Picosecond semiconductor lasers with a multisection saturable absorber, fabricated by heavy ion implantation

A method of implanting high-energy heavy ions across the emitter layer of a semiconductor laser has been developed to create distributed regions of ultrafast saturable absorber integrated into the cavity of a quantum-well laser. This method was used to fabricate picosecond laser diodes having high average powers and to demonstrate colliding-pulse mode-locking of a diode laser with a multisection saturable absorber. Pis’ma Zh. Tekh. Fiz. 25, 4–9 (July 12, 1999)     

Microfluidic sorting with a moving array of optical traps

Optical sorting was demonstrated by selective trapping of a set of microspheres (having specific size or composition) from a flowing mixture and guiding these in the desired direction by a moving array of optical traps. The approach exploits the fact that whereas the fluid drag force varies linearly with particle size, the optical gradient force has a more complex dependence on the particle size and also on its optical properties. Therefore, the ratio of these two forces is unique for different types of flowing particles. Selective trapping of a particular type of particles can thus be achieved by ensuring that the ratio between fluid drag and optical gradient force on these is below unity whereas for others it exceeds unity. Thereafter, the trapped particles can be sorted using a motion of the trapping sites towards the output. Because in this method the trapping force seen by the selected fraction of particles can be suitably higher than the fluid drag force, the particles can be captured and sorted from a fast fluid flow (about 150 μm/s). Therefore, even when using a dilute particle suspension, where the colloidal trafficking issues are naturally minimized, due to high flow rate a good throughput (about 30 particles/s) can be obtained. Experiments were performed to demonstrate sorting between silica spheres of different sizes (2, 3, and 5 μm) and between 3 μm size silica and polystyrene spheres.     

A novel microsensor fabricated with charge-flow transistor and a Langmuir-Blodgett organic semiconductor film

Microsensor for nitrogen dioxide (NO₂) fabricated by Langmuir-Boldgett (LB) films of Samarium bis[2,3,9,10,16,17,23,24-octakis(octyloxy)phthalocyaninato] complex Sm[Pc*]₂ (Pc*=Pc(OC₈H₁₇)₈) and charge-flow transistor (CFT) are evaluated. Pure Sm[Pc*]₂ and mixture of Sm[Pc*]₂ and octadecanol (OA) deposited from both pure water and 10⁻⁴ M Cd²⁺ subphases were investigated. It is found that a mixture of 1:3 Sm[Pc*]₂:OA forms an excellent material for the fabrication of gas-sensing LB film by studying the film-forming characteristic. Such novel microsensor has been fabricated by incorporating the multilayer LB film into the gate electrode of a metal-oxide-semiconductor field effect transistor, forming an array of CFT. On the application of a gate voltage (V_GS), greater than the threshold voltage (V_TH), a delay was observed in the response of the drain current. It is due to the time taken for the resistive gas-sensing film to charge up to V_GS. This delay characteristic was found to depend on the concentration of NO₂. Results are presented showing that the device can detect reversibly concentration of NO₂ gas down to 5 ppm at room temperature.     

A novel fast disintegrating tablet fabricated by three-dimensional printing

Background: Based on computer-aided models, three-dimensional printing (3DP) technology can exercise local control over the material composition, microstructure, and surface texture during it layer-by-layer manufacturing process to endow the products with special properties. It can be a useful tool in the development of novel solid dosage forms. Method: In this study, a novel fast disintegrating tablet (FDT) with loose powders in it was designed and fabricated using 3DP process. The inner powder regions were formed automatically by depositing the binder solutions onto selected regions during the layer-printing processes. Results: Environmental scanning electron microscope images clearly showed that the printed regions were bound together. The particle size was reduced or individual particles could no longer be distinguished. In contrast, the unprinted regions were uncompacted with cracks and fissures among the loose powders. The tablets had a hardness value of 54.5 N/cm² and 0.92%?mass loss during the friability tests. The disintegration time of the tablets was 21.8 seconds and the wetting time was 51.7 seconds. The in vitro dissolution tests showed that 97.7%?acetaminophen was released in the initial 2 min. Conclusion: 3DP process is able to offer novel methods for preparing FDTs.     

Index-distributed planar microlenses for three-dimensional micro-optics fabricated by silver-sodium ion exchange in BGG35 substrates

Fabrication of planar microlens arrays by silver-sodium ion exchange is possible by using a new glass type, optimized for this technology. Because of its nonlinear diffusion response it is well suited to the fabrication of microlens arrays. We show that the diffusion coefficient can be described theoretically by an exponential concentration dependence. The parameters of the planar microlenses are measured interferometrically and by imaging experiments. Because of the specific index distribution, new evaluation techniques for the determination of lens parameters from interferometric measurements have been applied. We also present a simple model that relates the achievable lens parameters to the diffusion conditions.     

Electrostatic ion traps with separation of variables in parabolic coordinates

Electrostatic traps with ideal spatiotemporal focusing in one of the directions of ion motion and the motion in transverse directions integrated using the Hamilton-Jacobi method are considered. Conditions for the ion confinement in the trap field are determined. The finite character of ion motion under confinement conditions is independently checked by the numerical integration of the Lagrange equations.     

A diffusion problem in semiconductor technology

Summary This paper deals with a mathematical model of a SEM-EBIC experiment devised to measure the diffusion length of semiconductor materials. In the model the semiconductor material occupies a half-space, of which the plane bounding surface is partly covered by a semi-infinite current-collecting junction, the Schottky diode. A scanning electron microscope (SEM) is used to inject minority carriers into the material. It is assumed that injection occurs at a single point only. The injected minority carriers diffuse through the material and recombine in the bulk at a rate proportional to their local concentration. Recombination also occurs at the free surface of the material, not covered by the junction, where its rate is determined by the surface recombination velocity v. The mathematical model gives rise to a mixed-boundary-value problem for the diffusion equation, which is solved by means of the Wiener-Hopf technique. An analytical expression is derived for the measurable electron-beam-induced current (EBIC) caused by the minority carriers reaching the junction. The solution obtained is valid for all values of v, and special attention is given to the limiting cases v= and v=0. Asymptotic expansions of the induced current, which are usable when the injection point is more than a few diffusion lengths away from the edge of the junction, are derived as well.     

A parallel miniature cylindrical ion trap array

A small mass spectrometer array is described in which each element is a cylindrical ion trap (CIT). The array contains four CITs, each having an inner radius of 2.5 mm, arranged in parallel and operated using a single electronics system under common conditions for trapping and mass analysis. By using an array of identically sized traps, higher ion capacity can be achieved than with a single miniature CIT, but the advantage of lower power and voltage requirements associated with the smaller ion trap is maintained. Overall signal intensity of the array of four traps is compared with that of a two-element CIT array to demonstrate the increased ion capacity of larger arrays. Resolution for m-dichlorobenzene is shown to be approximately 180 (full width at half-maximum), with no significant loss in resolution as a result of using multiple CITs. The detection of 5 x 10(-9) Torr partial pressure of krypton in argon with a signal-to-noise ratio of approximately 30 for the most abundant isotope is shown, demonstrating the applicability of the device for process gas monitoring. Also, a preliminary spectrum from a 10-CIT array with each element having an inner radius of 1.5 mm is reported.     

A resonant accelerometer with two-stage microleverage mechanisms fabricated by SOI-MEMS technology

We present the design, fabrication, and testing of a push-pull differential resonant accelerometer with double-ended-tuning-fork (DETF) as the inertial force sensor. The accelerometer is fabricated with the silicon-on-insulator microelectromechanical systems (MEMS) technology that bridges surface micromachining and bulk micromachining by integrating the 50-/spl mu/m-thick high-aspect ratio MEMS structure with the standard circuit foundry process. Two DETF resonators serve as the force sensor measuring the acceleration through a frequency shift caused by the inertial force acting as axial loading. Two-stage microleverage mechanisms with an amplification factor of 80 are designed for force amplification to increase the overall sensitivity to 160 Hz/g, which is confirmed by the experimental value of 158 Hz/g. Trans-resistance amplifiers are designed and integrated on the same chip for output signal amplification and processing. The 50-/spl mu/m thickness of the high-aspect ratio MEMS structure has no effect on the amplification factor of the mechanism but contributes to a greater capacitance force; therefore, the resonator can be actuated by a much lower ac voltage comparing to the 2-/spl mu/m-thick DETF resonators. The testing results agree with the designed sensitivity for static acceleration.     

Microassembly of semiconductor three-dimensional photonic crystals

Electronic devices and their highly integrated components formed from semiconductor crystals contain complex three-dimensional (3D) arrangements of elements and wiring. Photonic crystals, being analogous to semiconductor crystals, are expected to require a 3D structure to form successful optoelectronic devices. Here, we report a novel fabrication technology for a semiconductor 3D photonic crystal by uniting integrated circuit processing technology with micromanipulation. Four- to twenty-layered (five periods) crystals, including one with a controlled defect, for infrared wavelengths of 3-4.5 microm, were integrated at predetermined positions on a chip (structural error <50 nm). Numerical calculations revealed that a transmission peak observed at the upper frequency edge of the bandgap originated from the excitation of a resonant guided mode in the defective layers. Despite their importance, detailed discussions on the defective modes of 3D photonic crystals for such short wavelengths have not been reported before. This technology offers great potential for the production of optical wavelength photonic crystal devices.     

脉冲阳极氧化工艺制作GaAs／AlGaAs宽条形半导体激光器

研究了脉冲阳极氧化工艺的特性,并利用脉冲阳极氧化工艺制作出激射波长为776nm的宽条形半导体激光器,器件阚值电流为0.35A,斜率效率为1.12W/A,与常规工艺制作的器件相比阈值电流降低了22%,斜率效率提高了18%.     

Spatial-mode control of vertical-cavity lasers with micromirrors fabricated and replicated in semiconductor materials

Micromirrors were fabricated in gallium phosphide by mass transport to provide spatial-mode control of vertical-cavity surface-emitting lasers (VCSEL's). The concave mirrors were used in an external-cavity configuration to provide spatial filtering in the far field. Single-mode cw lasing was demonstrated in 15-mum-diameter VCSEL's with currents as high as 6 times threshold. The fabrication process was extended to micromirrors in gallium arsenide by use of a replication and dry-etch transfer process.     

Particle-size analysis by laser diffraction with a complementary metal-oxide semiconductor pixel array

Existing laser-diffraction instruments that use photodiode detectors have a limited resolution for particle sizing. We attempt the implementation of a complementary metal-oxide semiconductor pixel sensor for particle-size measurement by laser diffraction. The sensor has unique features: high resolution, no blooming, and a wide dynamic range (i.e., direct measurement of the scattering pattern). The calibration of the sensor is based on each pixel. The signal-processing and the inversion schemes for obtaining the particle-size distribution are described. The results indicate an improved size resolution and an increased flexibility of application.     

锗表面二维亚波长结构的反应离子刻蚀制备

为了获得具有金字塔结构的二维亚波长结构表面,提高其高宽比,用掩模曝光光刻及反应离子刻蚀技术,以SF6和O2为反应气体,在Ge衬底上制备了二维亚波长结构.用扫描电镜对刻蚀图形的形貌进行了观察,研究了功率、气压、气体流量及掩模图形对刻蚀图形的影响.结果表明:刻蚀图形腰部被优先刻蚀,形成凹陷的侧壁轮廓;O2流量增大有利于在侧壁形成保护层,从而减小腰部刻蚀、增大顶部及根部刻蚀;功率及气压过大或过小均会使侧壁刻蚀较大;方形图案比圆形图案掩模更有利于刻蚀出深度较大的亚波长结构.     

Criterion of transverse stability for ion traps with transverse motion integrable in elliptic coordinates

Electrostatic traps with ideal spatiotemporal focusing of ions in one direction of their motion and the motion in transverse directions integrable in the elliptic coordinates are considered. Conditions for the confinement of ions in the trapping field are determined. The finite character of ion motion under the proposed confinement conditions is independently checked by numerical integration of the Lagrange equations.     

Multichannel microchip electrophoresis device fabricated in polycarbonate with an integrated contact conductivity sensor array

A 16-channel microfluidic chip with an integrated contact conductivity sensor array is presented. The microfluidic network consisted of 16 separation channels that were hot-embossed into polycarbonate (PC) using a high-precision micromilled metal master. All channels were 40 microm deep and 60 microm wide with an effective separation length of 40 mm. A gold (Au) sensor array was lithographically patterned onto a PC cover plate and assembled to the fluidic chip via thermal bonding in such a way that a pair of Au microelectrodes (60 microm wide with a 5 microm spacing) was incorporated into each of the 16 channels and served as independent contact conductivity detectors. The spacing between the corresponding fluidic reservoirs for each separation channel was set to 9 mm, which allowed for loading samples and buffers to all 40 reservoirs situated on the microchip in only five pipetting steps using an 8-channel pipettor. A printed circuit board (PCB) with platinum (Pt) wires was used to distribute the electrophoresis high-voltage to all reservoirs situated on the fluidic chip. Another PCB was used for collecting the conductivity signals from the patterned Au microelectrodes. The device performance was evaluated using microchip capillary zone electrophoresis (mu-CZE) of amino acid, peptide, and protein mixtures as well as oligonucleotides that were separated via microchip capillary electrochromatography (mu-CEC). The separations were performed with an electric field (E) of 90 V/cm and were completed in less than 4 min in all cases. The conductivity detection was carried out using a bipolar pulse voltage waveform with a pulse amplitude of +/-0.6 V and a frequency of 6.0 kHz. The conductivity sensor array concentration limit of detection (SNR = 3) was determined to be 7.1 microM for alanine. The separation efficiency was found to be 6.4 x 10(4), 2.0 x 10(3), 4.8 x 10(3), and 3.4 x 10(2) plates for the mu-CEC of the oligonucleotides and mu-CZE of the amino acids, peptides, and proteins, respectively, with an average channel-to-channel migration time reproducibility of 2.8%. The average resolution obtained for mu-CEC of the oligonucleotides and mu-CZE of the amino acids, peptides, and proteins was 4.6, 1.0, 0.9, and 1.0, respectively. To the best of our knowledge, this report is the first to describe a multichannel microchip electrophoresis device with integrated contact conductivity sensor array.     

Modified emission of semiconductor nano-dots in three-dimensional photonic crystals

Mono-dispersed polystyrene spheres were used to grow synthetic opal photonic crystals on glass substrates using controlled evaporation. Commercially available cadmium telluride (CdTe) semiconductor nano-dots with emission in the visible were infiltrated into a slice of opal voids by capillary action. The optical properties of CdTe dot loaded opal were studied by using a white light source and showed a red shift of the opal stop-band due to an increase in the effective refractive index. The emission of the CdTe dots was matched with the edge of the (111) direction stop-band of bare opal. Stop-band confined emission from CdTe dots was observed by pumping with an argon-ion laser. The full width at half maximum of the CdTe emission from an infiltrated section of the opal was significantly reduced due to the stop-band effect of bare opal.     

Growth of carbon nanofibers on nanoscale catalyst strips fabricated with a focused ion beam

We studied the growth mode of vertically aligned carbon nanofibers (CNFs) on Ni catalyst strips fabricated using a focused ion beam (FIB). We found that the CNF growth on Ni catalysts was strongly affected by the geometry of the microfabricated Ni catalyst strips. Selective growth of vertically aligned CNFs requires ion milling from the outside edge of the sample so that the milled materials are effectively evacuated. The CNF diameter and density on the strip depends on its width. Possible mechanisms to control CNF growth using microfabricated catalysts are analyzed with a liquid model using surface free energies.     

Focused ion beam nanofabrication: a comparison with conventional processing techniques

Focused ion beam and dual platform systems are versatile tools for nanoengineering and nanoscience applications. These systems complement conventional processing methods and can be used to prototype and modify a diverse range of nano-devices and sensors. This article discusses FIB nanofabrication and compares it with other fabrication techniques such as electron beam lithography and reactive ion etching. Aspects such as the minimum feature size and side wall profiles are discussed and compared. In addition, the limitations and detrimental effects of FIB processes are discussed.