Volume 103, Issue 19, 04 November 2013
- photonics and optoelectronics
- surfaces and interfaces
- structural, mechanical, optical, and thermodynamic properties of advanced materials
- magnetics and spintronics
- 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:
This work shows that a nano-coating of electrochromic polymer grown onto the ligaments of nanoporous gold causes reversible dimensional and color changes during electrochemical actuation. This combination of electromechanical and optical properties opens additional avenues for the applications of artificial muscles, i.e., a metallic muscle exhibits its progress during work by changing color that can be detected by optical means.
- PHOTONICS AND OPTOELECTRONICS
Efficiency droop and incomplete carrier localization in InGaN/GaN quantum well light-emitting diodes103(2013); http://dx.doi.org/10.1063/1.4828780View Description Hide Description
A direct correlation between efficiency droop and broadening of emission spectrum of InGaN/GaN quantum wells (QWs) with increasing current density is found. A model of incomplete carrier localization in InGaN/GaN QWs is proposed. At low injection, the strong carrier localization and high-energy cutoff of emission spectrum results from fast carrier energy relaxation due to carrier hopping between localized tail states in QWs. At high level injection, the energy relaxation rate decreases due to the partial filling of tail states and high energy slope of the spectrum starts to determine by Boltzmann occupancy of tail states. This results in the incomplete carrier localization and the efficiency droop.
103(2013); http://dx.doi.org/10.1063/1.4828997View Description Hide Description
This study demonstrates a room-temperature ultraviolet GaN/Al nanorod (NR) metal laser with an optimized sidewall. A wet-chemical etching process with potassium hydroxide was used to control the GaN NR sidewall angle and polish the NR surface. The lasing action was observed near a wavelength of 365 nm with a low threshold power density of 5.2 mJ/cm2. The high-quality factor (Q) surface plasmon lasing modes were characterized with experiments and three-dimensional finite-element method simulations. We also studied the optical modes in GaN metal-coated NR with and without an Al layer and verified the metal layer is necessary for high-Q resonant modes.
103(2013); http://dx.doi.org/10.1063/1.4827422View Description Hide Description
A continuous-wave index-antiguided planar waveguide laser with a 220 -μm Nd:YAG active layer is investigated. Robust fundamental mode oscillation in the antiguided direction is demonstrated with negligible thermal index focusing while in the orthogonal direction the laser is dominantly thermally guided. A maximum output power of 2.25 W is obtained with a slope efficiency of 43% and an optical efficiency of 39%. A model is developed for finding the fundamental mode of laser oscillation with arbitrary index antiguiding and gain guiding in orthogonal directions with simultaneous thermal index guiding. Experimental results are compared to theory with good agreement.
103(2013); http://dx.doi.org/10.1063/1.4828351View Description Hide Description
Metallic nanocavities have been actively studied for realizing nanolasers with low threshold. Presence of resonance modes with high cavity Q values is the indication of low internal loss that leads to low threshold lasing. However, cavity Q values observed in metallic nanocavities below lasing threshold remain low at present on the order of 100 to 500. We study the possibility to realize higher resonance Q values with a metallic nanocavity. For probing purpose of cavity modes we propose to employ broad mid-gap-state optical emission of n-type GaAs. With this method we report the observation of a resonance mode with the high Q value of 3800 at room temperature with the metallic nanocavity. The cavity mode is identified as a whispering-gallery mode with finite-element-method simulation.
Damage threshold and focusability of mid-infrared free-electron laser pulses gated by a plasma mirror with nanosecond switching pulses103(2013); http://dx.doi.org/10.1063/1.4828995View Description Hide Description
The presence of a pulse train structure of an oscillator-type free-electron laser (FEL) results in the immediate damage of a solid target upon focusing. We demonstrate that the laser-induced damage threshold can be significantly improved by gating the mid-infrared FEL pulses with a plasma mirror. Although the switching pulses we employ have a nanosecond duration which does not guarantee the clean wavefront of the gated FEL pulses, the high focusability is experimentally confirmed through the observation of spectral broadening by a factor of 2.1 when we tightly focus the gated FEL pulses onto the Ge plate.
103(2013); http://dx.doi.org/10.1063/1.4829026View Description Hide Description
The recombination processes of excitons in hexagonal boron nitride (hBN) have been probed using time-resolved photoluminescence. It was found that the theory for two-dimensional (2D) exciton recombination describes well the exciton dynamics in three-dimensional hBN. The exciton Bohr radius and binding energy deduced from the temperature dependent exciton recombination lifetime is around 8 Å and 740 meV, respectively. The effective masses of electrons and holes in 2D hBN deduced from the generalized relativistic dispersion relation of 2D systems are 0.54mo, which are remarkably consistent with the exciton reduced mass deduced from the experimental data. Our results illustrate that hBN represents an ideal platform to study the 2D optical properties as well as the relativistic properties of particles in a condensed matter system.
103(2013); http://dx.doi.org/10.1063/1.4829036View Description Hide Description
Chirped laser dispersion spectroscopy (CLaDS) utilizing direct modulation of a quantum cascade laser (QCL) is presented. By controlling the laser bias nearly single- and dual-sideband CLaDS operation can be realized in an extremely simplified optical setup with no external optical modulators. Capability of direct single-sideband modulation is a unique feature of QCLs that exhibit a low linewidth enhancement factor. The developed analytical model shows excellent agreement with the experimental, directly modulated CLaDS spectra. This method overcomes major technical limitations of mid-infrared CLaDS systems by allowing significantly higher modulation frequencies and eliminating optical fringes introduced by external modulators.
103(2013); http://dx.doi.org/10.1063/1.4829065View Description Hide Description
We report on high performance Al xGa1−xN-based solar-blind ultraviolet photodetector (PD) array grown on sapphire substrate. First, high quality, crack-free AlN template layer is grown via metalorganic chemical vapor deposition. Then, we systematically optimized the device design and material doping through the growth and processing of multiple devices. After optimization, uniform and solar-blind operation is observed throughout the array; at the peak detection wavelength of 275 nm, 729 μm2 area PD showed unbiased peak external quantum efficiency and responsivity of ∼80% and ∼176 mA/W, respectively, increasing to 89% under 5 V of reverse bias. Taking the reflection loses into consideration, the internal quantum efficiency of these optimized PD can be estimated to be as high as ∼98%. The visible rejection ratio measured to be more than six orders of magnitude. Electrical measurements yielded a low-dark current density: <2 × 10−9 A/cm2, at 10 V of reverse bias.
103(2013); http://dx.doi.org/10.1063/1.4829142View Description Hide Description
We report on room-temperature 1.5 μm electroluminescence from trivalent erbium (Er3+) ions embedded in three different CMOS-compatible silicon-based hosts: SiO2, Si3N4, and SiNx. We show that although the insertion of either nitrogen or excess silicon helps enhance electrical conduction and reduce the onset voltage for electroluminescence, it drastically decreases the external quantum efficiency of Er3+ ions from 2% in SiO2 to 0.001% and 0.0004% in SiNx and Si3N4, respectively. Furthermore, we present strong evidence that hot carrier injection is significantly more efficient than defect-assisted conduction for the electrical excitation of Er3+ ions. These results suggest strategies to optimize the engineering of on-chip electrically excited silicon-based nanophotonic light sources.
103(2013); http://dx.doi.org/10.1063/1.4829363View Description Hide Description
We propose theoretically an optical diode based on exciton-polaritons in semiconductor microcavities. A flow of polaritons in the bistable regime is used to send signals through an asymmetric fixed potential that favours the bridging of particles in one direction. Through dynamic modelling of the coherent polariton field, we demonstrate the characteristics of an ideal diode, namely, that the forward signal is fully transmitted while the transmission in the reverse direction tends to zero, without any additional external control. Moreover, the system proves to be robust to the presence of disorder, intrinsic to microcavities, and can function at gigahertz repetition rates.
Optical tuning of dielectric properties of Ba0.6Sr0.4TiO3-La(Mg0.5Ti0.5)O3 ceramics in the terahertz range103(2013); http://dx.doi.org/10.1063/1.4829479View Description Hide Description
The dielectric properties of 0.4Ba0.6Sr0.4TiO3-0.6La(Mg0.5Ti0.5)O3 ceramics and their tunability under external optical fields are investigated at room temperature by means of terahertz time-domain spectroscopy. Application of the optical field leads to an appreciable tuning of the permittivity, which reaches up to 16%. Meanwhile, the dielectric loss changes about 21%. From the results, we find that the change of refractive index has a linear relationship on scale with the applied light power. These findings are attributed to the internal space charge field in the ceramic caused by the excited free carriers.
103(2013); http://dx.doi.org/10.1063/1.4829487View Description Hide Description
We investigated the role of ambient gas pressure on the expansion and the emission features during ultrafast laser ablation of metal target. Plasma plumes were generated using 800 nm, 40 fs laser pulses on a copper target and the ambient air pressure was varied more than seven orders (1 × 10−5 – 760 Torr) of magnitude. Fast-gated images showed a complex interaction between the plume and ambient leading to changes in the plume geometry with pressure as well as time. The ambient pressure levels are found to affect both the line intensities and broadening along with signal to noise (S/N) and signal to background (S/B) ratios. The optimum pressure condition for analytical applications is found to be ∼100 Torr.
103(2013); http://dx.doi.org/10.1063/1.4828354View Description Hide Description
We report on the optical characterization of site-controlled InP/GaInP quantum dots (QDs). Spatially resolved low temperature cathodoluminescence proves the long-range ordering of the buried emitters, revealing a yield of ∼90% of optically active, positioned QDs and a strong suppression of emitters on interstitial positions. The emission of single QDs shows a pronounced degree of linear polarization along the [0,−1,1] crystal axis with an average degree of polarization of 94%. Photon correlation measurements of the emission from a single QD indicate the single-photon character of the exciton and biexciton emission lines as well as the cascaded nature of the photon pair.
103(2013); http://dx.doi.org/10.1063/1.4829575View Description Hide Description
THz meta-foils have been introduced as an electromagnetic metamaterial, and their properties depend only on the geometric structure and the metal. Upon showing that conventional parallel-string meta-foils exhibit a strong polarization dependence, we propose and manufacture crossed meta-foils and demonstrate experimentally and numerically that their transmission spectra are polarization-independent. In this way, polarization-independent, all-metal, self-supported, free-standing, left-handed metamaterials are achieved, which provide a versatile platform for developing practical applications of metamaterials.
103(2013); http://dx.doi.org/10.1063/1.4829641View Description Hide Description
A planar optical concentrator comprising a periodic multilayered isotropic dielectric material backed by a metallic surface-relief grating was theoretically examined for silicon photovoltaics. The concentrator was optimized using a differential evolution algorithm for solar-spectrum-integrated power-flux density. Further optimization was carried out for tolerance to variations in the incidence angle, spatial dimensions, and dielectric properties. The average electron-hole pair density in a silicon solar cell can be doubled, and the material costs substantially diminished by this concentrator, whose efficacy is due to the excitation of waveguide modes and multiple surface-plasmon-polariton waves in a broad spectral regime.
103(2013); http://dx.doi.org/10.1063/1.4829655View Description Hide Description
Dual Fano resonances are realized in a nonlinear photonic metamaterial consisting of periodic arrays of asymmetrical meta-molecules etched in a gold film coated with azobenzene polymer layer made of poly[(methyl methacrylate)-co-(disperse red 13 acrylate)]. Enormously enhanced photoisomerization associated with resonant excitation brings about a large refractive index variation in the azobenzene polymer. Under excitation of a weak pump light as low as 0.61 kW/cm2, a large shift of 50 nm in the Fano resonance wavelength is obtained. Compared with previous reports, the threshold pump intensity is reduced by seven orders of magnitude while a large tunability is maintained simultaneously.
Identification of several propagation regimes for terahertz surface waves guided by planar Goubau lines103(2013); http://dx.doi.org/10.1063/1.4829744View Description Hide Description
We report on the experimental and numerical characterization of planar Goubau lines in the terahertz frequency range. We demonstrate the existence of three propagation regimes that depend on the ratio between the thickness of the dielectric slab and the operation wavelength. Although two of these regimes are dispersive, it is possible to mitigate this undesirable property by a judicious choice of the materials used to fabricate the structures. As an example, we show that Au planar Goubau lines supported by thin layers of Kapton have very interesting characteristics including a modest dispersion coefficient (0.28 ps/mm) and low losses (<0.4 mm−1) up to 0.8 THz.
103(2013); http://dx.doi.org/10.1063/1.4829489View Description Hide Description
The development of a few-cycle optical probe-pulse for the investigation of laser-plasma interactions driven by a Ti:sapphire, 30 Terawatt (TW) laser system is described. The probe is seeded by a fraction of the driving laser's energy and is spectrally broadened via self-phase modulation in a hollow core fiber filled with a rare gas, then temporally compressed to a few optical cycles via chirped mirrors. Shadowgrams of the laser-driven plasma wave created in relativistic electron acceleration experiments are presented with few-fs temporal resolution, which is shown to be independent of post-interaction spectral filtering of the probe-beam.
Dynamic plasmonic tweezers enabled single-particle-film-system gap-mode Surface-enhanced Raman scattering103(2013); http://dx.doi.org/10.1063/1.4829617View Description Hide Description
Based on numerical simulation and experiment, we demonstrate a dynamic single-particle-film Surface-enhanced Raman scattering (SERS) system enabled by manipulation of a single gold nanoparticle by plasmonic nano-tweezers (PNT). A corresponding dynamic plasmonic gap-mode is induced by the hybridization of the surface plasmon polaritons (SPPs) on the film and the localized surface plasmon of the particle. This gap-mode produces an additional enhancement of ∼104 compared to the bare SPPs without the particle, reaching a final SERS enhancement factor of ∼109. Enabled by nano-manipulation with PNT, this dynamic single-particle-film-system provides a promising route to controllable SERS detection in aqueous environments.
- SURFACES AND INTERFACES
103(2013); http://dx.doi.org/10.1063/1.4828794View Description Hide Description
Growing a crystalline oxide film on III-V semiconductor renders possible approaches to improve operation of electronics and optoelectronics heterostructures such as oxide/semiconductor junctions for transistors and window layers for solar cells. We demonstrate the growth of crystalline barium oxide (BaO) on GaAs(100) at low temperatures, even down to room temperature. Photoluminescence (PL) measurements reveal that the amount of interface defects is reduced for BaO/GaAs, compared to Al2O3/GaAs, suggesting that BaO is a useful buffer layer to passivate the surface of the III-V device material. PL and photoemission data show that the produced junction tolerates the post heating around 600 °C.