Hamid T. Chorsi

Apertureless Near-Field Scanning Probes Based on Graphene Plasmonics

We present a novel approach in designing high-throughput high-resolution apertureless near-field scanning probes with enhanced nanofocusing based on graphene plasmonics. Extremely localized plasmons on graphene are mingled with nanofocusing of surface plasmon polaritons to confine and steer the plasmon waves into the apex of a near-field scanning optical microscopy tip. The Fermi level, localized emission sites on graphene, and the angle of excitation play a critical role in exciting graphene surface plasmons on the lateral walls of the designed conical probes. The optimized probes feature full-width at half-maximum (FWHM) around 25 nm, which is at least two times smaller than conventional metallic plasmonic tips. The near-field electromagnetic properties of the designed probes are characterized in detail and compared to the conventional single-aperture and typical apertureless metallic plasmonic (silver and gold) probes. Over three orders of magnitude electric field enhancement compared to metallic probes on SiO 2 substrate has been achieved.

Using graphene plasmonics to boost biosensor sensitivity

The specific detection of biomolecules in biological samples is extremely important in medical diagnosis,1 single-cell analysis,2, 3 and monitoring disease recurrence.4 However, difficulty in measuring the inherent physical features of biological analytes means that label-based techniques—i.e., in which some sort of tag (or ‘label’) is attached to molecules, viruses, or cells to capture them—have primarily been used to date for biosensors. The label in this technique may be a metal nanoparticle, fluorophore-embedded bead, semiconductor quantum dot, or a particular antibody that can initiate an enzymatic reaction. In practice, however, the labels can physically and functionally interfere with the assay and they require a high degree of development. In contrast to label-based measurement techniques, label-free detection approaches are based on the use of a transducer to measure the physical properties of biological analytes. With such transducers it is possible to directly measure the bulk physical properties (e.g., electrical impedance, heat capacity, or refractive index) of a sample. Surface plasmons (SPs)—collective oscillations of conducting electrons at the interface between a metal and a dielectric—are thought to be theoretically ideal elements within transducers.5 The resonant frequency of SPs is strongly dependent on the shape and the refractive index of the surrounding media, and this remarkable property has already been exploited in SP resonance (SPR) biosensors. Metals such as gold and silver are commonly used for plasmonic applications.6–11 Although silver has better optical properties than gold, it has so far rarely been used for biosensing applications because of its high oxidation susceptibility. Gold (i.e., the popular choice), on the other hand, presents high losses at IR frequencies because of intraband transitions that limit the confinement of SPs and reduce their sensitivity. Moreover, gold does not Figure 1. Schematic diagram of the proposed graphene plasmonic biosensor.

Patterned Plasmonic Surfaces—Theory, Fabrication, and Applications in Biosensing

Low-profile patterned plasmonic surfaces are synergized with a broad class of silicon microstructures to greatly enhance near-field nanoscale imaging, sensing, and energy harvesting coupled with far-field free-space detection. This concept has a clear impact on several key areas of interest for the microelectromechanical systems community, including but not limited to ultra-compact microsystems for sensitive detection of small number of target molecules, and “surface” devices for optical data storage, micro-imaging, and displaying. In this paper, we review the current state-of-the-art in plasmonic theory, as well as derive design guidance for plasmonic integration with microsystems, fabrication techniques, and selected applications in biosensing, including refractive-index based label-free biosensing, plasmonic integrated lab-on-chip systems, plasmonic near-field scanning optical microscopy, and plasmonics on-chip systems for cellular imaging. This paradigm enables low-profile conformal surfaces on microdevices, rather than bulk material or coatings, which provide clear advantages for physical, chemical, and biological-related sensing, imaging, and light harvesting, in addition to easier realization, enhanced flexibility, and tunability. [2016-0294]

Tunable plasmonic substrates with ultrahigh Q-factor resonances

Precisely tailored plasmonic substrates can provide a platform for a variety of enhanced plasmonic applications in sensing and imaging. Despite the significant advances made in plasmonics, most plasmonic devices suffer critically from intrinsic absorption losses at optical frequencies, fatally restricting their efficiency. Here, we describe and engineer plasmonic substrates based on metal-insulator-metal (MIM) plasmon resonances with ultra-sharp optical transmission responses. Due to their sharp transmission spectrum, the proposed substrates can be utilized for high quality (Q)-factor multi-functional plasmonic applications. Analytical and numerical methods are exploited to investigate the optical properties of the substrates. The optical response of the substrate can be tuned by adjusting the periodicity of the nanograting patterned on the substrate. Fabricated substrates present Q-factors as high as ∼40 and refractive index sensing of the surrounding medium as high as 1245 nm/RIU. Our results indicate that by engineering the substrate geometry, the dielectric thickness and incident angle, the radiation losses can be greatly diminished, thus enabling the design of plasmonic substrates with large Q factor and strong sensitivity to the environment.