Ferroelectric crystals are commonly used in nonlinear optics for frequency conversion of laser radiation. The quasi-phase matching (QPM) approach uses a periodically modulated nonlinearity that can be achieved by periodically inverting domains in ferroelectric crystals and allows versatile and efficient frequency conversion in the whole transparency region of the material. KTiOPO4 (KTP) is one of the most attractive ferroelectric non-linear optical material for periodic domain-inversion engineering due to its excellent non-linearity, high resistance for photorefractive damage, and its relatively low coercive field. A periodic structure of reversed domains can be created in the crystal by lithographic patterning with subsequent electric field poling. The performance of the periodically poled KTP crystals (PPKTP) as frequency converters rely directly upon the poling quality. Therefore, characterization methods that lead to a deeper understanding of the polarization switching process are of utmost importance. In this work, several techniques have been used and developed to study domain structure in KTP, both in-situ and ex-situ. The results obtained have been utilized to characterize different aspects of the polarization switching processes in KTP, both for patterned and unpatterned samples. It has also been demonstrated that it is possible to fabricate sub-micrometer (sub-μm) PPKTP for novel optical devices. Lithographic processes based on e-beam lithography and deep UV-laser lithography have been developed and proven useful to pattern sub- μm pitches, where the later has been the most convenient method. A poling method based on a periodical modulation of the K-stoichiometry has been developed, and it has resulted in a sub-μm domain grating with a period of 720 nm for a 1 mm thick KTP crystal. To the best of our knowledge, this is the largest domain aspect-ratio achieved for a bulk ferroelectric crystal. The sub-micrometer PPKTP samples have been used for demonstration of 6:th and 7:th QPM order backward second-harmonic generation with continuous wave laser excitation, as well as a demonstration of narrow wavelength electrically-adjustable Bragg reflectivity. / QC 20100930

Lithium niobate (LiNbO3) and KTiOPO4 (KTP) are ferroelectric crystals of considerable interest in different fields of optics and optoelectronics. Due to its large values of the nonlinear optical, electro-optic (EO), piezoelectric and acousto-optical coefficients, LiNbO3 is widely used for laser frequency conversion using the quasiphase matching (QPM) approach where the sign of nonlinearity has been periodically modulated by electric field poling (EFP). In the microwave and telecommunication field LiNbO3 is used for surface acoustic devices and integrated optical modulators. KTP and its isomorphs, on the other hand, exhibit slightly lower nonlinear coefficients but have much higher photorefractive damage thresholds, so that it is mainly used in the fabrication of QPM devices for both UV, IR and visible light generation and in high power applications. This thesis focus on different key issues: (1) accurate characterization of specific optical properties of LiNbO3, which are of interest in nonlinear and EO applications; (2) in-situ visualization and characterization of domain reversal by EFP in LiNbO3 and KTP crystals for a through understanding of the ferroelectric domain switching; (3) fabrication of periodic surface structures at sub-micron scale in LiNbO for photonic applications. An interferometric method is used for accurate measurement of ordinary and extraordinary refractive indices in uniaxial crystals, which is of great interest in the proper design of QPM crystals. A digital holography (DH) based method is presented here for 2D characterization of the EO properties of LiNbO , which is considerably interesting in the applications where the proper design of the EO device requires a spatially resolved information about the EO behaviour and the existing pointwise techniques are not sufficient. A DH method for novel in-situ monitoring of domain reversal by EFP in both LiNbO3 and KTP, is also presented here. The technqiue could be used as a tool for high fidelity periodic domain engineering but also provides information about domain kinetics, internal field and crystals defects. 3 3 3 Finally this thesis presents novel results concerning nanoscale periodic surface structuring of congruent LiNbO3. Holographic lithography (HL) is used for sub-micron period resist patterning and electric overpoling for surface domain reversal. Surface structures are obtained by selective etching. Moiré effect is also used in the HL to fabricate complicated structures with multiple periods. The depth compatibility with waveguide implementation allows foreseeing possible applications of these structures for Bragg gratings or innovative photonic crystal devices, exploiting the additional nonlinear and EO properties typical of LiNbO3. / QC 20100824

Nonlinear optical interaction in quasi-phase-matched structures opens up unique possibilities to build compact and efficient parametric devices such as optical parametric oscillators, generators, and amplifiers with tailored spectral properties. The focus of this thesis is on novel parametric interactions with periodically-poled KTiOPO4 (PPKTP) as the parametric gain medium. Optical parametric oscillators (OPOs) are attractive light sources for many applications, particularly in spectroscopy, and plays a central role in this thesis. Special attention is put on simple, yet powerful, spectral-manipulation and bandwidth-narrowing techniques for OPOs. The overall knowledge gained from these studies has been used for device construction of several tunable ultraviolet sources for biological sensing. In the case of bandwidth narrowing, the observation of decreasing spectral bandwidth in a noncollinear, idler-resonant OPO, as compared with a signal-resonant one, has been found to be due to the interplay between the material properties and the angular dispersion of PPKTP. To further reduce the bandwidth, we have shown that it is very beneficial to replace the output mirror in an OPO with a bulk Bragg grating. In fact, even close to degeneracy, where the bandwidth is typically wide, this approach is able to decrease the bandwidth drastically. Moreover, different OPO cavity designs have been examined in order to spectrally manipulate the resonant waves. By deploying a grating in a ring OPO cavity, it becomes possible to access the resonant wave and spectrally manipulated it in a zero-dispersion arrangement; the filtered wave is subsequently sent back into its own cavity as a seed signal, in a self-seeding arrangement. This particular cavity design decreases the bandwidth close to ~ 1000 times as compare to the free-running mode. An interesting phenomenon arises when two mutually coherent laser beams are used to pump a linear OPO cavity. When the pump beams intersect within the PPKTP crystal, an interference grating is formed and acts as a catalyst for the generation of new spectral sidebands through multiple cascaded four-wave mixing, in the pump, the idler and the signal directions. The spacing of these sidebands is determined geometrically by the incident pump angle, while the signals are continuously tunable over the c-band telecom window (λ ~ 1.5 μm) by rotating the cavity. Ultrabroad bandwidths have been generated in an optical parametric generator (OPG) pumped by an amplified picosecond Ti:sapphire laser. In the collinear direction the output spectrum extends over three octaves in the mid-infrared region. This enormously broad spectrum is also Fourier-filtered and subsequently used for narrowband seeding of an optical parametric amplifier (OPA). Finally, the spectral range between 285 nm and 340 nm is of importance for detection of biological substances through fluorescence spectroscopy. With this spectral region in mind a practical way to generate a tunable parametric device in the ultraviolet region is presented in the thesis. The developed ultraviolet laser is used for studies of the characteristics of biological particles. The ultraviolet source and the results from these studies, will be utilized in an integrated detection system, a so called early-warning system. / QC 20100923

Nonlinear optical frequency conversion is one of the most important key techniques in order to obtain lasers with wavelengths targeted for specific applications. In order to realize efficient and tailored lasers, the quasi-phase-matching (QPM) approach using periodically-poled ferroelectric crystals is getting increasingly important. Also understanding of damage mechanisms in nonlinear materials is necessary to be able to design reliable and well working lasers. This is especially true for high power application lasers, which is a rapidly growing field, where the damage problem normally is the ultimate limiting factor. In this thesis work, several promising novel ferroelectric materials have been investigated for nonlinear optical applications and the emphasis has been put on QPM devices consisting of periodically-poled structures. The materials were selected from three different types of ferroelectric materials: 1) MgO-doped stoichiometric LiNbO3 (MgO:SLN) and LiTaO3 (MgO:SLT), and non-doped stoichiometric LiTaO3 (SLT), 2) KTiOPO4 (KTP) and its isomorphs RbTiOPO4 (RTP), and 3) KNbO3 (KN). The focus in our investigations have been put on the spontaneous polarization switching phenomena, optimization of the periodic poling conditions, and the photochromic optical damage properties which were characterized by the help of blue light-induced infrared absorption (BLIIRA) measurements. With electrical studies of the spontaneous polarization switching, we were able to determine quantitatively, and compare, the coercive field values of different materials by applying triangularly shaped electric fields. We found that the values of the coercive fields depended on the increase rate of the applied electric field. The coercive field of KN was the lowest (less than 0.5 kV/mm) followed by the ones of KTP, SLT, and MgO:SLT (1.5 to 2.5 kV/mm). MgO:SLN, and RTP had relatively high coercive fields, approximately 5.0 to 6.0 kV/mm, respectively. Based on the domain switching characteristics we found, we successfully fabricated periodically-poled devices in all of the investigated materials with 30 μm periodicities and sample thickness of 1 mm. Blue light-induced infrared absorption (BLIIRA) has been characterized for unpoled bulk and periodically-poled samples using a high-sensitivity, thermal-lens spectroscopy technique. SLT showed a large photorefraction effect and the BLIIRA signal could not be properly measured because of the large distortion of the probe beam. The rise and relaxation time of BLIIRA, after switching the blue light on and off was in a time span of 10 to 30 sec except for KTP and its isomorphs, which needed minutes to hours in order to saturate at a fixed value. KN and MgO:SLN showed the lowest susceptibility to the induced absorption. Periodic poling slightly increased the susceptibility of KTP, MgO:SLT, and KN. Relatively high thresholds were observed in MgO:SLT and KN. By increasing the peak-power intensity of the blue light, the induced absorption for MgO:SLN, KTP and KN saturated at a constant value while that of MgO:SLT increase in a constant fashion. This trend is critical issue for the device reliability at high-power applications. / QC 20100830

Photodetectors are a kind of semiconductor devices that convert incoming light to an electrical signal. Photodetectors are classified based on their different structure, fabrication technology, applications and different sensitivity. Infrared photodetectors are widely used in many applications such as night vision, thermal cameras, remote temperature sensing, and medical diagnosis etc.   All detectors have material inside that is sensitive to incoming light. It will absorb the photons and, if the incoming photons have enough energy, electrons will be excited to higher energy levels and if these electrons are free to move, under the effect of an external electric field, a photocurrent is generated.   In this project Fourier Transform Infrared (FT-IR) Spectroscopy is used to investigate a new kind of photodiodes that are based on self-assembled semiconductor nanowires (NWs) which are grown directly on the substrate without any epi-layer. The spectrally resolved photocurrent (at different applied biases) and IV curves (in darkness and illumination) for different temperatures have been studied respectively. Polarization effects (at low and high Temperatures) have been investigated.  The experiments are conducted for different samples with high concentration of NWs as well as with lower concentration of NWs in the temperature range from 78 K (-195ºC) to 300 (27ºC). These photodiodes are designed to work in near infrared (NIR) spectral range.   The results show that the NW photodetectors indeed are promising devices with fairly high break down voltage, change of photocurrent spectra with polarized light, low and constant reverse saturation current (Is). The impact of different polarized light on photocurrent spectra has been investigated and an attempt has been made to clarify the observed double peak of InP photocurrent spectrum. Our investigations also include a comparison to a conventional planar InP p-i-n photodetector.

Early detection is vital to successfully treating breast cancer, and mammography screening is the most efficient and wide-spread method to reach this goal. Imaging low-contrast targets, while minimizing the radiation exposure to a large population is, however, a major challenge. Optimizing the image quality per unit radiation dose is therefore essential. In this thesis, two optimization schemes with respect to x-ray photon energy have been investigated: filtering the incident spectrum with refractive x-ray optics (spectral shaping), and utilizing the transmitted spectrum with energy-resolved photon-counting detectors (spectral imaging). Two types of x-ray lenses were experimentally characterized, and modeled using ray tracing, field propagation, and geometrical optics. Spectral shaping reduced dose approximately 20% compared to an absorption-filtered reference system with the same signal-to-noise ratio, scan time, and spatial resolution. In addition, a focusing pre-object collimator based on the same type of optics reduced divergence of the radiation and improved photon economy by about 50%. A photon-counting silicon detector was investigated in terms of energy resolution and its feasibility for spectral imaging. Contrast-enhanced tumor imaging with a system based on the detector was characterized and optimized with a model that took anatomical noise into account. Improvement in an ideal-observer detectability index by a factor of 2 to 8 over that obtained by conventional absorption imaging was found for different levels of anatomical noise and breast density. Increased conspicuity was confirmed by experiment. Further, the model was extended to include imaging of unenhanced lesions. Detectability of microcalcifications increased no more than a few percent, whereas the ability to detect large tumors might improve on the order of 50% despite the low attenuation difference between glandular and cancerous tissue. It is clear that inclusion of anatomical noise and imaging task in spectral optimization may yield completely different results than an analysis based solely on quantum noise. / QC 20100714