Browsing by Author "Nyamuda, Gibson Peter"
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- ItemDesign and development of an external cavity diode laser for laser cooling and spectroscopy applications(Stellenbosch : University of Stellenbosch, 2006-12) Nyamuda, Gibson Peter; Steenkamp, Christine M.; Rohwer, Erich G.; University of Stellenbosch. Faculty of Science. Dept. of Physics.External cavity diode lasers are used increasingly as sources of light in applications ranging from industrial photonic systems to basic laboratory research on the interaction of light and atoms. External cavity diode lasers offer more stable output frequency and narrow spectral bandwidth than the typical free-running diode lasers. These characteristics are achieved by exploiting the sensitivity of diode lasers to external optical feedback. In this study the design and development of an external cavity diode laser system for future applications in spectroscopy and laser cooling of rubidium atoms is presented. The external cavity diode laser including mechanical components and control electronics of the system is developed from basic components. The system uses frequency selective optical feedback from a diffraction grating in a Littrow configuration to provide collimated, narrow-band, frequency tunable light near 780 nm. The external cavity diode laser is designed to increase the mode-hop-free frequency tuning range, and allow accurate frequency tuning and stabilisation. A low-noise current source and a temperature controller for thermal stability were developed as part of the system since the output frequency changes with temperature and current. The temperature controller is optimised experimentally for the thermal characteristics of the external cavity. An electronic sidelock servo circuit for frequency locking of the external cavity diode laser to an external reference for long term frequency stabilisation is proposed and discussed. The servo circuit electrically controls the grating tilt and the current through the diode laser in order to lock the frequency of the diode laser. The external cavity diode laser is optimised and characterised near 780 nm. Results obtained in this study indicate that the external cavity diode laser is suitable for future applications in spectroscopy and laser cooling of neutral rubidium atoms.
- ItemOptical second harmonic generation and pump-probe reflectivity measurements from Si/SiO2 interfaces(Stellenbosch : University of Stellenbosch, 2010-12) Nyamuda, Gibson Peter; Rohwer, Erich G.; Steenkamp, Christine M.; Stafast, H.; University of Stellenbosch. Faculty of Science. Dept. of Physics.ENGLISH ABSTRACT: Silicon/silicon dioxide (Si/SiO2) interface is widely used in microelectronics as the gate between the drain and source of most metal oxide semiconductor field effect transistors (MOSFETs). The functionality, reliability and electrical properties of such transistors are strongly dependent on the quality of the Si/SiO2 structure forming the gate. Characterization of the Si/SiO2 interface is important in understanding device degradation therefore the Si/SiO2 interface is a subject of intensive investigation. Research studies of Si/SiO2 interfaces using optical methods have been reported by many groups around the world but to date many open questions still exist. The physics of photoinduced trap or defect generation processes and the subsequent trapping of charge carriers, the precise role of photoinduced interfacial electric field in altering optical properties of the Si/SiO2 interface and its role in affecting the second harmonic (SH) yield measurements are not well understood. In this work a commercial near infrared femtosecond (fs) laser source [1.55 eV, 75 ± 5 fs, 10 nJ, 80 MHz] is used to study native Si/SiO2 interfaces of free standing single crystalline Si membrane and bulk Si. Optical second harmonic (SH) generated at the Si/SiO2 interfaces of a Si membrane in reflection and for the first time in transmission is demonstrated as well as stationary, single colour, pump-probe reflectivity measurements from the Si/SiO2 interface of bulk n-type Si. The experimental setups for the second harmonic generation (SHG) and pump-probe techniques were designed and implemented, and measurements were recorded by a computer controlled data acquisition system. Free standing Si membrane samples were successfully produced at the Institut f¨ur Photonische Technologien (IPHT) in Jena, Germany from bulk Si using a chemical etching process and were characterised using the z-scan technique. The penetration depth of light with a photon energy of 1.55 eV in silicon allows transmission of the fundamental fs laser pulses through the Si membrane (∼ 10 μm in thickness) and this is exploited to generate a SH signal in transmission from the Si/SiO2 interfaces of the Si membrane. In the presence of sufficiently intense fs laser light defects are created at the interfaces and populated by multiphoton transfer of charges from Si to SiO2 where they are subsequently trapped. The transfer of charge establishes interfacial electric fields across the interfaces of the Si membrane and this enhances SHG. This phenomenon is called electric field induced second harmonic (EFISH) generation. To our knowledge, EFISH measurements from interfaces of Si membrane performed in transmission are demonstrated for the first time in the present study. The demonstration of EFISH in transmission revealed new results which allowed us to provide additional perspectives on the EFISH generation process at Si/SiO2 interfaces never reported before. The temporal response of SH signals from virgin spots were recorded at different incident laser powers for both reflection and transmission geometries. The SH responses measured in transmission were observed to be time dependent and show an increase during irradiation of the sample corresponding to EFISH process. A series of SH measurements were recorded at different laser powers to compare the magnitudes of SH yield in each detection geometry for a single Si/SiO2 interface. The magnitude of the SH yield measured in transmission was higher than expected and surpassed the SH yield measured in reflection. The expectation is based on the fact that the local intensity of the fundamental beam at the second interface where the SH in transmission is generated is low compared to the local intensity at the first interface where the SH in reflection originates. A physical model is developed to consistently interpret the experimental results obtained in this study. In this model we established the origin of EFISH signals in each detection geometry, explain the unexpected high SH signals measured in transmission and provide an analysis of the time constants extracted from SH response in transmission and reflection. In addition, we also report for the first time stationary pump-probe reflectivity measurements from bulk n-type Si(111) samples with native oxide. A strong pump beam was focused on the same spot as a weak probe beam from the same fs laser source. The change in reflectivity of the Si(111)/SiO2 system was recorded by monitoring the change in intensity of the weak reflected probe beam. The temporal evolutions of the reflectivity of the material were recorded at different pump powers. The reflectivity of the material increases over several minutes of irradiation and reaches steady-state after long time irradiation. The change in reflectivity of the material is attributed to a nonlinear process called Kerr effect, and the temporal response arises from the photoinduced interfacial electric field across the Si(111)/SiO2 interface caused by multiphoton charge transfer from bulk Si(111) to the SiO2 layer. The results reported in this study contribute to the understanding of the photoinduced interfacial electric field caused by charge carrier separation across buried solid-solid interfaces. They also reveal nonlinear optical processes such as the Kerr effect caused by charge dynamics across the interface in addition to the well known SHG process.