This dissertation contains the synthesis and characterization of squaraine based new functional materials. In the first part of this thesis work, a water soluble benzothiazolium squaraine dye was synthesized with pyridium pendents, and controlled aggregation properties were achieved. After formation of partially reversible J-aggregation on a polyelectrolyte (poly(acryl acid) sodium salt) template, the nonlinear, two-photon absorption cross section per repeat unit was found to be above 30-fold enhanced compared with nonaggregate and/or low aggregates. Using a similar strategy, sulfonate anions were introduced into the squaraine structure, and the resulting compounds exhibited good water solubilities. A ‘turn on’ fluorescence was discovered when these squaraine dyes interacted with bovine serum albumin (BSA), titration studies by BSA site selective reagents show these squaraine dyes can bind to both site I and II of BSA, with a preference of site II. Introduction of these squaraine dyes to BSA nanoparticles generated near-IR protein nano fabricates, and cell images were collected. Metal sensing properties were also studied using the sulfonates containing a benzoindolium squaraine dye, and the linear response of the absorption of the squaraine dye to the concentration of Hg2+ makes it a good heavy metal-selective sensing material that can be carried out in aqueous solution. Later, a squaraine scaffold was attached to deoxyribonucleosides by Sonogashira coupling reactions, in which the reaction conditions were modified. Iodo-deoxyuridine and bromo-deoxyadenosine were used as the deoxyribonucleosides building blocks, and the resulting squaraine dye-modified deoxyribonucleosides exhibited near-IR absorption and emission properties due to the squaraine chromophore. Interestingly, these non-natural deoxyribonucleosdies showed viscosity dependent photophysical properties, which make them nice candidates for fluorescence viscosity sensors at the cellular level. After incubation with cells, these iv viscosity sensors were readily uptaken by cell, and images were obtained showing regions of high viscosity in cells.

This work establishes a solid foundation for the use of indirect band gap semiconductors for light emitting application and presents the work on development of white light emitting diodes (LEDs) in silicon carbide (SiC). Novel laser doping has been utilized to fabricate white light emitting diodes in 6H-SiC (n-type N) and 4H-SiC (p-type Al) wafers. The emission of different colors to ultimately generate white light is tailored on the basis of donor acceptor pair (DAP) recombination mechanism for luminescence. A Q-switched Nd:YAG pulse laser (1064 nm wavelength) was used to carry out the doping experiments. The p and n regions of the white SiC LED were fabricated by laser doping an n-type 6H-SiC and p-type 4H-SiC wafer substrates with respective dopants. Cr, B and Al were used as p-type dopants (acceptors) while N and Se were used as n-type dopants (donors). Deep and shallow donor and acceptor impurity level states formed by these dopants tailor the color properties for pure white light emission. The electromagnetic field of lasers and non-equilibrium doping conditions enable laser doping of SiC with increased dopant diffusivity and enhanced solid solubility. A thermal model is utilized to determine the laser doping parameters for temperature distribution at various depths of the wafer and a diffusion model is presented including the effects of Fick's diffusion, laser electromagnetic field and thermal stresses due to localized laser heating on the mass flux of dopant atoms. The dopant diffusivity is calculated as a function of temperature at different depths of the wafer based on measured dopant concentration profile. The maximum diffusivities achieved in this study are 4.61x10-10 cm2/s at 2898 K and 6.92x10-12 cm2/s at 3046 K for Cr in 6H-SiC and 4H-SiC respectively. Secondary ion mass spectrometric (SIMS) analysis showed the concentration profile of Cr in SiC having a penetration depth ranging from 80 nm in p-type 4H-SiC to 1.5 [micro]m in n-type 6H-SiC substrates respectively. The SIMS data revealed enhanced solid solubility (2.29x1019 cm-3 in 6H-SiC and 1.42x1919 cm-3 in 4H-SiC) beyond the equilibrium limit (3x1017 cm-3 in 6H-SiC above 2500 [degrees]C) for Cr in SiC. It also revealed similar effects for Al and N. The roughness, surface chemistry and crystalline integrity of the doped sample were examined by optical interferometer, energy dispersive X-ray spectrometry (EDS) and transmission electron microscopy (TEM) respectively. Inspite of the larger atomic size of Cr compared to Si and C, the non-equilibrium conditions during laser doping allow effective incorporation of dopant atoms into the SiC lattice without causing any damage to the surface or crystal lattice. Deep Level Transient Spectroscopy (DLTS) confirmed the deep level acceptor state of Cr with activation energies of Ev+0.80 eV in 4H-SiC and Ev+0.45 eV in 6H-SiC. The Hall Effect measurements showed the hole concentration to be 1.98x1019 cm-3 which is almost twice the average Cr concentration (1x1019 cm-3) obtained from the SIMS data. These data confirmed that almost all of the Cr atoms were completely activated to the double acceptor state by the laser doping process without requiring any subsequent annealing step. Electroluminescence studies showed blue (460-498 nm), blue-green (500-520 nm) green (521-575 nm), and orange (650-690 nm) wavelengths due to radiative recombination transitions between donor-acceptors pairs of N-Al, N-B, N-Cr and Cr-Al respectively, while a prominent violet (408 nm) wavelength was observed due to transitions from the nitrogen level to the valence band level. The red (698-738 nm) luminescence was mainly due to metastable mid-bandgap states, however under high injection current it was due to the quantum mechanical phenomenon pertaining to band broadening and overlapping. This RGB combination produced a broadband white light spectrum extending from 380 to 900 nm. The color space tri-stimulus values for 4H-SiC doped with Cr and N were X = 0.3322, Y = 0.3320 and Z = 0.3358 as per 1931 CIE (International Commission on Illumination) corresponding to a color rendering index of 96.56 and the color temperature of 5510 K. And for 6H-SiC n-type doped with Cr and Al, the color space tri-stimulus values are X = 0.3322, Y = 0.3320 and Z = 0.3358. The CCT was 5338 K, which is very close to the incandescent lamp (or black body) and lies between bright midday sun (5200 K) and average daylight (5500 K) while CRI was 98.32. Similar white LED's were also fabricated using Cr, Al, Se as one set of dopants and B, Al, N as another.

In this dissertation, a novel semiconductor mode-locked oscillator which is an extension of eXtreme Chirped Pulse Amplification (XCPA) is investigated. An eXtreme Chirped Pulse Oscillator (XCPO) implemented with a Theta cavity also based on a semiconductor gain is presented for generating more than 30ns frequency-swept pulses with more than 100pJ of pulse energy and 3.6ps compressed pulses directly from the oscillator. The XCPO shows the two distinct characteristics which are the scalability of the output energy and the mode-locked spectrum with respect to repetition rate. The laser cavity design allows for low repetition rate operation < 100MHz. The cavity significantly reduces nonlinear carrier dynamics, integrated self phase modulation (SPM), and fast gain recovery in a Semiconductor optical Amplifier (SOA). Secondly, a functional device, called a Grating Coupled Surface Emitting Laser (GCSEL) is investigated. For the first time, passive and hybrid mode-locking of a GCSEL is achieved by using saturable absorption in the passive section of GCSEL. To verify the present limitation of the GCSEL for passive and hybrid mode-locking, a dispersion matched cavity is explored. In addition, a Grating Coupled surface emitting Semiconductor Optical Amplifier (GCSOA) is also investigated to achieve high energy pulse. An energy extraction experiment for GCSOA using stretched pulses generated from the colliding pulse semiconductor mode-locked laser via a chirped fiber bragg grating, which exploits the XCPA advantages is also demonstrated. Finally, passive optical cavity amplification using an enhancement cavity is presented. In order to achieve the interferometric stability, the Hänsch-Couillaud Method is employed to stabilize the passive optical cavity. The astigmatism-free optical cavity employing an acousto-optic modulator (AOM) is designed and demonstrated. In the passive optical cavity, a 7.2 of amplification factor is achieved with a 50 KHz dumping rate.

From space exploration and advanced aircraft to next generation weapons, achieving hypersonic speed is becoming increasingly important across a range of research domains. The immense challenge associated with this goal involves the development of suitable materials and systems for the different components of a hypersonic vehicle, each of which must have the inherent capability to resist extreme temperatures, high thermal shock due to high heat flux, and high oxidation and ablation. First, the ultra-high temperature ceramic (UHTC) zirconium diboride or ZrB2 was sintered by ultra-fast high temperature sintering (UHS). The UHS process was optimized and the sintering parameters for ZrB2 and other UHTCs were studied. ZrB2 is an ultra-high temperature ceramic (UHTC) with a very high melting point; thus, its densification is difficult, energy intensive, and time-consuming. Commercial ZrB2 powders were rapidly densified via UHS to >90% relative density within 60 second in vacuum without pressure. The effect of sintering time on densification and final grain size were studied. An innovative process for manufacturing bulk UHTC materials was studied and is detailed herein. Second, the work function (W_f) of electron emitting materials was reduced to improved performance. A reduction of W_f in multicomponent hexaborides was achieved by doping with highly electropositive Ba, which enhances electron emission. Single-phase bulk multicomponent polycrystalline hexaborides of La0.5Ba0.5B6, Ce0.5Ba0.5B6, and BaB6 powders were first synthesized and then densified by UHS sintering. W_f measurements were obtained by Kelvin probe force microscopy. Ba-substitution was found to lower W_f (~25%) in synthesized multicomponent hexaborides. The specific techniques required to engineer the W_f of these materials are also provided herein. Finally, combining low W_f materials with UHTCs was explored for thin film systems for the exterior surface of hypersonic vehicles. The thin films of CeB6, a low W_f material, was deposited on sintered ZrB2 by RF-sputtering and single crystalline SrTiO3 (STO) substrates. Epitaxial thin films of SrHfO3 (SHO) were also deposited on (100), (110) and (111) STO substrates at 600°C. X-ray diffraction (XRD) results confirmed the formation of epitaxial layer, and reciprocal space mapping (RSM) was used to characterize film's mosaicity / texture on different substrates. XRD and RSM data demonstrated that the most favorable film growth direction was (110). As detailed herein, an inexpensive thin film production process, RF-sputtering, was exploited to manufacture various epitaxial and non-epitaxial layers of low W_f materials on UHTC and single-crystal substrates for hypersonic vehicles. To summarize, a range of bulk UHTCs and low W_f materials were prepared by UHS, and various thin films of low W_f material were produced on UHTC. Thereafter, the properties of synthesized materials were studied to develop new material systems for hypersonic applications. The findings from this research shed light on the development of suitable materials for implementation of electron transpiration cooling for hypersonic vehicle development. / Doctor of Philosophy / Rapid sintering of ultra-high temperature ceramics (UHTCs) and synthesis of low work-function electron emitting materials have been performed by ultra-fast high temperature sintering technique (UHS). Sintering of UHTCs is a difficult process, due to their high melting temperature, presence of covalent bond, and slower diffusion coefficient. A long sintering duration is used to achieve a high relative density along with adding sintering aid, using fine powder (produced by milling), and utilizing pressure (such as field assisted sintering and hot-pressing technology) during sintering. Synthesis and densification of multicomponent hexaboride is difficult, involves multi-steps and complicated processes. These long and complicated processes not only prolong development of new materials but also cause chemical wastes. To overcome all the aforementioned processing issues, an advanced processing technique, UHS, is used and densified pure and commercially available UHTCs to >90% within 60 second without applying sintering aid, powder milling, and pressure. The outcome of this research demonstrates the potential for a simple, cost-effective, fast, and adjustable processes, UHS, to develop a wide range of bulk UHTCs and other technical ceramics, and it gives new insight into the mechanisms of rapid sintering of UHTCs by rapid heating. The first detailed studies (experimental report) on rapid sintering of ZrB2 (and other UHTCs) by UHS technique and a through characterizations of the UHS sintered sample were performed to understand rapid sintering mechanism and how the processing effects the microstructure and properties of UHS ZrB2. The rapid microstructural evolution during the UHS sintering is investigated at 10, 30, and 60 second sintering interval. The UHS technique enables a heating rate of 103 - 104 °C/min and reaches a sintering temperature of 2600 °C in 30 seconds. Microstructural analysis was conducted on polished sample surfaces by using ImageJ software (National Institutes of Health, version 1.53e), measuring the grain size perpendicular to two diagonals of each grain. A comparison of grain size from sample center and periphery showed a homogeneous microstructure after sintering. Furthermore, the rapid sintering did not change/effect crystallinity, boron to metal stoichiometry, and grain boundary elemental composition as observed by XRD and EDS analysis. Additional characterization of the UHS sintered ZrB2 shows a hardness and elastic modulus of 30 GPa and 412 GPa respectively by nanoindentation method. Finally, the oxidation test at 1100 °C in isothermal condition showed a weight gain of 1.4% in air. The low work-function (W_f) materials are famous for electron emitting applications like electron guns for scanning electron microscopy. DFT simulation predicts the W_f of the widely used electron emitters (such as LaB6 and CeB6) can be reduced by changing their compositions, which increase electron generation efficiency of those materials. Previously, those materials were synthesized by long processes that involved multiple processing steps, which required expensive starting materials and yielded chemical wastes. The advantages of rapid sintering technique, UHS, had been exploited to synthesize low work function electron emitting materials. Single-phase bulk polycrystalline hexaborides were produced by using electrically powered UHS technique using a vacuum atmosphere. A reaction synthesis route: B4C reduction technique was first used to form pure phase hexaboride. Then, the synthesized compositions were densified to ~90% theoretical density in 180 seconds by UHS densification. After UHS sintering, XRD analysis confirmed the presence of a phase pure cubic BaB6, La0.5Ba0.5B6, and Ce0.5Ba0.5B6. Additional analyses were conducted to determine an optimum reaction temperature 1500 and 2100 °C for the formation BaB6 and multi-component hexaborides. Microstructural analyses were conducted to observe both reaction-synthesized and densified products. EDS compositional analysis and elemental mapping revealed a stoichiometric reaction product with homogeneous metal cation and boron distributions. The W_f of BaB6, La0.5Ba0.5B6, and Ce0.5Ba0.5B6 was determined to be 1.95 ± 0.1, 2.05 ± 0.1 and 2.0 ± 0.1 eV, respectively. The addition of BaB6 in La0.5Ba0.5B6, and Ce0.5Ba0.5B6 resulted in a 25% decrease in W_f for LaB6 from 2.7 ± 0.1 to 2.00 ± 0.1 eV and a 23% decrease in W_f for CeB6 from 2.68 ± 0.08 to 2.05 ± 0.1 eV. Ba substitution is shown to be a general method for lowering W_f in a variety of multicomponent hexaborides. Finally, the polycrystalline thin films of CeB6, a low W_f material, was deposited on sintered ZrB2 by RF-sputtering technique. Additionally, epitaxial thin films of SrHfO3 (SHO) were also deposited on (100), (110) and (111) STO single crystalline substrates. Both types of thin films were deposited at 600 °C temperature and at a vacuum pressure of 10-3 Torr. After deposition of the SHO films, X-ray diffraction (XRD) was conducted to confirm the formation of epitaxial layer, and reciprocal space mapping (RSM) was used to characterize film's mosaicity / texture on different substrates. XRD and RSM data demonstrated that the most favorable film growth direction was (110). The XRD of the CeB6 film showed highly crystalline film was formed. For both the films, a detailed microstructural analysis was performed by scanning electron microscopy and film smoothness was characterized by atomic force microscopy method. As detailed herein, an inexpensive thin film production process, RF-sputtering, was exploited to manufacture various epitaxial and non-epitaxial layers of low W_f materials on UHTC and single-crystal substrates for hypersonic vehicles applications.

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