Volume 103, Issue 21, 18 November 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
- energy conversion and storage
- interdisciplinary and general physics
Index of content:
Plasmonic nanohelix arrays are shown to interact with electromagnetic fields in ways not typically seen with ordinary matter. Chiral metamaterials (CMMs) with feature sizes small with respect to the wavelength of visible light are a promising route to experimentally achieve such phenomena as negative refraction without the need for simultaneously negative ε and μ. Here we not only show that giant circular dichroism in the visible is achievable with hexagonally arranged plasmonic nanohelix arrays, but that we can precisely tune the optical activity via morphology and lattice spacing. The discrete dipole approximation is implemented to support experimental data.
- PHOTONICS AND OPTOELECTRONICS
103(2013); http://dx.doi.org/10.1063/1.4831982View Description Hide Description
The efficiency of photocurrent extraction was studied directly inside operating Colloidal Quantum Dot (CQD) photovoltaic devices. A model was derived from first principles for a thin film p-n junction with a linearly spatially dependent electric field. Using this model, we were able to clarify the origins of recent improvement in CQD solar cell performance. From current-voltage diode characteristics under 1 sun conditions, we extracted transport lengths ranging from 39 nm to 86 nm for these materials. Characterization of the intensity dependence of photocurrent extraction revealed that the dominant loss mechanism limiting the transport length is trap-mediated recombination.
103(2013); http://dx.doi.org/10.1063/1.4830040View Description Hide Description
It has recently been demonstrated that adiabatic nanofocusing of light by gap-plasmon modes in ultra-sharp convex metal grooves can turn metallic surfaces with high reflectivity in the visible and near-infrared into excellent absorbers of light polarized perpendicular to the groove direction. Here we demonstrate that this effect can be used to design broadband linear polarizers, operating in reflection and inducing negligible dispersive stretching of ultra-short (5–10 fs) laser pulses.
Sub-250 nm low-threshold deep-ultraviolet AlGaN-based heterostructure laser employing HfO2/SiO2 dielectric mirrors103(2013); http://dx.doi.org/10.1063/1.4829477View Description Hide Description
We report a sub-250-nm, optically pumped, deep-ultraviolet laser using an Al xGa1−xN-based multi-quantum-well structure grown on a bulk Al-polar c-plane AlN substrate. TE-polarization-dominant lasing action was observed at room temperature with a threshold pumping power density of 250 kW/cm2. After employing high-reflectivity SiO2/HfO2 dielectric mirrors on both facets, the threshold pumping power density was further reduced to 180 kW/cm2. The internal loss and threshold modal gain can be calculated as 2 cm−1 and 10.9 cm−1, respectively.
103(2013); http://dx.doi.org/10.1063/1.4831741View Description Hide Description
We investigate numerically the edge modes supported by graphene ribbons and the planar band-stop filter consisting of a graphene ribbon lateral coupled a graphene ring resonator by using the finite-difference time-domain method. Simulation results reveal that the edge modes can enhance the electromagnetic coupling between objects indeed and this structure realizes perfect, tunable filtering effect. Successively, the channel-drop filter is constructed. Especially, the proposed structures can be designed and the size of the ring is changed by creating non-uniform conductivity patterns on monolayer graphene. Our studies will benefit the fabrication of the planar, ultra-compact devices in the mid-infrared region.
103(2013); http://dx.doi.org/10.1063/1.4832071View Description Hide Description
We have studied how two-dimensional arrays of metallodielectric core-shell microspheres on a metal substrate can efficiently absorb infrared electromagnetic radiation in a narrow wavelength range under normal incidence. Our simulations indicate that perfect absorption efficiencies can be achieved for resonance wavelengths. The influence of core-shell microspheres geometry and lattice geometry is studied on absorption properties. For wavelength from 1.2 μm to 2.6 μm, an optimal combination of sphere and core radius was obtained to provide perfect absorption which can be wavelength adjusted.
Fabrication of terahertz metamaterial with high refractive index using high-resolution electrohydrodynamic jet printing103(2013); http://dx.doi.org/10.1063/1.4832197View Description Hide Description
Metamaterial is an engineered material whose electromagnetic properties can be determined by the unit structure. Lithography is one of main methods to fabricate metamaterials for fine patterning which has limitations in large-area fabrication. We present a direct fabrication method for metamaterial using the electrohydrodynamic jet printing. An electrical pulse was controlled to make drop-on-demand operation, through which flexible high refractive-index metamaterial could be fabricated in the form of I-shaped silver electrodes with 10-μm widths and 5-μm gaps on polyimide substrate. The peak value of the refractive index was 18.4 at a frequency of around 0.48 THz.
Polarization- and angle-dependent characteristics in two dimensional photonic crystal membrane reflectors103(2013); http://dx.doi.org/10.1063/1.4832221View Description Hide Description
We report here measured angle- and polarization-dependent reflection properties in two-dimensional photonic crystal broadband membrane reflectors. Polarization independent reflection was obtained at surface normal direction. High reflection can also be obtained for one transverse electric (TE) polarization state over a wide range of incident angles. With the target of 1550 nm spectral band, the measured spectral bandwidths for TE polarization with reflectivity greater than 90% are 180 nm and 56 nm at surface normal and oblique incidence of 45°, respectively. The experimental spectrum approximates the theoretical spectral response of this membrane reflector.
103(2013); http://dx.doi.org/10.1063/1.4832355View Description Hide Description
Temperature-dependent terahertz magnetic dipole radiation in antiferromagnetic GdFeO3 ceramic is investigated both theoretically and experimentally in this work. A two-level quantum transition mechanism is introduced to describe the excitation-radiation process, and radiative lifetime is derived analytically from the change of spin state density during this process. Terahertz spectral measurements demonstrate that the radiative frequency exhibits a red-shift and lifetime shortens as temperature increases, which is in good agreement with theoretical predictions. The temperature-sensitive radiative frequency and excellent terahertz emission mean that the antiferromagnetic ceramics show potential for application in terahertz sensors and frequency-tunable terahertz lasers.
Low repetition rate and broad frequency tuning from a grating-coupled passively mode-locked quantum dot laser103(2013); http://dx.doi.org/10.1063/1.4833025View Description Hide Description
Passively mode-locked quantum dot lasers with a grating-coupled external cavity arrangement are investigated. A broad repetition-rate tuning range of fundamental mode-locking from 2 GHz to a record-low frequency of 79.3 MHz is achieved with selecting the wavelength at 1.28 μm. A narrow RF linewidth of ∼25 Hz and an intrinsic linewidth as low as 0.15 Hz are also obtained.
103(2013); http://dx.doi.org/10.1063/1.4832216View Description Hide Description
A theoretical analysis of light trapping properties of Gallium arsenide (GaAs) nanoneedle arrays (NNAs) solar cells is presented. The effect of geometric parameters on the optical absorption of NNAs has been analyzed by using rigorous coupled wave analysis algorithm and finite element method. Compared with nanowire arrays and thin-film layer counterpart, higher light absorption efficiency can be achieved in GaAs NNAs, due to the graded refractive index of NNAs that incident light can be coupled into the NNAs efficiently. The absorption profiles at different wavelength and angle-dependant optical properties of NNAs are also evaluated. Meanwhile, the short-circuit current of GaAs NNAs for various lengths at fixed filling factor is obtained.
Small-signal modulation and differential gain of red-emitting (λ = 630 nm) InGaN/GaN quantum dot lasers103(2013); http://dx.doi.org/10.1063/1.4832332View Description Hide Description
We report small-signal modulation bandwidth and differential gain measurements of a ridge waveguide In0.4Ga0.6N/GaN quantum dot laser grown by molecular beam epitaxy. The laser peak emission is at λ = 630 nm. The −3 dB bandwidth of an 800 μm long device was measured to be 2.4 GHz at 250 mA under pulsed biasing, demonstrating the possibility of high-speed operation of these devices. The differential gain was measured to be 5.3 × 10−17 cm2, and a gain compression factor of 2.87 × 10−17 cm3 is also derived from the small-signal modulation response.
Wide single-mode tuning in quantum cascade lasers with asymmetric Mach-Zehnder interferometer type cavities with separately biased arms103(2013); http://dx.doi.org/10.1063/1.4832337View Description Hide Description
We report on the experimental demonstration of a widely tunable single mode quantum cascade laser with Asymmetric Mach-Zehnder (AMZ) interferometer type cavities with separately biased arms. Current and, consequently, temperature tuning of the two arms of the AMZ type cavity resulted in a single mode tuning range of 20 cm−1 at 80 K in continuous-wave mode operation, a ten-fold improvement from the lasers under a single bias current. In addition, we also observed a five fold increase in the tuning rate as compared to the AMZ cavities controlled by one bias current.
103(2013); http://dx.doi.org/10.1063/1.4832342View Description Hide Description
We demonstrate a fiber-based vibration sensor that involves an enclosed suspended microcantilever integrated with a readout fiber, providing in-line measurement of frequency and amplitude of vibration. The microcantilever is fabricated from a bend-insensitive fiber by chemical etching and fused to a single-mode fiber. Vibration induced periodic bending of the microcantilever angularly scans the readout fiber core resulting in output power modulation corresponding to certain drive frequencies and voltages of a shaker. Experimental results agree well with the theoretical analysis and demonstrate a continuous vibration frequency range of 5 Hz–10 kHz with a maximum signal-to-noise ratio of 68 dB.
103(2013); http://dx.doi.org/10.1063/1.4832636View Description Hide Description
Observation of the optical Rashba effect in plasmonics is reported. Polarization helicity degeneracy removal, associated with the inversion symmetry violation, is attributed to the surface symmetry design via anisotropic nanoantennas with space-variant orientations. By utilizing the Rashba-induced momentum in a nanoscale kagome metastructure, we demonstrated a spin-based surface plasmon multidirectional excitation under a normal-incidence illumination. The spin-controlled plasmonics via spinoptical metasurfaces provides a route for spin-based surface-integrated photonic nanodevices and light-matter interaction control, extending the light manipulation capabilities.
103(2013); http://dx.doi.org/10.1063/1.4832858View Description Hide Description
The electro-optical effects occurring under the action of AC and DC electric field in hybrid aligned nematic layers placed between crossed polarizers were simulated numerically. Switching between dark and bright states of such systems was analyzed. The role of flexoelectricity in the behavior of the layers was studied, thanks to the fact that the flexoelectric contribution to switching was present in the DC case and cancelled in the AC case. It was found that fast switching was favored by negative flexoelectric parameters or by large difference between voltages at which the both states existed.
103(2013); http://dx.doi.org/10.1063/1.4833539View Description Hide Description
We report a tuning method for ultrahigh-quality factor toroidal optical microcavities capable of rapid modulation and resonance position control over multiple decades. A free-space laser is focused onto the resonator's silicon support pillar, rapidly heating the mode-containing silica. Microcavity photothermal response is spatially mapped. Resonance shift varies inversely with pillar diameter, reaching 1.5 × 105 fm/mW at 2 μm diameter, allowing switching with 1 μW control power. Larger pillar resonators can be modulated at high speeds (>4 kHz). Heat flow simulations accurately model observed shifts. This versatile approach fulfills an outstanding need for fast, flexible control over toroid resonances.
103(2013); http://dx.doi.org/10.1063/1.4833545View Description Hide Description
We demonstrate second harmonic generation at telecommunications wavelengths in photonic crystal cavities in (111)-oriented GaAs. We fabricate 30 photonic crystal structures in both (111)- and (100)-oriented GaAs and observe an increase in generated second harmonic power in the (111) orientation, with the mean power increased by a factor of 3, although there is a large scatter in the measured values. We discuss possible reasons for this increase, in particular, the reduced two photon absorption for transverse electric modes in (111) orientation, as well as a potential increase due to improved mode overlap.
103(2013); http://dx.doi.org/10.1063/1.4833155View Description Hide Description
The intrinsic polarization is generally considered a nuisance in III-nitride devices, but recent studies have shown that it can be used to enhance p- and n-type conductivity and even to replace impurity doping. We show by numerical simulations that polarization-doped light-emitting diode (LED) structures have a significant performance advantage over conventional impurity-doped LED structures. Our results indicate that polarization doping decreases electric fields inside the active region and potential barriers in the depletion region, as well as the magnitude of the quantum-confined Stark effect. The simulations also predict at least an order of magnitude increase in the current density corresponding to the maximum efficiency (i.e., smaller droop) as compared to impurity-doped structures. The obtained high doping concentrations could also enable, e.g., fabrication of III-N resonant tunneling diodes and improved ohmic contacts.
103(2013); http://dx.doi.org/10.1063/1.4832328View Description Hide Description
We present a robust imaging method based on time-correspondence imaging and normalized ghost imaging (GI) that sets two thresholds to select the reference frame exposures for image reconstruction. This double-threshold time-correspondence imaging protocol always gives better quality and signal-to-noise ratio than previous GI schemes, and is insensitive to surrounding noise. Moreover, only simple add and minus operations are required while less data storage space and computing time are consumed; thus, faster imaging speeds are attainable. The protocol offers a general approach applicable to all GI techniques and marks a further step forward towards real-time practical applications of correlation imaging.
103(2013); http://dx.doi.org/10.1063/1.4831682View Description Hide Description
Graphene is a promising candidate for the development of detectors of Terahertz (THz) radiation. A well-known detection scheme due to Dyakonov and Shur exploits plasma waves in a field-effect transistor (FET), whereby a dc photovoltage is generated in response to a THz field. In the quest for devices with a better signal-to-noise ratio, we theoretically investigate a plasma-wave photodetector in which a dc photocurrent is generated in a graphene FET. The noise equivalent power of our device is shown to be much smaller than that of a Dyakonov-Shur detector in a wide spectral range.