Optical Tweezing Terahertz Probing for a Single Metal Nanoparticle.

Recently, extensive research has been reported on the detection of metal nanoparticles using terahertz waves, due to their potential for efficient and nondestructive detection of chemical and biological samples without labeling. Resonant terahertz nanoantennas can be used to detect a small amount of molecules whose vibrational modes are in the terahertz frequency range with high sensitivity. However, the positioning of target molecules is critical to obtaining a reasonable signal because the field distribution is inhomogeneous over the antenna structure. Here, we combine an optical tweezing technique and terahertz spectroscopy based on nanoplasmonics, resulting in extensive controllable tweezing and sensitive detection at the same time. We observed optical tweezing of a gold nanoparticle and detected it with terahertz waves by using a single bowtie nanoantenna. Furthermore, the calculations confirm that molecular fingerprinting is possible by using our technique. This study will be a prestep of biomolecular detection using gold nanoparticles in terahertz spectroscopy.

Trench Formation under the Tunable Nanogap: Its Depth Depends on Maximum Strain and Periodicity.

Metallic nanogaps have been studied for many years in the context of a significant amount of field enhancements. Nanogaps of macroscopic lengths for long-wave applications have attracted much interest, and recently one dimensional tunable nanogaps have been demonstrated using flexible PET substrates. For nanogaps on flexible substrates with applied tensile strain, large stress is expected in the vicinity of the gap, and it has been confirmed that several hundred nanometer-deep trenches form beneath the position of the nanogap because of this stress singularity. Here, we studied trench formation under nanogap structures using COMSOL Multiphysics 6.1. We constructed a 2D nanogap unit cell, consisting of gold film with a crack on a PDMS substrate containing a trench beneath the crack. Then, we calculated the von Mises stress at the bottom of the trench for various depths and spatial periods. Based on it, we derived the dependence of the trench depth on the strain and periodicity for various yield strengths. It was revealed that as the maximum tensile strain increases, the trench deepens and then diverges. Moreover, longer periods lead to larger depths for the given maximum strain and larger gap widths. These results could be applied to roughly estimate achievable gap widths and trench depths for stretchable zerogap devices.

Strain versus Tunable Terahertz Nanogap Width: A Simple Formula and a Trench below.

A flexible zerogap metallic structure is periodically formed, healing metal cracks on a flexible substrate. Zerogap is continuously tunable from nearly zero to one hundred nanometers by applying compressive strains on the flexible substrate. However, there have been few studies on how the gap width is related to the strain and periodicity, nor the mechanism of tunability itself. Here, based on atomic force microscopy (AFM) measurements, we found that 200 nm-deep nano-trenches are periodically generated on the polymer substrate below the zerogap owing to the strain singularities extant between the first and the second metallic deposition layers. Terahertz and visible transmission properties are consistent with this picture whereby the outer-bending polyethylene terephthalate (PET) substrate controls the gap size linearly with the inverse of the radius of the curvature.

Antenna-based reduced IR absorbers for high-performance microbolometers.

Absorbers for long-wavelength infrared (LWIR) are designed to have a reduced geometry fitted to a gold cross antenna and numerically studied. Compared to the square membrane geometry widely used in conventional microbolometers, the reduced geometry results in smaller thermal capacities of the vanadium dioxide (VO2) and silicon nitride (Si3N4) layers. However, near-field focusing by the cross antenna leads to a high LWIR absorption. Calculations show that the temperature change per incident energy increases with a decrease in the arm width, and the reduced absorber surpasses the square geometry for all incident angles and polarizations. The antenna-based reduced absorber studied here could serve as an alternative geometry for high-performance microbolometers.

Broadband high-performance terahertz polarizer based on a dense array of 5†nm gap slit antennas.

Critical factors for terahertz polarizers include broadband operation, high transmittance, and a good extinction ratio. In this paper, using a 5 nm-wide metallic slit array with a 200 nm periodicity as a wire grid polarizer, we achieved over 95% transmittance with an average extinction ratio of 40 dB, over the entire spectrum as defined by the terahertz time-domain spectroscopy (0.4 ∼ 2 THz). Theoretical calculations revealed that the slit array can show 100% transmission up to 5 THz, and wider bandwidths with a higher cutoff frequency can be achieved by reducing the slit periodicity. These results provide a novel approach for achieving a broadband THz polarizer and open a new path for seamless integration of the polarizers with nanophotonic applications.

Angstrom-Scale Active Width Control of Nano Slits for Variable Plasmonic Cavity.

Nanogap slits can operate as a plasmonic Fabry-Perot cavity in the visible and infrared ranges due to the gap plasmon with an increased wavenumber. Although the properties of gap plasmon are highly dependent on the gap width, active width tuning of the plasmonic cavity over the wafer length scale was barely realized. Recently, the fabrication of nanogap slits on a flexible substrate was demonstrated to show that the width can be adjusted by bending the flexible substrate. In this work, by conducting finite element method (FEM) simulation, we investigated the structural deformation of nanogap slit arrays on an outer bent polydimethylsiloxane (PDMS) substrate and the change of the optical properties. We found that the tensile deformation is concentrated in the vicinity of the gap bottom to widen the gap width proportionally to the substrate curvature. The width widening leads to resonance blueshift and field enhancement decrease. Displacement ratio ((width change)/(supporting stage translation)), which was identified to be proportional to the substrate thickness and slit period, is on the order of 10-5 enabling angstrom-scale width control. This low displacement ratio comparable to a mechanically controllable break junction highlights the great potential of nanogap slit structures on a flexible substrate, particularly in quantum plasmonics.

High sensitivity bolometers based on metal nanoantenna dimers with a nanogap filled with vanadium dioxide.

One critical factor for bolometer sensitivity is efficient electromagnetic heating of thermistor materials, which plasmonic nanogap structures can provide through the electric field enhancement. In this report, using finite element method simulation, electromagnetic heating of nanorod dimer antennas with a nanogap filled with vanadium dioxide (VO2) was studied for long-wavelength infrared detection. Because VO2 is a thermistor material, the electrical resistance between the two dimer ends depends on the dimer's temperature. The simulation results show that, due to the high heating ability of the nanogap, the temperature rise is several times higher than expected from the areal coverage. This excellent performance is observed over various nanorod lengths and gap widths, ensuring wavelength tunability and ultrafast operating speed, thereby making the dimer structures a promising candidate for high sensitivity bolometers.

Effective-zero-thickness terahertz slot antennas using stepped structures.

Metallic nanostructures play an essential role in electromagnetic manipulations due to the localization and enhancement of electromagnetic waves in nanogaps. Scaling down the dimensions of the gap, such as the gap width and the thickness, is an effective way to enhance light-matter interaction with colossal field enhancement. However, reducing the thickness below 10 nanometers still suffers from fabrication difficulty and unintended direct transmission through metals. Here, we fabricate effective-zero-thickness slot antennas by stepping metals in the vicinity of the gaps to confine electromagnetic waves in tiny volumes. We analyze and simulate terahertz transmission, and demonstrate the absorption enhancement of molecules in the slot antennas. Our fabrication technique provides a simple but versatile tool for maximum field enhancement and molecular sensing.

Topology-Changing Broadband Metamaterials Enabled by Closable Nanotrenches.

One of the most straightforward methods to actively control optical functionalities of metamaterials is to apply mechanical strain deforming the geometries. These deformations, however, leave symmetries and topologies largely intact, limiting the multifunctional horizon. Here, we present topology manipulation of metamaterials fabricated on flexible substrates by mechanically closing/opening embedded nanotrenches of various geometries. When an inner bending is applied on the substrate, the nanotrench closes and the accompanying topological change results in abrupt switching of metamaterial functionalities such as resonance, chirality, and polarization selectivity. Closable nanotrenches can be embedded in metamaterials of broadband spectrum, ranging from visible to microwave. The 99.9% extinction performance is robust, enduring more than a thousand bending cycles. Our work provides a wafer-scale platform for active quantum plasmonics and photonic application of subnanometer phenomena.

Rectangular plasmonic interferometer for high sensitive glycerol sensor.

A novel plasmonic interferometric sensor intended for application to biochemical sensing has been investigated experimentally and theoretically. The sensor was included a slit surrounded by rectangular grooves using a thick gold film. A three-dimensional finite difference time-domain commercial software package was applied to simulate the structure. The Focused ion beam milling has been used as a mean to fabricate series of rectangular plasmonic interferometer with varying slit-groove distance L. Oscillation behavior is shown by transmission spectra in a broadband wavelength range between 400 nm and 800 nm in the distance between slit and grooves. Red-shifted interference spectrum is the result of increasing refractive indices. The proposed structure is functional from visible to near-infrared wavelength range and yields a sensitivity of 4923 nm/RIU and a figure of merit as high as 214 at 729 nm wavelength. In conclusion, this study indicates the possibility of fabricating a low cost, compact, and real-time high-throughput plasmonic interferometer.

Magnetic Nature of Light Transmission through a 5-nm Gap.

Slot antennas have been exploited as important building blocks of optical magnetism because their radiations are invoked by the magnetic fields along the axes, as vectorial Babinet principle predicts. However, optical magnetism of a few-nanometer-width slit, for which fascinating applications are found due to the colossal field enhancement but Babinet principle fails due to the nonnegligible thickness, has not been investigated. In this paper, we demonstrated that the magnetic field plays a dominant role in light transmission through a 5-nm slit on a 150-nm-thick gold film. The 5-nm slit was fabricated by atomic layer lithography, and the transmission was investigated for various incident angles by experiment and simulation at 785-nm wavelength. We found that, due to the deep subwavelength gap width, the transmission has the same incident angle dependence as the tangential magnetic field on the metal surface and this magnetic nature of a nanogap holds up to ~100-nm width. Our analysis establishes conditions for nanogap optical magnetism and suggests new possibilities in realizing magnetic-field-driven optical nonlinearities.

Light scattering of rectangular slot antennas: parallel magnetic vector vs perpendicular electric vector.

We study light scattering off rectangular slot nano antennas on a metal film varying incident polarization and incident angle, to examine which field vector of light is more important: electric vector perpendicular to, versus magnetic vector parallel to the long axis of the rectangle. While vector Babinet's principle would prefer magnetic field along the long axis for optimizing slot antenna function, convention and intuition most often refer to the electric field perpendicular to it. Here, we demonstrate experimentally that in accordance with vector Babinet's principle, the incident magnetic vector parallel to the long axis is the dominant component, with the perpendicular incident electric field making a small contribution of the factor of 1/|ε|, the reciprocal of the absolute value of the dielectric constant of the metal, owing to the non-perfectness of metals at optical frequencies.