Georgia Tech-CNRS Laboratory

We report on the implementation of a reverse-reconciliated coherent-state continuous-variable quantum key distribution system, with which we generated secret keys at a rate of more than $2\phantom{\rule{0.3em}{0ex}}\mathrm{kb}∕\mathrm{s}$ over $25\phantom{\rule{0.3em}{0ex}}\mathrm{km}$ of optical fiber. Time multiplexing is used to transmit both the signal and phase reference in the same optical fiber. Our system includes all experimental aspects required for a field implementation of a quantum key distribution setup. Real-time reverse reconciliation is achieved by using fast and efficient low-density parity check error correcting codes.

Solid state UV emitters have many advantages over conventional UV sources. The (Al,In,Ga)N material system is best suited to produce LEDs and laser diodes from 400 nm down to 210 nm-due to its large and tuneable direct band gap, n-and p-doping capability up to the largest bandgap material AlN and a growth and fabrication technology compatible with the current visible InGaN-based LED production. However AlGaN based UV-emitters still suffer from numerous challenges compared to their visible counterparts that become most obvious by consideration of their light output power, operation voltage and long term stability. Most of these challenges are related to the large bandgap of the materials. However, the development since the first realization of UV electroluminescence in the 1970s shows that an improvement in understanding and technology allows the performance of UV emitters to be pushed far beyond the current state. One example is the very recent realization of edge emitting laser diodes emitting in the UVC at 271.8 nm and in the UVB spectral range at 298 nm. This roadmap summarizes the current state of the art for the most important aspects of UV emitters, their challenges and provides an outlook for future developments.

We propose a method for encrypting a signal within the high dimensional chaotic fluctuations of the wavelength from a delayed feedback tunable laser diode. Decoding is performed remotely by using a slave laser diode fully synchronized with the master one. No additional synchronization channel is required.

Although flakes of two-dimensional (2D) heterostructures at the micrometer scale can be formed with adhesive-tape exfoliation methods, isolation of 2D flakes into monolayers is extremely time consuming because it is a trial-and-error process. Controlling the number of 2D layers through direct growth also presents difficulty because of the high nucleation barrier on 2D materials. We demonstrate a layer-resolved 2D material splitting technique that permits high-throughput production of multiple monolayers of wafer-scale (5-centimeter diameter) 2D materials by splitting single stacks of thick 2D materials grown on a single wafer. Wafer-scale uniformity of hexagonal boron nitride, tungsten disulfide, tungsten diselenide, molybdenum disulfide, and molybdenum diselenide monolayers was verified by photoluminescence response and by substantial retention of electronic conductivity. We fabricated wafer-scale van der Waals heterostructures, including field-effect transistors, with single-atom thickness resolution.

We report on the experimental study of the propagation of surface guided waves in a periodic arrangement of pillars on a semi-infinite medium. Samples composed of nickel pillars grown on a lithium niobate substrate were prepared and wide bandwidth transducers were used for the electrical generation of surface elastic waves. We identify a complete band gap for surface guided waves appearing at frequencies markedly lower than the Bragg band gap. Using optical measurements of the surface vibrations and by comparison with a finite element model, we argue that the low frequency band gap arises because of local resonances in the pillars. When resonance is reached, the acoustic energy is confined inside the pillars and transmission through the array is strongly reduced. At higher frequencies and inside the Bragg band gap, the incident surface elastic waves are almost completely reflected and the observed exponential decay of the transmission is similar to the case of phononic crystals made of holes in a substrate.

We investigate theoretically the propagation of acoustic waves in a two-dimensional array of cylindrical pillars on the surface of a semi-infinite substrate. Through the computation of the band structure of the periodic array and of the transmission of waves through a finite length array, we show that the phononic crystal can support a number of surface propagating modes in the nonradiative region of the substrate, or sound cone, as limited by the slowest bulk acoustic wave. The modal shape and the polarization of these guided modes are more complex than those of classical surface waves propagating on a homogeneous surface. Significantly, an in-plane polarized wave and a transverse wave with sagittal polarization appear that are not supported by the free surface. In the band structure, guided modes define band gaps that appear at frequencies markedly lower than those expected from the Bragg interference condition. We identify them as originating from local resonances of the individual cylindrical pillars and show their dependence on the geometrical parameters, in particular with the height of the pillars. The transmission of surface acoustic waves across a finite array of pillars shows the signature of the locally resonant band gaps for surface modes and their dependence on the symmetry of the source and its polarization. Numerical simulations are performed by using the finite element method and considering silicon pillars on a silicon substrate.

High- and ultrahigh-Q whispering-gallery mode resonators have the capability to trap photons by total internal reflection for a duration ranging from nanoseconds to milliseconds. These exceptionally long photon lifetimes enhance the light–matter interactions at all scales, namely at the electronic, molecular, and lattice levels. As a consequence, nonlinear photon scattering can be triggered with very low threshold powers, down to a few microwatts. The possibility to efficiently harness photon–photon interactions with a system optimizing size, weight, power, and cost constraints has created a new, quickly thriving research area in photonics science and technology. This topic is inherently cross-disciplinary, as it stands at the intersection of nonlinear and quantum optics, crystallography, optoelectronics, and microwave photonics. From a fundamental perspective, high-Q whispering-gallery mode resonators have emerged as an ideal platform to investigate light–matter interactions in nonlinear bulk materials. From an applied viewpoint, technological applications include time-metrology, aerospace engineering, coherent optical fiber communications, or nonclassical light generation, among others. The aim of this paper is to provide an overview of the most recent advances in this area, which is increasingly gaining importance in contemporary photonics.

A critical issue in optical chaos-based communications is the possibility to identify the parameters of the chaotic emitter and, hence, to break the security. In this paper, we study theoretically the identification of a chaotic emitter that consists of a semiconductor laser with an optical feedback. The identification of a critical security parameter, the external-cavity round-trip time (the time delay in the laser dynamics), is performed using both the auto-correlation function and delayed mutual information methods applied to the chaotic time-series. The influence on the time-delay identification of the experimentally tunable parameters, i.e., the feedback rate, the pumping current, and the time-delay value, is carefully studied. We show that difficult time-delay-identification scenarios strongly depend on the time-scales of the system dynamics as it undergoes a route to chaos, in particular on how close the relaxation oscillation period is from the external-cavity round-trip time.

We investigate theoretically the possibility of retrieving the value of the time delay of a semiconductor laser with an external optical feedback from the analysis of its intensity time series. When the feedback rate is moderate and the injection current set such that the laser relaxation-oscillation period is close to the delay, then the time-delay identification becomes extremely difficult, thus improving the security of chaos-based communications using external-cavity lasers.

Electron and x-ray diffraction are well-established experimental methods used to explore the atomic scale structure of materials. In this work, a computational method is implemented to produce virtual electron and x-ray diffraction patterns directly from atomistic simulations without a priori knowledge of the unit cell. This method is applied to study the structure of [0 1 0] symmetric tilt low-angle and large-angle grain boundaries in Ni. Virtual electron diffraction patterns and x-ray diffraction 2θ line profiles show that this method can distinguish between low-angle grain boundaries with different misorientations and between low-angle boundaries with the same misorientation but different dislocation configurations. For large-angle Σ5 (2 1 0), Σ29 (5 2 0) and Σ5 (3 1 0) coincident site lattice [0 1 0] symmetric tilt grain boundaries, virtual diffraction methods can identify the misorientation of the grain boundary and show subtle differences between grain boundaries in the x-ray 2θ line profiles. A thorough analysis of the effects of simulation size on the relrod structure in the electron diffraction patterns is presented.

Abstract Autonomous robotic weeding systems in precision farming have demonstrated their full potential to alleviate the current dependency on agrochemicals such as herbicides and pesticides, thus reducing environmental pollution and improving sustainability. However, most previous works require fast and constant‐time weed detection systems to achieve real‐time treatment, which forecloses the implementation of more capable but time‐consuming algorithms, for example, learning‐based methods. In this paper, a nonoverlapping multicamera system is applied to provide flexibility for the weed control system in dealing with the indeterminate classification delays. The design, implementation, and testing of our proposed modular weed control unit with mechanical and chemical weeding tools are presented. A framework that performs naive Bayes filtering, 3D direct intra‐ and inter‐camera visual tracking, and predictive control, while integrating state‐of‐the‐art crop/weed detection algorithms, is developed to guide the tools to achieve high‐precision weed removal. The experimental results show that our proposed fully operational weed control system is capable of performing selective mechanical as well as chemical in‐row weeding with indeterminate detection delays in different terrain conditions and crop growth stages.

While secrecy in communication systems has historically been obtained through cryptographic means in the upper layers, recent research efforts have focused on the physical layer and have unveiled ample opportunities for security design. In particular, the combination of signal processing techniques with channel coding for secrecy has been central to the development of physical-layer security efforts. Although implicit coding techniques for secrecy have been known since the 1970s, explicit code constructions have only been discovered within the last decade. The purpose of this article is to provide a synopsis of the state of the art in coding for secrecy. We discuss the general principles of coding, and we illustrate them with several examples. In particular, we discuss the importance of a nested code structure and stochastic encoding, which allow for both data reliability and security.

This paper reports the experimental investigation of two different approaches to random bit generation based on the chaotic dynamics of a semiconductor laser with optical feedback. By computing high-order finite differences of the chaotic laser intensity time series, we obtain time series with symmetric statistical distributions that are more conducive to ultrafast random bit generation. The first approach is guided by information-theoretic considerations and could potentially reach random bit generation rates as high as 160 Gb/s by extracting 4 bits per sample. The second approach is based on pragmatic considerations and could lead to rates of 2.2 Tb/s by extracting 55 bits per sample. The randomness of the bit sequences obtained from the two approaches is tested against three standard randomness tests (ENT, Diehard, and NIST tests), as well as by calculating the statistical bias and the serial correlation coefficients on longer sequences of random bits than those used in the standard tests.

We propose a method based on a scalar second-order difference-differential equation to obtain intensity chaos from a laser diode with a nonlinear delayed feedback. The method can be used for encrypting, transmitting, and decrypting a signal in a chaos-based communication system. The core of the chaotic transmitter and receiver is formed by an electrooptic modulator that is used to generate a strong reproducible nonlinearity and chaotic waveforms of extremely high Lyapunov dimensionality. The system opens the way to ultrafast chaotic communications.

This article reports on two-dimensional (2D) layered hexagonal BN (h-BN) grown on sapphire by metalorganic vapor phase epitaxy (MOVPE). The highly oriented lattice and hexagonal phase of the epitaxial layers were confirmed by X-ray diffraction, Raman spectrum, and cross-section scanning transmission electron microscopy. The surface of BN over a 2-in. wafer exhibits specific 2D material morphology features for different BN thicknesses, from an atomically flat surface to a honeycomb wrinkle network. The grown epitaxial layers demonstrate a large absorption coefficient (∼106 cm–1) above the bandgap energy of 5.87 eV with direct band transition behavior. Near-bandgap luminescence at 216.5 nm (5.73 eV) and characteristic defect band recombination at longer wavelengths were observed by cathodoluminescence at 77 K. This wafer-scale MOVPE-grown layered h-BN with different 2D morphology and with near bandgap emission can facilitate applications such as graphene-based electronics, advanced van der Waals heterostructures, and deep UV photonics.

The influence of material loss on the complex band structure of two-dimensional phononic crystals is investigated. A viscoelasticity model is added to the extended plane-wave expansion (EPWE) method, with viscosity proportional to the frequency. It is found that losses have a stronger influence on the real than on the imaginary part of Bloch waves, in contrast with propagation in homogeneous media. Flat bands, i.e., bands initially showing low group velocity without losses, acquire an enhanced damping as compared to bands with larger group velocities. Losses are also found to limit the appearance of large group slownesses, or conversely small group velocities.

We report single-photon interference in the sidebands of modulated light. The relative phase of interacting quantum states is reliably controlled by the phase of a low-frequency modulating signal. We show how this type of interference can be used to build a robust system for quantum cryptography. An experiment was implemented at 1540 nm, over a 20-km-long standard single-mode fiber, using a germanium singlephoton avalanche photodiode.

Semiconductor quantum well electroabsorption modulators are widely used to modulate near-infrared (NIR) radiation at frequencies below 0.1 terahertz (THz). Here, the NIR absorption of undoped quantum wells was modulated by strong electric fields with frequencies between 1.5 and 3.9 THz. The THz field coupled two excited states (excitons) of the quantum wells, as manifested by a new THz frequency- and power-dependent NIR absorption line. Nonperturbative theory and experiment indicate that the THz field generated a coherent quantum superposition of an absorbing and a nonabsorbing exciton. This quantum coherence may yield new applications for quantum well modulators in optical communications.

While the conceptual framework for nanoplasmonic waveguides composed of a chain of noncontacting metal nanoparticles usually neglects the effects of the ends, the long-range nature of the interparticle coupling underlying the electromagnetic transport means that finite chain length can play an important role. Here, the complex energies of the plasmon-polariton modes in finite-length nanoparticle chains are calculated to ascertain the effects of chain length on the mode dispersion and the radiative contribution to the attenuation. The results indicate that, for typical parameters, the infinite-chain limit is reached with approximately 10 nanoparticles. Thus, even for chain lengths well exceeding the attenuation length, long-range coupling of distant nanoparticles is shown to impact the dispersion and radiative loss.

A new type of two-dimensional photonic-crystal (PC) structure called annular PC composed of a dielectric-rod and a circular-air-hole array in a square or triangular lattice such that a dielectric rod is centered within each air hole is studied. The dielectric rods within the air holes greatly modify the dispersion diagram of the photonic crystal despite the fact that the percentage of volume occupied by the dielectric rods may be small (<12%). Increasing the radius of the inner-dielectric rod, starting from zero to a critical value, reduces the band gap and closes it completely as expected, because of the addition of more dielectric material inside the unit cell. Continuing to increase the radius of the rod above the critical value surprisingly creates another photonic band gap. Comparison of the dispersion diagrams of the new structure and the original lattice (circular air hole square/triangular array in dielectric background) reveals that the photonic band gap is considerably enhanced in size for both square and triangular lattice with the new structure. This approach preserves the symmetry of the structure and provides a complete photonic band gap away from the close-packed condition and at low normalized frequencies.