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The Laser-Induced Forward Transfer (LIFT) technique [1] exists as a simple method for the direct-writing of a wide range of materials with sub-micron to 100s of microns feature sizes. A thin film of the material to be deposited (the donor) is coated onto one face of a transparent carrier substrate and transferred to another substrate (the receiver) placed some microns away by irradiating the carrier-donor interface through the carrier with a CW or pulsed laser. The process is inherently thermal in nature, with melting and ablation of the donor required to provide the necessary thrust for transferring material. Hence, the technique is not readily applied to the deposition of thermo-sensitive materials.

Measurement of spectral interference with a near-field scanning optical microscope is used to determine directly the variation of group velocity between modes of a planar slab waveguide as the modes propagate along the guide.

We describe the first non-destructive measurements of the evolution of the optical continuum along a non-linear waveguide. By utilizing near-field microscopy, the spectral variation can be imaged along and across the waveguide with sub-wavelength resolution.

We describe spectral measurements using NSOM of the nonlinear propagation of femtosecond pulses in waveguides. Nonlinearity produces self phase modulation, which broadens the spectrum of the propagating light as it travels along the guide. This process is the basis for supercontinuum sources that span octaves in frequency, which are important for metrology[1] and carrier-envelope phase measurement[2]. Our experiments uniquely measure the spectral evolution of this continuum as the light propagates along the waveguide by sampling the evanescent field with an uncoated NSOM tip, allowing measurement of the spectrum as it develops over long (mm) length scales, and also allowing the study of the submicron evolution of the spectra both along and across the guide. The evolution in one example is shown in figure 1(a). We will compare the measured continuum generation with our nonlinear propagation modeling, and discuss how the observed spectra differ from predictions of simple models.

The use of lasers to directly pattern optoelectronic devices primarily utilizes direct irradiation by UV light. We present here an alternative route using multi-photon absorption within a spherical focus in 3D space, thus allowing complex embedded structures to be directly written. In wide-bandgap materials such as chalcogenide, fluoride and silica glasses, our observations suggest free electrons are produced within the focus of a high-power infrared ultrashort pulse. The anisotropic interaction of this plasma with the incident pulse produces micron-sized DBR gratings of a 150nm pitch. An amplified Ti:S laser with 250kHz repetition rate, 150fs pulse duration, and wavelength tuned from 800-850nml is used to write embedded diffraction gratings and arrays of dots. The laser beam is focussed with a 50x objective into transparent polished samples, with pulse energies ranging from 0.1-1.1pJ (Fig.la). During the writing process broadband sub-bandgap UV light is emitted from a micron-sized spot at the sample focus. The written structures are permanent, typically with large refractive index changes on the order of Delta.n = +0.01 depending on the material.

Thermal poling of silica glass modified by femtosecond laser irradiation has been demonstrated. Evidence of second-order nonlinearity modulation between the treated and non-treated regions has been observed. The technique was used for χ(2) grating fabrication.

In recent years, the use of a focused femtosecond laser to directly write structures deep within transparent media has attracted much attention due to its ability to write in three-dimensions [1]. By utilizing an amplified Ti:sapphire laser (pulse duration 150fs, repetition rate 250kHz, λ=850nm), we have developed a novel technique that enables us to write grating structures within the bulk of an optical fiber through its cleaved face, allowing control over light subsequently exiting the fiber. Fig 1(a) shows a microscope image of an embedded diffraction grating having a 5µm pitch and buried 25µm below a fiber's cleaved face. Fig 1(b) displays a far-field pattern created by a single-mode fiber which has a two-dimensional grating written 5µm below its cleaved face. The controllability of the power and direction of diffracting orders offers tantalizing opportunities for new methods of optical routing. A further grating of 20µm period and 4µm line width has been directly-written into a silica glass plate (Herasil 1). Thermal poling was carried out on the plate in air for 15 minutes at 280°C with 4kV applied [2]. After poling the grating was investigated using a Nd:YAG laser (λ=1064nm), with a focused spot sue of 20µm. The second harmonic was subsequently imaged by CCD camera. Fig 2(a) shows the second harmonic produced in a Gaussian beam away from the grating region as expected. However, Fig 2(b) shows the second harmonic produced at the position of the irradiated lines to be much stronger indicating that the χ(2) is larger in the Ti:Sapphire irradiated regions. The incident beam is large enough to encompass two grating lines as indicated by the arrows. In Fig 2(a) the brightness is enhanced compared to Fig 2(b) to make the spot visible. By direct-writing periodic structures into silica fibers from the side and subsequent poling, work is currently underway to investigate the feasibility of achieving efficient quasi-phase-matching in silica fibres [3]. Fig.1. (a) A 5µm period diffraction grating written 25µm below a fiber's cleaved face. (b) Far-field pattern of single-mode light exiting a fiber containing an embedded 2-d grating. Fig.2. Imaging of the SHG (a) Inclined incidence and well away from the grating. (b) Inclined incidence and on the grating.

New phenomena of light scattering and Cherenkov third-harmonic generation peaking in the plane of polarization during direct writing with ultrashort light pulses in glass have been reported. These observations were unexpected because the scattering of polarized light in the plane of light polarization in isotropic medium such as glass is always weaker compared to the orthogonal plane, since a dipole does not radiate in the direction of its axis. The phenomena were interpreted in terms of angular distribution of photoelectrons and sub-wavelength anisotropic index inhomogenities. Another experiments demonstrated uniaxial birefringence of structures in fused silica written by femtosecond light pulses. The index change for light polarized along the direction of polarization of writing beam was much stronger than for the orthogonal polarization. The origin of this anisotropic phenomenon remained a mystery. Recently we observed a further anisotropic property in silica after being irradiated by a femtosecond laser - strong reflection from the modified region occurring only along the direction of polarization of the writing laser. We show this can arise from a self-organized periodic sub-wavelength refractive index modulation. We speculate they arise from a mechanism associated with the creation of a hot electron plasma by multiphoton absorption of incident light. Anisotropic index inhomogeneities are then induced by electrons moving along the direction of light polarization. Self-organized nanostructures are in turn produced by a pattern of interference between the incident laser radiation and a plasmon-polariton wave generated within the sample. Positive feedback leads to exponential growth of the periodic nanostructures in the plane of light polarization, which become frozen within the material. The femtosecond-laser-induced birefringence is therefore likely to be caused by these laterally-oriented small-period grating structures. Birefringence of this nature is well known as 'form' birefringence. Our analysis suggests that this effect is also the primary cause of all anisotropic phenomena reported in the experiments on direct writing with ultrashort pulses in glass. The anisotropic phenomena and micro-reflectors described here should be useful in many monolithic photonic devices and can be harnessed for information storage, MEMS applications or quasi-phase matching where nanoscale periodic structuring is required.

Recently, the use of a focused femtosecond laser to directly write devices into the bulk of glass has become increasingly prevalent [1]. The physical damage created by such lasers, especially in the high power regime has been identified to display anisotropic properties such as anomalous anisotropic light scattering [2] and uniaxial birefringence [3]. So far, the microscopic processes underlying such anisotropies remain unclear. By writing embedded structures with a Ti:sapphire laser (repetition rate 250kHz pulse duration 150fs, λ=850nm, and beam width ~1.5µm), we have identified a further anisotropic property in silica - strong reflection from the modified region occurring only along the direction of polarization of the writing laser. Our analysis suggests that this effect is also the primary cause of all previously reported anisotropic phenomena. Fig. 1, which shows a schematic of embedded diffraction gratings and an array of 'dots' written into a silica plate, demonstrates the geometry of the phenomenon. We believe that the anisotropic reflectivity can only be explained as a consequence of Bragg reflection from a self organized periodic structure, produced by interference between the incident laser radiation and a plasmon-polariton wave generated within the sample. This is demonstrated in the magnified region of Fig. 1 which shows the structure within a single 1.5µm 'dot'. Figure 2(a) shows a microscope image of the edge of an array of 'dots'. Here, the writing polarization is directed out of the page. The observed anisotropic reflectivity (1% per 1.5µm 'dot') can be compared to Figure 2(b) which shows no reflection from the same object, viewed from an orthogonal direction. The spectrum given in Fig 2(c) shows the peak of reflection to be 460nm, indicating that the self organized structures have a characteristic period of ~150nm, and modulating refractive index variation of amplitude Δn~10. Fig. 1. Anisotropic reflection from embedded structures. The magnified region illustrates the self-organized periodic structuring Fig.2. Reflection from a 2-d array of 'dots' in directions (a) parallel and (b) perpendicular to writing laser. (c) Spectrum of reflection.

In recent years, near-field microscopy has been utilized for assessing the properties of optical wave-guides at an increasing rate. Here, a Scanning Near-field Optical Microscope (SNOM) has been designed and constructed in order to expand this work into an analysis of the optical and structural properties of fibre Bragg gratings, which are used throughout the optical fibre telecommunications network. By imaging the evanescent fields of Bragg gratings, a characterization technique has been developed which has enabled the acquisition of sub-wavelength information about the optical field distribution within a fibre grating and its refractive index structure. Six separate fibre grating samples have been examined, demonstrating the feasibility of the developed scanning technique to become a useful characterization tool. In particular, the study has enabled grating standing wave fringes to be imaged relative to corresponding refractive index fringes, for the first time. The SNOM has also been utilized to map free-space diffraction patterns close to a phase-mask (transmission diffraction grating). The patterns are normally used to create fibre gratings via UV photosensitivity mechanisms. The field distributions have been imaged under various experimental conditions and have revealed some of the technical problems that might occur during the writing of gratings. The measured patterns have also served to confirm existing diffraction grating theory, which has been expanded during the course of this work to produce a new expression for the 'Talbot length', originally formulated by Rayleigh in 1881.

Epilepsy is a neurological disease, characterized by an enduring predisposition to generate epileptic seizures, that affects approximately 65 million people worldwide. While anti-epileptic drugs can suppress seizures, approximately 30 % of patients present with drug-resistant epilepsy, requiring alternative treatments. The etiology of epilepsy is diverse, ranging from genetic mutations, such as in mTORopathies due to mutations in the mammalian target of rapamycin (mTOR) signaling pathway, to acquired epilepsy in response to i.a. acute brain injuries. In recent years, overproduction of seizure-mediated reactive oxygen species and oxidative stress (OS) have been appreciated as important contributers to epilepsy. Therefore, we aimed to investigate OS, its relation to inflammation and iron metabolism, and its contribution to cellular damage and altered neuronal circuitry in epileptogenesis and epilepsy.. In cortical brain tissue from the mTORopathies focal cortical dysplasia type 2 and tuberous sclerosis complex we identified a strong interconnection between OS and neuroinflammation, which was mediated by the ROS-sensitive transcription factor SPI1/PU.1 in cells with mTOR activation. Moreover, we discovered that pro-inflammatory factors produced by cells with mTOR activation may attract peripheral immune cells to the brain, exacerbating neuroinflammation. Additionally, we found chronic activation of the antioxidant transcription factor NRF2 via the inflammation-induced transcriptional modulator microRNA 155 which promotes release of pro-oxidant iron. Altered iron metabolism was also identified in the hippocampus of patients with temporal lobe epilepsy and accompanied by oxidative damage and overexpression of neuroinflammatory factors. We conclude that therapies targeting OS, neuroinflammation and/or iron metabolism warrant further exploration as novel epilepsy treatments.