Volume 90, Issue 9, 26 February 2007
- lasers, optics, and optoelectronics
- structural, mechanical, thermodynamic, and optical properties of condensed matter
- electronic transport and semiconductors
- magnetism and superconductivity
- dielectrics and ferroelectricity
- nanoscale science and design
- device physics
- applied biophysics
- interdisciplinary and general physics
Index of content:
The authors present the results of heteroepitaxialgrowth of single-crystalline ZnSnanowire arrays on CdSnanoribbon substrates by the metal-catalyzed vapor-liquid-solidgrowth method. ZnSnanowire arrays were vertically or crosswise aligned to the surface of CdSnanoribbon substrates. Room-temperature lasing from ZnSnanowire arrays was demonstrated. The present synthesis provides a new approach to the rational design of building blocks for nanodevices.
- LASERS, OPTICS, AND OPTOELECTRONICS
90(2007); http://dx.doi.org/10.1063/1.2709944View Description Hide Description
The optimization of a waveguidephotodiode (WGPD) having a thin absorption layer is discussed. With an absorption layer thickness of less than , responsivity of was achieved for lensed-fiber coupling. For flat-ended fiber coupling, responsivity of was achieved. An absorption layer thickness of more than is required for high responsivity and low polarization dependency. A formula for frequency response in a WGPD with a thin absorption layer is also obtained. This formula gives the optimized WGPD structure for high speed operation with high responsivity.
90(2007); http://dx.doi.org/10.1063/1.2709920View Description Hide Description
The authors demonstrate a method of texturing a meshed surface on a poly(dimethyl siloxane) (PDMS) film for improving light extraction. This meshed surface is fabricated through a casting process by using a self-organized porous film as a template. Experimental results show that the light outcoupling efficiency increases on the meshed surface of a freestanding PDMS film with large incident angles. The external quantum efficiency of an organic light-emitting diode with the texturedPDMS film was also demonstrated to have an enhancement of 46%.
90(2007); http://dx.doi.org/10.1063/1.2709994View Description Hide Description
The critical voltage with an arbitrary pretilt angle is given analytically which is consistent with the numerical results [Acosta et al., Liq. Cryst.27, 977 (2000)]. The author’s results show that the critical voltage is dependent on the parameters of liquid crystal and pretilt angle but is independent of the cell gap. By numerical simulation the authors find that the on time of pi cell decreases with the increasing pretilt at the same voltage, and the off time of the cell driven by the undershoot method is much faster than that of the normal driven method for the cell with nonzero critical voltage.
90(2007); http://dx.doi.org/10.1063/1.2709970View Description Hide Description
This letter reports on a quantum cascade laser that exhibits simultaneous dual-wavelength emission from two consecutive optical transitions in each active region. These “cascaded” transitions—a second-excited state to first-excited state and a first-excited state to ground state—yield light at and , respectively, in good agreement with simulations. The two lasing wavelengths have similar thresholds at the leading edge of a current pulse.
90(2007); http://dx.doi.org/10.1063/1.2710002View Description Hide Description
A one-dimensional (1D) photonic metal parallel plate waveguide is presented in the spectral range of that has high throughput and stop bands with up to the experimental limit of of dynamic range. By incorporating a defect into the periodic bottom plate of the waveguide, a transmission resonance is generated in the first stop band with a value of 120 and a dynamic range of over . The 1D geometry allows the utilization of the mode matching technique to analytically calculate the transmission of the photonicwaveguide.
Destruction of amplified spontaneous emission via chemical doping at low-work-function metal/conjugated polymer interfaces90(2007); http://dx.doi.org/10.1063/1.2710188View Description Hide Description
The authors investigate how the use of different metal electrodes affects the ability of poly[2-methoxy-5-(-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) films to undergo amplified spontaneous emission(ASE). High-work-function metals such as Ag or Au have little effect on the ASE threshold, but low-work-function metals such as Ca or Al completely shut off ASE.ASE is restored when a thin spacer layer, such as a few nanometers of polystyrene or oxidized Ca, is introduced between the MEH-PPV film and the Ca or Al electrode. This suggests that low-work-function metals chemically dope the polymer, creating polarons that destroy ASE not only by lowering the gain through emission quenching but primarily by increasing the loss via optical absorption. Thus, the exponential sensitivity of ASE to optical losses provides a spectroscopic probe of conjugated polymer/metal interfaces.
90(2007); http://dx.doi.org/10.1063/1.2695682View Description Hide Description
In this letter the authors experimentally demonstrate the strong coupling regime at room temperature between a surface plasmon and an exciton from an auto-organized quantum well. The sample is formed by a two dimensional layered perovskite-type semiconductor:, spin coated onto a silver film. The dispersion lines resulting from reflectometry experiments performed at room temperature in the Kretschmann geometry present an anticrossing with a Rabi splitting of . The emission of the low energy polariton is presented.
Temperature-dependent current injection and lasing in T-shaped quantum-wire laser diodes with perpendicular - and -doping layers90(2007); http://dx.doi.org/10.1063/1.2709521View Description Hide Description
The authors measured the temperature dependence of the lasing properties of current-injection T-shaped quantum-wire (T-wire) lasers with perpendicular - and -doping layers. The T-wire lasers with high-reflectivity coatings on both cleaved facets achieved continuous-wave single-mode operation between 5 and . The lowest threshold current was at . The temperature dependences of differential quantum efficiency and threshold current were attributed mainly to that of current-injection efficiency.
90(2007); http://dx.doi.org/10.1063/1.2709964View Description Hide Description
A quantum well(QW)transistor laser with the capability of high current density, a collector current ensuring minimal operational change (beyond laser threshold) in base QW carrier population, a favorable long narrow emitter laser geometry, and built-in base-collector Franz-Keldysh absorption (with ), is measured for chirp-related behavior. A relatively low transistor laser linewidth enhancement factor is obtained despite the resolution limitations of the measurement apparatus.
90(2007); http://dx.doi.org/10.1063/1.2710769View Description Hide Description
The authors demonstrate a photodetection configuration where the responsivity in the ultraviolet spectral region is limited to a few nanometers, representing high-quality-factor, narrow-band detection together with polarization sensitivity. Both features are obtained by utilizing a polarization-sensitive photodectector in combination with a polarization filter made from two identical -plane GaN films on (100) substrate. The optical band gap of these films depends on the direction of the in-plane polarization vector of the incident light beam with respect to the axis. Electronic-band-structure calculations show that the naturally present anisotropic in-plane strain in these films is the crucial parameter to achieve both a high responsivity and a high polarization contrast.
90(2007); http://dx.doi.org/10.1063/1.2711183View Description Hide Description
The authors have developed a hybrid system for coherent raster-scan imaging at . It combines a high-power electronic source (a multiplied Gunn emitter) with a femtosecond Ti:sapphire laser in order to achieve a high dynamic range via electro-optic detection. The single-scan dynamic range of at a lock-in time constant of is sufficient to permit detection of scattered terahertz radiation in addition to specularly reflected/transmitted light. Active synchronization of the electronic source and the laser is not needed because of the remarkably low jitter between the two radiationsources.
Terahertz microcavity lasers with subwavelength mode volumes and thresholds in the milliampere range90(2007); http://dx.doi.org/10.1063/1.2710754View Description Hide Description
The authors demonstrate terahertz microcavity lasers with ultralow current thresholds and with reduced mode volumes of , i.e., less than one cubic wavelength. A double metalwaveguide with reduced active core thickness is used to achieve confinement in the vertical direction, without compromising the laser performances. Confinement in the longitudinal direction is obtained using microdisk resonators. The guiding properties of surface plasmons are exploited to guide the mode with the metal contact. This makes the use of a resonator with vertical and smooth sidewalls unnecessary. The emission wavelength is . The devices lase up to in pulsed mode, and they achieve continuous-wave operation up to .
90(2007); http://dx.doi.org/10.1063/1.2710777View Description Hide Description
A low threshold and high efficiency laser based on dye-doped cholesteric liquid crystals (CLCs) is demonstrated using an input excitation with the same handedness of circular polarization as the helical structure of the sample at the shorter wavelength band edge of the reflection band. The responsible mechanism originates from the dramatic increase of the optical density of state (DOS) at the band edges. The calculated DOS of the CLC system confirms the authors’ experimental results.
Tolerance of microfabricated wedge for the generation of geometrical stable optical vortex at focal vicinity90(2007); http://dx.doi.org/10.1063/1.2539704View Description Hide Description
The geometrical stability of the optical vorticesgenerated by an optical wedge at the focal vicinity is investigated. It is found that an optical wedge generates an optical vortex beam in the free-space propagation for a wide range of imparted phase shift, however, it possesses a smaller tolerance zone of induced phase shift within which it generates structurally stable optical vortex at focal vicinity. Furthermore, the wedge was employed for the generation of the femtosecond optical vortex beam free from appreciable spatial chirp.
90(2007); http://dx.doi.org/10.1063/1.2437053View Description Hide Description
The simulations and experiments on light emitters within a planar, metal/insulator/metal cavity demonstrate an omnidirectional resonance. This is a resonance for which the maximum decay rate enhancement in the cavity occurs at a well-defined wavelength independent of emission angle. It is shown numerically that the resonance occurs when the cavity thickness is a quarter of the surface plasmon wavelength. Experiments on a gold/blue-emitting polymer/gold cavity support this. Further simulations show that low loss omnidirectional cavities have an emission intensity that is largely independent of angle, which may find application for solid state light sources that require a broad emission cone.
Light emission and negative differential conductance of -type nanoporous silicon with buried -layer assistance90(2007); http://dx.doi.org/10.1063/1.2709632View Description Hide Description
The light-emission and current-voltage properties of -type nanoporous silicon (-NPS) with a hole assistance of buried layer are explored. The influences of anodic current density on the formation, morphology, and properties of -NPS are measured. Such -NPS films have nanoscaled pores and high-aspect-ratio pillars. Since the anisotype junction is forward biased during the anodization process, many holes can drift straightupward from layer and participate in the electrochemical reaction. At room temperature, high peak-to-valley current ratios of about 117.3 can be obtained in negative difference conductance region as well as strong visible light emissions are clearly observed under ultraviolet excitation.
90(2007); http://dx.doi.org/10.1063/1.2710080View Description Hide Description
The authors demonstrate fast (up to ), low-power ( , pulses) modulation of photonic crystal cavities in GaAs containing InAs quantum dots. Rapid modulation through blueshifting of the cavity resonance is achieved via free-carrier injection by an above-band picosecond laser pulse. Slow tuning by several linewidths due to laser-induced heating is also demonstrated.
90(2007); http://dx.doi.org/10.1063/1.2710775View Description Hide Description
The authors demonstrate a mechanism for focusing at optical frequencies based on the use of nanohole quasiperiodic arrays in metal screens. Using coherent illumination at and scanning apertureoptical microscopy, “hot spots” were observed at a distance of from the array. Even smaller hot spots of about in waist were observed closer to the plane of the array.
90(2007); http://dx.doi.org/10.1063/1.2711186View Description Hide Description
Pillar microcavities are subject to two common fabrication artifacts: Bragg mirror corrugation and oxide deposit cladding. In this letter the authors investigate the impact of these features on the quality factor. A quasiperiodic variation of the quality factor as a function of the pillar diameter is experimentally observed and well described by theory. Moreover, observation of quality factors in excess of 1500, close to the theoretical limit, is reported for -diameter GaAs micropillars bounded by Bragg mirrors.