Solid solution GaSe1−xSx single crystals for THz generation

A table top source of coherent Terahertz (30-1000 µm) radiation, which is high power, narrow bandwidth, and broadly tunable, is high desired for applications in imaging, non-destructive testing (NDT), quantum, security and biomedical technologies. In spite of intensive research over many decades such a device remains elusive. Sulpher doped Gallium Selenide (GaSex−1Sx) solid solution ε-polytype crystals are an outstanding candidate for the efficient generation of radiation and tunability throughout the majority of the Terahertz (THz) regime; thanks to the prodigious linear and nonlinear optical properties of the Gallium Selenide (GaSe) parent crystal. Close control of doping and the crystal growth process enable the manufacture of superior quality nonlinear crystals, where the optical properties may be engineered and the mechanical properties vastly improved. Thus overcoming many of the physical issues that, despite its exceptional optical properties, have frustrated the widespread adoption of GaSe for laser frequency down conversion to the THz regime. In order to fully exploit the potential of GaSex−1Sx crystals and successfully design efficient sources for THz generation the optical properties of these crystals must be accurately determined and their transformation with doping well understood. The work in this thesis aims to accurately determine the optical properties of GaSe, Gallium Sulphide (GaS) and GaSex−1Sx crystals in the Far-Infrared and THz regimes to enable this exploitation. In the first phase of investigation we determine the linear refractive index (n) and absorption (α) coefficient for both the o and e waves in the THz regime (0.14.5 THz) using Terahertz - Time Domain Spectroscopy (THz-TDS) for GaSe, and a dense set of GaSex−1Sx crystals (x = 0.05 0.11 0.22 0.29 0.44). Measurements of THz dispersion and absorption properties of GaS crystals are performed for the first time. The transformation of the optical properties of the crystals and their phonon structure is studied. We examine the sources of inaccuracy in the THzTDs measurements of high refractive index birefringent crystals and propose a set of criteria for the selection of adequate data. The nonlinear Figure of Merit (FOM) of available high quality GaSex−1Sx crystals is found to be an order of magnitude less than that predicted in the literature, with FOM = 19.8 for GaSe, FOM = 17 for GaSex−1Sx, on the other hand estimates for double doping with Sulphur and Aluminium predict significant enhanced of these FOM values, up to 5-10 times. In the second phase of investigation we examine the phonon band of the GaSe, GaS and GaSex−1Sx by FTIR and Raman spectroscopy. For the first time we determine the absorption coefficients of the main phonon peak in the set of GaSex−1Sx crystals. The transformation of the phonon band with doping is studied. In the third phase of investigation we attempt to determine the nonlinear optical properties deff and n2 of GaSe and GaSex−1Sx in the Far Infra-Red (FIR) and THz regimes using the Maker fringe and Z-scan methods on the FELIX free electron laser

Ultrafast Mid-infrared Fibre Lasers

Laser light has enabled some of the most important scientific discoveries and innovations, and research in newer types of lasers continues to reveal more applications. Mid-infrared light, which interacts strongly with naturally occurring molecules, holds many promises in sensing and medical technologies and their applications are now beginning to appear. This surging development will strongly benefit with the availability of field-usable, compact and robust sources of mid-infrared light. The fibre lasers have the extreme potential to achieve this; however their performance in terms of generating ultrafast pulses of mid-infrared light has not yet been demonstrated. Here we explore the potential fluoride fibre lasers have in the ultrafast pulsed regime. We have demonstrated this via the production of nanosecond, to picosecond, and finally femtosecond pulses from fluoride fibre lasers using a variety of methods. These demonstrations reveal that fluoride fibres lasers are strong candidates to be an ultrafast mid-infrared source, with applications ranging from frequency comb based molecular sensing to next generation laser scalpels

Development of optical parametric chirped-pulse amplifiers and their applications

In this work, optical pulse amplification by parametric chirped-pulse amplification (OPCPA) has been applied to the generation of high-energy, few-cycle optical pulses in the near-infrared (NIR) and infrared (IR) spectral regions. Amplification of such pulses is ordinarily difficult to achieve by existing techniques of pulse amplification based on standard laser gain media followed by external compression. Potential applications of few-cycle pulses in the IR have also been demonstrated. The NIR OPCPA system produces 0.5-terawatt (10 fs, 5 mJ) pulses by use of noncollinearly phase-matched optical parametric amplification and a down-chirping stretcher and upchirping compressor pair. An IR OPCPA system was also developed which produces 20-gigawatt (20 fs, 350 uJ pulses at 2.1 um. The IR seed pulse is generated by optical rectification of a broadband pulse and therefore it exhibits a self-stabilized carrier-envelope phase (CEP). In the IR OPCPA a common laser source is used to generate the pump and seed resulting in an inherent sub-picosecond optical synchronization between the two pulses. This was achieved by use of a custom-built Nd:YLF picosecond pump pulse amplifier that is directly seeded with optical pulses from a custom-built ultrabroadband Ti:sapphire oscillator. Synchronization between the pump and seed pulses is critical for efficient and stable amplification. Two spectroscopic applications which utilize these unique sources have been demonstrated. First, the visible supercontinuum was generated in a solid-state media by the infrared optical pulses and through which the carrier-envelope phase (CEP) of the driving pulse was measured with an f-to-3f interferometer. This measurement confirms the self-stabilization mechanism of the CEP in a difference frequency generation process and the preservation of the CEP during optical parametric amplification. Second, high-order harmonics with energies extending beyond 200 eV were generated with the few-cycle infrared pulses in an argon target. Because of the longer carrier period, the IR pulses transfer more quiver energy to ionized free electrons compared to conventional NIR pulses. Therefore, higher energy radiation is emitted upon recombination of the accelerated electrons. This result shows the highest photon energy generated by a laser excitation in neutral argon

Laboratory Directed Research and Development Program FY 2004 Annual Report

The Oak Ridge National Laboratory (ORNL) Laboratory Directed Research and Development (LDRD) Program reports its status to the U.S. Department of Energy (DOE) in March of each year. The program operates under the authority of DOE Order 413.2A, 'Laboratory Directed Research and Development' (January 8, 2001), which establishes DOE's requirements for the program while providing the Laboratory Director broad flexibility for program implementation. LDRD funds are obtained through a charge to all Laboratory programs. This report describes all ORNL LDRD research activities supported during FY 2004 and includes final reports for completed projects and shorter progress reports for projects that were active, but not completed, during this period. The FY 2004 ORNL LDRD Self-Assessment (ORNL/PPA-2005/2) provides financial data about the FY 2004 projects and an internal evaluation of the program's management process. ORNL is a DOE multiprogram science, technology, and energy laboratory with distinctive capabilities in materials science and engineering, neutron science and technology, energy production and end-use technologies, biological and environmental science, and scientific computing. With these capabilities ORNL conducts basic and applied research and development (R&D) to support DOE's overarching national security mission, which encompasses science, energy resources, environmental quality, and national nuclear security. As a national resource, the Laboratory also applies its capabilities and skills to the specific needs of other federal agencies and customers through the DOE Work For Others (WFO) program. Information about the Laboratory and its programs is available on the Internet at <http://www.ornl.gov/>. LDRD is a relatively small but vital DOE program that allows ORNL, as well as other multiprogram DOE laboratories, to select a limited number of R&D projects for the purpose of: (1) maintaining the scientific and technical vitality of the Laboratory; (2) enhancing the Laboratory's ability to address future DOE missions; (3) fostering creativity and stimulating exploration of forefront science and technology; (4) serving as a proving ground for new research; and (5) supporting high-risk, potentially high-value R&D. Through LDRD the Laboratory is able to improve its distinctive capabilities and enhance its ability to conduct cutting-edge R&D for its DOE and WFO sponsors. To meet the LDRD objectives and fulfill the particular needs of the Laboratory, ORNL has established a program with two components: the Director's R&D Fund and the Seed Money Fund. As outlined in Table 1, these two funds are complementary. The Director's R&D Fund develops new capabilities in support of the Laboratory initiatives, while the Seed Money Fund is open to all innovative ideas that have the potential for enhancing the Laboratory's core scientific and technical competencies. Provision for multiple routes of access to ORNL LDRD funds maximizes the likelihood that novel and seminal ideas with scientific and technological merit will be recognized and supported