Ultratrumpiesiems šviesos impulsams sklindant skaidriose terpėse dėl jų saviveikos gali formuotis ypatingi šviesos dariniai – šviesos gijos, kurios pirmą kartą stebėtos 1995 m. Braun ir kt. Pirmuosius šviesos gijų stebėjimus sekė eilė tyrimų, kurie atskleidė įdomius femtosekundinių lazerinių impulsų saviveikos reiškinius, kartais apimančius platesnes netiesinės optikos tyrimų sritis. Šios disertacijos tikslas – išsamiai ištirti dinaminius erdvėlaikinius reiškinius (šviesos gijų sklidimą, erdvėlaikines transformacijas, impulsų skilimą ir spūdą, superkontinuumo generaciją ir daugelio šviesos gijų atsiradimą) įvairiuose intensyvių femtosekundinių lazerio impulsų saviveikos režimuose skaidriose dielektrinėse terpėse su momentiniu Kero netiesiškumu. Tyrimams naudoti įvairūs netiesinės optikos metodai (trimatė lazerinė tomografija, auto- ir kryžminės koreliacijos matavimai, dažninės skyros optinės sklendės autokoreliaciniai matavimai), erdvinės-dažninės skyros spektrų matavimas, statistinė spektrinių, energijos matavimų analizė, skaitmeninis modeliavimas. Atliktų tyrimų visuma išplečia esamas žinias apie femtosekundinių šviesos impulsų saviveiką ir netiesinių optinių sistemų dinamiką normalios ir anomalios grupinių greičių dispersijos atvejais, vedančią prie nestabilumo ir ekstremalių optinių įvykių žadinimo. Kita vertus, ištirti veiksniai naudingi praktiniuose taikymuose, kai siekiama formuoti tvarkias erdvines-laikines struktūras, žadinti ekstremalias optines bangas arba... [toliau žr. visą tekstą] / In 1995 Braun and co-workers reported the observation of self-channeling of femtosecond laser pulses over 20 m in air. This first observation of femtosecond filament triggered a series of studies, which discovered an exciting physics beyond the interaction of ultrashort laser pulses with transparent dielectric media. This dissertation aims at comprehensive study of transient spatiotemporal phenomena (space-time transformations, pulse splitting and compression, filament propagation dynamics, supercontinuum generation and multiple filamentation) that take place during self-focusing of intense femtosecond laser pulses in various self-action regimes in transparent dielectric media with instantaneous Kerr nonlinearity. Various nonlinear optics diagnostic methods (three dimensional mapping, auto- and cross-correlation measurements, frequency resolved optical gating measurements), spatially resolved frequency spectra measurements, statistical analysis of spectral intensities and energies, numerical simulations were employed. The dissertation addresses important issues regarding complete physical understanding of the evolution cycle of femtosecond filaments in normally dispersive media, physical nature of spatiotemporal light bullets generated by filamentation in the anomalous group velocity regime, detailed statistical aspects of supercontinuum generation, and spatiotemporal characterization of multiple filaments. Analysed factors are useful for practical applications when... [to full text]

Unique applications of Terahertz radiation in various fields such as biology and medical sciences, remote sensing, and chemical detection have motivated researchers to develop compact and coherent sources for this least touched region of electromagnetic spectrum. Of the many techniques for generating terahertz signals, difference- frequency generation (DFG) in various crystals is one of the mostly explored methods. Various phase matching methodologies, including phase matching in bulk crystals based on birefringence, and quasi-phase matching have been proposed for this purpose. Although GaAs has an order of magnitude higher second-order nonlinear coefficient in comparison with other crystals, it is one of the least employed crystals for DFG due to phase-matching difficulties. First, it does not provide birefringence in the bulk crystal for birefringence phase matching. Second, GaAs quasi-phase matching has been shown only in few works because patterning the nonlinear susceptibilities in semiconductors is not easily achieved. In this thesis, integration of a GaAs optical waveguide and a terahertz waveguide is proposed as a wide-band phase matching technique for DFG to generate high power coherent terahertz radiation. Using waveguides for both optical and terahertz waves allows for tailoring the phase matching and increasing the interaction length to get high conversion efficiency. Using pump wavelengths between 1.5-1.6 um, where low cost and high optical powers are available, we obtained phase matching for terahertz generation in the range of 0-3.5 THz. We exploit the differences between the GaAs dielectric constant in optical and terahertz range, a high second order nonlinear coefficient, and low terahertz absorption. Simulation results show the appropriate behavior of the proposed devices for both optical and terahertz waves. The proposed waveguide phase matching can be useful for other types of devices using similar nonlinear phenomena, such as coherent detection, electro-optic modulation, and ultra-short pulse generation.

The work presented in this thesis describes an investigation into the properties and behaviour of a new class of nonlinear optical organic chromophores. This study contributes to the optimisation of nonlinear optical molecules through an improved understanding of the relationships between the molecular nonlinear optical properties and the measured macroscopic quantities. A series of highly dipolar non-linear optical chromophores with absorption typically in the range of 350-500 nm have been synthesised by the reactions of amines with tetracyanoquinodimethane (TCNQ). One of the advantages of these materials is the large molecular figure of merit (μβ where μ is the molecular dipole moment and P is the second order polarisability), which theoretically allows large nonlinear optical coefficients to be obtained. The molecular dipole moments of these chromophores were determined both experimentally and theoretically, and were found to agree. The nonlinear optical properties of these compounds in solution were studied using an electric field induced second harmonic generation (EFISH) technique. The measurements of μβ at 1064 nm and 1907 nm in chloroform and acetone are presented. Moderate μβ values were obtained but β is found to be unexpectedly small in chloroform and shows unusual dispersion characteristics in this solvent compared to acetone. Further concentration investigations revealed features that suggest the presence of aggregates within solution. Optical spectroscopy measurements provide evidence of new species whose presence and conformation were found to be solvent-dependent. The results of this work highlight the need for an entire concentration range to be studied if accurate determination of molecular properties of highly dipolar molecules is required. Guest-host polymer films of these materials have been corona poled using a constant current corona triode. Detailed characterisation studies of the second order nonlinearities using second harmonic generation (SHG) were compared to a less dipolar molecule. These investigations showed that the highly dipolar TCNQ derivatives show severe aggregation within the polymer films. The magnitude of the SHG that can be obtained from such systems is therefore limited by this aggregation.

No description available.

This thesis explored the theory, design, fabrication and characterization of AlGaAs Bragg reflection waveguides (BRW) towards the goal of a platform for monolithic integration of active and optically nonlinear devices. Through integration of a diode laser and nonlinear phase-matched cavity, the possibility of on-chip nonlinear frequency generation was explored. Such integrated devices would be highly useful as a robust, alignment free, small footprint and electrically injected alternative to bulk optic systems. A theoretical framework for modal analysis of arbitrary 1-D photonic crystal defect waveguides is developed. This method relies on the transverse resonance condition. It is then demonstrated in the context of several types of Bragg reflection waveguides. The framework is then extended to phase-match second-order nonlinearities and incorporating quantum-wells for diode lasers. Experiments within a slab and ridge waveguide demonstrated phase-matched Type-I second harmonic generation at fundamental wavelength of 1587 and 1600 nm, respectively; a first for this type of waveguide. For the slab waveguide, conversion efficiency was 0.1 %/W. In the more strongly confined ridge waveguides, efficiency increased to 8.6 %/W owing to the increased intensity. The normalized conversion efficiency was estimated to be at 600 %/Wcm^2. Diode lasers emitting at 980 nm in the BRW mode were also fabricated. Verification of the Bragg mode was performed through imaging the near- field of the mode. Propagation loss of this type of mode was measured directly for the first time at 14 cm^-1. The lasers were found to be very insensitive with characteristic temperature at 215 K. Two designs incorporating both laser and phase-matched nonlinearity within the same cavity were fabricated, for degenerate and non-degenerate down-conversion. Though the lasers were sub-optimal, a parametric fluorescence signal was readily detected. Fluorescence power as high as 4 nW for the degenerate design and 5 nW for the non-degenerate design were detected. The conversion efficiency was 4176 %/Wcm^2 and 874 %/Wcm^2, respectively. Neither design was found to emit near the design wavelength. In general, the signal is between 1600-1800 nm and the idler is between 2200-2400 nm. Improvements in laser performance are expected to drastically increase the conversion efficiency.

This thesis explored the theory, design, fabrication and characterization of AlGaAs Bragg reflection waveguides (BRW) towards the goal of a platform for monolithic integration of active and optically nonlinear devices. Through integration of a diode laser and nonlinear phase-matched cavity, the possibility of on-chip nonlinear frequency generation was explored. Such integrated devices would be highly useful as a robust, alignment free, small footprint and electrically injected alternative to bulk optic systems. A theoretical framework for modal analysis of arbitrary 1-D photonic crystal defect waveguides is developed. This method relies on the transverse resonance condition. It is then demonstrated in the context of several types of Bragg reflection waveguides. The framework is then extended to phase-match second-order nonlinearities and incorporating quantum-wells for diode lasers. Experiments within a slab and ridge waveguide demonstrated phase-matched Type-I second harmonic generation at fundamental wavelength of 1587 and 1600 nm, respectively; a first for this type of waveguide. For the slab waveguide, conversion efficiency was 0.1 %/W. In the more strongly confined ridge waveguides, efficiency increased to 8.6 %/W owing to the increased intensity. The normalized conversion efficiency was estimated to be at 600 %/Wcm^2. Diode lasers emitting at 980 nm in the BRW mode were also fabricated. Verification of the Bragg mode was performed through imaging the near- field of the mode. Propagation loss of this type of mode was measured directly for the first time at 14 cm^-1. The lasers were found to be very insensitive with characteristic temperature at 215 K. Two designs incorporating both laser and phase-matched nonlinearity within the same cavity were fabricated, for degenerate and non-degenerate down-conversion. Though the lasers were sub-optimal, a parametric fluorescence signal was readily detected. Fluorescence power as high as 4 nW for the degenerate design and 5 nW for the non-degenerate design were detected. The conversion efficiency was 4176 %/Wcm^2 and 874 %/Wcm^2, respectively. Neither design was found to emit near the design wavelength. In general, the signal is between 1600-1800 nm and the idler is between 2200-2400 nm. Improvements in laser performance are expected to drastically increase the conversion efficiency.

Silicon Nanowires (SiNWs) with ability to confine carriers and photons in two directions while allowing propagation in third dimension offer interesting modified optical properties such as increased material absorption and optical non-linearities with regard to that of bulk silicon. Enhanced optical properties in SiNWs open a window not only to improve the performance of existing devices but also to realize novel structures. As such, I chose to investigate SiNWs for their applications in photonics, especially for sensing and non-linear devices. My goal was to conduct fundamental research on the optical properties of these SiNWs, and then develop an integration platform to realize practical devices. The platform should be compatible with IC manufacturing. Electron Beam Lithography (EBL) using a Poly Methyl Methacrylate (PMMA) resist followed by Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE) is used for SiNWs fabrication. Now we are able to fabricate nanowires as small as 15 nm in diameter with the smallest separation of 50 nm. In addition, the interface between SiNWs and Si substrate is optically smooth enabling us to fundamentally understand optical properties of these structures. During the course of this project, I have contributed new fundamental knowledge about SiNWs. For example, Second Harmonic Generation (SHG) is demonstrated in SiNWs, which is absent in bulk silicon. This is achieved by self-straining the nanowires and is the first demonstration of this kind. Second-order non-linearities are more efficient for optical signal processing than third-order ones (which have been used for silicon photonics devices so far). Therefore, these results open a new area of research in silicon. In addition to second order nonlinearity, high enhancement of Raman scattering is achieved in SiNWs fabricated on Silicon on Insulator (SOI) substrate. This can find promising applications in sensing and nonlinear based devices such as optical switches and logic gates. Further, polarization resolved reflections from these nanowire arrays were measured and significant differences were observed for the reflection characteristics for the sand p-polarized beams. In order to understand these reflections, an effective index model is proposed based on calculations using Finite Difference Time Domain (FDTD) method. Results of this analysis provide useful information for designing of many optical devices using SiNWs such as solar cells and photodetectors. As another part of this thesis, vivid colors in mutually coupled SiNWs is demonstrated where nanowire diameters range from 105 nm to 345 nm. A simple sensor is demonstrated by observing the change in the reflected color with changing refractive index of the surrounding medium. A refractive index resolution of 5×10−5 is achieved using a simple charge coupled device (CCD) camera. Although, there were some paradigm shifting results during my fundamental studies, it became very apparent that SiNWs suffer from a major issue inhibiting their use in photonics devices. Below the diameter of 100 nm where these enhanced material properties were observed, SiNW is a poor optical waveguide with less than 1 % of light confined. The low confinement factor means that though the intrinsic properties of SiNWs increase, the overall device performance is not significantly enhanced. To overcome this issue, a new platform technology is invented, called Silicon Nanowire Optical Waveguide (SNOW). It combines the material advantages of nanostructures with the optical properties of conventional waveguides, and consists of arrays of nanowires in close proximity. It is shown that such a structure can guide an optical mode using the FDTD method. This waveguide structure can be used as a versatile platform to manufacture various devices such as sensors, switches, modulators, grating, and delay lines. For instance, a novel bio-sensor is proposed and designed whose sensitivity is enhanced by a factor of 20, compared to conventional silicon-wire waveguides.

The nonlinear optical (NLO) properties were investigated in various extended　π-conjugated chromophores: cyanine and alkyne carbocations; porphyrin dimers; and squaraine compounds that possess electronic, double resonance, and vibronic based NLO properties. In summary: (i) It was demonstrated that the alkyne carbocations have very similar optical properties to traditional cyanine dyes. Our theoretical results establish that the alkyne carbocations, in spite of their significant degree of bond-length alternation, behave in the same way as cyanine dyes. (ii) The nature of the -bridge in porphyrin dimers tunes the electronic coupling strength, which in turn determines the splitting of the energy levels and the (non)linear optical properties. (iii) We have shown that the origin of the lowest TPA-active states in squaraines is dependent on the nature of substituent donor moiety, changing from predominantly electronic to vibronic in character. For all squaraines containing indolinylidenemethyl donors, a vibronic origin for the TPA peak, and the energy and lineshape of the experimentally observed lowest TPA peak in these compounds were confirmed.

The subject of this theoretical  study is the role ofelectronic structure as well as of rotational and vibrational motionson interactions between molecules and electromagnetic radiation,ranging from optical to x-ray. The thesis concerns both linear and nonlinear regimes of the light-matter interaction. The first part of the thesis is devoted to propagation of opticalpulses with different time-structure through various nonlinear absorbers.First we explain the double-exponential decay of fluorescence caused by photobleaching of pyrylium  salt irradiated by a train of short (100 fs) optical pulses. The main reason for this effect is the transversal inhomogeneity of the light beam which makes the dynamics of the photobleaching differ in the core of the pulse and on its periphery. We also explore the optical power limitingof C60 fullerene irradiated by either microsecond optical pulses or a picosecond pulse trains. Enhancement of nonlinear absorption is caused by strong triplet-triplet absorption that becomes important due toelongation of the interaction time.Here we show the importance of the repetitionrate for the optical power limiting performance.The second part of the thesis addresses the interaction of optical and x-rayfields with rotational degrees of freedom of molecules. In this part the main attention is paid to the rotational heating caused by the recoil, experienced by molecules due to the ejection of photoelectrons. We have quantitatively explained two qualitatively different experiments with the N2 molecule.We predict the interference modulation of the recoil-induced shift,which is a shift of the photoelectron line caused by the rotational recoil effect, as a function of the photon energy.The developed theory also explains the rotational heating ofmolecules observed in the optical fluorescence induced by x-ray radiation.Based on this explanation, we suggest a new scheme of the optical fluorescence induced by x-rays that allows to detect the recoil effect via the recoil-inducedsplitting of the optical resonance.The last part of the thesis focuses on multi-mode nuclear dynamics of the resonant Auger scattering from the C2H2 molecule, that was the subject of a recent experimental study.Here we develop a theory that explains the observed vibrationalscattering anisotropy. We have found that three qualitatively different mechanisms are responsible for this phenomenon. The first mechanism is the interference of the direct and resonance scattering channels. The second mechanismis the interference of the resonant scattering channels through core excitedstate with the orthogonal orientation of the vibrational modes of core excitedstate. The Young's double slit like interference of the quantum pathways through the double-well potential of the bending motion of core excited state is the third mechanism of the vibrational scattering anisotropy. / QC 20100713

The present thesis is devoted to theoretical studies of pulse propagation of light through linear and nonlinear media, and of light-induced nuclear dynamics. The first part of the thesis addresses propagation of light pulses in linear periodical media - photonic crystals. The main accent was put on studies of the angular properties of two qualitatively different types of photonic crystals: holographic photonic crystals, and impurity band based photonic crystals. The anisotropy of band structure, group velocity and pulse delay with respect to the light polarization are analyzed. In the second part of the thesis a strict theory of nonlinear propagation of a few strong interacting light beams is presented. The key idea of this approach is a self-consistent solution of the nonlinear wave equation and the density matrix equations of the material. This technique is applied to studies of dynamics of cavityless lasing generated by ultra-fast multi-photon excitation. It is shown that interaction of co- and counter-propagating pulses of amplified spontaneous emission (ASE) affects the dynamics and efficiency of nonlinear conversion. Our dynamical theory allows to explain the asymmetric spectral properties of the forward and backward ASE pulses, which were observed in recent experiment with different dye molecules. It is shown that the ASE spectral profile changes drastically when the pump intensity approaches the threshold level. The effect of the temporal self-pulsation of ASE is studied in detail. The third part of the thesis is devoted to light-induced nuclear dynamics. Time- and frequency-resolved X-ray spectroscopy of molecules driven by strong and coherent infrared (IR) pulses shows that the phase of the IR field strongly influences the trajectory of the nuclear wave packet, and hence, the X-ray spectrum. Such a dependence arises due to the interference of one (X-ray) and two-photon (X-ray + IR) excitation channels. The phase of the light influences the dynamics also when the Rabi frequency approaches the vibrational frequency, breaking down the rotating-wave approximation. The probe X-ray spectra are also sensitive to the delay time, the duration, and the shape of the pulses. The evolution of the nuclear wave packets in the dissociative core-excited state affects the dynamics of resonant Auger scattering from fixed-in-space molecules. One of the important dynamical effects is the atomic-like resonance which experiences electronic Doppler shift. We predict that the scattering of the Auger electrons by nearby atoms leads to new Doppler shifted resonances. These extra resonances show sharp maxima in the bond directions, which makes them very promising as probes for local molecular structure using energy and angular resolved electron-ion coincidence techniques. Our theory provides prediction of several new effects, but also results that are in good agreement with the available experimental data. / QC 20100906