We study the behavior of electric fields in and around dielectric and metal nanoparticles, and prepare the ground for their applications to a variety of systems viz. photovoltaics, imaging and detection techniques, and molecular spectroscopy. We exploit the property of nanoparticles being able to focus the radiation field into small regions and study some of the interesting nonlinear, and quantum coherence and interference phenomena near them. The traditional approach to study the nonlinear light-matter interactions involves the use of the slowly varying amplitude approximation (SVAA) as it simplifies the theoretical analysis. However, SVVA cannot be used for systems which are of the order of the wavelength of the light. We use the exact solutions of the Maxwell's equations to obtain the fields created due to metal and dielectric nanoparticles, and study nonlinear and quantum optical phenomena near these nanoparticles. We begin with the theoretical description of the electromagnetic fields created due to the nonlinear wavemixing process, namely, second-order nonlinearity in an nonlinear sphere. The phase-matching condition has been revisited in such particles and we found that it is not satisfied in the sphere. We have suggested a way to obtain optimal conditions for any type and size of material medium. We have also studied the modifications of the electromagnetic fields in a collection of nanoparticles due to strong near field nonlinear interactions using the generalized Mie theory for the case of many particles applicable in photovoltaics (PV). We also consider quantum coherence phenomena such as modification of dark states, stimulated Raman adiabatic passage (STIRAP), optical pumping in $4$-level atoms near nanoparticles by using rotating wave approximation to describe the Hamiltonian of the atomic system. We also considered the behavior of atomic and the averaged atomic polarization in $7$-level atoms near nanoparticles. This could be used as a prototype to study any $n-$level atomic system experimentally in the presence of ensembles of quantum emitters. In the last chapter, we suggested a variant of a pulse-shaping technique applicable in stimulated Raman spectroscopy (SRS) for detection of atoms and molecules in multiscattering media. We used discrete-dipole approximation to obtain the fields created by the nanoparticles.

No description available.

Upconverted light from nanostructured metal surfaces can be produced by harmonic generation and multi-photon luminescence; however, these are weak processes and require extremely high field intensities to produce a measurable signal. Here we report on bright emission, five orders of magnitude greater than harmonic generation, that can be seen from metal tunnel junctions due to light-induced inelastic tunneling. Like inelastic tunneling light emission, which was recently reported to have 2% conversion efficiency per tunneling event, the emission wavelength recorded varies with the local electric field applied; however, here the field is from a 1560 nm femtosecond pulsed laser source. Finite-difference time-domain simulations of the experimental conditions show the local field is sufficient to generate tunneling-based inelastic light emission in the visible regime. This phenomenon is promising for producing ultrafast upconverted light emission with higher efficiency than conventional nonlinear processes. / Graduate

No description available.

Quadratic spatial soliton interactions were investigated in this Dissertation. The first part deals with characterizing the principal features of multi-soliton generation and soliton self-reflection. The second deals with two beam processes leading to soliton interactions and collisions. These subjects were investigated both theoretically and experimentally. The experiments were performed by using potassium niobate (KNBO3) and periodically poled potassium titanyl phosphate (KTP) crystals. These particular crystals were desirable for these experiments because of their large nonlinear coefficients and, more importantly, because the experiments could be performed under non-critical-phase-matching (NCPM) conditions. The single soliton generation measurements, performed on KNBO3 by launching the fundamental component only, showed a broad angular acceptance bandwidth which was important for the soliton collisions performed later. Furthermore, at high input intensities multi-soliton generation was observed for the first time. The influence on the multi-soliton patterns generated of the input intensity and beam symmetry was investigated. The combined experimental and theoretical efforts indicated that spatial and temporal noise on the input laser beam induced multi-soliton patterns. Another research direction pursued was intensity dependent soliton routing by using of a specially engineered quadratically nonlinear interface within a periodically poled KTP sample. This was the first time demonstration of the self-reflection phenomenon in a system with a quadratic nonlinearity. The feature investigated is believed to have a great potential for soliton routing and manipulation by engineered structures. A detailed investigation was conducted on two soliton interaction and collision processes. Birth of an additional soliton resulting from a two soliton collision was observed and characterized for the special case of a non-planar geometry. A small amount of spiraling, up to 30 degrees rotation, was measured in the experiments performed. The parameters relevant for characterizing soliton collision processes were also studied in detail. Measurements were performed for various collision angles (from 0.2 to 4 degrees), phase mismatch, relative phase between the solitons and the distance to the collision point within the sample (which affects soliton formation). Both the individual and combined effects of these collision variables were investigated. Based on the research conducted, several all-optical switching scenarios were proposed.

Attosecond pulses have been developed as a means for investigating phenomena that proceed on the order of the atomic unit of time (24 as). Unfortunately, these extreme ultraviolet (XUV) pulses by themselves contain too few photons to initiate nonlinear dynamics or dress states in an attosecond pump--attosecond probe scheme. As a result, most attosecond experiments thus far have featured complementary near infrared (NIR) femtosecond lasers for instigating electron dynamics. In order to access the benefits of all-attosecond measurements and open attosecond physics to new fields of exploration, the photon flux of these pulses must be increased. One way to boost the attosecond pulse energy is to scale up the energy of the NIR pulse responsible for driving high-harmonic generation (HHG). With generalized double optical gating (GDOG), isolated attosecond pulses can be generated with multi-cycle laser systems, wherein the pulse energy can be boosted more easily than in the few-cycle laser systems required by other gating methods. At the Institute for the Frontier of Attosecond Science and Technology (IFAST), this scalability was demonstrated using a 350 mJ, 15 fs (10 TW) Ti:sapphire laser, which was used to generate a 100 nJ XUV continuum. This represented an order-of-magnitude improvement over typical attosecond pulse energies achievable by millijoule-level few-cycle lasers. To obtain the microjoule-level attosecond pulse energy required for performing all-attosecond experiments, the attosecond flux generated by the IFAST 10 TW system was still deficient by an order of magnitude. To this end, the laser system was upgraded to provide joule-level output energies while maintaining pulse compression to 15 fs, with a targeted peak power of 200 TW. This was accomplished by adding an additional Ti:sapphire amplifier to the existing 10 TW system and implementing a new pulse compression system to accommodate the higher pulse energy. Because this system operated at a 10 Hz repetition rate, stabilization of the carrier-envelope phase (CEP) -- important for controlling attosecond pulse production -- could not be achieved using traditional methods. Therefore, a new scheme was developed, demonstrating the first-ever control of CEP in a chirped-pulse amplifier (CPA) at low repetition rates. Finally, a new variation of optical gating was proposed as a way to improve the efficiency of the attosecond pulse generation process. This method was also predicted to allow for the generation of isolated attosecond pulses with longer driving laser pulses, as well as the extension of the high-energy photon cut-off of the XUV continuum.

In this thesis, I develop a novel time-domain approach for nonlinear scattering theory (NLST), a previously frequency domain method for estimating the nonlinear generation from a nanostructure. Due to a gap in literature, I then perform a full comparison of this novel time domain approach to the existing one in the frequency domain. Using the example scenario of third harmonic generation from various media in 1D and 3D, I compare - quantitatively - the NLST estimated nonlinear spectra to two types of direct nonlinear simulations: one using an experimental value for the nonlinear optical susceptibility, and, for plasmonic systems, another using a hydrodynamics model for the nonlinear plasmonic response. Through testing differing NLST approaches on these systems, I demonstrate the effectiveness of the novel time-domain NLST and assess the use cases for this method as well as the pre-existing ones. Lastly, I discuss the applicability of NLST in future works involving the inverse design process, and high-order harmonic generation.

The semiconductor Bloch equations (SBEs) are an insightful and well-established formalism for studying light-matter interactions in solids. When Coulomb interactions between electrons are omitted, the SBEs are simplified to a single particle model. The SBEs in this single electron approximation have been used extensively to model strong-field interactions in condensed matter. The SBEs in the length gauge provide an intuitive and numerically efficient model of high harmonic generation (HHG) in solids. In this approach, the SBEs involve Berry connections and transition dipole moments, which are gauge dependent structural quantities. This thesis studies the role of gauge symmetry in the SBEs, and how it can be exploited to facilitate efficient numerical analysis of HHG in solids. In the length gauge, the macroscopic current describing HHG can be decomposed into physically intuitive contributions. In particular, this leads to a contribution known as the "mixture" current, which has been overlooked by the HHG community until recently. We study the influence of this contribution using the analytic tight-binding model for gapped graphene. We derive an analytic gauge transformation that removes singular behaviour from the gapped graphene model, thus enabling efficient numerical integration of the SBEs. We also present an alternative approach for simulating dynamics in tight-binding models. Instead of simulating the SBEs in the usual basis of Bloch functions, we transform to the basis in which the tight-binding Hamiltonian is represented. The dipole matrix elements necessarily vanish in this basis, and the SBEs can be integrated using only the Hamiltonian matrix elements. We first generalize the SBEs to accomodate a non-diagonal Hamiltonian matrix, and we demonstrate this formalism numerically using two different tight-binding models. Finally, we derive a novel formulation of the SBEs which involve only gauge invariant matrix elements. Specifically, the Berry connections and transition dipole phases are replaced by a gauge invariant quantity known as the shift vector. This yields a fully gauge invariant description of HHG in solids, and the shift vector provides intuitive insight for HHG in systems with broken inversion symmetry. Further, the ability to describe HHG solely in terms of gauge invariant quantities raises new possibilities for tomographic reconstruction of crystal band structure, and this idea is discussed as a possible direction of future work.

Single-pass frequency conversion by a nonlinear optical crystal is an attractive method to generate coherent radiation in various spectral domains from ultraviolet to mid-infrared. Wavelength converters based on quasi-phase matched (QPM) periodically poled lithium niobate (PPLN) have proved to be important wavelength conversion devices for many useful applications. This thesis develops a novel integration design for temperature controlling and temperature wavelength tuning of a QPM-PPLN waveguide wavelength converter. A Cr/Pt/Au thin film alloy layer is deposited on a PPLN device with a polymer buffer layer to work as a micro-heater and a temperature sensor at the same time. The temperature of the device can be tailored by applying current to the micro-heater layer, which changes the effective period of the QPM grating and thus the QPM wavelength through the thermal optical effect (TOE). The device's temperature can be monitored by measuring the resistance change of the alloy layer. Micro-heater design and mode profile simulation are involved in the thesis. The entire device fabrication process is introduced. Both electrical and optical features of the device are characterized and discussed. In contrast to the conventional temperature tuning method based on a bulky oven, the proposed design has some excellent characteristics such as compact package size and low power consumption. / Thesis / Master of Applied Science (MASc)

In this thesis, detailed studies are presented into self-assembled, noncentrosymmetric, optically active films. Second harmonic generation (SHG) is used to measure the second order nonlinear optical susceptibility (?(2)) of ionically self-assembled monolayer (ISAM) thin films. Conventional ISAM films are fabricated by alternately immersing a substrate into oppositely-charged polyelectrolyte solutions. The polyelectrolytes bind electrostatically to the oppositely-charged substrate, and thus reverse the charge of the substrate. The charge reversal limits the amount of adsorbed material and primes the substrate for the next layer. During the deposition of the nonlinear optical (NLO) active layer, the chromophores are attracted to the oppositely-charged surface, which results in net orientation of the chromophores. Some of the net orientation is lost during the deposition of the next NLO-inactive layer as this layer orients some of the chromophores away from the substrate. A disadvantage of the polymer ISAM deposition method is that although there is a net orientation toward the substrate, a large number of chromophores are randomly or oppositely oriented. This reduces the nonlinear optical response. To overcome this problem, two alternative methods with a better net orientation are discussed: hybrid covalent / ionic deposition and multivalent monomer deposition. In both deposition methods, the NLO-active material is a monomer instead of a polymer. In hybrid covalent / ionic deposition, the NLO-inactive polymer is deposited using electrostatic attraction while the NLO-active monomer is deposited covalently. This forces alignment of the chromophores. The multivalent method uses chromophores with multiple charges on one side of the molecule and one charge (same sign) on the other. The difference in electrostatic attraction causes a preferential orientation of the chromophores during deposition. Attempts have been made to further improve the net orientation by complexation of the monomers with cyclodextrins (cone shaped organic molecules), so far with only limited success. The SHG response of NLO-active layers near the glass and air interfaces is much stronger than the SHG response of layers in the bulk of the film for all deposition methods and NLO-active materials investigated in this thesis. For larger number of bilayers (the bulk regime), the square root of the SHG signal increases linearly with the number of bilayers as expected for a uniform chromophore orientation. We isolated the interface effects through use of buffer layers of NLO-inactive polymers. The glass interface effect extends roughly one bilayer deep for all investigated materials. The air interface effect is different for polymers and monomers. For monomers, this effect extends only one bilayer deep, while it extends multiple layers deep for polymers. Using glass cells to contain the polyelectrolyte solutions, we were able to measure the SHG signal in situ, which proved to be a powerful tool to monitor the deposition rate as a function of chosen parameters. All depositions were rapid, on the order of one minute or less. Provided that a minimum concentration is met, the deposition rate and final SHG values are independent of concentration. Bulk layers deposit at the same rate as layers near the interface. For polymer NLO-active layers a secondary, slower growth of SHG is observed that is presumably due to reorganization of the adsorbed polymer layer. This secondary growth is not observed in the deposition of NLO-active monomers. / Ph. D.