APPLICATION OF DIFFRACTIVE OPTICAL ELEMENT ON SPECTROSCOPY AND IMAGING

Zhenkun Guo: Application of Diffractive Optical Element on Spectroscopy and Imaging (Under the direction of Andrew Moran) Diffractive optical elements (DOE) are optical components that manipulate light by diffraction, interference, and other phase control methods. The application of DOE in multi-dimensional spectroscopy could significantly reduce the efforts required for conducting experiments and enhance the signal-to-noise ratio with high efficiency. In this dissertation, DOE-based two-dimensional resonance Raman spectroscopy was developed and implemented in two model systems, triiodide and myoglobin. This new technique uncovers new dimensions of information, which were not available with previous one-dimensional spectroscopy techniques. The DOE was also applied to the wide-field transient absorption microscopy. Conducting a large number of experiments simultaneously is possible in this configuration. Analysis of parallel measurements provides statistical information essential to comprehensively study heterogeneous samples. After absorbing an ultraviolet photon, triiodide undergoes photodissociation to produce diiodide and radical iodine on a time scale comparable to the period of triiodide’s nuclear motion, which could impulsively activate a vibrational coherence in the diiodide. In this dissertation, the ability of 2DRR to capture coherent reaction mechanisms is demonstrated by directly establishing a correlation, for the first time, between the nonequilibrium geometry of triiodide at photodissociation and the stretching frequency of diiodide. Ligand binding and dissociation processes are crucial to the functions of heme proteins. The recovery of the protein matrix involves fast energy dissipation from the heme group to solvent, facilitated by the propionic acid side chains as an effective “gateway”. In this dissertation, we found that the propionic chains possess significant structural heterogeneity, which could be induced by the thermal fluctuation in geometries. It is interesting to consider whether the variation in conformation could relate to the vibrational cooling rate distributions. Carrier diffusion is imaged in a perovskite film and crystal using a newly developed DOE-based wide-field transient absorption microscopy technique. The function of the instrument is illustrated with 41 parallel measurements conducted on methylammonium lead iodide perovskite films and single crystals in a single experiment. Obvious carrier diffusion is observed in the crystal. However, results indicate that the carrier dynamics in the film are dominated by many-body interactions instead. The grain boundaries in the film contribute to this difference in behavior.Doctor of Philosoph

HIGHER-ORDER EFFECTS IN CONDENSED PHASE SPECTROSCOPY AND DYNAMICS

ABSTRACT Thomas Paul Cheshire: Higher-Order Effects in Condensed Phase Spectroscopy and Dynamics (Under the direction of Andrew M. Moran) Researchers in the 1970s wondered whether traditional Raman experiments could distinguish homogeneous and inhomogeneous line broadening mechanisms. Since then, a feedback between experiment and theory has spawned and matured the field of multidimensional Raman spectroscopy and laid the groundwork for modeling nonlinear photoinduced reaction pathways. Here two-dimensional resonance Raman (2DRR) spectroscopy is developed to investigate photochemical reaction mechanisms and structural heterogeneity in condensed phase systems. Models are developed to understand 2DRR spectra and extended to incorporate non-radiative transitions. The photodissociation reaction of triiodide serves to uncover the capabilities of 2DRR. A unique pattern of 2DRR resonances is associated with the transition of a nuclear wavepacket from reactant to product. The pattern of resonances is reproduced by modeling the photodissociation as a vibronic coherence transfer. Transient absorption experiments performed on a transition metal complex composed of titanium and catechol, [Ti(cat)3]2-, exhibit signatures of coherent wavepacket motion initiated by back-electron transfer. The model used for triiodide photodissociation applies to this system, and calculations predict that vibrational coherences in the product are independent of whether the reactant undergoes coherent nuclear motion. Vibrational population-to-coherence transitions could accelerate the electron transfer (ET) process, regardless if vibrational dephasing is faster than the reaction rate--a prediction not captured by traditional ET models. 2DRR spectroscopy is further used to investigate oxygen- and water-ligated myoglobin line broadening mechanisms. Vibrational modes proximal to propionic acid side chains of the heme exhibit significant heterogeneity in the 2DRR spectra. A hydrophobic pocket encompasses the heme, but the side chains are exposed to solvent. Molecular dynamics (MD) simulations suggest that fluctuations in the side chain geometries are correlated with the heterogeneity. 2DRR spectra and MD simulations reveal that the side chains function as effective pathways for thermal relaxation. Despite progress, a major challenge still plagues multidimensional Raman spectroscopy. Cascading signals radiated in the same direction as the desired signal can render a signal impossible to analyze. Simulations of 2DRR, femtosecond stimulated Raman spectroscopy, and the accompanying artifacts suggest solute-solute and solute-solvent interactions can significantly affect measured signals if experimental parameters are not carefully selected.Doctor of Philosoph

Elucidation of Chemical Reactions by Two-Dimensional Resonance Raman Spectroscopy

It has been shown for many systems, including photosynthetic complexes, molecule-semiconductor interfaces, and bulk heterojunctions, that interaction between electronic and nuclear dynamics may heavily influence chemical mechanisms. Four-wave-mixing spectroscopies (i.e. transient absorption, two-dimensional spectroscopy) provide some insight into such non-equilibrium processes but are limited by the single “population time” available in these types of experiments. In this dissertation, two-dimensional resonance Raman spectroscopy (2DRR) is developed to obtain new information regarding chemical reactions that possess time coincident electronic and nuclear evolution. These new insights can only be acquired through higher-order techniques possessing two “population times”. Specifically, the coherent reaction mechanism in triiodide photodissociation and structural heterogeneity in myoglobin are investigated. All multidimensional spectroscopies have roots in the off-resonant multidimensional Raman techniques developed from the late 1980’s to the early 2000’s. Throughout their development these experiments were plagued with technical challenges that eventually halted further use. In this dissertation it is shown through rigorous experimental tests that the technical challenges of the past are obviated for 2DRR, which is done under electronically resonant conditions. The key is that under electronic resonance the harmonic character of vibrational modes contributes to the signal. Under off-resonant conditions signal generation depends on much weaker effects. Upon absorption of light ranging from ~250 to ~500 nm triiodide photodissociates into diiodide and radical iodine on the same time scale as the period of triiodide’s symmetric stretch, impulsively initiating coherence in the stretching coordinate of diiodide. In this dissertation, the sensitivity of 2DRR to coherent reaction mechanisms is shown by directly measuring, for the first time, how the nonequilibrium geometry of triiodide at the moment of photodissociation determines the stretching frequency of diiodide. The functions of heme proteins involve ligand binding and dissociation events, which are facilitated by the fast exchange of energy between the heme and aqueous solvent. It is known that the heme’s propionic acid side chains act as an effective “gateway” for this fast energy exchange. In this dissertation it is shown that the propionic chains within myoglobin posses significant structural heterogeneity, suggesting that this may be an important factor in facilitating the functions of heme proteins.Doctor of Philosoph

Micro-combs: a novel generation of optical sources

The quest towards the integration of ultra-fast, high-precision optical clocks is reflected in the large number of high-impact papers on the topic published in the last few years. This interest has been catalysed by the impact that high-precision optical frequency combs (OFCs) have had on metrology and spectroscopy in the last decade [1–5]. OFCs are often referred to as optical rulers: their spectra consist of a precise sequence of discrete and equally-spaced spectral lines that represent precise marks in frequency. Their importance was recognised worldwide with the 2005 Nobel Prize being awarded to T.W. Hänsch and J. Hall for their breakthrough in OFC science [5]. They demonstrated that a coherent OFC source with a large spectrum – covering at least one octave – can be stabilised with a self-referenced approach, where the frequency and the phase do not vary and are completely determined by the source physical parameters. These fully stabilised OFCs solved the challenge of directly measuring optical frequencies and are now exploited as the most accurate time references available, ready to replace the current standard for time. Very recent advancements in the fabrication technology of optical micro-cavities [6] are contributing to the development of OFC sources. These efforts may open up the way to realise ultra-fast and stable optical clocks and pulsed sources with extremely high repetition-rates, in the form of compact and integrated devices. Indeed, the fabrication of high-quality factor (high-Q) micro-resonators, capable of dramatically amplifying the optical field, can be considered a photonics breakthrough that has boosted not only the scientific investigation of OFC sources [7–13] but also of optical sensors and compact light modulators [6,14]

Nonlinear and Quantum Optics with Whispering Gallery Resonators

Optical Whispering Gallery Modes (WGMs) derive their name from a famous acoustic phenomenon of guiding a wave by a curved boundary observed nearly a century ago. This phenomenon has a rather general nature, equally applicable to sound and all other waves. It enables resonators of unique properties attractive both in science and engineering. Very high quality factors of optical WGM resonators persisting in a wide wavelength range spanning from radio frequencies to ultraviolet light, their small mode volume, and tunable in- and out- coupling make them exceptionally efficient for nonlinear optical applications. Nonlinear optics facilitates interaction of photons with each other and with other physical systems, and is of prime importance in quantum optics. In this paper we review numerous applications of WGM resonators in nonlinear and quantum optics. We outline the current areas of interest, summarize progress, highlight difficulties, and discuss possible future development trends in these areas.Comment: This is a review paper with 615 references, submitted to J. Op

Silicon Photonic Waveguides and Devices for Near- and Mid-IR Applications

Silicon photonics has been a very buoyant research field in the last several years mainly because of its potential for telecom and datacom applications. However, prospects of using silicon photonics for sensing in the mid-IR have also attracted interest lately. In this paper, we present our recent results on waveguide-based devices for near- and mid-infrared applications. The silicon-on-insulator platform can be used for wavelengths up to 4 μm; therefore, different solutions are needed for longer wavelengths. We show results on passive Si devices such as couplers, filters, and multiplexers, particularly for extended wavelength regions and finally present integration of photonics and electronics integrated circuits for high-speed applications

Cryogenic fibre-fed laser metrology

Cryogenic cooling is a fundamental requirement for broadband far-infrared spectroscopic instrumentation to benefit from state-of-the-art far-infrared detectors. The precision to which the moving cryogenic components of the instrument can be measured and controlled affects its ability to recover the spectrum and exacts a low power robust position metrology system. This thesis explores a number of laser-based position metrology solutions and shows that a fibre-fed range-resolved interferometer meets the stringent precision and low power requirements of a metrology system for future space missions. Two cryogenic fibre-fed range-resoled interferometers are theoretically discussed and subsequently constructed; the first using the Clarke transform to decode three-phase signals, and the second based on sinusoidal laser frequency modulation. Experimental results of room and cryogenic (<4 K) temperature testing for both systems are presented. Lessons learned, suggested improvements, and the employment of a range-resolved interferometer for cryogenic accelerometry, lunar seismology, and other applications are discussed

Ultra-Narrow Bandwidth Optical Resonators for Integrated Low Frequency Noise Lasers

The development of narrowband resonators has far reaching applications in integrated optics. As a precise reference of wavelength, filters can be used in sensors, metrology, nonlinear optics, microwave photonics, and laser stabilization. In this work, we develop record high quality factor (Q) Si3N4 waveguide resonators, and utilize them to stabilize a heterogeneously integrated Si/III V laser. To increase the Q factor of waveguide resonators, particular attention is given to loss mechanisms. Propagation loss of &lt;0.1 dB/m is demonstrated on the ultra low loss waveguide platform, a low index contrast, high aspect ratio Si3N4 waveguide geometry fabricated with high quality materials and high temperature anneals. Ideality in the directional couplers used for coupling to the resonators is studied and losses are reduced such that 81 million intrinsic Q factor is achieved. Additional results include 1×16 resonant splitters, low κ narrowband gratings, and a dual layer waveguide technology for low loss and low bend radius in separate regions of the same device layer. We then combine an ultra high Q resonator and a heterogeneous Si/III V laser in a Pound Drever Hall (PDH) frequency stabilization system to yield narrow linewidth characteristics for a stable on chip laser reference. The high frequency noise filtering is performed with Si resonant mirrors in the laser cavity. A 30 million Q factor Si3N4 resonator is used with electrical feedback to reduce close in noise and frequency walk off. The laser shows high frequency noise levels of 60×10^3 Hz^2/Hz corresponding to 160 kHz linewidth, and the low frequency noise is suppressed 33 dB to 10^3 Hz^2/Hz with the PDH system