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We demonstrate a chip-scale germanium-silicon optical phased array (OPA) fabricated on a CMOS-compatible platform capable of 2D beam steering in the mid-infrared wavelength range. The OPA included a specially designed grating emitter waveguide array with uniform emission intensity along the mm-length waveguide propagation to realize very sharp instantaneous field-of-view (IFOV) and wide beam-steering total-field-of-view (TFOV). The experimental results indicated lateral beam-steering TFOV up to 12.7° by phase-tuning the waveguide array and longitudinal TFOV up to 12° by wavelength tuning. The 3-dB beam divergence is 3.08° × 0.18°. The demonstrated OPA architecture can employ wafer-scale fabrication and integration while supporting sensing and imaging applications in the mid-infrared spectral range.

Our Universe is most radiant at terahertz frequencies (0.1–10.0 THz) (ref. 1), providing critical information on the formation of the planets, stars and galaxies, as well as the atmospheric constituents of the planets, their moons, comets and asteroids2–9. The detection of faint fluxes of photons at terahertz frequencies is crucial for many planetary, cosmological and astrophysical studies10–14. For example, understanding the physics and molecular chemistry of the life cycle of stars and their relationship with the interstellar medium in galaxies requires heterodyne detectors with noise temperatures close to the quantum limit15. Near-quantum-limited heterodyne terahertz detection has so far been possible only through the use of cryogenically cooled superconducting mixers as frequency downconverters15–18. Here we introduce a heterodyne terahertz detection scheme that uses plasmonic photomixing for frequency downconversion to offer quantum-level sensitivities at room temperature. Frequency downconversion is achieved by mixing terahertz radiation and a heterodyning optical beam with a terahertz beat frequency in a plasmonics-enhanced semiconductor active region. We demonstrate terahertz detection sensitivities down to three times the quantum limit at room temperature. With a versatile design capable of broadband operation over a 0.1–5.0 THz bandwidth, this plasmonic photomixer has broad applicability to astronomy, cosmology, atmospheric studies, gas sensing and quantum optics. The measurement of faint fluxes of photons from across the Universe requires sensitive terahertz detectors, cooled to minimize noise. By using a photomixer with plasmonics-based semiconductors to downconvert the high frequencies, terahertz detection can be accomplished at room temperature.

Stability of optical beats between longitudinal laser modes in a chaotically oscillated laser diodes for THz wave generation and detection is demonstrated compared to a free-running continuous-wave laser conventional laser. The high stability of chaotically oscillating lasers makes these lasers promising candidates for optical pump sources in terahertz time-domain spectroscopy systems.

Stability of optical beats in a chaotically oscillating laser is compared to that of a free-running continuous-wave laser using a highly efficient plasmonic photomixer (Anttena). The high stability of optical beats in chaotically oscillating lasers is verified.

The stability of optical beats in a chaotically oscillating laser is compared to that of a free-running continuous-wave laser using a highly efficient plasmonic photomixer. Using a chaotically oscillating laser diode, stable optical beats are observed over an operation current range of 60-90 mA. The optical spectra are stable even with frequent mode hopping. In contrast, optical beats in a free-running continuous-wave laser are not stable compared to those of a chaotically oscillating laser, because of intermittent hopping of the laser modes. The high stability of chaotically oscillating lasers makes these lasers promising candidates for optical pump sources in terahertz time-domain spectroscopy systems.

The scope and potential uses of time-domain terahertz imaging and spectroscopy are mainly limited by the low optical-to-terahertz conversion efficiency of photoconductive terahertz sources. State-of-the-art photoconductive sources utilize short-carrier-lifetime semiconductors to recombine carriers that cannot contribute to efficient terahertz generation and cause additional thermal dissipation. Here, we present a novel photoconductive terahertz source that offers a significantly higher efficiency compared with terahertz sources fabricated on short-carrier-lifetime substrates. The key innovative feature of this source is the tight three-dimensional confinement of the optical pump beam around the terahertz nanoantennas that are used as radiating elements. This is achieved by means of a nanocavity formed by plasmonic structures and a distributed Bragg reflector. Consequently, almost all of the photo-generated carriers can be routed to the terahertz nanoantennas within a sub-picosecond time-scale. This results in a very strong, ultrafast current that drives the nanoantennas to produce broadband terahertz radiation. We experimentally demonstrate that this terahertz source can generate 4 mW pulsed terahertz radiation under an optical pump power of 720 mW over the 0.1–4 THz frequency range. This is the highest reported power level for terahertz radiation from a photoconductive terahertz source, representing more than an order of magnitude of enhancement in the optical-to-terahertz conversion efficiency compared with state-of-the-art photoconductive terahertz sources fabricated on short-carrier-lifetime substrates.

We present a heterodyne terahertz spectrometry platform based on plasmonic photomixing, which enables the resolution of narrow spectral signatures of gases over a broad terahertz frequency range. This plasmonic heterodyne spectrometer replaces the terahertz mixer and local oscillator of conventional heterodyne spectrometers with a plasmonic photomixer and a heterodyning optical pump beam, respectively. The heterodyning optical pump beam is formed by two continuous-wave, wavelength-tunable lasers with a broadly tunable terahertz beat frequency. This broadly tunable terahertz beat frequency enables spectrometry over a broad bandwidth, which is not restricted by the bandwidth limitations of conventional terahertz mixers and local oscillators. We use this plasmonic heterodyne spectrometry platform to resolve the spectral signatures of ammonia over a 1-4.5 THz frequency range.

We present a high-responsivity and broadband photoconductive terahertz detector based on a plasmonic nanocavity, which enables high quantum efficiency and ultrafast operation without using short-carrier-lifetime substrates. We experimentally demonstrate that a terahertz time-domain spectroscopy setup employing the presented detector can offer 102 dB dynamic range over 0.1-4.5 THz.

We present a photoconductive terahertz detector, which offers high-responsivity and broadband operation without using a short-carrier-lifetime substrate, for the first time. To achieve this unprecedented performance, we utilize a plasmonic nanocavity to confine the majority of optical pump photons in a thin photoconductive active region to maintain a sub-picosecond transport time for the majority of photocarrier to the device contact electrodes. Therefore, an ultrafast operation is achieved without using a short-carrier-lifetime semiconductor. We demonstrate terahertz detection with dynamic ranges exceeding 100 dB over a 0.1-4.5 THz frequency range.

Graphene is a very attractive material for broadband photodetection in hyperspectral imaging and sensing systems. However, its potential use has been hindered by tradeoffs between the responsivity, bandwidth, and operation speed of existing graphene photodetectors. Here, we present engineered photoconductive nanostructures based on gold-patched graphene nano-stripes, which enable simultaneous broadband and ultrafast photodetection with high responsivity. These nanostructures merge the advantages of broadband optical absorption, ultrafast photocarrier transport, and carrier multiplication in graphene nano-stripes with the ultrafast transport of photocarriers to the gold patches before recombination. Through this approach, high-responsivity operation is achieved without the use of bandwidth- and speed-limiting quantum dots, defect states, or tunneling barriers. We demonstrate high-responsivity photodetection from the visible to the infrared regime (0.6 A/W at 0.8 μm and 11.5 A/W at 20 μm) with operation speeds exceeding 50 GHz. Our results demonstrate an improvement of the response times by more than seven orders of magnitude and an increase in bandwidths of one order of magnitude compared to those of higher-responsivity graphene photodetectors based on quantum dots and tunneling barriers.

Utilizing short-carrier-lifetime semiconductors as the photo-absorbing substrate of photoconductive terahertz detectors has been considered a necessity to enable ultrafast operation and to recombine the slow photo-generated carriers that increase the detector noise and reduce the detector responsivity. However, most of the techniques used for growing short-carrier-lifetime semiconductors introduce a high density of defects in the semiconductor lattice, degrading the carrier mobility and drift velocity and, thus, the detector responsivity. To eliminate the need for a short-carrier-lifetime semiconductor, we present a novel photoconductive terahertz detector based on a nanocavity-coupled plasmonic nanoantenna array. The presented photoconductive terahertz detector uses an undoped GaAs layer embedded inside a nanocavity as the photoconductive active region. The nanocavity is specifically designed to confine the optical pump photons very tightly inside the undoped GaAs layer so that all the photo-generated carriers concentrate around an array of plasmonic nanoantennas, which are also designed to operate as broadband terahertz antennas. Therefore, the presented nanocavity-coupled plasmonic nanoantenna array maximizes the photo-generated carrier concentration and the induced terahertz electric field in response to an incident terahertz radiation near the plasmonic nanoantenna contact electrodes. This significantly increases the detector responsivity and offers photo-generated carrier transport times comparable to photoconductive terahertz detectors based on short-carrier-lifetime semiconductors. By using the presented detector in a time-domain terahertz spectroscopy system, we demonstrate resolving terahertz spectra with a large dynamic range over the 0.1-5 THz frequency range.

We demonstrate a chip-scale germanium-silicon optical phased array fabricated on a Bi-CMOS compatible platform capable of beam steering in the mid-infrared wavelength. We achieved beam steering angle up to 12.7° with beam divergence of 0.47°×2.86°.

We characterize sensitivity of plasmonic heterodyne terahertz spectrometers using standard Y-factor noise temperature measurements and demonstrate near-quantum-limited sensitivities at room temperature. We demonstrate double-sideband noise temperatures of 120–270 Kover 0.1-2 THz.

We present a photoconductive terahertz source that offers broadband pulsed terahertz radiation with enhanced optical-to-terahertz conversion efficiencies compared to photoconductive terahertz sources based on short-carrier-lifetime semiconductors. The performance enhancement is achieved by utilizing a plasmonic nanocavity that tightly confines optical pump photons inside a photoconductive layer near the terahertz radiating elements. The plasmonic nanocavity is implemented by sandwiching the photoconductive layer between a distributed Bragg reflector and plasmonic metallic structures, which are optimized to be resonant at the optical pump wavelength. The plasmonic structures are also designed as a broadband terahertz nanoantenna array. A thin undoped GaAs film is used as the photoconductive layer offering much higher carrier drift velocities compared to short-carrier-lifetime GaAs substrates. The tight confinement of the optical pump photons and the use of a low-defect photoconductive semiconductor layer allow drift of almost all of the photo-generated carriers to the terahertz nanoantennas in a sub-picosecond time scale to efficiently contribute to pulsed terahertz radiation. We experimentally demonstrate that the presented terahertz source offers 60 times higher optical-to-terahertz conversion efficiency compared to a similar terahertz nanoantenna array fabricated on a short-carrier-lifetime semiconductor. We demonstrate pulsed terahertz radiation with powers exceeding 4 mW over 0.1-4 THz frequency range.

We demonstrate a new class of terahertz spectrometers that use plasmonic photomixers as frequency downconverters to offer quantum-level sensitivity at room temperature. We demonstrated double-sideband noise temperatures of 120-270 K (10hv/k -2hv/k) over 0.1-2 THz, equivalent to 10-2 photons.

We present a high responsivity and bias-free graphene photodetector with responsivity levels as high as 225 mA/W at 800 nm wavelength, which uses nano-grating contact electrodes to enhance optical absorption and photocarrier extraction.

Metamaterials bring sub-wavelength resonating structures together to overcome the limitations of conventional materials. The realization of active metadevices has been an outstanding challenge that requires electrically reconfigurable components operating over a broad spectrum with a wide dynamic range. The existing capability of metamaterials, however, is not sufficient to realize this goal. Here, by integrating passive metamaterials with active graphene devices, we demonstrate a new class of electrically controlled active metadevices working in microwave frequencies. The fabricated active metadevices enable efficient control of both amplitude (> 50 dB) and phase (> 90��) of electromagnetic waves. In this hybrid system, graphene operates as a tunable Drude metal that controls the radiation of the passive metamaterials. Furthermore, by integrating individually addressable arrays of metadevices, we demonstrate a new class of spatially varying digital metasurfaces where the local dielectric constant can be reconfigured with applied bias voltages. Additionally, we reconfigure resonance frequency of split ring resonators without changing its amplitude by damping one of the two coupled metasurfaces via graphene. Our approach is general enough to implement various metamaterial systems that could yield new applications ranging from electrically switchable cloaking devices to adaptive camouflage systems., 18 pages, 5 figures

We present a novel photoconductive terahertz source that can generate high power and broadband terahertz radiation with high optical-to-terahertz conversion efficiency. The high optical-to-terahertz conversion efficiency is achieved by confining the optical pump photons very tightly around the radiating elements of the photoconductive source. A distributed Bragg reflector and plasmonic nanostructures are used for three-dimensional confinement of light. We experimentally demonstrate a record-high terahertz radiation power of 4 mW over a 0.1–4 THz frequency range.

We present a novel optically-pumped terahertz antenna array, which offers much higher radiation efficiencies compared to the state-of-the-art. The key novelty of the device is tight confinement of the optical pump beam around the antenna elements. The tight light confinement is provided by a specially-designed plasmonic nano-antenna array and photo-absorbing semiconductor substrate, which operates as a reflector. We experimentally demonstrate a record-high pulsed terahertz radiation power of 4 mW and a radiation bandwidth of 0.1–5 THz, exhibiting one order of magnitude enhancement in radiation efficiency compared to the state-of-the-art.

We present an ultrafast and high responsivity graphene photodetector that operates over a broad bandwidth from visible to infrared regime. Use of plasmonic nanoantennas as photodetector contact electrodes provides a strong concentration of photo-generated carriers near the contact electrodes. As a result, a large number of the photocarriers drift to the photodetector contact electrodes despite the short carrier lifetime of graphene, providing high responsivity levels. The photodetector is also designed to offer high speed operation by minimizing capacitive parasitics. We demonstrate broadband photodetection from 800 nm to 20 μm with operation speeds exceeding 50 GHz and responsivity levels as high as 0.6 A/W at 800 nm and 11.5 A/W at 20 μm. These results are the first demonstration of high-responsivity photodetection with such a broad operation bandwidth and high speed, enabled by plasmonic nanoantennas.

We present a novel photoconductive terahertz emitter, which offers significantly high terahertz radiation power levels through three-dimensional light confinement near terahertz radiating elements. Arrays of plasmonic nano-antennas fabricated on a photo-absorbing semiconductor substrate are used as the terahertz radiating elements. The plasmonic nano-antenna arrays are designed to offer high radiation resistance over a broad terahertz frequency range. An optical reflector layer is embedded inside the substrate to spatially confine and absorb a major portion of an incident optical pump beam near the plasmonic nano-antennas. Therefore, very efficient ultrafast photocurrent can be generated and coupled to the plasmonic nano-antennas for high-efficiency terahertz radiation generation. We experimentally demonstrate record-high terahertz radiation powers as high as 11.4 mW over 0.1–5 THz frequency range with 2.3% optical-to-terahertz conversion efficiency.

Graphene is a promising two-dimensional material for photo-detection owing to its high mobility, broadband optical absorption, zero band gap nature, and tunable carrier concentration through electrical gating. Despite these unique properties, its 2.3% optical absorption from ultraviolet to infrared wavelengths and short carrier lifetime has limited its usage for practical applications. In this work, we present a broadband, high responsivity, and high speed graphene photodetector. By use of plasmonic nanoantennas, an incident optical field can be strongly concentrated in close proximity to the metallic nanoantennas. This significantly reduces the drift path length of the majority of photo-generated carriers in graphene to the plasmonic nanoantennas that serve as the photodetector contact electrodes. As a result, a large number of the photo-generated carriers can drift to the photodetector contact electrodes despite the short carrier lifetime of graphene, offering high responsivity levels. Moreover, the photodetector is designed to offer high speed operation by minimizing the capacitive parasitics induced by the plasmonic nanoantennas. We demonstrate a broadband photo-detection operation covering the wavelength regime from 800 nm to 1800 nm. We achieve responsivity levels as high as 0.6 A/W at 800 nm, which is close to the theoretical limit of 0.65 A/W. In summary, the combination of the high-responsivity, broad bandwidth, and high-speed performance of the presented plasmonics-enhanced graphene photodetector could find many applications in future optical communication, imaging and sensing systems.

In this work, we report the nanofabrication, optical characterization, and electromagnetic modeling of various nanostructure arrays for localized surface plasmon resonance (LSPR) based biosensing studies. Comparison of the experimental results and simulation outputs of various nanostructure arrays was made and a good correspondence was achieved.

WOS: 000393815100004, The electronic transport properties of Dirac fermions in chemical vapour-deposited single-layer epitaxial graphene on an SiO2/Si substrate have been investigated using the Shubnikov-de Haas (SdH) oscillations technique. The magnetoresistance measurements were performed in the temperature range between 1.8 and 43 K and at magnetic fields up to 11 T. The 2D carrier density and the Fermi energy have been determined from the period of the SdH oscillations. In addition, the in-plane effective mass as well as the quantum lifetime of 2D carriers have been calculated from the temperature and magnetic field dependences of the SdH oscillation amplitude. The sheet carrier density (1.42 x 10(13) cm(-2) at 1.8 K), obtained from the lowfield Hall Effect measurements, is larger than that of 2D carrier density (8.13 x 10(12) cm(-2)). On the other hand, the magnetoresistance includes strong magnetic field dependent positive, non-oscillatory background magnetoresistance. The strong magnetic field dependence of the magnetoresistance and the differences between sheet carrier and 2D carrier density can be attributed to the 3D carriers between the graphene sheet and the SiO2/Si substrate., European Union; TUBITAK [105E066, 105A005, 106E198, 106A017], This work is supported by the European Union under the projects EU-METAMORPHOSE, EU-PHOREMOST, EU-PHOME, and EU-ECONAM, and TUBITAK [project number 105E066], project number 105A005], [project number 106E198], and [project number 106A017].

We present a highly efficient photoconductive terahertz generation scheme based on photon trapping. We utilize this scheme to demonstrate record-high pulsed terahertz radiation powers as high as 4 mW over 0.1–5 THz frequency.

We present broadband photodetection from 800 nm to 20 μm with operation speeds exceeding 50 GHz and responsivity levels as high as 0.6 A/W at 0.8 μm and 11.5 A/W at 20 μm by using plasmonic nanoantennas as photodetector contact electrodes on graphene.

The evolution of SiC surface morphology during graphene growth process has been studied through the comparison of substrate surface step structure after in-situ etching and graphene growth in vacuum. Influence of in-situ substrate surface preparation on the properties of graphene was studied through the comparison of graphene layers on etched and un-etched substrates grown under same conditions.

We present a photoconductive terahertz detector to be used in terahertz time-domain imaging and spectroscopy systems without utilizing a short-carrier lifetime semiconductor for the photoconductive active region. A plasmonic nanocavity is used to form the photoconductive active region of the detector to limit the transport time of the majority of the photo-generated carrier to a sub-picosecond order and eliminate the need for a short-carrier lifetime semiconductor. We demonstrate that the presented detector is capable of detecting terahertz pulses over a 0.1–4.5 THz frequency band with more than a 100 dB dynamic range under a 5 mW optical pump power, exhibiting a comparable performance with the state-of-the-art photoconductive terahertz detectors based on short-carrier lifetime substrates.

In this work, fractal metasurfaces that consist of periodic gold squares on graphene are used to increase light-graphene interaction. We show by simulations and experiments that higher level fractal structures result in higher spectral tunability of resonance wavelength. This is explained by higher field localization for higher level fractal structures. Furthermore, spectral tunability of fractal metasurfaces integrated with graphene is investigated comparing two different schemes for electrostatic gating. Experiment results show that a top-gated device yields more spectral tunability (8% of resonance wavelength) while requiring much smaller gate voltages compared to the back-gated device. © 2016 American Chemical Society.

Surface Enhanced Raman Spectroscopy (SERS) is a popular method that amplifies weak Raman signals from Raman-active analyte molecules making use of certain specially-prepared metallic surfaces. The main challenge in SERS is to design and fabricate highly repeatable, predictable, and sensitive substrates. There are many fabrication methods that strive to achieve this goal, which are briefly summarized in this paper. The E-beam lithography method is proposed to be superior to the mentioned techniques. In this paper, we review how EBL can be utilized in the preparation of SERS substrates and we discuss the contributions to the field by the Ozbay group. (C) 2015 Elsevier B.V. All rights reserved.

Reconfigurable metamaterials have emerged as promising platforms for manipulating the spectral and spatial properties of terahertz waves without being limited by the characteristics of naturally existing materials. Here, we present a comprehensive overview of various types of reconfigurable metamaterials that are utilized to manipulate the intensity, phase, polarization, and propagation direction of terahertz waves. We discuss various reconfiguration mechanisms based on optical, electrical, thermal, and mechanical stimuli while using semiconductors, superconductors, phase-change materials, graphene, and electromechanical structures. The advantages and disadvantages of different reconfigurable metamaterial designs in terms of modulation efficiency, modulation bandwidth, modulation speed, and system complexity are discussed in detail.

WOS: 000333004300012, Hall effect measurements on epitaxial graphene (EG) on SiC substrate have been carried out as a function of temperature. The mobility and concentration of electrons within the two-dimensional electron gas (2DEG) at the EG layers and within the underlying SiC substrate are readily separated and characterized by the simple parallel conduction extraction method (SPCEM). Two electron carriers are identified in the EG/SiC sample: one high-mobility carrier (3493 cm(2)/Vs at 300 K) and one low-mobility carrier (1115 cm(2)/Vs at 300 K). The high mobility carrier can be assigned to the graphene layers. The second carrier has been assigned to the SiC substrate., project DPT-HAMIT; project ESF-EPIGRAT; project EU-N4E; TUBITAK [107A004, 107A012, 109E301]; Turkish Academy of Sciences; [NATO-SET-181], This work is supported by the projects DPT-HAMIT, ESF-EPIGRAT, EU-N4E, and NATO-SET-181, and TUBITAK under the Project Nos. 107A004, 107A012, and 109E301. One of the authors (E.O.) also acknowledges partial support from the Turkish Academy of Sciences.

Hall effect measurements of a graphene-on-SiC system were carried out as a function of temperature (1.8-200 K) at a static magnetic field (0.51) With the analysis of temperature dependent single-field Hall data with the Simple Parallel Conduction Extraction Method (SPCEM), bulk and two-dimensional (2D) carrier densities and mobilities were extracted successfully. Bulk carrier is attributed to SIC substrate and 2D carrier is attributed to the graphene layer. For each SPCEM extracted carrier data, relevant three-dimensional or 2D scattering analyses were performed. Each SPCEM extracted carrier data were explained with the related scattering analyses. A temperature independent mobility component, which may related to an interaction between graphene and SIC, was observed for both scattering analyses with the same mobility limiting value. With the SPCEM, effective ionized impurity concentration of SiC substrate, extracted 2D-mobility, and sheet carrier density of the graphene layer are calculated with using temperature dependent static magnetic field Hall data. Funding Agencies|DPT-HAMIT; DPT-FOTON; TUBITAK [113F364, 113E331, 109A015, 109E301]; Turkish Academy of Sciences; [NATO-SET-193]

Cataloged from PDF version of article. We introduced fractal geometry to the conventional bowtie antennas. We experimentally and numerically showed that the resonance of the bowtie antennas goes to longer wavelengths, after each fractalization step, which is considered a tool to miniaturize the main bowtie structure. We also showed that the fractal geometry provides multiple hot spots on the surface, and it can be used as an efficient SERS substrate. (C)2014 Optical Society of America.

The transport properties of quasi-free-standing (QFS) bilayer graphene on SiC depend on a range of scattering mechanisms. Most of them are isotropic in nature. However, the SiC substrate morphology marked by a distinctive pattern of the terraces gives rise to an anisotropy in graphene's sheet resistance, which may be considered an additional scattering mechanism. At a technological level, the growth-preceding in situ etching of the SiC surface promotes step bunching which results in macro steps similar to 10 nm in height. In this report, we study the qualitative and quantitative effects of SiC steps edges on the resistance of epitaxial graphene grown by chemical vapor deposition. We experimentally determine the value of step edge resistivity in hydrogen-intercalated QFS-bilayer graphene to be similar to 190 Omega mu m for step height h(S) = 10 nm and provide proof that it cannot originate from mechanical deformation of graphene but is likely to arise from lowered carrier concentration in the step area. Our results are confronted with the previously reported values of the step edge resistivity in monolayer graphene over SiC atomic steps. In our analysis, we focus on large-scale, statistical properties to foster the scalable technology of industrial graphene for electronics and sensor applications. (C) 2014 AIP Publishing LLC.

Metallic bowtie antennas are used in nanophotonics applications in order to confine the electromagnetic field into volumes much smaller than that of the incident wavelength. Electrically controllable carrier concentration of graphene opens the door to the use of plasmonic nanoantenna structures with graphene so that the resonant nature of nanoantennas can be tuned. In this study, we demonstrated with the Fourier transform infrared (FTIR) spectroscopy and the Finite Difference Time Domain (FDTD) method that the intensity and resonance peak of bowtie nanoantennas on monolayer graphene can be tuned at mid-infrared (MIR) wavelength regime by applying a gate voltage, since the optical properties of graphene change by changing the carrier concentration. (C) 2014 Elsevier B.V. All rights reserved.

We demonstrate theoretically and experimentally that Fano resonances can be obtained in terahertz metamaterials that are composed of periodic continuous metallic wires dressed with periodic split ring resonators. An asymmetric Fano lineshape has been found in a narrow frequency range of the transmission curve. By using a transmission line combined with lumped element model, we are able to not only fit the transmission spectra of Fano resonance which is attributed to the coupling and interference between the transmission continuum of continuous metallic wires and the bright resonant mode of split ring resonators, but also reveal the capacitance change of the split ring resonators induced frequency shift of the Fano resonance. Therefore, the proposed theoretical model shows more capabilities than conventional coupled oscillator model in the design of Fano structures. The effective parameters of group refractive index of the Fano structure are retrieved, and a large group index more than 800 is obtained at the Fano resonance, which could be used for slow light devices. (C) 2014 Optical Society of America

One-way excitation of spoof surface plasmons (SPs) and strongly pronounced diodelike extraordinary transmission of linearly polarized waves in the beaming regime can be obtained by combining spoof SPs and cross-polarization conversion resonances. The reciprocal composite structure that is suggested to realize this mechanism consists of a symmetric metallic grating with a subwavelength slit and a metamaterial based ultrathin 90â̂̃ polarization rotator and, therefore, shows the broken spatial inversion symmetry. In contrast to the earlier studies of SP inspired transmission through subwavelength slits, asymmetric (one-way) beaming is demonstrated at normal incidence and for both s- and p-polarized incident waves. Furthermore, as an implication of Lorentz reciprocity, the studied diodelike mechanism is reversible, which manifests itself in that transmission is significant for one of the two opposite illumination directions at s polarization and for the other direction at p polarization. The obtained numerical and experimental results verify the general idea and enable us to select the optimal operation regimes. © 2013 American Physical Society.

Cataloged from PDF version of article. Split ring resonators (SRRs) are subwavelength structures that are able to localize and enhance the electromagnetic wave. Controlling the plasmonic resonance behavior of metallic nanostructures, such as SRRs, plays an important role in optoelectronics and nanophotonics applications. Electrically tunable carrier concentration of graphene provides hybrid devices, where the plasmonic structures and graphene are combined. In this paper, we report the design, fabrication, and measurement of a device comprising a SRR array on epitaxial graphene. We obtained resonance broadening and tuning of split ring resonators by utilizing an epitaxial graphene transistor with transparent top-gate. © 2013 AIP Publishing LLC.

WOS: 000314724500045, Shubnikov-de Haas (SdH) and Hall effect measurements performed in a temperature range between 1.8 and 275 K, at an electric field up to 35 kV m(-1) and magnetic fields up to 11 T, have been used to investigate the electronic transport properties of monolayer graphene on SiC substrate. The number of layers was determined by the use of the Raman spectroscopy. The carrier density and in-plane effective mass of electrons have been obtained from the periods and temperature dependencies of the amplitude of the SdH oscillations, respectively. The effective mass is in good agreement with the current results in the literature. The two-dimensional (2D) electron energy relaxations in monolayer graphene were also investigated experimentally. The electron temperature (T-e) of hot electrons was obtained from the lattice temperature (T-L) and the applied electric field dependencies of the amplitude of SdH oscillations. The experimental results for the electron temperature dependence of power loss indicate that the energy relaxation of electrons is due to acoustic phonon emission via mixed unscreened piezoelectric interaction and deformation-potential scattering

Cataloged from PDF version of article. Diode and collimator/multiplexer functions are suggested to be combined in one device that is based on a thin metallic grating with a single subwavelength slit. A proper choice of the structural (a) symmetry of the grating can result in obtaining one-way collimation and multiplexing with a single on-axis or off-axis, or two off-axis narrow outgoing beams. It is possible due to freedom in utilizing different combinations of the excitation conditions of the spoof surface plasmons at the four grating parts - right and left front-side and right and left back-side ones. Such a combining provides one with an efficient tool to engineer one-way collimators and multiplexers with the desired characteristics. Strong asymmetry in transmission with respect to the incidence direction (forward vs backward case) can be obtained within a wide range of variation of the incident beam parameters, i.e., angle of incidence and frequency, while the outgoing radiation is concentrated within a narrow range of the observation angle variation. Most of the observed asymmetric transmission features can be qualitatively explained using the concept of the equivalent source placed inside the slit. (C) 2012 Optical Society of America