Choon How Gan

Analysis of surface plasmon excitation at terahertz frequencies with highly doped graphene sheets via attenuated total reflection

Excitation of surface plasmons supported by doped graphene sheets at terahertz frequencies is investigated numerically. To alleviate the momentum mismatch between the highly confined plasmon modes and the incident radiation, it is proposed to increase the surface conductivity of graphene through high doping levels or with few-layer graphene. For currently achievable doping levels, our analysis shows that surface plasmons on monolayer graphene may be excited at operating frequencies up to about 10 THz (∼41.3 meV) with a high-index coupling prism, and higher frequencies/energies are possible for few-layer graphene. These highly confined surface modes are promising for sensing and waveguiding applications in the terahertz regime.

Active plasmonic switching at mid-infrared wavelengths with graphene ribbon arrays

An active plasmonic switch based on single- and few-layer doped graphene ribbon array operating in the mid-infrared spectrum is investigated with theoretical and numerical calculations. It is shown that significant resonance wavelength shifts and modulation depths can be achieved with a slight variation of the doping concentration of the graphene ribbon. The few-layer graphene ribbon array device outperforms the single-layer one in terms of the achievable modulation depth. Our simulations reveal that, by modulating the Fermi-energy level between 0.2 eV and 0.25 eV, a four-layer graphene ribbon array device can achieve a modulation depth and resonance wavelength shift of ∼13 dB and 0.94 μm, respectively, compared to ∼2.8 dB and 1.85 μm for a single-layer device. Additionally, simple fitting models to predict the modulation depth and the resonance wavelength shift are proposed. These prospects pave the way towards ultrafast active graphene-based plasmonic devices for infrared and THz applications.

Plasmon switching: Observation of dynamic surface plasmon steering by selective mode excitation in a sub-wavelength slit

We report a plasmon steering method that enables us to dynamically control the direction of surface plasmons generated by a two-mode slit in a thin metal film. By varying the phase between different coherent beams that are incident on the slit, individual waveguide modes are excited. Different linear combinations of the two modes lead to different diffracted fields at the exit of the slit. As a result, the direction in which surface plasmons are launched can be controlled. Experiments confirm that it is possible to distribute an approximately constant surface plasmon intensity in any desired proportion over the two launching directions. We also find that the anti-symmetric mode generates surface plasmons more efficiently than the fundamental symmetric mode.

Well-confined surface plasmon polaritons for sensing applications in the near-infrared

The surface plasmon polariton (SPP) dispersion at the interface between a dielectric half-space and a layered metallodielectric metamaterial is investigated. By varying the material constituants, it is shown that the SPP resonance frequency can be readily shifted to the near-IR. Through numerical simulations, the validity domain of homogenization and the effects of the finite number of layers in the metamaterial are studied. It is found that as few as N=2 periods are sufficient for practical operation. These results reveal the potential of employing metallodielectric stacks for sensing applications in the near-IR regime.

Coherence converting plasmonic hole arrays

Simulations are presented that demonstrate that the global state of spatial coherence of an optical wavefield can be altered on transmission through an array of subwavelength-sized holes in a metal plate that supports surface plasmons. It is found that the state of coherence of the emergent field strongly depends on the separation between the holes and their scattering strength. Our findings suggest that subwavelength hole arrays on a metal film can be potentially employed as a plasmon-assisted coherence converting device, useful in modifying the directionality, spectrum, and polarization of the transmitted wave.

Proposal for Compact Solid-State III-V Single-Plasmon Sources

We propose a compact single-plasmon source operating at near-infrared wavelengths on an integrated III-V semiconductor platform, with a thin ridge waveguide serving as the plasmon channel. By attaching an ultra-small cavity to the channel, it is shown that both the plasmon generation efficiency ({\beta}) and the spontaneous-decay rate into the channel can be significantly enhanced. An analytical model derived with the Lorentz reciprocity theorem captures the main physics involved in the design of the source and yields results in good agreement with fully-vectorial simulations of the device. At resonance, it is predicted that the ultra-small cavity increases the {\beta}-factor by 70% and boosts the spontaneous decay rate by a factor 20. The proposed design could pave the way towards integrated and scalable plasmonic quantum networks. Comparison of the present design with other fully-dielectric competing approaches is addressed.

Strategies for employing surface plasmons in near-field optical readout systems

The enhanced transmission of light through subwavelength-size holes in a metal plate is well-known to be associated with surface plasmons. We have undertaken a systematic theoretical study of several strategies for applying these plasmon effects in a near-field optical readout system using an exact Greens tensor formulation. Based on the results of our simulations with light of wavelength lambda = 500 nm, data structures separated by 120 nm could be clearly resolved, and asymmetries of about +/-10 nm in the optical readout system could be tolerated without serious degradation of the performance. Advantages and disadvantages of each strategy are discussed.

Extraordinary optical transmission through multi-layered systems of corrugated metallic thin films

Optical transmission through multi-layered systems of corrugated metallic thin films is investigated by rigorous electromagnetic simulations based on an exact Green tensor method. Compared to a single metal slab of equivalent thickness and volume, it was found that the multi-layered system can significantly impede the field decay, often leading to transmission greater than that expected from the Fabry-Perot resonance-like behavior exhibited by subwavelength slits in a single slab. Extraordinary optical transmission is also observable for systems of layers whose combined thicknesses are much greater than the skin depth of the metal. Structures consisting of up to five layers with a net thickness of 500 nm for the metal films were considered in our study. These findings demonstrate that an appreciable fraction of the optical power that is incident on the thin metal films can be transmitted over distances greater than their skin depth using plasmonic resonances.

Diffraction of evanescent waves and nanomechanical displacement detection

Sensitive displacement detection has emerged as a significant technological challenge in mechanical resonators with nanometer-scale dimensions. A novel nanomechanical displacement detection scheme based upon the scattering of focused evanescent fields is proposed. The sensitivity of the proposed approach is studied using diffraction theory of evanescent waves. Diffraction theory results are compared with numerical simulations.

Plasmon Switching: Observation of Dynamic Surface Plasmon Steering by Selective Mode Excitation in a Sub-wavelength Slit

By selectively exciting the modes in a sub-wavelength slit in a gold film, an approximately constant surface plasmon intensity can be distributed in any desired proportion over two launching directions.