Nonlinear Optics and Carrier Dynamics in Nanostructured and Two-Dimensional Materials

Understanding and measuring the interaction of light with sub-wavelength structures and atomically thin materials is of critical importance for the development of next generation photonic devices.  One approach to achieve the desired optical properties in a material is to manipulate its mesoscopic structure or its composition in order to affect the properties of the light-matter interaction.  There has been tremendous recent interest in so called two-dimensional materials, consisting of only a single to a few layers of atoms arranged in a planar sheet.  These materials have demonstrated great promise as a platform for studying unique phenomena arising from the low-dimensionality of the material and for developing new types of devices based on these effects.  A thorough investigation of the optical and electronic properties of these new materials is essential to realizing their potential.  In this work we present studies that explore the nonlinear optical properties and carrier dynamics in nanoporous silicon waveguides, two-dimensional graphite (graphene), and atomically thin black phosphorus. We first present an investigation of the nonlinear response of nanoporous silicon optical waveguides using a novel pump-probe method. A two-frequency heterodyne technique is developed in order to measure the pump-induced transient change in phase and intensity in a single measurement. The experimental data reveal a characteristic material response time and temporally resolved intensity and phase behavior matching a physical model dominated by free-carrier effects that are significantly stronger and faster than those observed in traditional silicon-based waveguides.  These results shed light on the large optical nonlinearity observed in nanoporous silicon and demonstrate a new measurement technique for heterodyne pump-probe spectroscopy. Next we explore the optical properties of low-doped graphene in the terahertz spectral regime, where both intraband and interband effects play a significant role. Probing the graphene at intermediate photon energies enables the investigation of the nonlinear optical properties in the graphene as its electron system is heated by the intense pump pulse. By simultaneously measuring the reflected and transmitted terahertz light, a precise determination of the pump-induced change in absorption can be made. We observe that as the intensity of the terahertz radiation is increased, the optical properties of the graphene change from interband, semiconductor-like absorption, to a more metallic behavior with increased intraband processes. This transition reveals itself in our measurements as an increase in the terahertz transmission through the graphene at low fluence, followed by a decrease in transmission and the onset of a large, photo-induced reflection as fluence is increased.  A hybrid optical-thermodynamic model successfully describes our observations and predicts this transition will persist across mid- and far-infrared frequencies.  This study further demonstrates the important role that reflection plays since the absorption saturation intensity (an important figure of merit for graphene-based saturable absorbers) can be underestimated if only the transmitted light is considered. These findings are expected to contribute to the development of new optoelectronic devices designed to operate in the mid- and far-infrared frequency range.  Lastly we discuss recent work with black phosphorus, a two-dimensional material that has recently attracted interest due to its high mobility and direct, configurable band gap (300 meV to 2eV), depending on the number of atomic layers comprising the sample. In this work we examine the pump-induced change in optical transmission of mechanically exfoliated black phosphorus flakes using a two-color optical pump-probe measurement. The time-resolved data reveal a fast pump-induced transparency accompanied by a slower absorption that we attribute to Pauli blocking and free-carrier absorption, respectively. Polarization studies show that these effects are also highly anisotropic - underscoring the importance of crystal orientation in the design of optical devices based on this material. We conclude our discussion of black phosphorus with a study that employs this material as the active element in a photoconductive detector capable of gigahertz class detection at room temperature for mid-infrared frequencies.

FLUTE MUSIC FROM THE MID-TWENTIETH CENTURY BY THREE FRENCH COMPOSERS: JEAN-MICHEL DAMASE, ANDRE JOLIVET, AND HENRI TOMASI

Jean-Michel Damase (b.1928), Andre Jolivet (1905-1974), and Henri Tomasi (1901-1971) are three prominent French composers ofthe twentieth century. Tomasi won the Prix de Rome in 1927, and Damase won the Prix de Rome in 1947. All three composers were educated and lived in Paris around the same period; however, their musical styles are quite distinct. Most of Jolivet's compositions for flute are well known and are often selected as international competition repertoire. The compositions for flute by Damase and Tomasi are not as recognized as those of Jolivet, and most of their works for flute still have not been commercially recorded. The purpose of this dissertation is to provide a more comprehensive guide to the compositions for flute by Damase, Jolivet and Tomasi, and, in addition, to make the works ofDamase and Tomasi familiar to flutists. This dissertation will focus on the compositions ofDamase, Jolivet, and Tomasi for flute alone and those for flute and piano, written between 1928 and 1971 (1928 is the year Damase was born, and 1971 is the year that Tomasi died). Damase continues French romanticism, and his music is always playful, elegant, and accessible with rhythmic and harmonic surprises, but with an underlying complexity. His compositions for flute include three concertos, two double concertos, one flute solo work, and nine works for flute and piano. Jolivet's compositions make use of ancient rituals, incantations, and spirituality, as well as repeated phrases and single notes, irregular rhythmic patterns, dissonant effects, and rhythmic drive. He composed one flute concerto, three works for flute solo, and four works for flute and piano. Tomasi's compositions also continue French romanticism and contain melodies which often seem to tell a story, and which are not only full of flourishes and vitality, but are also delicate, colorful, and romantic. Virtuosic technical demand is another characteristic of his style. Tomasi composed three flute concertos, three works for solo flute, and one work for flute and piano. Appendix I is a list of the compositions for flute by Damase, Jolivet, and Tomasi, and Appendix II is a discography of their works.