Volume 101, Issue 15, 08 October 2012
- 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:
The properties of carbon nanotube-graphene junctions are investigated with first-principles electronic structure and electron transport calculations. Contact properties are found to be key factors in determining the performance of nanotube based electronic devices. In a typical single-walled carbon nanotube-metal junction, there is a p-type Schottky barrier of up to which depends on the nanotube diameter. Calculations of the Schottky barrier height in carbon nanotube-graphene contacts indicate that low barriers of and 0.04 eV are present in nanotube-graphene contacts ((8,0) and (10,0) nanotubes, respectively). Junctions with a finite contact region are investigated with simulations of the current-voltage characteristics. The results suggest the suitability of the junctions for applications and provide insight to explain recent experimental findings.
- PHOTONICS AND OPTOELECTRONICS
101(2012); http://dx.doi.org/10.1063/1.4757872View Description Hide Description
Random lasing is reported in a dye-circulated structured polymericmicrofluidic channel. The role of disorder, which results from limited accuracy of photolithographic process, is demonstrated by the variation of the emission spectrum with local-pump position and by the extreme sensitivity to a local perturbation of the structure. Thresholds comparable to those of conventional microfluidic lasers are achieved, without the hurdle of state-of-the-art cavity fabrication. Potential applications of optofluidic random lasers for on-chip sensors are discussed. Introduction of random lasers in the field of optofluidics is a promising alternative to on-chip laser integration with light and fluidic functionalities.
Efficient Cherenkov-type terahertz generation in Si-prism-LiNbO3-slab structure pumped by nanojoule-level ultrashort laser pulses101(2012); http://dx.doi.org/10.1063/1.4757882View Description Hide Description
We demonstrate, both theoretically and experimentally, that a sandwich-type structure consisting of a thin slab and Si prism outcoupler can be an efficient convertor of unamplified laser pulses into broadband terahertz radiation.Pumping a 1 cm long sandwich structure with a 35 μm thick slab by 8 nJ, 100 fs optical pulses from Ti:sapphire oscillator we achieved the conversion efficiency of , i.e., two orders of magnitude higher than in the conventional generation scheme with ZnTe crystal. Using laser oscillator as a pump has an advantage of high repetition rate and, therefore, potentially high signal-to-noise ratio (∼50 dB in our experiment).
Gallium nitride-based light-emitting diodes with embedded air voids grown on Ar-implanted AlN/sapphire substrate101(2012); http://dx.doi.org/10.1063/1.4757996View Description Hide Description
GaN-based light-emitting diodes(LEDs)grown on sapphire with ex situAlNnucleation layer prepared by radio-frequency sputtering were investigated. GaN-based epitaxial layers grown on the Ar-implanted AlN/sapphire (AIAS) substrates exhibited selective growth and subsequent lateral growth due to the difference of lattice constants between the implanted and implantation-free regions. Consequently, air voids over the implanted regions were formed around the GaN/AlN/sapphire interfaces. We proposed the growth mechanisms of the GaN layer on the AIAS substrates and characterized the LEDs with embedded air voids. With a 20 mA current injection, experimental results indicate that the light output power of LEDsgrown on the AIAS substrates was enhanced by 25% compared with those of conventional LEDs. This enhancement can be attributed to the light scattering at the GaN/air voidinterfaces to increase the light extraction efficiency of the LEDs.
Analysis of electron and light scattering in a fluorescent thin film by combination of Monte Carlo simulation and finite-difference time-domain method101(2012); http://dx.doi.org/10.1063/1.4758290View Description Hide Description
We analyzed light intensity distributions in a subwavelength fluorescent film, which was excited by a focused electron beam. We have developed an analyzing method using Monte Carlo simulation and the finite-difference time-domain(FDTD) method. Electron scattering and trajectories were calculated by Monte Carlo simulation. Propagation and scattering of light excited with the electrons was calculated by FDTD method. A nanometric light spot was formed on the fluorescent film surface and its light intensity and its full width at half maximum (FWHM) were evaluated. We discuss the intensity and the FWHM dependence on the thickness of the fluorescent thin film and the acceleration voltage of an incident electron beam.
101(2012); http://dx.doi.org/10.1063/1.4758295View Description Hide Description
We model analytically light harvesting in realistic solar cells by extending a formalism suggested by Deckman et al. [Appl. Phys. Lett. 42, 110968 (1983)], based on tracing of an average ray of light. Arbitrary light scattering schemes and parasitic absorption are implemented in the model, and we validate our approach by comparing with experimental measurements from microcrystalline silicondevices. The intuitive understanding obtained with this extended model is discussed. This approach enables identifying parasitic absorption as main limitation of state-of-the-art light harvesting schemes, and highlights that a better light trapping requires improving the first scattering events.
101(2012); http://dx.doi.org/10.1063/1.4754665View Description Hide Description
Deposition of vertical, cone-shaped plasmonicnanorod arrays onto sub-50 nm polythiophene films on Ag substrates is shown to result in significant absorption enhancement (>12 at the polythiophene band edge) and spectral broadening (more than 250 nm increase) relative to polythiophene/Ag films without plasmonicnanorod arrays. Full-field electromagnetic simulations are used to identify the modes of the plasmonicnanorod array/polythiophene/Ag film system. Both gap modes and longitudinal monopole antenna modes give rise to highly localized electric fields in the polythiophene film and are the primary contributors to polythiophene absorption enhancement. This approach is suitable for large area optoelectronic applications where light management in ultrathin active layers is desired.
Localized optical resonances in low refractive index rolled-up microtube cavity for liquid-core optofluidic detection101(2012); http://dx.doi.org/10.1063/1.4758992View Description Hide Description
Spatially localized high order axial resonant modes are observed in a SiO/SiO2-based microtube cavity rolled-up from a prestrained nanomembrane. A diameter variation of the tube structure is revealed which provides localized axial confinement. Optofluidic detection is demonstrated by filling the tube core with salt solution and subsequently detecting the shifts of spectral mode positions and axial mode spacings. A sensing methodology is proposed by monitoring axial mode spacing changes.
101(2012); http://dx.doi.org/10.1063/1.4759043View Description Hide Description
A terahertz quantum cascade laser (THz QCL) architecture is presented in which only the ground state subbands of each quantum well are involved in the transport and lasing transition. Compared to state-of-the art THz QCLs based on the resonant-phonon scheme, ground stateQCLs employ narrower wells so that all high-energy subbands are pushed up far above the occupied subband levels, significantly reducing parasitic interactions. Data on the experimental realization of two types of ground stateQCLs are presented, in which the result of lasing above 5 THz is demonstrated.
101(2012); http://dx.doi.org/10.1063/1.4758467View Description Hide Description
We propose an approach to scale the frequency of surface phononpolariton to megahertz-gigahertz region via an artificial microstructure, ionic-type phononic crystal (ITPC). The period of ITPC can be intentionally controlled on all relevant length scales, which allows the creation of surface phononpolariton with almost arbitrary dispersion in frequency and space. A field of surface phononpolariton optics in microwave band is expected with similar optical properties to those of ionic crystals in infrared.
101(2012); http://dx.doi.org/10.1063/1.4758471View Description Hide Description
Broadband switching functionality realized by an ultra-compact W2 photonic crystal waveguide (PCW) is demonstrated with an integrated titanium/aluminum microheater on its surface. Due to the enhanced coupling between the defect modes in W2 PCW, switching functionality with bandwidth up to 24 nm is achieved by the PCW with footprint of only 8 μm × 17.6 μm, while the extinction ratio is in excess of 15 dB over the entire bandwidth. Moreover, the switching speed is measured by alternating current modulation. Response time for this thermo-optic switch is 11.0 ± 3.0 μs for rise time and 40.3 ± 5.3 μs for fall time, respectively.
101(2012); http://dx.doi.org/10.1063/1.4758687View Description Hide Description
We report on high power ultrashort Bessel-X pulses directly from a compact fiber laser system. For spatial profiles, non-diffracting Bessel-like wavepackets were generated with the combination of a collimating lens and a silica-based inverse micro-axicon, which is specially designed and fabricated on the facet of an amplifying fiber doped by ytterbium. For temporal profiles, the seed pulses from a mode-locked dissipative solitonfiber laser at a repetition rate of 56.9 MHz were pre-shaped by a grating pair and a spectral filter. Amplified 18.7 W average power chirped ps pulses were obtained and then dechirped to a duration of 38 fs.
101(2012); http://dx.doi.org/10.1063/1.4757866View Description Hide Description
The recently reported quantum junction architecture represents a promising approach to building a rectifying photovoltaic device that employs colloidalquantum dot layers on each side of the p-n junction. Here, we report an optimized quantum junctionsolar cell that leverages an improved aluminum zinc oxide electrode for a stable contact to the n-side of the quantum junction and silverdoping of the p-layer that greatly enhances the photocurrent by expanding the depletion region in the n-side of the device. These improvements result in greater stability and a power conversion efficiency of 6.1% under AM1.5 simulated solar illumination.
Blue monolithic AlInN-based vertical cavity surface emitting laser diode on free-standing GaN substrate101(2012); http://dx.doi.org/10.1063/1.4757873View Description Hide Description
We report on III-nitride based blue vertical cavity surface emitting lasers using defect-free highly reflective AlInN/GaN distributed Bragg reflectorsgrown on c-plane free-standing GaN substrates. Lasing is demonstrated at room temperature under pulsed electrical injection. The high lasing threshold current density still prevents devices from continuous wave lasing because of large self-heating. The reasons for such a high threshold are discussed and we show that it mainly comes from large light absorption in the indium tin oxide current spreading layer. Properly tuning both its thickness and its position with respect to the electrical field could remarkably decrease the threshold.
Reduced temperature sensitivity of the polarization properties of hydrogenated InGaAsN V-groove quantum wires101(2012); http://dx.doi.org/10.1063/1.4758685View Description Hide Description
We investigated the effects of hydrogen irradiation on the degree of linear polarization, ρ, of the light emitted by site-controlled, dilute-nitride InGaAsN V-groove quantum wires (QWRs). While in the as-grown sample the polarization of the QWR emission is highly sensitive to the increasing temperature (T), after sample hydrogenation the value of ρ remains nearly unchanged (and ∼25%) for T ≤ 220 K. This observation—potentially important for the development of devices based on the QWR polarization—points to a larger energy separation between hole subbands in the hydrogenated QWRs, due to the strain increase associated with the H-induced passivation of nitrogen.
101(2012); http://dx.doi.org/10.1063/1.4759034View Description Hide Description
The diffusion of electron-hole pairs, which are excited in an intrinsic graphene by the ultrashort focused laser pulse in mid-IR or visible spectral region, is described for the cases of peak-like or spread over the passive region distributions of carriers. The spatio-temporal transient optical response on a high-frequency probe beam appears to be strongly dependent on the regime of diffusion and can be used for verification of the elasic relaxation mechanism. Sign flip of the differential transmission coefficient takes place due to interplay of the carrier-induced contribution and weak dynamic conductivity of undoped graphene.
101(2012); http://dx.doi.org/10.1063/1.4759042View Description Hide Description
We demonstrate free-space focusing of terahertz (THz) radiation by scatteringplasmonic surface-waves into the air. We use a grating of shallow holes which contains non-equidistant defects which act as scattering centers. The scattering occurs with defined phase delays such that the waves emitted in free-space interfere constructively to form a focus above the waveguidesurface. In contrast to conventional lenses, this structure does not require any free-space on its backside and has great potential for integrated THz optics.
Lithium niobate photonic crystal wire cavity: Realization of a compact electro-optically tunable filter101(2012); http://dx.doi.org/10.1063/1.4760224View Description Hide Description
We report an electro-optically tunable filter using a lithium niobate photonic crystalcavity configuration with an efficient optical guiding geometry. The compact device (5.5 μm × 2.8 μm) was made on a hybrid waveguide combining an annealed proton exchange waveguide and a ridge waveguide realized by focused ion beam with vertically deposited electrodes. Due to the slow light and nonlinear effect in lithium niobate photonic crystal, experimental results show an enhanced tunability of ∼0.56 nm/V. This compact tunable photonic crystalcavity demonstration opens the path for the development of micro and nano-scale low-power driving active photonic devices.
101(2012); http://dx.doi.org/10.1063/1.4760225View Description Hide Description
A quantum welltransistor laser with a base cavity length L = 300 μm has been designed, fabricated, and operated at threshold ITH = 25 mA (0 °C). As a consequence of the inherent advantage of the picosecond base recombination lifetime, the transistor laser is able to achieve nearly a quantum shot-noise limited laser relative intensity noise (RIN) with a peak amplitude of −151 dB/Hz at frequency 8.6 GHz. Compared with a diode laser (a charge storage device) at the same output power, the transistor laser (a charge flow device) has a better than 28 dB (number dependent on the laser device design) peak RIN advantage.
101(2012); http://dx.doi.org/10.1063/1.4760230View Description Hide Description
We show that a graphene sheet perforated with micro- or nano-size antidots has prominent absorption resonances in the microwave and terahertz regions. These resonances correspond to surface plasmons of a continuous sheet “perturbed” by a lattice. They are excited in different diffraction orders, in contrast to cavity surface plasmon modes existing in disconnected graphene structures. The resonant absorption by the antidot array can essentially exceed the absorption by a continuous graphene sheet, even for high antidot diameter-to-period aspect ratios. Surface plasmon-enhanced absorption and suppressed transmission are more efficient for higher relaxation times of the charge carriers.
101(2012); http://dx.doi.org/10.1063/1.4760231View Description Hide Description
This letter introduces a semiconductor-under-insulator (SUI) technology in InP for designing strip waveguides that interfaceInPphotonic crystal membrane structures. Strip waveguides in InP-SUI are supported under an atomic layer deposited insulator layer in contrast to strip waveguides in silicon supported oninsulator. We show a substantial improvement in optical transmission when using InP-SUI strip waveguides interfaced with localized photonic crystal membrane structures when compared with extended photonic crystal waveguide membranes. Furthermore, SUI makes available various fiber-coupling techniques used in SOI, such as sub-micron coupling, for planar membrane III-V systems.