Applied Physics B最新文献

A high-energy acousto-optic Q-switched dual-crystal Tm:YAP laser at 1 kHz repetition rate was investigated. Using two b-cut Tm:YAP crystals in an L-shaped resonator with laser diode dual-end pumped at 793 nm, a pulse energy of 22.31 mJ at 1938.86 nm with 0.19 nm linewidth was achieved, corresponding to a 65 ns pulse width and 343.2 kW peak power, with beam quality factor M² ~ 3.4. In addition, when the pump power was 137.4 W, an average power of 29.96 W was obtained in continuous wave mode. To the best of our knowledge, this work represents the highest pulse energy reported of acousto-optic Q-switched Tm:YAP oscillator lasers at 1.94 μm.

This paper proposes and designs a dynamic radiative cooling smart window based on a metal-dielectric multilayer structure. An improved genetic algorithm is employed to optimize the number of layers, their thickness, and material type to achieve different target visible transmittance levels (from 30 to 60%). The optimal structure identified is a 10-layer stack of VO₂ and BaF₂ on a 1-μm-thick BaF₂ substrate, achieving an average visible transmittance of 50%. This design provides significant spectral control capabilities. Below its phase-transition temperature, the window is in an insulating state with a high near-infrared (NIR, 0.78–2.5 μm) transmittance of 80.8% and a low mid-infrared (MIR, 8–13 μm) emissivity of 6.7%, facilitating passive solar heating. Above the phase-transition temperature, it switches to a metallic state with a low NIR transmittance of 10.6% and a high MIR emissivity of 88.3%, enabling effective radiative cooling. The window exhibits substantial modulation abilities, with |ΔT_NIR| and Δε values of 0.70 and 0.82, respectively. This energy-free, self-adaptive technology, which dynamically regulates thermal radiation in response to ambient temperature, is ideally suited for enhancing energy efficiency in green buildings and smart windows under varying weather conditions.

We propose a technique for wavefront (W) sensing of optical systems using both Bi-Ronchi and Hartmann tests, modified by incorporating the classical tests by including a Spatial Light Modulator (SLM). To this aim, we replace the conventional Hartmann screens with structured apertures with a transmission Liquid Crystal Spatial Light Modulator (LC-SLM) using binary patterns of either circular or square apertures. We build overlapping horizontal and vertical fringe patterns to obtain equidistant square apertures (Bi-Ronchi). In the case of the Hartmann test, we substitute the square apertures with circles serving as binary masks, which we refer to as BRM (Bi-Ronchi Mask) and HM (Hartmann mask). We design an experimental setup in which a collimated laser illuminates the SLM, considering the diffractive effects of the SLM to optimize the identification of centroids, which yields transversal aberrations ((bold{TA}({rho},{theta}))). We assume a vector formulation suitable to interpret TA and thus directly solve (bold{W}({rho }, {theta})) by two-dimensional numerical integration applying directional derivative and Gaussian quadrature. We perform this process for both the Hartmann and Bi-Ronchi tests simultaneously, leveraging the spatial resolution and flexibility of the SLM. We present and discuss our results and compare them with those obtained using dedicated wavefront sensing software using the Ronchi test (under the conditions of our proposal) with the SLM.

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.