Volume 100, Issue 2, 15 July 2006
- lasers, optics, and optoelectronics
- plasmas and electrical discharges
- structural, mechanical, thermodynamic, and optical properties of condensed matter
- electronic structure and transport
- magnetism and superconductivity
- dielectrics and ferroelectricity
- nanoscale science and design
- device physics
- interdisciplinary and general physics
Index of content:
- LASERS, OPTICS, AND OPTOELECTRONICS
Comparison of quantum well intermixing in GaAs structures using a low temperature grown epitaxial layer or a cap100(2006); http://dx.doi.org/10.1063/1.2214360View Description Hide Description
Studies of quantum well intermixing (QWI) have been performed on Al-free GaAs based structures in which InGaAsquantum wells(QWs) have either GaAs barriers or InGaAsP quaternary barriers such that the barrier-QW compositional change consists solely of a group III change (GaAs barrier) or a group V change (quaternary barrier). These structures permit identification of the sublattice upon which intermixing occurs when the point defects responsible for the QWI are created by annealing in the presence of a (conventional) dielectric cap layer versus an InGaP cap layer grown at low temperature (LT-InGaP). QWI occurs on the group III sublattice via vacancydiffusion in both the LT-InGaP and capped samples with identical group V compositions in the QW and barrier layers. For the samples with identical group III compositions for the QW and barriers, QWI is only observed with the LT-InGaP capping and occurs via group V interstitialdiffusion and P–As exchange in the QW.
Optical properties and electric field enhancement in cholesteric liquid crystal containing different periodicities100(2006); http://dx.doi.org/10.1063/1.2215124View Description Hide Description
We study a defect mode in a one-dimensional photonic band gap of a cholesteric liquid crystal (CLC) consisting of two helicoidal periodicities. The optical properties of this CLC are analyzed using transfer matrix and finite difference time domain(FDTD) methods. In calculated transmission spectra of this CLC, one of the defect modes always appears at the band edge wavelength of the inner CLC having a different helix to that of two sides of CLCs. Furthermore, the electric field analysis of this CLC has also been demonstrated by the FDTD method. At the defect mode wavelength, the electric field enhancement is found to be significant larger than a normal CLC.
The ultrafast excitation processes in femtosecond laser-induced damage in dielectric omnidirectional reflectors100(2006); http://dx.doi.org/10.1063/1.2215217View Description Hide Description
A pump and probe system is developed, where the probe pulse duration is less than while the pump pulse is stretched up to . The time-resolved excitation processes and damage mechanisms in the omnidirectional reflectors and are studied. It is found that as the pump pulse energy is higher than the threshold value, the reflectivity of the probe pulse decreases rapidly during the former half, rather than around the peak of the pump pulse. A coupled dynamic model based on the avalanche ionization (AI) theory is used to study the excitation processes in the sample and its inverse influences on the pump pulse. The results indicate that as pulse duration is longer than , photoionization (PI) and AI both play important roles in the generation of conduction band electrons (CBEs); the CBE density generated via AI is higher than that via PI by a factor of . The theory explains well the experimental results about the ultrafast excitation processes and the threshold fluences.
100(2006); http://dx.doi.org/10.1063/1.2212147View Description Hide Description
An analytical theory is developed for the thermal properties of a large optical cavitysemiconductor laser high above threshold. The performance of symmetric and asymmetric waveguide structures is analyzed and compared. Narrow asymmetric waveguides are predicted to be advantageous from the thermal point of view.
Electromagnetism theory of waveguide array electro-optical scanner: Model and characteristics of optical field distribution in the waveguide array100(2006); http://dx.doi.org/10.1063/1.2219152View Description Hide Description
Waveguide array electro-opticalscanner (WAEOS) technology is presented. Based on the theory of the dielectric slab optical waveguide and the coupled wave equations of infinite waveguide array given by Yariv and Yeh [Optical Waves in Crystals (Wiley, New York, 1983)], the coupled wave equations are modified for adapting the finite waveguide array of the WAEOS. A model of the optical field distribution in the waveguide array is derived, which determines the output field of the WAEOS. Some characteristics of the optical field distribution with uniform incidence and single waveguide incidence are given.
100(2006); http://dx.doi.org/10.1063/1.2214302View Description Hide Description
Ultraviolet (UV)-light-induced optical absorption in the near infrared(NIR) region was observed in diluted single crystals at low temperatures. Illumination by wideband light (, ) is accompanied by the appearance of a broad NIRabsorption band with the position of the maxima varying in the (, ) region for different Nb concentrations. This UV-light-induced absorption is absent in nominally pure , as well as in all Nb diluted specimens at elevated temperatures. The centers responsible for the photochromic NIRabsorption bands are tied to interband optical transitions of pair electronic polarons. The photochromic experimental data, supplemented by luminescence studies in the visible range, evidence the strong localization of the photocharge carriers by pair polarons at low temperatures. It is suggested that namely the strong localization of the photocarriers plays a crucial role in photoinduced gigantic dielectric effects and possible phase transitions, which have been recognized recently in incipient ferroelectrics at low temperatures.
Raman characteristics and nonlinear optical properties of tellurite and phosphotellurite glasses containing heavy metal oxides with ultrabroad Raman bands100(2006); http://dx.doi.org/10.1063/1.2215218View Description Hide Description
A family of glasses suitable for ultrabroadband Raman amplifications was designed and fabricated. In addition to the bandwidth the gain characteristics of these glasses are expected to be much higher than those of the silica based materials. The thermal stabilities of these glasses in view of fiber fabrication were discussed. The various structural units which contribute to the overall Raman band were unraveled. Accordingly the various bands were tailored in such a way to get a wide continuous Raman band. The nonlinear optical properties which are vital for optical switching and other nonlinear processes were studied via third-harmonic generation measurements.
Nonlinear spectroscopy of PbS quantum-dot-doped glasses as saturable absorbers for the mode locking of solid-state lasers100(2006); http://dx.doi.org/10.1063/1.2215353View Description Hide Description
The bleaching relaxation characteristics of PbSquantum dots with diameters in the range of synthesized in silicate and phosphate glasses have been studied under the conditions of femtosecond excitation. It has been observed that increase of pump fluence from leads to the appearance of a second (slow) component in the bleaching relaxation kinetics and to the shortening of both the fast (from several tens of picoseconds to several picoseconds) and slow relaxation times. An absorption saturation fluence of with a nonsaturable absorption coefficient of at has been evaluated for the PbS quantum-dot-doped glasses studied. The nature of the nonsaturable absorption and the mechanism of the bleaching relaxation in PbSquantum dots have been discussed.
100(2006); http://dx.doi.org/10.1063/1.2209089View Description Hide Description
The exciton and biexciton emissions of a series of single quantum dots of InAs in an AlAs matrix have been studied. These emissions consist of linear cross polarized doublets showing large values of both the biexciton binding energy and the fine-structure splitting. At increasing exciton emission energy, corresponding to decreasing dot size, the biexciton binding energy of decreases down to zero, reflecting a possible crossover to an antibinding regime. Simultaneously the fine-structure splitting diminishes from a value of down to zero, at the same energy, suggesting a common origin for the two effects.
100(2006); http://dx.doi.org/10.1063/1.2210589View Description Hide Description
We present a detailed experimental and theoretical investigation of the lasing characteristics of organic photonic crystal lasers. These lasers are based on strongly modulated two-dimensional polymer surface relief structures on which thin films of optically active organic materials have been deposited. We determine the in-plane photonic band structure of the corresponding quasiguided modes within an effective two-dimensional model. In addition, we calculate the total (three-dimensional) losses associated with these modes. This allows us to identify the lasing thresholds for square lattice geometries and to understand the emission pattern.
100(2006); http://dx.doi.org/10.1063/1.2210591View Description Hide Description
Thermal dependent changes of light depolarization degree were observed in the (ITO) layers deposited onto the flat glass substrates which are used usually as electrodes for optoelectronic devices. The observed effect is reversible. It can be attributed to the changes of nanostructure geometry in the bulk of the ITO layer as well as on its surface. Such geometric changes involve dispersion of polarized light. The investigated effect should be taken into consideration when developing optoelectronic devices because it can provoke distortion of the optical information field.
Using a quantum well heterostructure to study the longitudinal and transverse electric field components of a strongly focused laser beam100(2006); http://dx.doi.org/10.1063/1.2214207View Description Hide Description
We report a method to measure the electric energy density of longitudinal and transverse electric field components of strongly focused polarized laser beams. We used a quantum well photodetector and exploited the polarization dependent optical transitions of light holes and heavy holes to probe the electric field distribution in the focal region. A comparison of the measuredphotocurrentspectra for radially and azimuthally polarized beams at the light and heavy hole absorption peaks provides a measure of the amount of the longitudinal electric field component.
100(2006); http://dx.doi.org/10.1063/1.2218030View Description Hide Description
An experimental study was made on lasing features in both scattering gain media and amplified spontaneous emission systems from a rhodamine-6G xanthene dye in ethanol solution. The emission intensity, spectral line-half-width, and lasing threshold are investigated in terms of dye concentration, cell thickness, and pump energy. The analysis shows that both systems behave quite different. It was found that the scattering gain media present best lasing properties which are enhanced to smaller cell thickness where the backscattering effect plays a significant role. In addition, its laserlike characteristics were found slightly dependent on dye concentration within the range in which the dimer species influence is absent. In return, the amplified spontaneous emission systems were observed strongly dependent on the product of cell thickness and dye concentration, namely, optical density. These systems present lasing properties only around an optimized value of optical density. Within this range, their emission intensity, spectral line-half-width, and threshold could overcome those obtained from scattering gain media.
100(2006); http://dx.doi.org/10.1063/1.2218038View Description Hide Description
Excess waveguide bend loss can be minimized through the use of offsets and air trenches. Offsets, used for reducing the junction loss between straight and curved waveguides, and air trenches, which prevent bend radiation loss, were simulated by a three-dimensional, semivectorial beam propagation method. Low loss polymer waveguide bend structures, employing both offsets and trenches, were fabricated. A reduction of the 180° bend insertion loss from with a bending radius (BR) of is experimentally confirmed at . BR ranging from are evaluated with decent match when compared with simulation results. The polarization dependent loss is BR dependent with a maximum value of when the BR is reduced to . The experimental results confirm that the joint use of air trenches and junction offsets is effective in reducing the bend radii of low index contrast polymer waveguides in planar lightwave circuits.
100(2006); http://dx.doi.org/10.1063/1.2214299View Description Hide Description
Extended x-rayabsorption fine structure(EXAFS)spectroscopy was employed to study the Er coordination in polycrystalline thin films, which was found to dictate their photoluminescence(PL) properties. Incorporation of Er with concentrations varying from was achieved by radical-enhanced atomic layer deposition at . In all samples, Er was found to be in the optically active trivalent state, confirmed by their x-rayabsorption near-edge spectroscopy spectra. Modeling of the EXAFS data revealed that the local structure of is similar to that of in . Specifically, is coordinated with six O at 2.24 and . Excellent fits to the EXAFS for samples with concentration less than were achieved when the second coordination shell was modeled as a mixture of and , indicating a complete miscibility of in the matrix under these experimental conditions. This behavior is attributed to the almost perfect ionic size match between and , having identical valence state and coordination characteristics. For thin films with higher Er concentrations, the EXAFS analysis revealed an exsolution with domain. Since there is no indication of Er clustering, it is concluded that the PL quenching observed in samples with the Er doping level exceeding is likely due to Er ion-ion interaction but not Er immiscibility in . Specifically, an increase in the concentration implied an increase in the average number of in the second coordination shell, thus making ion-ion interaction possible. The critical interionic distance between two was determined to be , thus setting an upper limit on the concentration in at , at least three orders of magnitude higher than the solubility limit in the conventional host .
100(2006); http://dx.doi.org/10.1063/1.2216882View Description Hide Description
In ultrashort pulse laser ablation of dielectrics, affected materials are first transformed into absorbing plasma with metallic properties and, then, the subsequent laser-plasma interaction causes material removals. For ultrashort-pulse laser ablation of dielectrics, this study proposes a model using the Fokker-Planck equation for electron density distribution, a plasma model for the optical properties of ionized dielectrics, and quantum treatments for electron heating and relaxation time. The free electron density distribution of the plasma within the pulse duration is then used to determine the ablation crater shape. The predicted threshold fluences and ablation depths for barium aluminum borosilicate and fused silica are in agreement with published experimental data. It is found that the significantly varying optical properties in time and space are the key factors determining the ablation crater shape. The effects of fluence and pulse duration are also studied.
100(2006); http://dx.doi.org/10.1063/1.2215355View Description Hide Description
In an electric discharge oxygen-iodine laser, laser action at on the transition of atomic iodine is obtained by a near resonant energy transfer from which is produced using a low-pressure electric discharge. The discharge production of atomic oxygen, ozone, and other excited species adds higher levels of complexity to the postdischarge kinetics which are not encountered in a classic purely chemical generation system. Mixing effects are also present. In this paper we present postdischarge modeling results obtained using a modified version of the BLAZE-IIgas laser code. A 28 species, 105 reactionchemical kineticreaction set for the postdischarge kinetics is presented. Calculations were performed to ascertain the impact of a two stream mixing mechanism on the numerical model and to study gain as a function of reactant mass flow rates. The calculations were compared with experimental data. Agreement with experimental data was improved with the addition of new kinetics and the mixing mechanism.
Ultraviolet pulse laser induced modifications of native silicon/silica interfaces analyzed by optical second harmonic generation100(2006); http://dx.doi.org/10.1063/1.2216873View Description Hide Description
Native silicon/silica interfaces are investigated by electric field induced second harmonic (EFISH) generation employing near infrared femtosecond laser pulses (, , , repetition rate). Here, the temporal EFISH evolution induced by the femtosecond laserirradiation is recorded with a time resolution of . Comparative EFISH studies are performed with interfaces, preexposed to ultraviolet (UV) laser pulses (, , ) as well as virgin . After UVirradiation the femtosecond laser induced electron injection and trapping in the ultrathin oxide is found to be drastically accelerated in contrast to the essentially unaffected interfacial hole dynamics. This result is explained by an enhanced interfacial electron trap density caused in most cases by the UV laser induced melting and recrystallization of the near-interface silicon. Furthermore, three-dimensional second harmonic imaging reveals a saturation effect of the UV induced sample modification due to the melting of the near-interface silicon, for which a threshold fluence of is extracted for xenon chloride (XeCl) laser irradiation. Below this threshold an accumulative behavior of the UV induced modification is observed for repeated exposure to several UV laser pulses. This is pointing to a dose dependent modification process presumably due to thermally driven interface chemistry and/or microscopic structural and electronic changes of the interface.
100(2006); http://dx.doi.org/10.1063/1.2216878View Description Hide Description
Fractalmultilayers of two distinct dielectric materials are structures exhibiting stop bands and transmission peaks in their electromagnetic response. In this work we analyze how the transmission spectrum of a Cantor fractalmultilayer changes when a defect, consisting of a dielectric layer different from those constituting the multilayer, is inserted midway through its thickness. Unlike a periodic multilayerstructure, no transmission peaks (defect modes) appear in the stop band by inserting a defect in a Cantor multilayer. We show that a defect in the fractalstructure results in a frequency shift of its transmission spectrum and in a change of the width of both stop bands and transmission peaks. The extent of such spectrum modifications depends on either the thickness and the refractive index of the defect layer. By suitably modulating these parameters a tunable narrow band filter can be realized to cover a desired range of frequency. This feature may hold attraction for certain applications such as dense wavelength division multiplexing systems and sensors.
100(2006); http://dx.doi.org/10.1063/1.2213150View Description Hide Description
We analyze the properties of quasi-phase-matched second-harmonic generation (SHG) from a defect waveguide in photonic-crystal (PhC) slabs embedded in a periodically inverted material with a collinear beam configuration. We show that by controlling the material dispersion with the structural dispersion of a defect waveguide in PhC slabs of infinite height, it is possible to realize a much wider frequency range for quasi-phase-matched SHG than without the PhC structure. Also, taking the fabrication of actual devices into consideration, we examine the case for PhC slabs of finite height and show that, although guided modes are index confined in the vertical direction (while they are confined by the PhC structure in the horizontal direction), the effect that the PhC structure has on the broadening of the frequency range remains and that the broadened range can be comparable to that for the PhC slabs of infinite height if appropriate structural parameters are taken.