We present experimentally determined photodisruption enhancement of 50 nm gold spheres irradiated with 780 nm 180 fs pulses using a pump-probe scattering system. Our results indicate a 300 nm cavitation bubble threshold reduction of 31 times when compared to an ultra-pure water base control solution. We utilize a method of matching time between bubble initiation in a continually circulated nanoparticle solution with theoretical focal volume size dependent time between particle-focal volume collision events based on simple particle kinetics. We propose the observed photodisruption is due to electrostatic particle ablation kinetics due to electron photoemission. We apply the Fowler-Dubridge theory for photoemission to nanospheres experiencing strong near-field enhancement to describe particle electric fields induced by non-zero particle charge densities resulting from emitted electrons. An apparent ultra-energy efficient photodisruption mechanism results from multiphoton emission processes in the sub 100 femtosecond pulse regime exceeding typical methods utilizing explosive boiling. In the process of explaining experimental results, we develop a near complete picture of nanoparticle mediated photodisruption as a function of identified relevant system non-dimensional groups and particle enhancement. These results may be used to guide the selection of laser and particle parameters for imaging and different photodisruption regimes. / text

This project explores optimization of two-photon excited fluorescence (TPEF) enhancement between tunable narrowband and un-tuned broadband femtosecond (fs) laser pulse excitations for two-photon microscopy (TPM). The research is conducted preliminarily in time domain and comprehensively in frequency domain to understand the physics behind TPEF enhancement by un-tuned sub-10 fs nearly transform-limited pulse (TLP) versus tunable 140 fs pulse. The preliminary study on inverse proportionality of TPEF yield to fs-pulse duration delimits a general lower-bound to narrowband fs-pulse regime (pulse duration > 40 fs) with assumption of dye-molecule frequency invariant response. Deviations from this inverse proportionality in broadband fs-pulse regime (pulse duration < 40 fs) highlights dye-molecule frequency variant response, necessity of group delay dispersion (GDD) compensation, and broadband TLP for TPEF enhancement. The follow-up comparative study is made on un-tuned sub-10 fs TLP versus tunable 140 fs pulse excitations using three dye-phantoms (Indo-1, FITC, and TRITC) representative of fluorescent probes with similar TPEF characteristics. The integrated experimental system, with custom-designed GDD compensation, dispersion-less laser-beam expanding and focusing, and compound-lens for efficient fluorescence collection with good spectral resolution, ensures accurate TPEF measurements. Differentiated TPEF enhancements of Indo-1 (1.6), FITC (6.7), and TRITC (5.2) proportionally agree with calculated ones due to the overlap of fs-pulse second harmonic (SH) power spectrum with dye-molecule two-photon excitation (TPE) spectrum. Physically speaking, with broadband sub-10 fs TLP readily involved in both degenerate (v1 = v2) and non-degenerate (v1 ≠ v2) two-photon absorption (TPA), this un-tuned ultrashort fs-pulse excitation simultaneously allows for more accessibility to TPA-associated final states and diversely promotes population of thus excited dye-molecules with the three dye-phantoms. Under environmental influences (mutual quenching through one-photon absorption(s) and solvent effect), multicolor TPEF enhancement observed from a mixture of the three dyes shows promise of sub-10 fs TLP as simultaneous excitation for multiple-dye labeled samples in contrast to compromised excitation with narrowband fs-pulse tuning. Both single- and multicolor TPEF enhancements clarify tradeoff between tunability of narrowband fs-pulse and un-tuned broadband fs-pulse excitations, being instructive to further considerations on optimization of TPEF enhancement by strategic utilization of broadband fs-pulse for better performance of TPM.

Thesis (M. S.)--Physics, Georgia Institute of Technology, 2009. / Committee Chair: Rick Trebino; Committee Member: Jennifer Curtis; Committee Member: John Buck; Committee Member: Mike Chapman; Committee Member: Stephen Ralph.

Thesis (Ph.D.) - University of St Andrews, November 2008.

Thesis (Ph.D.) - University of St Andrews, November 2009. / Restricted until 5th November 2011.

Graphene is a single hexagonal atomic carbon layer. Since its discovery, graphene is emerging as an exciting and promising new material to impact various areas of fundamental research and technology. It has potentially useful electrical properties for device applications such as graphene photodetectors and graphene-based sensors. This thesis focuses on the femtosecond laser processing of graphene from both scientific and industrial points of view. Started from the manufacturing process, a new manufacturing route for graphene devices based on a femtosecond laser system is explored. In this thesis, the graphene ablation threshold was determined in the range of 100 mJ/cm2. In this deposited fluence range, selective removal of graphene was achieved using femtosecond laser processing with little damage to the SiO2 /Si substrate. This finding supports the feasibility of direct patterning of graphene for silicon-substrate field effect transistors (FETs) as the gate dielectric, silicon dioxide is only negligibly removed (2~10 nm) and no damage occurs to the silicon. Beyond the selective removal of graphene, the effects of exposing femtosecond laser pulses on a monolayer of graphene deposited on a SiO2/Si substrate is also studied under subthreshold irradiation conditions. It has been demonstrated that a femtosecond laser can induce defects on exposure. The dependence of the D, G, and 2D Raman spectrum lines on various laser pulse energies was evaluated using Raman Spectroscopy. The I (D)/I (G) ratio was seen to increase with increasing laser energy. The increase in the D’ (intravalley phonon and defect scattering) peak at 1620 cm-1 appeared as defective graphene. These findings provide an opportunity for tuning graphene properties locally by applying femtosecond laser pulses. Applications might include p-n junctions, and the graphene doping process. To explore the power absorption process in graphene and the SiO2/Si substrate, a theoretical model was developed based on the transfer-matrix method. The results revealed that the most significant absorption was in the silicon substrate. The light reflection form each layer was considered. The model shows the temperature oscillations are more significant in the silicon layer compared to the silicon dioxide which can provide a theoretical rationale for the swelling effect observed in the experiments. This model can assist in the choice of laser parameters chosen for future laser systems used in the production of graphene devices.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第15385号 / 工博第3264号 / 新制||工||1491(附属図書館) / 27863 / 京都大学大学院工学研究科材料化学専攻 / (主査)教授 平尾 一之, 教授 横尾 俊信, 教授 田中 勝久 / 学位規則第4条第1項該当

Kyoto University (京都大学) / 0048 / 新制・論文博士 / 博士(工学) / 乙第12459号 / 論工博第4041号 / 新制||工||1497(附属図書館) / 28069 / (主査)教授 平尾 一之, 教授 横尾 俊信, 教授 田中 勝久 / 学位規則第4条第2項該当

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第16865号 / 工博第3586号 / 新制||工||1542(附属図書館) / 29540 / 京都大学大学院工学研究科材料化学専攻 / (主査)教授 平尾 一之, 教授 三浦 清貴, 教授 田中 勝久 / 学位規則第4条第1項該当

Femtosecond laser interaction with dielectrics has unique characteristics for micromachining, notably non-thermal interaction with materials, precision and flexibility. The nature of this interaction is highly nonlinear due to multiphoton ionization, so the laser energy can be nonlinearly absorbed by the material, leading to permanent change in the material properties in a localized region of Mu-m3. This dissertation demonstrated the potential of these nonlinear interactions induced changes (index modification and ablation for machining) in the dielectrics and explored several practical applications. We studied femtosecond laser ablation of Poly-methayl methacrylate (PMMA) under single and multiple pulse irradiation regimes. We demonstrated that the onset of surface ablation in dielectric surface is associated with surface swelling, followed by material removal. Also, the shape of the ablation craters becomes polarization dependent with increasing fluence, except for circular polarization. The morphology of the damaged/ablated material was examined by optical and scanning electron microscopy. The dynamics of laser ablation of PMMA was simulated using a 2 dimensional Molecular Dynamics model and a 3 dimensional Finite Difference Time Domain model. The results from numerical simulations agreed well with experimental results presented in this thesis. We also demonstrated the formation of nano-pillar within the ablation crater when the surface of bulk-PMMA was irradiated by two femtosecond pulses at a certain delay with energies below single shot ablation threshold. With increasing fluence, the nano-pillar vanished and the structure within the ablation crater resembled volcanic eruption. At higher fluences we demonstrated nanoscale porosity in PMMA. For application, a novel in-line fiber micro-cantilever was fabricated in bend insensitive fiber, that provides details of in-line measurement of frequency and amplitude of vibration, and can be further extended to be used as chemical/bio and temperature sensors. By modifying the refractive index at random spacing within the single mode fiber core, a unique quasi-random micro-cavities fiber laser was fabricated, which exhibits comparable characteristics with a commercial fiber laser in terms of narrow linewidth and frequency stability.