Graphene modelocked VECSELs

In the past decade, passively modelocked optically pumped vertical external cavity surface emitting lasers (OPVECSELs), sometimes referred to as semiconductor disk lasers (OP-SDLs), impressively demonstrated the potential for generating femtosecond pulses at multi-Watt average output powers with gigahertz repetition rates. Passive modelocking with a semiconductor saturable absorber mirror (SESAM) is well established and offers many advantages such as a flexible design of the parameters and low non-saturable losses. Recently, graphene has emerged as an attractive wavelength-independent alternative saturable absorber for passive modelocking in various lasers such as fiber or solid-state bulk lasers because of its unique optical properties. Here, we present and discuss the modelocked VECSELs using graphene saturable absorbers. The broadband absorption due to the linear dispersion of the Dirac electrons in graphene makes this absorber interesting for wavelength tunable ultrafast VECSELs. Such widely tunable modelocked sources are in particularly interesting for bio-medical imaging applications. We present a straightforward approach to design the optical properties of single layer graphene saturable absorber mirrors (GSAMs) suitable for passive modelocking of VECSELs. We demonstrate sub-500 fs pulses from a GSAM modelocked VECSEL. The potential for broadband wavelength tuning is confirmed by covering 46 nm in modelocked operation using three different VECSEL chips and up to 21 nm tuning in pulsed operation is achieved with one single gain chip. A linear and nonlinear optical characterization of different GSAMs with different absorption properties is discussed and can be compared to SESAMs. © 2014 SPIE.

Passively modelocked VECSEL using a single-layer graphene saturable absorber mirror

Optically pumped ultrafast vertical external cavity surface emitting lasers (VECSELs), also referred to as semiconductor disk lasers (SDLs), are very attractive sources for ps- and fs-pulses in the near infrared [1]. So far VECSELs have been passively modelocked with semiconductor saturable absorber mirrors (SESAMs, [2]). Graphene has emerged as a promising saturable absorber (SA) for a variety of applications [3-5], since it offers an almost unlimited bandwidth and a fast recovery time [3-5]. A number of different laser types and gain materials have been modelocked with graphene SAs [3-4], including fiber [5] and solid-state bulk lasers [6-7]. Ultrafast VECSELs are based on a high-Q cavity, which requires very low-loss SAs compared to other lasers (e.g., fiber lasers). Here we develop a single-layer graphene saturable absorber mirror (GSAM) and use it to passively modelock a VECSEL. © 2013 IEEE.

Ultrafast non-thermal electron dynamics in single layer graphene

The impulsive optical excitation of carriers in graphene creates an out-of-equilibrium distribution, which thermalizes on an ultrafast timescale [1-4]. This hot Fermi-Dirac (FD) distribution subsequently cools via phonon emission within few hundreds of femtoseconds. While the relaxation mechanisms mediated by phonons have been extensively investigated, the initial stages, ruled by fundamental electron-electron (e-e) interactions still pose a challenge. © 2013 IEEE.

Doping dependence of the Raman spectrum of defected graphene.

We investigate the evolution of the Raman spectrum of defected graphene as a function of doping. Polymer electrolyte gating allows us to move the Fermi level up to 0.7 eV, as directly monitored by in situ Hall-effect measurements. For a given number of defects, we find that the intensities of the D and D' peaks decrease with increasing doping. We assign this to an increased total scattering rate of the photoexcited electrons and holes, due to the doping-dependent strength of electron-electron scattering. We present a general relation between D peak intensity and defects valid for any doping level.

Effect of the interplay between protein and surface on the properties of adsorbed protein layers

Although protein adsorption to surface is a common phenomenon, investigation of the process is challenging due to the complexity of the interplay between external factors, protein and surface properties. Therefore experimental approaches have to measure the properties of adsorbed protein layers with high accuracy in order to achieve a comprehensive description of the process. To this end, we used a combination of two biosensing techniques, dual polarization interferometry and quartz crystal microbalance with dissipation. From this, we are able to extract surface coverage values, layer structural parameters, water content and viscoelastic properties to examine the properties of protein layers formed at the liquid/solid interface. Layer parameters were examined upon adsorption of proteins of varying size and structural properties, on surfaces with opposite polarity. We show that "soft" proteins such as unfolded α-synuclein and high molecular weight albumin are highly influenced by the surface polarity, as they form a highly diffuse and hydrated layer on the hydrophilic silica surface as opposed to the denser, less hydrated layer formed on a hydrophobic methylated surface. These layer properties are a result of different orientations and packing of the proteins. By contrast, lysozyme is barely influenced by the surface polarity due to its intrinsic structural stability. Interestingly, we show that for a similar molecular weight, the unfolded α-synuclein forms a layer with the highest percentage of solvation not related to surface coverage but resulting from the highest water content trapped within the protein. Together, these data reveal a trend in layer properties highlighting the importance of the interplay between protein and surface for the design of biomaterials. © 2014 The Authors.

Electrical control of nanoscale functionalization in graphene by the scanning probe technique

Functionalized graphene is a versatile material that has well-known physical and chemical properties depending on functional groups and their coverage. However, selective control of functional groups on the nanoscale is hardly achievable by conventional methods utilizing chemical modifications. We demonstrate electrical control of nanoscale functionalization of graphene with the desired chemical coverage of a selective functional group by atomic force microscopy (AFM) lithography and their full recovery through moderate thermal treatments. Surprisingly, our controlled coverage of functional groups can reach 94.9% for oxygen and 49.0% for hydrogen, respectively, well beyond those achieved by conventional methods. This coverage is almost at the theoretical maximum, which is verified through scanning photoelectron microscope measurements as well as first-principles calculations. We believe that the present method is now ready to realize 'chemical pencil drawing' of atomically defined circuit devices on top of a monolayer of graphene. © 2014 Nature Publishing Group All rights reserved.

Terahertz optical modulator based on metamaterial split-ring resonators and graphene

The integration of quantum cascade lasers with devices capable of efficiently manipulating terahertz light represents a fundamental step for many different applications. Split-ring resonators, subwavelength metamaterial elements exhibiting broad resonances that are easily tuned lithographically, represent the ideal route to achieve such optical control of the incident light. We have realized a design based on the interplay between metallic split rings and the electronic properties of a graphene monolayer integrated into a single device. By acting on the doping level of graphene, an active modulation of the optical intensity was achieved in the frequency range between 2.2 and 3.1 THz, with a maximum modulation depth of 18%. © 2014 Society of Photo-Optical Instrumentation Engineers.

Performance of hybrid buffer Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) layers doped with plasmonic silver nanoparticles

We compare the performance of a typical hole transport layer for organic photovoltaics (OPVs), Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) thin film with a series of PEDOT:PSS layers doped with silver (Ag) nanoparticles (NPs) of various size distributions. These hybrid layers have attracted great attention as buffer layers in plasmonic OPVs, although there is no report up to date on their isolated performance. In the present study we prepared a series of PEDOT:PSS layers sandwiched between indium tin oxide (ITO) and gold (Au) electrodes. Ag NPs were deposited on top of the ITO by electron beam evaporation followed by spin coating of PEDOT:PSS. Electrical characterization performed in the dark showed linear resistive behavior for all the samples; lower resistance was observed for the hybrid ones. It was found that the resistivity of the samples decreases with increasing the particle's size. A substantial increase of the electric field between the ITO and the Au electrodes was seen through the formation of current paths through the Ag NPs. A striking observation is the slight increase in the slope of the current density versus voltage curves when measured under illumination for the case of the plasmonic layers, indicating that changes in the electric field in the vicinity of the NP due to plasmonic excitation is a non-vanishing factor. © 2014 Published by Elsevier B.V. All rights reserved.

Graphene-based optical modulator realized in metamaterial split-ring resonators operating in the THz frequency range

The integration of quantum cascade lasers with devices capable of efficiently manipulating terahertz light, represents a fundamental step for many different applications. Split-ring resonators, sub-wavelength metamaterial elements exhibiting broad resonances that are easily tuned lithographically, represent the ideal route to achieve such optical control of the incident light. We have realized a design based on the interplay between metallic split rings and the electronic properties of a graphene monolayer integrated into a single device. By acting on the doping level of graphene, an active modulation of the optical intensity was achieved in the frequency range between 2.2 THz and 3.1 THz, with a maximum modulation depth of 18%.

Quantum tunneling through graphene nanorings

We investigate theoretically quantum transport through graphene nanorings in the presence of a perpendicular magnetic field. Our theoretical results demonstrate that the graphene nanorings behave like a resonant tunneling device, contrary to the Aharonov-Bohm oscillations found in conventional semiconductor rings. The resonant tunneling can be tuned by the Fermi energy, the size of the central part of the graphene nanorings and the external magnetic field.

Carrier capture by threading dislocations in (In,Ga)N/GaN heteroepitaxial layers

Using spatially resolved cathodoluminescence spectroscopy, we investigate the spatial luminescence distribution in a fully strained (In,Ga)N layer, in particular, its correlation with the distribution of threading dislocations (TDs). Regarding the impact of TDs on the luminescence properties, we can clearly distinguish between pure edge-type TDs and TDs with screw component. At the positions of both types of TDs, we establish nonradiative recombination sinks. The radius for carrier capture is at least four times larger for TDs with screw component as for pure edge-type TDs. The large capture radius of the former is due to a spiral-like growth mode resulting in an increase in the In content in the center of the spiral domains in comparison to their periphery.

Raman scattering characterization of Mn composition and strain in Ga1-xMnxSb/GaSb epitaxial layers

Raman scattering (RS) experiments have been performed for simultaneous determination of Mn composition and strain in Ga1-xMnxSb thin films grown on GaSb substrate by liquid phase epitaxy technique. The Raman spectra obtained from various Ga1-xMnxSb samples show only GaSb-like phonon modes whose frequency positions are found to have Mn compositional dependence. With the combination of epilayer strain model, RS and energy dispersive x-ray (EDX) experiments, the compositional dependence of GaSb-like LO phonon frequency is proposed both in strained and unstrained conditions. The proposed relationships are used to evaluate Mn composition and strain from the Ga1-xMnxSb samples. The results obtained from the RS data are found to be in good agreement with those determined independently by the EDX analysis. Furthermore, the frequency positions of MnSb-like phonon modes are suggested by reduced-mass model. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Resonant tunneling through double-bended graphene nanoribbons

We investigate theoretically resonant tunneling through double-bended graphene nanoribbon (GNR) structures, i.e., armchair-edged GNRs (AGNRs) in between two semi-infinite zigzag GNR leads. Our numerical results demonstrate that the resonant tunneling can be tuned dramatically by the Fermi energy and the length and/or widths of the AGNR for both the metallic and semiconductorlike AGNRs. The structure can also be use to control the valley polarization of the tunneling currents and could be useful for potential application in valleytronics devices. (C) 2008 American Institute of Physics.

Tuning of energy levels and optical properties of graphene quantum dots

We investigate theoretically the magnetic levels and optical properties of zigzag- and armchair-edged hexagonal graphene quantum dots (GQDs) utilizing the tight-binding method. A bound edge state at zero energy appears for the zigzag GQDs in the absence of a magnetic field. The magnetic levels of GQDs exhibit a Hofstadter-butterfly spectrum and approach the Landau levels of two-dimensional graphene as the magnetic field increases. The optical properties are tuned by the size, the type of the edge, and the external magnetic field.

Theory of huge tunneling magnetoresistance in graphene

We investigate theoretically the spin-independent tunneling magnetoresistance effect in a graphene monolayer modulated by two parallel ferromagnets deposited on a dielectric layer. For the parallel magnetization configuration, Klein tunneling can be observed in the transmission spectrum but at specific oblique incident angles. For the antiparallel magnetization configuration, the transmission can be blocked by the magneticelectric barrier provided by the ferromagnets. Such a transmission discrepancy results in a tremendous magnetoresistance ratio and can be tuned by the inclusion of an electric barrier.