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  • Borgentun, Carl, 1979- (författare)
  • Design and Performance of Optically Pumped Semiconductor Disk Lasers with Wide Tuning Ranges
  • 2010
  • Licentiatavhandling (övrigt vetenskapligt)abstract
    • In this work I present a strategy for providing the possibility to select the wavelength, or color, of a semiconductor laser over a very large range, by cleverly designing the vital part of the laser: the gain element of a so called optically pumped semiconductor disk laser. Optically pumped semiconductor disk lasers (OP-SDLs) are a relatively new class of lasers showing great promise for future applications. The advantages include the wavelength versatility that is common for all semiconductor lasers, but also adds the ability to deliver multi-Watt output powers into a nearly diffraction-limited beam, and a free-space cavity for the easy insertion of various optical elements. These properties have generated great interest in the OP-SDL for use in life science, metrology, entertainment applications, forensics, and many other fields. Recently, efforts have also been made to extend the tuning range for use in spectroscopic applications such as intra-cavity laser absorption spectroscopy. The work underlying this thesis has focused on the design of the gain element of an OP-SDL and how to obtain a wide tuning range while keeping the output power at a high level. The strategy has been to balance the effects of the spectral dependencies of material gain, subcavity resonance, and spatial overlap of quantum wells and optical field. Experimental evaluations show that the strategy has been successful and a relative tuning range of 4.3% with a maximum output power of 2.6 W was obtained. Furthermore, a new measurement technique for the full characterization of a laser beam has been developed. This technique is well suited for the high-intensity beam from an OP-SDL.
  • Proceedings of the 2nd International Symposium on Terahertz Science and Technology between Japan and Sweden
  • 2010
  • Samlingsverk (redaktörskap) (övrigt vetenskapligt)abstract
    • Preface - Japan-Sweden Terahertz Symposium (JSTS) 2009. “BRIDGING THE GAP BETWEEN RADIO WAVES AND LIGHT”. The second International Symposium on Terahertz Science and Technology between Japan and Sweden, was held in Göteborg, Sweden, November 18-20, 2009. Chalmers vice president Stefan Bengtsson followed by two keynote lectures, presented by Professor Nishizawa and Professor Kollberg, opened the symposium. Successive invited lectures were held in Chalmerska huset, where scientists from both Japan and Sweden presented frontier research covering a wide range of aspects in the field of terahertz science, techniques and applications. The aim of this forum is to provide a platform where researchers from the two countries can discuss latest results and trends in the field of terahertz science and technology. Sandwiched between the optical on the short wavelength side and radio on the long wavelength extreme, the terahertz (THz) or far-infrared has long been considered the last remaining scientific gap in the electromagnetic spectrum. The broad spectrum of THz applications has attracted researchers from different disciplines dealing with optics and photonics, microwave engineering and semiconductor physics. The role of this meeting is not only bridging the THz-gap and expanding the use of the electromagnetic spectrum but also strengthening the collaboration between Japan and Sweden.
  • Song, Yuxin, 1981- (författare)
  • Optimization of Metamorphic Materials on GaAs Grown by MBE
  • 2010
  • Licentiatavhandling (övrigt vetenskapligt)abstract
    • Advanced epitaxial technologies such as molecular beam epitaxy (MBE) and metal-organic vapor phase epitaxy (MOVPE) have enabled the idea of semiconductor heterostructures, which built up the foundation of the fast developing information and communication technology nowadays. Lattice mismatch has been a problem limiting designs of semiconductor heterostructures. Restricted by the availability of large and high-quality commercial substrates, only a small range of materials with a lattice constant close to certain substrate, such as GaAs and InP, could be chosen. Metamorphic growth is one of the solutions by which a virtual substrate with a desired lattice constant can be obtained after growing a mismatched but nearly relaxed buffer layer on a conventional substrate. The main challenges of this method are the rough interface and a high threading dislocation (TD) density in the active region of the devices. The TD problem is more severe for optoelectronic devices, such as lasers, which have a large device area, and therefore can easily contain TDs in the active region. Although there have been notable progresses for metamorphic optoelectronic devices in recent years, further reduction of the TD density is still required to improve the performance and make them competitive with existing products. In this work, we investigate and optimize growth schemes of metamorphic buffer layers grown on GaAs substrate by MBE. Effects of both n- and p-type doping on material quality in alloy graded InGaAs buffers with different parameters, such as grading profiles, grading slopes, In compositions and thicknesses are studied systemically. Moreover, further TD reduction by nitrogen incorporation in metamorphic buffers is demonstrated. The physical origin is found to be due to both the strain and the lattice hardening effect. These results show that by proper designs of the metamorphic buffers there are great potentials to further improve the quality of metamorphic heterostructures and enhance the performance of metamorphic optoelectronic devices.
  • Adolfsson, Göran, 1981- (författare)
  • Threshold and Temperature Characteristics of InGa(N)As-GaAs Multiple Quantum Well Lasers
  • 2008
  • Licentiatavhandling (övrigt vetenskapligt)abstract
    • Semiconductor lasers emitting in the 1.3 μm regime are of interest for applications in access-networks like fiber-to-the-home and radio-over-fiber systems. Such fiber optical networks are expected to replace the copper-based access-networks currently in use due to a continuously increasing demand on user bandwidth. To facilitate a widespread implementation of such networks, low-cost semiconductor lasers emitting at 1.3 μm are needed. A significant improvement in cost efficiency is obtained with lasers capable of un-cooled operation. For this reason much research has been devoted to GaAs-based lasers which offer an inherent improved temperature stability compared to the temperature-sensitive InP-based lasers traditionally used. The work presented in this thesis deals with InGa(N)As multiple quantum-well (QW) lasers grown on GaAs, with the aim of improving and understanding their temperature characteristics. A performance comparison between InGaNAs/GaAs lasers and other GaAs and InP-based lasers is presented. The epitaxial material is grown by molecular beam epitaxy (MBE). By optimizing MBE growth conditions we have obtained record low values of threshold current density of 107 and 133 A/cm2/QW for triple QW 1.2 μm InGaAs and 1.3 μm InGaNAs lasers, respectively. A thorough investigation of the temperature dependence of the threshold current (Ith) for ridge wave guide InGaNAs double QW lasers is presented. The good temperature stability of such lasers is usually ttributed to large amounts of defect recombination as well as a large conduction band offset. This work, however, reveals that their good temperature stability also to a large extent arises from a significant and weakly temperature dependent lateral diffusion current, which is not an effect intrinsic to InGaNAs but rather related to the geometry of the laser cavity. By impeding lateral diffusion it should be possible to reduce Ith and still obtain a relatively good temperature performance.
  • Agulo, Ian, 1974- (författare)
  • Ultrasensitive Cold-Electron Bolometer
  • 2007
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • The Cold-Electron Bolometer (CEB) is an ultrasensitive device designed for the detection of cosmic microwave background radiation. The key to its sensitivity is the electron cooling of the absorber by the superconductor-insulator-normal metal (SIN) tunnel junction. At a voltage near and below the superconducting gap, the electrons in the absorber are cooled well below the phonon temperature of the normal metal. This translates to the enhanced sensitivity of the CEB. This thesis describes the work we have done on the optimization of electron cooling of the normal metal absorber, and our measurement of the sensitivity of the CEB. We have optimized the electron cooling of the absorber by SIN tunnel junctions. The best electron cooling was achieved when normal metal traps were added in proximity to the superconducting electrodes in addition to the advanced geometry of the superconducting electrodes. With these modifications, we have decreased the electron temperature by 198 mK. With just the advanced geometry, the electron temperature drop was 129 mK. With just a simple geometry, the drop in temperature was 56 mK. The noise equivalent power (NEP) of the CEB was also measured at 100 mK to be at the level of 10^-18 W/Hz^1/2 at 1 kHz. The NEP was obtained by measuring the noise of the CEB, and then dividing that by its power responsivity, dV/dP. The main limitation in our measurements was the noise component from the amplifier. Finally, we have made measurements on the temperature sensitivity of the SIN tunnel junctions. We have compared the sensitivity between single and ten SIN junctions in series and found that it increases proportionally to the number of junctions. The best temperature responsivity obtained for 10 junctions was approximately 15 μV/mK. Using such thermometer, we have been able to measure the temperature stability of the Oxford Instruments cryogenfree refrigerator to be plus/minus 250 μK for a period of 8 hours. The resolution of the thermometer was measured to be plus/minus100 μK, limited by the noise due to the amplifier. The current and future works are also discussed. These include: a CEB array designed to distribute the total power load, improving further the sensitivity; a one-thousand SIN tunnel junction series thermometer in order to realize and understand the fundamental limitation of such a device; a superconducting cold-electron bolometer (SCEB) for high background power loads; and different read-out systems including an on-chip quasiparticle amplifier (QPA).
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