Applied Physics B最新文献

THz generation from chirped and delayed laser pulses using periodically poled lithium niobate (PPLN) traditionally requires high-energy sources and bulky PPLN crystals. However, for compact, integrated, and miniaturized THz-based applications, such systems must be downsized. In this work, we numerically and experimentally demonstrate THz generation in a PPLN waveguide with a cross-section of 500 × 500 µm², pumped with low pulse energies in the microjoule range at a wavelength of 1 µm. Simulations show that tightly focused optical pulses in this configuration achieve significantly higher THz generation efficiency compared to collimated pumping. For a propagation length of 1.6 cm, optimal parameters are identified as a pulse duration of 5 ps and a pump energy between 0.5 and 1 µJ. Experimental validation confirms these findings, yielding a narrowband THz spectrum centered around 488 GHz. This work opens new perspectives for the development of efficient, integrated THz sources for advanced applications in THz photonics, spectroscopy, and high-speed communications.

We present the temporally resolved measurements of electron number density and temperature over timescales on the order of 1 ns to 10 ns in a femtosecond (fs) laser-induced plasma (LIP). Additionally, we also report on the neutral gas number density and rotational temperature over timescales on the order of 1 ns to 1000 ns. The fs LIP was generated in ambient air and investigated using laser Thomson and rotational Raman scattering. Electron number densities were observed to decrease from (sim 4times 10^{21}) (hbox {m}^{-3}) at early times to (sim 7times 10^{20}) (hbox {m}^{-3}) at later times, while the electron temperature remained within (0.5-1) eV throughout the probed range. The neutral gas number density remained nearly constant at (sim 2times 10^{25}) (hbox {m}^{-3}) for the first 50 ns before declining to (sim 1.1times 10^{25}) (hbox {m}^{-3}) after 500 ns. The rotational temperature initially decreased from (sim 700) K within the first 30 ns to (sim 500) K at 100 ns, before rising back to (sim 700) K. Finally, the experimental results were compared with simulations, showing good agreement.

Short, highly doped Cr:LiCAF crystals can exhibit very low passive loss and high efficiency but are prone to thermally induced internal damage. Here we instead use a 15 mm, low-ion-doped (1.25%) Cr:LiCAF crystal, accepting somewhat higher passive loss in exchange for improved thermal robustness and longer expected lifetime, and demonstrate its continuous-wave (CW) intracavity second-harmonic generation (SHG) into the violet. The Cr:LiCAF oscillator delivers 1.85 W of CW output at 790 nm with 5.8 W of absorbed pump power, corresponding to a 38% slope efficiency, and is continuously tunable from 747 nm to 870 nm using a 2-mm off-axis birefringent filter (BRF). Efficient SHG to the blue–violet is achieved by placing Type-I BBO/BiBO crystals at a secondary intracavity waist, yielding up to 440 mW of CW power near 400 nm (7.6% optical-to-optical efficiency) with 1.1% RMS stability over 10 min. By combining crystals optimized for fundamentals near 775, 800, and 825 nm and adjusting phase-matching, we achieve continuous SH tuning from 377 nm to 424 nm. The 400 nm SH beam exhibits walk-off-induced ellipticity with beam-quality factors of M² ≈ 12.2 (sagittal) and M² ≈ 1.9 (tangential), while the 800 nm fundamental shows M² ≈ 5.5 and 1.2, respectively. Beyond performance metrics, we provide a systematic analysis linking crystal length and focusing to conversion efficiency, offering practical guidelines for compact, broadly tunable violet sources based on intracavity-doubled Cr:LiCAF lasers.

An innovative interference-based method for detecting degenerate index states of polarization singularity is presented here. Index degeneracy occurs when different combinations of spin and orbital angular momentum states are superposed to form the polarization singularity. The technique presented here utilizes the interference of a polarization singular beam with a spherical reference beam to create distinct interference patterns. The interference fringes generated by the setup consist of the fork as well as spiral fringes. Two observations, one on the interference pattern and the other on the projection of the interference pattern on one of the linear basis states pertaining to (S_{2}) Stokes polarization parameter. This allows straightforward identification and quantification of degenerate index states of polarization singular beam. Beyond resolving index degeneracies, the method can also be extended to determine the coordinates on the Hybrid-order Poincaré Sphere as well as the relative phase shift between orthogonal components in the input polarization beam. Both simulation and experimental results validate the proposed method.

We derive an explicit analytical expression for the distribution of longitudinal component of the canonical energy flux vector near the initial plane (in the near field). We consider a nonparaxial light beam with linear polarization and with an amplitude of the plane waves spectrum in the form of a circle. This distribution has the shape of an ellipse elongated along the direction of the linear polarization vector. If the plane waves spectrum includes evanescent waves, then in the near field, a reverse canonical energy flux arises. This reverse flux almost coincides with the first side lobe of the intensity distribution in the near field. The reverse flux arises near intensity zeros. Computer simulations is performed using two independent methods: plane-wave spectral decomposition and Rayleigh-Sommerfeld integral. The simulation results agree with theoretical predictions.

Laser phase retrieval plays a crucial role in the evaluation of laser beam quality, yet model-based approaches are often limited by their sensitivity to initial conditions and susceptibility to local minima. To address these challenges, we propose AttentionPD-ResUNet, a phase retrieval framework that integrates the Phase Diversity (PD) method with attention mechanisms. Specifically, focused and defocused intensity images acquired via PD are employed as inputs to the network, which incorporates SE channel recalibration, ASPP-based multi-scale sampling, and spatial attention modules. This design enables the establishment of an end-to-end nonlinear mapping from the measured intensity distributions to the underlying wavefront phase. In comparative experiments, the proposed method achieves an RMSE of 0.068(lambda ) and an MAE of 0.041(lambda ) relative to the ground truth, with an average inference time of 0.41 s, thereby presenting a promising approach for reliable laser beam quality assessment.

Understanding preferential evaporation in multi-component fuel sprays is critical for optimizing combustion efficiency and reducing emissions in internal combustion engines. This study focuses on the development of a novel approach that allows for the simultanous detailed characterization of the Sauter Mean Diameter (SMD) and relative ethanol/isooctane volume fraction to elucidate the mechanisms governing preferential evaporation in binary fuel mixtures. This is achieved by combining Planar Droplet Sizing (PDS), a technique based on the ratio between the laser-induced fluorescence (LIF) signal of Nile red-doped fuel and the elastic scattering signal, with a two-color-LIF approach. As Nile red is a solvatochromic dye, i.e., it exhibits a shift in its fluorescence signal with changes in solvent polarity and temperature, the mole fraction of ethanol and isooctane in the spray can be determined, if the spray temperature is known. We performed extensive calibration on various Nile red-doped fuel mixtures in a heated cuvette, as well as in a droplet generator. Further, we minimized morphology-dependent resonance (MDR) effects in the LIF signal of the spray by the selection of spectral filters designed to cover all measured temperatures and concentrations. We found that in the ethanol spray temperatures decrease toward the spray edge. For the fuel-mixed samples, this coincides with a smaller overall SMD and a shift in the ethanol volume fraction in this region.

A Ho:YAG amplifier in-band pumped by a home-built high power and narrow-linewidth thulium-doped fiber laser (TDFL) at 1907.5 nm is demonstrated. Using a single-end-pumped configuration with the TDFL, a maximum output power of 135 W is achieved at 2090.48 nm of the Ho:YAG amplifier. The amplifier delivers a maximum pulse energy of 67.5 mJ with a pulse width of 30 ns at a repetition rate of 2 kHz, corresponding to a peak power of 2.25 MW. The slope efficiency relative to the absorbed pump power reaches 61.9%.

Nitrogen dioxide (NO₂) is a priority air pollutant strongly associated with transport emissions, but its high spatial and temporal variability are challenging to measure. High quality instruments are costly and often unsuited to fast, portable measurements, while low-cost sensors are limited by measurement accuracy and are too slow for measurements on a second time scale. Here we present a strategy for a sensor based on cavity-enhanced absorption spectroscopy (CEAS) to achieve low ppb sensitivity to NO₂ in tens of seconds. The approach uses a single, broadband wavelength channel (420–460 nm), a spectral region in which NO₂ is the dominant absorber in urban settings. We describe two CEAS instruments that differ in cavity length and signal detection approach. One instrument had 43 cm cavity length, a photodiode detector and lock-in amplification, and achieved a precision of 3 ppb in 10 s. The second system had a 100 cm cavity and photomultiplier tube detector and achieved a precision of 1.1 ppb (in 10 s). Instrument accuracy was assessed against reference measurements in chamber experiments and one instrument was applied in an 11 week study of the effects of vehicle traffic on NO₂ levels around a primary school in Cork city, Ireland. Despite moderate reflectivity cavity mirrors, the performance of these systems demonstrates that broadband, non-spectrally resolved CEAS is a fruitful approach to fast, modest cost urban measurements.

While time-resolved laser-induced incandescence (TiRe-LII) has become a standard laser-based diagnostic for soot, there remain unexplained observations in some datasets. One such effect is the so-called “anomalous cooling”, in which the pyrometric temperature decays faster than can be explained by conventional heat transfer models immediately following the peak temperature. This work investigates this phenomenon through experiments on soot entrained in different bath gases and irradiated in the low-fluence regime, where particle sublimation is minimal. The anomalous cooling phenomenon is caused by the contribution of particles in the probe volume that have been heated beyond the sublimation threshold to the overall incandescence signal, due to nonuniform laser fluence. Particles in these “hot spot” regions feature a faster cooling rate due to sublimation, contributing to the effect of apparent anomalous cooling. Particle-size polydispersity also plays a notable but minor role. The effect depends on the bath-gas composition, which is attributed to differences in species-specific heat transfer.