A graphene-based broadband optical modulator.

Integrated optical modulators with high modulation speed, small footprint and large optical bandwidth are poised to be the enabling devices for on-chip optical interconnects. Semiconductor modulators have therefore been heavily researched over the past few years. However, the device footprint of silicon-based modulators is of the order of millimetres, owing to its weak electro-optical properties. Germanium and compound semiconductors, on the other hand, face the major challenge of integration with existing silicon electronics and photonics platforms. Integrating silicon modulators with high-quality-factor optical resonators increases the modulation strength, but these devices suffer from intrinsic narrow bandwidth and require sophisticated optical design; they also have stringent fabrication requirements and limited temperature tolerances. Finding a complementary metal-oxide-semiconductor (CMOS)-compatible material with adequate modulation speed and strength has therefore become a task of not only scientific interest, but also industrial importance. Here we experimentally demonstrate a broadband, high-speed, waveguide-integrated electroabsorption modulator based on monolayer graphene. By electrically tuning the Fermi level of the graphene sheet, we demonstrate modulation of the guided light at frequencies over 1 GHz, together with a broad operation spectrum that ranges from 1.35 to 1.6 µm under ambient conditions. The high modulation efficiency of graphene results in an active device area of merely 25 µm(2), which is among the smallest to date. This graphene-based optical modulation mechanism, with combined advantages of compact footprint, low operation voltage and ultrafast modulation speed across a broad range of wavelengths, can enable novel architectures for on-chip optical communications.

Maskless plasmonic lithography at 22 nm resolution.

Optical imaging and photolithography promise broad applications in nano-electronics, metrologies, and single-molecule biology. Light diffraction however sets a fundamental limit on optical resolution, and it poses a critical challenge to the down-scaling of nano-scale manufacturing. Surface plasmons have been used to circumvent the diffraction limit as they have shorter wavelengths. However, this approach has a trade-off between resolution and energy efficiency that arises from the substantial momentum mismatch. Here we report a novel multi-stage scheme that is capable of efficiently compressing the optical energy at deep sub-wavelength scales through the progressive coupling of propagating surface plasmons (PSPs) and localized surface plasmons (LSPs). Combining this with airbearing surface technology, we demonstrate a plasmonic lithography with 22 nm half-pitch resolution at scanning speeds up to 10 m/s. This low-cost scheme has the potential of higher throughput than current photolithography, and it opens a new approach towards the next generation semiconductor manufacturing.

General properties of surface modes in binary metal-dielectric metamaterials.

We present general properties of surface modes in binary metal-dielectric metamaterials. We show mechanism for surface mode formation and analyze their existence conditions for semi-infinite metamaterials in the frame of couple mode theory.

Deep subwavelength surface modes in metal-dielectric metamaterials.

We present what we believe to be the first study of deep subwavelength surface modes in binary metal-dielectric metamaterials. By employing anomalous coupling in binary periodicity, peculiar properties of band structure and eigenmode symmetry are obtained. We show that strongly confined plasmonic Tamm-like and Shockley-like surface modes can be formed at the termination of the array. We clarify the character of each surface mode and analyze its unique symmetry with the corresponding band structure.

High-Q surface-plasmon-polariton whispering-gallery microcavity.

Surface plasmon polaritons (SPPs) are electron density waves excited at the interfaces between metals and dielectric materials. Owing to their highly localized electromagnetic fields, they may be used for the transport and manipulation of photons on subwavelength scales. In particular, plasmonic resonant cavities represent an application that could exploit this field compression to create ultrasmall-mode-volume devices. A key figure of merit in this regard is the ratio of cavity quality factor, Q (related to the dissipation rate of photons confined to the cavity), to cavity mode volume, V (refs 10, 11). However, plasmonic cavity Q factors have so far been limited to values less than 100 both for visible and near-infrared wavelengths. Significantly, such values are far below the theoretically achievable Q factors for plasmonic resonant structures. Here we demonstrate a high-Q SPP whispering-gallery microcavity that is made by coating the surface of a high-Q silica microresonator with a thin layer of a noble metal. Using this structure, Q factors of 1,376 +/- 65 can be achieved in the near infrared for surface-plasmonic whispering-gallery modes at room temperature. This nearly ideal value, which is close to the theoretical metal-loss-limited Q factor, is attributed to the suppression and minimization of radiation and scattering losses that are made possible by the geometrical structure and the fabrication method. The SPP eigenmodes, as well as the dielectric eigenmodes, are confined within the whispering-gallery microcavity and accessed evanescently using a single strand of low-loss, tapered optical waveguide. This coupling scheme provides a convenient way of selectively exciting and probing confined SPP eigenmodes. Up to 49.7 per cent of input power is coupled by phase-matching control between the microcavity SPP and the tapered fibre eigenmodes.

Observation of stimulated emission of surface plasmon polaritons.

We report a direct experimental evidence of stimulated emission of surface plasmon polaritons (SPPs) at telecom wavelengths (1532 nm) with erbium doped glass as a gain medium. We observe an increase in the propagation length of signal surface plasmons when erbium ions are excited optically using pump SPP. The design, fabrication, and characterization of SPP waveguides, thin gold metal strips, embedded in erbium (Er) doped phosphate glass is presented. Such systems can be suitable as integrated devices coupling electronic and photonic data transmissions as well as SPP amplifiers and SPP lasers.

Three-dimensional optical metamaterial with a negative refractive index.

Metamaterials are artificially engineered structures that have properties, such as a negative refractive index, not attainable with naturally occurring materials. Negative-index metamaterials (NIMs) were first demonstrated for microwave frequencies, but it has been challenging to design NIMs for optical frequencies and they have so far been limited to optically thin samples because of significant fabrication challenges and strong energy dissipation in metals. Such thin structures are analogous to a monolayer of atoms, making it difficult to assign bulk properties such as the index of refraction. Negative refraction of surface plasmons was recently demonstrated but was confined to a two-dimensional waveguide. Three-dimensional (3D) optical metamaterials have come into focus recently, including the realization of negative refraction by using layered semiconductor metamaterials and a 3D magnetic metamaterial in the infrared frequencies; however, neither of these had a negative index of refraction. Here we report a 3D optical metamaterial having negative refractive index with a very high figure of merit of 3.5 (that is, low loss). This metamaterial is made of cascaded 'fishnet' structures, with a negative index existing over a broad spectral range. Moreover, it can readily be probed from free space, making it functional for optical devices. We construct a prism made of this optical NIM to demonstrate negative refractive index at optical frequencies, resulting unambiguously from the negative phase evolution of the wave propagating inside the metamaterial. Bulk optical metamaterials open up prospects for studies of 3D optical effects and applications associated with NIMs and zero-index materials such as reversed Doppler effect, superlenses, optical tunnelling devices, compact resonators and highly directional sources.

Coverage and nearest-neighbor dependence of adsorbate diffusion.

We present data on the coverage and nearest-neighbor dependences of the diffusion of CO on Cu(111) by time-lapsed scanning tunneling microscope (STM) imaging. Most notable is a maximum in diffusivity of CO at a local coverage of one molecule per 20 substrate atoms and a repulsion between CO molecules upon approach closer than three adsites, which in combination with a less pronounced increase in potential energy at the diffusion transition state, leads to rapid diffusion of CO molecules around one another. We propose a new method of evaluating STM-based diffusion data that provides all parameters necessary for the modeling of the dynamics of an adsorbate population.