Volume 94, Issue 6, 09 February 2009
- 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:
The self-assembledGaN hexagonal micropyramid and microdisk were grown on by plasma-assisted molecular-beam epitaxy. It was found that the disk was established with the capture of N atoms by most-outside Ga atoms as the surface was constructing, while the pyramid was obtained due to the missing of most-outside N atoms. The intensity of cathode luminescence excited from the microdisk was one order of amplitude greater than that from -plane GaN.
- LASERS, OPTICS, AND OPTOELECTRONICS
94(2009); http://dx.doi.org/10.1063/1.3080688View Description Hide Description
We demonstrate room temperature midinfrared electroluminescence from intersublevel transitions in self-assembled InAsquantum dots. The dots are grown in GaAs/AlGaAs heterostructures designed to maximize current injection into dot excited states while preferentially removing electrons from the ground states. As such, these devices resemble quantum cascade lasers. However, rigorous modeling of carrier transport through the devices indicates that the current transport mechanism for quantum dot active regions differs from that of quantum-well-based midinfrared lasers. We present the calculated energy states and transport mechanism for an intersublevel quantum dot emitter, as well as experimental electroluminescence data for these structures.
94(2009); http://dx.doi.org/10.1063/1.3080689View Description Hide Description
Room-temperature multimode laser emission is observed in a microcavity consisting of dielectric mirrors and small-molecular-weight organic photonic dots as a cavity layer. The structure shows simultaneous lasing of a wide variety of transverse modes. A comparison of the laser operating characteristics with those of unpatterned structures shows an enhancement in the spontaneous emission coupling factor by more than two orders of magnitude due to the lateral confinement. The spectral features are in quantitative agreement with calculations of quantized photonic states in three-dimensional optical cavities.
94(2009); http://dx.doi.org/10.1063/1.3079398View Description Hide Description
Wide-band gap ZnOsemiconductors are attractive materials for the investigation of microcavity excitonpolaritons due to the large exciton binding energy and oscillator strength. We report the growth and characterization of bulk ZnO-based hybrid microcavity. The phenomenon of strong exciton-photon coupling at room temperature has been observed in the ZnO-based hybrid microcavity structure, which consists of 30 pair epitaxially grown AlN/AlGaN distributed Bragg reflector (DBR) on the bottom side of the thick ZnO cavity and 9 pair DBR as the top mirror. The cavity quality factor is about 221. The experimental results show good agreement with theoretically calculated exciton-polariton dispersion curves based on transfer matrix method. From the theoretical and experimental exciton-polariton dispersion curves with two different cavity-exciton detuning values, the large vacuum Rabi splitting is estimated to be about 58 meV in the ZnO-based hybrid microcavity.
94(2009); http://dx.doi.org/10.1063/1.3079403View Description Hide Description
We demonstrated the mode locking of an external-cavity 404 nm GaInN semiconductor laserdiode with an internal saturable absorber layer for the first time. Stable passive mode locking was confirmed at a repetition rate of 840 MHz. Furthermore, hybrid mode-locking operation incorporating rf current modulation generated optical pulses of 20 ps duration and 0.4 W peak power.
Structural tuning of color chromaticity through nonradiative energy transfer by interspacing CdTe nanocrystal monolayers94(2009); http://dx.doi.org/10.1063/1.3079679View Description Hide Description
We proposed and demonstrated architectural tuning of color chromaticity by controlling photoluminescence decay kinetics through nonradiative Förster resonance energy transfer in the heterostructure of layer-by-layer spaced CdTenanocrystal (NC) solids. We achieved highly sensitive tuning by precisely adjusting the energy transfer efficiency from donor NCs to acceptor NCs via controlling interspacing between them at the nanoscale. By modifying decay lifetimes of donors from 12.05 to 2.96 ns and acceptors from 3.68 to 14.57 ns, we fine-tuned chromaticity coordinates from to (0.632, 0.367). This structural adjustment enabled a postsynthesis color tuning capability, alternative or additive to using the size, shape, and composition of NCs.
94(2009); http://dx.doi.org/10.1063/1.3080216View Description Hide Description
Mid-infrared diode lasers with type-II “W” active regions are analyzed using a fully microscopic many-body theory. The Auger carrier losses are found to dominate over radiative losses even at low temperatures. The experimentally observed strong temperature-dependent increase in Auger losses is shown to be a consequence of thermal gain reduction causing increased threshold carrier densities. Good agreement between theory and experiment is demonstrated for temperature-dependent photoluminescence spectra as well as threshold loss currents.
Focusing and spectral enhancement of a repetition-rated, laser-driven, divergent multi-MeV proton beam using permanent quadrupole magnets94(2009); http://dx.doi.org/10.1063/1.3078291View Description Hide Description
A pair of conventional permanent magnetquadrupoles is used to focus a 2.4 MeV laser-driven proton beam at a 1 Hz repetition rate. The magnetic field strengths are 55 and 60 T/m for the first and second quadrupoles, respectively. The proton beam is focused to a spot with a size of less than at a distance of 650 mm from the source. This result is in good agreement with the Monte Carlo particle trajectory simulation.
94(2009); http://dx.doi.org/10.1063/1.3081030View Description Hide Description
We report on the observation of oscillatory variations in the quality factor of quantum dot-micropillarcavities based on planar Bragg reflectors. The oscillatory behavior in the versus diameter dependence appears in the diameter range between 1.0 and , has a characteristic period of a few hundred nanometers and increases in amplitude with increasing reflectivity of the planar microcavity structures. The experimental results are well reproduced by numerical calculations which support the interpretation that the oscillations are caused by coupling of propagating Bloch modes of different orders at the mirror interfaces.
94(2009); http://dx.doi.org/10.1063/1.3081106View Description Hide Description
We report single-mode lasing in subwavelength GaAs disks under optical pumping. The disks are fabricated by standard photolithography and two steps of wet chemical etching. The simple fabrication method can produce submicron disks with good circularity, smooth boundary, and vertical sidewalls. The smallest lasing disks have a diameter of 627 nm and thickness of 265 nm. The ratio of the disk diameter to the vacuum lasing wavelength is about 0.7. Our numerical simulations confirm that the lasing modes are whispering-gallery modes with the azimuthal number as small as 4 and a modal volume of .
94(2009); http://dx.doi.org/10.1063/1.3081109View Description Hide Description
We report bidirectional direct-current electroluminescence(EL) from double-barrier heterostructures on Si. When the heterostructure-based device is forward biased with negative voltage applied on Si, ultraviolet (UV) emission is more intense than visible emissions. The visible emissions appear only at sufficiently high currents. As the device is reverse biased, the visible emissions dominate the EL at low currents. However, they are gradually overridden by the UV emission with increasing current. The EL mechanism has been discussed in terms of energy band structures of the device under forward and reverse biases.
94(2009); http://dx.doi.org/10.1063/1.3080198View Description Hide Description
We investigate the behavior of liquid crystaldynamic flow in a cell with a bidirectional alignment (BDA) surface. Numerical simulations show that with a BDA surface having a pitch comparable to the cell gap , the liquid crystaldynamic flow direction can be controlled by the driving voltage. Such an effect can be applied to bistable twisted nematic displays without the need for anchoring breaking.
Giant birefringence and tunable differential group delay in Bragg reflector based on tapered three-dimensional hollow waveguide94(2009); http://dx.doi.org/10.1063/1.3075058View Description Hide Description
A tunable Bragg reflector based on a tapered three-dimensional (3D) hollow waveguide (HWG) with variable taper angle has been proposed and demonstrated. A large grating coupling coefficient for a large reflection band and a giant birefringence of 0.01 have been achieved by optimizing the structure of the 3D HWG. The large birefringence causes a delay difference between the orthogonal polarizations and the variable taper angle provides tuning in the delay difference. A 13 ps tuning in differential group delay has been reported with a 3 mm long compact device, which can be used for adjustable compensation of polarization mode dispersion in optical fiber links.
94(2009); http://dx.doi.org/10.1063/1.3080218View Description Hide Description
Identifying neuronal connections is regarded as essential in understanding the structure and functions of neural circuits. However, the issue is difficult to resolve. Due to its ultrahigh peak power, the femtosecond laser was used to stimulate neuron circuits in this study. Our results showed that optical stimulation of one neuron triggered significant calcium responses in the neighboring neurons. According to the responses, neural connections were estimated and the functional topology of the neural circuit was mapped. The optical identification of neural connections proved to be noncontact, nondestructive, and highly reproducible, which would enable characterizing the dynamics of neural circuits.
94(2009); http://dx.doi.org/10.1063/1.3077313View Description Hide Description
Si-based photodetectors for narrow-band ultraviolet light (319 nm) and green light (500 nm) detection are demonstrated using a metal-oxide-semiconductortunneling structure. By using appropriate selection of gate metal, the metal-oxide-semiconductortunneling diode can detect specific range of light. Due to the spectral dependence of absorption and reflection of the Ag and Au as gate electrodes, the narrow-band detection of ultraviolet and green light can be achieved, respectively. The photodetectors with 130 nm thick Ag gate and 70 nm thick Au gate exhibit peak responsivities of 5.1 and 0.3 mA/W at 319 and 500 nm, respectively.
94(2009); http://dx.doi.org/10.1063/1.3078409View Description Hide Description
Titanium filmssputtered on heavily boron-doped silicon substrates were thermally oxidized to form electroluminescent heterostructures. The electroluminescence(EL) features a broad spectrum covering red, green, and blue regions. We believe that in recombinations between electrons at oxygen-vacancy-related energy levels and holes in the valence band result in the EL. Furthermore, the EL mechanism has been explained in terms of the energy band diagram of the heterostructure, which possesses an intermediate ultrathin layer revealed by high resolution transmission microscopy.
94(2009); http://dx.doi.org/10.1063/1.3081059View Description Hide Description
Test structures were developed to experimentally measure the presence of electron overflow in light-emitting diodes(LEDs) under typical bias conditions. These test structures are comprised of a standard LED structure with an extra Mg-doped quantum well inserted on the -type side of the electron blocking layer. Electrons escaping the active region recombine in the extra quantum well and the corresponding photon emission is observed. No electron overflow was observed at low current densities. At intermediate current densities where efficiency droop occurs, overflow was observed and increased with increasing current density. The onset of electron overflow occurred at slightly lower current densities than the onset of efficiency droop. Auger-assisted overflow, a by-product of the Auger process, is considered in addition to traditional overflow mechanisms.
AlN/AlGaN short-period superlattice sacrificial layers in laser lift-off for vertical-type AlGaN-based deep ultraviolet light emitting diodes94(2009); http://dx.doi.org/10.1063/1.3081060View Description Hide Description
Large-area laser lift-off (LLO) wafer separation of layers from AlN/sapphire templates has been demonstrated by using 200-period short-period superlattice (SPSL) sacrificial layers instead of conventional GaN photoabsorbing layers. The SPSL functions as the photoabsorbing and mechanically weakened layer in the LLO process. This SPSL-assisted LLO technique promises future progress of vertical-type deep ultraviolet light emitting diodes and freestanding AlN–AlGaN bulk substrates.
94(2009); http://dx.doi.org/10.1063/1.3081400View Description Hide Description
The photoelectric properties of heterostructures with different substrate thicknesses were systematically investigated, in which the thin films were epitaxially grown on -type Si substrates by a computer-controlled laser molecular-beam epitaxy system. Picosecond photoelectric response was observed, and the photoelectric sensitivity was improved greatly by decreasing the thickness of the Si substrates. The maximum photoelectric sensitivity reached 85.6 V/W, and faster photoelectric response was obtained with thinner Si substrate. The experimental results demonstrate that the photoelectrical effects on heterostructures consisting of perovskite oxide and thin silicon substrate are not only with fast response but also with high sensitivity.
Velocity-matched terahertz generation by optical rectification in an organic nonlinear optical crystal using a Ti:sapphire laser94(2009); http://dx.doi.org/10.1063/1.3080214View Description Hide Description
Broadband terahertz pulses have been generated in 2-cyclooctylamino-5-nitropyridine (COANP) single crystals by optical rectification of 150 fs laser pulses from an amplified Ti:sapphire laser operating at 776 nm. Due to better velocity-matching and a higher figure of merit, COANP allows more efficient terahertz generation between 0.2 and 2.4 THz than the benchmark electro-optic crystal ZnTe at this laser wavelength. The peak amplitude of the terahertz signal obtained in COANP was 2.5 times larger in the time-domain and 2.1 times larger in the frequency-domain compared to ZnTe. We achieved a 4.3 times higher energy conversion efficiency in COANP than in ZnTe.
Analysis of optical gain property in the InGaN/GaN triangular shaped quantum well under the piezoelectric field94(2009); http://dx.doi.org/10.1063/1.3075862View Description Hide Description
InGaN/GaN triangular quantum wells(QWs) are investigated theoretically, and the electron-hole wave-function overlap and optical gain characteristics are analyzed. The strong internal piezoelectric field is taken into account to explain the major difference between the optical properties of triangular QWs and conventional rectangular QWs. Our calculations reveal that triangular QWs, in comparison to rectangular QWs, provide higher electron-hole wave-function overlap and are less affected by the quantum confined Stark effect. Furthermore, triangular QWs exhibit increased optical gain and higher polarization degree, which are beneficial to GaN-based light emitting devices.