Volume 93, Issue 13, 29 September 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
- applied biophysics
- interdisciplinary and general physics
Index of content:
We propose to observe the retardation effect between two quantum dots in a one-dimensional waveguide. The effect of retardation is more pronounced comparing to that in free space. If the photons are to be reflected by a mirror at one of the ends, the interference role played by the reflecting photon is found to be destructive. With the combination of junction, the retardation effect can be read out via current-noise spectrums.
- LASERS, OPTICS, AND OPTOELECTRONICS
93(2008); http://dx.doi.org/10.1063/1.2990646View Description Hide Description
We have fabricated superconducting nanowire single photon detectors made of NbTiN on a silicon substrate. This type of material reduces the dark count rate by a factor of 10 compared to identical NbN detectors, enabling single photon detection with unprecedented signal to noise ratio: we report a noise equivalent power of at 4.2 K. The compatibility of our superconducting device with silicon enables its integration with complex structures.
93(2008); http://dx.doi.org/10.1063/1.2991446View Description Hide Description
We measure the spectral dependence of the degenerate three photon absorption coefficient, , for a Si  wafer using pulses in the range , i.e., photon energy between half and one-third the indirect band gap. For pulses linearly polarized along the  crystal axis increases from a value of near at to a peak value of at before decreasing with shorter wavelength; this is consistent with the dispersion expected from allowed-allowed-allowed transitions. At the value is larger for light polarized along  than along .
Transverse mode competition effects on the dynamics of gain-switched vertical-cavity surface-emitting lasers93(2008); http://dx.doi.org/10.1063/1.2992630View Description Hide Description
An experimental analysis of the nonlinear dynamics of a multi-transverse-mode vertical-cavity surface-emitting laser (VCSEL) when subject to a high-frequency current modulation is performed. Regular periodic dynamics—with periods equal to the modulation period or twice the modulation period—and irregular pulsating dynamics are obtained. Our results show that the irregular pulsating dynamics in multimode VCSELs subject to large-signal current modulation is due to the competition between different transverse modes.
93(2008); http://dx.doi.org/10.1063/1.2993232View Description Hide Description
We suggest to use InAs/GaSb coupled quantum wells for terahertz lasing. In these heterostructures terahertz lasing is based not on intersubband but on interband transitions. Crucial advantages of this design in comparison with intersubband lasers are (i) a large value of the interband dipole matrix element and (ii) easier maintenance of population inversion. These advantages lead to a gain of two orders of magnitude higher than that for intersubband lasing. Even higher gain can be obtained in special design InAs/GaSb -structures where a hybridization gap of 1–3 THz is formed and optical density of states is singular.
Generation of tunable Fourier-transform-limited terahertz pulses in -dimethylamino--methyl stilbazolium tosylate crystals93(2008); http://dx.doi.org/10.1063/1.2977490View Description Hide Description
We report difference-frequency generation (DFG) of widely tunable, pulsed, Fourier-transform-limited terahertz radiation (bandwidth of , peak power up to ) from pulse-amplified near-infrared laser radiation using a crystal of the organic salt -dimethylamino--methyl stilbazolium tosylate (DAST) cut along the axis to fulfill the phase-matching condition. The broad tunability and narrow bandwidth of the terahertz radiationgenerated by DFG in DAST are illustrated by a single scan of the terahertz spectrum of from 0.1 to 11 THz, and a recording of the absorptionspectrum of a pure rotational transition of HF near 5 THz.
Direct probe of the built-in electric field of Mg-doped -plane wurtzite InN surfaces with time-resolved electric-field-induced second harmonic generation93(2008); http://dx.doi.org/10.1063/1.2979238View Description Hide Description
The exceptionally large electron affinity of InN causes the pinning of surfaceFermi level well above the conduction band minimum. This unique electronic property leads to the electron accumulation at InN surfaces and a large built-in electric field in the topmost few nanometers of InN surfaces. In this letter, we demonstrate that this surfaceelectric field can be unambiguously determined and monitored in -plane wurtzite InN surface via time-resolved electric-field-induced second harmonic generation. This finding makes it possible to directly probe and characterize the surface electronic properties of Mg-doped InN with an all-optical technique in ambient environment.
93(2008); http://dx.doi.org/10.1063/1.2990756View Description Hide Description
InSbN alloys are fabricated by two-step nitrogen ion implantation into InSb (111) wafers. X-ray photoelectron spectroscopy indicates that most of the implanted nitrogen ions substitute Sb to form In–N bonds. The percentage of the In–N bonds is found to decrease with the increase in the implanted nitrogen. Such alloys can effectively detect long wavelength infrared radiation and the absorption peak energies can be controlled by monitoring the implanted nitrogen dose. The measured peak wavelengths are consistent with the band gaps of the alloys calculated using a ten-band model.
93(2008); http://dx.doi.org/10.1063/1.2991447View Description Hide Description
An experimental investigation on the influence of the material systems on the optical properties of quantum cascade structures is presented. Three electroluminescent quantum cascade devices have been grown using , , and heterostructures. The devices emit at and are based on a similar bandstructure design. Our results verify that the optical quantum efficiency has the predicted dependence on the electron effective mass. We also demonstrate that the shape of the electroluminescence spectra is independent from the particular material parameters and mainly depends on the tunnel coupling between the injector state and the upper state of the radiative transition.
Amplified dispersive Fourier-transform imaging for ultrafast displacement sensing and barcode reading93(2008); http://dx.doi.org/10.1063/1.2992064View Description Hide Description
Dispersive Fourier transformation is a powerful technique in which the spectrum of an optical pulse is mapped into a time-domain waveform using chromatic dispersion. It replaces a diffraction grating and detector array with a dispersive fiber and single photodetector. This simplifies the system and, more importantly, enables fast real-time measurements. Here we describe a novel ultrafast barcode reader and displacement sensor that employs internally amplified dispersive Fourier transformation. This technique amplifies and simultaneously maps the spectrally encoded barcode into a temporal waveform. It achieves a record acquisition speed of —four orders of magnitude faster than the current state of the art.
93(2008); http://dx.doi.org/10.1063/1.2995863View Description Hide Description
Transverse acoustic resonances at gigahertz frequencies are excited by electrostriction in the few-micrometer-thick waists of low-loss optical fiber tapers of up to 40 cm long. A pump-probe technique is used in which the resonances are excited by a train of optical pulses and probed in a Sagnac interferometer. Strong radially symmetric acoustic resonances are observed and the dependence of their frequencies on taper thickness is investigated. Such easily reconfigurable acousto-optic interactions may have applications in the high-frequency mode locking of fiber lasers.
93(2008); http://dx.doi.org/10.1063/1.2992198View Description Hide Description
Supercollimation is the propagation of light without diffraction using the properties of photonic crystals. We present the first experimental demonstration of supercollimation in a planar photonic crystal composed of nanoscale rods. Supercollimation was observed over distances of up to 1000 lattice periods.
93(2008); http://dx.doi.org/10.1063/1.2993352View Description Hide Description
Here, we report the gigahertz-rate modulation of a tunable terahertz carrier. Terahertz radiation, tunable from 300 GHz to 1.2 THz, is generated by mixing two telecom lasers with an offset frequency in an ultrafast photoconductive device coupled to a broadband antenna. The microwave modulation applied to the telecom lasers is directly transferred to the terahertz carrier. A maximum modulation rate of 20 GHz has been achieved, and the bandwidth is independent of the carrier frequency.
peak-power duration optical pulse generation by gain switching of a single-transverse-mode GaInN blue laser diode93(2008); http://dx.doi.org/10.1063/1.2996025View Description Hide Description
We have generated optical pulses at from a single-transverse-mode GaInN blue laser diode under intensive gain-switching operation with a minimum pulse duration of less than . The maximum optical peak power was as high as with a pulse width of . The peak power obtained is the highest value for optical pulses ever generated from a single transverse-mode GaInN laser diode.
Near-ultraviolet excitable orange-yellow phosphor for potential application in light-emitting diodes93(2008); http://dx.doi.org/10.1063/1.2996278View Description Hide Description
phosphordoped with was prepared by a soli-state reaction. This phosphor emits a broad orange-yellow luminescence with a peak wavelength of and a full width at half maximum of about under near-ultraviolet (NUV) excitation at . Yellow light-emitting diodes(LEDs) for general lighting were fabricated by combining phosphor with an NUV chip. The phosphor-converted LEDs had a color temperature of about and their color rendering index was 74.
Reduction in dark current using resonant tunneling barriers in quantum dots-in-a-well long wavelength infrared photodetector93(2008); http://dx.doi.org/10.1063/1.2996410View Description Hide Description
We report the use of resonant tunneling (RT) assisted barriers to reduce the dark current in quantum dots-in-a-well (DWELL) infrared photodetectors. Designed RT barriers allow energy-selective extraction of photoexcited carriers while blocking a continuum of energies. Over two orders of magnitude reduction in the dark current in the RT-DWELL device over a control sample without RT-DWELL at has been demonstrated. Specific detectivity of at at with a conversion efficiency of 5.3% was obtained in the RT-DWELL device. for the RT-DWELL device is five times higher than that of the control sample.
93(2008); http://dx.doi.org/10.1063/1.2996566View Description Hide Description
Low dislocation density ultraviolet (UV) multiple quantum well(MQW) structure was grown using atomic layer deposition(ALD) technique. The MQWgrown on the sapphire substrate consisted of three GaN QWs and four AlGaN barriers formed by ALDgrownsuperlattices. The as-grown sample showed smooth surface morphology with a root-mean-square roughness value of only , and no surface cracks were found. The dislocation density was estimated to be as low as . X-ray and transmission electron microscope data showed the MQW had sharp interfaces with good periodicity. The sample had an UV photoluminescence emission at with a very narrow linewidth of at . The cathodoluminescence image revealed a fairly uniform luminescence pattern at room temperature. The MQWgrown by ALD technique should be useful for providing high crystalline quality for fabrication of various optical devices.
Suppression of the subband parasitic peak by interlayer in AlGaN deep ultraviolet light-emitting diodes93(2008); http://dx.doi.org/10.1063/1.2996580View Description Hide Description
The origin and suppression of the subband parasitic peak in AlGaN deep ultraviolet light-emitting diodes have been studied. The parasitic peak is clarified to come from a cladding layer and to be related to Mgdopants. By using as an interlayer between the active region and the cladding layer, this peak is suppressed efficiently. The devices with such an interlayer show improved output power by a factor of 4 at injection current density of , except that the series resistance and turn-on voltage are slightly increased.
- PLASMAS AND ELECTRICAL DISCHARGES
Plasma-liquid electrochemistry: Rapid synthesis of colloidal metal nanoparticles by microplasma reduction of aqueous cations93(2008); http://dx.doi.org/10.1063/1.2988283View Description Hide Description
We demonstrate operation of an aqueous electrochemical cell with an atmospheric-pressure microplasma cathode and a solid metal anode for the rapid production of colloidal metal nanoparticles. Microplasmas are miniaturized versions of low-pressure glow discharges that operate nonthermally at high pressures and contain energetic electrons. Aqueous metal cations are directly reduced by electrons in the microplasma without the presence of a solid cathode or chemical reducing agents. Kinetic studies performed by UV-visible absorbance spectroscopy suggest that particle nucleation and growth are coupled to redox reactions initiated in the electrochemical cell through plasma-liquid interactions.
Direct measurements of the energy flux due to chemical reactions at the surface of a silicon sample interacting with a plasma93(2008); http://dx.doi.org/10.1063/1.2995988View Description Hide Description
Energy exchanges due to chemical reactions between a siliconsurface and a plasma were directly measured using a heat flux microsensor (HFM). The energy flux evolution was compared with those obtained when only few reactions occur at the surface to show the part of chemical reactions. At 800 W, the measuredenergy flux due to chemical reactions is estimated at about against for ion bombardment and other contributions. Time evolution of the HFM signal is also studied. The molar enthalpy of the reaction giving molecules was evaluated and is consistent with values given in literature.
Absolute atomic oxygen density profiles in the discharge core of a microscale atmospheric pressure plasma jet93(2008); http://dx.doi.org/10.1063/1.2995983View Description Hide Description
The micro atmospheric pressure plasma jet is an rf driven (13.56 MHz, ) capacitively coupled discharge producing a homogeneous plasma at ambient pressure when fed with a gas flow of helium (1.4 slm) containing small admixtures of oxygen . The design provides excellent optical access to the plasma core. Ground state atomic oxygen densities up to are measured spatially resolved in the discharge core by absolutely calibrated two-photon absorption laser-induced fluorescence spectroscopy. The atomic oxygen density builds up over the first 8 mm of the discharge channel before saturating at a maximum level. The absolute value increases linearly with applied power.