Three-dimensional femtosecond laser nanolithography of crystals

Nanostructuring hard optical crystals has so far been exclusively feasible at their surface, as stress induced crack formation and propagation has rendered high precision volume processes ineffective. We show that the inner chemical etching reactivity of a crystal can be enhanced at the nanoscale by more than five orders of magnitude by means of direct laser writing. The process allows to produce cm-scale arbitrary three-dimensional nanostructures with 100 nm feature sizes inside large crystals in absence of brittle fracture. To showcase the unique potential of the technique, we fabricate photonic structures such as sub-wavelength diffraction gratings and nanostructured optical waveguides capable of sustaining sub-wavelength propagating modes inside yttrium aluminum garnet crystals. This technique could enable the transfer of concepts from nanophotonics to the fields of solid state lasers and crystal optics.Comment: Submitted Manuscript and Supplementary Informatio

Ultrashort filaments of light in weakly-ionized, optically-transparent media

Modern laser sources nowadays deliver ultrashort light pulses reaching few cycles in duration, high energies beyond the Joule level and peak powers exceeding several terawatt (TW). When such pulses propagate through optically-transparent media, they first self-focus in space and grow in intensity, until they generate a tenuous plasma by photo-ionization. For free electron densities and beam intensities below their breakdown limits, these pulses evolve as self-guided objects, resulting from successive equilibria between the Kerr focusing process, the chromatic dispersion of the medium, and the defocusing action of the electron plasma. Discovered one decade ago, this self-channeling mechanism reveals a new physics, widely extending the frontiers of nonlinear optics. Implications include long-distance propagation of TW beams in the atmosphere, supercontinuum emission, pulse shortening as well as high-order harmonic generation. This review presents the landmarks of the 10-odd-year progress in this field. Particular emphasis is laid to the theoretical modeling of the propagation equations, whose physical ingredients are discussed from numerical simulations. Differences between femtosecond pulses propagating in gaseous or condensed materials are underlined. Attention is also paid to the multifilamentation instability of broad, powerful beams, breaking up the energy distribution into small-scale cells along the optical path. The robustness of the resulting filaments in adverse weathers, their large conical emission exploited for multipollutant remote sensing, nonlinear spectroscopy, and the possibility to guide electric discharges in air are finally addressed on the basis of experimental results.Comment: 50 pages, 38 figure

Nonlinear increase, invisibility, and sign inversion of a localized fs-laser-induced refractive index change in crystals and glasses

Multiphoton absorption via ultrafast laser focusing is the only technology that allows a three-dimensional structural modification of transparent materials. However, the magnitude of the refractive index change is rather limited, preventing the technology from being a tool of choice for the manufacture of compact photonic integrated circuits. We propose to address this issue by employing a femtosecond-laser-induced electronic band-gap shift (FLIBGS), which has an exponential impact on the refractive index change for propagating wavelengths approaching the material electronic resonance, as predicted by the Kramers–Kronig relations. Supported by theoretical calculations, based on a modified Sellmeier equation, the Tauc law, and waveguide bend loss calculations, we experimentally show that several applications could take advantage of this phenomenon. First, we demonstrate waveguide bends down to a submillimeter radius, which is of great interest for higher-density integration of fs-laser-written quantum and photonic circuits. We also demonstrate that the refractive index contrast can be switched from negative to positive, allowing direct waveguide inscription in crystals. Finally, the effect of the FLIBGS can compensate for the fs-laser-induced negative refractive index change, resulting in a zero refractive index change at specific wavelengths, paving the way for new invisibility applications

Optical Propagation of Self-sustaining Wavefronts and Nonlinear Dynamics in Parabolic Multimode Fibers

The aim of this thesis is to introduce my work which has generally been focused on optical wavefronts that have the unusual property of resisting commonplace phenomena such as diffraction and dispersion. Interestingly, these special beams are found both in linear and nonlinear situations. For example, in the linear regime, localized spatio-temporal waves which resemble the spherical harmonic symmetries of the hydrogen quantum orbitals can simultaneously negotiate both diffractive and dispersive effects. In the nonlinear regime, dressed optical filaments can be arranged to propagate multi-photon produced plasma channels orders of magnitude longer than expected. The first portion of this dissertation will begin by surveying the history of diffraction-free beams and introducing some of their mathematical treatments. Interjected throughout this discussion will be several relevant concepts which I explored during my first years a CREOL. The discussion will then be steered into a detailed account of diffraction/dispersion free wavefronts which display hydrogen-like symmetries. The second segment of the document will cover the highly nonlinear process of optical filamentation. This chapter will almost entirely investigate the idea of the dressed filament, an entity which allows for substantial prolongation of this light string. I will then conclude by delving into the topic of supercontinuum generation in parabolic multimode fibers which, in the upcoming years, has great potential of becoming important in optics

Rapid high-resolution mid-IR imaging for molecular spectral histopathological diagnosis of oesophageal cancers

This thesis is written as part of Marie-Curie international training network called Mid-TECH. Mid-TECH is devoted to improve mid-infrared (MIR) technologies and consists of 15 PhD projects across European universities. This thesis aims to evaluate new technologies and concepts developed by the project partners for their applicability in a biomedical setting. The clinical problem to diagnose oesophageal cancers serves as an example case for this. The thesis consists of three projects all aimed to further the understanding of MIR hyperspectral imaging. The first project discussed in chapter 5 demonstrates the use of an new design of the United States Airforce resolution test chart. The new test chart is developed to evaluate spatial resolution of MIR hyperspectral imaging systems. The use of different materials is discussed and the new iteration of the thes chart is evaluated using a state of the art MIR imaging system. The second project discussed in chapter 6 evaluates the technical differences and their practical implications of discrete frequency MIR imaging systems compared to continuum source systems. A comparison of the two system types is drawn for imaging paraffin embedded sections of oesophageal tissue. Furthermore the effect of chemically removing the paraffin from the sample is compared to a mathematical correction algorithm. The system performance is compared based on their ability to differentiate healthy from cancerous tissue. The third project discussed in chapter 7 evaluates the potential of a new MIR detection scheme called upconversion in combination with a novel MIR laser source. It is a prove of concept study demonstrating that those two technologies can be deployed to do hyperspectral imaging in the MIR.European Commissio

Hawking radiation and mode conversion at optically induced horizons.

The scattering of electromagnetic radiation is an essential and fundamental tool by which we may study light-matter interaction. In usual conditions, the medium is considered as stationary and scattering is characterized by a change in the wavevector spectrum of the light beam. The frequency of the beam is not affected. In this thesis we study the problem of scattering from a moving dielectric perturbation (DP) induced by the nonlinear Kerr effect. Light is scattered and resonantly transferred to two output modes identified by distinct frequencies, one positive and the other negative in the comoving reference frame. Experiments confirm generation of negative resonant radiation in a variety of settings, ranging from optical fibers to bulk Kerr media. A first Born approximation analysis, verified by numerical simulations, predicts that the mixing of the positive and negative modes during the scattering process leads to amplification at the expense of the DP. This provides a link with the spontaneous emission of photons, known as “Hawking radiation”, excited at the event horizon of a gravitational black hole. The moving DP is thus described using the tools of transformation optics and general relativity, in terms of a flowing medium which curves the effective space-time metric as seen by the light rays. Our experimental results show evidences of spontaneous photon emission by the analogue horizon associated to the moving DP, which are in quantitative agreement with the Hawking model

Mid-infrared integrated photonic sensors of water and ice films for harsh environments

Quan una aeronau travessa núvols situats a mitja altitud que estan principalment compostos d'incomptables gotes d'aigua en subfusió, corre el risc d’engelament en les seves parts més exposades. Fins i tot una capa submilimètrica de gel formada sobre elements com ales, motors o tubs de pitot pot reduir dràsticament la seva sustentació i rendiment i, per tant, afectar a la seva eficiència i maniobrabilitat fins al punt de causar una pèrdua de control i posterior accident. Les tecnologies anti-gel instal·lades en aeronaus modernes busquen reduir aquest problema mitjançant sistemes que no solament fonguin el gel acumulat si no que ho detectin abans que resulti perillós. En aquesta Tesi es presenta un nou i ultrasensible, però a la vegada robust, sensor de gel basat en fotònica integrada d’IR-mitjà que pot ser muntat en qualsevol superfície sense pèrdua aerodinàmica. A partir de la detecció de canvis en les característiques de l'aigua a l’IR-mitjà que estan associats intrínsecament a variacions en la seva estructura molecular per a les seves diferents fases, es demostra la detecció de gel amb alta sensibilitat i rapidesa per aconseguir una alerta prematura d’engelament en vol. Al llarg d'aquesta Tesi, es tracta, des de les perspectives fonamental i aplicada, el desenvolupament complet del sensor abastant des de la concepció teòrica fins a la demonstració experimental del seu rendiment i fiabilitat en un túnel de vent. Finalment, es presenta un segon sensor amb millor sensibilitat a costa d'una menor robustesa per a la detecció de petits volums de contaminants químics en aigua. El sensor aprofita l'excitació d'un plasmó superficial a l’IR-mitjà que intensifica la interacció entre llum i anàlit al voltant de ressonàncies moleculars. Això emfatitza encara més els enormes beneficis que operar a l’IR-mitjà pot aportar a futurs sensors fotònics integrats.Al atravesar una aeronave nubes situadas a media altitud que están principalmente compuestas de incontables gotas de agua en subfusión, corre el riesgo de engelamiento en sus partes más expuestas. Incluso una capa submilimétrica de hielo formada sobre elementos como alas, motores o tubos de pitot puede reducir drásticamente su sustentación y rendimiento y, por tanto, afectar a su eficiencia y maniobrabilidad hasta el punto de causar una pérdida de control y posterior accidente. Las tecnologías anti-hielo instaladas en aeronaves modernas buscan reducir este problema mediante sistemas que no solo derritan el hielo acumulado si no que lo detecten antes de que resulte peligroso. En esta Tesis se presenta un nuevo y ultrasensible, pero a su vez robusto, sensor de hielo basado en fotónica integrada de IR-medio que puede ser montado en cualquier superficie sin apenas pérdida aerodinámica. A partir de la detección de cambios en los rasgos característicos del agua en el IR-medio que están asociados intrínsecamente a variaciones en su estructura molecular para sus diferentes fases, se demuestra la detección de hielo con alta sensibilidad y rapidez para conseguir una alerta temprana de engelamiento en vuelo. A lo largo de esta Tesis, se trata, desde las perspectivas fundamental y aplicada, el desarrollo completo del sensor abarcando desde la concepción teórica hasta la demonstración experimental de su rendimiento y fiabilidad en un túnel de viento. Por último, se presenta un segundo sensor con mejor sensibilidad a costa de una robustez reducida para la detección de pequeños volúmenes de contaminantes químicos en agua. El sensor aprovecha la excitación de un plasmón superficial en el IR-medio que intensifica la interacción entre luz y analito alrededor de resonancias moleculares. Esto enfatiza más si cabe los enormes beneficios que operar el IR-medio puede aportar a futuros sensores fotónicos integrados.Al atravesar una aeronave nubes situadas a media altitud que están principalmente compuestas de incontables gotas de agua en subfusión, corre el riesgo de engelamiento en sus partes más expuestas. Incluso una capa submilimétrica de hielo formada sobre elementos como alas, motores o tubos de pitot puede reducir drásticamente su sustentación y rendimiento y, por tanto, afectar a su eficiencia y maniobrabilidad hasta el punto de causar una pérdida de control y posterior accidente. Las tecnologías anti-hielo instaladas en aeronaves modernas buscan reducir este problema mediante sistemas que no solo derritan el hielo acumulado si no que lo detecten antes de que resulte peligroso. En esta Tesis se presenta un nuevo y ultrasensible, pero a su vez robusto, sensor de hielo basado en fotónica integrada de IR-medio que puede ser montado en cualquier superficie sin apenas pérdida aerodinámica. A partir de la detección de cambios en los rasgos característicos del agua en el IR-medio que están asociados intrínsecamente a variaciones en su estructura molecular para sus diferentes fases, se demuestra la detección de hielo con alta sensibilidad y rapidez para conseguir una alerta temprana de engelamiento en vuelo. A lo largo de esta Tesis, se trata, desde las perspectivas fundamental y aplicada, el desarrollo completo del sensor abarcando desde la concepción teórica hasta la demonstración experimental de su rendimiento y fiabilidad en un túnel de viento. Por último, se presenta un segundo sensor con mejor sensibilidad a costa de una robustez reducida para la detección de pequeños volúmenes de contaminantes químicos en agua. El sensor aprovecha la excitación de un plasmón superficial en el IR-medio que intensifica la interacción entre luz y analito alrededor de resonancias moleculares. Esto enfatiza más si cabe los enormes beneficios que operar el IR-medio puede aportar a futuros sensores fotónicos integrados