Preliminary assessment of ship detection and trajectory evaluation using distributed acoustic sensing on an optical fiber telecom cable.

Distributed acoustic sensing (DAS) is a recent instrumental approach allowing the conversion of fiber-optic cables into dense arrays of acoustic sensors. This technology is attractive in marine environments where instrumentation is difficult to implement. A promising application is the monitoring of environmental and anthropic noise, leveraging existing telecommunication cables on the seafloor. We assess the ability of DAS to monitor such noise using a 41.5 km-long cable offshore of Toulon, France, focusing on a known and localized source. We analyze the noise emitted by the same tanker cruising above the cable, first 5.8 km offshore in 85 m deep bathymetry, and then 20 km offshore, where the seafloor is at a depth of 2000 m. The spectral analysis, the Doppler shift, and the apparent velocity of the acoustic waves striking the fiber allow us to separate the ship radiated noise from other noise. At 85 m water depth, the signal-to-noise ratio is high, and the trajectory of the boat is recovered with beamforming analysis. At 2000 m water depth, although the acoustic signal of the ship is more attenuated, signals below 50 Hz are detected. These results confirm the potential of DAS applied to seafloor cables for remote monitoring of acoustic noise even at intermediate depth.

Nanoscale Mapping and Control of Antenna-Coupling Strength for Bright Single Photon Sources.

Cavity quantum electrodynamics is the art of enhancing light-matter interaction of photon emitters in cavities with opportunities for sensing, quantum information, and energy capture technologies. To boost emitter-cavity interaction, that is, coupling strength g, ultrahigh quality cavities have been concocted yielding photon trapping times of microsecondsy to milliseconds. However, such high- Q cavities give poor photon output, hindering applications. To preserve high photon output, it is advantageous to strive for highly localized electric fields in radiatively lossy cavities. Nanophotonic antennas are ideal candidates combining low- Q factors with deeply localized mode volumes, allowing large g, provided the emitter is positioned exactly right inside the nanoscale mode volume. Here, with nanometer resolution, we map and tune the coupling strength between a dipole nanoantenna-cavity and a single molecule, obtaining a coupling rate of gmax ∼ 200 GHz. Together with accelerated single photon output, this provides ideal conditions for fast and pure nonclassical single photon emission with brightness exceeding 109 photons/sec. Clearly, nanoantennas acting as "bad" cavities offer an optimal regime for strong coupling g to deliver bright on-demand and ultrafast single photon nanosources for quantum technologies.

Vectorial nanoscale mapping of optical antenna fields by single molecule dipoles.

Optical nanoantennas confine light on the nanoscale, enabling strong light-matter interactions and ultracompact optical devices. Such confined nanovolumes of light have nonzero field components in all directions (x, y, and z). Unfortunately mapping of the actual nanoscale field vectors has so far remained elusive, though antenna hotspots have been explored by several techniques. In this paper, we present a novel method to probe all three components of the local antenna field. To this end a resonant nanoantenna is fabricated at the vertex of a scanning tip. Next, the nanoantenna is deterministically scanned in close proximity to single fluorescent molecules, whose fixed excitation dipole moment reads out the local field vector. With nanometer molecular resolution, we distinctly map x-, y-, and z-field components of the dipole antenna, i.e. a full vectorial mode map, and show good agreement with full 3D FDTD simulations. Moreover, the fluorescence polarization maps the localized coupling, with emission through the longitudinal antenna mode. Finally, the resonant antenna probe is used for single molecule imaging with 40 nm fwhm response function. The total fluorescence enhancement is 7.6 times, while out-of-plane molecules, almost undetectable in far-field, are made visible by the strong antenna z-field with a fluorescence enhancement up to 100 times. Interestingly, the apparent position of molecules shifts up to 20 nm depending on their orientation. The capability to resolve orientational information on the single molecule level makes the scanning resonant antenna an ideal tool for extreme resolution bioimaging.

Ultrafast stimulated emission microscopy of single nanocrystals.

Single-molecule detection is a powerful method used to distinguish different species and follow time trajectories within the ensemble average. However, such detection capability requires efficient emitters and is prone to photobleaching, and the slow, nanosecond spontaneous emission process only reports on the lowest excited state. We demonstrate direct detection of stimulated emission from individual colloidal nanocrystals at room temperature while simultaneously recording the depleted spontaneous emission, enabling us to trace the carrier population through the entire photocycle. By capturing the femtosecond evolution of the stimulated emission signal, together with the nanosecond fluorescence, we can disentangle the ultrafast charge trajectories in the excited state and determine the populations that experience stimulated emission, spontaneous emission, and excited-state absorption processes.