Hasan Guner

Reversible electrical reduction and oxidation of graphene oxide.

We demonstrate that graphene oxide can be reversibly reduced and oxidized using electrical stimulus. Controlled reduction and oxidation in two-terminal devices containing multilayer graphene oxide films are shown to result in switching between partially reduced graphene oxide and graphene, a process which modifies the electronic and optical properties. High-resolution tunneling current and electrostatic force imaging reveal that graphene oxide islands are formed on multilayer graphene, turning graphene into a self-assembled heterostructure random nanomesh. Charge storage and resistive switching behavior is observed in two-terminal devices made of multilayer graphene oxide films, correlated with electrochromic effects. Tip-induced reduction and oxidation are also demonstrated. Results are discussed in terms of thermodynamics of oxidation and reduction reactions.

Electrochemically tunable ultrafast optical response of graphene oxide

dation. Here, we study the effect of the oxidation level on nonlinear optical properties of GO. We demonstrate that both electrochemically induced reversible reduction and optically induced photoreduction in GO result in changes in the nonlinear optical properties of GO thin films. We present the carrier dynamics and nonlinear optical properties of such films, studied by ultrafast wavelength-dependent pumpprobe spectroscopy. We show that ultrafast response of GO can be tuned by both reduction procedure. The preparation, characterization, linear optical, and electrochromism properties of GO were very recently reported. 10 We study the electrical reduction in GO in air, using multilayer GO thin films deposited on metalized glass substrates. The two terminal devices consist of thin 10‐50 nm Pd/Au planar contacts, separated by 0.3‐0.6 mm, with a thin multilayer GO film covering both contacts and in between Fig. 1a. The degree of chemical reduction and linear absorption spectrum can be tuned by applying a

Raman Enhancement on a Broadband Meta-Surface

Plasmonic metamaterials allow confinement of light to deep subwavelength dimensions, while allowing for the tailoring of dispersion and electromagnetic mode density to enhance specific photonic properties. Optical resonances of plasmonic molecules have been extensively investigated; however, benefits of strong coupling of dimers have been overlooked. Here, we construct a plasmonic meta-surface through coupling of diatomic plasmonic molecules which contain a heavy and light meta-atom. Presence and coupling of two distinct types of localized modes in the plasmonic molecule allow formation and engineering of a rich band structure in a seemingly simple and common geometry, resulting in a broadband and quasi-omni-directional meta-surface. Surface-enhanced Raman scattering benefits from the simultaneous presence of plasmonic resonances at the excitation and scattering frequencies, and by proper design of the band structure to satisfy this condition, highly repeatable and spatially uniform Raman enhancement is demonstrated. On the basis of calculations of the field enhancement distribution within a unit cell, spatial uniformity of the enhancement at the nanoscale is discussed. Raman scattering constitutes an example of nonlinear optical processes, where the wavelength conversion during scattering may be viewed as a photonic transition between the bands of the meta-material.

Infrared absorption spectroscopy and sensing of protein monolayers using high performance enhancing substrates and a mobile phone(Conference Presentation)

Infrared absorption spectroscopy has greatly benefited from the electromagnetic field enhancement offered by plasmonic surfaces. However, because of the localized nature of plasmonic fields, such field enhancements are limited to nm-scale volumes. Here, we demonstrate that a relatively small, but spatially-uniform field enhancement can yield a superior infrared detection performance compared to the plasmonic field enhancement exhibited by optimized infrared nanoantennas. A specifically designed CaF2/Al thin film surface is shown to enable observation of stronger vibrational signals from the probe material, with wider bandwidth and a deeper spatial extent of the field enhancement as compared to optimized plasmonic surfaces. It is demonstrated that the surface structure presented here can enable chemically specific and label-free detection of organic monolayers using surface enhanced infrared spectroscopy. Also, a low cost hand held infrared absorption measurement setup is demonstrated using a miniature bolometric sensor and a mobile phone. A specifically designed grating in combination with an IR light source yields an IR spectrometer covering 7-12 um range, with about 100 cm-1 resolution. Combining the enhancing substrates with the spectroscopy setup, low cost, high sensitivity mobile infrared sensing is enabled. The results have implications in homeland security and environmental monitoring as well as chemical analysis.

Plasmonic absorbers for multispectral and broadband absorption

We present polarization dependent multispectral and broadband plasmonic absorbers in the visible spectrum. The spectral characteristics of these structures are tunable over a broad spectrum. Experimental results are verified with the FDTD and RCWA analysis methods. These structures are used as surface enhanced raman spectroscopy(SERS) substrates. Designed structures have resonances that span the Raman Stokes and excitation wavelength. Such structures can be used for energy, LED and other spectroscopy applications.