G. R. Nash

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 of confining light in subwavelength dimensions and enhancing light–matter interactions. Recently, the technological potential of graphene-based plasmonics has been recognized as the latter features large tunability, higher field-confinement and lower loss compared with metal-based plasmonics. Here, we introduce hybrid structures comprising graphene plasmonic resonators 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 ∼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.

Strong Coupling in the Far-Infrared between Graphene Plasmons and the Surface Optical Phonons of Silicon Dioxide

We study plasmonic resonances in electrostatically gated graphene nanoribbons on silicon dioxide substrates. Absorption spectra are measured in the mid-far-infrared and reveal multiple peaks, with width-dependent resonant frequencies. We calculate the dielectric function within the random phase approximation and show that the observed spectra can be explained by surface-plasmon–phonon–polariton modes, which arise from coupling of the graphene plasmon to three surface optical phonon modes in silicon dioxide.

Macroscopic acoustoelectric charge transport in graphene

We demonstrate macroscopic acoustoelectric transport in graphene, transferred onto piezoelectric lithium niobate substrates, between electrodes up to 500 μm apart. Using double finger interdigital transducers we have characterised the acoustoelectric current as a function of both surface acoustic wave intensity and frequency. The results are consistent with a relatively simple classical relaxation model, in which the acoustoelectric current is proportional to both the surface acoustic wave intensity and the attenuation of the wave caused by the charge transport.

Activation energy for fluorine transport in amorphous silicon

The transport of ion implanted F in amorphous Si is studied using secondary ion mass spectroscopy and transmission electron microscopy. Significant redistribution of F is observed at temperatures in the range 600°C to 700°C. The measured F depth-profiles are modelled using a simple Gaussian solution to the diffusion equation, and the diffusion coefficient is deduced at each temperature. An activation energy of 2.2eV±0.4eV for F transport is extracted from an Arrhenius plot of the diffusion coefficients. It is shown that the F transport is influenced by implantation induced defects.

Long-wavelength HgCdTe negative luminescent devices

We have investigated the negative luminescent properties of a HgCdTe device, fabricated from a 1 mm diameter array of photodiodes having peak emission at a wavelength of 8.5 μm. This long-wavelength luminescence is of sufficient efficiency and area to be useful in device applications.

Midinfrared GaInSb/AlGaInSb quantum well laser diodes operating above 200 K

Electroluminescence from GaInSb/AlGaInSb type I quantum well diode lasers, grown on GaAs, has been investigated as a function of strain in the quantum wells. Lasing was observed, in pulsed operation, up to temperatures of 161, 208, 219, and 202 K for structures containing 0.55%, 0.62%, 0.78%, and 1.1% strain, respectively, with lasing occurring at ∼3.3 μm at 200 K for the 1.1% structure.

Acoustoelectric photoresponse in graphene

The acoustoelectric current in graphene has been investigated as a function of illumination, using blue (450 nm) and red (735 nm) light-emitting diodes (LEDs), and surface acoustic wave (SAW) intensity and frequency. The measured acoustoelectric current increases with illumination, more than the measured change in the conductivity of the graphene, whilst retaining a linear dependence on the SAW intensity. The latter is consistent with the interaction between the carriers and SAWs being described by a relatively simple classical relaxation model suggesting that the change in the acoustoelectric current is caused by the effect of the illumination on the electronic properties of the graphene. The increase in the acoustoelectric current is greatest under illumination with the blue LED, consistent with the creation of a hot electron distribution.

Micromachined optical concentrators for IR negative luminescent devices

Negative luminescent (NL) devices, which to an IR observer appear colder than they actually are, have a wide range of possible applications, including use as thermal radiation shields in IR cameras, and as IR sources in gas-sensing systems. For many of these applications a large area (>1 cm2) device which displays as large as possible apparent temperature range is required. However, under reverse bias, significant currents are required to reduce the carrier concentrations to the levels needed for maximum possible absorption. We have therefore used a novel micromachining technique to fabricate integrated optical concentrators in InSb/InAlSb and HgCdTe NL devices. Smaller area diodes can then be used to achieve the same absorption (e.g. for InSb an area reduction of 16 is possible) and the required currents are thus reduced. To fabricate the concentrators, spherical resist masks are first produced, which are ∼10 μm high and ∼53 μm wide, by resist reflow at 120°C. Inductively coupled plasma (ICP) etching is then used to etch alternately the resist mask and the semiconductor, with oxygen and methane/hydrogen respectively, producing concentrators with almost parabolic profiles. Currently, the concentrators are typically 30 μm high, with a top diameter of ∼15 μm. Continuing optimization of the process to reach the theoretical limits of optical gain is described.

Design of practicable phase-change metadevices for near-infrared absorber and modulator applications

Phase-change chalcogenide alloys, such as Ge2Sb2Te5 (GST), have very different optical properties in their amorphous and crystalline phases. The fact that such alloys can be switched, optically or electrically, between such phases rapidly and repeatedly means that they have much potential for applications as tunable photonic devices. Here we incorporate chalcogenide phase-change films into a metal-dielectric-metal metamaterial electromagnetic absorber structure and design absorbers and modulators for operation at technologically important near-infrared wavelengths, specifically 1550 nm. Our design not only exhibits excellent performance (e.g. a modulation depth of ~77% and an extinction ratio of ~20 dB) but also includes a suitable means for protecting the GST layer from environmental oxidation and is well-suited, as confirmed by electro-thermal and phase-transformation simulations, to in situ electrical switching. We also present a systematic study of design optimization, including the effects of expected manufacturing tolerances on device performance and, by means of a sensitivity analysis, identify the most critical design parameters.

Temperature dependence of the acoustoelectric current in graphene

The acoustoelectric current in graphene has been investigated as a function of temperature, surface acoustic wave (SAW) intensity, and frequency. At high SAW frequencies, the measured acoustoelectric current decreases with decreasing temperature, but remains positive, which corresponds to the transport of holes, over the whole temperature range studied. The current also exhibits a linear dependence on the SAW intensity, consistent with the interaction between the carriers and SAWs being described by a relatively simple classical relaxation model. At low temperatures and SAW frequencies, the measured acoustoelectric current no longer exhibits a simple linear dependence on the SAW intensity, and the direction of the acoustoelectric current is also observed to reverse under certain experimental conditions.