Intensity dependences of the nonlinear optical excitation of plasmons in graphene

Recently, we demonstrated an all-optical coupling scheme for plasmons, which takes advantage of the intrinsic nonlinear optical response of graphene. Frequency mixing using free-space, visible light pulses generates surface plasmons in a planar graphene sample, where the phase matching condition can define both the wavevector and energy of surface waves and intraband transitions. Here, we also show that the plasmon generation process is strongly intensity-dependent, with resonance features washed out for absorbed pulse fluences greater than 0.1 J m−2. This implies a subtle interplay between the nonlinear generation process and sample heating. We discuss these effects in terms of a non-equilibrium charge distribution using a two-temperature model.Peer ReviewedPostprint (author's final draft

Hot phonon decay in supported and suspended exfoliated graphene

Near infrared pump-probe spectroscopy has been used to measure the ultrafast dynamics of photoexcited charge carriers in monolayer and multilayer graphene. We observe two decay processes occurring on 100 fs and 2 ps timescales. The first is attributed to the rapid electron-phonon thermalisation in the system. The second timescale is found to be due to the slow decay of hot phonons. Using a simple theoretical model we calculate the hot phonon decay rate and show that it is significantly faster in monolayer flakes than in multilayer ones. In contrast to recent claims, we show that this enhanced decay rate is not due to the coupling to substrate phonons, since we have also seen the same effect in suspended flakes. Possible intrinsic decay mechanisms that could cause such an effect are discussed.Comment: 4 pages, 3 figure

Non-linear resistivity and heat dissipation in monolayer graphene

We have experimentally studied the nonlinear nature of electrical conduction in monolayer graphene devices on silica substrates. This nonlinearity manifests itself as a nonmonotonic dependence of the differential resistance on applied DC voltage bias across the sample. At temperatures below ~70K, the differential resistance exhibits a peak near zero bias that can be attributed to self-heating of the charge carriers. We show that the shape of this peak arises from a combination of different energy dissipation mechanisms of the carriers. The energy dissipation at higher carrier temperatures depends critically on the length of the sample. For samples longer than 10um the heat loss is shown to be determined by optical phonons at the silica-graphene interface.Comment: 4 pages, 4 figure

Non-invasive, near-field terahertz imaging of hidden objects using a single pixel detector

Terahertz (THz) imaging has the ability to see through otherwise opaque materials. However, due to the long wavelengths of THz radiation ({\lambda}=300{\mu}m at 1THz), far-field THz imaging techniques are heavily outperformed by optical imaging in regards to the obtained resolution. In this work we demonstrate near-field THz imaging with a single-pixel detector. We project a time-varying optical mask onto a silicon wafer which is used to spatially modulate a pulse of THz radiation. The far-field transmission corresponding to each mask is recorded by a single element detector and this data is used to reconstruct the image of an object placed on the far side of the silicon wafer. We demonstrate a proof of principal application where we image a printed circuit board on the underside of a 115{\mu}m thick silicon wafer with ~100{\mu}m ({\lambda}/4) resolution. With subwavelength resolution and the inherent sensitivity to local conductivity provided by the THz probe frequencies, we show that it is possible to detect fissures in the circuitry wiring of a few microns in size. Imaging systems of this type could have other uses where non-invasive measurement or imaging of concealed structures with high resolution is necessary, such as in semiconductor manufacturing or in bio-imaging

The College News, 1936-04-08, Vol. 22, No. 19

Bryn Mawr College student newspaper. Merged with The Haverford News in 1968 to form the Bi-college News (with various titles from 1968 on). Published weekly (except holidays) during the academic year

A pyrene-appended spiropyran for selective photo-switchable binding of Zn(II): UV-visible and fluorescence spectroscopy studies of binding and non-covalent attachment to graphene, graphene oxide and carbon nanotubes

PublishedArticleSynthesis of photo-switchable, Zn2+ sensitive hybrid materials was achieved by facile non-covalent functionalization of graphene, graphene oxide and carbon nanotubes with a pyrene-appended spiropyran. Solution phase binding studies, using UV–visible and fluorescence spectroscopy, indicated that the pyrene-spiropyran dyad was highly selective for Zn2+ over a range of potentially competitive cations and that binding occurred with 1:1 stoichiometry and a binding constant of K=1.4×104 mol−1 dm3 at 295 K. Zn2+ binding was promoted by UV irradiation or in darkness and reversed upon irradiation with visible light.Engineering & Physical Sciences Research Council (EPSRC

Impact of pump wavelength on terahertz emission of a cavity-enhanced spintronic trilayer

This is the final version. Available on open access from AIP Publishing via the DOI in this recordWe systematically study the pump-wavelength dependence of terahertz pulse generation in thin-film spintronic THz emitters composed of a ferromagnetic Fe layer between adjacent nonmagnetic W and Pt layers. We find that the efficiency of THz generation is essentially at for excitation by 150 fs pulses with center wavelengths ranging from 900 to 1500 nm, demonstrating that the spin current does not depend strongly on the pump photon energy. We show that the inclusion of dielectric overlayers of TiO2 and SiO2, designed for a particular excitation wavelength, can enhance the terahertz emission by a factor of of up to two in field.The authors like to acknowledge support via the EPSRC Centre for Doctoral Training in Metamaterials (Grant No. EP/L015331/1). EH acknowledges support from EPSRC fellowship (EP/K041215/1). TK, TSS, MK and GJ acknowledge the German Research Foundation for funding through the collaborative research centers SFB TRR 227 Ultrafast spin dynamics (project B02) and SFB TRR 173 Spin+X as well as the Graduate School of Excellence Materials Science in Mainz (MAINZ, GSC 266). TK also acknowledges funding through the ERC H2020 CoG project TERAMAG/Grant No. 681917

Efficient mm-wave photomodulation via coupled Fabry–Perot cavities

This is the author accepted manuscript. The final version is available from AIP Publishing via the DOI in this recordData availability: The data that support the findings of this study are available from the corresponding author upon reasonable request.An efficient mm-wave photomodulator is designed based on coupled Fabry–Perot modes in a low-lifetime silicon wafer and an adjacent cavity formed from a transparent reflector, such as indium tin oxide. The modulation of a reflected beam using this coupled-cavity design is increased by a factor of 7 compared with that from an isolated silicon wafer, while also introducing a degree of tunability and maintaining low angular dispersion. For the particular design built and tested, a modulation of 32% is achieved for an extremely low optical illumination of just 0.006W/cm2 and with a maximum operation rate of more than 3 kHz. The large increase in modulation, coupled with the flexibility of the design and the fact that all components can be industrially manufactured, makes this photomodulator a promising candidate for many communication, imaging, and sensing applications.Engineering and Physical Sciences Research Council (EPSRC)QinetiQ Ltd

The ultrafast dynamics and conductivity of photoexcited graphene at different Fermi energies

This is the final version. A vailable from the American Association for the Advancement of Science via the DOI in this record.For many of the envisioned optoelectronic applications of graphene it is crucial to understand the sub-picosecond carrier dynamics immediately following photoexcitation, as well as the effect on the electrical conductivity - the photoconductivity. Whereas these topics have been studied using various ultrafast experiments and theoretical approaches, controversial and incomplete explanations have been put forward concerning the sign of the photoconductivity, the occurrence and significance of the creation of additional electron-hole pairs, and, in particular, how the relevant processes depend on Fermi energy. Here, we present a unified and intuitive physical picture of the ultrafast carrier dynamics and the photoconductivity, combining optical pump - terahertz probe measurements on a gate-tunable graphene device, with numerical calculations using the Boltzmann equation. We distinguish two types of ultrafast photo-induced carrier heating processes: At low (equilibrium) Fermi energy ($E_{\rm F} \lesssim$ 0.1 eV for our experiments) broadening of the carrier distribution involves interband transitions - interband heating. At higher Fermi energy ($E_{\rm F} \gtrsim$ 0.15 eV) broadening of the carrier distribution involves intraband transitions - intraband heating. Under certain conditions, additional electron-hole pairs can be created (carrier multiplication) for low $E_{\rm F}$, and hot carriers (hot-carrier multiplication) for higher $E_{\rm F}$. The resultant photoconductivity is positive (negative) for low (high) $E_{\rm F}$, which originates from the effect of the heated carrier distributions on the screening of impurities, consistent with the DC conductivity being mostly due to impurity scattering. The importance of these insights is highlighted by a discussion of the implications for graphene photodetector applications.European Union Horizon 2020Severo Ochoa Programme for Centres of Excellence in R&DMineco grants Ramon y CajalSpanish Ministry of Economy and CompetitivenessGovernment of CataloniaEuropean Research Council StG CarbonLightGerman Research FoundationState Research Centre for Innovative and Emerging Materials and the Graduate School of Excellence Materials Science in Mainz (MAINZ)Engineering and Physical Sciences Research Council (EPSRC)Minec