Volume 103, Issue 23, 02 December 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:
We report 90° polarization switching in a BaTiO3 crystal without domain wall (DW) motion by prefabricating samples with interlocking domains via compression. During electric re-poling of the depoled and aged crystals, 90° domain nucleation still exists, but 90° DW motion is inhibited by the strong constraints from surrounding domains, leading to DW-free 90° polarization switching. The measured coercive field of 500 V/mm for the DW-free 90° switching is close to the intrinsic values and much larger than that of 80 V/mm via 90° DW motion. Compared to the rather difficult domain-free 180° polarization switching in ultrathin films, 90° DW-free switching is easier.
- PHOTONICS AND OPTOELECTRONICS
103(2013); http://dx.doi.org/10.1063/1.4837716View Description Hide Description
The noise properties of the pulse trains of passively mode-locked 40 GHz quantum-dot lasers subject to optical feedback are investigated in detail. Five different feedback regimes are discovered and the clearly identified regime of resonant optical feedback is further examined. The feedback parameters yielding minimum phase noise are determined. Here, the radio-frequency (RF)-line-width is reduced from its original value by 99% to 1.9 kHz. The corresponding pulse-to pulse jitter of 23 fs is a record low value for passively mode-locked 40 GHz quantum-dot lasers.
Generation of picosecond pulses and frequency combs in actively mode locked external ring cavity quantum cascade lasers103(2013); http://dx.doi.org/10.1063/1.4838275View Description Hide Description
We propose a robust and reliable method of active mode locking of mid-infrared quantum cascade lasers and develop its theoretical description. Its key element is the use of an external ring cavity, which circumvents fundamental issues undermining the stability of mode locking in quantum cascade lasers. We show that active mode locking can give rise to the generation of picosecond pulses and phase-locked frequency combs containing thousands of the ring cavity modes.
Multifunctional Bi2ZnOB2O6 single crystals for second and third order nonlinear optical applications103(2013); http://dx.doi.org/10.1063/1.4837055View Description Hide Description
Bi2ZnOB2O6 nonlinear optical single crystals were grown by means of the Kyropoulos method from stoichiometric melt. The second and third harmonic generation (SHG/THG) of Bi2ZnOB2O6 crystals were investigated by the SHG/THG Maker fringes technique. Moreover, SHG microscopy studies were carried out providing two-dimensional SHG images as a function of the incident laser polarization. The high nonlinear optical efficiency combined with the possibility to grow high quality crystals make Bi2ZnOB2O6 an excellent candidate for photonic applications.
High multi-photon visible upconversion emissions of Er3+ singly doped BiOCl microcrystals: A photon avalanche of Er3+ induced by 980 nm excitation103(2013); http://dx.doi.org/10.1063/1.4838636View Description Hide Description
Under 980 nm excitation, high multi-photon upconversion (UC) emission from the 2H11/2/4S3/2 (green) and 4F9/2 (red) levels of Er3+ ions were observed from Er3+ singly doped BiOCl microcrystals. These high-energy excited states were populated by a three to ten photon UC process conditionally, which depended on the pump power density and the Er3+ ion doping concentration, characterizing as a hetero-looping enhanced energy transfer avalanche UC process. UC emission lifetime and Raman analysis suggest that the unusual UC phenomena are initiated by the new and intense phonon vibration modes of BiOCl lattices due to Er3+ ions doping.
Enhancing photoresponse by synergy of gate and illumination in electric double layer field effect transistors fabricated on n-ZnO103(2013); http://dx.doi.org/10.1063/1.4838656View Description Hide Description
We report that photoresponse of ZnO in ultraviolet (UV) can be enhanced substantially by simultaneously applying a gate bias in an Electric Double Layer Field Effect Transistor configuration fabricated on ZnO as a channel. The effect arises from synergy between UV illumination and applied gate bias, which leads to a substantial enhancement in the device current. We propose that large carrier density created by the illumination and the gate leads to neutralization of some of the oxygen charged vacancies which in turn reduce potential scattering leading to enhanced field effect mobility. This is verified by gate bias controlled Photo Luminescence experiment.
Detection and measurement of electroreflectance on quantum cascade laser device using Fourier transform infrared microscope103(2013); http://dx.doi.org/10.1063/1.4839421View Description Hide Description
We demonstrate the use of a Fourier Transform Infrared microscope system to detect and measure electroreflectance (ER) from mid-infrared quantum cascade laser (QCL) device. To characterize intersubband transition (ISBT) energies in a functioning QCL device, a microscope is used to focus the probe on the QCL cleaved mirror. The measured ER spectra exhibit resonance features associated to ISBTs under applied electric field in agreement with the numerical calculations and comparable to observed photocurrent, and emission peaks. The method demonstrates the potential as a characterization tool for QCL devices.
103(2013); http://dx.doi.org/10.1063/1.4839535View Description Hide Description
In this Letter, we demonstrate the self-mixing effect in an interband cascade laser. We show that a viable self-mixing signal can be acquired through the variation in voltage across the laser terminals, thereby removing the need for an external detector. Using this interferometric technique, we have measured the displacement of a remote target, and also demonstrated high resolution imaging of a target. The proposed scheme represents a highly sensitive, compact, and self-aligned sensing technique with potential for materials analysis in the mid-infrared.
Inversion by metalorganic chemical vapor deposition from N- to Ga-polar gallium nitride and its application to multiple quantum well light-emitting diodes103(2013); http://dx.doi.org/10.1063/1.4841755View Description Hide Description
We demonstrate a metalorganic chemical vapor deposition growth approach for inverting N-polar to Ga-polar GaN by using a thin inversion layer grown with high Mg flux. The introduction of this inversion layer allowed us to grow p-GaN films on N-polar GaN thin film. We have studied the dependence of hole concentration, surface morphology, and degree of polarity inversion for the inverted Ga-polar surface on the thickness of the inversion layer. We then use this approach to grow a light emitting diode structure which has the MQW active region grown on the advantageous N-polar surface and the p-layer grown on the inverted Ga-polar surface.
103(2013); http://dx.doi.org/10.1063/1.4837219View Description Hide Description
Significant polarity-related differences in the near-band-edge photoluminescence from the Zn-polar and O-polar faces of hydrothermally grown ZnO single crystals, particularly in the ionized donor bound and free exciton recombination regions, were initially enhanced and then extinguished on annealing in oxygen at 400 °C and 600 °C, respectively. Polarity effects were also observed in the defect band emission with a structured green band associated with deep copper acceptor impurities appearing at lower annealing temperatures on the O-polar face. The loss of hydrogen is implicated in both these behaviors and in a sudden semiconductor-to-insulator transition between 200–300 °C.
103(2013); http://dx.doi.org/10.1063/1.4841635View Description Hide Description
In this report, the sampled grating distributed feedback laser architecture is modified with digital concatenated gratings to partially compensate for the wavelength dependence of optical gain in a standard high efficiency quantum cascade laser core. This allows equalization of laser threshold over a wide wavelength range and demonstration of wide electrical tuning. With only two control currents, a full tuning range of 500 nm (236 cm−1) has been demonstrated. Emission is single mode, with a side mode suppression of >20 dB.
103(2013); http://dx.doi.org/10.1063/1.4842055View Description Hide Description
We demonstrate the controllable broadband enhancement of two-photon absorption in a wide spectral range from 710 nm to 960 nm by controlling the surface states of aqueous Co2+ doped CdTe quantum dots, which is consistent with the measurement results of surface potential and fluorescence decay. The enhancement can be tuned in the range between 1 and 1.7 by changing the dopant concentrations that determine the surface states.
Enhancement of light-extraction efficiency in AlGaInP light-emitting diodes using evanescent wave coupling effect103(2013); http://dx.doi.org/10.1063/1.4842215View Description Hide Description
Sub-wavelength-sized ridge structures that satisfy evanescent wave coupling effect requirements at the emission wavelength were fabricated through the photolithography and wet chemical etching of the light-extraction surface of a thin-film type AlGaInP light-emitting diode (LED). By doing this, the light output power relative to a reference LED with a flat light-extraction surface was increased by a factor of approximately 3.8, corresponding to an equivalent enhancement in light-extraction efficiency that is possibly attributable to an evanescent wave coupling effect occurring on the surfaces of the minute ridge structures.
103(2013); http://dx.doi.org/10.1063/1.4844655View Description Hide Description
Resonance cavity modes enhance optical transmission through sub-wavelength metallic apertures but their role in absorption remains unclear. Here, we use full field simulation and a semi-analytical model to report absorption and transmission enhancement in transmission gratings under transverse electric illumination. The fundamental cavity mode of the sub-wavelength grating cavities is the major contributor to absorption. We demonstrate the possibility of tailoring such cavity resonances to induce total absorption in reflection gratings. Our method advances the understanding of transmittance and absorption enhancing mechanisms in metallic nanostructures which constitute fundamental components in energy harvesting, sensing, and spectroscopic applications.
Anomalous optical switching and thermal hysteresis during semiconductor-metal phase transition of VO2 films on Si substrate103(2013); http://dx.doi.org/10.1063/1.4838395View Description Hide Description
We present a detailed infrared study of the semiconductor-to-metal transition (SMT) in a vanadium dioxide (VO2) film deposited on silicon wafer. The VO2 phase transition is studied in the mid-infrared region by analyzing the transmittance and the reflectance measurements, and the calculated emissivity. The reflectance has been measured in two configurations: from the side of the VO2 film and from that of Si wafer. The results show a strong asymmetry between the emissivity in the two configurations, and the fact that the emissivity dynamic range from the silicon side is twice as large than that from the VO2 side. The temperature behaviour of the emissivity during the SMT put into evidence the phenomenon of the anomalous absorption in VO2, which has been explained by applying the Maxwell Garnett effective medium approximation theory.
- SURFACES AND INTERFACES
103(2013); http://dx.doi.org/10.1063/1.4838637View Description Hide Description
The voltage-spectral density SV (f) of the 2-dimensional electron gas formed at the interface of LaAlO3/SrTiO3 has been thoroughly investigated. The low-frequency component has a clear 1/f behavior with a quadratic bias current dependence, attributed to resistance fluctuations. However, its temperature dependence is inconsistent with the classical Hooge model, based on carrier-mobility fluctuations. The experimental results are, instead, explained in terms of carrier-number fluctuations, due to an excitation-trapping mechanism of the 2-dimensional electron gas.
Scanning tunneling spectroscopy studies of angle-dependent van Hove singularities on twisted graphite surface layer103(2013); http://dx.doi.org/10.1063/1.4839419View Description Hide Description
Slightly misoriented surface graphene layer on highly ordered pyrolytic graphite (HOPG) can result in Moiré patterns (MP). Two van Hove singularity (VHS) peaks in the density of states are observed with scanning tunneling spectroscopy on a series of the MPs with different periods on HOPG surface, flanking the Fermi energy. The energy difference between the two VHS peaks (ΔEVHS ) is linearly proportional to the sine of interlayer rotation angle. A similar relation between ΔEVHS and MP periods has recently been reported on twisted graphene layers. Compared to the case of twisted graphene layers, the Fermi velocity of MPs on a HOPG surface is found to be larger, and the interlayer hopping is enhanced.
Enhanced sensitivity and contrast with bimodal atomic force microscopy with small and ultra-small amplitudes in ambient conditions103(2013); http://dx.doi.org/10.1063/1.4840075View Description Hide Description
Here, we introduce bimodal atomic force microscopy operated with sub-nm and ultra-small, i.e., sub-angstrom, first and second mode amplitudes in ambient conditions. We show how the tip can be made to oscillate in the proximity of the surface and in perpetual contact with the adsorbed water layers while the second mode amplitude and phase provide enhanced contrast and sensitivity. Nonlinear and nonmonotonic behavior of the experimental observables is discussed theoretically with a view to high resolution, enhanced contrast, and minimally invasive mapping. Fractions of meV of energy dissipation are shown to provide contrast above the noise level.
103(2013); http://dx.doi.org/10.1063/1.4841335View Description Hide Description
The electronic band structure of monolayer (4 × 4) silicene on Ag(111) is imaged by angle resolved photoelectron spectroscopy. A dominant hybrid surface metallic band is observed to be located near the bulk Ag sp-band which is also faintly visible. The two-dimensional character of the hybrid band has been distinguished against the bulk character of the Ag(111) sp-band by means of photon energy dependence experiments. The surface band exhibits a steep linear dispersion around the point and has a saddle point near the point of Ag(111) resembling the π-band dispersion in graphene.
103(2013); http://dx.doi.org/10.1063/1.4841955View Description Hide Description
Ultra-thin tri-color (tri-layer) titanate superlattices ([3 u.c. LaTiO3/2 u.c. SrTiO3/3 u.c. YTiO3], u.c. = unit cells) were grown in a layer-by-layer way on single crystal TbScO3 (110) substrates by pulsed laser deposition. High sample quality and electronic structure were characterized by the combination of in-situ photoelectron and ex-situ structure and surface morphology probes. Temperature-dependent sheet resistance indicates the presence of metallic interfaces in both [3 u.c. LaTiO3/2 u.c. SrTiO3] bi-layers and all the tri-color structures, whereas a [3 u.c. YTiO3/2 u.c. SrTiO3] bi-layer shows insulating behavior. Considering that in the bulk YTiO3 is ferromagnetic below 30 K, the tri-color titanate superlattices provide an opportunity to induce tunable spin-polarization into the two-dimensional electron gas with Mott carriers.
The effects of three-dimensional shaping of vertically aligned carbon-nanotube contacts for micro-electro-mechanical switches103(2013); http://dx.doi.org/10.1063/1.4844695View Description Hide Description
A micro-electro-mechanical switch integrated with vertically aligned carbon nanotubes (CNTs) as the contact material is presented. Arrays of the CNTs are three-dimensionally micropatterned using a pulsed micro-discharge process to have tapered contact surfaces with controlled angles, achieving maximized contact areas, while providing contact resistances in the 10 Ω range with an enhanced current capacity. A shape-memory-alloy actuator is integrated to demonstrate stable switching for ∼1.4 × 106 ON-OFF cycles with no sign of damage. The results prove that post-growth micropatterning of CNTs is a promising path to improved and reliable micro contact switches enabled by arrayed CNT contacts for high-power applications.