Recently, with the escalating demands for optical communications, the need for bandwidth in optical communication network has increased. The technology breakthrough in dry fiber fabrication opens the possibility for fiber bandwidth from 1.3 to 1.6 £gm. Cr4+:YAG double-clad crystal fiber (DCF) grown by the co-drawing laser-heated pedestal growth method has a strong spontaneous emission spectrum from 1.3 to 1.6 £gm. Such fiber is, therefore, eminently suitable for broadband optical amplifier, amplifier spontaneous emission (ASE) light source, tunable solid-state laser, and optical coherence tomography (OCT) applications. In this thesis, multilayer dielectric thin films were directly deposited by E-gun coating onto the end faces of the heterostructure Cr4+:YAG DCF. In this way we have successfully improved the extracted ASE power by the high reflection (HR) coatings. The backward ASE in the fiber reflected and propagates with gain through the fiber in the forward direction. In dual-pump scheme, as much as 1.7 mW power (DCF length is 9.5 cm) of collimated output ASE was achieved. The dual-pump scheme and HR thin films provided 1.6 time improvements of the ASE output power. For broadband optical amplifier in dual-pump and double-pass scheme, a 3.7-dB gross gain and a 0.7-dB net loss (DCF length is 8.7 cm) at 1.4-£gm signal wavelength have been successfully developed with HR coatings onto one of the Cr4+:YAG DCF end faces. In addition, we have successfully developed the Cr4+:YAG DCF fiber laser by direct HR coatings onto fiber end faces. A record-low threshold of 96 mW (DCF length is 1.6 cm) with a slope efficiency of 6.9% was achieved at room temperature. It is more than four times lower than any previously reported Cr4+:YAG lasers.

Surface plasmon-polaritons are optical surface waves formed through the interaction of photons with free electrons at the surface of metals. They offer interesting applications in a broad range of scientific fields such as physics, chemistry, biology, and material science. However, many of such applications face limitations imposed by the high propagation losses of these waves at visible and near-infrared wavelengths, which result mainly from power dissipation in the metal. In principle, the propagation losses of surface plasmon-polaritons can be compensated through optical amplification. The objective of this thesis is to provide deeper insights on the physics of surface plasmon-polariton amplification and spontaneous emission in surface plasmon-polariton amplifiers through theoretical and experimental vehicles applied (but not necessarily restricted) to a particular plasmonic mode termed long-range surface plasmon-polariton. On the theoretical side, the objective is approached by developing a realistic theoretical model to describe the small-signal amplification of surface plasmon-polaritons in planar structures incorporating dipolar gain media such as organic dye molecules, rare-earth ions, and quantum dots. This model takes into account the inhomogeneous gain distribution formed near the metal surface due to a non-uniform excitation of dipoles and due to a position-dependent excited-state dipole lifetime that results from near-field interactions between the excited dipoles and the metal. Also, a theoretical model to describe the amplified spontaneous emission of surface plasmon-polaritons supported by planar metallic structures is developed. This model takes into account the different energy decay channels into which an exited dipole located in the vicinity of the metal can relax. The validity of this model is confirmed through experimentation. On the experimental side, the objective is approached by providing a direct experimental demonstration of complete loss compensation in a plasmonic waveguide. The experiments are conducted using the long-range surface plasmon-polariton supported by a symmetric thin gold waveguide incorporating optically pumped organic dye molecules in solution as the gain medium. Also, an experimental study of spontaneous emission in a long-range surface plasmon-polariton amplifier is presented. It is shown that this amplifier benefits from a low spontaneous emission into the amplified mode, which leads to an optical amplifier with low noise characteristics. The experimental setup and techniques are explained in detail.

京都大学 / 新制・課程博士 / 博士(工学) / 甲第23160号 / 工博第4804号 / 新制||工||1751(附属図書館) / 京都大学大学院工学研究科分子工学専攻 / (主査)教授 関 修平, 教授 今堀 博, 教授 梶 弘典 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

The process of spontaneous emission can be dramatically modified by optical micro- and nanostructures. We studied the modification of fluorescence dynamics using a polymer spacer layer fabricated through layer-by-layer (LbL) self-assembly. The advantages of this method are numerous: The self-assembled spacers can possess exceptional smooth surface morphology; The thickness of the spacer can be controlled with nanometer accuracy; And depending on fabrication conditions, the spacer layer is stimuli responsive and its thickness can be dynamically tuned. This thesis contains three interlinked components. First, we vary LbL spacer layer thickness and explore the change in fluorescence lifetime induced by the modified photonic density of states (PDOS), i.e., Purcell effects. Our experimental results agree well with theoretical predictions based on a classical dipole model, which also yields consistent values for the fluorophores' intrinsic fluorescence lifetime and quantum yield near a dielectric as well as a plasmonic interface. Based on this observation, we further demonstrate that self-assembled fluorophores can be used to probe the modified PDOS near optical micro- and nano-structures. These results naturally lead to the second component of our research. In particularly, based on the PDOS-induced changes in fluorescent lifetime, we develop a non-contact method that can measure morphological changes with nanoscale resolution. Our method relies on quantitatively linking fluorophore position with PDOS, and is validated through direct comparison with ellipsometry and atomic force microscopy (AFM) measurements. To demonstrate the potential application of this method, we investigated the swelling/deswelling of LbL films induced by pH changes. Our results indicate significant difference between a LbL film composed of a single polymer monolayer and a LbL film with 3 monolayers. Such stimuli-responsive polymers can be used to construct active and tunable plasmonic nano-devices. As a proof-of-principle demonstration, we experimentally confirm that it is possible to utilize the swelling/deswelling behavior of stimuli-responsive films to dynamically control the separation between Au nanoparticles and Texas Red (TR) dyes. This result is based on the strong correlation of TR fluorescence lifetime and nanoparticles-TR separation. Finally, we investigate the impact of different lithography processes on the fluorescence properties of self-assembled fluorophores. We consider three methods: direct fluorophore patterning through ultraviolet (UV) ablation, focused ion beam (FIB) milling of self-assembled fluorophores, and self-assembly of fluorescent materials over plasmonic nano-patterns. / Ph. D.

No description available.

Surface plasmon-polaritons are optical surface waves formed through the interaction of photons with free electrons at the surface of metals. They offer interesting applications in a broad range of scientific fields such as physics, chemistry, biology, and material science. However, many of such applications face limitations imposed by the high propagation losses of these waves at visible and near-infrared wavelengths, which result mainly from power dissipation in the metal. In principle, the propagation losses of surface plasmon-polaritons can be compensated through optical amplification. The objective of this thesis is to provide deeper insights on the physics of surface plasmon-polariton amplification and spontaneous emission in surface plasmon-polariton amplifiers through theoretical and experimental vehicles applied (but not necessarily restricted) to a particular plasmonic mode termed long-range surface plasmon-polariton. On the theoretical side, the objective is approached by developing a realistic theoretical model to describe the small-signal amplification of surface plasmon-polaritons in planar structures incorporating dipolar gain media such as organic dye molecules, rare-earth ions, and quantum dots. This model takes into account the inhomogeneous gain distribution formed near the metal surface due to a non-uniform excitation of dipoles and due to a position-dependent excited-state dipole lifetime that results from near-field interactions between the excited dipoles and the metal. Also, a theoretical model to describe the amplified spontaneous emission of surface plasmon-polaritons supported by planar metallic structures is developed. This model takes into account the different energy decay channels into which an exited dipole located in the vicinity of the metal can relax. The validity of this model is confirmed through experimentation. On the experimental side, the objective is approached by providing a direct experimental demonstration of complete loss compensation in a plasmonic waveguide. The experiments are conducted using the long-range surface plasmon-polariton supported by a symmetric thin gold waveguide incorporating optically pumped organic dye molecules in solution as the gain medium. Also, an experimental study of spontaneous emission in a long-range surface plasmon-polariton amplifier is presented. It is shown that this amplifier benefits from a low spontaneous emission into the amplified mode, which leads to an optical amplifier with low noise characteristics. The experimental setup and techniques are explained in detail.

The work described in this thesis involves the measurements of the optical gain and amplified spontaneous emission (A.S.E) spectrum of Pb1-xSnxTe epilayers and the establishment of conditions under which optically pumped Pb1-xSnxTe laser using a CO2 laser as pump source can be produced. Pb1-xSnxTe epilayers have been grown by a hot wall epitaxy (HWE) technique on BaF2 single crystal substrates and the optical gain which can be produced in these layers has been measured by pumping the films transversely with a N2 laser. A model for optical gain and stimulated emission as a function of pump intensity has been developed which has permitted for the first time in these materials, a direct comparison between the magnitude of the gain pumping rate, and the optical gain generated. The measured optical gain is in very good agreement with the model predicted gain. Good fits to the measured stimulated emission spectra were also obtained from the model prediction. It is shown that the gain for a given pump wavelength has a drastic dependence on the material doping density. According to the model, CO2 laser optically pumped Pb1-xSnxTe laser can be readily achieved, provided that epilayer doping densities can be reduced to values of 1017 cm-3 or less. Nevertheless, doping densities even in nominally undoped layers are generally at least an order of magnitude too high. In some initial attempts to achieve lower doping densities, using a thermal annealing technique, doping densities as low as 2 x 10 17 cm-3 have been obtained and significant pump absorption was achieved at CO2 laser wavelength, as predicted by the model. / Thesis / Master of Engineering (ME)

Narrow silicon nanowires host a rich set of physical phenomena. Understanding these phenomena will open new opportunities for applications of silicon nanowires in optoelectronic devices and adds more functionality to silicon especially in those realms that bulk silicon may not operate remarkably. Compatibility of silicon nanowires with the mainstream fabrication technology is also advantageous. The main theme of this thesis is finding the possibility of using silicon nanowires in light sources; laser and light emitting diodes. Using Tight Binding (TB) and ab-initio Density Functional Theory (DFT) methods it was shown that axial strain can induce significant changes in the effective mass, density of states and bandgap of silicon nanowires. Generality of the observed effects was proven by investigating nanowires of different crystallography, diameter and material (e.g. germanium nanowires). The observed direct to indirect bandgap conversion suggests that strain is able to modulate the light emission properties of silicon nanowires. To investigate this possibility, spontaneous emission time was formulated using perturbation theory including Longitudinal Optical (LO) and Acoustic (LA) phonons. It was observed that corresponding to bandgap conversion, the spontaneous emission time can be modulated by more than one order of magnitude. This emanates from bandgap conversion and symmetry change of wave function in response to strain. A mechanism for population inversion was proposed in the thesis which is based on the Ensemble Monte Carlo (EMC) study of carrier statistics in direct and indirect conduction sub bands. By calculating all possible electron-phonon scattering mechanisms which may deplete the already populated indirect subband, it was shown that at different temperatures and under different electric fields there is a factor of 10 difference between the population of indirect and direct sub bands. This suggests that population inversion can be achieved by biasing an already strained nanowire in its indirect bandgap state. The light emission is possible then by releasing or inverting the strain direction. A few ideas of implementing this experiment were proposed as a patent application. Furthermore the photo absorption of silicon nanowires was calculated using TB method and the role of diameter, optical anisotropy and strain were investigated on band-edge absorption.

Narrow silicon nanowires host a rich set of physical phenomena. Understanding these phenomena will open new opportunities for applications of silicon nanowires in optoelectronic devices and adds more functionality to silicon especially in those realms that bulk silicon may not operate remarkably. Compatibility of silicon nanowires with the mainstream fabrication technology is also advantageous. The main theme of this thesis is finding the possibility of using silicon nanowires in light sources; laser and light emitting diodes. Using Tight Binding (TB) and ab-initio Density Functional Theory (DFT) methods it was shown that axial strain can induce significant changes in the effective mass, density of states and bandgap of silicon nanowires. Generality of the observed effects was proven by investigating nanowires of different crystallography, diameter and material (e.g. germanium nanowires). The observed direct to indirect bandgap conversion suggests that strain is able to modulate the light emission properties of silicon nanowires. To investigate this possibility, spontaneous emission time was formulated using perturbation theory including Longitudinal Optical (LO) and Acoustic (LA) phonons. It was observed that corresponding to bandgap conversion, the spontaneous emission time can be modulated by more than one order of magnitude. This emanates from bandgap conversion and symmetry change of wave function in response to strain. A mechanism for population inversion was proposed in the thesis which is based on the Ensemble Monte Carlo (EMC) study of carrier statistics in direct and indirect conduction sub bands. By calculating all possible electron-phonon scattering mechanisms which may deplete the already populated indirect subband, it was shown that at different temperatures and under different electric fields there is a factor of 10 difference between the population of indirect and direct sub bands. This suggests that population inversion can be achieved by biasing an already strained nanowire in its indirect bandgap state. The light emission is possible then by releasing or inverting the strain direction. A few ideas of implementing this experiment were proposed as a patent application. Furthermore the photo absorption of silicon nanowires was calculated using TB method and the role of diameter, optical anisotropy and strain were investigated on band-edge absorption.

In this work, we study the nonlinear propagation of light in liquid crystals (LCs) and the optical gain provided by LCs when they are polymer- or dye-doped.We will focus on nematic LCs, which are characterized by a mean orientation (also called director) of the elongated molecules and by a subsequent birefringence. After a general introduction on LCs, we focus on the nonlinear propagation of light in nematic LCs, and in particular the soliton-like propagation (nematicon). Indeed, if the light injected in the cell is intense enough, it can create a waveguide that counteracts the diffraction of the light. The light then propagates with an almost constant (or periodic) transverse profile.Our contribution to the subject starts with the numerical modeling of the thermal noise that characterizes the nematic LCs and the study of spatial instabilities of the soliton propagation caused by that noise. In Ch.3 we show that, by explicitly implementing the spatial correlation of the director in the LC thermal noise, it is possible to reproduce some of the features that characterize the LC response, such as the speckle generation or the fluctuating trajectory of the spatial optical soliton in LCs. Indeed, when the nematicon diameter is of the same order ofmagnitude as or smaller than the refractive index perturbations caused by the thermal noise, the nematicon starts to fluctuate in space. These fluctuations are not present when the noise is not correlated, indicating that the long-range interactions in LCs are crucial to explain the fluctuations. The model also allows us to introduce the propagation losses experienced by the nematicon without the use of an ad-hoc term. The simulations are in agreement with the experimental results. This method could also help the modeling of complex nonlinear phenomena in LCs that rely on noise, such as modulation instabilities or filamentation.Then, the optical gain is included in the LCs by dissolving photoluminescent polymers or dyes in it. In particular, we show that a particular polymer, the polyfuorene, when dissolved in nematic LCs, creates an intricate supramolecular pattern composed by homogeneous LC-rich regions surrounded by polymer-rich boundaries. The study of these structures through an ultra-fast spectroscopic technique (the pump-probe technique) and confocal microscopy reveals that the boundaries are composed by ordered and isolated chains of polymers. This particular morphology allows the observation of the optical gain from an oxidized unit of the polymeric chain (keto defects). This signal is usually covered by the absorption caused by the chain aggregation in solid state samples, while in LCs it is clearly visible. The optical gain from the keto defects appears also to be polarized orthogonal to the LC director, which is also the orientation of most of the boundaries. When a dye, one of the pyrromethenes, is dissolved in the LCs, the sample appears to be homogeneous. The optical gain from the dye ispolarized along the LC director and it shows an important spectral blue-shift (10 nm) passing from a polarization parallel to orthogonal to the LC director. The amplified spontaneous emission (ASE) shows the same shift when changing the direction of the sample excitation.When the ASE and the nematicon are generated in the same sample, it is possible to study the interaction between the two. In particular, the waveguide induced by the soliton can be used to guide another signal at another wavelength. We show that the nematicon can collect the ASE generated in the same device and guide it to the same fiber used to inject the nematicon in the LC cell. The extraction of the ASE from the device increases almost one order of magnitude when the soliton is present. However, due to the nematicon spatial fluctuations in LCs, an optimal nematicon power has to be found. Indeed, by increasing the soliton power, the light guiding is improved since the refractive index contrast of the nematicon-induced waveguide is increased. However, very high soliton powers have to be avoided, since the power-dependent soliton fluctuations prevent an optimal collection of the light. The nematicon is also found to increase the spectral purity and the polarization degree of the guided signal.Another LC system is studied, the chiral nematic LCs. In this system, the molecules are disposed following an helicoidal distribution. Due to their optical anisotropy and the periodic distribution, the system presents an optical band-gap. If the LC is also dye-doped, the combination of optical band-gap and gain generates laser emission. We are interested in a fast (sub-ms) reorientation of the helix, with the aim of studying the effect of this reorientation on the laser emission. The first step is the alignment of the LC helix (without the dye) with its axis parallel to the glass plates that constitute the cell, which is difficult to obtain with a high optical quality. For this reason, an innovative method is developed to align LCs through directional solvent evaporation. The solvent-induced method allows us to obtain particularly homogeneous textures, with a contrast ratio between the bright and the dark states that is a factor of 4 greater than that obtained with traditional methods. The LC samples based on solvent-induced alignment are then stabilized via two-photon photo-polymerization. This technique allows us to polymerize small regions of the device while the rest of the sample can be washed out in a solvent bath. When an achiral material is used to refill the device, it assumes a chiral alignment in the polymerized regions and an achiral nematic distribution in the rest. The first characterization of the laser emission is then presented in the last Chapter, with the aim of achieving sub-ms electrical tuning in future works.In this work a wide range of aspects have been investigated, leading to the realization of novel techniques for the fabrication of liquid crystal devices, the demonstration of novel phenomena for light amplification in liquid crystals and the experimental verification of new numerical modeling tools for light propagation in liquid crystals. The three key aspects of the work are nonlinear propagation, optical amplification and electrical response of different LC-based mixtures. Although the first few chapters deal with some of the aspects separately, in the last chapter these aspects are combined, revealing interesting new phenomena and pointing out a number of new aspects that could be part of future work. The results in this work have potential applications in fast tunable lasers, optical communication systems and lab-on-chip components. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished