Modulation characteristics of graphene-based thermal emitters

© 2016 The Japan Society of Applied Physics. We have investigated the modulation characteristics of the emission from a graphene-based thermal emitter both experimentally and through simulations using finite element method modelling. Measurements were performed on devices containing square multilayer graphene emitting areas, with the devices driven by a pulsed DC drive current over a range of frequencies. Simulations show that the dominant heat path is from the emitter to the underlying substrate, and that the thermal resistance between the graphene and the substrate determines the modulation characteristics. This is confirmed by measurements made on devices in which the emitting area is encapsulated by hexagonal boron nitride.This work has been undertaken as part of a UK EPSRC Fellowship in Frontier Manufacturing (GRN) grant no. EP=J018651=1, and of the project “GOSFEL”, which has received funding from the European Union for this research. The authors would also like to thank Choon How Gan for useful discussions

Publisher’s Erratum to: Metamaterial-based graphene thermal emitter

This is the final version. Available from Springer Verlag via the DOI in this record.The article to which this is the erratum is in ORE: http://hdl.handle.net/10871/30747The article Metamaterial-based graphene thermal emitter, written by Cheng Shi, Nathan H. Mahlmeister, Isaac J. Luxmoore, and Geoffrey R. Nash, was originally published electronically on the publisher’s internet portal (currently SpringerLink) on December 6th 2017 without open access. With the author(s)’ decision to opt for Open Choice the copyright of the article changed in February 2018 to © The Author(s) 2018 and the article is forthwith distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, duplication, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made. The original article has been corrected

Visible light emitting waveguide on Si chip

This is the author accepted manuscript. The final version is available from SPIE via the DOI in this record.Photonic lab-on-a-chip portable platforms have proved to be very sensitive, rapid in analysis and easy-to-use. However, they still rely on a bulk light source to operate, thus hindering the actual portability and potential for commercial realization. In the present paper we have proposed a design for a light emitting structure that could be easily implemented on chip. The design consists of a Si3N4 strip waveguide on SiO2 substrate, with an active material that emits light as top and lateral cladding. The cross-section of the waveguide was optimised to support both excitation and emission as guided modes, with a high mutual overlap and high confinement to the cladding. This ensures an efficient light emission activation from the cladding and a stable propagation along the waveguide. The proposed structure shows to be operative along the visible range; demonstrated from 400nm to 633nm. The procedure we have followed along this report can be virtually used for designing the cross-section geometry of any strip waveguide system so that the performance is optimised for a given cladding refractive index and emission and excitation wavelengths. In addition we have proposed the use of polymeric quantum dots as the gain material to be used as active cladding. The ease of on-chip integration of this gain material via spin-coating, together with the simplicity of our light emitting waveguide, makes our light source design suitable for large-scale integration on Si chip. Specially, for lab-on-chip applications where multiplexed operation is essential.This research was possible thanks to the Engineering and Physical Sciences Research Council (EPSRC) Centre for Doctoral Training in Electromagnetic Metamateriales at University of Exeter (Grant No. EP/L015331/1) and also via the EPSRC Grant EP/N035569/1

Graphene–Metamaterial Photodetectors for Integrated Infrared Sensing

PublishedIn this work we study metamaterial-enhanced graphene photodetectors operating in the mid-IR to THz. The detector element consists of a graphene ribbon embedded within a dual-metal split ring resonator, which acts like a cavity to enhance the absorption of electromagnetic radiation by the graphene ribbon, while the asymmetric metal contacts enable photothermoelectric detection. Detectors designed for the mid-IR demonstrate peak responsivity (referenced to total power) of ∼120 mV/W at 1500 cm–1 and are employed in the spectroscopic evaluation of vibrational resonances, thus demonstrating a key step toward a platform for integrated surface-enhanced sensing.The authors thank Johanna Wolf for providing the QCL used for the detector characterization. This research was supported by the European Union under the FET-open grant GOSFEL and the Swiss National Science Foundation through NCCR QSIT. G.R.N. also gratefully acknowledges the support of the UK Engineering and Physical Sciences Research Council through a fellowship in Frontier Manufacturing (Grant No. EP/J018651/1)

Optical gating of photoluminescence from color centers in hexagonal boron nitride.

This is the final version. Available from the publisher via the DOI in this record.We report on multicolor excitation experiments with color centers in hexagonal boron nitride at cryogenic temperatures. We demonstrate controllable optical switching between bright and dark states of color centers emitting around 2eV. Resonant, or quasi-resonant excitation of photoluminescence also pumps the color center, via a two-photon process, into a dark state, where it becomes trapped. Repumping back into the bright state has a step-like spectrum with a defect dependent threshold between 2.25 and 2.6eV. This behavior is consistent with photoionization and charging between optically bright and dark states of the defect. Furthermore, a second zero phonon line, detuned by +0.4eV, is observed in absorption with orthogonal polarization to the emission, evidencing an additional energy level in the color center.Engineering and Physical Sciences Research Council (EPSRC

Inverse design of whispering-gallery nanolasers with tailored beam shape and polarization

This is the final version. Available from the American Chemical Society via the DOI in this record. Control over the shape and polarization of the beam emitted by a laser source is important in applications such as optical communications, optical manipulation and high-resolution optical imaging. In this paper, we present the inverse design of monolithic whispering-gallery nanolasers which emit along their axial direction with a tailored laser beam shape and polarization. We design and experimentally verify three types of submicron cavities, each one emitting into a different laser radiation mode: an azimuthally polarized doughnut beam, a radially polarized doughnut beam and a linearly polarized Gaussian-like beam. The measured output laser beams yield a field overlap with respect to the target mode of 92%, 96%, and 85% for the azimuthal, radial, and linearly polarized cases, respectively, thereby demonstrating the generality of the method in the design of ultracompact lasers with tailored beams.Engineering and Physical Sciences Research Council (EPSRC)Engineering and Physical Sciences Research Counci

Boron nitride encapsulated graphene infrared emitters

This is the final version of the article. Available from AIP Publishing via the DOI in this record.The spatial and spectral characteristics of mid-infrared thermal emission from devices containing a large area multilayer graphene layer, encapsulated using hexagonal boron nitride, have been investigated. The devices were run continuously in air for over 1000 h, with the emission spectrum covering the absorption bands of many important gases. An approximate solution to the heat equation was used to simulate the measured emission profile across the devices yielding an estimated value of the characteristic length, which defines the exponential rise/fall of the temperature profile across the device, of 40 μm. This is much larger than values obtained in smaller exfoliated graphene devices and reflects the device geometry, and the increase in lateral heat conduction within the devices due to the multilayer graphene and boron nitride layers.This work has been undertaken as part of an UK Engineering and Physical Sciences Research Council Fellowship (GRN) in Frontier Manufacturing (Grant No. EP/J018651/1)

Single vs double anti-crossing in the strong coupling between surface plasmons and molecular excitons (article)

This is the final version. Available on open access from AIP Publishing via the DOI in this recordThe dataset associated with this article is located in ORE at: https://doi.org/10.24378/exe.3023Strong coupling between surface plasmons and molecular excitons may lead to the formation of new hybrid states—polaritons—that are part light and part matter in character. A key signature of this strong coupling is an anti-crossing of the exciton and surface plasmon modes on a dispersion diagram. In a recent report on strong coupling between the plasmon modes of a small silver nano-rod and a molecular dye, it was shown that when the oscillator strength of the exciton is large enough, an additional anti-crossing feature may arise in the spectral region where the real part of the permittivity of the excitonic material is zero. However, the physics behind this double anti-crossing feature is still unclear. Here, we make use of extensive transfer matrix simulations to explore this phenomenon. We show that for low oscillator strengths of the excitonic resonance, there is a single anti-crossing arising from strong coupling between the surface plasmon and the excitonic resonance, which is associated with the formation of upper and lower plasmon–exciton polaritons. As the oscillator strength is increased, we find that a new mode emerges between these upper and lower polariton states and show that this new mode is an excitonic surface mode. Our study also features an exploration of the role played by the orientation of the excitonic dipole moment and the relationship between the modes we observe and the transverse and longitudinal resonances associated with the excitonic response. We also investigate why this type of double splitting is rarely observed in experiments.Engineering and Physical Sciences Research Council (EPSRC)European Research Council (ERC

Strong coupling in the far-infrared between graphene plasmons and the surface optical phonons of silicon dioxide

This is the author accepted manuscript. The final version is available from American Chemical Society via the DOI in this record.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 the silicon dioxide.This research was supported by the UK Engineering and Physical Sciences Research Council, via the award of a Fellowship in Frontier Manufacturing (EP/J018651/1) to G.N., and the European Union under the FET-open grant GOSFEL

Stimulated emission depletion spectroscopy of color centers in hexagonal boron nitride

This is the final version. Available on open access from the American Chemical Society via the DOI in this recordWe demonstrate the use of Stimulated Emission Depletion (STED) spectroscopy to map the electron-optical-phonon sideband of the ground state of the radiative transition of color centers in hexagonal boron nitride emitting at 2.0–2.2 eV, with in-plane linear polarization. The measurements are compared to photoluminescence of excitation (PLE) spectra that maps the electron-optical-phonon sideband of the excited state. The main qualitative difference is a red-shift in the longitudinal optical phonon peak associated with E1u symmetry at the zone center. We compare our results to theoretical work on different defect species in hBN and find they are consistent with a carbon-based defect.Engineering and Physical Sciences Research Council (EPSRC