Resolution dependence of rough surface scattering using a power law roughness spectrum

Contemporary high-resolution sonar systems use broadband pulses and long arrays to achieve high resolution. It is important to understand effects that high-resolution sonar systems might have on quantitative measures of the scattered field due to the seafloor. A quantity called the broadband scattering cross section is defined, appropriate for high-resolution measurements. The dependence of the broadband scattering cross section, $\sigma_{bb}$ and the scintillation index, $SI$ on resolution was investigated for one-dimensional rough surfaces with power-law spectra and backscattering geometries. Using integral equations and Fourier synthesis, no resolution dependence of $\sigma_{bb}$ was found. The incoherently-averaged frequency-domain scattering cross section has negligible bandwidth dependence. $SI$ increases as resolution increases, grazing angle decreases, and spectral strength increases. This trend is confirmed for center frequencies of 100 kHz and 10 kHz, as well as for power-law spectral exponents of 1.5, 2, and 2.5. The hypothesis that local tilting at the scale of the acoustic resolution is responsible for intensity fluctuations was examined using a representative model for the effect of slopes (inspired by the composite roughness approximation). It was found that slopes are responsible in part for the fluctuations, but other effects, such as multiple scattering and shadowing may also play a role.Comment: 22 pages, 10 figures, preprint version of paper published in the Journal of the Acoustical Society of America, at \url{https://doi.org/10.1121/10.0002974

The Kuramoto model: A simple paradigm for synchronization phenomena

Synchronization phenomena in large populations of interacting elements are the subject of intense research efforts in physical, biological, chemical, and social systems. A successful approach to the problem of synchronization consists of modeling each member of the population as a phase oscillator. In this review, synchronization is analyzed in one of the most representative models of coupled phase oscillators, the Kuramoto model. A rigorous mathematical treatment, specific numerical methods, and many variations and extensions of the original model that have appeared in the last few years are presented. Relevant applications of the model in different contexts are also included

Exciting with quantum light

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física Teórica de la Materia Condensada. Fecha de lectura: 22-11-2019A two-level system—the idealization of an atom with only two energy levels—is the most fundamental quantum object. As such, it has long been at the forefront of the research in Quantum Optics: its emission spectrum is simply a Lorentzian distribution, and the light it produces is the most quantum that can be. The temporal distribution of the photon emission displays a perfect antibunching, meaning that such a system will never emit two (or more) photons simultaneously, which is consistent with the intuition that the two-level system can only sustain a single excitation at any given time. Although these two properties have been known for decades, it was not until the advent of the Theory of Frequency-filtered and Time-resolved Correlations that it was observed that the perfect antibunching is not the end of the story: the correlations between photons possess an underlying structure, which is unveiled when one retains the information about the color of the photons. This is a consequence of the Heisenberg uncertainty principle: measuring perfect antibunching implies an absolute knowledge about the time at which the photons have been emitted, which in turn implies an absolute uncertainty on their energy. Thus, keeping some information about the frequency of the emitted photons affects the correlations between them. This means that a two-level system can be turned into a versatile source of quantum light, providing light with a large breadth of correlation types well beyond simply antibunching. Furthermore, when the two-level system is driven coherently in the so-called Mollow regime (in which the two-level system becomes dressed by the laser and the emission line is split into three), the correlations blossom: one can find every type of statistics—from antibunching to super-bunching—provided that one measures the photons emitted at the adequate frequency window of the triplet. In fact, the process of filtering the emission at the frequencies corresponding to N-photon transitions is the idea behind the Bundler, a source of light whose emission is always in bundles of exactly N photons. The versatility of the correlations decking the emitted light motivates the topic of this Dissertation, in which I focus on the theoretical study of the behaviour that arises when physical systems are driven with quantum light, i.e., with light that cannot be described through the classical theory of electromagnetism. As the canon of excitation used in the literature is restricted to classical sources, namely lasers and thermal reservoirs, our description starts with the most fundamental objects that can be considered as the optical targets: a harmonic oscillator (which represents the field for non-interacting bosonic particles) and a two-level system (which in turn represents the field for fermionic particles). We describe which regions of the Harmonic oscillator’s Hilbert space can be accessed by driving the harmonic oscillator with the light emitted by a two-level system, i.e., which quantum steady states can be realized. Analogously, we find that the quality of the single-photon emission from a two-level system can be enhanced when it is driven by quantum light. Once the advantages of using quantum, rather than classical, sources of light are demonstrated with the fundamental optical targets, we turn to the quantum excitation of more involved systems, such as the strong coupling between a harmonic oscillator and either a two-level system—whose description is made through the Jaynes-Cummings model—or a nonlinear harmonic oscillator—which can be realized in systems of, e.g., exciton-polaritons. Here we find that the statistical versatility of the light emitted by the Mollow triplet allows to perform Quantum Spectroscopy on these systems, thus gaining knowledge of its internal structure and dynamics, and in particular to probe their interactions with the least possible amount of particles: two. In the process of exciting with quantum light, we are called to further examine the source itself. In fact, there is even the need to revisit the concept of a single-photon source, for which we propose more robust criterion than g(2). We also turn to toy-models of the Bundler so as to use it effectively as an optical source. We can then xix study the advantages that one gets and shortcomings that one faces when using this source of light to drive all the systems considered on excitation with the emission of a two-level system. Finally, we go from the continuous to the pulsed regime of excitation, which is of higher interest for applications and comes with its own set of fundamental questions

From Quantum to Classical Interactions Between a Free Electron and a Surface

Quantum theory is often cited as being one of the most empirically validated theories in terms of its predictive power and precision. These attributes have led to numerous scientific discoveries and technological advancements. However, the precise relationship between quantum and classical physics remains obscure. The prevailing description is known as decoherence theory, where classical physics emerges from a more general quantum theory through environmental interaction. Sometimes referred to as the decoherence program, it does not solve the quantum measurement problem. We believe experiments performed between the microscopic and macroscopic world may help finish the program. The following considers a free electron that interacts with a surface (the environment), providing a controlled decoherence mechanism. There are non-decohering interactions to be examined and quantified before the weaker decohering effects are filtered out. In the first experiment, an electron beam passes over a surface that’s illuminated by low-power laser light. This induces a surface charge redistribution causing the electron deflection. This phenomenon’s parameters are investigated. This system can be well understood in terms of classical electrodynamics, A second experiment tests decoherence theory by introducing a nanofabricated diffraction grating before the surface. The electron undergoes diffraction through the grating, but as the electron passes over the surface it’s predicted by various physical models that the electron will lose its wave interference property. Image charge based models, which predict a larger loss of contrast than what is observed, are falsified (despite experiencing an image charge force). A theoretical study demonstrates how a loss of contrast may not be due to the irreversible process decoherence, but dephasing (a reversible process due to randomization of the wavefunction’s phase). To resolve this ambiguity, a correlation function on an ensemble of diffraction patterns is analyzed after an electron undergoes either process in a path integral calculation. The diffraction pattern is successfully recovered for dephasing, but not for decoherence, thus verifying it as a potential tool in experimental studies to determine the nature of the observed process

Multidimensional Spectroscopy of Mixed-Cation Perovskite Thin Films

Metal halide perovskite (MHP) thin films are currently undergoing an intense re- search thrust due to the excellent performance of MHP based photovoltaic (PV) devices, which have the potential to revolutionize the worlds energy production via a unique combination of low-cost fabrication and high power conversion efficiency (PCE). However, the vast majority of research is currently aimed at incremental improvements in device PCE, resulting in a body of work without the foundational understanding of the charge-carrier dynamics of the system upon photoexcitation. This thesis begins with the development of a phase-modulated multidimensional coherent spectroscopy (PM-MDCS) experiment. PM-MDCS is an ultrafast multidi- mensional coherent spectroscopy (MDCS) technique that can identify photophysical processes unavailable to one-dimensional spectroscopies. The thesis then goes on to describe the development of a novel data acquisition scheme and data process- ing technique, diagonal slice four-wave mixing (DS-FWM). Next, a description of calibrating the absolute phase in PM-MDCS experiments is presented. Finally, the thesis discusses the application of steady-state photoluminescence (PL), MDCS, and DS-FWM to study the charge-carrier dynamics in MHP thin films at 5K. These studies provide crucial information to building a fundamental understanding of the photophysical processes in MHP films under illumination, providing direction for targeted research toward improved MHP PV performance. A novel technique for collection of PM-MDCS data, and analysis of all MDCS data, DS-FWM succeeds where other MDCS lineshape analyses have failed, analyti- cal separation of broadening mechanisms in MDCS data. This technique significantly shortens data acquisition time for time-domain coherent spectroscopies, such as PM- MDCS, and provides direct access to relevant material paramters, such as the pure dephasing rate in the studied system, without the need for any assumptions. Phasing PM-MDCS spectra is a central concern because the interpretation of spectra rely critically on the phase. We developed a method of calibrating the absolute phase in PM-MDCS that reconstructs all phase contributions to the signal and removes all but the phase of the material response. The PM-MDCS data presented on MHP thin films clearly show long-lived exci- tons in the system, the existence of which has long been debated in the literature, with surprisingly long dephasing times up to ~ 1 ps. These excitons show clear in- homogeneous broadening, likely due to the large amount of disorder intrinsic to the MHP system, disproving a widely cited finding in the literature that the emission of the MHP system is homogeneously broadened and the system is well ordered. The data also show multiple isolated states that appear as one peak in steady-state PL data, likely due to defect states in the imperfect MHP lattice. PM-MDCS has the capability to disentangle spectrally broad resonances in ways that steady-state mea- surements cannot, allowing the studies performed to access the individual response of these states directly. Time dependent studies spanning hundreds of femtoseconds to about one nanosecond show multiple relaxation pathways and timescales between these states and some coherent coupling. The interaction of the states and transfer pathways of the charge-carriers are of vital importance, as the coupling of defect- states to current-generating states could lead to marked improvements in MHP PV performance

GNSS reflectometry for land remote sensing applications

Soil moisture and vegetation biomass are two essential parameters from a scienti c and economical point of view. On one hand, they are key for the understanding of the hydrological and carbon cycle. On the other hand, soil moisture is essential for agricultural applications and water management, and vegetation biomass is crucial for regional development programs. Several remote sensing techniques have been used to measure these two parameters. However, retrieving soil moisture and vegetation biomass with the required accuracy, and the appropriate spatial and temporal resolutions still remains a major challenge. The use of Global Navigation Satellite Systems (GNSS) reflected signals as sources of opportunity for measuring soil moisture and vegetation biomass is assessed in this PhD Thesis. This technique, commonly known as GNSS-Reflectometry (GNSS-R), has gained increasing interest among the scienti c community during the last two decades due to its unique characteristics. Previous experimental works have already shown the capabilities of GNSS-R to sense small reflectivity changes on the surface. The use of the co- and cross-polarized reflected signals was also proposed to mitigate nuisance parameters, such as soil surface roughness, in the determination of soil moisture. However, experimental evidence of the suitability of that technique could not be demonstrated. This work analyses from a theoretical and an experimental point of view the capabilities of polarimetric observations of GNSS reflected signals for monitoring soil moisture and vegetation biomass. The Thesis is structured in four main parts. The fi rst part examines the fundamental aspects of the technique and provides a detailed review of the GNSS-R state of the art for soil moisture and vegetation monitoring. The second part deals with the scattering models from land surfaces. A comprehensive description of the formation of scattered signals from rough surfaces is provided. Simulations with current state of the art models for bare and vegetated soils were performed in order to analyze the scattering components of GNSS reflected signals. A simpli ed scattering model was also developed in order to relate in a straightforward way experimental measurements to soil bio-geophysical parameters. The third part reviews the experimental work performed within this research. The development of a GNSS-R instrument for land applications is described, together with the three experimental campaigns carried out in the frame of this PhD Thesis. The analysis of the GNSS-R and ground truth data is also discussed within this part. As predicted by models, it was observed that GNSS scattered signals from natural surfaces are a combination of a coherent and an incoherent scattering components. A data analysis technique was proposed to separate both scattering contributions. The use of polarimetric observations for the determination of soil moisture was demonstrated to be useful under most soil conditions. It was also observed that forests with high levels of biomass could be observed with GNSS reflected signals. The fourth and last part of the Thesis provides an analysis of the technology perspectives. A GNSS-R End-to-End simulator was used to determine the capabilities of the technique to observe di erent soil reflectivity conditions from a low Earth orbiting satellite. It was determined that high accuracy in the estimation of reflectivity could be achieved within reasonable on-ground resolution, as the coherent scattering component is expected to be the predominant one in a spaceborne scenario. The results obtained in this PhD Thesis show the promising potential of GNSS-R measurements for land remote sensing applications, which could represent an excellent complementary observation for a wide range of Earth Observation missions such as SMOS, SMAP, and the recently approved ESA Earth Explorer Mission Biomass.La humedad del suelo y la biomasa de la vegetaci on son dos parametros clave desde un punto de vista tanto cient co como econ omico. Por una parte son esenciales para el estudio del ciclo del agua y del carbono. Por otra parte, la humedad del suelo es esencial para la gesti on de las cosechas y los recursos h dricos, mientras que la biomasa es un par ametro fundamental para ciertos programas de desarrollo. Varias formas de teledetección se han utilizado para la observaci on remota de estos par ametros, sin embargo, su monitorizaci on con la precisi on y resoluci on necesarias es todav a un importante reto tecnol ogico. Esta Tesis evalua la capacidad de medir humedad del suelo y biomasa de la vegetaci on con señales de Sistemas Satelitales de Posicionamiento Global (GNSS, en sus siglas en ingl es) reflejadas sobre la Tierra. La t ecnica se conoce como Reflectometr í a GNSS (GNSS-R), la cual ha ganado un creciente inter es dentro de la comunidad científ ca durante las dos ultimas d ecadas. Experimentos previos a este trabajo ya demostraron la capacidad de observar cambios en la reflectividad del terreno con GNSS-R. El uso de la componente copolar y contrapolar de la señal reflejada fue propuesto para independizar la medida de humedad del suelo de otros par ametros como la rugosidad del terreno. Sin embargo, no se pudo demostrar una evidencia experimental de la viabilidad de la t ecnica. En este trabajo se analiza desde un punto de vista te orico y experimental el uso de la informaci on polarim etrica de la señales GNSS reflejadas sobre el suelo para la determinaci on de humedad y biomasa de la vegetaci on. La Tesis se estructura en cuatro partes principales. En la primera parte se eval uan los aspectos fundamentales de la t ecnica y se da una revisi on detallada del estado del arte para la observaci on de humedad y vegetaci on. En la segunda parte se discuten los modelos de dispersi on electromagn etica sobre el suelo. Simulaciones con estos modelos fueron realizadas para analizar las componentes coherente e incoherente de la dispersi on de la señal reflejada sobre distintos tipos de terreno. Durante este trabajo se desarroll o un modelo de reflexi on simpli cado para poder relacionar de forma directa las observaciones con los par ametros geof sicos del suelo. La tercera parte describe las campañas experimentales realizadas durante este trabajo y discute el an alisis y la comparaci on de los datos GNSS-R con las mediciones in-situ. Como se predice por los modelos, se comprob o experimentalmente que la señal reflejada est a formada por una componente coherente y otra incoherente. Una t ecnica de an alisis de datos se propuso para la separacióon de estas dos contribuciones. Con los datos de las campañas experimentales se demonstr o el bene cio del uso de la informaci on polarim etrica en las señales GNSS reflejadas para la medici on de humedad del suelo, para la mayor a de las condiciones de rugosidad observadas. Tambi en se demostr o la capacidad de este tipo de observaciones para medir zonas boscosas densamente pobladas. La cuarta parte de la tesis analiza la capacidad de la t ecnica para observar cambios en la reflectividad del suelo desde un sat elite en orbita baja. Los resultados obtenidos muestran que la reflectividad del terreno podr a medirse con gran precisi on ya que la componente coherente del scattering ser a la predominante en ese tipo de escenarios. En este trabajo de doctorado se muestran la potencialidades de la t ecnica GNSS-R para observar remotamente par ametros del suelo tan importantes como la humedad del suelo y la biomasa de la vegetaci on. Este tipo de medidas pueden complementar un amplio rango de misiones de observaci on de la Tierra como SMOS, SMAP, y Biomass, esta ultima recientemente aprobada para la siguiente misi on Earth Explorer de la ESA

Interfacing GHz-bandwidth heralded single photons with a room-temperature Raman quantum memory

Photonics is a promising platform for quantum technologies. However, photon sources and two-photon gates currently only operate probabilistically. Large-scale photonic processing will therefore be impossible without a multiplexing strategy to actively select successful events. High time-bandwidth-product quantum memories - devices that store and retrieve single photons on-demand - provide an efficient remedy via active synchronisation. Here we interface a GHz-bandwidth heralded single-photon source and a room-temperature Raman memory with a time-bandwidth product exceeding 1000. We store heralded single photons and observe a clear influence of the input photon statistics on the retrieved light, which agrees with our theoretical model. The preservation of the stored field's statistics is limited by four-wave-mixing noise, which we identify as the key remaining challenge in the development of practical memories for scalable photonic information processing