Volume 104, Issue 5, 03 February 2014
- 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:
Based on hydrogen-mediated ferromagnetism and a selective hydrogen exposure technique, i.e., hydrogen lithography, we attempted to produce magnetic domains in a paramagnetic host. Hydrogen lithography on Co-doped ZnO with an anodic aluminum oxide template was used to produce nanomagnetic domains in paramagnetic Co-doped ZnO. The domains showed in-plane magnetization with a head-to-tail configuration at room temperature, which is consistent with the object-oriented micro-magnetic framework simulations.
- PHOTONICS AND OPTOELECTRONICS
104(2014); http://dx.doi.org/10.1063/1.4863504View Description Hide Description
We introduce a simple technique to improve the beam quality of broad area quantum cascade lasers. Moderately tilted front facets of the laser provide suppression of higher order lateral waveguide modes. A device with a width of 60 μm and a front facet angle of 17° shows a nearly diffraction limited beam profile. In addition, the peak output power and the slope efficiency of the device are increased since most of the light inside the cavity is emitted through the tilted front facet by an asymmetric light intensity distribution along the cavity.
Mode switching in a multi-wavelength distributed feedback quantum cascade laser using an external micro-cavity104(2014); http://dx.doi.org/10.1063/1.4863663View Description Hide Description
We demonstrate a multi-wavelength distributed feedback (DFB) quantum cascade laser (QCL) operating in a lensless external micro-cavity and achieve switchable single-mode emission at three distinct wavelengths selected by the DFB grating, each with a side-mode suppression ratio larger than 30 dB. Discrete wavelength tuning is achieved by modulating the feedback experienced by each mode of the multi-wavelength DFB QCL, resulting from a variation of the external cavity length. This method also provides a post-fabrication control of the lasing modes to correct for fabrication inhomogeneities, in particular, related to the cleaved facets position.
Ultra-broadband terahertz time-domain ellipsometric spectroscopy utilizing GaP and GaSe emitters and an epitaxial layer transferred photoconductive detector104(2014); http://dx.doi.org/10.1063/1.4862974View Description Hide Description
We present a reflection-type ultra-broadband terahertz (THz) time-domain spectroscopic ellipsometry system covering the frequency range of 0.5–30 THz. GaP (110) and z-cut GaSe crystals are used as emitters to generate the THz and mid-infrared pulses, respectively, and a photoconductive antenna switch using a low-temperature grown GaAs epitaxial layer transferred on Si substrate was used as a detector. By changing the emitter between the GaP and GaSe crystals, the measurable frequency range can be easily switched from the 0.5–7.8 THz range to the 7.8–30 THz range without additional optical alignment. We demonstrated the measurement of the dielectric function in a p-type InAs wafer and the optical conductivity of an indium tin oxide (ITO) thin film. The obtained carrier density and the mobility of the ITO thin film show good agreement with that obtained by the Hall measurement.
Stability enhancement of an electrically tunable colloidal photonic crystal using modified electrodes with a large electrochemical potential window104(2014); http://dx.doi.org/10.1063/1.4863735View Description Hide Description
The color tuning behavior and switching stability of an electrically tunable colloidal photonic crystal system were studied with particular focus on the electrochemical aspects. Photonic color tuning of the colloidal arrays composed of monodisperse particles dispersed in water was achieved using external electric field through lattice constant manipulation. However, the number of effective color tuning cycle was limited due to generation of unwanted ions by electrolysis of the water medium during electrical switching. By introducing larger electrochemical potential window electrodes, such as conductive diamond-like carbon or boron-doped diamond, the switching stability was appreciably enhanced through reducing the number of ions generated.
Enhancing the Cu2ZnSnS4 solar cell efficiency by back contact modification: Inserting a thin TiB2 intermediate layer at Cu2ZnSnS4/Mo interface104(2014); http://dx.doi.org/10.1063/1.4863736View Description Hide Description
In this work, TiB2 thin films have been employed as intermediate layer between absorber and back contact in Cu 2ZnSnS4 (CZTS) thin film solar cells for interface optimization. It is found that the TiB2 intermediate layer can significantly inhibit the formation of MoS2 layer at absorber/back contact interface region, greatly reduces the series resistance and thereby increases the device efficiency by short current density (Jsc) and fill factor boost. However, introducing TiB2 degrades the crystal quality of absorber, which is detrimental to device performance especially Voc. The careful control of the thickness of TiB2 intermediate layer is required to ensure both MoS2 with minimal thickness and CZTS absorber with large grain microstructure according to the absorber growth process.
104(2014); http://dx.doi.org/10.1063/1.4863838View Description Hide Description
We examined the potential of stacked multilayer graphene as broadband terahertz (THz) antireflection coating based on the impedance matching effect in experiment and theory. The reflected pulses from the quartz and silicon substrates were observed to change with the layer number and doping concentration of the graphene coating. Remarkable broadband impedance matching was achieved due to optimized THz conductivity. Theoretical analysis based on Drude model and thin film Fresnel coefficients have been used to explain the experimental phenomena, which indicated the shift of Fermi level caused by chemical doping. This work paves the way for graphene-based broadband THz antireflection coating.
104(2014); http://dx.doi.org/10.1063/1.4863925View Description Hide Description
A hexagonally close-packed microlens array has been integrated onto the sapphire face of a flip-chip bonded InGaN light-emitting diode (LED). The micro-optics is formed by etching a self-assembled monolayer of 1-μm silica microspheres coated on the sapphire substrate, producing hemispherical sapphire lenses. Without degrading electrical characteristic, the light output power of the lensed LED is increased by more than a quarter compared with the unlensed LED. Enhanced light extraction via micro-optics is verified by rigorous coupled wave analysis. The focusing behavior of the micro-lenses, as well as the emission characteristics of the lensed LED, is studied by confocal microscopy.
104(2014); http://dx.doi.org/10.1063/1.4863927View Description Hide Description
We investigate theoretically the optical bistability of reflection at the interface between graphene and Kerr-type nonlinear substrates. We derive a simple procedure to calculate the nonlinear reflectivity with graphene, and discuss the influence of the graphene sheets on the hysteretic response of the TM-polarization reflected light. It is found that the bistable behavior of the reflected light can be electrically controlled via suitably varying the applied voltage on the graphene. In THz, the bistable thresholds can be lowered markedly by increasing the Fermi energy. However, in near-infrared frequency, it requires multiple graphene layers to exhibit significant influence on the bistable thresholds.
104(2014); http://dx.doi.org/10.1063/1.4863932View Description Hide Description
The exquisite mechanical properties of SiC have made it an important industrial material with applications in microelectromechanical devices and high power electronics. Recently, the optical properties of SiC have garnered attention for applications in photonics, quantum information, and spintronics. This work demonstrates the fabrication of microdisks formed from a p-N SiC epilayer material. The microdisk cavities fabricated from the SiC epilayer material exhibit quality factors of as high as 9200 and the approach is easily adaptable to the fabrication of SiC-based photonic crystals and other photonic and optomechanical devices.
Practical and fast quantum random number generation based on photon arrival time relative to external reference104(2014); http://dx.doi.org/10.1063/1.4863224View Description Hide Description
We present a practical high-speed quantum random number generator, where the timing of single-photon detection relative to an external time reference is measured as the raw data. The bias of the raw data can be substantially reduced compared with the previous realizations. The raw random bit rate of our generator can reach 109 Mbps. We develop a model for the generator and evaluate the min-entropy of the raw data. Toeplitz matrix hashing is applied for randomness extraction, after which the final random bits are able to pass the standard randomness tests.
104(2014); http://dx.doi.org/10.1063/1.4863853View Description Hide Description
We report on the strong coupling between the Bloch surface wave supported by an inorganic multilayer structure and J-aggregate excitons in an organic semiconductor. The dispersion curves of the resulting polariton modes are investigated by means of angle-resolved attenuated total reflectance, as well as photoluminescence experiments. The measured Rabi splitting is 290 meV. These results are in good agreement with those obtained from our theoretical model.
104(2014); http://dx.doi.org/10.1063/1.4863953View Description Hide Description
We demonstrate control of the charge transfer process in PbS and PbSe quantum dot assemblies. We first demonstrate efficient charge transfer from donor quantum dots to acceptor quantum dots in a multi-layer PbSe cascade structure. Then, we assemble type-I and type-II heterostructures using both PbS and PbSe quantum dots via careful control of the band alignment. In type-I structures, photo-generated carriers are transferred and localized in the smaller bandgap (acceptor) quantum dots, resulting in a significant luminescence enhancement. In contrast, a significant luminescence quenching and shorter emission lifetime confirms an efficient separation of photo-generated carriers in the type-II architecture.
Electronic-state-controlled reset operation in quantum dot resonant-tunneling single-photon detectors104(2014); http://dx.doi.org/10.1063/1.4863954View Description Hide Description
The authors present a systematic study of an introduced reset operation on quantum dot (QD) single photon detectors operating at 77 K. The detectors are based on an AlAs/GaAs/AlAs double-barrier resonant tunneling diode with an adjacent layer of self-assembled InAs QDs. Sensitive single-photon detection in high region with suppressed current fluctuations is achieved. The dynamic detection range is extended up to at least 104 photons/s for sensitive imaging applications by keeping the device far from saturation by employing an appropriate reset frequency.
Enhanced efficiency of the dye-sensitized solar cells by excimer laser irradiated carbon nanotube network counter electrode104(2014); http://dx.doi.org/10.1063/1.4864059View Description Hide Description
The carbon nanotube network decorated with Pt nanoparticles (PtCNT) irradiated by excimer laser as counter electrode (CE) of dye-sensitized solar cells (DSSCs) has been systematically demonstrated. The conversion efficiency would be improved from 7.12% to 9.28% with respect to conventional Pt-film one. It was attributed to the enhanced catalytic surface from Pt nanoparticles and the improved conductivity due to the adjoining phenomenon of PtCNTs irradiated by laser. Moreover, the laser annealing could also promote the interface contact between CE and conductive glass. Therefore, such a simple laser-irradiated PtCNT network is promising for the future flexible DSSCs applications.
A four-pixel single-photon pulse-position array fabricated from WSi superconducting nanowire single-photon detectors104(2014); http://dx.doi.org/10.1063/1.4864075View Description Hide Description
We demonstrate a scalable readout scheme for an infrared single-photon pulse-position camera consisting of WSi superconducting nanowire single-photon detectors. For an N × N array, only 2 × N wires are required to obtain the position of a detection event. As a proof-of-principle, we show results from a 2 × 2 array.
How much better are InGaN/GaN nanodisks than quantum wells—Oscillator strength enhancement and changes in optical properties104(2014); http://dx.doi.org/10.1063/1.4864083View Description Hide Description
We show over 100-fold enhancement of the exciton oscillator strength as the diameter of an InGaN nanodisk in a GaN nanopillar is reduced from a few micrometers to less than 40 nm, corresponding to the quantum dot limit. The enhancement results from significant strain relaxation in nanodisks less than 100 nm in diameter. Meanwhile, the radiative decay rate is only improved by 10 folds due to strong reduction of the local density of photon states in small nanodisks. Further increase in the radiative decay rate can be achieved by engineering the local density of photon states, such as adding a dielectric coating.
104(2014); http://dx.doi.org/10.1063/1.4864095View Description Hide Description
The inherent uncertainty in quantum mechanics offers a source of true randomness which can be used to produce unbreakable cryptographic keys. We discuss the development of a high-speed random number generator based on the quantum phase fluctuations in spontaneously initiated stimulated Raman scattering (SISRS). We utilize the tight confinement and long interaction length available in a Potassium Titanyl Phosphate waveguide to generate highly efficient SISRS using nanojoule pulse energies, reducing the high pump power requirements of the previous approaches. We measure the random phase of the Stokes output using a simple interferometric setup to yield quantum random numbers at 145 Mbps.
Origin of InGaN/GaN light-emitting diode efficiency improvements using tunnel-junction-cascaded active regions104(2014); http://dx.doi.org/10.1063/1.4864311View Description Hide Description
This Letter investigates the efficiency enhancement achieved by tunnel junction insertion into the InGaN/GaN multi-quantum well (MQW) active region of blue light emitting diodes (LEDs). The peak quantum efficiency of such LED exceeds 100%, but the maximum wall-plug efficiency (WPE) hardly changes. However, due to the increased bias, the WPE peaks at much higher input power, i.e., the WPE droop is significantly delayed, and the output power is strongly enhanced. The main physical reason for this improvement lies in the non-uniform vertical carrier distribution typically observed within InGaN MQWs.
104(2014); http://dx.doi.org/10.1063/1.4864267View Description Hide Description
We investigate the specific optical regime occurring at short wavelengths, in the high absorption regime, in silicon thin-films patterned by periodically arranged nano-holes. Near-field scanning optical microscopy indicates that the incoming light is coupled to vertically channelling modes. Optical modelling and simulations show that the light, travelling inside the low-index regions, is absorbed at the direct vicinity of the nano-holes sidewalls. This channelling regime should be taken into account for light management in optoelectronic devices.
104(2014); http://dx.doi.org/10.1063/1.4864282View Description Hide Description
We report on ultraviolet emission from a multi-layer graphene (MLG)/MgZnO/ZnO light-emitting diodes (LED). The p-type MLG and MgZnO in the MLG/MgZnO/ZnO LED are used as transparent hole injection and electron blocking layers, respectively. The current-voltage characteristics of the MLG/MgZnO/ZnO LED show that current transport is dominated by tunneling processes in the MgZnO barrier layer under forward bias conditions. The holes injected from p-type MLG recombine efficiently with the electrons accumulated in ZnO, and the MLG/MgZnO/ZnO LED shows strong ultraviolet emission from the band edge of ZnO and weak red-orange emission from the deep levels of ZnO.