Gap Plasmons and Near-Field Enhancement in Closely Packed Sub-10 nm Gap Resonators

Pairs of metal nanoparticles with a sub-10 nm gap are an efficient way to achieve extreme near-field enhancement for sensing applications. We demonstrate an attractive alternative based on Fabry–Perot type nanogap resonators, where the resonance is defined by the gap width and vertical elongation instead of the particle geometry. We discuss the crucial design parameters for such gap plasmons to produce maximum near-field enhancement for surface-enhanced Raman scattering and show compatibility of the pattern processing with low-cost and low-resolution lithography. We find a minimum critical metal thickness of 80 nm and observe that the mode coupling from the far field increases by tapering the gap opening. We also show the saturation of the Raman signal for nanogap periodicities below 1 μm, demonstrating efficient funneling of light into such nanogap arrays

Engineering Metal Adhesion Layers That Do Not Deteriorate Plasmon Resonances

Adhesion layers, required to stabilize metallic nanostructures, dramatically deteriorate the performances of plasmonic sensors, by severely damping the plasmon modes. In this article, we show that these detrimental effects critically depend on the overlap of the electromagnetic near-field of the resonant plasmon mode with the adhesion layer and can be minimized by careful engineering of the latter. We study the dependence of the geometrical parameters such as layer thickness and shape on the near-field of localized plasmon resonances for traditional adhesion layers such as Cr, Ti, and TiO₂. Our experiments and simulations reveal a strong dependence of the damping on the layer thickness, in agreement with the exponential decay of the plasmon near-field. We developed a method to minimize the damping by selective deposition of thin adhesion layers (<1 nm) in a manner that prevents the layer to overlap with the hotspots of the plasmonic structure. Such a designed structure enables the use of standard Cr and Ti adhesion materials to fabricate robust plasmonic sensors without deteriorating their sensitivity

Metal Double Layers with Sub-10 nm Channels

Double-layer plasmonic nanostructures are fabricated by depositing metal at normal incidence onto various resist masks, forming an antenna layer on top of the resist post and a hole layer on the substrate. Antenna plasmon resonances are found to couple to the hole layer, inducing image charges which enhance the near-field for small layer spacings. For continued evaporation above the resist height, a sub-10 nm gap channel develops due to a self-aligned process and a minimal undercut of the resist sidewall. For such double layers with nanogap channels, the average surface-enhanced Raman scattering intensity is improved by a factor in excess of 60 in comparison to a single-layer antenna with the same dimensions. The proposed design principle is compatible with low-cost fabrication, straightforward to implement, and applicable over large areas. Moreover, it can be applied for any particular antenna shape to improve the signals in surface-enhanced spectroscopy applications

Extreme ultraviolet lithography reaches 5 nm resolution

Extreme ultraviolet (EUV) lithography is the leading lithography technique in CMOS mass production, moving towards the sub-10 nm half-pitch (HP) regime with the ongoing development of the next generation high-numerical aperture (high-NA) EUV scanners. Hitherto, EUV interference lithography (EUV-IL) utilizing transmission gratings has been a powerful patterning tool for the early development of EUV resists and related processes, playing a key role in exploring and pushing the boundaries of photon-based lithography. However, achieving pattering with HPs well below 10 nm using this method presents significant challenges. In response, our study introduces a novel EUV-IL setup that employs mirror-based technology and circumvents the limitations of diffraction efficiency towards the diffraction limit that is inherent in conventional grating-based approaches. We present line/space patterning of HSQ resist down to HP 5 nm using the standard EUV wavelength 13.5 nm, and the compatibility of the tool with shorter wavelengths beyond EUV. The mirror-based interference lithography tool paves the way towards the ultimate photon-based resolution at EUV wavelengths and beyond. This advancement is vital for scientific and industrial research, addressing the increasingly challenging needs of nanoscience and technology and future technology nodes of CMOS manufacturing in the few-nanometer HP regime

Poly(methyl methacrylate)-Based Nanofluidic Device for Rapid and Multiplexed Serological Antibody Detection of SARS-CoV‑2

The outbreak of SARS-CoV-2 has emphasized the value of point-of-care diagnostics, as well as reliable and cost-effective serological antibody tests, to monitor the viral spread and contain pandemics and endemics. Here, we present a three-dimensional (3D) nanofluidic device for rapid and multiplexed detection of viral antibodies. The device is made from poly­(methyl methacrylate) and contains 3D fluidic channels with nanoscale topography variations on the millimeter length scale, enabled by combining gray-scale e-beam lithography and nanoimprint lithography. It works with capillary pumps only and does not require a complex microfluidic setup and pumps, which hinder the widespread usage of micro- and nanofluidic devices. The device is designed to size dependently immobilize particles from a multiparticle mixture at predefined positions in nanochannels, resulting in distinct trapping lines. We show that it can be used as an on-chip fluorescence-linked immunosorbent assay for highly specific and sensitive multiplexed detection of serological antibodies against different viral proteins. Further test flexibility is demonstrated by on-bead color multiplexing for simultaneous detection of IgG and IgM antibodies in convalescent human serum. The particle sorting is further leveraged to enable concurrent detection of anti-spike (SARS-CoV-2) and anti-hemagglutinin (influenza A) antibodies. The device’s applications can be further extended to detect a large variety of diseases simultaneously in a reliable and straightforward manner

Poly(methyl methacrylate)-Based Nanofluidic Device for Rapid and Multiplexed Serological Antibody Detection of SARS-CoV‑2

The outbreak of SARS-CoV-2 has emphasized the value of point-of-care diagnostics, as well as reliable and cost-effective serological antibody tests, to monitor the viral spread and contain pandemics and endemics. Here, we present a three-dimensional (3D) nanofluidic device for rapid and multiplexed detection of viral antibodies. The device is made from poly­(methyl methacrylate) and contains 3D fluidic channels with nanoscale topography variations on the millimeter length scale, enabled by combining gray-scale e-beam lithography and nanoimprint lithography. It works with capillary pumps only and does not require a complex microfluidic setup and pumps, which hinder the widespread usage of micro- and nanofluidic devices. The device is designed to size dependently immobilize particles from a multiparticle mixture at predefined positions in nanochannels, resulting in distinct trapping lines. We show that it can be used as an on-chip fluorescence-linked immunosorbent assay for highly specific and sensitive multiplexed detection of serological antibodies against different viral proteins. Further test flexibility is demonstrated by on-bead color multiplexing for simultaneous detection of IgG and IgM antibodies in convalescent human serum. The particle sorting is further leveraged to enable concurrent detection of anti-spike (SARS-CoV-2) and anti-hemagglutinin (influenza A) antibodies. The device’s applications can be further extended to detect a large variety of diseases simultaneously in a reliable and straightforward manner

Deep-UV Surface-Enhanced Resonance Raman Scattering of Adenine on Aluminum Nanoparticle Arrays

We report the ultrasensitive detection of adenine using deep-UV surface-enhanced resonance Raman scattering on aluminum nanostructures. Well-defined Al nanoparticle arrays fabricated over large areas using extreme-UV interference lithography exhibited sharp and tunable plasmon resonances in the UV and deep-UV wavelength ranges. Theoretical modeling based on the finite-difference time-domain method was used to understand the near-field and far-field optical properties of the nanoparticle arrays. Raman measurements were performed on adenine molecules coated uniformly on the Al nanoparticle arrays at a laser excitation wavelength of 257.2 nm. With this technique, less than 10 amol of label-free adenine molecules could be detected reproducibly in real time. Zeptomole (∼30 000 molecules) detection sensitivity was readily achieved proving that deep-UV surface-enhanced resonance Raman scattering is an extremely sensitive tool for the detection of biomolecules

Poly(methyl methacrylate)-Based Nanofluidic Device for Rapid and Multiplexed Serological Antibody Detection of SARS-CoV‑2

The outbreak of SARS-CoV-2 has emphasized the value of point-of-care diagnostics, as well as reliable and cost-effective serological antibody tests, to monitor the viral spread and contain pandemics and endemics. Here, we present a three-dimensional (3D) nanofluidic device for rapid and multiplexed detection of viral antibodies. The device is made from poly­(methyl methacrylate) and contains 3D fluidic channels with nanoscale topography variations on the millimeter length scale, enabled by combining gray-scale e-beam lithography and nanoimprint lithography. It works with capillary pumps only and does not require a complex microfluidic setup and pumps, which hinder the widespread usage of micro- and nanofluidic devices. The device is designed to size dependently immobilize particles from a multiparticle mixture at predefined positions in nanochannels, resulting in distinct trapping lines. We show that it can be used as an on-chip fluorescence-linked immunosorbent assay for highly specific and sensitive multiplexed detection of serological antibodies against different viral proteins. Further test flexibility is demonstrated by on-bead color multiplexing for simultaneous detection of IgG and IgM antibodies in convalescent human serum. The particle sorting is further leveraged to enable concurrent detection of anti-spike (SARS-CoV-2) and anti-hemagglutinin (influenza A) antibodies. The device’s applications can be further extended to detect a large variety of diseases simultaneously in a reliable and straightforward manner

Magnetic Hot Spots in Closely Spaced Thick Gold Nanorings

Light–matter interaction at optical frequencies is mostly mediated by the electric component of the electromagnetic field, with the magnetic component usually being considered negligible. Recently, it has been shown that properly engineered metallic nanostructures can provide a magnetic response at optical frequencies originated from real or virtual flows of electric current in the structure. In this work, we demonstrate a magnetic plasmonic mode which emerges in closely spaced thick gold nanorings. The plasmonic resonance obtains a magnetic dipole character by sufficiently increasing the height of the nanorings. Numerical simulations show that a virtual current loop appears at resonance for sufficiently thick nanorings, resulting in a strong concentration of the magnetic field in the gap region (magnetic hot spot). We find that there is an optimum thickness that provides the maximum magnetic intensity enhancement (over 200-fold enhancement) and give an explanation of this observation. This strong magnetic resonance, observed both experimentally and theoretically, can be used to build new metamaterials and resonant loop nanoantennas at optical frequencies

Electrically Tunable Multicolored Filter Using Birefringent Plasmonic Resonators and Liquid Crystals

Dynamic tuning of color filters finds numerous applications including displays or image sensors. Plasmonic resonators are subwavelength nanostructures which can tailor the phase, polarization, and amplitude of the optical field, but they are limited in color vibrancy when used as filters. In this work, birefringence-induced colors of plasmonic resonators and a fast switching thin liquid crystal cell are combined in a multicolored electrically tunable filter. With this mechanism, the color gamut of the plasmonic surface and the liquid crystal cell is mutually enhanced in order to generate all primary additive and subtractive colors with high saturation as well as different tones of white. A single filter is able to cover more than 70% of the color gamut of standard RGB filters by applying a voltage ranging between 2 and 6.5 V. This spectral selectivity is added in transmission without any loss in the image resolution. The presented approach is foreseen to be implemented in a variety of devices including miniature sensors or smart-phone cameras to enhance the color information, ultraflat multispectral imagers, wearable or head-worn displays, as well as high resolution display panels