Volume 101, Issue 3, 01 February 2007
- 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
- applied biophysics
- interdisciplinary and general physics
Index of content:
- LASERS, OPTICS, AND OPTOELECTRONICS
101(2007); http://dx.doi.org/10.1063/1.2432308View Description Hide Description
We experimentally and theoretically investigate the wavelength switching dynamics of an injection-seeded Er doped fiber ring laser. The fiber ring laser is seeded by an external cavity laser diode that is amplitude modulated with a long period 50% duty cycle wave form so that the return to steady state can be investigated. It is found that the laser suffers from severe large-amplitude relaxation oscillations and slow decay times that limit the available wavelength switching speed.
Efficient blue upconversion emission in via energy transfer from doped in lithium modified tellurite glass101(2007); http://dx.doi.org/10.1063/1.2432305View Description Hide Description
Intense blue and red upconversion emissions were obtained in doped tellurite glass when codoped with ions on pumping with radiation. The highest intensity of the blue emission was found for glasses with and . From a study of the dependence of the upconversion intensity on excitation intensity, it is concluded that emissions are due to ions and involve cooperative energy transfer as well as energy transfer from to ions. Lifetime of the level, which is responsible for the intense blue emission, has been measured for various concentrations of .
Efficient blue lasers based on gain structure optimizing of vertical-external-cavity surface-emitting laser with second harmonic generation101(2007); http://dx.doi.org/10.1063/1.2432366View Description Hide Description
We report on the demonstration of highly efficient blue lasers based on intracavity frequency doubling vertical-external-cavity surface-emitting lasers (VECSELs). By optimizing the number of InGaAsquantum wells and employing carrier blocking layers in resonant periodic gain structures, we observed the pump-power-limited output power of at for an quantum well VECSEL. With a frequency doubling crystal inside the cavity, continuous-wave blue output was demonstrated. Power conversion efficiencies ( power∕pump input power) of 22.5% and 9.5% are realized for and , respectively.
Improvements of external quantum efficiency of InGaN-based blue light-emitting diodes at high current density using GaN substrates101(2007); http://dx.doi.org/10.1063/1.2432307View Description Hide Description
InGaN-based blue light-emitting diodes(LEDs) with different quantum well(QW) thicknesses were grown on freestanding GaN substrates with low threading dislocation densities (TDDs) and on -plane sapphire substrates. In the case of thin QWs of in thickness, the external quantum efficiencies (EQEs) of LEDs on GaN substrates, as well as those on sapphire substrates, decreased with increasing forward current, indicating that carrier localization is in play on both types of substrates. For thicker -thick QWs, the EQEs of LEDsgrown on GaN substrates improved at high current densities, while those on sapphire substrates decreased even at low current densities. The LED with -thick QWs on the GaN substrate mounted -side down and molded with epoxy showed EQE as high as 26% at . Cathodoluminescence observations of the active layers on GaN substrates revealed that the expansion of nonradiative areas related to TDDs, which are responsible for the deterioration of the EQE of the LED on the sapphire substrate, had been suppressed.
Picosecond blue-light-induced infrared absorption in single-domain and periodically poled ferroelectrics101(2007); http://dx.doi.org/10.1063/1.2434007View Description Hide Description
Picosecond blue-light-induced infrared absorption is investigated in bulk and periodically poled ferroelectrics known to have high photorefractive resistance (, MgO-doped congruent , MgO-doped stoichiometric, MgO-doped stoichiometric, and ). and MgO-doped congruent and stoichiometric show the lowest susceptibility to induced absorption. Periodic poling slightly increases the susceptibility to the induced absorption in all materials but most noticeably in , MgO-doped stoichiometric, and . Different dynamics of induced absorption are investigated. Relatively high thresholds for induced absorption were observed in MgO-doped stoichiometric and . By increasing the peak power intensity of blue light, the induced absorption for , , and are saturated, while in MgO-doped stoichiometric, the induced absorption increases almost linearly with the blue peak intensity. The low susceptibility to induced absorption observed in corroborates well with the earlier proposed existence of very shallow traps close to the valence band.
Analysis of plume following ultraviolet laser ablation of doped polymers: Dependence on polymer molecular weight101(2007); http://dx.doi.org/10.1063/1.2434013View Description Hide Description
This work investigates the effect of polymer molecular weight on the plume characteristics of poly(methyl methacrylate) (PMMA) and polystyrene (PS)filmsdoped with iodonaphthalene (NapI) and iodophenanthrene (PhenI) following irradiation in vacuum at . Laser-induced fluorescence probing of the plume reveals the presence of ArH products (NapH and PhenH from, respectively, NapI- and PhenI-doped films). While a bimodal translational distribution of these products is observed in all cases, on average, a slower translational distribution is observed in the low system. The extent of the observed dependence is reduced as the optical absorption coefficient of the film increases, i.e., in the sequence NapI∕PMMA, PhenI∕PMMA, and PS-doped films. Further confirmation of the bimodal translational distributions is provided by monitoring in situ the temporally resolved attenuation by the plume as it expands in vacuum of a continuous wave helium–neon laser propagating parallel to the substrate. Results are discussed in the framework of the bulk photothermal model, according to which ejection requires that a critical number of bonds are broken.
101(2007); http://dx.doi.org/10.1063/1.2432484View Description Hide Description
To widen the alignment tolerance of a InGaAs photodiode, which typically only has an optical coupling aperture of in diameter, we have developed a self-positioning ball-lens-on-chip scheme for enlarging the effective coupling aperture. According to the measured results, the 4.5- and 5.3-fold improvements along the transverse and optical axes, respectively, in alignment tolerance have been demonstrated without sacrificing the diode efficiency as a commercially available ruby micro-ball-lens (MBL) with a lens diameter integrated on the high-speed photodiode. To further explore the aperture enlargement capability and optimize the structural design, we have constructed a ray trace model based on the Monte Carlo method for the optical coupling system as well. Using this well-constructed simulation model, we further predict that, by integrating a diametric ruby MBL on the photodiode with a high lens socket, the alignment tolerance can have 7.1- and 10-fold improvements along the transverse and optical axes, respectively. Such a large alignment tolerance permits the usage of conventional passive scheme for photodiode package.
101(2007); http://dx.doi.org/10.1063/1.2433131View Description Hide Description
By exciting at , we have characterized the near infrared emission of trivalent thulium ions in , single crystals as a function of the dopant concentration and temperature, from to room temperature. An overall Stark splitting of for the ions in the , was obtained. We also studied the blue emission at and the near infrared emissions at and as a function of the dopant concentration. Experimental decay times of the , , and and excited states have been measured as a function of and ion concentrations. For the transition of ions, we used the reciprocity method to calculate the maximum emission cross section of at for the polarization parallel to the principal optical direction.
Analytic determination of , , and of an absorbing film from polarimetric data in the thin-film limit101(2007); http://dx.doi.org/10.1063/1.2434004View Description Hide Description
We obtain a solution of the three-phase model in the limit , where the complex refractive index and thickness of an isotropic film on an isotropic substrate are given analytically at any single wavelength from polarimetric data and , where is the complex reflectance ratio and is either the - or -polarized reflectance. We describe several procedures for extending the range of validity of the solution.Analysis of correlations shows that the uncertainty of is significantly more important than the of , which allows us to obtain an expression for the uncertainties , , and of , , and , and to identify conditions that optimize the determination of the layer parameters. We find that the relative uncertainties and are not equal, as would be expected if they were determined by the optical thickness nd measured by ellipsometry, but that breaks the connection. We verify our results by measurements of reversibly physisorbed on oxidized GaAs, finding, for example, that for our conditions is determined more accurately than , and more accurately than . These data and model calculations show that fluctuations in parameters, particularly , are asymmetric, leading in principle to inaccurate average values. However, we show that the importance of the data together with the remaining high correlation between and allows us to define a characteristic curve that can be used to correct the results for this nonlinearity. Finally, we extend our analysis to determine the orthogonal linear combinations of , , and that the data actually determine, which explains why the data fit the characteristic curve so well. Our results will be useful in various contexts for the analysis of films less than thick, for example, in applications involving preparation of next-generation electronic and optoelectronic devices with complicated multilayer structures, real-time control of deposition, and the identification of physisorbed and chemisorbed layers on the monolayer scale.
Self-assembled spontaneous structures induced by a pulsed laser on a surface of azobenzene polymer film101(2007); http://dx.doi.org/10.1063/1.2423083View Description Hide Description
The paper presents the self-assembled regularly patterned unique structures induced by the irradiation of pulsed laser beam on a surface of azobenzene polymer film. Self-assembled patterns depend on the polarization of laser beam. Linearly polarized beam gave rise to the ripple structures whose grating vector is perpendicular to the polarization of laser beam. Pitch of the ripple structures was related to the wavelength of irradiatedlaser beam;. for and and for were obtained. These results implied that the ripple structures were produced by the interference of the incident beam with the optical wave scattered in a surface (waveguide mode light). The difference of value is explained by the wavelength dispersion of refractive indices in the azobenzene polymeric matrix. Circularly polarized beam gave rise to the concentric wavelike structure. Pitch of wavelike structure was analyzed using two dimensional Fourier transform imaging techniques. Pitch of wavelike structure increases with increasing the number of laser shots. Furthermore, with increasing irradiation time, well-defined honeycomblike structure was also developed.
Resonant control of the characteristic temperatures and of AlInGaAs semiconductor lasers with delta-doped tunneling quantum wells101(2007); http://dx.doi.org/10.1063/1.2433769View Description Hide Description
It is demonstrated experimentally that lasers with Te -type -doped resonant tunnelingquantum wells(QWs), tuned to operate at resonance at elevated temperature, exhibit higher external efficiency and improved temperature stability. These lasers were designed to ensure that the decrease in the band bending (which results from the increased current density at threshold due to the elevated temperature) would cause the ground-state levels in the two QWs to coincide, thus obtaining enhanced coupling between the QWs. The enhanced coupling increases the efficiency and reduces the transparency current which curbs the increase of the threshold current due to the elevated temperature.
101(2007); http://dx.doi.org/10.1063/1.2433748View Description Hide Description
Er–Tm codoped thin films prepared by alternate pulsed laser deposition show a broad photoluminescence band with two characteristic peaks at 1540 and at , respectively, related to and emissions. Two series of films have been prepared. For the first series, the Tm concentration [Tm] has been increased while keeping constant the Er concentration [Er]. The results show that the photoluminescence intensity at to that at decreases and that at increases, i.e., the ratio increases as [Tm] is increased. For , a fairly flat emission spectrum with a full width at half maximum of is achieved. For the second series both [Tm] and [Er] are increased while keeping the ratio constant. The ratio tends to be constant with a full width at half maximum of . The lifetime values decrease in all cases as [Tm] increases. The evolution of the and lifetimes as a function of [Tm] as well as the analysis of the latter considering Er as a donor and Tm as an acceptor evidence that there is an efficient energy transfer from to . These results suggest that enhanced performance with a flat broadband emission useful for planar integrated devices is achievable by further dopant engineering in the nanoscale.
101(2007); http://dx.doi.org/10.1063/1.2434010View Description Hide Description
A strong intensity modulation is found in spatially and angular resolved photoluminescence spectra of heterostructures and quantum wells epitaxially grown on Si(111) substrates. This Fabry-Perot effect results from the high refractive index contrasts at the and the Air/InGaN interfaces. It can be used for a wavelength stabilization of the sample upon temperature change and, e.g., in the case of light emitting diodes, to additionally reduce the blueshift at increasing injection currents. A simple geometric approach has been chosen to calculate the influence of layer thickness, absorption and refractive indices, as well as detection angle. The cavity can be described quantitatively by a simple three layer Fabry-Perot model. An analytical expression is derived for the external luminescence line shape. Microphotoluminescence measurements at samples with the silicon substrate locally removed corroborate the model.
Theoretical study of quantum well infrared photodetectors with asymmetric well and barrier structures for broadband photodetection101(2007); http://dx.doi.org/10.1063/1.2434938View Description Hide Description
The -type quantum well infrared photodetector(QWIP) with asymmetric graded barriers for broadband detection has been investigated theoretically based on the eight-band model. It is found that the intersubband transitions from the ground state to all bound and continuum excited states contribute to the overall absorption and the bound-to-continuum (B-to-C) transitions dominate. The superposition of the bound-to-bound and B-to-C transitions results in a broad detection bandwidth, and both the detected wavelength and bandwidth can be tuned by the applied voltage. The analysis method is also applicable to the QWIP with step quantum wells. The calculated results are consistent with the reported experimental observations.
Third-order nonlinear optical properties of bismuth-borate glasses measured by conventional and thermally managed eclipse scan101(2007); http://dx.doi.org/10.1063/1.2434940View Description Hide Description
Third-order nonlinearity one order of magnitude larger than silica is measured in bismuth-borate glasses presenting a fast response . The results for the sign and magnitude of the nonlinearity were obtained using a combination of the eclipse scan with thermal nonlinearity managed scan, whereas the Kerr shutter technique was employed to obtain the electronic time response of the nonlinearity, all performed with repetition rate pulses at . Conventional scans in the picosecond regime at 532 and were also independently performed, yielding the values of the third-order nonlinear susceptibilities at those wavelengths. The results obtained for the femtosecond response, enhanced third-order nonlinearity of this glass (with respect to silica), place this glass system as an important tool in the development of photonics devices. Electro-optical modulators, optical switches, and frequency converters are some of the applications using second-order nonlinear properties of the Bi-glass based on the rectification model.
101(2007); http://dx.doi.org/10.1063/1.2434941View Description Hide Description
Phthalocyanines are promising molecules which present very attractive properties for nonlinear optical applications of both second and third orders. Different solutions of zinc phthalocyanine in toluene at increasing concentration were investigated by spectroscopic analysis in order to get the best conditions for the realization of films in poly(methyl methacrylate). Some films of zinc phthalocyanine in polymeric matrix were realized and their nonlinear optical behavior was characterized. The ratio by weight of the compound to the host polymer was varied between 0.004% and 0.15%. In this range all samples show a low degree of aggregation. The nonlinear optical behavior of the films was evaluated by measuring nonlinear transmission. The measurements of nonlinear transmission were carried out at using a nanosecond Nd:YAG laser followed by an optical parametric oscillator, and a nonlinear response was observed for the two highest concentrations (0.083% and 0.15%). Finally, the nonlinear absorption coefficient was retrieved by fitting experimental data and found to be 29 and , respectively.
101(2007); http://dx.doi.org/10.1063/1.2433768View Description Hide Description
The broadband absorption of metal-oxide-semiconductorquantum dot infrared photodetectors is demonstrated using borondoping in the Si spacer. The peak at results from the intersubband transition in the SiGe quantum dot layers. The other peak at mainly comes from the intraband transition in the boron-doping wells in the Si spacers. Since the atmospheric transmission windows are located at 3–5.3 and , broadband detection is feasible using this device. The doping in SiGe quantum dots and quantum wells is also investigated to identify the origin of the absorption.
101(2007); http://dx.doi.org/10.1063/1.2433715View Description Hide Description
A series of samples with varying GaAs spacer width are studied by electric-field modulated absorption (EA) and reflectance spectroscopy and modeled using a microscopic theory. The analysis of the Franz–Keldysh oscillations of GaAs capping layer and of the quantum-confined Stark shift of the lowest quantum well(QW) transitions shows the strong inhomogeneity of the built-in electric field indicating that the field modulation due to an external bias voltage differs significantly for the various regions of the structures. The calculations demonstrate that the line shape of the EA spectra of these samples is extremely sensitive to the value of the small conduction band offset between GaAs and as well as to the magnitude of the internal electric field changes caused by the external voltage modulation in the QW region. The EA spectra of the entire series of samples are modeled by the microscopic theory. The good agreement between experiment and theory allows us to extract the strength of the modulation of the built-in electric field in the QW region and to show that the band alignment between GaAs and is of type II with a conduction band offset of approximately 40 meV.
101(2007); http://dx.doi.org/10.1063/1.2434803View Description Hide Description
A theoretical study of the dynamics of semiconductor lasers subjected to pseudorandom digital modulation at gigabit rates is presented. The eye diagram, turn-on jitter (TOJ), and power fluctuations in the modulated laser wave form are analyzed. The study is based on numerical large-signal analysis of the laser rate equations. Influences of the biasing and modulation currents on the eye diagram and TOJ are examined. The degree of eye opening is measured in terms of a factor of the laser signal analogous to the factor determining the bit-error rate in transmission systems. Influence of optimizing both the sampling and decision times on the signal factor is modeled. We show that the most eye opening corresponds to shortening the sampling time associated with lengthening the decision time. We also assess the relative contributions of the laser intrinsic noise and pseudorandom bit pattern to the TOJ. The results show that the bit pattern is the major contributor to the TOJ when the setting time of the relaxation oscillation is longer than the bit slot.
Linearly polarized lasing in one-dimensional hybrid photonic crystal containing cholesteric liquid crystal101(2007); http://dx.doi.org/10.1063/1.2434835View Description Hide Description
We have achieved single-mode laser action in a one-dimensional hybrid photonic crystal(HPC) containing cholesteric liquid crystal (CLC) as a defect and experimentally investigated the polarization characteristics of the laser light. The laser light emitted from the HPC containing the CLC was linearly polarized, which was different from that of the simple CLC, despite the CLC being contained in this structure. We have theoretically calculated the optical propagation to justify the experimental result. The calculated result was in good agreement with the experimental result and showed the difference in optical characteristics between the simple CLC and the HPC containing the CLC.