Non-contact ultrasonic inspection by Gas-Coupled Laser Acoustic Detection (GCLAD)

Gas-Coupled Laser Acoustic Detection (GCLAD) is an ultrasonic, non-contact detection technique that has been recently proven to be applicable to the inspection of mechanical components. GCLAD response raises as the intersection length between the probe laser beam and the acoustic wavefront propagating in the air increases; such feature differentiates the GCLAD device from other optical detection instruments, making it a line detection system rather than a point detector. During the inspection of structures mainly extending in two dimensions, the capability to evidence presence of defects in whichever point over a line would enable moving the emitter and the detector along a single direction: this translates in the possibility to decrease the overall required time for interrogation of components compared to point detectors, as well as generating simpler automated monitoring layouts. Based on this assumption, the present study highlights the possibility of employing the GCLAD device as a line inspection tool. To this end, preliminary concepts are provided allowing maximization of the GCLAD response for the non-destructive testing of components which predominantly extend in two dimensions. Afterwards, the GCLAD device is employed in pulse-echo mode for the detection of artificial defects machined on a 12 mm-thick steel plate: the GCLAD probe laser beam is inclined to be perpendicular to the propagation direction of the airborne ultrasound, generated by surface acoustic waves (SAWs) in the solid which are first reflected by the defect flanks and subsequently refracted in the air. Numerical results are provided highlighting the SAW reflection patterns, originated by 3 mm deep surface and subsurface defects, that the GCLAD should interpret. The subsequent experimental campaign highlights that the GCLAD device can identify echoes associated with surface and subsurface defects, located in eight different positions on the plate. B-scan of the component ultimately demonstrates the GCLAD performance in accomplishing the inspection task.

Dual-comb Photothermal Spectroscopy

Dual-comb spectroscopy (DCS) has revolutionized optical spectroscopy by providing broadband spectral measurements with unprecedent resolution and fast response. Photothermal spectroscopy (PTS) offers an ultrasensitive and background-free gas sensing method, which is normally performed using a single-wavelength pump laser. The merging of PTS with DCS may enable a new spectroscopic method by taking advantage of both technologies, which has never been studied yet. Here, we report dual-comb photothermal spectroscopy (DC-PTS) by passing dual combs and a probe laser through a gas-filled anti-resonant hollow-core fiber, where the generated multi-heterodyne modulation of the refractive index is sensitively detected by an in-line interferometer. As an example, we have measured photothermal spectra of acetylene over 1 THz, showing a good agreement with the spectral database. Our proposed DC-PTS provides new opportunities for broadband gas sensing with super-fine resolution and high sensitivity, as well as with a small sample volume and compact configuration.

Retrieval of Angle-Dependent Strong-Field Ionization by Using High Harmonics Generated from Aligned N2 Molecules

We propose a method to retrieve the angle-dependent strong-field ionization of highest occupied molecular orbital (HOMO) from high-order harmonic generation (HHG) of aligned molecules. This method is based on the single-molecule quantitative rescattering model with known alignment distribution and photo-recombination cross sections of fixed-in-space molecules. With the macroscopic HHG of aligned N2 molecules, we show that angle-dependent ionization of HOMO can be successfully retrieved at both low and high degrees of alignment. We then show that the error in the retrieved angular dependence of ionization becomes larger if the uncertainty in the alignment distribution is introduced in the retrieval procedure. We also examine that the retrieved ionization of HOMO is much deviated from the accurate one if the intensity of probe laser becomes higher such that inner HOMO-1 can contribute to HHG.

Micromotion minimization using Ramsey interferometry

We minimize the stray electric field in a linear Paul trap quickly and accurately, by applying interferometry pulse sequences to a trapped ion optical qubit. The interferometry sequences are sensitive to the change of ion equilibrium position when the trap stiffness is changed, and we use this to determine the stray electric field. The simplest pulse sequence is a two-pulse Ramsey sequence, and longer sequences with multiple pulses offer a higher precision. The methods allow the stray field strength to be minimized beyond state-of-the-art levels. Using a sequence of nine pulses we reduce the 2D stray field strength to (10.5±0.8)mVm-1 in 11s measurement time. The pulse sequences are easy to implement and automate, and they are robust against laser detuning and pulse area errors. We use interferometry sequences with different lengths and precisions to measure the stray field with an uncertainty below the standard quantum limit. This marks a real-world case in which quantum metrology offers a significant enhancement. Also, we minimize micromotion in 2D using a single probe laser, by using an interferometry method together with the resolved sideband method; this is useful for experiments with restricted optical access. Furthermore, a technique presented in this work is related to quantum protocols for synchronizing clocks; we demonstrate these protocols here.

A NIR-II Fluorophores for Multiple Myeloma Imaging and Photodynamic Therapy

Background: Multiple myeloma is a hematological malignant disease in which clonal plasma cells proliferate abnormally. NIR-II fluorophores have great application prospects for cancer treatment. Methods: Multiple detection techniques were used to evaluate effect and mechanisms of the novel small-molecule NIR-II fluorophores probe on multiple myeloma cells in vitro experiment. In vivo MM.1S Luc+ B-NSG mouse model was built to assess the role of probe in multiple myeloma treatment. Results: The novel small-molecule NIR-II fluorophores probe shown high affinity with multiple myeloma cells and specific myeloma imaging in vivo. More importantly, the probe could effectively induce tumor death with 808 nm laser irradiation in a dose-dependent manner. Further, Confocal found that the probe was mainly distributed in mitochondria and significantly reduced the mitochondrial membrane potential with laser irradiation. Moreover, Reactive Oxygen Species (ROS) level was notably increased and the up-regulation expression of cleaved poly ADP-ribose polymerase (PARP) was detected after treated with the probe and laser irradiation. Consistent with the previous results, the disease was alleviated obviously in probe laser-treated group. Conclusions: Our work confirm for the first time that the specific imaging and remarkable photodynamic therapy (PDT) effect of a novel NIR-II fluorophores probe to multiple myeloma. It provides the foundation for application of NIR-II fluorophores from solid tumor to non-solid tumor. Disclosures No relevant conflicts of interest to declare.

An estimate of size of copper nanoparticles levitating over the melt surface using the measurements of spectral reflectance

A strong decrease in normal reflectance of a probe laser beam of 660 nm wavelength reflected from the surface of copper sample just after the beginning of the sample melting in a rarefied argon atmosphere has been observed recently by the authors. A similar time dependence of the reflectance is obtained in the laboratory experiments of the present paper at the wavelengths of 532 nm. The additional spectral measurements enable the authors to estimate the size of condensed nanoparticles levitating over the copper melt.

On the Determination of Thermal Conductivity From Frequency Domain Thermoreflectance Experiments

The Fourier and the hyperbolic heat conduction equations were solved numerically to simulate a frequency-domain thermoreflectance (FDTR) experiment. Numerical solutions enable isolation of pump and probe laser spot size effects, and use of realistic boundary conditions. The equations were solved in time domain and the phase lag between the temperature of the transducer (averaged over the probe laser spot) and the modulated pump laser signal, were computed for a modulation frequency range of 200 kHz to 200 MHz. Numerical calculations showed that extracted values of the thermal conductivity are sensitive to both the pump and probe laser spot sizes, while analytical solutions (based on Hankel transform) cannot isolate the two effects, although for the same effective (combined) spot size, the two solutions are found to be in excellent agreement. If the substrate (computational domain) is sufficiently large, the far-field boundary conditions were found to have no effect on the computed phase lag. The interface conductance between the transducer and the substrate was found to have some effect on the extracted thermal conductivity. The hyperbolic heat conduction equation yielded almost the same results as the Fourier heat conduction equation for the particular case studied. The numerically extracted thermal conductivity value (best fit) for the silicon substrate considered in this study was found to be about 82-108 W/m/K, depending on the pump and probe laser spot sizes used.