Electrical & Computer Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2765
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Item INTEGRATION OF ATOMIC EMITTERS IN PHOTONIC PLATFORMS FOR CLASSICAL AND QUANTUM INFORMATION APPLICATIONS(2024) Zhao, Yuqi; Waks, Edo; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Integrated photonics provide a powerful toolbox for a wide range of classical and nonclassical applications. In addition to their scalability and significantly lower power consumption, integrated photonic structures enable new design knobs and functionalities that are inaccessible in their bulk counterparts.Solid-state atomic emitters, such as rare-earth ions (REIs) and quantum dots, serve as excellent sources for scalable quantum memories and exhibit strong nonlinear resonant absorption. Integrating atomic emitters with photonic devices enhances light-matter interactions, unlocking new opportunities for advanced optoelectronic systems in both classical and quantum regimes. This thesis tackles two main challenges utilizing the integration of photonic devices and atomic emitters: (1) developing scalable quantum network components, and (2) creating low-power nonlinear components for classical on-chip optical signal processing. Specifically, we focus on a platform of rare-earth ion doped thin-film lithium niobate (TFLN), leveraging the ions’ stable optical transitions with thin-film lithium niobate’s rich toolbox of high-performance photonics. We first demonstrate an integrated atomic frequency comb (AFC) memory in this platform, an essential component for quantum networks. This memory exhibits a broad storage bandwidth exceeding 100 MHz and optical storage time as long as 250 ns. As the first demonstrated integrated AFC memory, it features a significantly enhanced optical confinement compared to the previously demonstrated REI memories based on ion-diffused waveguides, leading to a three orders of magnitude reduction in optical power required for a coherent control. Next, we develop reconfigurable narrowband spectral filters using ring resonators in the REI:TFLN platform. These on-chip optical filters, with linewidths in the MHz and kHz range and extinction ratios of 13 dB – 20 dB, are crucial for reducing background noise in quantum frequency conversion. By spectral hole burning at 100 mK temperature in a critical-coupled resonance mode, we achieve bandpass filters with a linewidth of as narrow as 681 kHz. Moreover, the cavity enables reconfigurable filtering by varying the cavity coupling rate. Such versatile integrated spectral filters with high extinction ratio and narrow linewidth could serve as fundamental component for optical signal processing and optical memories on-a-chip. We also demonstrate picowatt-threshold power nonlinearity in TFLN, utilizing the strong resonant nonlinear absorption induced by three-level REIs and enhanced by TFLN ring resonators. This work presents three distinct nonlinear transmission functions by adjusting the ring’s coupling strength. The lifetime of the nonlinear transmission is measured to be ~3 ms, determined by the ion’s third-level lifetime. Finally, we propose a novel nonlinear device design based on a different material system and mechanism - an ultrathin optical limiter with low threshold intensity (0.45 kW/cm2), utilizing a 500 nm-thick GaAs zone plate embedded with InAs quantum dots. The optical limiting performance, enabled by the zone plate’s nonlinear focusing behavior, is investigated using FDTD simulations. We also explore the effects of the zone plate’s thickness and radius on its optical limiting performance.Item Development of Photonic Reservoir Computers for Radiofrequency Spectrum Awareness(2024) Klimko, Benjamin; Chembo, Yanne K.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this dissertation, we study the use of several optoelectronic oscillator architectures for physical reservoir computing tasks. While optoelectronic oscillator-based reservoir computers have been reported in the literature for over a decade, all reported experimental results have been processed using wideband systems with baseband data sets. Our work focuses on two majorinnovations for physical reservoir computing: (i) narrowband reservoir computers allowing computing tasks to be performed natively on radiofrequency signals and (ii) illustrating that “simplified” optoelectronic oscillators, without external optical modulators, are capable of meeting or exceeding the results from more complex photonic reservoir computers. By their nature, optoelectronic oscillators operate in the radio passband regime and reservoir computers have been shown to be capable on time-series tasks such as waveform prediction and classification data sets. We demonstrated that the optoelectronic oscillator-based reservoir computer can effectively process signals in the radio passband, which is a novel result that could provide an enabling technology for next-generation communication methods such as cognitive networks. The benefits of this innovation would scale with increasing frequency, such as potential use with millimeter-wave cellular networks. In our second physical reservoir innovation, we have shown that external optical modulators, nearly ubiquitous devices in optoelectronic oscillators, may be excluded from the design of a physical reservoir computer without decreasing its accuracy. This is a major result as a reservoir without active optical components could be built on a single integrated circuit using modern semiconductor manufacturing processes. Such integration and miniaturization would be a large step towards photonic reservoir systems that could be more easily put into an operational environment. Up to this point, there has been minimal work on transitioning the optoelectronic oscillator from a benchtop, experimental system to one useful in the real world. Lastly, we investigated the relationship between computational power of the reservoir computer and task error. This is a crucial finding since reservoir computing is often touted as an alternative computing paradigm that is less resource-intensive than other computing methods. By determining a threshold on computational needs for a photonic reservoir computer, we ensure that such systems are utilized efficiently and do not unnecessarily use resources.Item Nonlinear and Stochastic Dynamics of Optoelectronic Oscillators(2024) Ha, Meenwook; Chembo, Yanne K.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Optoelectronic oscillators (OEOs) are nonlinear, time-delayed and self-sustained microwave photonic systems capable of generating ultrapure radiofrequency (RF) signals with extensive frequency tunabilities. Their hybrid architectures, comprising both optical and electronic paths, underscore their merits. One of the most notable points of OEOs can be unprecedentedly high quality factors, achieved by storing optical energies for RF signal generations. Thanks to their low phase noise and broad frequency tunabilities, OEOs have found diverse applications including chaos cryptography, reservoir computing, radar communications, parametric oscillator, clock recovery, and frequency comb generation. This thesis pursues two primary objectives. Firstly, we delve into the nonlinear dynamics of various OEO configurations, elucidating their universal behaviors by deriving corresponding envelope equations. Secondly, we present a stochastic equation delineating the dynamics of phases and explore the intricacies of the phase dynamics. The outputs of OEOs are defined in their RF ports, with our primary focus directed towards understanding the dynamics of these RF signals. Regardless of their structural complexities, we employ a consistent framework to explore these dynamics, relying on the same underlying principles that determine the oscillation frequencies of OEOs. To comprehend behaviors of OEOs, we analyze the dynamics of a variety of OEOs. For simpler systems, we can utilize the dynamic equations of bandpass filters, whereas more complex physics are required for expressing microwave photonic filtering. Utilizing an envelope approach, which characterizes the dynamics of OEOs in terms of complex envelopes of their RF signals, has proven to be an effective method for studying them. Consequently, we derive envelope equations of these systems and research nonlinear behaviors through analyses such as investigating bifurcations, stability evaluations, and numerical simulations. Comparing the envelope equations of different models reveals similarities in their dynamic equations, suggesting that their dynamics can be governed by a generalized universal form. Thus, we introduce the universal equation, which we refer to as the universal microwave envelope equation and conduct analytical investigations to further understand its implications. While the deterministic universal equation offers a comprehensive tool for simultaneous exploration of various OEO dynamics, it falls short in describing the stochastic phase dynamics. Our secondary focus lies in investigating phase dynamics through the implementation of a stochastic approach, enabling us to optimize and comprehend phase noise performance effectively. We transform the deterministic universal envelope equation into a stochastic delay differential form, effectively describing the phase dynamics. In our analysis of the oscillators, we categorize noise sources into two types: additive noise contribution, due to random environmental and internal fluctuations, and multiplicative noise contribution, arising from noisy loop gains. The existence of the additive noise is independent of oscillation existence, while the multiplicative noise is intertwined with the noisy loop gains, nonlinearly mixing with signals above the threshold. Therefore, we investigate both sub- and above-threshold regimes separately, where the multiplicative noise can be characterized as white noise and colored noise in respective regimes. For the above-threshold regime, we present the stochastic phase equation and derive an equation for describing phase noise spectra. We conduct thorough investigations into this equation and validate our approaches through experimental verification. In the sub-threshold regime, we introduce frameworks to experimentally quantify the noise contributions discussed in the above-threshold part. Since no signal is present here and the oscillator is solely driven by the stochastic noise, it becomes feasible to reverse-engineer the noise powers using a Fourier transform formalism. Here, we introduce a stochastic expression written in terms of the real-valued RF signals, not the envelopes, and the transformation facilitates the expressions of additive and multiplicative noise contributions as functions of noisy RF output powers. The additive noise can be defined by deactivating the laser source or operating the intensity modulator at the minimum transmission point, given its independence from the loop gains. Conversely, the expression for the multiplicative noise indicates a dependence on the gain, however, experimental observations suggest that its magnitude may remain relatively constant beyond the threshold.Item Quantum and Stochastic Dynamics of Kerr Microcombs(2024) Liu, Fengyu; Chembo, Yanne K.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Kerr microcombs are sets of discrete, equidistant spectral lines and are typically generated by pumping a high-quality factor optical resonator with a continuous- or pulse-wave resonant laser. They have emerged as one of the most important research topics in photonics nowadays, with applications related to spectroscopy, sensing, aerospace, and communication engineering. A key characteristic of these microcombs is the threshold pump power. Below the threshold, two pump photons are symmetrically up- and down-converted as twin photons via spontaneous four-wave mixing, and they can be entangled across up to a hundred eigenmodes. These chipscale, high-dimensional, and room-temperature systems are expected to play a major role in quantum engineering. Above the threshold, the four-wave mixing process is stimulated, ultimately leading to the formation of various types of patterns in the spatio-temporal domain, which can be extended (such as roll patterns), or localized (bright or dark solitons). The semiclassical dynamics of Kerr microcombs have been studied extensively in the last ten years and the deterministic characteristics are well understood. However, the quantum dynamics of the twin-photon generation process, and the stochastic dynamics led by the noise-driven fluctuations, are still not so clear. In the first part of our investigation, we introduce the theoretical framework to study the semiclassical dynamics of the Kerr microcombs based on the slowly varying envelope of the intracavity electrical fields. Two equivalent models -- the coupled-mode model and the Lugiato-Lefever model are used to analyze the spectro- and spatio-temporal dynamics, respectively. These models can determine the impact of key parameters on the Kerr microcomb generation process, such as detuning, losses, and pump power, as well as critical values of the system, such as threshold power. Various types of patterns and combs can be observed through simulations that follow experimental parameters. Furthermore, we show an eigenvalue analysis method to determine the stability of the microcomb, and this method is applied to an unstable microcomb solution to understand the generation of subcombs surrounding the primary comb. In the second and third parts, we investigate a stochastic model where noise is added to the coupled-mode equations governing the microcomb dynamics to monitor the influence of random noise on the comb dynamics. We find the model with additive Gaussian white noise allows us to characterize the noise-induced broadening of spectral lines and permits us to determine the phase noise spectra of the microwaves generated via comb photodetection. Our analysis indicates that the low-frequency part of the phase noise spectra is dominated by pattern drift while the high-frequency part is dominated by pattern deformation. The dynamics of the Kerr microcomb with multiplicative noises, including thermal and photothermal fluctuations, are also investigated in the end. We propose that the dynamics of the noise can be included in the simulation of stochastic dynamics equations, introduce the methods to solve the dynamics of the noise, and study a quiet point method for phase noise reduction. In the fourth part, we use canonical quantization to obtain the quantum dynamics for Kerr microcombs generated by spontaneous four-wave mixing below the threshold and develop the study of them using frequency-bin quantum states. We introduce a method to find the quantum expansion of the output state and explore the properties of the eigenkets. A theoretical framework is also developed to obtain explicit solutions for density operators of quantum microcombs, which allows us to obtain their complete characterization, as well as for the analytical determination of various performance metrics such as fidelity, purity, and entropy. Finally, we describe a quantum Kerr microcomb generator with a pulse-wave laser and propose the time-bin entangled states generated by it.Item Dynamics and applications of long-distance laser filamentation in air(2024) Goffin, Andrew; Milchberg, Howard; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Femtosecond laser pulses with sufficient power will form long, narrow high-intensity light channels in a propagation medium. These structures, called “filaments”, form due to nonlinear self-focusing collapse in a runaway process that is arrested by a mechanism that limits the peak intensity. For near-infrared pulses in air, the arrest mechanism is photoionization of air molecules and the resulting plasma-induced defocusing. The interplay between plasma-induced defocusing and nonlinear self-focusing enables high-intensity filament propagation over long distances in air, much longer than the Rayleigh range (~4 cm) corresponding to the ~200 µm diameter filament core. In this thesis, the physics of atmospheric filaments is studied in detail along with several applications. Among the topics of this thesis: (1) Using experiments and simulations, we studied the pulse duration dependence of filament length and energy deposition in the atmosphere, revealing characteristic axial oscillations intimately connected to the delayed rotational response of air molecules. This measurement used a microphone array to record long segments of the filament propagation path in a single shot. These results have immediate application to the efficient generation of long air waveguides. (2) We investigated the long-advertised ability of filaments to clear fog by measuring the dynamics of single water droplets in controlled locations near a filament. We found that despite claims in the literature that droplets are cleared by filament-induced acoustic waves, they are primarily cleared through optical shattering. (3) We demonstrated optical guiding in the longest-filament induced air waveguides to date (~50 m, a length increase of ~60×) using multi-filamentation of Laguerre-Gaussian LG01 modes with pulse durations informed by experiment (1). (4) We demonstrated the first continuously operating air waveguide, using a high-repetition-rate laser to replenish the waveguide faster than it could thermally dissipate. For each of the air waveguide experiments, extension to much longer ranges and steady state operation is discussed.Item MONTE CARLO SIMULATIONS OF BRILLOUIN SCATTERING IN TURBID MEDIA(2023) Lashley, Stephanie; Chembo, Yanne K; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Brillouin microscopy is a non-invasive, label-free optical elastography method for measuring mechanical properties of cells. It provides information on the longitudinal modulus and viscosity of a medium, which can be indicators of traumatic brain injury, cancerous tumors, or fibrosis. All optical techniques face difficulties imaging turbid media, and Monte Carlo simulations are considered the gold-standard to model these scenarios. Brillouin microscopy adds a unique challenge to this problem due to the angular dependence of the scattering event. This thesis extends a traditional Monte Carlo simulation software by adding the capability to simulate Brillouin scattering in turbid media, which provides a method to test strategies to mitigate the effects of multiple elastic scattering without the time and cost associated with physical experiments. Experimental results have shown potential methods to alleviate the problems caused by multiple elastic scattering, and this thesis will verify the simulation results against the experimental findings.Item INTEGRATION OF CLASSICAL/NONCLASSICAL OPTICAL NONLINEARITIES WITH PHOTONIC CIRCUITS(2023) Buyukkaya, Mustafa A; Waks, Edo; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Recent developments in nanofabrication have opened opportunities for strong light-matter interactions that can enhance optical nonlinearities, both classical and non-classical, for applications such as optical computing, quantum communication, and quantum computing. However, the challenge lies in integrating these optical nonlinearities efficiently and practically with fiber-based and silicon-based photonic circuits on a large scale and at low power. In this thesis, we aimed to achieve this integration of classical and quantum optical nonlinearities with fiber-based and silicon-based photonic circuits.For classical optical applications, optical bistability is a well-researched nonlinear optical phenomenon that has hysteresis in the output light intensity, resulting from two stable electromagnetic states. This can be utilized in various applications such as optical switches, memories, and differential amplifiers. However, integrating these applications on a large scale requires low-power optical nonlinearity, fast modulation speeds, and photonic designs with small footprints that are compatible with fiber optics or silicon photonic circuits. Thermo-optic devices are an effective means of producing optical bistability through thermally induced refractive index changes caused by optical absorption. The materials used must have high absorption coefficients and strong thermo-optic effects to realize low-power optical bistability. For this purpose, we choose high-density semiconductor quantum dots as the material platform and engineer nanobeam photonic crystal structures that can efficiently be coupled to an optical fiber while achieving low-power thermo-optical bistability. For applications that require non-classical nonlinearities such as quantum communication and quantum computing, single photons are promising carriers of quantum information due to their ability to propagate over long distances in optical fibers with extremely low loss. However, the efficient coupling of single photons to optical fibers is crucial for the successful transmission of quantum information. Semiconductor quantum dots that emit around telecom wavelengths have emerged as a popular choice for single photon sources due to their ability to produce bright and indistinguishable single photons, and travel long distances in fiber optics. Here, we present our advances in integrating telecom wavelength single photons from semiconductor quantum dots to optical fibers to realize efficient fiber-integrated on-demand single photon sources at telecom wavelengths. Finally, using the same methodology, we demonstrate the integration of these quantum dots with CMOS foundry-made silicon photonic circuits. The foundry chip is designed to individually tune quantum dots using the quantum confined stark shift with localized electric fields at different sections of the chip. This feature could potentially enable the tuning of multiple quantum emitters for large-scale integration of single photon sources for on-chip quantum information processing.Item STRONG OPTICAL NONLINEARITY AND SPIN CONTROL WITH PHOTONIC STRUCTURES(2023) singh, harjot; Waks, Edo; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Photons are excellent carriers of quantum information. Since they hardly interact with each other, they can maintain their quantum state over long distances. However, this poses a challenge if one wishes to create entanglement between the degrees of freedom of photons. Creating entangled states of photons is essential for quantum information processing with photons. One way to create interactions between photons is to create strong interactions between two level atoms and modes of electromagnetic radiation. This can be achieved by coupling optical transitions of two-level atoms with modes of optical cavities or waveguides. The nonlinearity of a two-level atom then effectively mediates interactions between two (or more) photons. However, due to a fundamental time-bandwidth limit, a two-level atom cannot enable arbitrary quantum operations on the states of two photons. In this thesis, we study theoretically the problem of splitting two indistinguishable photons to distinguishable output modes with a two-level atom. Due to the time-bandwidth limit, the achieved splitting efficiency is fundamentally limited to 82% using just a two-level atom. We show that a linear optical unitary transformation on the output modes of the two-level atom can exceed this limit. Via optimization of the input two photon wavefunction and the parameters of the linear optical unitary, we calculated a splitting efficiency of 92%. For experimental realization of strong atom-light interactions, we used InGaAs quantum dots coupled to a bullseye cavity. Bullseye cavities are promising towards realization of efficient collection of light due to their near Gaussian far field emission. We demonstrated a strong interaction between the quantum dot exciton and the Bullseye cavity mode, quantified by a Cooperativity of ~8. This high cooperativity with a low-quality factor cavity can be attributed to the charge stabilization enabled by the diode heterostructure of the quantum dot samples we used. Finally, we focus on the electron spin ground states of a negatively charged InGaAs quantum dot. The electron spin interacts with the nuclear spins of the In, Ga and As. We measure the spectrum of this interaction using all optical dynamical decoupling pulse sequencies. This work lays a path forward to realizing efficient and coherent spin-photon interfaces with InGaAs quantum dots.Item QUANTUM MODEM AND ROUTER FOR THE QUANTUM INTERNET(2022) Saha, Uday; Waks, Edo; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Like the internet, the quantum internet can change the world by connecting quantum computers over long distances. This connectivity can revolutionize different industries like banking, healthcare, and data analytics that utilize quantum computing and simulations. Additionally, it would enable us to connect multiple small quantum computers into powerful distributed quantum computers that can solve problems of significant societal impact. Despite the rise of excellent quantum computers, we don't yet have the core technologies to connect them. This is because modems and routers we use to connect classical computers don't work for quantum information. They destroy the coherence and entanglement of quantum information, which is vital for connecting quantum computers. In my thesis, I developed a quantum modem and router that can connect quantum computers and create a scalable quantum network. I have conceived the modem and router for the trapped ion quantum computers, the most promising quantum computing platform. However, we can easily use my developed concepts to connect different quantum computing platforms. I accomplished a quantum modem that provides an interface between a quantum computer and a fiber-optic network by generating telecommunication photons from the computer. I used a two-stage quantum frequency conversion scheme to realize the quantum modem. By calculating the second-order correlation function, I experimentally verified single-photon characteristics retained after the frequency conversion process. Telecommunication photons generated by the quantum modem can carry quantum information from ions over long distances. This will allow a long-distance quantum network to realize the quantum internet. On the other hand, I implemented a quantum router with photonic integrated circuits. Utilizing the thermo-optic property, I route photons from a trapped barium ion into different output ports of the quantum router in a programmable manner. This router can connect multiple quantum computers on-demand and in a scalable way. We are the first group to demonstrate a quantum modem and router working together with a quantum computer. This demonstration could lead to a scalable quantum network where photons from different quantum computers can be interfered with a programmable photonic chip to herald entanglement. Additionally, I developed visible photonic circuits for quantum data centers. In a quantum data center, there can be multiple trapped ion quantum computers that need to be connected. For this purpose, I designed a photonic circuit on a thin-film lithium niobate platform that can entangle two trapped ion quantum computers with >99% fidelity. Apart from achieving high fidelity entanglement, the circuit can achieve any polarization-independent power splitting ratio, which can have extensive use in integrated photonic technology. Finally, I invented a multiplexing scheme by which we can send quantum information from multiple quantum computers using a single fiber-optic cable. That will increase the channel capacity, where multiple quantum computers can communicate through the same channel. By encoding quantum information into the different wavelengths of photons, I devised my idea of multiplexing quantum information. These results will enable us to achieve a programmable and scalable network of quantum computers to increase the capability of quantum computing and quantum simulations and lead us to the future quantum internet.Item SILICON NITRIDE INTEGRATED PHOTONIC DEVICES AND THEIR APPLICATIONS IN ASTRONOMY AND QUANTUM PHYSICS(2022) Zhan, Jiahao; Dagenais, Mario; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The photonics technology has revolutionized the telecommunication industry in the past 40 years with the deployment of the undersea fiber-optic network. Nowadays, with the maturity of silicon photonics technology, the integrated photonic platform is enabling more and more cutting-edge technologies, such as optical transceivers for data center connectivity, automotive LiDARs for self-driving vehicles, the next-generation astronomical instrumentation and nearterm photonic quantum computers, to name a few. In recent years, silicon nitride (Si3N4) material has attracted a significant amount of attention mainly due to the ultra-low loss that can be achieved. Compared to silicon, Si3N4 has a much wider transparency window, and does not suffer from two-photon absorption and free-carrier absorption over the telecommunication band. The relatively low refractive index of Si3N4 also means less sensitivity of optical modes to the waveguide sidewall roughness, therefore reducing the scattering loss. In this dissertation, I will first give an introduction of integrated photonics, and a brief overview of some novel applications and current trends. Next I will graphically show our methods for device fabrication and characterization, and then demonstrate a few integrated photonic devices implemented on the Si3N4 material platform, including Bragg gratings, multimode interferometers, polarization beam splitters, and polarization rotators, with an in-depth discussion of their potentialapplications, principles of operation, simulation and experimental results. I will then embark on a new chapter on arrayed waveguide gratings (AWGs), with emphasis on its application in integrated astronomical spectrometers. To obtain a continuous two-dimensional spectrum, cleaving at the output focal plane of the AWGis required. I will discuss and demonstrate a three-stigmatic-point AWG, which provides an elegant solution to the non-flat focal plane issue in traditional Rowland AWGs. This work is a critical step towards the development of an efficientand miniaturized astronomical spectrograph for the upcoming extremely-large telescopes. Next, I will introduce a one-dimensional nanobeam cavity enabled by a slow-light waveguide. A cubic relation between the quality factor and the length of the cavity will be derived and experimental verification will be demonstrated. The current progress towards the investigation of the Purcell effect of this nanobeam cavity will be discussed, including the platform and the loss characterization of the deposited amorphous silicon material. In the final chapter, I will first summarize the major conclusions from the previous chapters. Then I will briefly discuss some future research directions extending the work in this thesis, including ultra-broadband polarization beam splitter, the development of an on-chip Bell state analyzer, and the design of a polarization-insensitive flat-focal-field spectrometer.