No description available.

Providing molecular fingerprint information, vibrational spectroscopy is a powerful tool for chemical analysis. In the mid-infrared window, FT-IR spectroscopy and microscopy have been routinely used for sample characterization. In the near-IR window, near-infrared spectroscopy has been widely used for tissue analysis and for the detection of lipids in the arterial walls. Yet, these traditional linear spectroscopies have intrinsic limitations. FT-IR spectroscopy suffers from a poor spatial resolution and strong water absorption for the study of living systems. Near-infrared spectroscopy avoids water absorption, yet it suffers from a poor, millimeter-scale spatial resolution in tissue analysis. My thesis focuses on breaking these limitations through photoacoustic and photothermal detection approaches. The first part of my thesis is on improving the spatial resolution in catheter-based intravascular photoacoustic (IVPA) imaging. By near-infrared excitation of lipids and acoustic detection, IVPA allows depth-resolved identification of lipid-laden atherosclerotic plaque. Thus far, most IVPA endoscopes use multimode fibers, which do not allow tight focusing of photons. Recent experiments on pulse propagation in multimode graded-index fibers have shown a nonlinear improvement in beam quality. Here, we harness this nonlinear phenomenon for the fiber-delivery of nanosecond laser pulses. We built a photoacoustic catheter 1.4 mm outer diameter, offering a lateral resolution as fine as 30 μm within a depth range of 2.5 mm. Such resolution is one order of magnitude better than current multi-mode fiber-based intravascular photoacoustic catheters. At the same time, the delivered pulse energy can reach as high as 20 μJ, which is two orders of magnitude higher than that of an optical resolution photoacoustic endoscope built with single-mode fiber. These improvements are expected to promote the biomedical application of photoacoustic endoscopes which require both high resolution and high pulse energy. Based on the technical advances, my thesis work further demonstrated longitudinal imaging of the same plaque in the same living animal. Recently developed mid-infrared photothermal (MIP) microscopy overcomes the limitations in FT-IR microscopy by probing the IR absorption-induced photothermal effect using visible light. MIP microscopy yields sub-micrometer spatial resolution with high spectral fidelity and much-reduced water background. The second part of my thesis work pushes the physical limits of MIP microscopy in aspects of detection sensitivity and imaging speed using two approaches. First, taking advantage of the interference scattering effect, the scattering signal from the sample can be greatly enhanced. Together with the relatively large infrared absorption coefficient, the sensitivity of the infrared spectrum is greatly improved, and single virus detection is achieved. Second, by using fluorescence as a thermo-sensitive probe, the temperature raise by infrared absorption can be retrieved in a more efficient way and much higher imaging speed and sensitivity are thus accomplished.

This thesis describes a series of experiments that were undertaken to extend the limits of output power and wavelength coverage of optically pumped mid-infrared lasers. Initially, two new cw Raman lasers operating at wavelengths of 11.5 and 12.5 pm were developed. Maximum output powers of 650 add 150 mW were produced, with pump powers of 11 and 3.3 W, respectively. The effect of the pump offset on the output power was then determined by measuring the efficiency of an NH3 laser pumped at frequency offsets of 94 and 274 MHz. In lasers ope rating in pure NH3 the larger pump offset required a greater pump intensity to reach threshold, but efficiency increased with pump offset. Higher NH3 pressures could be used at larger pump offsets and the improved efficiency was attributed to reduced saturation effects at the higher operating pressures. Experiments carried out with NH3 inversion lasers have greatly increased the output powers available at a large number of wavelengths in the 10 to 14 um range. In a buffered NH3 mixture, the sR(5,0) transition was pumped on resonance. Collisions with either N2 or Ar buffer gases were effective in thermalizing the rotational populations in the v2=1 vibrational level and producing gain on a wide range of frequencies. Output powers as large as 3.5 W on a single Tine and greater than 5 W multi-line were produced, at efficiencies of 20 and 30 % respectively. The number of lasing wavelengths increased substantially, as more than forty ortho-NH3 transitions were observed to lase in a grating-tuned cavity. The optical pumping technique was then used for the first time to produce line-tunable lasing on para-NH3 transitions. The sR(5,1) transition was pumped near resonance and 24 para-transitions were observed to lase. In total, lasing was achieved on 65 different transitions in 14NHg, with wave lengths of 10.3 to 13.8 pm. / Thesis / Master of Science (MS)

Silicon has been the material of choice of the photonics industry over the last decade due to its easy integration with silicon electronics as well as its optical transparency in the near-infrared telecom wavelengths. Besides these, it has very high refractive index, and also a broad optical transparency window over the entire mid-IR till about 8[Mu]m. Photonic crystal is well known that it can slow down the speed of light. It also can provide a universal platform for microcavity optical resonators with high quality factor Q and small modal volumes. The slow light effect, high Q and small modal volumes enhance light-matter interaction, together with high refractive index of silicon can be utilized to build a highly sensitive, high throughput sensor with small footprint. In this research, we have demonstrated highly compact and sensitive silicon based photonic crystal biosensor by engineering the photonic crystal microcavity in both cavity size and cavity-waveguide coupling condition. We have developed solutions to increase biosensor throughput by integrating multimode interference device and improving the coupling efficiency to a slow light photonic crystal waveguides. We have also performed detailed investigations on silicon based photonic devices at mid-infrared region to develop an ideal platform for highly sensitive optical absorption spectroscopy on chip. The studies have led to the demonstration of the first slot waveguide, the first photonic crystal waveguide, and the first holey photonic crystal waveguide and first slotted photonic crystal waveguide in silicon-on-sapphire at mid-infrared. The solutions and devices we developed in our research could be very useful for people to realize an integrated photonic circuit for biological and chemical sensing in the future. / text

The mid-infrared wavelength region (2-20 µm) is of great utility for a number of applications, including chemical bond spectroscopy, trace gas sensing, and medical diagnostics. Despite this wealth of applications, the on-chip mid-IR photonics platform needed to access them is relatively undeveloped. Silicon is an attractive material of choice for the mid-IR, as it exhibits low loss through much of the mid-IR. Using silicon allows us to take advantage of well-developed fabrication techniques and CMOS compatibility, making the realization of on-chip integrated mid-IR devices more realistic. The mid-IR wavelengths also afford the opportunity to exploit Si's high third-order optical nonlinearity for nonlinear frequency generation applications. In this work, we present a Si-based platform for mid-IR photonics, with a special focus on micro-resonators for strong on-chip light confinement in the 4-5 μm range. Additionally, we develop experimental optical characterization techniques to overcome the inherent difficulties of working in this wavelength regime. First, we demonstrate the design, fabrication, and characterization of photonic crystal cavities in a silicon membrane platform, operational at 4.4 μm (Chapter 2). By transferring the technique known as resonant scattering to the mid-IR, we measure quality (Q) factors of up to 13,600 in these photonic crystal cavities. We also develop a technique known as scanning resonant scattering microscopy to image our cavity modes and optimize alignment to our devices. Next, we demonstrate the electro-optic tuning of these mid-IR Si photonic crystal cavities using gated graphene (Chapter 3). We demonstrate a tuning of about 4 nm, and demonstrate the principle of on-chip mid-IR modulation using these devices. We then investigate the phenomenon of optical bistability seen in our photonic crystal cavities (Chapter 4). We discover that our bistability is thermal in origin and use post-processing techniques to mitigate bistability and increase Q-factors. We then demonstrate the design, fabrication, and characterization grating-coupled ring resonators in a silicon-on-sapphire (SOS) platform at 4.4 μm, achieving intrinsic Q-factors as high as 278,000 in these devices (Chapter 5). Finally, we provide a quantitative analysis of the potential of our SOS devices for nonlinear frequency generation and describe ongoing experiments in this regard (Chapter 6). / Engineering and Applied Sciences

No description available.

This thesis describes a series of experiments that were undertaken to extend the limits of output power and wavelength coverage of optically pumped mid-infrared lasers. Initially, two new CW Raman lasers operating at wavelengths of 11.5 and 12.5 pm were developed. Maximum powers of 650 and 150 mW were produced, with pump powers of 11 and 3.3 W, respectively. The effect of the pump offset on the output power was then determined by measuring the efficiency of an NHg laser pumped at frequency offsets of 94 and 274 MHz. In lasers operating in pure NH3, the larger pump offset required a greater pump intensity to reach threshold, but efficiency increased with pump offset. Higher NH3 pressures could be used at larger pump offsets and the improved efficiency was attributed to reduced saturation effects at the higher operating pressures. Experiments carried out with NHg inversion lasers have greatly increased the output powers available at a large number of wavelengths in the 10 to.14 pm range. In a buffered NH3 mixture, the sR(5,0) transition was pumped on resonance. C^llis;o^r^s with either — or Ar buffer gases were effective in thermalizing the rotational populations in the v>2=1 vibrational level and producing gain on a wide range of frequencies. Output powers as large as 3.5 W on a single line and greater than 5 W multi-line were produced, at efficiencies of 20 and 30 % respectively. The number of lasing wavelengths increased substantially, as more than forty ortho-NH3 transitions were observed to lase in a grating-tuned cavity. The optical pumping technique was then used for the first time to produce line-tunable lasing on para-NH3 transitions. The sR(5,l) transition was pumped near resonance and 24 para-transst'lons were observed to lase. In total, lasing was achieved on 65 different transitions in 14NH3, with wavelengths of 10.3 to 13.8 pm. / Thesis / Master of Science (MSc)

The near- through mid-infrared offers a unique and, as this thesis aims to show, essential view of extragalactic stellar populations both nearby, at intermediate redshifts and at very high redshift. In chapter 2, I demonstrate that rest-frame near-IR photometry obtained by the Spitzer Space Telescope provides more robust stellar mass estimates for a spectroscopic sample of ~100 galaxies in the redshift desert (0.5<z<2), and is crucial for modeling galaxies with young star-forming populations. From this analysis, a surprising result emerges in the data. Although the rest-frame light short of 2 micron improves stellar mass estimates, the models and observations disagree beyond 2 micron and emission from non-stellar sources becomes significant. At wavelengths from 1-30 micron, stellar and non-stellar emission contribute equally to a galaxy's global spectral energy distribution. This is unlike visible wavelengths where stellar emission dominates or the far-IR where dust emission provides the bulk of a galaxy's luminosity. Using the sample of high-z galaxies, in chapter 3, I quantify the statistical significance of the excess emission at 2-5 micron and find the emission to correlate with the OII luminosity, suggesting a link between the excess emission and star formation. The origin of the excess emission is not clear, although I explore a number of non-stellar candidates in this chapter. Nearby resolved observations provide a clearer picture of the excess by spatially resolving 68 nearby galaxies. By analyzing the pixel-by-pixel near-IR colours within each galaxy at ~1-5 micron, increasingly red near-IR colors are mapped to spatial regions in chapter 4. For regions with red NIR colors and high star formation rates, I find the broad near- through mid-IR spectrum is constant, varying only in amplitude as a function of the intensity of star formation, suggesting the infrared emission of a young, dusty stellar populations can be added to stellar population synthesis models as an additional component tied to the star formation rate. In closing the thesis, the focus is moved to the detection of stellar populations in the earliest star-forming galaxies. By z>6, all visible wavelength emission is redshifted into near-IR wavelengths. In chapter 5, I show how a tunable near-IR filter I have helped develop holds promise for finding bright Lyman alpha emitting galaxies at redshifts of 8<z<11.

The objective of this Ph.D. project is to investigate Arctic middle atmosphere chemistry using solar infrared absorption spectroscopy. These measurements were made at the Polar Environment Atmospheric Research Laboratory (PEARL) at Eureka, Nunavut, which is operated by the Canadian Network for the Detection of Atmospheric Change (CANDAC). This research is part of the CANDAC/PEARL Arctic Middle Atmosphere Chemistry theme and aims to improve our understanding of the processes controlling the stratospheric ozone budget using measurements of the concentrations of stratospheric constituents. The instrument, a Bruker IFS 125HR Fourier transform infrared (FTIR) spectrometer, has been specifically designed for high-resolution measurements over a broad spectral range and has been used to measure reactive species, source gases, reservoirs, and dynamical tracers at PEARL since August 2006. The first part of this research focuses on the optimization of ozone retrievals, for which 22 microwindows were studied and compared. The spectral region from 1000 to 1005 cm-1 was found to be the most sensitive in both the stratosphere and troposphere, giving the highest number of independent pieces of information and the smallest total error for retrievals at Eureka. iii Similar studies were performed in coordination with the Network for the Detection of Atmospheric Composition Change for nine other species, with the goal of improving and harmonizing the retrieval parameters among all Infrared Working Group sites. Previous satellite validation exercises have identified the highly variable polar conditions of the spring period to be a challenge. In this work, comparisons between the 125HR and ACE-FTS (Atmospheric Chemistry Experiment-Fourier transform spectrometer) from 2007 to 2010 have been used to develop strict criteria that allow the ground and satellite-based instruments to be confidently compared. After applying these criteria, the differences between the two instruments were generally small and in good agreement with previous ground-based FTIR/ACE-FTS comparisons. No clear bias was seen from year-to-year, and, in all cases, the difference between the measurements was within one standard deviation. The mean biases between the ACE-FTS and 125HR partial columns for 2007-2010 were -5.61 to 1.11%, -0.23 to 4.86%, -15.33 to -2.86%, -4.77 to 1.09%, and -0.34 to 5.23% for O3, HCl, ClONO2, HNO3, and HF, respectively. The 125HR measurements and three atmospheric models (CMAM-DAS, GEM-BACH, and SLIMCAT) were used to derive an NOy partial column data product for Eureka. This data product includes the five primary species NO, NO2, HNO3, N2O5, and ClONO2 and was used to study the seasonal and interannual variability of NOy from 2007 to 2010. The NOy 15-40 km partial column was found to be approximately constant through the sunlit part of the year, with greater variability during the spring. The mean partial column averaged for the spring period was (2.5±0.2)x1016 molec cm-2, while for the summer, it was (2.3±0.1)x1016 molec cm-2. The springtime evolution of NOy and its constituent nitrogen species, was also examined for all four years. The variability of the 5-NOy partial column was seen to be dominated by that of HNO3. iv The evolution of the individual nitrogen species was found to be consistent with the current understanding of the chemical and dynamical processes that occur in the polar stratosphere. Unusually low ozone columns were measured at Eureka from mid-February to late March 2011 and compared to the previous 14 years of measurements by the 125HR and its predecessor, Environment Canada’s Bomem DA8. The normalized O3/HF, HCl/HF, and HNO3/HF ratios, for which the effects of dynamics have been reduced, also showed record minima over this period. The SLIMCAT chemical transport model was used to quantify chemical ozone loss using the passive subtraction method. Chemical ozone depletion inside the vortex above Eureka was estimated to be 35%, which is the largest observed there in the past 15 years.

The objective of this Ph.D. project is to investigate Arctic middle atmosphere chemistry using solar infrared absorption spectroscopy. These measurements were made at the Polar Environment Atmospheric Research Laboratory (PEARL) at Eureka, Nunavut, which is operated by the Canadian Network for the Detection of Atmospheric Change (CANDAC). This research is part of the CANDAC/PEARL Arctic Middle Atmosphere Chemistry theme and aims to improve our understanding of the processes controlling the stratospheric ozone budget using measurements of the concentrations of stratospheric constituents. The instrument, a Bruker IFS 125HR Fourier transform infrared (FTIR) spectrometer, has been specifically designed for high-resolution measurements over a broad spectral range and has been used to measure reactive species, source gases, reservoirs, and dynamical tracers at PEARL since August 2006. The first part of this research focuses on the optimization of ozone retrievals, for which 22 microwindows were studied and compared. The spectral region from 1000 to 1005 cm-1 was found to be the most sensitive in both the stratosphere and troposphere, giving the highest number of independent pieces of information and the smallest total error for retrievals at Eureka. iii Similar studies were performed in coordination with the Network for the Detection of Atmospheric Composition Change for nine other species, with the goal of improving and harmonizing the retrieval parameters among all Infrared Working Group sites. Previous satellite validation exercises have identified the highly variable polar conditions of the spring period to be a challenge. In this work, comparisons between the 125HR and ACE-FTS (Atmospheric Chemistry Experiment-Fourier transform spectrometer) from 2007 to 2010 have been used to develop strict criteria that allow the ground and satellite-based instruments to be confidently compared. After applying these criteria, the differences between the two instruments were generally small and in good agreement with previous ground-based FTIR/ACE-FTS comparisons. No clear bias was seen from year-to-year, and, in all cases, the difference between the measurements was within one standard deviation. The mean biases between the ACE-FTS and 125HR partial columns for 2007-2010 were -5.61 to 1.11%, -0.23 to 4.86%, -15.33 to -2.86%, -4.77 to 1.09%, and -0.34 to 5.23% for O3, HCl, ClONO2, HNO3, and HF, respectively. The 125HR measurements and three atmospheric models (CMAM-DAS, GEM-BACH, and SLIMCAT) were used to derive an NOy partial column data product for Eureka. This data product includes the five primary species NO, NO2, HNO3, N2O5, and ClONO2 and was used to study the seasonal and interannual variability of NOy from 2007 to 2010. The NOy 15-40 km partial column was found to be approximately constant through the sunlit part of the year, with greater variability during the spring. The mean partial column averaged for the spring period was (2.5±0.2)x1016 molec cm-2, while for the summer, it was (2.3±0.1)x1016 molec cm-2. The springtime evolution of NOy and its constituent nitrogen species, was also examined for all four years. The variability of the 5-NOy partial column was seen to be dominated by that of HNO3. iv The evolution of the individual nitrogen species was found to be consistent with the current understanding of the chemical and dynamical processes that occur in the polar stratosphere. Unusually low ozone columns were measured at Eureka from mid-February to late March 2011 and compared to the previous 14 years of measurements by the 125HR and its predecessor, Environment Canada’s Bomem DA8. The normalized O3/HF, HCl/HF, and HNO3/HF ratios, for which the effects of dynamics have been reduced, also showed record minima over this period. The SLIMCAT chemical transport model was used to quantify chemical ozone loss using the passive subtraction method. Chemical ozone depletion inside the vortex above Eureka was estimated to be 35%, which is the largest observed there in the past 15 years.