Volume 101, Issue 3, 16 July 2012
- 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:
In this work, hybrid heterojunction solar cells are demonstrated based on a conjugate polymer poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) directly spun-cast on micro-textured n-type crystalline silicon wafers. The fabrication conditions suggest that the organic coverage on the micro-textured surface is excellent and key to achieve high efficiency, leading to an average power conversion efficiency of 9.84%. A one-dimensional drift-diffusion model is then developed based on fitting the device characteristics with experimentally determined PEDOT:PSS parameters and projects an ultimate efficiency above 20% for organic/inorganic hybrid photovoltaics. The simulation results reveal the impacts of defect densities, back surface recombination, doping concentration, and band alignment.
- PHOTONICS AND OPTOELECTRONICS
Nanofocusing enhancement in a tapered slit by using a dielectric micro isosceles triangle prism and tuning the entrance aperture101(2012); http://dx.doi.org/10.1063/1.4737206View Description Hide Description
We present a combined configuration consisting of a dielectric micro isosceles triangular prism and a metallic tapered slit for enhanced nanofocusing. The dielectric micro isosceles triangular prism collects the incident light first and then the tapered slit guides the beam to form naofocusing with higher intensity at the exit aperture of the tapered slit, which overcomes the limit of the extremely small entrance aperture diaphragm of the slit. With the help of the dielectric micro isosceles triangular prism, the intensity of the nanofocused electric field at the exit aperture of the tapered slit can be increased more than 4 folds.
101(2012); http://dx.doi.org/10.1063/1.4736862View Description Hide Description
Sequential radio-frequency sputtering was used to produce CdTe-ZnO nanocompositethin films with varied semiconductor-phase extended structures. Control of the spatial distribution of CdTenanoparticles within the ZnO embedding phase was used to influence the semiconductor phase connectivity, contributing to both changes in quantum confinement induced spectralabsorption and carrier transport characteristics of the resulting nanocomposite. An increased number density of CdTe particles deposited along the applied field direction produced an enhancement in the photo-induced current observed. These results highlight the opportunity to employ long-range phase assembly as a means to control optoelectronic properties of significant interest for photovoltaic applications.
101(2012); http://dx.doi.org/10.1063/1.4737164View Description Hide Description
The measurement of the wavefront of a terahertz (THz) beam is essential for the development of any optical instrument operating at THz frequencies. We have realized a Hartmann wavefront sensor for the THz frequency range. The sensor is based on an aperture plate consisting of a regular square pattern of holes and a microbolometer camera. The performance of the sensor is demonstrated by characterizing the wavefront of a THz beam emitted by a quantum-cascade laser. The wavefront determined by the sensor agrees well with that expected from a Gaussian-shaped beam. The spatial resolution is 1 mm, and a single-wavefront measurement takes less than 1 s.
101(2012); http://dx.doi.org/10.1063/1.4737425View Description Hide Description
We report the achievement of a semiconductor tube laser that can operate in the optical communication wavelength range for applications in the emerging Si-photonics. Such nanoscale devices are fabricated from self-organized InAs/InGaAsP quantum dotnanomembranes through a strain-driven self-rolling mechanism using standard photolithography process. Under continuous wave optical pumping, the devices exhibit an ultralow lasing threshold of ∼1.26 μW at 82 K, with multiple emission wavelengths in the S band of optical communications. The spontaneous emission coupling factor and Purcell factor are estimated to be ∼0.30 and ∼4.8, respectively.
101(2012); http://dx.doi.org/10.1063/1.4737441View Description Hide Description
We study the opto-mechanical properties of coupled chiral meta-atoms based on a pair of twisted split-ring resonators. By using a simple analytical model in conjunction with the Maxwell stress tensor, we capture insight into the mechanism and find that this structure can be used as a general prototype of subwavelength light-driven actuators over a wide range of frequencies. This coupled structure can provide a strong and tunable torque, and can support different opto-mechanical modes, including uniform rotation, periodically variable rotation and damped oscillations. Our results suggest that chiral meta-atoms are good candidates for creating sub-wavelength motors or wrenches controlled by light.
101(2012); http://dx.doi.org/10.1063/1.4731762View Description Hide Description
Zero-phonon-line (ZPL) emission of nitrogen vacancies (NVs) is coupled to the guided modes of solid- and hollow-core nanodiamond-doped photonic-crystal fibers (PCFs). Both types of PCFs are tailored toward enhancing ZPL emission coupling to the fiber modes. In solid-core PCFs, this involves enhancing the evanescent field of the waveguide modes supported by an ultrasmall fiber core. In hollow-core PCFs, the NV emission spectrum is matched with the transmission band of the fiber, controlled by the photonic bands of the fiber cladding.
101(2012); http://dx.doi.org/10.1063/1.4736411View Description Hide Description
The National Ignition Campaign has the goal of developing a burning plasma platform producing up to ∼1019neutrons in ∼20 picoseconds, ps. Diagnosis will require instruments operating with a time resolution of a few ps within this extremely large neutron flux environment. A diagnostic, which performs an ultrafast conversion of the x-ray signals into the optical regime, has been developed using a linearly chirped probe beam to measure the temporal history of the x-ray pulse. This diagnostic technique was tested on a laser-produced x-ray source and obtained a measurement of the full-width-at-half-maximum, FWHM, of the x-ray pulse of ∼7.2 ps.
101(2012); http://dx.doi.org/10.1063/1.4733981View Description Hide Description
We report that the oxidation of graphene-based highly transparent conductive layers to AlGaN/GaN/AlGaN ultra-violet (UV)light-emitting diodes(LEDs) was suppressed by the use of SiNXpassivation layers. Although graphene is considered to be an ideal candidate as the transparent conductive layer to UV-LEDs, oxidation of these layers at high operating temperatures has been an issue. The oxidation is initiated at the un-saturated carbon atoms at the edges of the graphene and reduces the UV light intensity and degrades the current-voltage (I-V) characteristics. The oxidation also can occur at defects, including vacancies. However, GaN-based UV-LEDs deposited with SiNX by plasma-enhanced chemical vapor deposition showed minimal degradation of light output intensity and I-V characteristics because the graphene-based UV transparent conductive layers were shielded from the oxygen molecules. This is a simple and effective approach for maintaining the advantages of graphene conducting layers as electrodes on UV-LEDs.
Complementary metal–oxide–semiconductor compatible high efficiency subwavelength grating couplers for silicon integrated photonics101(2012); http://dx.doi.org/10.1063/1.4737412View Description Hide Description
We demonstrate a through-etched grating coupler based on subwavelength nanostructure. The grating consists of arrays of 80 nm × 343 nm rectangular air holes, which can be patterned in a single lithography/etch. A peak coupling efficiency of 59% at 1551.6 nm and a 3 dB bandwidth of 60 nm are achieved utilizing the silicon-on-insulator platform with a 1 μm thick buried-oxide layer for transverse electric mode. The performance is comparable to gratings requiring much more complicated fabrication processes.
101(2012); http://dx.doi.org/10.1063/1.4737426View Description Hide Description
Intraband photocurrent spectroscopy of site-controlled pyramidal quantum dots by inserting them into the intrinsic region of n-i-n like quantum dot infrared photodetector structure is reported. The photovoltaic response is observed in the mid-infrared region. A peak responsivity of 0.4 mA/W at 120 meV (λ = 10 μm) is observed at 10 K at −2 V bias. The ability to engineer states in the conduction band of the QDs has been exploited to tune their photocurrent response from 10 μm to 18 μm with a narrow spectral width of Δλ/λ = 0.17.
Subpicosecond electron-hole recombination time and terahertz-bandwidth photoresponse in freestanding GaAs epitaxial mesoscopic structures101(2012); http://dx.doi.org/10.1063/1.4737442View Description Hide Description
We present the ultrafast (THz-bandwidth) photoresponse from GaAs single-crystal mesoscopic structures, such as freestanding whiskers and platelets fabricated by the top-down technique, transferred onto a substrate of choice, and incorporated into a coplanar strip line. We recorded electrical transients as short as ∼600 fs from an individual whisker photodetector. Analysis of the frequency spectrum of the photoresponse electrical signal showed that, intrinsically, our device was characterized by an ∼150-fs carrier lifetime and an overall 320-fs response. The corresponding 3-dB frequency bandwidth was 1.3 THz—the highest bandwidth ever reported for a GaAs-based photodetector. Simultaneously, as high-quality, epitaxially grown crystals, our GaAs structures exhibited mobility values as high as ∼7300 cm2/V·s, extremely low dark currents, and ∼7% intrinsic detection efficiency, which, together with their experimentally measured photoresponse repetition time of ∼1 ps, makes them uniquely suitable for terahertz-frequency optoelectronic applications, ranging from ultrafast photon detectors and counters to THz-bandwidth optical-to-electrical transducers and photomixers.
101(2012); http://dx.doi.org/10.1063/1.4737642View Description Hide Description
We demonstrate a fast switchable grating based on ferroelectric liquid crystals and orthogonal planar alignment by means of photo alignments. Both 1D and 2D gratings have been constructed. The proposed diffracting element provides fast response time of around 20 μs, contrast of 7000:1 and high diffraction efficiency, at the electric field of 6 V/μm. The saturated electro-optical (EO) states up to very high frequency (≈5 kHz) are the real advantage of the proposed switchable grating, which opens several opportunities to improve the quality of existing devices and to find new applications.
101(2012); http://dx.doi.org/10.1063/1.4737943View Description Hide Description
We demonstrate the direct generation of optical vortex pulses with stable energy and changeable orbital angular momentum. Single Laguerre-Gaussian (LG0,l) laser modes were directly generated using a laser diode with output intensity profile of doughnut distribution. With passive Q-switching, vortex pulses with stable energy were obtained. Moreover, the topological charge was changeable by variation of the pump power. By a mode-converter and second harmonic generation, the LG0,2 l mode was identified. It can be proposed that this pulsed laser should have promising applications in various fields based on its compact structure, stable and high pulse energy, and changeable orbital angular momentum.
Local rotational symmetry effects on Fano resonances with constant non-resonant transmission channel101(2012); http://dx.doi.org/10.1063/1.4737178View Description Hide Description
Three kinds of 12-fold quasi-periodic subwavelength hole arrays have been designed using the same dodecahedral supercell arranged with different local rotational symmetries. Fano resonances associated with spoof surface plasmons in these structures have been studied by far-infrared transmission measurements. The resonant transmission channels of the lowest-order Fano resonance mode have been compared directly between these structures, benefitting from constant non-resonant transmission channel. It is found that the higher is the local rotational symmetry of the supercell array, the higher the transmission intensity and the narrower the linewidth of the Fano resonance.
101(2012); http://dx.doi.org/10.1063/1.4737637View Description Hide Description
We studied GaN-based optoelectronic devices such as light-emitting diodes (LEDs) and solar cells (SCs) with graphene electrodes. A decoration of Au nanoparticles (NPs) on multi-layer graphene films improved the electrical conductivity and modified the work function of the graphene films. The Au NP-decorated graphene film enhanced the current injection and electroluminescence of GaN-based LEDs through low contact resistance and improved the power conversion efficiency of GaN-based SCs through additional light absorption and energy band alignment. Our study will enhance the understanding of the role of Au NP-decorated graphene electrodes for GaN-based optoelectronic device applications.
101(2012); http://dx.doi.org/10.1063/1.4736406View Description Hide Description
The relative coherence and phase are extracted from two-element, coherently coupled, vertical cavity surface emitting laser arrays. The array elements are defined optically by a photonic crystal pattern and electrically by ion implantation. We obtain the near and far fields experimentally under varying current injection. The Fraunhofer approximation is used to simulate propagation from the near to far field. The phase and coherence are extracted as fitting parameters to match the experimental and propagated far field patterns. The phase and coherence will aid in future array designs and in elucidating the phase-shifting mechanism.
101(2012); http://dx.doi.org/10.1063/1.4736408View Description Hide Description
Amplified spontaneous emissions(ASEs) with a tuned wavelength have been observed in a thin film composite made by spin-coating a dye-doped polymer layer (thickness ∼80 nm) on Ag nanoparticles deposited on a SiO2 substrate. The samples having a varied wavelength of the localized surface plasmon resonance (LSPR) are excited with second harmonic pulses from a doped yttrium aluminum garnet laser. When the pump energy exceeds a threshold, a spectrally narrow peak appears on a broad emission from dye molecules with a nonlinear increase in emission intensity, indicating an onset of ASE. It is found that the peak wavelength of ASE shifts with the change of the wavelength of LSPR.
101(2012); http://dx.doi.org/10.1063/1.4737648View Description Hide Description
We report a ZnO-based thin film transistorUVphotodetector with a back gate configuration. The thin-film transistor(TFT) aspect ratio W/L is 150 μm/5 μm and has a current on-off ratio of 1010. The detector shows UV-visible rejection ratio of 104 and cut-off wavelength of 376 nm. The device has low dark current of 5 × 10−14 A. The persistent photoconductivity is suppressed through oxygen plasma treatment of the channel surface which significantly reduces the density of oxygen vacancy confirmed by XPS measurements. The proper gate bias-control further reduces recovery time. The UV-TFT configuration is particularly suitable for making large-area imaging arrays.
101(2012); http://dx.doi.org/10.1063/1.4738765View Description Hide Description
We report the design and fabrication of the complete visible range omnidirectional mirror, with a dielectric multilayered structure based on porous silicon(PS). The refractive index profile consisted of Bragg type chirped layers with an increasing thicknesses modulated by a potential envelop function . The omnidirectional photonic band gap (OPBG) was measured from 396 to 805 nm, with more than 95% of the reflectivity, for 8° and 68°. The theoretical simulations based on the transfer matrix method along with a photographic sequence of the sample confirmed the OPBG for higher incident angles. It was enhanced by a factor of 19 as compared to the last reported visible region OPBG by PSdielectric multilayered structure.
- SURFACES AND INTERFACES
101(2012); http://dx.doi.org/10.1063/1.4737210View Description Hide Description
The interaction between solid particles and gas-liquid interfaces is relevant in technological applications. Former studies did focus on detachment-dynamics of particles from thin liquid films or on attachment-dynamics of particles to gas bubbles. Here, we investigated snap-in dynamics of individual micron-sized particles to water drops by means of the colloidal probe technique. The snap-in time (∼0.1 ms) and the snap-in force of hydrophilic and hydrophobic particles were measured. The snap-in time increased with particle size regardless of wettability. The snap-in force increased with particle size and wettability. We show that the snap-in dynamics is dominated by capillarity and inertia.