Arrays of nanoantennas consisting of plasmonic dipole pairs have been widely used in surface-enhanced Raman spectroscopy (SERS). Fine-tuned structures that can efficiently convert incident electromagnetic energy to excite molecules and provide enhanced detection. However, this tuning mechanism also has its disadvantages. In order to prevent the cross coupling, the distance between each individual element must be increased. This leads to low packing density values which in turn results in a reduction of the overall enhanced Raman signal when these structures are compared to broadly tuned aggregates of particles such as those obtained through metal sputtering or colloidal deposition. In this work we demonstrate through simulations and experimental work that it is possible to increase the reflected signal of an array of nanoantennas by reducing the distance between them in the direction both perpendicular and parallel to the orientation of the incident electric field. It is shown the resonant wavelength shifts in two different spectral directions depending in how the intercell distance was reduced. These resultant shifts can reduce the tuning capabilities of the structures but also can increase the SERS intensity due to close coupling of the dipole pairs. We believe that these results will enable the design and fabrication of structures possessing a greater degree of tunability together with an overall enhanced Raman signal that can rival aggregated SERS substrates.
The deposition of organic molecules on gold nanoantennas is reported through chemisorption for sensing in the midinfrared (mid-IR) spectral range. The specific nanostructures are gold asymmetric-split ring resonators (A-SRRs) based on circular-geometry with two different ‘arc’ lengths. The plasmonic resonant coupling technique was used to match the vibrational responses of the targeted molecules for their enhanced detection. Gold nanostructures are functionalised through chemisorption of octadecanethiol (ODT) in ethanol solution. The molecular vibrational responses were measured using a microscope coupled Fourier Transform Infrared (FTIR) spectroscopy. The experimental findings are closely supported using FDTD simulation. The modified nanoantennas surfaces are capable of supporting wide range of organic-sensing applications.
Metamaterials are being increasingly used as highly sensitive detection devices. The design of these structures and the
ability to effect changes in response through small changes in the geometry of their constituent elements allow for the
enhancement of known analysis techniques such as infrared or Raman spectroscopy. High electromagnetic fields have
been shown to occur in features such as small gaps and sharp tips and these so called “hot-spots” are the main focus of
recent work in Surface Enhanced Raman Spectroscopy (SERS). Previous work has shown dipole pairs with small gaps
between them to be suitable for the SERS detection of very small amounts of organic compounds. The main difficulties
lie in the small dimensions (≤100 nm) necessary to attain a significant response at the typical Raman pump wavelengths.
Also the small size of the gaps is a challenge when it comes to prevent “bridging” between the structures during the
fabrication process. In this work we show, through simulations, that carefully controlling the length of dipolar structures
as well as the gap between these dipoles a resonant response can be achieved close to the pump Raman wavelengths.
Also, we see that increasing the width of the dipole pair shifts the resonant peaks to longer wavelengths. By optimizing
their geometry, more efficient and easier to fabricate structures can be used as environmental organic sensors.
The effects of two different type of asymmetric nano-antenna that produce distinct resonance peaks were experimentally and numerically observed. At mid-infrared wavelengths broad resonances based upon various metamaterials structures have been previously reported. Here we show that introducing a crossbar on vertical asymmetric dipole nano-antenna can produce narrower resonance peaks than the dipole alone. Our approach to investigating different asymmetric nanoantenna structures yielded quality factor values more than twice the existing values reported within this region of the electromagnetic spectrum.
We tune nanoantennas to resonate within mid-infrared wavelengths to match the vibrational resonances of C=C and C-H of the hormone estradiol. Modelling and fabrication of the nanoantennas produce plasmon resonances between 2 μm to 7 μm. The hormone estradiol was dissolved in ethanol and evaporated, leaving thickness of a few hundreds of nanometres on top of gold asymmetric split H-like shaped on a fused silica substrate. The reflectance was measured and a red-shift is recorded from the resonators plasmonic peaks. Fourier transform infrared spectroscopy is use to observe enhanced spectra of the stretching modes for the analyte which belongs to alkenyl biochemical group.
This presentation is concerned with nanophotonic structures, especially with arrays of asymmetric split-ring resonator (ASRR) structures, that may be exploited in a variety of sensing applications. These applications include bio-medical sensing, organic material sensing more generally - and environmental sensing. Specific attention has been paid to the identification of molecules of interest via their bond-resonance spectral signatures.
Recent advances have seen asymmetric split ring resonators (A-SRRs) developed as sensing elements to record a shift in their peaks when there is a corresponding change in the surrounding environment. These studies have led to the investigation of Fano resonances associated with the coupling of the resonances of the A-SRRs with the molecular resonances of the analyte. The hormone estradiol (E2) was dissolved in ethanol and evaporated, leaving thickness of a few hundreds of nanometres on top of gold A-SRRs on a silica substrate. The reflectance was measured and a red shift is recorded from the resonators plasmonic peaks. The geometric sizes of the ASRRs are calculated to tune the plasmonic resonances near the molecular resonance of the C-H stretch at nominally 3.33 microns. Corresponding Lumerical modelling of the experimental data is performed using only the intensity and wavelength to match the Fano resonance at modified wavelengths of 3.42 and 3.49 microns.
Asymmetric split ring resonators (A-SRRs) are formed when two separate metallic arcs of different lengths
share the same centre-of-curvature. The resonances of the two arcs interact to produce steep slopes in the
reflection spectrum. Due to their size they are also known as nano antennas. By depositing very thin films of
poly-methyl-methacrylate (PMMA), a shift in resonance reflection spectra is obtained. Similarly, it is known
that the spectral position of the A-SRR resonances can be tuned with size. We show that, when PMMA is
used as an organic probe (analyte) on top of an A-SRR array, the phase and amplitude of a characteristic
molecular bond resonance associated with PMMA changes the appearance of the observed Fano resonance,
according to the spectral position of the plasmonic reflection peaks. This effect can be utilized to give
characteristic signatures for the purpose of detection. We also show the effectiveness of localizing different
blocks of PMMA at different places on the A-SRR array to detect very small amounts of non-uniformly
distributed analytes. Finally we show that even though the resonance Q-factor is much smaller when
compared to values achievable in photonic crystal microcavities, the plasmonic nano-antenna arrays can be
used to provide highly sensitive detection of organic compounds.
Planar devices that can be categorised as having a nanophotonic dimension constitute an increasingly important area of
photonics research. Device structures that come under the headings of photonic crystals, photonic wires and
metamaterials are all of interest - and devices based on combinations of these conceptual approaches may also play an
important role. Planar micro-/nano-photonic devices seem likely to be exploited across a wide spectrum of applications
in optoelectronics and photonics. This spectrum includes the domains of display devices, biomedical sensing and sensing
more generally, advanced fibre-optical communications systems - and even communications down to the local area
network (LAN) level. This article will review both device concepts and the applications possibilities of the various
Double asymmetric split ring resonators (DA-SRRs) are composed of four separate asymmetric metallic arcs that share
the same centre-of-curvature. These four arcs interact to produce very steep slopes in the reflection spectrum and also
increase the number of trapped modes. This combination produces larger resonance quality-factors (Q-factors), making
arrays of such resonators potentially useful for optical sensing.
The response of metallic split ring resonators (SRRs) scales linearly with their dimensions. At higher frequencies, metals
do not behave like perfect conductors but display properties characterized by the Drude model. In this paper we compare
the responses of nano-sized gold-based SRRs at near infra-red wavelengths. Deposition of gold SRRs onto dielectric
substrates typically involves the use of an additional adhesion layer. We have employed the commonly used metal
titanium (Ti) to provide an adhesive layer for sticking gold SRRs to silicon substrates - and have investigated the effect
of this adhesion layer on the overall response of these gold SRRs. Both experimental and theoretical results show that
even a two nm thick titanium adhesion layer can shift the overall SRR response by 20 nm.
Metamaterials based on single-layer metallic Split Ring Resonators (SRR) and Wires have been
demonstrated to have a resonant response in the near infra-red wavelength range. The use of
semiconductor substrates gives the potential for control of the resonant properties of split-ring
resonator (SRR) structures by means of active changes in the carrier concentration obtained using
either electrical injection or photo-excitation. We examine the influence of extended wires that are
either parallel or perpendicular to the gap of the SRRs and report on an equivalent circuit model that
provides an accurate method of determining the polarisation dependent resonant response for
incident light perpendicular to the surface. Good agreement is obtained for the substantial shift
observed in the position of the resonances when the planar metalisation is changed from gold to
We report a novel method for modeling the resonant frequency response of infra-red light, in the range of 2 to 10
microns, reflected from metallic spilt ring resonators (SRRs) fabricated on a silicon substrate. The calculated positions of
the TM and TE peaks are determined from the plasma frequency associated with the filling fraction of the metal array
and the equivalent LC circuit defined by the SRR elements. The capacitance of the equivalent circuit is calculated using
conformal mapping techniques to determine the co-planar capacitance associated with both the individual and the
neighbouring elements. The inductance of the equivalent circuit is based on the self-inductance of the individual
elements and the mutual inductance of the neighboring elements.
The results obtained from the method are in good agreement with experimental results and simulation results obtained
from a commercial FDTD simulation software package. The method allows the frequency response of a SRR to be
readily calculated without complex computational methods and enables new designs to be optimised for a particular
frequency response by tuning the LC circuit.
Gold Split Ring Resonators (SRRs) were fabricated on silicon substrates by electron beam lithography and lift-off, with overall dimensions of approximately 200 nm. Reflectance spectra from the SRRs are similar to those published elsewhere. New devices are proposed based on the additional functionality afforded by the use of a silicon substrate.