Alp Artar

Seeing protein monolayers with naked eye through plasmonic Fano resonances

We introduce an ultrasensitive label-free detection technique based on asymmetric Fano resonances in plasmonic nanoholes with far reaching implications for point-of-care diagnostics. By exploiting extraordinary light transmission phenomena through high-quality factor (Qsolution ∼ 200) subradiant dark modes, we experimentally demonstrate record high figures of merits (FOMs as high as 162) for intrinsic detection limits surpassing that of the gold standard prism coupled surface-plasmon sensors (Kretschmann configuration). Our experimental record high sensitivities are attributed to the nearly complete suppression of the radiative losses that are made possible by the high structural quality of the fabricated devices as well as the subradiant nature of the resonances. Steep dispersion of the plasmonic Fano resonance profiles in high-quality plasmonic sensors exhibit dramatic light intensity changes to the slightest perturbations within their local environment. As a spectacular demonstration of the extraordinary sensitivity and the quality of the fabricated biosensors, we show direct detection of a single monolayer of biomolecules with naked eye using these Fano resonances and the associated Wood’s anomalies. To fabricate high optical-quality sensors, we introduce a high-throughput lift-off free evaporation fabrication technique with extremely uniform and precisely controlled nanofeatures over large areas, leading to resonance line-widths comparable to that of the ideally uniform structures as confirmed by our time-domain simulations. The demonstrated label-free sensing platform offers unique opportunities for point-of-care diagnostics in resource poor settings by eliminating the need for fluorescent labeling and optical detection instrumentation (camera, spectrometer, etc.) as well as mechanical and light isolation.

An Optofluidic Nanoplasmonic Biosensor for Direct Detection of Live Viruses from Biological Media

Fast and sensitive virus detection techniques, which can be rapidly deployed at multiple sites, are essential to prevent and control future epidemics and bioterrorism threats. In this Letter, we demonstrate a label-free optofluidic nanoplasmonic sensor that can directly detect intact viruses from biological media at clinically relevant concentrations with little to no sample preparation. Our sensing platform is based on an extraordinary light transmission effect in plasmonic nanoholes and utilizes group-specific antibodies for highly divergent strains of rapidly evolving viruses. So far, the questions remain for the possible limitations of this technique for virus detection, as the penetration depths of the surface plasmon polaritons are comparable to the dimensions of the pathogens. Here, we demonstrate detection and recognition of small enveloped RNA viruses (vesicular stomatitis virus and pseudotyped Ebola) as well as large enveloped DNA viruses (vaccinia virus) within a dynamic range spanning 3 orders of magnitude. Our platform, by enabling high signal to noise measurements without any mechanical or optical isolation, opens up opportunities for detection of a broad range of pathogens in typical biology laboratory settings.

Multispectral plasmon induced transparency in coupled meta-atoms.

We introduce an approach enabling construction of a scalable metamaterial media supporting multispectral plasmon induced transparency. The composite multilayered media consist of coupled meta-atoms with radiant and subradiant hybridized plasmonic modes interacting through the structural asymmetry. A perturbative model incorporating hybridization and mode coupling is introduced to explain the observed novel spectral features. The proposed scheme is demonstrated experimentally by developing a lift-off-free fabrication scheme that can automatically register multiple metamaterial layers in the transverse plane. This metamaterial which can simultaneously enhance nonlinear processes at multiple frequency domains could open up new possibilities in optical information processing.

Integrated nanoplasmonic-nanofluidic biosensors with targeted delivery of analytes

Performances of the biosensors are often limited by the depletion zones created around the sensing area which impede the effective analyte transport. To overcome this limitation, we propose and demonstrate a nanoplasmonic-nanofluidic sensor enabling targeted delivery of analytes to the sensor surface with dramatic improvements in mass transport efficiency. Our sensing platform is based on extraordinary light transmission effect in suspended plasmonic nanoholes. This scheme allows three-dimensional control of the fluidic flow by connecting separate layers of microfluidic channels through plasmonic/nanofluidic holes. To implement the proposed sensor platform, we also introduce a lift-off free nanofabrication method.

Flexible Plasmonics on Unconventional and Nonplanar Substrates

Flexible plasmonics and metamaterials on polymeric and nonplanar substrates are demonstrated using nanostencil lithography. High-resolution fabrication with 10 nm accuracy is achieved at high throughput and low cost in a single fabrication step. Optical tuning is shown with mechanical stretching of the polymer substrate. Patterning of nanostructures on curved surfaces, including optical fibers, is demonstrated.

Directional Double Fano Resonances in Plasmonic Hetero-Oligomers

We experimentally demonstrate for the first time a very compact plasmonic hetero-oligomer structure where the multiple radiant and subradiant modes can be tailored independently. Unlike previous approaches based on collective excitations in complex plasmonic systems, we show precise engineering of resonances leading to simultaneous spectral overlap of multiple plasmonic modes with opposite radiative character. This asymmetric behavior combined with inherent spatial features of the structure leads to directional double Fano resonances as shown with numerical analysis. A model based on temporal coupled mode theory is also provided to describe the double Fano behavior.

Fabry–Pérot nanocavities in multilayered plasmonic crystals for enhanced biosensing

We have demonstrated extraordinary light transmission effect through Fabry–Perot cavities in multilayered plasmonic crystals formed by coupling two physically separated metallic nanohole and nanodisk array layers. Superior field-medium overlap is observed with Fabry–Perot resonances as a result of stronger electromagnetic field confinement in the dielectric region far from the metallic surfaces. We show that these cavity resonances are highly sensitive to refractive index changes. The large field-material overlap combined with simple fabrication scheme used here makes these structures an ideal candidate for biosensing applications.

Engineered Absorption Enhancement and Induced Transparency in Coupled Molecular and Plasmonic Resonator Systems

Coupled plasmonic resonators have become the subject of significant research interest in recent years as they provide a route to dramatically enhanced light-matter interactions. Often, the design of these coupled mode systems draws intuition and inspiration from analogies to atomic and molecular physics systems. In particular, they have been shown to mimic quantum interference effects, such as electromagnetically induced transparency (EIT) and Fano resonances. This analogy also been used to describe the surface-enhanced absorption effect where a plasmonic resonance is coupled to a weak molecular resonance. These important phenomena are typically described using simple driven harmonic (or linear) oscillators (i.e., mass-on-a-spring) coupled to each other. In this work, we demonstrate the importance of an essential interdependence between the rate at which the system can be driven by an external field and its damping rate through radiative loss. This link is required in systems exhibiting time-reversal symmetry and energy conservation. Not only does it ensure an accurate and physically consistent description of resonant systems but leads directly to interesting new effects. Significantly, we demonstrate this dependence to predict a transition between EIT and electromagnetically induced absorption that is solely a function of the ratio of the radiative to intrinsic loss rates in coupled resonator systems. Leveraging the temporal coupled mode theory, we introduce a unique and intuitive picture that accurately describes these effects in coupled plasmonic/molecular and fully plasmonic systems. We demonstrate our approachs key features and advantages analytically as well as experimentally through surface-enhanced absorption spectroscopy and plasmonic metamaterial applications.

Plasmon induced transparency in cascaded π-shaped metamaterials

We experimentally and numerically demonstrate a planar metamaterial consisting of two asymmetrically positioned π-structures in a single unit that exhibits plasmonic analogue of electromagnetically induced transparency (EIT). Through the coupling of the constituent nanorod elements, the proposed structure enables fine spectral tuning of the EIT-like behavior and controlling the location of near field enhancement. Originated from the asymmetric cascaded π-structures, we introduce a more compact system which possesses the EIT-like characteristics and as well as much smaller mode volumes. Due to these properties, the proposed metamaterials can be utilized for a wide range of applications including bio-chemical sensors, optical filters and modulators and enhancement of non-linear processes.

Reusable nanostencils for creating multiple biofunctional molecular nanopatterns on polymer substrate.

In this paper, we demonstrate a novel method for high throughput patterning of bioprobes with nanoscale features on biocompatible polymer substrate. Our technique, based on nanostencil lithography, employs high resolution and robust masks integrated with array of reservoirs. We show that the smallest pattern size can reach down to 100 nm. We also show that different types of biomolecules can be patterned on the same substrate simultaneously. Furthermore, the stencil can be reused multiple times to generate a series of identical patterns at low cost. Finally, we demonstrate that biomolecules can be covalently patterned on the surface while retaining their biofunctionalities. By offering the flexibility on the nanopattern design and enabling the reusability of the stencil, our approach significantly simplifies the bionanopatterning process and therefore could have profound implications in diverse biological and medical applications.