Search

Gallium nitride nanowire and nanorod substrates are prospective platforms allowing to control the local strain distribution in graphene films on top of them, resulting in an induction of pseudomagnetic fields. AFM measurements performed in a HybriD mode complemented by SEM allow for a detailed visualization of the strain distribution on graphene surface. Graphene in direct contact with supporting regions is tensile strained, while graphene located in-between is characterized by lower strain. Characteristic tensile strained wrinkles also appear in the areas between the supporting regions. A positive correlation between strain gradient and distances between borders of supporting regions is observed. These results are confirmed by analysis of the Raman D’ band intensity, which is affected by an enhancement of intravalley scattering. Furthermore, scanning tunneling spectroscopy shows a local modification of the density of states near the graphene wrinkle and weak localization measurements indicate the enhancement of pseudomagnetic field-induced scattering. Therefore, we show that nanowire and nanorod substrates provide strain engineering and induction of pseudomagnetic fields in graphene. The control of graphene morphology by modification of distances between supporting regions is promising for both further fundamental research and the exploration of innovative ways to fabricate pseudomagnetic field-based devices like sensors or filters.

Thin layers of silver nanowires are commonly studied for transparent electronics. However, reports of their terahertz (THz) properties are scarce. Here, we present the electrical and optical properties of thin silver nanowire layers with increasing densities at THz frequencies. We demonstrate that the absorbance, transmittance and reflectance of the metal nanowire layers in the frequency range of 0.2 THz to 1.3 THz is non-monotonic and depends on the nanowire dimensions and filling factor. We also present and validate a theoretical approach describing well the experimental results and allowing the fitting of the THz response of the nanowire layers by a Drude–Smith model of conductivity. Our results pave the way toward the application of silver nanowires as a prospective material for transparent and conductive coatings, and printable antennas operating in the terahertz range—significant for future wireless communication devices.

RF switches, which use a combination of graphene and two-dimensional high-density electron gas (2DEG) in the AlGaN/GaN system, were proposed and studied in the frequency band from 10 MHz to 114.5 GHz. The switches were integrated into the coplanar waveguide, which allows them to be used in any system without the use of, e.g., bonding, flip-chip and other technologies and avoiding the matching problems. The on-state insertion losses for the designed switches were measured to range from 7.4 to 19.4 dB, depending on the frequency and switch design. Although, at frequencies above 70 GHz, the switches were less effective, the switching effect was still evident with an approximately 4 dB on–off ratio. The best switches exhibited rise and fall switching times of ~25 ns and ~17 ns, respectively. The use of such a switch can provide up to 20 MHz of bandwidth in time-modulated systems, which is an outstanding result for such systems. The proposed equivalent circuit describes well the switching characteristics and can be used to design switches with required parameters.

We present the study of the influence of different thin silver nanowire layers on electrical and optical properties in the Terahertz (THz) frequency range. We demonstrate that the absorbance, transmittance and reflectance of the metal nanowire layers in the frequency range of 0.2~THz to 1.2~THz is non-monotonic and depends on the nanowire dimensions and density. We present and validate also a theoretical approach describing well the experimental results and allowing to model the THz response as function the nanowire layer structure. Our results pave the way toward the application of silver nanowires as a perspective material for transparent and conductive coatings, and printable antennas operating in the terahertz range – significant for future wireless communication devices.

By using four layered graphene/gallium nitride (GaN) Schottky diodes with an undoped GaN spacer, we demonstrate highly effective gating of graphene at low bias rendering this type of structure very promising for potential applications. An observed Raman G band position shift larger than 8.5 cm-1 corresponds to an increase in carrier concentration of about 1.2x10^13 cm-2. The presence of a distinct G band splitting together with a narrow symmetric 2D band indicates turbostratic layer stacking and suggests the presence of a high potential gradient near the Schottky junction even at zero bias. The subbands characterized by the highest Raman energies correspond to the largest concentration of electrons. An analysis based on electroreflectance measurements and a modified Richardson equation confirmed that graphene on n-GaN separated by an undoped GaN spacer behaves like a capacitor at reverse bias. At least 60% of G subband position shifts occur at forward bias, which is related to a rapid reduction of electric field near the Schottky junction. Raman micromapping shows a high uniformity of gating efficiency on the surface. Therefore, our studies demonstrate the usefulness of few layer turbostratic graphene deposited on GaN for tracing electron-phonon coupling in graphene. Multilayer graphene also provides uniform and stable electric contacts. Moreover, the observed bias sensitive G band splitting can be used as an indicator of charge transfer in sensor applications in the low bias regime.

We present the performance of an all polarization-maintaining Erbium-doped fiber laser mode-locked by a multilayer graphene based saturable absorber. The cavity incorporates a Martinez-type compressor, based on a transmission grating, which allows for continuous tuning of the net cavity dispersion as well as spectral filtering. By adjusting the dispersion and the spectral bandwidth, we could operate the laser in the soliton, the dispersion managed soliton and the absorber-limited pulse regimes. The pulse durations from the laser ranged between 338 fs to 2.1 ps and the laser could be operated at nearly constant output powers in the anomalous and the normal dispersion regimes. We also conducted stability analyses based on dispersive Fourier transform measurements in the different operating regimes. Comparison of the collected spectra reveals that the soliton regime had the best overall stability.

In this paper, new cost-effective platinum-free and flexible counter electrodes (CEs) for dye-sensitized solar cells (DSSCs) were reported. The ITO/PET CEs were produced using graphene films synthesized by CVD method and graphene flakes with addition PEDOT: PSS or PEDOT: PSS/PVP. The influence of mechanical stress arising from the bending of a flexible substrate on morphology, resistance of CEs as well as electrical properties of DSSCs was analyzed. The testing of these electrodes in DSSCs have demonstrated a power conversion efficiency of up to 3.95% to compare with the power conversion efficiency of 4.39% for DSSC with a standard Pt-based CE. After 100 bending cycles the electrodes demonstrated power conversion efficiency in range the 2.37–3.23% to compare with the power conversion efficiency of 2.08% for DSSC with a standard Pt-based CE. HRTEM investigation confirms the crystallographic structure of graphene. The obtained results demonstrate the possibility of replacing expensive platinum in DSCC by using graphene-based counter electrode.

The one-port configuration of a microwave device is the simplest architecture for graphene ultrafast (photo-)resistors, (photo-)diodes, sensors, and photodetectors. Here, this configuration is realized by a segment of coplanar waveguide (CPW) loaded with a CVD graphene monolayer shorted to the ground. The magnitude and phase of the reflection coefficient ( ${S}_{11}$ parameter) measured in the $0.1~\div ~26$ GHz frequency range are presented for undisturbed structures as well as for structures illuminated by white light and electrostatically polarized with the backgate. A simple and robust de-embedding procedure based on the signal flow graph and allowing for a simultaneous extraction of the parameters characterizing the CPW segment and the graphene monolayer is proposed. The graphene impedance at microwave frequencies is in good agreement with the value of dc sheet resistance obtained from van der Pauw measurements. The back-gate voltage modulation is an efficient way to modulate both the dc sheet resistance and the impedance. In particular, the Dirac point can be achieved both at microwave frequencies and at dc. An equivalent circuit model consists of only the resistance and capacitance connected in parallel, because of good quality ohmic contacts and a negligible inductance as a result of low-current flow in high-resistance graphene samples.

We present a cross-correlation characterization of bound soliton states emerging in a polarization-multiplexed single-cavity dual-comb laser. High temporal and spectral resolution measurements over long optical delays are obtained through multi-heterodyne beating of the laser output.

Graphene due to its properties, such as high sensitivity and biocompatibility finds application in instruments that are used to cooperation with organic substances. At the same time, from the point of view of sensory devices, it is a material with high absorption potential that is able to improve sensitivity and selectivity of these devices. Another benefit of graphene application may be to use its properties in connection with ISFET – Ion Sensitive Field Effect Transistor, which operation principle is based mostly on detection of changes in hydrogen ions concentration. ISFET transistors ale produced in MOS technology, the difference between them and classic MOSFET (Metal-OxideSemiconductor Field Effect Transistor) structures is gate area, where gate metallization was replaced with reference electrode submerged in solution applied in this area. Properties of the solution determine transistor’s action. It is possible to make modifications in gate area of the structure which effects in changes of transistor’s properties. Example of such a modification may be application of graphene layer, which properties may significantly improve detecting capabilities of ISFET devices. For the needs of the research described in this work, graphene was deposited in gate area of transistors through transfer from cooper and germanium surfaces. To check correctness of ISFETs with graphene layer work, current – voltage characteristics of them were determined. Standard I-V characteristics with SiO2 as gate dielectric were compared with these where gate area was enriched with a graphene layer. Structures with graphene mostly worked properly. Thanks to the results presented in this work, it is possible to carry out further experiments using this structures and organic substances.

Dual-comb spectroscopy (DCS) has experienced a remarkable growth propelled by recent advances in fiber and semiconductor technology. A key enabler for DCS is mutual stability between two repetition-rate-mismatched frequency combs, which conventionally relies on feedback loops employing additional optics and electronics to synchronize a pair of sources. This limitation can be overcome in several ways, of which one of the most promising is the generation of two frequency combs in a single free-running fiber laser cavity [1–3]. To date, however, the only configuration proven to be compatible with Doppler-limited spectroscopy was the dual-color laser presented in Ref. [2] requiring external spectral broadening, thus adding an extra layer of complexity.

This paper presents the study on power scaling of an all-polarization maintaining (PM) fiber mode-locked laser using the multilayer chemical vapor deposition graphene/poly(methyl methacrylate) composite as a saturable absorber by changing the coupling ratio of the laser output coupler. Pulses with the duration of 148 fs were generated using 10% output coupler, which are the shortest pulses obtained from mode-locked erbium doped fiber laser working in anomalous dispersion regime, additionally the highest average output power of 15.66 mW was achieved by applying 70% output coupler. All-PM fiber resonator configuration provides self-start and stable operation. To our knowledge, this is the first comprehensive study of power scaling of erbium doped fiber laser with graphene saturable absorber. Conducted experimental work is in very good agreement with numerical simulations.

Dual-comb spectroscopy has emerged as an indispensable analytical technique in applications that require high resolution and broadband coverage within short acquisition times. Its experimental realization, however, remains hampered by intricate experimental setups with large power consumption. Here, we demonstrate an ultra-simple free-running dual-comb spectrometer realized in a single all-fiber cavity suitable for the most demanding Doppler-limited measurements. Our dual-comb laser utilizes just a few basic fiber components, allows to tailor the repetition rate difference, and requires only 350 mW of electrical power for sustained operation over a dozen of hours. As a demonstration, we measure low-pressure hydrogen cyanide within 1.7 THz bandwidth, and obtain better than 1% precision over a terahertz in 200 ms enabled by a drastically simplified all-computational phase correction algorithm. The combination of the unprecedented setup simplicity, comb tooth resolution and high spectroscopic precision paves the way for proliferation of frequency comb spectroscopy even outside the laboratory., Comment: 15 pages, 7 figures, presented on 05/10 at CLEO 2019

We present a graphene-based holmium-doped fiber laser incorporating a Martinez-style compressor which enables precise and continuous dispersion control of the cavity. Solitonic operation of the laser for a wide range of net cavity dispersion values is achieved without significantly altering the repetition frequency. The compressor also provides the possibility to introduce additional spectral filtering - stable mode-locking operation in the 2049- 2093 nm wavelength span is presented.

In this work we propose a simple, all-fiber dispersion managed Ho-doped laser mode-locked with graphene saturable absorber (SA). Dispersion compensating fiber was used to balance the anomalous dispersion of the active and standard single mode fibers, which allows to overcome the limitations set by soliton area theorem. The laser was pumped by a custom-made 1940 nm thulium-doped fiber laser delivering up to 3 W of power. Graphene/PMMA composite sandwiched between fiber connectors serves as a fully fiberized SA. The generated Gaussian pulses centered at 2059 nm reached 53.6 nm of bandwidth. Compressing the pulses by guiding it through SMF-28 fiber yields pulse durations of 190 fs. We also compare the graphene-based laser with a setup mode-locked with nonlinear polarization rotation - an artificial SA. To the best of our knowledge, this is a first demonstration of a nanomaterial-based stretched pulse fiber laser operating beyond 2 μm. The all-fiber cavity is a more robust alternative to setups that include bulk optics, and requires significantly less alignment compared to lasers mode-locked with real SAs

In this Letter, we demonstrate an all-fiber holmium-doped laser operating in the stretched-pulse regime. As a result of dispersion management, the laser is capable of generating 190 fs pulses with a bandwidth of 53.6 nm. The pulses centered at 2060 nm reach 2.55 nJ of energy. Mode-locking is achieved with a multilayer graphene saturable absorber (SA). The Letter also presents the measurement of group velocity dispersion of active (Nufern SM-HDF-10/130), passive (SMF28), and dispersion-compensating (Nufern UHNA4) fibers in a 1.8–2.1 μm range. To the best of our knowledge, this is the first report on an all-fiber, stretched-pulse laser operating beyond 2 μm with nanomaterial-based SA.

The surface-enhanced Raman scattering in graphene deposited on AlxGa1−xN/GaN axial heterostructure nanowires was investigated. It was found that the intensity of graphene Raman spectra is not correlated with aluminium content. Analysis of graphene Raman band parameters, Kelvin probe force microscopy, and electroreflectance showed a screening of polarisation charges. Theoretical calculations demonstrated that plasmon resonance in graphene is far beyond the Raman spectral range. This excludes the presence of an electromagnetic mechanism of surface-enhanced Raman scattering, hence suggesting a chemical mechanism of enhancement.

In this Letter, we demonstrate an all-polarization-maintaining, stretched-pulse Tm-doped fiber laser generating ∼200 fs pulses centered at 1945 nm. As a saturable absorber, a graphene/poly(methyl methacrylate) composite was used. To the best of our knowledge, this is the first demonstration of stretched-pulse operation of a graphene-based fiber laser at 2 μm.

In this report, we demonstrate the preparation method of a multi-layer stack with a pre-defined number of graphene layers, which was obtained using chemical vapor deposition graphene deposited on a copper substrate and subsequently transferred onto a poly(methyl methacrylate) (PMMA) substrate. The prepared multi-layer stack can also be transferred onto an arbitrary substrate and in the end, the polymer can be removed, which in consequence significantly increases the range of possible graphene applications. The multi-layer character was confirmed by optical transmittance measurements and Raman spectroscopy, whereas the microstructure of the multi-layer graphene stack was investigated using Scanning Electron Microscopy. The electrical properties in the function of the number of graphene layers were assessed with standard Hall Effect measurements. Finally, we showed the practical application of the multi-layer graphene stack as a saturable absorber of a mode-locked Er-doped fiber laser.

We demonstrate the generation of 98 fs soliton pulses in all-polarization maintaining fiber laser mode-locked by graphene saturable absorber. The experimental results are supported by detailed numerical simulations. We confirmed soliton-like pulse shaping in the cavity with strong dispersion-management. The developed computer model was used to study the intra-cavity pulse dynamics. These are the shortest soliton pulses generated in a fiber laser with graphene-based saturable absorber up to date (with the broadest optical spectrum).