Broadband Coherent Raman Scattering Microscopy

Spontaneous Raman (SR) microscopy allows label-free chemically specificimaging based on the vibrational response of molecules; however, due to thelow Raman scattering cross section, it is intrinsically slow. Coherent Ramanscattering (CRS) techniques, by coherently exciting vibrational oscillators inthe focal volume, increase signal levels by several orders of magnitude underappropriate conditions. In its single-frequency version, CRS microscopy hasreached very high imaging speeds, up to the video rate; however, it providesinformation which is not sufficient to distinguish spectrally overlappedchemical species within complex heterogeneous systems, such as cells andtissues. Broadband CRS combines the acquisition speed of CRS with theinformation content of SR, but it is technically very demanding. In this Review,the current state of the art in broadband CRS microscopy, both in the coherentanti-Stokes Raman scattering (CARS) and the stimulated Raman scattering(SRS) versions are reviewed. Different technical solutions for broadband CARSand SRS, working both in the frequency and in the time domains, arecompared and their merits and drawbacks assesse

Raman optical frequency comb generation in hydrogen-filled hollow-core fiber

xiv, 138 p. : ill. (some col.)In this dissertation, we demonstrate the generation of optical Raman frequency combs by a single laser pump pulse traveling in hydrogen-filled hollow-core optical fibers. This comb generation process is a cascaded stimulated Raman scattering effect, where higher-order sidebands are produced by lower orders scattered from hydrogen molecules. We observe more than 4 vibrational and 20 rotational Raman sidebands in the comb. They span more than three octaves in optical wavelength, largely thanks to the broadband transmission property of the fiber. We found that there are phase correlations between the generated Raman comb sidebands (spectral lines), although their phases are fluctuating from one pump pulse to another due to the inherit spontaneous initiation of Raman scattering. In the experiment, we generated two Raman combs independently from two fibers and simultaneously observed the single-shot interferences between Stokes and anti-Stokes components from the two fibers. The experimental results clearly showed the strong phase anti-correlation between first-order side bands. We also developed a quantum theory to describe this Raman comb generation process, and it predicts and explains the phase correlations we observe. The phase correlation that we found in optical Raman combs may allow us to synthesize single-cycle optical pulse trains, creating attosecond pulses. However, the vacuum fluctuation in stimulated Raman scattering will result in the fluctuation of carrier envelope phase of the pulse trains. We propose that we can stabilize the comb by simultaneously injecting an auxiliary optical beam, mutually coherent with the main Raman pump laser pulse, which is resonant with the third anti-Stokes field.Committee in Charge: Dr. Steven van Enk, Chair; Dr. Michael G. Raymer; Dr. Daniel A. Steck; Dr. David M. Strom; Dr. Andrew H. Marcu

Nonlinear integrated photonics on silicon and gallium arsenide substrates

Silicon photonics is nowadays a mature technology and is on the verge of becoming a blossoming industry. Silicon photonics has also been pursued as a platform for integrated nonlinear optics based on Raman and Kerr effects. In recent years, more futuristic directions have been pursued by various groups. For instance, the realm of silicon photonics has been expanded beyond the well-established near-infrared wavelengths and into the mid-infrared (3 - 5 µm). In this wavelength range, the omnipresent hurdle of nonlinear silicon photonics in the telecommunication band, i.e., nonlinear losses due to two-photon absorption, is inherently nonexistent. With the lack of efficient light-emission capability and second-order optical nonlinearity in silicon, heterogeneous integration with other material systems has been another direction pursued. Finally, several approaches have been proposed and demonstrated to address the energy efficiency of silicon photonic devices in the near-infrared wavelength range. In this dissertation, theoretical and experimental works are conducted to extend applications of integrated photonics into mid-infrared wavelengths based on silicon, demonstrate heterogeneous integration of tantalum pentoxide and lithium niobate photonics on silicon substrates, and study two-photon photovoltaic effect in gallium arsenide and plasmonic-enhanced structures. Specifically, performance and noise properties of nonlinear silicon photonic devices, such as Raman lasers and optical parametric amplifiers, based on novel and reliable waveguide technologies are studied. Both near-infrared and mid-infrared nonlinear silicon devices have been studied for comparison. Novel tantalum-pentoxide- and lithium-niobate-on-silicon platforms are developed for compact microring resonators and Mach-Zehnder modulators. Third- and second-harmonic generations are theoretical studied based on these two platforms, respectively. Also, the two-photon photovoltaic effect is studied in gallium arsenide waveguides for the first time. The effect, which was first demonstrated in silicon, is the nonlinear equivalent of the photovoltaic effect of solar cells and offers a viable solution for achieving energy-efficient photonic devices. The measured power efficiency achieved in gallium arsenide is higher than that in silicon and even higher efficiency is theoretically predicted with optimized designs. Finally, plasmonic-enhanced photovoltaic power converters, based on the two-photon photovoltaic effect in silicon using subwavelength apertures in metallic films, are proposed and theoretically studied

Pushing the physical limits of infrared chemical imaging: intravascular photoacoustic & mid-infrared photothermal

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

Photo-Induced Cell Damage Analysis for Single- and Multifocus Coherent Anti-Stokes Raman Scattering Microscopy

In this study, we investigated photo-induced damage to living cells during single- and multifocus excitations for coherent anti-Stokes Raman scattering (CARS) imaging. A near-infrared pulsed laser (709 nm) was used to induce cell damage. We compared the photo-induced cell damage in the single- and the multifocus excitation schemes with the condition to obtain the same CARS signal in the same frame rate. For the evaluation of cell viability, we employed 4',6-diamidino-2-phenylindole (DAPI) fluorophores that predominantly stained the damaged cells. One- and two-photon fluorescence of DAPI fluorophores were, respectively, excited by an ultraviolet light source and the same near-infrared light source and were monitored to evaluate the cell viability during near-infrared pulsed laser irradiation. We found lower uptake of DAPI fluorophores into HeLa cells during the multifocus excitation compared with the single-focus excitation scheme in both the one- and the two-photon fluorescence examinations. This indicates a reduction of photo-induced cell damage in the multifocus excitation. Our findings suggested that the multifocus excitation scheme is expected to be suitable for CARS microscopy in terms of minimal invasiveness

Laser Systems for Applications

This book addresses topics related to various laser systems intended for the applications in science and various industries. Some of them are very recent achievements in laser physics (e.g. laser pulse cleaning), while others face their renaissance in industrial applications (e.g. CO2 lasers). This book has been divided into four different sections: (1) Laser and terahertz sources, (2) Laser beam manipulation, (3) Intense pulse propagation phenomena, and (4) Metrology. The book addresses such topics like: Q-switching, mode-locking, various laser systems, terahertz source driven by lasers, micro-lasers, fiber lasers, pulse and beam shaping techniques, pulse contrast metrology, and improvement techniques. This book is a great starting point for newcomers to laser physics

Modern Applications in Optics and Photonics: From Sensing and Analytics to Communication

Optics and photonics are among the key technologies of the 21st century, and offer potential for novel applications in areas such as sensing and spectroscopy, analytics, monitoring, biomedical imaging/diagnostics, and optical communication technology. The high degree of control over light fields, together with the capabilities of modern processing and integration technology, enables new optical measurement systems with enhanced functionality and sensitivity. They are attractive for a range of applications that were previously inaccessible. This Special Issue aims to provide an overview of some of the most advanced application areas in optics and photonics and indicate the broad potential for the future