Volume 103, Issue 17, 21 October 2013
- photonics and optoelectronics
- surfaces and interfaces
- structural, mechanical, optical, and thermodynamic properties of advanced materials
- magnetics and spintronics
- superconductivity and superconducting electronics
- dielectrics, ferroelectrics, and multiferroics
- nanoscale science and technology
- organic electronics and photonics
- device physics
- biophysics and bio-inspired systems
- energy conversion and storage
- interdisciplinary and general physics
Index of content:
Flexible integration of a microfluidic system comprising pumps, valves, and microchannels was realized by an optoelectronic reconfigurable microchannels (OERM) technique. Projecting a low light fluidic device pattern—e.g., pumps, valves, and channels—onto an OERM platform generates Joule heating and melts the substrate in the bright area on the platform; thus, the fluidic system can be reconfigured by changing the projected light pattern. Hexadecane was used as the substrate of the microfluidic system. The volume change of hexadecane during the liquid–solid phase transition was utilized to generate pumping pressure. The system can pump nanoliters of water within several seconds.
- PHOTONICS AND OPTOELECTRONICS
Electro-optic polymer/silicon hybrid slow light modulator based on one-dimensional photonic crystal waveguides103(2013); http://dx.doi.org/10.1063/1.4824421View Description Hide Description
An electro-optic (EO) modulator composed of EO polymer/silicon hybrid one-dimensional photonic crystal nanobeam waveguides is proposed and experimentally demonstrated. The optical field of the photonic crystal nanobeam is designed to concentrate at the nanoscale low refractive index EO polymer region. We have demonstrated enhanced EO modulation efficiencies as a result of the slower group velocity in the 100-μm-long photonic crystal nanobeam phase-shifters. The results agree well with the behavior predicted from band dispersion characteristics, indicating that EO polymer/silicon hybrid photonic crystal nanobeam waveguides are an effective platform for realizing extremely small and ultrafast EO modulators with low operational power.
Observation of whispering gallery modes from hexagonal ZnO microdisks using cathodoluminescence spectroscopy103(2013); http://dx.doi.org/10.1063/1.4826481View Description Hide Description
Zinc oxide hexagonal microdisks with diameters ranging from 3 μm up to 15 μm were fabricated by thermal chemical vapour deposition. Optical characterisation of ZnO microdisks was performed using low temperature (80 K) cathodoluminescence (CL) imaging and spectroscopy. The microdisks exhibited green luminescence locally distributed near the hexagonal boundary of the ZnO microdisks. High resolution CL spectra of the ZnO microdisks revealed whispering gallery modes (WGMs) emission. The experimentally observed WGMs were in excellent agreement with the predicted theoretical positions calculated using a plane wave model. This work could provide the means for ZnO microdisk devices operating in the green spectral range.
An all-silicon optical platform based on linear array of vertical high-aspect-ratio silicon/air photonic crystals103(2013); http://dx.doi.org/10.1063/1.4826146View Description Hide Description
An all-silicon optical platform (SiOP) that integrates a linear array of vertical (100-μm–deep) one-dimensional photonic crystals (1D-PhCs), with a different number of elementary silicon/air cells (from 2.5 to 11.5) and featuring a transmission peak around 1.55 μm, together with U-grooves (125-μm-wide) and end-stop-spacers for coupling/positioning/alignment of readout optical fibers in front of 1D-PhCs is reported. The SiOP is fabricated by electrochemical micromachining and characterized by measuring both reflection and transmission spectra of 1D-PhCs. An experimental/theoretical analysis of 1D-PhC features (transmissivity, quality factor, full-width-half-maximum) in transmission, around 1.55 μm, as a function of the number of elementary cells is reported.
Highly efficient, spatially coherent distributed feedback lasers from dense colloidal quantum dot films103(2013); http://dx.doi.org/10.1063/1.4826147View Description Hide Description
Colloidal quantum dots (CQD) are now making their entry to full-color displays, endowed by their brightness and single-material base. By contrast, many obstacles have been encountered in their use towards lasers. We demonstrate here optically pumped distributed feedback (DFB) lasers, based on close-packed, solid films self-assembled from type-I CQDs. Notably, the single mode CQD-DFB lasers could reach such a low threshold as to be pumpable with a compact pulsed source in a quasi-continuous wave regime. Our results show the spatially and temporally coherent laser beam outputs with power of 400 μW and a quantum efficiency of 32%.
103(2013); http://dx.doi.org/10.1063/1.4826273View Description Hide Description
An optical alignment and a corresponding reconstruction algorithm are proposed to realize a single shot Ptychographical Iterative Engine (PIE). Multiple light beams generated by a cross grating are used to illuminate the specimen, and the resulting diffraction patterns formed on the detector plane by each beam are recorded simultaneously. The modulus and phase images are properly reconstructed with standard PIE algorithm. The proposed single shot method omits the needs for a mechanical x-y scanning of standard PIE algorithm and hence dramatically reduces the data acquisition time.
Evidence for critical scaling of plasmonic modes at the percolation threshold in metallic nanostructures103(2013); http://dx.doi.org/10.1063/1.4826535View Description Hide Description
In this work we provide the experimental demonstration of critical scaling of plasmonic resonances in a percolation series of periodic structures which evolve from arrays of holes to arrays of quasi-triangles. Our observations are in agreement with the general percolation theory and could lead to sensor and detector applications.
103(2013); http://dx.doi.org/10.1063/1.4826536View Description Hide Description
Polarization conveys valuable information for electromagnetic signal processing exhibiting tremendous potential in developing application driven photonic devices. Manipulation of polarization state of an electromagnetic wave has drawn a lot of research interests in many different fields, especially in the terahertz regime. Here, we propose a unique approach to efficiently rotate the linear polarization of terahertz wave in a broadband configuration with tri-layer metasurfaces. We experimentally observe a nearly perfect orthogonal polarization conversion with an ultrahigh efficiency, demonstrating a ultrathin terahetz rotator. The Fabry-Perot cavity effect in the tri-layer metasurfaces is attributed to the underlying mechanism of high transmittance and polarization rotation.
Experimental demonstration of small-angle bending in an active direct-coupled chain of spherical microcavities103(2013); http://dx.doi.org/10.1063/1.4826577View Description Hide Description
We experimentally demonstrate collective amplified modes along bent chains of directly coupled, amplifying spherical microdroplet resonators. The chains, comprising ∼40 non-contacting resonators, were bent through angles up to ∼25°. The modal probability of the system shows a sharp drop upon bending through small angles (∼10°), and thereafter changes minimally under further bending. The frequency response is significantly maintained under bending. We numerically study the transmittance of a chain of non-contacting amplifying resonators using finite-difference-time-domain calculations, and observe that nanojet filamentation influences coupling at the bend. A self-correcting mechanism of propagation is observed, originating from the lensing effect of the spherical resonator.
103(2013); http://dx.doi.org/10.1063/1.4826272View Description Hide Description
We demonstrate remarkably strong nonlinear terahertz (THz) effects in an intrinsic GaAs wafer patterned with a nanometer-width slot antenna array. The antenna near-field reaches 20 MV/cm due to the huge field enhancement in the plasmonic nano-structure (field enhancement factor, α ≅ 50). The THz fields are strong enough to generate high density free carriers (Ne > 1017 cm−3) via interband excitations associated with impact ionizations and thus to induce large absorption of the THz radiation (>35%). The nonlinear THz interactions take place in the confined region of nanometer-scale layer adjacent to the antenna.
103(2013); http://dx.doi.org/10.1063/1.4826458View Description Hide Description
The background noise in solar-blind ultraviolet (UV) detectors from the solar-irradiance leakage photons has been compared with the detector noise from the dark current. It has been found that the background noise is the deterministic limiting factor in solar-blind UV photodetection. The detector performance of background limited ultraviolet photodetector limit and the ultimate signal fluctuation limit are therefore proposed. It proves to be an effective method for device optimization by suppressing the declining tails above 285 nm in the response curves. The absorptive spectral filter requirement has been discussed for common solar-blind UV detectors to achieve the detectivity improvement.
103(2013); http://dx.doi.org/10.1063/1.4826596View Description Hide Description
We report on the fabrication of photodiodes, employing rectifying contacts based on ternary (Mg,Zn)O thin films in wurtzite modification. We utilize a concept for forming a compositionally graded active layer, allowing the design of energy-selective, monolithic, and multichannel ultraviolet metal-semiconductor-metal photodetectors. In our device design, the filter layer blocking high energy irradiation is separated from the active layer and allows the tuning of cutoff energies and bandwidth of the photodiode. Here, the onset of absorption was tuned over 230 meV, and the bandwidth of the photodiodes was varied by 140 meV within the continuous composition spread of the active layer.
103(2013); http://dx.doi.org/10.1063/1.4826613View Description Hide Description
Spectral properties, both the optical spectrum and the intensity noise spectrum, of a broad-area diode laser with off-axis external-cavity feedback are presented. We show that the optical spectrum of the diode laser system is shifted to longer wavelengths due to the external-cavity feedback. The intensity noise spectrum of the diode laser shows that the intensity noise is increased strongly by the external-cavity feedback. External-cavity modes are excited in the external cavity even in the off-axis configuration. The peak spacing of the intensity noise spectrum shows that single roundtrip external-cavity modes are excited. We believe that the four-wave mixing process in the broad-area diode laser is responsible for the establishment of the external-cavity mode.
103(2013); http://dx.doi.org/10.1063/1.4826943View Description Hide Description
We increased the active region/waveguide thickness of terahertz quantum cascade lasers with semi-insulating surface plasmon waveguides by stacking two symmetric active regions on top of each other, via a direct wafer bonding technique. In this way, we enhance the generated optical power in the cavity and the mode confinement. We achieved 470 mW peak output power in pulsed mode from a single facet at a heat sink temperature of 5 K and a maximum operation temperature of 122 K. Furthermore, the devices show a broad band emission spectrum over a range of 420 GHz, centered around 3.9 THz.
- SURFACES AND INTERFACES
103(2013); http://dx.doi.org/10.1063/1.4826517View Description Hide Description
Priming dynamics is one of the critical parameters in designing a capillary-driven thermal management system. We report both an experimental and simulation study of hierarchical structures with silicon pillars and silicon nanowires on the pillar surface. Liquid front velocity covered and uncovered was characterized using capillary wetting experiments and validated by numerical simulation and theoretical prediction. The water under cover moves one order of magnitude faster than the water in the uncovered case. The experimental results and the prediction are in good agreement for flow regimes in both the covered and the uncovered regions.
103(2013); http://dx.doi.org/10.1063/1.4826457View Description Hide Description
The role of back channel surface chemistry on amorphous zinc tin oxide (ZTO) bottom gate thin film transistors (TFTs) has been characterized by positive bias-stress measurements and x-ray photoelectron spectroscopy. Positive bias-stress turn-on voltage shifts for ZTO-TFTs were significantly reduced by passivation of back channel surfaces with self-assembled monolayers of n-hexylphosphonic acid when compared to ZTO-TFTs with no passivation. These results indicate that adsorption of molecular species on the exposed back channel of ZTO-TFTs strongly influence observed turn-on voltage shifts, as opposed to charge injection into the dielectric or trapping due to oxygen vacancies.
103(2013); http://dx.doi.org/10.1063/1.4826518View Description Hide Description
A scanning microscopy probe based on three-dimensionally shaped carbon nanotube (CNT) forests and its application to atomic-force microscopy (AFM) are reported. Micro-scale CNT forests directly grown on silicon cantilevers are patterned into cone shapes with the tips of a few individual nanotubes. The CNT-forest-based probes provide significantly higher mechanical stability/robustness than the common single-CNT probes. AFM imaging using the fabricated probes reveals their imaging ability comparable to that of commercial probes. The patterning process also improves the uniformity of the CNT forests grown on each cantilever. The results suggest a promising future for CNT scanning probes and their production approach.
103(2013); http://dx.doi.org/10.1063/1.4826642View Description Hide Description
We report the tuning of electrical properties of single layer graphene by α-beam irradiation. As the defect density increases upon irradiation, the surface potential of the graphene changes, as determined by Kelvin probe force microscopy and Raman spectroscopy studies. X-ray photoelectron spectroscopy studies indicate that the formation of C/O bonding is promoted as the dose of irradiation increases when at atmospheric conditions. Our results show that the surface potential of the graphene can be engineered by introducing atomic-scale defects via irradiation with high-energy particles.
103(2013); http://dx.doi.org/10.1063/1.4826685View Description Hide Description
The position-dependent oxygen vacancy dynamics induced by a biased scanning probe microscopy tip in Samarium doped ceria thin films grown on MgO (100) substrates is investigated. The granularity of the samples gives rise to spatially dependent local electrochemical activity, as explored by electrochemical strain microscopy. The kinetics of the oxygen vacancy relaxation process is investigated separately for grain boundaries and grains. Higher oxygen vacancy concentration variation and slower diffusion are observed in the grain boundary regions as compared to the grains.
103(2013); http://dx.doi.org/10.1063/1.4826946View Description Hide Description
We evaluate the autocorrelation function of the electrostatic potential in doped graphene due to nearby charged impurities. The screening of those impurities is described by a combination of the polarization function for graphene in random phase approximation with the electrostatic Green's function of the surrounding dielectrics. Using the hard-disk model for a two-dimensional distribution of impurities, we show that large correlation lengths between impurities can give rise to anti-correlation in the electrostatic potential, in agreement with recent experiments.
103(2013); http://dx.doi.org/10.1063/1.4827336View Description Hide Description
We present a three-dimensional bandpass frequency selective structure (FSS) with high selectivity and wide out-of-band rejection. Furthermore, the structure is insensitive to the variation of incident angle and polarization of a striking plane electromagnetic wave. By studying the electric fields and currents excited in the proposed structure, it is revealed that four resonators are constructed to produce the desired filtering response. Experiments show that the fabricated FSS has a 3 dB fractional bandwidth of 18.75% for the passband and a bandwidth of 268.25% for the upper out-of-band rejection better than 20 dB under the normal incidence.