Volume 78, Issue 25, 18 June 2001
- 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
Index of content:
- LASERS, OPTICS, AND OPTOELECTRONICS
Milliwatt operation of AlGaN-based single-quantum-well light emitting diode in the ultraviolet region78(2001); http://dx.doi.org/10.1063/1.1377854View Description Hide Description
By introducing a single-quantum-well active layer and a high-Al-content carrier blocking layer, the output power of an AlGaN-based ultraviolet light-emitting diode has been improved by one order of magnitude. Optical output of 1 mW was achieved at the emission peak wavelength of 341–343 nm.
Femtosecond switching with semiconductor-optical-amplifier-based Symmetric Mach–Zehnder-type all-optical switch78(2001); http://dx.doi.org/10.1063/1.1379790View Description Hide Description
We investigate the effect of intraband carrier dynamics on a nonlinear phase shift induced in a semiconductor optical amplifier (SOA) in terms of its applicability to the Symmetric Mach–Zehnder (SMZ) all-optical switch. Nonlinear phase shifts in an SOA and a passive semiconductorwaveguide are compared under control-pulse durations ranging from 3.2 to 0.4 ps. The results show that femtosecond switching with higher efficiency is still possible by using the SOA. We experimentally achieve femtosecond (670 fs), femtojoule (140 fJ) switching with the SOA-based SMZ all-optical switch.
78(2001); http://dx.doi.org/10.1063/1.1379987View Description Hide Description
We investigate cavity-quantum electrodynamics (QED)effects in an all-semiconductor nanostructure by tuning a single self-assembledInAsquantum dot(QD) into resonance with a high quality factor microdisk whispering gallery mode(WGM). The stronger temperature dependence of the QD single-exciton (1X) resonance allows us to change the relative energy of the WGM and the 1X transitions by varying the sample temperature. The two coupled resonances exhibit crossing behavior due to the weak coupling cavity-QED regime. We demonstrate exciton lifetime reduction by 6 due to the Purcell effect by tuning the QD into resonance with the WGM. Our experiments also show that single-exciton lifetime is independent of temperature up to 50 K.
Room-temperature operation of photopumped monolithic InP vertical-cavity laser with two air-gap Bragg reflectors78(2001); http://dx.doi.org/10.1063/1.1379983View Description Hide Description
We demonstrate a long wavelength vertical-cavity laser built on a low-loss resonator formed by two InP/air-gap Bragg reflectors. The monolithic, InP-based structure uses a periodic gain active region with six strain-compensated quantum wells. The photopumped vertical-cavity laser requires record low power density of only 370 W/cm2 to reach threshold at 25 °C. The equivalent threshold current density is estimated to be as low as 400 A/cm2. Continuous-wave operation is demonstrated up to 32 °C despite the low heat conductivity of the reflectors. The emission is single mode and a power of up to 110 μW has been coupled into a single-mode fiber.
78(2001); http://dx.doi.org/10.1063/1.1379060View Description Hide Description
The introduction of patterned current fronts that modulate the transition between directly modulated optical bits allows for a strong reduction of the optical chirp in a single-mode semiconductor laser, as shown in the numerical integration of a standard model. Hence, optical transmission of information over much longer distances without signal deterioration and a substantial increase in data transmission speed are possible.
78(2001); http://dx.doi.org/10.1063/1.1378316View Description Hide Description
We demonstrate coherent control of the cavity-polariton modes of a quantum-well semiconductormicrocavity in a two-color scheme. The cavity enhancement of the excitonic nonlinearity gives rise to a large signal; modulating the relative phase of the excitation pulses between zero and π produces a differential reflectivity of up to 20%. The maximum nonlinear signal is obtained for cocircular pump and probe polarization. Excitation-induced dephasing is responsible for the incoherent nonlinear response, and limits the contrast ratio of the optical switching.
High diffraction efficiency polarization gratings recorded by biphotonic holography in an azobenzene liquid crystalline polyester78(2001); http://dx.doi.org/10.1063/1.1379791View Description Hide Description
High diffraction efficiencies have been achieved with polarizationgratings recorded in thin films of an azobenzene side-chain liquid crystalline polyester by means of biphotonic processes. Efficiency values up to 30% have been reached after an induction period of 300 s and subsequent evolution with the sample in darkness. These values are at least two orders of magnitude higher than those previously reported for biphotonic recording. The gratings can be erased with unpolarized blue light and partial recovery of the diffraction efficiency has been observed after the erasure process when the sample is kept in darkness. Red light illumination of the erased film increases the recovered efficiency value and the recovery rate.
White light emission from exciplex using tris-(8-hydroxyquinoline)aluminum as chromaticity-tuning layer78(2001); http://dx.doi.org/10.1063/1.1379788View Description Hide Description
We demonstrate efficient organic white light-emitting devices(LEDs), using N,N′-diphenyl-N,N′-bis(1-naphthyl)–(1,1′-biphenyl)-4,4′-diamine (NPB) as the hole-transporting layer, 1,6-bis(2-hydroxyphenyl)pyridine boron complex [(dppy)BF)] as the emitting layer, tris-(8-hydroxyquinoline)aluminum (Alq) as the electron-transporting and chromaticity-tuning layer. The white light comes from exciplex emission at the solid-state interface between (dppy)BF and NPB in addition to the exciton emission from NPB and (dppy)BF, respectively. The chromaticity of white emission can be tuned by adjusting the thickness of the Alq layer. The white LEDs with an Alq thickness of 15 nm exhibit a maximum luminescence of 2000 cd/m2 and efficiency of 0.58 lm/W, and the Commission Internationale De l’Eclairage coordinates of resulting emission vary from (0.29,0.33) to (0.31,0.35) with increasing forward bias from 10 to 25 V. The region is very close to the equienergy white point (0.33,0.33).
78(2001); http://dx.doi.org/10.1063/1.1378805View Description Hide Description
An “injectorless” quantum-cascade (QC) laser is presented. The requirement of using injector regions to transport electrons from the lower laser level and other low-lying energy levels of one active region to the upper laser level of the next electron-downstream active region was eliminated by using an appropriately designed double-quantum-well “chirped” superlattice active region. The major advantage of the “injectorless” QC laser is the close packing of the active regions and the concomitant large optical confinement factor. Using a cascade of 75 consecutive active regions, designed for emission at a pulsed peak output power of 270 mW is achieved at 7 K and approximately 10 mW at the maximum operating temperature of 195 K.
78(2001); http://dx.doi.org/10.1063/1.1380243View Description Hide Description
We used the Z-scan technique for measuring the nonlinear refractive index of a thin composite film formed by coppernanoparticles embedded in silicaglass. By varying the number of pulses of the laser shot, we evidenced heating effects induced by the laser during measurements. We were able to estimate the nonthermal refractive-index value,
- STRUCTURAL, MECHANICAL, THERMODYNAMIC, AND OPTICAL PROPERTIES OF CONDENSED MATTER
78(2001); http://dx.doi.org/10.1063/1.1378315View Description Hide Description
Crystalline SiGe alloy sheet polymers were topotactically prepared from epitaxially grown calcium germanosilicide precursor films in the whole composition range. These polygermanosilynes are found to be a well-defined mixture of the known siloxene and polygermyne sheet polymers with the OH groups exclusively bonded to silicon. The optical properties determined by photoluminescence and optical reflection measurements identify the mixed SiGe sheet polymers as direct semiconductors with efficient luminescence tunable in the energy range between 2.4 and 1.3 eV.
78(2001); http://dx.doi.org/10.1063/1.1380244View Description Hide Description
Raman measurements in backscattering configuration are employed to characterize the effect of nitrogen on the structural properties of a alloy with The following effects of N incorporation on the vibrational spectra of GaNP are observed. First, frequencies of GaP-like and GaN-like longitudinal optical phonons exhibit strong compositional dependence, due to a combined effect of alloying and biaxial strain. Second, a dramatic quenching of two-phonon Raman scattering and an emergence of zone-edge GaP-like vibrations are observed. These effects are tentatively attributed to a local distortion of the GaNP lattice and/or compositional disorder in the alloy.
Determination of optical properties of nitrogen-doped hydrogenated amorphous carbon films by spectroscopic ellipsometry78(2001); http://dx.doi.org/10.1063/1.1374501View Description Hide Description
Nitrogen-doped hydrogenated amorphouscarbonfilms have been deposited on silicon substrates by radio-frequencyplasma-enhanced chemical vapor deposition using different gas ratios from 0 to 3. The real and imaginary parts, n and k, of the complex index of refraction of these films have been determined for wavelengths between 300 and 830 nm by spectroscopicellipsometry. Excellent agreement has been found between measured and modeled spectra, in which an empirical dielectric function based on classical Lorentz oscillator and Tauc joint density of states, and a linear void distribution along the thickness of the films have been assumed. Decrease in the optical energy gap and increase in the extinction coefficient, k, with increase in nitrogen concentration have been observed. Refractive index,n, increases rapidly with increase in nitrogen concentration up to 6.8 at. % (∼7.0 at. %) and then increases slowly with further increase in nitrogen concentration. For all the samples, n is found to be highest at the film-substrate interface which gradually decreases towards the film surface.
78(2001); http://dx.doi.org/10.1063/1.1379782View Description Hide Description
The effect of defects on the dynamics of photoexcited carriers and coherent acoustic phonon in graphite is investigated by means of reflectivitymeasurements with femtosecond time resolution. Point defects are introduced by irradiating graphite with 5 keV ions. Introduction of the defects enhances the carrier relaxation by opening a decay channel via vacancy states, which competes efficiently with carrier–phonon scattering. The coherent phonon relaxation is also accelerated due to an additional scattering by defects. The linear fluence dependence of the decay rate is understood as scattering of propagating acoustic phonon by single vacancies.
78(2001); http://dx.doi.org/10.1063/1.1380730View Description Hide Description
Enhanced surface reaction efficiency has been demonstrated in the photocatalysis of obliquely depositedthin films with variously shaped columns such as zigzag, cylinder, and helix. The columnar thickness and spacing play an important role in the enhancement of the effective surface area, while the columnar shape is less important. The optimum morphology for a surface reaction has been obtained at the deposition angle where the photocatalytic activity is 2.5 times larger than that at The morphology controlled obliquely depositedthin films are applicable for solar cell, electro- and photochromicdevices besides photocatalyst.
78(2001); http://dx.doi.org/10.1063/1.1380728View Description Hide Description
Recent experiments show that, when a two-phase fluid confined between parallel substrates is subject to an electric field, one phase can self-assemble into a triangular lattice of islands in another phase. We describe a theory of the stability of the island lattice. It is well known that the total interface energy reduces when the island diameter increases. We show that, under certain conditions, the electrostaticfree energy reduces when the island diameter decreases. The islands select the equilibrium diameter to minimize the combined interface energy and electrostatic energy. We describe the conditions for electrostatic field to stabilize the island lattice, and analyze an idealized model. The theory suggests considerable experimental control over stable island size.
78(2001); http://dx.doi.org/10.1063/1.1380731View Description Hide Description
GaNfilms were grown in pairs on two opposite c faces of substrate by low-pressure metal-organic vapor phase epitaxy, and studied by scanning electron microscopy and converged beamelectron diffraction. It is found that GaNfilm on the whose c face is forward to its crystal seed has  polarity, and the other film on the whose c face is backward to its crystal seed has polarity.
78(2001); http://dx.doi.org/10.1063/1.1381041View Description Hide Description
We present an ab initio study of sulfur (S)–hydrogen (H)–vacancy complexes in diamond. An defect may become a much shallower donor than an isolated substitutional S defect when S in the complex is either three or five connected. Upon annealing the S-doped crystal, preferential formation of other deep-level defects would deactivate the shallow S-complex donors.
Effect of growth stoichiometry on the electrical activity of screw dislocations in GaN films grown by molecular-beam epitaxy78(2001); http://dx.doi.org/10.1063/1.1379789View Description Hide Description
The impact of the Ga/N ratio on the structure and electrical activity of threading dislocations in GaNfilmsgrown by molecular-beam epitaxy is reported. Electrical measurements performed on samples grown under Ga-rich conditions show three orders of magnitude higher reverse bias leakage compared with those grown under Ga-lean conditions. Transmission electron microscopy(TEM) studies reveal excess Ga at the surface termination of pure screw dislocations accompanied by a change in the screw dislocation core structure in Ga-rich films. The correlation of transport and TEM results indicates that dislocation electrical activity depends sensitively on dislocation type and growth stoichiometry.
78(2001); http://dx.doi.org/10.1063/1.1378807View Description Hide Description
In this letter, we show that normal-incidence spectroscopicellipsometry can be used for high-accuracy topographymeasurements on surface relief gratings. We present both experimental and theoretical results which show that spectroscopicellipsometry or reflectance-difference spectroscopy at near-normal incidence coupled with vector diffraction theory for data analysis is capable of high-accuracy critical dimension (CD), feature height, and sidewall angle measurements in the extreme submicron regime. Quantitative comparisons of optical and cross-sectional scanning electron microscopy(SEM)topographymeasurements from a number of 350 nm line/space reactive-ion-etched Si gratings demonstrate the strong potential for in situ etching monitoring. This technique can be used for both ex situ and in situ applications and has the potential to replace the use of CD-SEM measurements in some applications.