Zhanghua Han

Novel surface plasmon waveguide for high integration

A novel surface plasmon waveguide structure is proposed for highly integrated planar lightwave circuits. By etching a small trench through a metallic thin film on a silica substrate, a guided mode with highly confined light fields is realized. The mode properties of the proposed structure are studied. The necessity of using a polymer upper-cladding is discussed. The coupling between two closely positioned waveguides and a 90o bending are also studied numerically. Sharp bending and high integration can be realized with the present surface plasmon waveguide. The proposed structure is easy to fabricate as compared with some other types of surface plasmon waveguides for high integration.

Surface Plasmon Bragg Gratings Formed in Metal-Insulator-Metal Waveguides

We propose and numerically analyze surface plasmon Bragg gratings formed by a periodic variation of the width of the insulator in a metal-insulator-metal waveguide. The results indicate that very good filtering characteristics can be achieved in these plasmonic Bragg gratings. To suppress the sidelobes in the transmission spectrum, we further propose S-shaped Bragg cells and find better performance. By introducing a defect into the grating, a defect state with high Q-value is introduced into the bandgap and a Fabry-Peacuterot-like structure is formed

Plasmon-induced transparency with detuned ultracompact Fabry-Perot resonators in integrated plasmonic devices

We demonstrate the realization of on-chip plasmonic analogue of electromagnetically induced transparency (EIT) in integrated plasmonic devices using detuned Fabry-Perot resonators aperture-side-coupled to a metal-insulator-metal (MIM) waveguide, with the transmission peak occurring at the intermediate wavelength. Strong MIM mode confinement along with localized side-coupling allows one to realize subwavelength photonic components with EIT-like transmission. Numerical results show that MIM components exhibiting pronounced EIT-like spectra in near infrared with the footprint of < 0.15 μm2 and group index of ~26 can be designed.

Plasmonic black gold by adiabatic nanofocusing and absorption of light in ultra-sharp convex grooves

Excitation of localized and delocalized surface plasmon resonances can be used for turning excellent reflectors of visible light, such as gold and silver, into efficient absorbers, whose wavelength, polarization or angular bandwidths are however necessarily limited owing to the resonant nature of surface plasmon excitations involved. Nonresonant absorption has so far been achieved by using combined nano- and micro-structural surface modifications and with composite materials involving metal nanoparticles embedded in dielectric layers. Here we realize nonresonant light absorption in a well-defined geometry by using ultra-sharp convex metal grooves via adiabatic nanofocusing of gap surface plasmon modes excited by scattering off subwavelength-sized wedges. We demonstrate experimentally that two-dimensional arrays of sharp convex grooves in gold ensure efficient (>87%) broadband (450-850 nm) absorption of unpolarized light, reaching an average level of 96%. Efficient absorption of visible light by nanostructured metal surfaces open new exciting perspectives within plasmonics, especially for thermophotovoltaics.

Conductor-gap-silicon plasmonic waveguides and passive components at subwavelength scale

Subwavelength conductor-gap-silicon plasmonic waveguides along with compact S-bends and Y-splitters were theoretically investigated and experimentally demonstrated on a silicon-on-insulator platform. A thin SiO2 gap between the conductor layer and silicon core provides subwavelength confinement of light while a long propagation length of 40 microm was achieved. Coupling of light between the plasmonic and conventional silicon photonic waveguides was also demonstrated with a high efficiency of 80%. The compact sizes, low loss operation, efficient input/output coupling, combined with a CMOS-compatible fabrication process, make these conductor-gap-silicon plasmonic devices a promising platform for realizing densely-integrated plasmonic circuits.

Radiation guiding with surface plasmon polaritons.

Surface plasmon polaritons (SPPs) are electromagnetic (EM) modes propagating along metal-dielectric interfaces, in which surface collective excitations of free electrons in the metal are coupled to evanescent EM fields in the dielectric. Various SPP modes can be supported by flat and curved, single and multiple surfaces, exhibiting remarkable properties, including the possibility of concentrating EM fields beyond the diffraction limit, i.e. on the nanoscale, while enhancing local field strengths by several orders of magnitude. This unique feature of SPP modes, along with the ever-increasing demands for miniaturization of photonic components and circuits, generates an exponentially growing interest in SPP-mediated radiation guiding and SPP-based waveguide components. Here we review the current status of this rapidly developing field, starting with a brief presentation of the main planar SPP modes along with the techniques employed for their excitation and manipulation by sets of nanoparticles. We then describe in detail various SPP-based waveguide configurations that ensure two-dimensional mode confinement in the plane perpendicular to the propagation direction and compare their characteristics. Excitation of SPP waveguide modes and recent progress in the development of SPP-based waveguide components are also discussed, concluding with our outlook on challenges and possible future developments in this field.

Experimental realization of subwavelength plasmonic slot waveguides on a silicon platform

We report the experimental realization of horizontal plasmonic slot waveguides capable of subdiffraction modal confinement at IR wavelengths. These waveguides have a propagation length of approximately 6 lambda(g) and are monolithically integrated with conventional silicon photonic waveguides on the same silicon-on-insulator platform. Direct coupling of light from the silicon waveguides to the plasmonic waveguides was achieved with an efficiency of 30% using taper-funnel couplers to obtain mode matching between the two waveguide systems.

Graphene-protected copper and silver plasmonics

Plasmonics has established itself as a branch of physics which promises to revolutionize data processing, improve photovoltaics, and increase sensitivity of bio-detection. A widespread use of plasmonic devices is notably hindered by high losses and the absence of stable and inexpensive metal films suitable for plasmonic applications. To this end, there has been a continuous search for alternative plasmonic materials that are also compatible with complementary metal oxide semiconductor technology. Here we show that copper and silver protected by graphene are viable candidates. Copper films covered with one to a few graphene layers show excellent plasmonic characteristics. They can be used to fabricate plasmonic devices and survive for at least a year, even in wet and corroding conditions. As a proof of concept, we use the graphene-protected copper to demonstrate dielectric loaded plasmonic waveguides and test sensitivity of surface plasmon resonances. Our results are likely to initiate wide use of graphene-protected plasmonics.

Hybrid graphene plasmonic waveguide modulators.

The unique optical and electronic properties of graphene make possible the fabrication of novel optoelectronic devices. One of the most exciting graphene characteristics is the tunability by gating which allows one to realize active optical devices. While several types of graphene-based photonic modulators have already been demonstrated, the potential of combining the versatility of graphene with subwavelength field confinement of plasmonic waveguides remains largely unexplored. Here we report fabrication and study of hybrid graphene–plasmonic waveguide modulators. We consider several types of modulators and identify the most promising one for telecom applications. The modulator working at the telecom range is demonstrated, showing a modulation depth of >0.03 dB μm−1 at low gating voltages for an active device area of just 10 μm2, characteristics which are already comparable to those of silicon-based waveguide modulators while retaining the benefit of further device miniaturization. Our proof-of-concept results pave the way towards on-chip realization of efficient graphene-based active plasmonic waveguide devices for optical communications.

Aperture-coupled MIM plasmonic ring resonators with sub-diffraction modal volumes

We propose and investigate ultracompact aperture-coupled plasmonic ring resonators with submicron bending radii based on strongly-confined metal-insulator-metal plasmonic waveguides. Enhanced coupling can be obtained via diffraction by small apertures having typical widths between 50-100 nm in the metallic sidewall between the ring and bus waveguides. Both analytical model and rigorous FDTD simulations show that 500 nm-radius ring resonators can be obtained with low insertion loss, wide free spectral range and sub-diffraction cavity volume of less than 0.1(lambda(0)/n(eff))(3).