Volume 86, Issue 21, 23 May 2005
- 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
- device physics
- interdisciplinary and general physics
Index of content:
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The adsorption of an armchair single-walled carbon nanotube(CNT) on fully and partially hydrogenated Si(001) surfaces are studied from first-principles calculations. Our results indicate that the electronic properties of the adsorbed CNT can be ruled by the H concentration along the contact region. On the fully hydrogenated Si(001), the CNT is physisorbed, preserving almost unchanged its metallic character. Removing a few of H atoms along the adsorption site, we find an enhancement on the CNT metallic character. In contrast, removing all the H atoms along the adsorption site, the adsorbed CNT becomes a semiconductor, exhibiting an energy gap. These results suggest that armchair CNTs adsorbed on could be transformed into metal–semiconductor junctions by grading the H concentration along the CNT–surface interface.
- LASERS, OPTICS, AND OPTOELECTRONICS
86(2005); http://dx.doi.org/10.1063/1.1935034View Description Hide Description
Index-guided two-dimensional (2D) optical waveguides are numerically studied to investigate propagation properties of guided optical waves. The 2D optical waveguide consists of a dielectric thin film sandwiched between two semi-infinite metals. We demonstrate that vertically localized 2D optical waves can be laterally confined and guided by index guiding when the dielectric film has a high-refractive-index core and claddings of a lower index. The index guiding provides good optical power transmittance, otherwise optical power is rapidly attenuated due to lateral beam divergence.
86(2005); http://dx.doi.org/10.1063/1.1929070View Description Hide Description
We present high-power surface-emitting second-order distributed feedback quantum-cascade lasers in GaAs and InPmaterial systems. The GaAs device, grown by molecular-beam epitaxy, showed single-mode peak output powers of 3 W at 78 K in pulsed operation. With the InP-based devices, which are grown by metalorganic vapor phase epitaxy, we obtained single-mode peak output powers of 1 W at room temperature. These are the highest output powers for surface emission of quantum-cascade lasers reported so far. The InP-based distributed feedback lasers also have very low threshold current densities and are working well above room temperature.
86(2005); http://dx.doi.org/10.1063/1.1931058View Description Hide Description
A two-step growth method, commonly used for GaN on sapphire, was applied to grow high-quality on sapphire. Comparing to the one grown on a low-temperature grownAlNbuffer layer, the decomposition, recrystallization, and islands coalescence processes of the two-step growth increased the surface flatness, the crystal quality, the electrical property, suppressed the phase separation, and released the biaxial tensile strain. A thick high-quality crack-free nearly GaN-free epilayer was obtained.
High-differential-quantum-efficiency, long-wavelength vertical-cavity lasers using five-stage bipolar-cascade active regions86(2005); http://dx.doi.org/10.1063/1.1931060View Description Hide Description
We present five-stage bipolar-cascade vertical-cavity surface-emitting lasers emitting at grown monolithically on an InP substrate by molecular beam epitaxy. A differential quantum efficiency of 120%, was measured with a threshold current density of and voltage of , only larger than , the aggregate photon energy. Diffraction loss study on deeply etched pillars indicates that diffraction loss is a major loss mechanism for such multiple-active region devices larger than . We also report a model on the relationship of diffraction loss to the number of active stages.
86(2005); http://dx.doi.org/10.1063/1.1938256View Description Hide Description
A type-II “W” diode laser with five quantum well periods and emitting at operated in cw mode to . A second device produced more than of cw power at . The threshold current density at was , and pulsed operation was observed to . Improvements over previous single-stage devices for this wavelength range may be attributed in part to high growth quality and also to the incorporation of transition regions that smooth discontinuities in the conduction-band profile.
86(2005); http://dx.doi.org/10.1063/1.1935770View Description Hide Description
Light propagation in photonic crystals(PhC) is both sensitive to incident angle and wavelength. By combining two different PhC lattices, we utilize this effect to demonstrate a wavelength-dependent beam splitter with enhanced angular separation. The first lattice acts as a superprism that separates the incoming light according to wavelength, whereas the second lattice acts as an angular amplifier. We obtain 90° angular separation for two wavelengths separated by 70 nm (1300 nm regime) in a structure that is less than 10 μm long.
86(2005); http://dx.doi.org/10.1063/1.1935749View Description Hide Description
We propose an adaptive liquid-filled lens, which consists of an elasticmembrane, a solid plate, and an annular sealing ring; a liquid with a fixed volume stored in lens chamber. The key part is the annular sealing ring which looks like an iris diaphragm. The surfaces of annular sealing ring are sealed with an elasticmembrane. The radius of the annular sealing ring is changeable. By tuning the radius of the annular sealing ring, the stored liquid in the lens will be redistributed, thus changing the curvature of the elasticmembrane. Therefore, the lens cell causes light to converge or diverge. A liquid lens with a positive variable focus was demonstrated, this kind of lens has the advantages of simple fabrication process, compact structure, easy operation, and low cost.
86(2005); http://dx.doi.org/10.1063/1.1935751View Description Hide Description
An optical switch structure which combines a planar lightwave circuit and liquid crystal is proposed and experimentally demonstrated. The switching is realized by inducing total internal reflection through controlling the refractive index of the liquid crystal. The principle and the design of the integrated optical switch are described. Experimental results suggest that anchoring of the liquid crystal molecules on nonplanar structures is very crucial in affecting the switching performance.
Single-mode InGaAs vertical cavity surface-emitting lasers with temperature-tolerant modulation characteristics86(2005); http://dx.doi.org/10.1063/1.1935755View Description Hide Description
The dynamic performance of single-mode vertical cavity surface emitting lasers(VCSELs) is presented. In order to reach such a long wavelength, the devices utilize highly strained double-quantum wells and a large detuning between the material gain peak and cavity resonance. It is found that the large detuning improves the temperature stability of both static and modulation characteristics. A resonance frequency of 7.8–9.5 GHz and optical power of 0.30 mW in fiber was maintained throughout the investigated temperature range of 20–90 °C. The intrinsic response of the device suggests that long-wavelength VCSELs have the potential to be used as low cost uncooled optical transmitters at .
86(2005); http://dx.doi.org/10.1063/1.1938004View Description Hide Description
We have fabricated and characterized ultrafast metal-semiconductor-metalphotodetectors based on low-temperature-grown (LT) GaN. The photodetector devices exhibit up to electric breakdown fields and subpicosecond carrier lifetime. We recorded as short as -wide electrical transients using -wavelength and -duration laser pulses, that is corresponding to the carrier lifetime of in our LT GaN material.
86(2005); http://dx.doi.org/10.1063/1.1937995View Description Hide Description
A truly monopolar GaAs Fabry–Perot cavity Gunn laser is demonstrated. The device is grown by metalorganic chemical vapor deposition on a semi-insulating GaAs substrate and consists of an dopedGaAsactive layer sandwiched between the waveguiding layers. The operation of the device is based on the band to band recombination of impact-ionized nonequilibrium electron-hole pairs in the propagating high field domains in the Gunn diode, which is placed in a Fabry–Perot cavity, and biased above the threshold of negative differential resistance. Lasing from the device is observed at temperature . The maximum power emitted from the device is at and .
Observation of the biexponential ground-state decay time behavior in InAs self-assembled quantum dots grown on misoriented substrates86(2005); http://dx.doi.org/10.1063/1.1938000View Description Hide Description
Biexponential behavior of the time-resolved photoluminescence decay from the ground state has been studied over a temperature range of 77–300 K on samples with varying sized self-assembled InAs∕GaAs quantum dot ensembles controlled by substrate misorientation alone. The slower second decay component is considerably longer than the first one, and has been measured to be as long as 300 ns. This slow component is attributed to carrier recapturing and indirect radiative recombination processes.
86(2005); http://dx.doi.org/10.1063/1.1935033View Description Hide Description
We present a device concept for scanning large laser beams using a domain microengineered ferroelectric device. In our design, the large input beam is divided into smaller beamlets, which are then individually deflected through an angle, and then recombined in the far-field to reconstruct the large beam. As a demonstration of this concept, a five-stage cascaded rectangular domain micropatterned scannerdevice with 13 beamlet channels was fabricated in and was demonstrated to deflect a infrared laser beam by a total of 10.3° at .
86(2005); http://dx.doi.org/10.1063/1.1929086View Description Hide Description
A voltage-tunable four-color quantum-well infrared photodetector(QWIP) has been demonstrated. The advantages of this detector include widely separated peaks, narrow linewidths, and negligible spectral cross-talk. The four colors demonstrated are widely distributed from mid-infrared to long-infrared, and are centered at 4.5, 5.3, 8.3, and . Each spectral peak is well defined and the corresponding linewidth is 0.58, 0.71, 0.83, and , respectively (corresponding to ). The detector structure consists of two stacks of superlattice materials that are separated by a middle contact layer. Each material is designed to detect two specific wavelengths, tunable by the bias polarity. Four-color detection is achieved by applying different combinations of top and bottom bias relative to the common middle contact and reading out the optical signals sequentially from these two contacts. Using an appropriate bias sequence, the detector can be made sensitive to only one color at a time and the signals from the other colors are negligible. With this detection scheme, one can select different combinations of detection wavelengths and linewidths over a wide spectral range. This design thus greatly increases the flexibility of QWIPs in multicolor imaging.
Static and dynamic measurements of the -factor of five-quantum-dot-layer single-mode lasers emitting at on GaAs86(2005); http://dx.doi.org/10.1063/1.1935754View Description Hide Description
The “material” and “device” linewidth enhancement factor of five-quantum-dot (QD)-layer single-mode lasers emitting at are investigated using two methods. The Hakki–Paoli method associated with pulsed analysis of Fabry–Perot modes below threshold demonstrates a record value of 0.7 at 1295 nm. High-frequency current modulation experiments showed a value of 2.0 just above threshold at with a steady increase with the current. Dynamic measurements on a three-QD layer device, with a reduced ground state optical gain, showed a similar increase of with current but at a higher rate.
Over 3 W high-efficiency vertical-external-cavity surface-emitting lasers and application as efficient fiber laser pump sources86(2005); http://dx.doi.org/10.1063/1.1935756View Description Hide Description
We report on the design and fabrication of high-power, high-brightness diode-pumped vertical-external-cavity surface-emitting lasers. Over 3 W continuous wave fundamental transverse mode output at 980 nm with a high slope efficiency of 44% is demonstrated at room temperature. The diffraction-limited beam with factor of 1.15 at high-power operation is achieved. A vertical-external-cavity surface-emitting laser operating near 976 nm with a diffraction-limited beam is used to pump the core of 3 cm long -codoped single-mode phosphate fiber lasers. An output power in excess of 250 mW at 1535 nm with a slope efficiency of 29% is obtained without any cooling.
Mechanisms of dynamic range limitations in GaAs∕AlGaAs quantum-cascade lasers: Influence of injector doping86(2005); http://dx.doi.org/10.1063/1.1937993View Description Hide Description
The influence of doping density on the performance of GaAs∕AlGaAs quantum-cascade lasers is presented. A fully self-consistent Schrödinger–Poisson analysis, based on a scattering rate equation approach, was employed to simulate the above threshold electron transport in laser devices. V-shaped local field domain formation was observed, preventing resonant subband level alignment in the high pumping-current regime. The resulting saturation of the maximal current, together with an increase of the threshold current, limits the dynamic working range under higher doping. Experimental measurements are in good agreement with the theoretical predictions.
One-step waveguide and optical circuit writing in photopolymerizable materials processed by two-photon absorption86(2005); http://dx.doi.org/10.1063/1.1915525View Description Hide Description
Two-photon absorption process is known to be a convenient tool to create three-dimensional microstructures in photopolymerizablematerials. In this context, we have fabricated stable optical waveguides. The features of these waveguides (in particular, transmission losses) have been compared to the results of numerical simulations. We have also demonstrated the possibility of connecting two optical fibers via a curved guide and to realize Y splitters. The technique allows one to fabricate operational integrated optical circuits in photopolymerizable resins.
86(2005); http://dx.doi.org/10.1063/1.1940127View Description Hide Description
A simple THz waveguide element has been used as a sensitive sensor for adsorbates on surfaces. The evanescent wave from total internal reflection off a siliconprism was used to couple pulsed THz radiation frequency selectively into the waveguide. The coupled frequencies were determined via time domain spectroscopy and react sensitively to any changes of thickness or phase shift upon reflection. In particular, the sensitivity to phase shifts makes this waveguidesensor attractive for the detection of very thin adsorbates. Typical sensitivity and reliability limits for the detection of adsorbates are discussed.
86(2005); http://dx.doi.org/10.1063/1.1940132View Description Hide Description
We demonstrate a multipass noncollinear optical parametric chirped pulseamplifier seeded by pulses from a femtosecond oscillator and pumped by a commercial -switched, frequency doubled laser. Amplification higher than and pulse energy exceeding 1.7 mJ are achieved with four passes through a single -barium borate crystal. Good beam quality and high gain, together with broad amplification bandwidth, make it an attractive alternative to chirped pulseamplifier systems.