Volume 93, Issue 4, 28 July 2008
- lasers, optics, and optoelectronics
- plasmas and electrical discharges
- structural, mechanical, thermodynamic, and optical properties of condensed matter
- electronic transport and semiconductors
- magnetism and superconductivity
- dielectrics and ferroelectricity
- nanoscale science and design
- organic electronics and photonics
- device physics
- interdisciplinary and general physics
Index of content:
Boron carbide nanowires with a fivefold twinned structure have been shown to have regular spaced microtwin lamellas localized in one of the crystalline segments. This defectstructure is interpreted as incoherent structural relaxation to relieve the angular excess. A structural model, in terms of a disclination core surrounded by a small angle grain boundary with an intersecting microtwin lamella pair structure, has been proposed and strain analyses suggest it could be a common phenomena for nanowires with large angular mismatch and small twinning formation energy.
- LASERS, OPTICS, AND OPTOELECTRONICS
93(2008); http://dx.doi.org/10.1063/1.2959854View Description Hide Description
We report on the origin of three additional low-energy spontaneously emitted bands in GaAs-based broad-area laser diodes. Spectrally and spatially resolved scanning optical microscopy and Fourier-transform infrared spectroscopy assign the different contributions to bandtail-related luminescence from the gain region as well as interband and deep-level-related luminescences from the GaAs substrate. The latter processes are photoexcited due to spontaneous emission from the active region followed by a cascaded photon-recycling process within the substrate.
93(2008); http://dx.doi.org/10.1063/1.2963029View Description Hide Description
Blue multi-quantum-well light-emitting diodes(LEDs) with GaInN quantum wells and polarization-matched AlGaInN barriers are grown by metal-organic chemical vapor deposition. The use of quaternary alloys enables an independent control over interface polarizationcharges and bandgap and has been suggested as a method to reduce electron leakage from the active region, a carrier loss mechanism that can reduce efficiency at high injection currents—an effect known as the efficiency droop. The LEDs show reduced forward voltage, reduced efficiency droop, and improved light-output power at large currents compared to conventional LEDs.
93(2008); http://dx.doi.org/10.1063/1.2950087View Description Hide Description
We report the enhancement of direct bandgap emission from germanium ring resonators based on silicon-on-insulator (SOI). As a consequence of their strong confinement, a record quality factor of 620 is obtained that is an order of magnitude higher than that previously characterized for crystalline germanium microcavities. We also describe a pump power dependency of due to bandedge shifts not previously reported for silicon-or germanium-based emitters. A decline in the relative peak to baseline intensities with lower is attributed to the Purcell effect on account of the wavelength-scale dimensions and high index contrast of our samples.
Simultaneous forward and backward terahertz generations in periodically poled stoichiometric crystal using femtosecond pulses93(2008); http://dx.doi.org/10.1063/1.2960999View Description Hide Description
Forward and counterpropagated backward terahertz generations were demonstrated via difference frequency generation using a femtosecond laser. By cooling the crystal to reduce terahertz losses, we achieved the terahertz generations of 1.36 and . Temperature-dependentmeasurements showed gradual intensity increase of the terahertz pulse and red shift of the center frequency as the temperature decrease from , although there were insignificant decreases of the spectral bandwidth.
Enhanced light extraction efficiency from AlGaInP thin-film light-emitting diodes with photonic crystals93(2008); http://dx.doi.org/10.1063/1.2963030View Description Hide Description
We investigate the use of photonic crystals for light extraction from high-brightness thin-film AlGaInPlight-emitting diodes with different etch depths, lattice constants, and two types of lattices (hexagonal and Archimedean). Both simulations and experimental results show that the extraction of high order modes with a low effective index is most efficient. The highest external quantum efficiency without encapsulation is 19% with an Archimedean A7 lattice with reciprocal lattice constant, which is 47% better than an unstructured reference device.
93(2008); http://dx.doi.org/10.1063/1.2964093View Description Hide Description
We demonstrate that the optical response of graphenenanoribbons in the terahertz to far-infrared regime can be significantly enhanced and tuned by an applied magnetic field. The dependence of the threshold frequency on the magnetic field is studied. The ribbons with the strongest terahertz conductance under a magnetic field are those with one-dimensional massless Dirac Fermion energy dispersion. For a given ribbon, there exists an optimal field under which the conductance resonance can occur at the lowest frequency.
93(2008); http://dx.doi.org/10.1063/1.2965124View Description Hide Description
Planar slab waveguides were fabricated by pulsed laser deposition from glass compounds with composition that lies very close to the floppy to rigid stiffness transition. These high quality active structures, which were deposited on cladding layers above silicon substrates, support several transverse-electric (TE) modes, and a loss of for the mode was measured at wavelength. The ability to exploit electron beam writing at these special Ge in Se compositions to create nanoscale surface motifs are promising advances to create unique miniature optical processing devices.
93(2008); http://dx.doi.org/10.1063/1.2965126View Description Hide Description
This letter has demonstrated a light emitting diode(LED) with a pure emission using a metal-oxide-silicon (MOS)tunneling structure based on dislocation network in direct silicon bond wafer. It is found that under negative gate bias, the electrons in the metal gate electrode tunnel through the thin oxide to silicon and then recombine radiatively with holes at the dislocation related states to emit the D1-line with a wavelength of . The calculation of energy band diagram indicates that a potential well for electrons forms at the charged bonding interface under negative bias, therefore, the electrons tunneled from the gate can rapidly be attracted by the electric field and then confined at the interface, which essentially increases the efficiency of D1 luminescence from MOStunnelingLED. These results are of interest for the development of silicon based photonics with light emission.
Impact of resistance on cathodoluminescence and its application for layer sheet-resistance measurements93(2008); http://dx.doi.org/10.1063/1.2966339View Description Hide Description
The dependence of cathodoluminescence(CL) on resistances in semiconductor structures, especially on layer resistances, is described. The effect can be taken advantage of and used for characterization of sheet resistance of thin layers in semiconductor devices, as illustrated in this paper by an assessment of lateral confinements in semiconductor-laser heterostructures. At the same time, the effect, if neglected, can be detrimental for accuracy of spatially or spectrally resolved CL studies.
93(2008); http://dx.doi.org/10.1063/1.2955518View Description Hide Description
The dual beam counterpropagating optical trap has found increased use in studies such as optical stretching, optical binding, Raman spectroscopy, and the trapping of high index particles. In this letter we demonstrate the use of photonic crystal fiber to realize a long range dual beam trap that may support multiple wavelengths simultaneously. We develop a dual wavelength conveyor belt for trapped particles and realize the first ever dual beam white light (supercontinuum) trap. This low coherence light trap permits long range longitudinal optical binding of microparticles in the trap with no deleterious interference effects.
93(2008); http://dx.doi.org/10.1063/1.2960352View Description Hide Description
Power scaling of an infrared (IR) parametric amplifier chain, pumped at by a terawatt Ti:sapphire laser system has been reported. A total output energy exceeding with pulse duration has been achieved in the IR region, which is the highest energy and peak power ever reported for an ultrafast optical parametric amplifier scheme. By applying the developed IR pulses to high-order harmonic generation, we have observed a significant cutoff extension compared to the case of driving wavelength. This source suits as a driver laser for extending high-order harmonic photonenergy into the kiloelectronvolts region.
93(2008); http://dx.doi.org/10.1063/1.2962991View Description Hide Description
Graded silicon germanium epilayer with a thin layer of high Ge concentration near the surface is obtained by laser thermal annealing (LTA). The graded layer is formed during a liquid phase regrowth after LTA. The relaxation in this graded epilayer is insignificant; therefore it can be integrated into the source/drain of the -type metal-oxide-semiconductor field effect transistor to induce high compressive strain to the Si channel. The thickness of the graded epilayer and the concentration of the Ge near the surface can be controlled by the laser fluence, which in turn changes the strain induced to the Si channel of strained devices.
93(2008); http://dx.doi.org/10.1063/1.2965614View Description Hide Description
Spatially resolved cathodoluminescence(CL) spectra of excitons in a ZnO microparticle, which consists of a number of small particles (primary particles), have been investigated at low temperature . The spatially resolved CL spectra can be interpreted in terms of the influence of an internal electric field, which is likely caused by transfer of electrons from donor defects to surface states near the surface, on the radiative recombination of free and bound excitons in the ZnO microparticle.
Probing coherently excited optical phonons by extreme ultraviolet radiation with femtosecond time resolution93(2008); http://dx.doi.org/10.1063/1.2966180View Description Hide Description
We present a pump-probe experimental approach to study time-resolve coherent optical phonons using light pulses in the extreme ultraviolet wavelength range with femtosecond time resolution. Using this technique, a Bi (111)-oriented single crystal was excited by near-infrared pulses and probed by a high-order harmonic generation source, whose intrinsic flux instability was reduced by making use of a normalization procedure. This unconventional approach allowed us to perform measurements in a previously inaccessible range of phonon wavevectors within the Brillouin zone.
93(2008); http://dx.doi.org/10.1063/1.2963981View Description Hide Description
Coherent anti-Stokes Raman scattering spectroscopy(CARS) was used for detection of solid particles of explosives and related compounds. The CARSspectra were recorded in the fingerprint region and were shown to exhibit the strong characteristic features of spontaneous Raman spectra of the respective compounds. This study demonstrates the applicability of narrowband backward-CARS spectroscopy for detection of explosives and specifically of its preference over spontaneous Raman scattering. This method has the potential to be applied to remote sensing of hazardous materials.
Three dimensional micromachining inside a transparent material by single pulse femtosecond laser through a hologram93(2008); http://dx.doi.org/10.1063/1.2965451View Description Hide Description
A basic pattern of a three dimensional (3D) photonic crystal, which consists of 16 capsule shaped elements in eight layers, is formed inside a silicaglass using only a single pulse femtosecond laser through a computer generated hologram. The three parameters indispensable to the completely flexible 3D patterning, namely, “absolute position,” “relative position,” and “shape” of each element, are discussed on the basis of the measured optical axis elongation as functions of pulse duration and fluence.
Noncontact inspection technique for electrical failures in semiconductor devices using a laser terahertz emission microscope93(2008); http://dx.doi.org/10.1063/1.2965810View Description Hide Description
We have proposed and demonstrated a novel technique for the noncontact inspection of electrical failures in semiconductor devices using a laser terahertz emission microscope. It was found that the waveforms of the terahertz pulses, emitted by exciting junctions in semiconductor circuits with focused ultrafast laser pulses, depend on the interconnection structures of the circuits. We successfully distinguished damaged silicon metal-oxide-semiconductor field effect transistor circuits with disconnected wires from normal ones by comparing the images of terahertz emission amplitudes between a normal chip and a defective one.
93(2008); http://dx.doi.org/10.1063/1.2967730View Description Hide Description
Lifetimes and Auger coefficients for type-II W interband cascade lasers are deduced from correlations of the experimental threshold current densities and slope efficiencies with calculated threshold carrier densities and optical gains. The room-temperature Auger coefficients for a number of low-threshold devices emitting at wavelengths from to fall in the narrow range of , which represents a much stronger suppression of Auger decay than was implied by most earlier experiments and theoretical projections. The Auger coefficient is nearly independent of the thicknesses and compositions of the layers in the W active region.
93(2008); http://dx.doi.org/10.1063/1.2966342View Description Hide Description
We report on terahertz generation by Cherenkov-type optical rectification in lithium niobate using an actively controlled femtosecond pumped enhancement cavity. In this way a much higher pump power is available inside the cavity and an increased terahertz output power is obtained. The advantages of terahertz generation in the Cherenkov geometry are verified by comparing it with other types of emitters by means of electro-optical detection as well as by bolometer measurements.
93(2008); http://dx.doi.org/10.1063/1.2968190View Description Hide Description
A powerful experimental method, based on holographic microscopy, is used to retrieve the spatiotemporal distribution of small refractive index perturbations induced by ultrafast laser pulses in transparent media. A numerical iterative wavefront propagation approach is used to accurately reconstruct both the phase and the amplitude of the perturbed probe beam wavefront, while Abel inversion recovers the exact three dimensional distribution of the perturbation. For demonstration, a laser filament induced plasma string in air is fully characterized.