Volume 106, Issue 4, 26 January 2015
- 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:
- PHOTONICS AND OPTOELECTRONICS
106(2015); http://dx.doi.org/10.1063/1.4906802View Description Hide Description
A method to improve the sensitivity and selectivity of a monolithically integrated mid-infrared sensor using a distributed feedback laser (DFB) is presented in this paper. The sensor is based on a quantum cascade laser/detector system built from the same epitaxial structure and with the same fabrication approach. The devices are connected via a dielectric-loaded surface plasmon polariton waveguide with a twofold function: it provides high light coupling efficiency and a strong interaction of the light with the environment (e.g., a surrounding fluid). The weakly coupled DFB quantum cascade laser emits narrow mode light with a FWHM of 2 cm−1 at 1586 cm−1. The room temperature laser threshold current density is 3 kA∕cm2 and a pulsed output power of around 200 mW was measured. With the superior laser noise performance, due to narrow mode emission and the compensation of thermal fluctuations, the lower limit of detection was expanded by one order of magnitude to the 10 ppm range.
Mobility enhancement in electric double layer gated n-ZnO ultraviolet photodetector by synergy of gate and illumination: A photo Hall study106(2015); http://dx.doi.org/10.1063/1.4906598View Description Hide Description
We report a large enhancement of the Hall mobility of a ZnO film (channel) by simultaneously application of an ultraviolet illumination along with a gate bias in an electric double layer field effect transistor configuration. The effect arises from a synergy between the illumination and the field effect (FE), leading to large enhancement of the channel conductivity and the photo response. We propose that large carrier density created by the simultaneous presence of the illumination and the FE leads to neutralization of some of the oxygen charged vacancies which in turn reduce potential scattering leading to the enhanced mobility.
106(2015); http://dx.doi.org/10.1063/1.4906611View Description Hide Description
We use a picosecond acoustics technique to modulate the laser output of electrically pumped GaAs/AlAs micropillar lasers with InGaAs quantum dots. The modulation of the emission wavelength takes place on the frequencies of the nanomechanical extensional and breathing (radial) modes of the micropillars. The amplitude of the modulation for various nanomechanical modes is different for every micropillar which is explained by a various elastic contact between the micropillar walls and polymer environment.
106(2015); http://dx.doi.org/10.1063/1.4906596View Description Hide Description
Here, we investigate both numerically and experimentally, the polarization conversion capabilities of a rectangular array of holes with two unequal orthogonal periodicities. We show that it is possible to tune the periodicities in such a way that the transmitted light is circularly polarized for a nominated wavelength, λ CPL, when the structure is illuminated with appropriately oriented linearly polarized light at normal incidence. A device was fabricated and experiments confirmed that a degree of circular polarization of 0.89 could be achieved at the resonant wavelength.
106(2015); http://dx.doi.org/10.1063/1.4906860View Description Hide Description
There has been a considerable amount of interest in the development of various types of electromagnetic wave absorbers for use in different wavelength ranges. In particular, infrared (IR) absorbers with wavelength selectivity can be applied to advanced uncooled IR sensors, which would be capable of identifying objects through their radiation spectrum. In the present study, mushroom plasmonic metamaterial absorbers (MPMAs) for the IR wavelength region were designed and fabricated. The MPMAs consist of a periodic array of thin metal micropatches connected to a thin metal plate with narrow silicon (Si) posts. A Si post height of 200 nm was achieved by isotropic XeF2 etching of a thin Si layer sandwiched between metal plates. This fabrication procedure is relatively simple and is consistent with complementary metal oxide semiconductor technology. The absorption spectra of the fabricated MPMAs were experimentally measured. In addition, theoretical calculations of their absorption properties were conducted using rigorous coupled wave analysis. Both the calculated and measured absorbance results demonstrated that these MPMAs can realize strong selective absorption at wavelengths beyond the period of the array by varying the micropatch width. Absorbance values greater than 90% were achieved. Dual- or single-mode absorption can also be selected by varying the width of the Si posts. Pixel structures using such MPMAs could be used as high responsivity, high resolution and fast uncooled IR sensors.
106(2015); http://dx.doi.org/10.1063/1.4906923View Description Hide Description
We demonstrate one-dimensional photonic-crystal nanobeam cavities in amorphous silicon carbide. The fundamental mode exhibits intrinsic optical quality factor as high as 7.69 × 104 with mode volume at wavelength 1.5 μm. A corresponding Purcell factor value of ∼104 is the highest reported to date in silicon carbide optical cavities. The device exhibits great potential for integrated nonlinear photonics and cavity nano-optomechanics.
106(2015); http://dx.doi.org/10.1063/1.4906924View Description Hide Description
An approach to the optimization of the spectral power distribution of solid-state light sources with the tunable non-image forming photobiological effect on the human circadian rhythm is proposed. For tetrachromatic clusters of model narrow-band (direct-emission) light-emitting diodes (LEDs), the limiting tunability of the circadian action factor (CAF), which is the ratio of the circadian efficacy to luminous efficacy of radiation, was established as a function of constraining color fidelity and luminous efficacy of radiation. For constant correlated color temperatures (CCTs), the CAF of the LED clusters can be tuned above and below that of the corresponding blackbody radiators, whereas for variable CCT, the clusters can have circadian tunability covering that of a temperature-tunable blackbody radiator.
InAs-based interband-cascade-lasers emitting around 7 μm with threshold current densities below 1 kA/cm2 at room temperature106(2015); http://dx.doi.org/10.1063/1.4907002View Description Hide Description
Interband cascade lasers (ICLs) grown on InAs substrates with threshold current densities below 1 kA/cm2 are presented. Two cascade designs with different lengths of the electron injector were investigated. Using a cascade design with 3 InAs quantum wells (QWs) in the electron injector, a device incorporating 22 stages in the active region exhibited a threshold current density of 940 A/cm2 at a record wavelength of 7 μm for ICLs operating in pulsed mode at room temperature. By investigating the influence of the number of stages on the device performance for a cascade design with 2 QWs in the electron injector, a further reduction of the threshold current density to 800 A/cm2 was achieved for a 30 stage device.
106(2015); http://dx.doi.org/10.1063/1.4907004View Description Hide Description
Films of the topological insulator Bi2Se3 are grown by molecular beam epitaxy with in-situ reflection high-energy electron diffraction. The films are shown to be high-quality by X-ray reflectivity and diffraction and atomic-force microscopy. Quantum interference control of photocurrents is observed by excitation with harmonically related pulses and detected by terahertz radiation. The injection current obeys the expected excitation irradiance dependence, showing linear dependence on the fundamental pulse irradiance and square-root irradiance dependence of the frequency-doubled optical pulses. The injection current also follows a sinusoidal relative-phase dependence between the two excitation pulses. These results confirm the third-order nonlinear optical origins of the coherently controlled injection current. Experiments are compared to a tight-binding band structure to illustrate the possible optical transitions that occur in creating the injection current.
106(2015); http://dx.doi.org/10.1063/1.4906460View Description Hide Description
Optical characterisation methodologies are employed to validate a nanorod self-alignment technique for use in luminescent solar concentrators (LSCs). The nanorods utilised in this work were CdSe/CdS core/shell nanorods, and the self-alignment technique relied on the evaporation of a highly concentrated nanorod/xylene solution onto a glass substrate. Position and angular dependent light absorptivity measurements revealed evidence of vertical nanorod alignment over a limited region at the centre of the LSC sample. Vertical nanorod alignment is beneficial for absorbing diffuse/scattered sunlight and provides for a high light trapping efficiency in the LSC.
Texturing the cathode of white organic light-emitting diodes with a lattice of nanoscale scatterers for enhanced light out-coupling106(2015); http://dx.doi.org/10.1063/1.4907253View Description Hide Description
The external quantum efficiency of white organic light-emitting diodes is often limited by light out-coupling losses due to surface plasmons. We demonstrate how texturing of the metal-cathode surface using a two-dimensionally periodic lattice of nanoscale scatterers with limited disorder can be used to reduce plasmonic losses while simultaneously enhancing both the light out-coupling and the spontaneous-emission rate of the excitons. We use electrodynamic simulations and statistical modeling to explore the relationship between the topology of the surface texture and its corresponding scattering efficiency. From this, we outline attributes of textures that can most enhance device performance.
106(2015); http://dx.doi.org/10.1063/1.4906921View Description Hide Description
We investigate the spontaneous emission (SE) of self-assembled InAs quantum dots (QDs) embedded in GaAs ridge waveguides that lay on a low index substrate. In thin enough waveguides, the coupling to the fundamental guided mode is vanishingly small. A pronounced anisotropy in the coupling to non-guided modes is then directly evidenced by normal-incidence photoluminescence polarization measurements. In this regime, a measurement of the QD decay rate reveals a SE inhibition by a factor up to 4. In larger wires, which ensure an optimal transverse confinement of the fundamental guided mode, the decay rate approaches the bulk value. Building on the good agreement with theoretical predictions, we infer from calculations the fraction β of SE coupled to the fundamental guided mode for some important QD excitonic complexes. For a charged exciton (isotropic in plane optical dipole), β reaches 0.61 at maximum for an on-axis QD. In the case of a purely transverse linear optical dipole, β increases up to 0.91. This optimal configuration is achievable through the selective excitation of one of the bright neutral excitons.
Enhanced single photon emission from positioned InP/GaInP quantum dots coupled to a confined Tamm-plasmon mode106(2015); http://dx.doi.org/10.1063/1.4907003View Description Hide Description
We report on the enhancement of the spontaneous emission in the visible red spectral range from site-controlled InP/GaInP quantum dots by resonant coupling to Tamm-plasmon modes confined beneath gold disks in a hybrid metal/semiconductor structure. The enhancement of the emission intensity is confirmed by spatially resolved micro-photoluminescence area scans and temperature dependent measurements. Single photon emission from our coupled system is verified via second order autocorrelation measurements. We observe bright single quantum dot emission of up to ∼173 000 detected photons per second at a repetition rate of the excitation source of 82 MHz, and calculate an extraction efficiency of our device as high as 7%.
Investigating the origin of efficiency droop by profiling the temperature across the multi-quantum well of an operating light-emitting diode106(2015); http://dx.doi.org/10.1063/1.4907177View Description Hide Description
Performance degradation resulting from efficiency droop during high-power operation is a critical problem in the development of high-efficiency light-emitting diodes (LEDs). In order to resolve the efficiency droop and increase the external quantum efficiency of LEDs, the droop's origin should be identified first. To experimentally investigate the cause of efficiency droop, we used null-point scanning thermal microscopy to quantitatively profile the temperature distribution on the cross section of the epi-layers of an operating GaN-based vertical LED with nanoscale spatial resolution at four different current densities. The movement of temperature peak towards the p-GaN side as the current density increases suggests that more heat is generated by leakage current than by Auger recombination. We therefore suspect that at higher current densities, current leakage becomes the dominant cause of the droop problem.
Demonstration of transverse-magnetic deep-ultraviolet stimulated emission from AlGaN multiple-quantum-well lasers grown on a sapphire substrate106(2015); http://dx.doi.org/10.1063/1.4906590View Description Hide Description
We demonstrate transverse-magnetic (TM) dominant deep-ultraviolet (DUV) stimulated emission from photo-pumped AlGaN multiple-quantum-well lasers grown pseudomorphically on an AlN/sapphire template by means of photoluminescence at room temperature. The TM-dominant stimulated emission was observed at wavelengths of 239, 242, and 243 nm with low thresholds of 280, 250, and 290 kW/cm2, respectively. In particular, the lasing wavelength of 239 nm is shorter compared to other reports for AlGaN lasers grown on foreign substrates including sapphire and SiC. The peak wavelength difference between the transverse-electric (TE)-polarized emission and TM-polarized emission was approximately zero for the lasers in this study, indicating the crossover of crystal-field split-off hole and heavy-hole valence bands. The rapid variation of polarization between TE- and TM-dominance versus the change in lasing wavelength from 243 to 249 nm can be attributed to a dramatic change in the TE-to-TM gain coefficient ratio for the sapphire-based DUV lasers in the vicinity of TE-TM switch.
Ultraviolet stimulated emission from high-temperature-annealed MgO microcrystals at room temperature106(2015); http://dx.doi.org/10.1063/1.4907321View Description Hide Description
Research on semiconductor nanowires underlies the development of the miniaturization of laser devices with low cost and low energy consumption. In general, nanowire lasers are made of direct band gap semiconductors, e.g., GaN, ZnO and CdS, and their band-edge emissions are used to achieve optically pumped laser emission. In addition to the existing class of nanowire lasers, we here show that air-annealed micrometer-sized MgO cubic crystals with well-defined facets exhibit room-temperature stimulated emission at 394 nm under pulsed laser pumping at ∼350 nm. Surface midgap states are assumed to be responsible for the excitation and emission processes. The present findings will not only provide opportunities for the development of miniaturized lasers composed of insulating oxides, but will also open up functionality in various families of cubic crystalline materials.
106(2015); http://dx.doi.org/10.1063/1.4907326View Description Hide Description
Interband cascade (IC) lasers have been demonstrated based on type-I InGaAsSb/AlAsSb quantum well (QW) active regions. These type-I IC lasers are composed of 6-cascade stages and InAs/AlSb superlattice cladding layers. In contrast to the use of quinary AlGaInAsSb barriers for active region in previous type-I QW lasers, the type-I QW active region in each stage is sandwiched by digitally graded multiple InAs/AlSb QW electron injector and GaSb/AlSb QW hole injector. The fabricated type-I IC lasers were able to operate in continuous wave and pulsed modes at temperatures up to 306 and 365 K, respectively. The threshold current densities of broad-area lasers were around 300 A/cm2 at 300 K with a lasing wavelength near 3.2 μm. The implications and prospects of these initial results are discussed.
- SURFACES AND INTERFACES
106(2015); http://dx.doi.org/10.1063/1.4906417View Description Hide Description
This letter reports observations of the breakdown characteristics of helium/nitrogen parallel jets when AC voltage was applied across them. Below a certain voltage level, the plasma emission was fully confined to the helium stream as in a traditional dielectric barrier discharge, where the role of the dielectric was played by an insulating gas barrier (nitrogen as compared to helium). The discharge had characteristics of self-organized patterns. Some differences to traditional solid dielectrics are discussed.
106(2015); http://dx.doi.org/10.1063/1.4906867View Description Hide Description
Electronic properties of fullerene derivatives containing oligothiophene pendant chain (1–3 thiophene moieties) were investigated using the Kelvin probe technique and quantum chemistry methods. For electrochemical examination of these systems, Langmuir–Blodgett (LB) layers were prepared by the deposition on a gold substrate. The analysis of the experimental data shows that the value of the work function depends strongly on the length of oligothiophene chain. Similar dependence was also found for the surface photovoltage measurements conducted for the layers consisting of multiple LB films of the examined compounds deposited on gold surfaces. The assumption has been made that these changes are associated with the influence of oligothiophene chain on the electrostatic potential distribution near the surface of the sample. The hypothesis was confirmed by the results of DFT calculations, which revealed that the value of Fermi level energy shifts in the opposite direction to the determined work function. The key highlights of this study are as follows: electronic structure tuning by oligothiophene side chain; DFT calculation on fullerene-thiophene system; work function measurements of thin molecular layers.
Direct observation of electron emission and recombination processes by time domain measurements of charge pumping current106(2015); http://dx.doi.org/10.1063/1.4906997View Description Hide Description
To analyze the charge pumping (CP) sequence in detail, the source/drain electron current and the substrate hole current under the CP mode of transistors are simultaneously monitored in the time domain. Peaks are observed in both the electron and hole currents, which are, respectively, attributed to the electron emission from the interface defects and to the recombination with holes. The peak caused by the electron emission is found to consist of two components, strongly suggesting that the present time-domain measurement can enable us to resolve different kinds of interface defects. Investigating the correlation between the number of emitted and recombined electrons reveals that only one of the two components contributes to the CP current for the gate-pulse fall time from 6.25 × 10−4 to 1.25 × 10−2 s.