Ablation depth enhancement on a copper surface using a dual-color double-pulse femtosecond laser.

We investigate the femtosecond laser ablation of copper with a dual-color double-pulse femtosecond laser at the wavelengths of 515 nm and 1030 nm. By properly choosing the energy of the 515 nm pulse, the optical properties such as surface reflectivity and absorption coefficient on copper surface can be modified to increase the absorption of the subsequent 1030 nm pulse. The ablation depth of dual-color double-pulse laser is at least 50% higher than the total ablation depth of both the 515 nm and 1030 nm pulses, provided that the inter-pulse delay of the double-pulse laser is within the electron-phonon coupling time. The ablation depth enhancement on a copper surface using a dual-color double-pulse femtosecond laser is of significant interest for scientific research and industrial application.

Vibrational Raman scattering from coherently excited molecular ions in a strong laser field.

We report on a pump-probe investigation of vibrational Raman scattering from coherently excited N2+ ions. It is found that the Raman signals produced by the inelastic scattering of the probe pulse from molecular ions can be dramatically enhanced when the probe laser is resonant with electronic transitions in N2+ ions. The Raman signal can be amplified at 428 nm wavelength due to the presence of population inversion in N2+ ions.

Free-space Ν2+ lasers generated in strong laser fields: the role of molecular vibration.

We investigate free-space lasing actions from molecular nitrogen ions (N2+) at the wavelengths of ~391 nm and ~428 nm. Our results show that pronounced gain can be measured at either 391 nm or 428 nm laser wavelength with a pump laser centered at 800 nm wavelength, whereas the gain at 391 nm laser wavelength completely disappears when the wavelength of the pump laser is tuned to 1500 nm. Our theoretical analysis reveals that the different gain behaviors can be attributed to the vibrational distribution of populations in X2Σg+(v=0) and X2Σg+(v=1) states as the N2+ ions are generated by photoionization in the laser fields, giving rise to more robust (i.e., less sensitive to the pump laser wavelength) population inversion for generating the 428 nm laser.

A Semi-supervised Four-Chamber Echocardiographic Video Segmentation Algorithm Based on Multilevel Edge Perception and Calibration Fusion.

OBJECTIVE: Echocardiographic videos are commonly used for automatic semantic segmentation of endocardium, which is crucial in evaluating cardiac function and assisting doctors to make accurate diagnoses of heart disease. However, this task faces two distinct challenges: one is the edge blurring, which is caused by the presence of speckle noise or excessive de-noising operation, and the other is the lack of an effective feature fusion approach for multilevel features for obtaining accurate endocardium. METHODS: In this study, a deep learning model, based on multilevel edge perception and calibration fusion is proposed to improve the segmentation performance. First, a multilevel edge perception module is proposed to comprehensively extract edge features through both a detail branch and a semantic branch to alleviate the adverse impact of noise. Second, a calibration fusion module is proposed that calibrates and integrates various features, including semantic and detailed information, to maximize segmentation performance. Furthermore, the features obtained from the calibration fusion module are stored by using a memory architecture to achieve semi-supervised segmentation through both labeled and unlabeled data. RESULTS: Our method is evaluated on two public echocardiography video data sets, achieving average Dice coefficients of 93.05% and 93.93%, respectively. Additionally, we validated our method on a local hospital clinical data set, achieving a Pearson correlation of 0.765 for predicting left ventricular ejection fraction. CONCLUSION: The proposed model effectively solves the challenges encountered in echocardiography by using semi-supervised networks, thereby improving the segmentation accuracy of the ventricles. This indicates that the proposed model can assist cardiologists in obtaining accurate and effective research and diagnostic results.

Photon retention in coherently excited nitrogen ions.

Quantum coherence in quantum optics is an essential part of optical information processing and light manipulation. Alkali metal vapors, despite the numerous shortcomings, are traditionally used in quantum optics as a working medium due to convenient near-infrared excitation, strong dipole transitions and long-lived coherence. Here, we proposed and experimentally demonstrated photon retention and subsequent re-emittance with the quantum coherence in a system of coherently excited molecular nitrogen ions (N2+) which are produced using a strong 800 nm femtosecond laser pulse. Such photon retention, facilitated by quantum coherence, keeps releasing directly-unmeasurable coherent photons for tens of picoseconds, but is able to be read out by a time-delayed femtosecond pulse centered at 1580 nm via two-photon resonant absorption, resulting in a strong radiation at 329.3 nm. We reveal a pivotal role of the excited-state population to transmit such extremely weak re-emitted photons in this system. This new finding unveils the nature of the coherent quantum control in N2+ for the potential platform for optical information storage in the remote atmosphere, and facilitates further exploration of fundamental interactions in the quantum optical platform with strong-field ionized molecules..