Highly tunable hybrid metamaterials employing split-ring resonators strongly coupled to graphene surface plasmons

Metamaterials and plasmonics are powerful tools for unconventional manipulation and harnessing of light. Metamaterials can be engineered to possess intriguing properties lacking in natural materials, such as negative refractive index. Plasmonics offers capabilities to confine light in subwavelength dimensions and to enhance light-matter interactions. Recently,graphene-based plasmonics has revealed emerging technological potential as it features large tunability, higher field-confinement and lower loss compared to metal-based plasmonics. Here,we introduce hybrid structures comprising graphene plasmonic resonators efficiently coupled to conventional split-ring resonators, thus demonstrating a type of highly tunable metamaterial, where the interaction between the two resonances reaches the strong-coupling regime. Such hybrid metamaterials are employed as high-speed THz modulators, exhibiting over 60% transmission modulation and operating speed in excess of 40 MHz. This device concept also provides a platform for exploring cavity-enhanced light-matter interactions and optical processes in graphene plasmonic structures for applications including sensing, photo-detection and nonlinear frequency generation

Nanostructure-enhanced infrared spectroscopy

While infrared spectroscopy is a powerful technique that provides molecular information such as chemical constituents and chemical structures of analytes, it suffers from low absorption cross-section resulting in low sensitivity and poor signal-to-noise or signal-to-background ratios. Surface-enhanced infrared absorption (SEIRA) spectroscopy, which is supported by nanometer scale structures, is a promising technology to overcome these problems in conventional infrared (IR) spectroscopy and enhances IR signals using the field enhancement properties of surface plasmon resonance. Recently resonant SEIRA technique was proposed, and signal enhancement factor was significantly improved. In this review, we present an overview of the recent progresses on resonant SEIRA technologies including nanoantenna- and metamaterial-based SEIRA, and also SEIRA techniques with nanoimaging capabilities

Two-dimensional material-based nanosensors for detection of low-molecular-weight molecules

Low-molecular-mass small molecules play important roles in biological processes and often serve as disease-related biomarkers for diagnosis. Accurate detection of small molecules remains challenging for conventional sensors due to their limited sensitivities. Two-dimensional (2D) materials, thanks to their atomic level thickness, can be extraordinarily sensitive to external perturbations and therefore well-suited for sensing applications. This dissertation explores the use of 2D materials, including primarily graphene and transition metal dichalcogenides, in the detection of low-molecular-weight and low-charge molecules. This work starts with the study of methods that allow for efficient and clean transfer of graphene grown on Cu using chemical vapor deposition (CVD), which is a critical step for achievement of large-area and high-quality graphene for device fabrication. In addition to the conventional wet-etching transfer method, we have studied on the method of electrochemical delamination, which is more time-efficient and allows for recycling of the Cu foil. Generation of bubbles during the electrochemical reaction is minimized by tuning the experimental parameters, thereby minimizing transfer-induced damages to graphene. We then fabricate the graphene-based field effect transistor (FET) and use the graphene FET as biosensors. First, the sensor is configured as an electrolyte-gated FET. With appropriate biochemical functionalization of the graphene, the FET sensors have been used to detect multiple small-molecule biomarkers including glucose and insulin via their affinity binding with receptors. Then, on a flexible substrate, we demonstrate real-time measurement of tumor necrosis factor alpha, a signal protein that regulates immune cells. We then simplified the sensor structure using a bottom local-gate to replace the external electrode as required in the previous electrolyte gated FET. Using the bottom local-gated FET sensor we have carried out real-time monitoring of the variation of pH in solutions. In addition to the electrical sensors, highly sensitive and multifunctional plasmonic sensors have also been developed by combining the unique optical properties of graphene with engineered metallic metasurfaces. The plasmonic sensors operating in mid-infrared region are configured as either metallic metasurface or hybrid graphene-metallic metasurface. Using a metallic metasurface, we demonstrate simultaneous quantification and fingerprinting of protein molecules. Using a hybrid graphene-metallic metasurface, we demonstrate optical conductivity-based ultrasensitive biosensing. In contrast to refractive-index-based sensors, the sensitivity of the hybrid metasurface sensor is not limited by the molecular masses of analytes. A monolayer of the sub-nanometer chemicals can be readily detected and differentiated on the hybrid metasurface. Reversible detection of glucose is carried out via the affinity binding of glucose with boronic acid immobilized on the graphene of the hybrid metasurface. The lowest detection limit achieved in our work is 36 pg/mL, which is considerably lower than that for the existing optical sensors. Despite the high sensitivity of graphene, the zero band-gap of graphene fundamentally impedes its use in digital electronic devices. In contrast, two-dimensional semiconductors, such as transition metal dichalcogenide (TMDC) with non-zero band gaps, holds great potential for developing practical electronic devices and sensors. Monolayers of TMDC materials are particularly attractive for development of deeply scaled devices, although the contact resistance between metal and the monolayer TMDC has been so large to significantly limit the performance of the devices. We present a high-performance monolayer MoS2 FET with a monolayer graphene as bottom local gate. The graphene gate is found to significantly improve the dielectric strength of the oxide layer compared to the lithographically patterned metal gate. This in turn allows for the use of very thin gate dielectric layer (~5 nm) and application of a strong displacement field to lower the contact resistance. Benefiting from the low contact resistance, the monolayer MoS2 FET offers a high on/off ratio (108) and low subthreshold slope (64 mV/decade). Additionally, thanks to the highly efficient electrostatic coupling through the ultrathin gate dielectric layer, short-channel (50 nm and 14 nm) devices are realized that exhibit excellent switching characteristics. In summary, this dissertation presents significant contributions to 2D material-based electronic and optoelectronic nanosensors, especially for detection of small molecules. Perspectives are made in the end of the thesis, on future studies needed to realize practical applications of these sensors and other 2D material-based products

Fast Room-Temperature Detection of Terahertz Quantum Cascade Lasers with Graphene-Loaded Bow-Tie Plasmonic Antenna Arrays

We present a fast room-temperature terahertz detector based on interdigitated bow-tie antennas contacting graphene. Highly efficient photodetection was achieved by using two metals with different work functions as the arms of a bow-tie antenna contacting graphene. Arrays of the bow-ties were fabricated in order to enhance the responsivity and coupling of the incoming light to the detector, realizing an efficient imaging system. The device has been characterized and tested with a terahertz quantum cascade laser emitting in single frequency around 2 THz, yielding a responsivity of ∼34 μA/W and a noise-equivalent power of ∼1.5 × 10$^{-7}$ W/Hz$^{1/2}$.R.D., Y.R., and H.E.B. acknowledge financial support from the Engineering and Physical Sciences Research Council (Grant No. EP/J017671/1, Coherent Terahertz Systems). S.H. acknowledges funding from EPSRC (Grant No. EP/K016636/1, GRAPHTED). H.L. and J.A.Z. acknowledge financial support from the EPSRC (Grant No. EP/L019922/1). J.A.A.-W. acknowledges a Research Fellowship from Churchill College, Cambridge. H.J.J. thanks the Royal Commission for the Exhibition of 1851 for her Research Fellowship.This is the final version of the article. It first appeared from American Chemical Society via https://doi.org/10.1021/acsphotonics.6b0040

Gold/graphene fractals as tunable plasmonic devices

Graphene, an atomically thin sheet of carbon atoms arranged in a honeycomb geometry, is attracting unique attention thanks to its extraordinary mechanical, electrical and optical properties. This thesis work concerns the realization of graphene-based nanoscale devices for novel plasmonic applications. We focus mainly on gold/graphene (Au/G) structures designed to display plasmonic multiresonances in the visible range thanks to the nanostructure geometry based on the Sierpinski carpet (SC) deterministic fractal

Optomechanical Devices and Sensors Based on Plasmonic Metamaterial Absorbers

Surface plasmon resonance is the resonant oscillations of the free electrons at the interface between two media with different signs in real permittivities, e.g. a metal and a dielectric, stimulated by light. Plasmonics is a promising field of study, because electron oscillations inside a subwavelength space at optical frequencies simultaneously overcome the limit of diffraction in conventional photonics and carrier mobilities in semiconductor electronics. Due to the subwavelength confinement, plasmonic resonances can strongly enhance local fields and, hence, magnify light-matter interactions. Optical absorbers based on plasmonic metamaterials can absorb light resonantly at the operating wavelengths with up to 100% efficiency. We have explored plasmonic absorbers at infrared wavelengths for thermal detectors, e.g. a gold nanostrip antenna absorber that can absorb 10-times light using only 2% of material consumption comparing to a uniform gold film. In an optomechanical device, the optical mode and mechanical mode are mutually influenced, through the optical forces exerted on the mechanical oscillator and the detuning of optical resonance by the mechanical oscillator, so that the mechanical oscillations are either amplified or suppressed by light. We designed an optomechanical device integrated with plasmonic metamaterial absorber on a membrane mechanical oscillator, wherein a tunable Fano-resonant absorption in the absorber arises from the coupling between the plasmonic and Fabry-Perot reonsances. The absorber traps the incident light and heat up the membrane, causing an increase in thermal stress and a normal plasmomechanical force on it. This is a light-absorption-dependent elastic force arising from the opto-thermo-mechanical interactions. Due to the finite thermal response time in the membrane, the elastic plasmomechanical force is delayed and, consequently, generates a viscous component modifying the damping rate of the mechanical oscillator. We have observed optomechanical amplification and cooling in the device at designed detuning conditions. In particular, on the condition that the optomechanical gain beats the intrinsic mechanical damping, the oscillation becomes coherent, i.e. phonon lasing. We successfully demonstrated phonon lasing with a threshold power of 19 μW. This device is promising as an integration-ready coherent phonon source and may set the stage for applications in fundamental studies and ultrasonic imaging modalities

Efficient probes for terahertz near-field microscopy and spectroscopy

This thesis focuses on improving the sensitivity and spatial resolution of two near-field microscopy techniques at terahertz (THz) frequencies: direct detection in the near-field using a collection-mode aperture probe, and using the apex of a metallic tip to scatter a near-field interaction into the far-field. The first technique is limited in spatial resolution primarily due to strong attenuation of THz fields transmitted through the subwavelength aperture. By integrating a terahertz detector with an optical metasurface it is possible to make nanoscale terahertz detectors, which can efficiently detect non-propagating THz fields close to the rear of the aperture, increasing probe sensitivity and spatial resolution. The scattering technique suffers from high background signals and weak scattering from the tip apex. By designing a scattering probe to act as a resonant dipole antenna, the efficiency of the scattering process can be improved, and by efficiently coupling THz radiation to the probe using a radially polarized THz source, interference from background signals can be reduced. These improvements can enable imaging of a variety of fascinating systems, including polaritons in monolayers and heterostructures of 2D materials, biological systems and topological insulators

Gradient metasurfaces: a review of fundamentals and applications

In the wake of intense research on metamaterials the two-dimensional analogue, known as metasurfaces, has attracted progressively increasing attention in recent years due to the ease of fabrication and smaller insertion losses, while enabling an unprecedented control over spatial distributions of transmitted and reflected optical fields. Metasurfaces represent optically thin planar arrays of resonant subwavelength elements that can be arranged in a strictly or quasi periodic fashion, or even in an aperiodic manner, depending on targeted optical wavefronts to be molded with their help. This paper reviews a broad subclass of metasurfaces, viz. gradient metasurfaces, which are devised to exhibit spatially varying optical responses resulting in spatially varying amplitudes, phases and polarizations of scattered fields. Starting with introducing the concept of gradient metasurfaces, we present classification of different metasurfaces from the viewpoint of their responses, differentiating electrical-dipole, geometric, reflective and Huygens' metasurfaces. The fundamental building blocks essential for the realization of metasurfaces are then discussed in order to elucidate the underlying physics of various physical realizations of both plasmonic and purely dielectric metasurfaces. We then overview the main applications of gradient metasurfaces, including waveplates, flat lenses, spiral phase plates, broadband absorbers, color printing, holograms, polarimeters and surface wave couplers. The review is terminated with a short section on recently developed nonlinear metasurfaces, followed by the outlook presenting our view on possible future developments and perspectives for future applications.Comment: Accepted for publication in Reports on Progress in Physic