James. E. Downes

We present the remote measurement of temperature vs length profiles in up to 50 m of clear (swimming pool) water. A pair of blue gallium-nitride laser diodes, with wavelengths separated by 4 nm, was multiplexed in time as the excitation source. Elastic and Raman returns were collected using two detector channels. The ratio of the Raman measurements can be calibrated to retrieve temperature as a function of depth, while the ratio of the elastic measurements can be used to correct for differential attenuation between the wavelengths. The temperature accuracy decreases with range and is typically better than 0.5 °C at 10 m and 1 °C at 50 m with depth resolution of 0.5-4 m, respectively, using 5 ns excitation pulses.

Gallium nitride (GaN) laser diodes are shown to offer a viable alternative to the solid-state lasers typically used in photon counting ocean LiDAR. With their ability to operate at various blue wavelengths, coupled with compactness and efficiency, they offer some considerable advantages over conventional solid-state sources.

We present a method to simultaneously determine water temperature and salinity, which uses a pulsed excitation laser and a three-channel Raman spectrometer. The method relies on the systematic dependence of the Raman OH stretching band on temperature and salinity, and is compatible with LiDAR techniques. We have measured the variation of the OH stretching band in two seawater samples and a NaCl solution, and constructed a linear mapping between signal ratios derived from the three spectral channels and the temperature and salinity of each sample. For the natural seawater this approach has been determined by cross-validation to have a predictive accuracy of ±1.6 PSU and ±0.5 °C.

We present the retrieval of depth-resolved temperature measurements in water using Raman LiDAR. Using a 5 m pipe of laboratory water, we recover non-homogeneous temperature profiles with a temperature accuracy ranging between 0.35°C and 0.85°C, and a position resolution of 28 cm.

Levitated systems in vacuum have many potential applications ranging from new types of inertial and magnetic sensors through to fundamental issues in quantum science, the generation of massive Schrödinger cats, and the connections between gravity and quantum physics. In this work, we demonstrate the passive, diamagnetic levitation of a centimeter-sized massive oscillator, which is fabricated using a method that ensures that the material, though highly diamagnetic, is an electrical insulator. Electrical conductors moving in a magnetic field experience eddy damping—which can severely reduce their motional quality factor. By chemically coating a powder of microscopic graphite beads with silica and embedding the coated powder in high-vacuum compatible wax, we form a centimeter-sized thin square plate which magnetically levitates over a checkerboard magnet array. The insulating coating reduces eddy damping by almost an order of magnitude compared to uncoated graphite with the same particle size. These plates exhibit a different equilibrium orientation from pyrolytic graphite due to their isotropic magnetic susceptibility. We measure the motional quality factor to be Q∼1.58×105 for an approximately centimeter-sized composite resonator with a mean particle size of 12 μm. Furthermore, we apply delayed feedback to cool the vertical motion of frequency ∼19 Hz and achieve center-of-mass temperature decrease by three orders of magnitude.

Manipulation and patterning of diamond surface chemistry is of interest for a wide range of diamond-based technologies. We report the patterned oxidation of hydrogen-terminated diamond surfaces with sub-monolayer (ML) precision by a deep-UV two-photon process performed in air. Using focused laser pulses of photon energy 4.66 eV (266 nm; below the diamond bandgap of 5.47 eV), hydrogen-terminated (001) surfaces were exposed with calibrated doses to remove carbon with a precision of 0.02 ML. The measurement of the electrical properties of the laser-exposed zone between ohmic electrodes enabled monitoring of the transition from a conducting H-terminated surface to insulating O-terminated. The surface resistance increases by more than 7 orders of magnitude for doses corresponding to 0.5 ML, and the I–V characteristics show a transition from linear to nonlinear for doses above 0.30 ML. We show that this behavior agrees well with a surface percolation model for carrier diffusion in which the laser etch rate for the H-terminated top layer is the same as for O-terminated. Hence, this work reveals an ultra-precise method for modifying the sub-monolayer surface chemistry with the practical advantages of a laser-induced mechanism compared to conventional plasma or chemical processing methods.

Researchers seek methods to levitate matter for a wide variety of purposes, ranging from exploring fundamental problems in science through to developing new sensors and mechanical actuators. Many levitation techniques require active driving and most can only be applied to objects smaller than a few micrometers. Diamagnetic levitation has the strong advantage of being the only form of levitation which is passive, requiring no energy input, while also supporting massive objects. Known diamagnetic materials which are electrical insulators are only weakly diamagnetic and require large magnetic field gradients to levitate. Strong diamagnetic materials which are electrical conductors, such as graphite, exhibit eddy damping, restricting motional freedom and reducing their potential for sensing applications. In this work, we describe a method to engineer the eddy damping while retaining the force characteristics provided by the diamagnetic material. We study, both experimentally and theoretically, the motional damping of a magnetically levitated graphite plate in high vacuum and demonstrate that one can control the eddy damping by patterning the plate with through-slots which interrupt the eddy currents. We find that we can control the motional quality factor over a wide range with excellent agreement between the experiment and numerical simulations.