Serap Aksu

High-Throughput Nanofabrication of Infrared Plasmonic Nanoantenna Arrays for Vibrational Nanospectroscopy

The introduction of high-throughput and high-resolution nanofabrication techniques operating at low cost and low complexity is essential for the advancement of nanoplasmonic and nanophotonic fields. In this paper, we demonstrate a novel fabrication approach based on nanostencil lithography for high-throughput fabrication of engineered infrared plasmonic nanorod antenna arrays. The technique relying on deposition of materials through a shadow mask enables plasmonic substrates supporting spectrally sharp collective resonances. We show that reflectance spectra of these antenna arrays are comparable to that of arrays fabricated by electron beam lithography. We also show that nanostencils can be reused multiple times to fabricate a series of infrared nanoantenna arrays with identical optical responses. Finally, we demonstrate fabrication of plasmonic nanostructures in a variety of shapes with a single metal deposition step on different substrates, including nonconducting ones. Our approach, by enabling the reusability of the stencil and offering flexibility on the substrate choice and nanopattern design, could facilitate the transition of plasmonic technologies to the real-world applications.

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.

Large-scale plasmonic microarrays for label-free high-throughput screening

Microarrays allowing simultaneous analysis of thousands of parameters can significantly accelerate screening of large libraries of pharmaceutical compounds and biomolecular interactions. For large-scale studies on diverse biomedical samples, reliable, label-free, and high-content microarrays are needed. In this work, using large-area plasmonic nanohole arrays, we demonstrate for the first time a large-scale label-free microarray technology with over one million sensors on a single microscope slide. A dual-color filter imaging method is introduced to dramatically increase the accuracy, reliability, and signal-to-noise ratio of the sensors in a highly multiplexed manner. We used our technology to quantitatively measure protein-protein interactions. Our platform, which is highly compatible with the current microarray scanning systems can enable a powerful screening technology and facilitate diagnosis and treatment of diseases.

Multi-resonant metamaterials based on UT-shaped nano-aperture antennas

We demonstrate a compact multi-resonant metamaterial structure based on integrated U- and T-shaped nano-aperture antennas. We investigate the physical origin of the multi-resonant behavior and determine the parameter dependence of the nano-aperture antennas both experimentally and numerically. We also show enhanced field distribution in the apertures at the corresponding resonance wavelengths. Both multi-spectral response and enhanced near field distributions can open up exciting new opportunities in applications ranging from subwavelength optics and optoelectronics to chemical and biosensing.

Nanoparticle-Based Metamaterials as Multiband Plasmonic Resonator Antennas

Plasmonic metamaterials based on metal-dielectric nanostructures exhibit unique optical properties such as high near-field enhancement, negative refractive indexing, and optical cloaking. In this paper, we present a plasmonic multiband metamaterial based on UT shaped nanoparticles. In order to understand the multispectral response, we analyze the near-field distributions at the corresponding resonance frequencies. In addition, we both numerically and experimentally, show the dependence of the spectral response on the geometrical parameters of the structure. By embedding the system in a dielectric cladding medium, we show strong sensitivities of the resonant behavior to the refractive index and thickness of the dielectric load. Due to its tunable multiband spectral characteristics, the proposed metamaterial antenna can be used for wide range of applications, such as wavelength-tunable active filters, optical modulators, ultrafast switching devices, and biosensing.

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.

Theoretical and experimental analysis of subwavelength bowtie-shaped antennas

Recently, bowtie-shaped apertures have received significant attention due to their extraordinary ability to generate dramatic field enhancement and light confinement in nanometer scale. In this article, we investigate both experimentally and theoretically nearfield and farfield responses of bowtie-shaped apertures in detail. We study the role of bowtie gap in creating large and highly accessible local electromagnetic fields. In order to experimentally excite strong local fields, we introduce a high-resolution and lift-off free fabrication method which enables bowtie apertures with gap sizes down to sub-10 nm. We also show that for identical geometries, bowtie-shaped apertures support much stronger local electromagnetic fields compared to particle-based bowtie-shaped antennas. We investigate the role of polarization on the gap effect, which plays the dominant role for creating strong nearfield intensities. Finally, we introduce a mechanism to fine-tune the optical response of bowtie apertures through geometrical parameters.

Nanostencil lithography for high-throughput fabrication of infrared plasmonic sensors

We demonstrate a novel fabrication approach for high-throughput fabrication of engineered infrared plasmonic nanorod antenna arrays with Nanostencil Lithography (NSL). NSL technique, relying on deposition of materials through a shadow mask, offers the flexibility and the resolution to fabricate radiatively engineer nanoantenna arrays for excitation of collective plasmonic resonances. Overlapping these collective plasmonic resonances with molecular specific absorption bands can enable ultrasensitive vibrational spectroscopy. First, nanorod antenna arrays fabricated using NSL are investigated using SEM and optical spectroscopy, and compared against the nanorods with the same dimensions fabricated using EBL. No irregularities on the periodicity or the physical dimensions are detected for NSL fabricated nanorods. We also confirmed that the antenna arrays fabricated by NSL shows high optical quality similar to EBL fabricated ones. Furthermore, we show nanostencils can be reused multiple times to fabricate selfsame structures with identical optical responses repeatedly and reliably. This capability is particularly useful when high-throughput replication of the optimized nanoparticle arrays is desired. In addition to its high-throughput capability, NSL permits fabrication of plasmonic devices on surfaces that are difficult to work with electron/ion beam techniques. Nanostencil lithography is a resist free process thus allows the transfer of the nanopatterns to any planar substrate whether it is conductive, insulating or magnetic. As proof of the versatility of the NSL technique, we show fabrication of plasmonic structures in variety of geometries. We also demonstrate that nanostencil lithography can be used to achieve functional plasmonic devices in a single fabrication step, on variety of substrates. We introduced NSL for fabrication of nanoplasmonic structures including antenna arrays on rigid surfaces such as silicon, CaF2 and glass. In conclusion, Nanostencil Lithography enables plasmonic substrates supporting spectrally narrow far-field resonances with enhanced near-field intensities which are very useful for vibrational spectroscopy. We believe this nanofabrication scheme, enabling the reusability of stencil and offering flexibility on the substrate choice and nano-pattern design could significantly enhance wide-use of plasmonics in sensing technologies.

U-Shaped Nano-Apertures for Enhanced Optical Transmission and Resolution

The subject of light transmission through optically thin metal films perforated with arrays of subwavelength nanoholes has recently attracted significant attention. In this work, we present experimental and calculated results on optical transmission/reflection of the U-shaped nanoapertures for enhanced optical transmission and resolution. We propose different structure designs in order to prove the effect of geometry on resonance and enhanced fields. Theoretical calculations of transmission/reflection spectra and field distributions of U-shaped nano-apertures are performed by using 3-dimensional finite-difference time-domain method. The results of these numerical calculations show that transmission through the apertures is indeed concentrated in the gap region. Added to theoretical calculations we also performed a liftoff free plasmonic device fabrication technique based on positive resist electron beam lithography and reactive ion etching in order to fabricate U-shaped nanostructures. After transferring nanopattern on 80 nm thick suspended SiNx membrane using EBL followed by dry etching, a directional metal deposition processes is used to deposit 5 nm thick Ti and 30 nm thick Au layers. Theoretical calculations are supported with experimental results to prove the tunability of resonances with the geometry at the mid-infrared wavelengths which could be used for infrared detection of biomolecules.

Optical properties of UT-shaped plasmonic nanoaperture antennas

In this paper, we present numerical and experimental results on optical properties of a multi-resonant UT-shaped plasmonic nanoaperture antenna for enhanced optical transmission and near-field resolution. We propose different structure designs in order to prove the effect of geometry on resonance spectrum and near-field enhancement. Theoretical calculations of transmission spectra and field distributions of UT-shaped nano-apertures are performed by using three-dimensional finite-difference time-domain method. The results of these numerical calculations show that transmission through the apertures is indeed concentrated in the gap region. In addition to theoretical calculations, we also performed a lift-off free plasmonic device fabrication technique based on positive resist electron beam lithography (EBL) and reactive ion etching in order to fabricate UT-shaped nanostructures. For further confirmation of the multiresonant behavior, we checked the individual U-and T-shaped nano-aperture antenna responses. We also studied the parameter dependence of the structure to determine the control mechanism of the spectral response. Theoretical calculations are supported with experimental results to prove the enhanced field distribution and multi-resonant behavior which can be suitable for infrared detection of biomolecules, wavelength-tunable filters, optical modulators, and ultrafast switching devices.teInp