This dissertation reports on the investigation of the relationships between molecular structure and two-photon absorption (2PA) properties for a series of polymethine and squaraine molecules. Current and emerging applications exploiting the quadratic dependence upon laser intensity, such as two-photon fluorescence imaging, three-dimensional microfabrication, optical data storage and optical limiting, have motivated researchers to find novel materials exhibiting strong 2PA. Organic materials are promising candidates because their linear and nonlinear optical properties can be optimized for applications by changing their structures through molecular engineering. Polymethine and squaraine dyes are particularly interesting because they are fluorescent and showing large 2PA. We used three independent nonlinear spectroscopic techniques (Z-scan, two-photon fluorescence and white-light continuum pump-probe spectroscopy) to obtain the 2PA spectra revealing 2PA bands, and we confirm the experimental data by comparing the results from the different methods mentioned. By systematically altering the structure of polyemthines and squaraines, we studied the effects of molecular symmetry, strength of donor terminal groups, conjugation length of the chromophore chain, polarity of solvents, and the effects of placing bridge molecules inside the chromophore chain on the 2PA properties. We also compared polymethine, squaraine, croconium and tetraon dyes with the same terminal groups to study the effects of the different additions inserted within the chromophore chain on their optical properties. Near IR absorbing squaraine dyes were experimentally observed to show extremely large 2PA cross sections ([approximately equal to] 30000GM). A simplified three-level model was used to fit the measured 2PA spectra and detailed quantum chemical calculations revealed the reasons for the squaraine to exhibit strong 2PA. In addition, two-photon excitation fluorescence anisotropy spectra were measured through multiple 2PA transitions. A theoretical model based on four-levels with two intermediate states was derived and used for analysis of the experimental data.

External cavity semiconductor mode-locked lasers can produce pulses of a few picoseconds. The pulses from these lasers are inherently chirped with a predominant linear chirp component that can be compensated resulting in sub-picosecond pulses. External cavity semiconductor mode-locked lasers can be configured as multiwavelength pulse sources and are good candidates for time and wavelength division multiplexing applications. The gain medium in external cavity semiconductor mode-locked lasers is a semiconductor optical amplifier (SOA), and passive and hybrid mode-locked operation are achieved by the introduction of a saturable absorber (SA) in the laser cavity. Pump-probe techniques were used to measure the intracavity absorption dynamics of a SA in an external cavity semiconductor mode-locked laser and the gain dynamics of a SOA for the amplification of diverse pulses. The SOA gain dynamics measurements include the amplification of 750 fs pulses, 6.5 ps pulses, multiwavelength pulses and the intracavity gain dynamics of an external cavity multiwavelength semiconductor mode-locked laser. The experimental results show how the inherent chirp on pulses from external cavity semiconductor mode-locked lasers results in a slow gain depletion without significant fast gain dynamics. In the multiwavelength operation regime of these lasers, the chirp broadens the temporal pulse profile and decreases the temporal beating resulting from the phase correlation among wavelength channels. This results in a slow gain depletion mitigating nonlinearities and gain competition among wavelength channels in the SOA supporting the multiwavelength operation of the laser. Numerical simulations support the experimental results.

Even though most of the properties of optical fields, such as wavelength, polarization, wavefront curvature or angular spectrum, have been commonly manipulated in a variety of remote sensing procedures, controlling the degree of coherence of light did not find wide applications until recently. Since the emergence of optical coherence tomography, a growing number of scattering techniques have relied on temporal coherence gating which provides efficient target selectivity in a way achieved only by bulky short pulse measurements. The spatial counterpart of temporal coherence, however, has barely been exploited in sensing applications. This dissertation examines, in different scattering regimes, a variety of inverse scattering problems based on variable spatial coherence gating. Within the framework of the radiative transfer theory, this dissertation demonstrates that the short range correlation properties of a medium under test can be recovered by varying the size of the coherence volume of an illuminating beam. Nonetheless, the radiative transfer formalism does not account for long range correlations and current methods for retrieving the correlation function of the complex susceptibility require cumbersome cross-spectral density measurements. Instead, a variable coherence tomographic procedure is proposed where spatial coherence gating is used to probe the structural properties of single scattering media over an extended volume and with a very simple detection system. Enhanced backscattering is a coherent phenomenon that survives strong multiple scattering. The variable coherence tomography approach is extended in this context to diffusive media and it is demonstrated that specific photon trajectories can be selected in order to achieve depth-resolved sensing. Probing the scattering properties of shallow and deeper layers is of considerable interest in biological applications such as diagnosis of skin related diseases. The spatial coherence properties of an illuminating field can be manipulated over dimensions much larger than the wavelength thus providing a large effective sensing area. This is a practical advantage over many near-field microscopic techniques, which offer a spatial resolution beyond the classical diffraction limit but, at the expense of scanning a probe over a large area of a sample which is time consuming, and, sometimes, practically impossible. Taking advantage of the large field of view accessible when using the spatial coherence gating, this dissertation introduces the principle of variable coherence scattering microscopy. In this approach, a subwavelength resolution is achieved from simple far-zone intensity measurements by shaping the degree of spatial coherence of an evanescent field. Furthermore, tomographic techniques based on spatial coherence gating are especially attractive because they rely on simple detection schemes which, in principle, do not require any optical elements such as lenses. To demonstrate this capability, a correlated lensless imaging method is proposed and implemented, where both amplitude and phase information of an object are obtained by varying the degree of spatial coherence of the incident beam. Finally, it should be noted that the idea of using the spatial coherence properties of fields in a tomographic procedure is applicable to any type of electromagnetic radiation. Operating on principles of statistical optics, these sensing procedures can become alternatives for various target detection schemes, cutting-edge microscopies or x-ray imaging methods.

Liquid crystals have been a major subject of research for the past decades. Aside from the variety of structures they can form, they exhibit a vast range of optical phenomena. Many of these phenomena found applications in technology and became an essential part of it. In this dissertation thesis we continue the line to propose a number of new applications of optical effects in liquid crystals and develop their theoretical framework. One such application is the possibility of beam combining using Orientational Stimulated Scattering in a nematic liquid crystal cell. Our numerical study of the OSS process shows that normally this possibility does not exist. However, we found that if a number of special conditions is satisfied efficient beam combining with OSS can be done. These conditions require a combination of special geometric arrangement of incident beams, their profiles, nematic material, and more. When these conditions are fulfilled, power of the beamlets can be coherently combined into a single beam, with high conversion efficiency while the shape and wave-front of the output beam are still of good quality. We also studied the dynamics of the OSS process itself and observed (in a numerical model) a number of notorious instabilities caused by effects of back-conversion iv process. Additionally, there was found a numerical solitary-wave solution associated with this back-conversion process. As a liquid crystal display application, we consider a nematic liquid crystal layer with the anisotropy axis modulated at a fixed rate in the transverse direction with respect to light propagation direction. If the layer locally constitutes a half-wave plate, then the thinscreen approximation predicts 100% -efficient diffraction of normal incident wave. If this diffracted light is blocked by an aperture only transmitting the zero-th order, the cell is in dark state. If now the periodic structure is washed out by applying voltage across the cell and light passes through the cell undiffracted, the light will pass through the aperture as well and the cell will be in its bright state. Such properties of this periodically aligned nematic layer suggest it as a candidate element in projection display cells. We studied the possibility to implement such layer through anchoring at both surfaces of the cell. It was found that each cell has a thickness threshold for which the periodic structure can exist. The anchored periodic structure cannot exist if thickness of the cell exceeds this threshold. For the case when the periodic structure exists, we found the structure distortion in comparison with the preferable ideal sinusoidal profile. To complete description of the electromechanical properties of the periodic cell, we studied its behavior at Freedericksz transition. Optical performance was successfully described with the coupled-mode theory. While influence of director distortion is shown to be negligibly small, the walk-off effects appear to be larger. In summary, there are good prospects for use of this periodically v aligned cell as a pixel in projection displays but experimental study and optimization need to be performed. In the next part we discuss another modulated liquid crystal structure in which the director periodically swings in the direction of light propagation. The main characteristic of such structure is the presence of bandgap. Cholesteric liquid crystals are known to possess bandgap for one of two circular polarizations of light. However, unlike the cholesterics the bandgap of the proposed structure is independent of polarization of normally incident light. This means that no preparation of light is needed in order for the structure to work in, for example, liquid crystal displays. The polarization universality comes at the cost of bandgap size, whose maximum possible value ∆ωPTN compared to that of cholesterics ∆ωCh is approximately twice smaller: ∆ωPTN ≈ 0.58∆ωCh if modulation profile is sinusoidal, and ∆ωPTN ≈ 0.64∆ωCh if it is rectangular. This structure has not yet been experimentally demonstrated, and we discuss possible ways to make it.

Photonic crystals and nanophotonics have received a great deal of attention over the last decade, largely due to improved numerical modeling and advances in fabrication technologies. To this day, fabrication and optical behavior remain decoupled during the design phase and numerous assumptions are made about "perfect" geometry. As research moves from theory to real devices, predicting device behavior based on realistic geometry becomes critical. In this dissertation, a set of numerical tools was developed to model micro and nano fabrication processes. They were combined with equally capable tools to model optical performance of the simulated structures. Using these tools, it was predicted and demonstrated that 3D nanostructures may be formed on a standard mask aligner. A space-variant photonic crystal filter was designed and optimized based on a simple fabrication method of etching holes through hetero-structured substrates. It was found that hole taper limited their optical performance and a method was developed to compensate. A method was developed to tune the spectral response of guided-mode resonance filters at the time of fabrication using models of etching and deposition. Autocloning was modeled and shown that it could be used to form extremely high aspect ratio structures to improve performance of form-birefringent devices. Finally, the numerical tools were applied to metallic photonic crystal devices.

The purpose of this dissertation is to provide a study and possible applications of two-photon absorption (2PA), in direct-gap semiconductors and quantum-well (QW) semiconductor structures. One application uses extremely nondegenerate (END) 2PA, for mid-infrared (mid-IR) detection in uncooled semiconductors. The use of END, where the two photons have very different energies gives strong enhancement comapared to degenerate 2PA. This END-2PA enhanced detection is also applied to mid-IR imaging and light detection and ranging (LIDAR) in uncooled direct-gap photodiodes. A theoretical study of degenerate 2PA (D-2PA) in quantum wells, QWs, is presented, along with a new theory of ND 2PA in QWs is developed. Pulsed mid-IR detection of femtosecond pulses is investigated in two different semiconductor p-i-n photodiodes (GaAs and GaN). With the smaller gap materials having larger ND-2PA, it is observed that they have better sensitivity to mid-IR detection, but unwanted background from D-2PA outweighs this advantage. A comparison of responsivity and signal-to-background ratio for GaAs and GaN in END-2PA based detection is presented. END-2PA enhancement is utilized for CW IR detection in uncooled GaAs and GaN p-i-n photodiodes. The pulsed mid-IR detection experiments are further extended to perform mid-IR imaging in uncooled GaN p-i-n photodetectors. A 3-D automated scanning gated imaging system is developed to obtain 3-D mid-IR images of various objects. The gated imaging system allows simultaneous 3-D and 2-D imaging of objects. The 3-D gated imaging system described in the dissertation could be used for examination of buried structures (microchannels, defects etc.) or laser written volumetric structures and could also be suitable for in-vivo imaging applications in biology in the mid-IR spectral region. As an example, 3-D imaging of buried semiconductor structures is presented. A theoretical study of D-2PA of QWs for transverse electric (TE) and transverse magnetic (TM) fields is carried out and an analytical expression for the D-2PA coefficient in QWs using second-order perturbation theory is derived. A theory for ND-2PA in QW semiconductor using second-order perturbation theory is developed for the first time and an analytical expression for the ND-2PA coefficient for TE, TM, and the mixed case of TE and TM is derived. The shape of the 2PA curve for the D-2PA and ND-2PA for QWs in the TE case is similar to that of bulk semiconductors. As governed by the selection rules both the D-2PA and ND-2PA curves for the TE case does not show a step-like signature for the density of states of the QWs whereas 2PA curve for the TM case shows such step like sharp features. The ND-2PA coefficient for TE, TM, and the mixed case is compared with that obtained for bulk semiconductors. Large enhancement in ND-2PA of QW semiconductors for the TM case over bulk semiconductors is predicted.

A true linearized interferometric intensity modulator for pulsed light has been proposed and experimentally presented in this thesis. This has been achieved by introducing a mode-locked laser into one of the arms of a Mach-Zehnder interferometer and injection-locking it to the input light (which is pulsed and periodic). By modulating the injection-locked laser, and combining its output light with the light from the other arm of interferometer in quadrature, one can achieve true linearized intensity modulator. This linearity comes from the arcsine phase response of the injection-locked mode-locked laser (as suggested by steady-state solution of Adler's equation) when it is being modulated. Mode-locked lasers are fabricated using a novel AlGaInAs-InP material system. By using the BCB for planarization and minimizing the metal pad size and directly modulating the laser, we have achieved very effective fundamental hybrid mode-locking at the repetition rate of ~ 23 GHz. This laser also provided the short pulses of 860 fs and 280 fs timing jitter integrated from 1 Hz- 100 MHz. The linearized intensity modulator has been built by using two identical two-section mode-locked lasers with the same length, one as the slave laser in one of the arms of the Mach-Zehnder interferometer injection-locked to the other one as the master which is the input light to the modulator. A low V? of 8.5 mV is achieved from this modulator. Also the current of the gain section or the voltage of the saturable absorber section of the slave laser has been used to apply the modulation signal. A spur free dynamic range of 70 dB.Hz2/3 is achieved when modulating the modulator through the saturable absorber. Modulating the saturable absorber provides a reduced third-order intermodulation tone with respect to modulating the gain. This is simply because of the unwanted amplitude modulation created when modulating the gain section current. Finally an improved design is proposed and demonstrated to improve the modulator performance. This is achieved by introducing a third section to the laser. Using the impurity free vacancy disordering technique the photoluminescence peak of this section is blue-shifted selectively and therefore there would not be any absorption in that passive section. By applying the modulation signal to this passive section rather than applying it to the gain section or saturable absorber section, the amplitude and phase modulation could be decoupled. The experimental results have presented here and an almost six-fold reduction in V? and 5 dB improvement in spur free dynamic range have been achieved. The proposed and demonstrated configuration as an analog optical link has the potential to increase the performance and resolution of photonic analog-to-digital converters.

Self-heating leads to temperature rise of laser diode and limits the output power, efficiency and modulation bandwidth due to increased loss and decreased differential gain. The main heat sources in laser diode during continuous wave operation are Joule heating and free carrier absorption loss. To control device self-heating, the epi structure needs to be designed with low electrical resistance and low absorption loss, while the heat flux must spread out of the device efficiently. This dissertation presents the control of self-heating of both edge emitting laser diodes and vertical cavity surface emitting lasers (VCSELs). For the 980nm high power edge emitting laser, asymmetric waveguide is used for low free carrier absorption loss. The waveguide and cladding materials are optimized for high injection efficiency. BeO heatsink is applied to spread the heat efficiently. Injection efficiency of 71% and internal loss of 0.3 cm-1 have been achieved. A total output power of 9.3 W is measured from 0.5cm long device at 14.5A injection current. To further reduce the internal loss, the development of 980nm quantum dot active region is studied. Threshold current density as low as 59A/cm2 is reached. For the VCSELs, oxide-free structure is used to solve the self-heating problem of oxide VCSELs. Removing the oxide layer and using AlAs in the DBRs leads to record low thermal resistance. Optimization of the DBRs leads to low resistance and low free carrier absorption. Power conversion efficiency higher than 50% is achieved. To further reduce device voltage and heat generation, the development of intracavity contacts devices is introduced.

One of the most intriguing properties of light-matter interaction is the ability of an electromagnetic field to exert mechanical forces on polarizable objects. This phenomenon is a direct consequence of the fact that light carries momentum, which in turn can be transferred to matter. Mediated by scattering, this interaction usually manifests itself as a “pushing force” in the direction of beam propagation. However, it is possible to judiciously engineer these optical forces, either by tailoring particle polarizability, and/or by structuring the incident light field. As a simple example, a tightly focused laser beam demonstrates strong gradient forces, which may attract and even trap particles with positive polarizability in the focal volume. The opposite occurs in the regime of negative polarizability, where particles are expelled from the regions of highest intensity. Based on this fundamental principle, one can actively shape the beam using spatial light modulators to manipulate individual objects as well as ensembles of particles suspended in a liquid. In the latter case, a modulation of the local particle concentration is associated with changes of the effective refractive index. The result is an artificial nonlinear medium, whose Kerr-type response can be readily tuned by the parameters of its constituent particles. In the course of this work, we introduce a new class of synthetic colloidal suspensions exhibiting negative polarizabilities, and observe for the first time robust propagation and enhanced transmission of self-trapped light over long distances. Such light penetration in strongly scattering environments is enabled by the interplay between optical forces and self-activated transparency effects. We explore various approaches to the design of negative-polarizability arrangements, including purely dielectric as well as metallic and hybrid nanoparticles. In particular, we find that plasmonic resonances allow for extremely high and spectrally tunable polarizabilities, leading to unique nonlinear light-matter interactions. Here, for the first time we were able to observe plasmonic resonant solitons over more than 25 diffraction lengths, in colloidal nanosuspensions.

Surface plasmon resonances of metal nanostructures have been studied intensely in recent years. The strong plasmon-mediated electric field enhancement and field confinement well beyond the diffraction limit has been demonstrated to improve the performance of optical devices including ultrasensitive sensors, light emitters, and optical absorbers. A plasmon resonance mode of particular recent interest is the gap plasmon resonance that occurs on closely spaced metallic structures. In contrast to plasmon resonances supported by isolated metal nanostructures, coupled nanostructures provide additional spectral and spatial control over the plasmon resonance response. For example, the resonance frequencies of metal nanoparticle dimers depend strongly on the gap size between the nanoparticles. Gap plasmons can produce local electric field enhancement factors that are several orders of magnitude stronger and more confined than surface plasmon resonances of isolated plasmonic nanospheres. The reliance of gap plasmons on few-nanometer separation between nanostructures makes it difficult to prepare gap-plasmon supporting structures with predictable resonance frequency and field enhancement. A structure that avoids this challenge is the film-coupled nanoparticle (NP). Similar to nanoparticle dimers, a nanoparticle on a supporting metallic film (or NP-on-a-mirror) can offer a strong coupling between the particle and its local environment, in this case the supporting film instead of adjacent nanoparticles, enabling strongly confined gap-plasmon modes. The NP-on-a-mirror geometry has been shown to produce reproducible gap plasmon resonances in a chemically and thermally robust, easy to fabricate structure. In this Thesis, we first present a scheme for controlling the gap plasmon resonance frequency of single gold nanoparticles using aluminum oxide coated metal films. We demonstrate experimentally and numerically that the gap-plasmon resonance of single gold nanoparticles can be tuned throughout the visible range by controlling the aluminum oxide thickness via anodization. In a separate study of Au NP on Al2O3 coated gold films it is shown that the oxide coating improves the stability of the structure under intense laser irradiation. An combined experimental and numerical analysis of the spectral response of Au NP on rough Au films shows that a film roughness of a few nanometer can affect the gap plasmon resonance in the absence of an oxide spacer layer. A photoluminescence study of single gold nanoparticles on an Al2O3 coated gold film shows that the gap-plasmon resonance of this type of plasmonic structure can increase gold photoluminescence by more than four orders of magnitude. Related numerical simulations reveal that the local photoluminescence enhancement of a gold nanoparticle on an Al2O3 coated gold film can be as high as one million near the particle-film junction. Finally, a new plasmonic sensing element was proposed based on our findings in the previous chapters. This proposed hole-in-one structure offers several attractive features including an easily optically accessible gap plasmon mode, while maintaining a relatively simple fabrication method. Taken together, the research presented in this Thesis demonstrates how the resonance frequency, field enhancement, mode polarization, structural stability, and structure reliability can be controlled at the nanoscale. The knowledge gained in the course of this work could lead to further development of nanophotonic devices that utilize extremely confined optical fields and precisely controlled resonance frequencies.