Xiaoshu Chen

Atomic layer lithography of wafer-scale nanogap arrays for extreme confinement of electromagnetic waves

Squeezing light through nanometre-wide gaps in metals can lead to extreme field enhancements, nonlocal electromagnetic effects and light-induced electron tunnelling. This intriguing regime, however, has not been readily accessible to experimentalists because of the lack of reliable technology to fabricate uniform nanogaps with atomic-scale resolution and high throughput. Here we introduce a new patterning technology based on atomic layer deposition and simple adhesive-tape-based planarization. Using this method, we create vertically oriented gaps in opaque metal films along the entire contour of a millimetre-sized pattern, with gap widths as narrow as 9.9 Å, and pack 150,000 such devices on a 4-inch wafer. Electromagnetic waves pass exclusively through the nanogaps, enabling background-free transmission measurements. We observe resonant transmission of near-infrared waves through 1.1-nm-wide gaps (λ/1,295) and measure an effective refractive index of 17.8. We also observe resonant transmission of millimetre waves through 1.1-nm-wide gaps (λ/4,000,000) and infer an unprecedented field enhancement factor of 25,000.

Nanogap-Enhanced Infrared Spectroscopy with Template-Stripped Wafer-Scale Arrays of Buried Plasmonic Cavities

We have combined atomic layer lithography and template stripping to produce a new class of substrates for surface-enhanced infrared absorption (SEIRA) spectroscopy. Our structure consists of a buried and U-shaped metal-insulator-metal waveguide whose folded vertical arms efficiently couple normally incident light. The insulator is formed by atomic layer deposition (ALD) of Al2O3 and precisely defines the gap size. The buried nanocavities are protected from contamination by a silicon template until ready for use and exposed by template stripping on demand. The exposed nanocavity generates strong infrared resonances, tightly confines infrared radiation into a gap that is as small as 3 nm (λ/3300), and creates a dense array of millimeter-long hotspots. After partial removal of the insulators, the gaps are backfilled with benzenethiol molecules, generating distinct Fano resonances due to strong coupling with gap plasmons, and a SEIRA enhancement factor of 10(5) is observed for a 3 nm gap. Because of the wafer-scale manufacturability, single-digit-nanometer control of the gap size via ALD, and long-term storage enabled by template stripping, our buried plasmonic nanocavity substrates will benefit broad applications in sensing and spectroscopy.

Film-coupled nanoparticles by atomic layer deposition: Comparison with organic spacing layers

Film-coupled nanoparticle systems have proven a reliable platform for exploring the field enhancement associated with sub-nanometer sized gaps between plasmonic nanostructures. In this Letter, we present a side-by-side comparison of the spectral properties of film-coupled plasmon-resonant, gold nanoparticles, with dielectric spacer layers fabricated either using atomic layer deposition or using organic layers (polyelectrolytes or self-assembled monolayers of molecules). In either case, large area, uniform spacer layers with sub-nanometer thicknesses can be accurately deposited, allowing extreme coupling regimes to be probed. The observed spectral shifts of the nanoparticles as a function of spacer layer thickness are similar for the organic and inorganic films and are consistent with numerical calculations taking into account the nonlocal response of the metal.

Squeezing Millimeter Waves through a Single, Nanometer-wide, Centimeter-long Slit

We demonstrate broadband non-resonant squeezing of terahertz (THz) waves through an isolated 2-nm-wide, 2-cm-long slit (aspect ratio of 107), representing a maximum intensity enhancement factor of one million. Unlike resonant nanogap structures, a single, effectively infinitely-long slit passes incident electromagnetic waves with no cutoff, enhances the electric field within the gap with a broad 1/f spectral response, and eliminates interference effects due to finite sample boundaries and adjacent elements. To construct such a uniform, isolated slit that is much longer than the millimeter-scale spot of a THz beam, we use atomic layer lithography to pattern vertical nanogaps in a metal film over an entire 4-inch wafer. We observe an increasing field enhancement as the slit width decreases from 20 nm to 2 nm, in agreement with numerical calculations.

Split-Wedge Antennas with Sub-5 nm Gaps for Plasmonic Nanofocusing

We present a novel plasmonic antenna structure, a split-wedge antenna, created by splitting an ultrasharp metallic wedge with a nanogap perpendicular to its apex. The nanogap can tightly confine gap plasmons and boost the local optical field intensity in and around these opposing metallic wedge tips. This three-dimensional split-wedge antenna integrates the key features of nanogaps and sharp tips, i.e., tight field confinement and three-dimensional nanofocusing, respectively, into a single platform. We fabricate split-wedge antennas with gaps that are as small as 1 nm in width at the wafer scale by combining silicon V-grooves with template stripping and atomic layer lithography. Computer simulations show that the field enhancement and confinement are stronger at the tip–gap interface compared to what standalone tips or nanogaps produce, with electric field amplitude enhancement factors exceeding 50 when near-infrared light is focused on the tip–gap geometry. The resulting nanometric hotspot volume is on the order of λ3/106. Experimentally, Raman enhancement factors exceeding 107 are observed from a 2 nm gap split-wedge antenna, demonstrating its potential for sensing and spectroscopy applications.

Low-temperature enhancement of plasmonic performance in silver films

While plasmonic metals can manipulate optical energy at the nanoscale, they suffer from significant losses at visible wavelengths. We investigate the potential of low temperature to decrease such losses in optically thick Ag films. We extract the complex dielectric function (or relative permittivity) from spectroscopic ellipsometry measurements for smooth single-crystalline, smooth polycrystalline, and rough polycrystalline films down to liquid-helium temperatures and fit these data to a temperature-dependent Drude model. Smooth single-crystalline films exhibited the largest improvements relative to room temperature. Below 50 K, the surface plasmon polariton propagation lengths increased by ~50% at 650 nm. In rough polycrystalline films, improvements of 10% are expected.