on-chip integration

CMOS compatible horizontal nanoplasmonic slot waveguides TE-pass polarizer on silicon-on-insulator platform

Near-infrared waveguide-based nickel silicide Schottky-barrier photodetector for optical communications

Theoretical investigation of silicon MOS-type plasmonic slot waveguide based MZI modulators.