Applied Physics B最新文献

We investigate the differential-reflectance signature of a Fano-type plasmonic response in a reflective plasmonic grating structure. A double-layer Au diffraction grating—designed to operate in contact with ethanol under 780-nm illumination from the analyte side—is fabricated on a sapphire substrate using electron-beam lithography and vacuum deposition. Rigorous coupled-wave analysis reveals an asymmetric reflectance peak with steep angular variation, while numerical differentiation of the reflectance spectrum produces a sharp differential reflectance dip. Reflectance mapping combined with Rayleigh anomaly and propagating surface plasmon dispersion relations, along with quantitative near-field analysis, indicates that the observed response is consistent with a Fano-type plasmonic response involving localized and propagating plasmonic components. Angular reflectance measurements reproduce both the asymmetric reflectance peak and differential reflectance dip, in good agreement with the simulations. The potential sensing performance is evaluated from the simulated refractive-index sensitivity and experimentally obtained linewidth. These results demonstrate that differential reflectance provides a useful readout of a Fano-type plasmonic response in reflective plasmonic grating sensors, particularly for low-refractive-index liquids.

We have developed a broadband light source based on an extended-cavity tapered amplifier, whose frequency is stabilized using a volume Bragg grating (VBG). The setup delivers up to 200 (text {mW}) of optical power over the 14640–14670 (text {cm}^{-1}) range, with a spectral bandwidth of about 2 (text {cm}^{-1}). We have characterized the source as a function of both the VBG angle and the tapered amplifier drive current, using a home-built virtually imaged phased array (VIPA) spectrometer. To demonstrate its capabilities, we carried out iodine absorption spectroscopy in the 14664.5–14666.5 (text {cm}^{-1}) interval. This broadband source is promising for a variety of applications, in particular for cooling the translational, vibrational, and rotational degrees of freedom of diatomic molecules.

The structural parameters and spectral performance of the pulse compression gratings are directly related to the design and output quality of high-power laser systems. In this paper, for the gold-coated gratings applicable to the 850–970 nm wavelength range, the effects of variations in groove density, incident angle, and out-of-plane angle on their diffraction efficiency and bandwidth were calculated and analyzed. Subsequently, the variation laws of diffraction efficiency and bandwidth of the 1200 lines/mm gold-coated grating with structural parameters were simulated and analyzed, with a focus on the diffraction efficiency variation at the 850 nm short wavelength, which is more sensitive to structural parameters. For the 1200 lines/mm grating, the simulated average diffraction efficiency over the 850–970 nm wavelength range under the out-of-plane Littrow geometry (α = α_L, γ = 10°) is almost equal to that under the in-plane geometry (α = α_L + 12°, γ = 0°), with a difference of only 0.04%. Finally, by adopting holographic interference exposure technology and magnetron sputtering coating technology, a 1200 lines/mm gold-coated grating was successfully fabricated, which had an average diffraction efficiency of 91.4% in the 850–970 nm wavelength range. The aperture of this gold-coated grating is 320 mm × 230 mm, and the area-scanned average diffraction efficiencies corresponding to the single wavelengths of 850 nm, 910 nm, and 970 nm are 85.5%, 91.8%, and 91.8%, respectively.

Naturally occurring phyllosilicates such as talc and kaolinite-type minerals represent intrinsic layered two-dimensional insulators with highly anisotropic electronic and optical properties. In this work, we perform a comparative first-principles study of talc and kaolinite, along with its structural polymorphs dickite and nacrite, using density functional theory within the PBE-GGA framework. Structural relaxations, electronic band structures, and frequency-dependent dielectric functions were calculated to further analyze the effect of interlayer bonding on optical response. Talc, stabilized by van der Waals interlayer interactions, exhibits pronounced optical anisotropy with sharper ultraviolet absorption features, whereas kaolinite-type minerals, characterized by interlayer hydrogen bonding, display enhanced dielectric polarization along the direction normal to the layered planes and broader absorption spectra. The results demonstrate a direct link between interlayer coupling and optical anisotropy in layered phyllosilicates, highlighting their potential as natural wide-gap dielectric materials for photonic and optoelectronic applications.

The nonlinear refraction in two organic semiconducting polymers, namely MEH-PPV and PBDB-T, has been studied via spatial self-phase modulation (SSPM) using a 532 nm CW laser excitation. The results indicate that SSPM arises from both the optical Kerr and thermo-optical effects, which contribute significantly to the refractive index modulation of the polymer solution dispersions. The intensity-dependent evolution of the SSPM patterns, particularly the increase in the number of concentric diffraction rings, was analyzed to determine the nonlinear refraction (NLR) coefficients of MEH-PPV and PBDB-T, yielding values of ~ 9.707 × 10^− 11 m²/W and ~ 1.281 × 10^− 10 m²/W, respectively. This implies that the strong third-order nonlinear optical response of these polymers makes them suitable for all-optical switching, optical modulation, and photonic devices. The time evolution of SSPM patterns was investigated in both polymers, revealing modulation of NLR coefficients, primarily due to thermally induced convection and fluid motion. It offers novel platforms for thermal metrology based on optical nonlinearity and temperature-sensitive all-optical switching. Furthermore, this study provides valuable insight into the SSPM mechanism in organic semiconducting polymer systems, paving the way for future studies and establishing a foundation for continued exploration of nonlinear optical phenomena.

A dual-core D-shaped photonic crystal fiber sensor based on surface plasmon resonance is proposed for simultaneous detection of refractive index and temperature. The sensor features a fully symmetrical dual-core configuration with uniform air-hole diameters arranged in a hexagonal lattice, wherein two open-ring channels are incorporated on the D-shaped polished surface. One channel is coated with a gold (Au) film for RI sensing, while the other is filled with ‘polydimethylsiloxane (PDMS) + Au film’ mainly for temperature sensing, thereby effectively suppressing cross-sensitivity of the two sensing parameters. First, the finite element method (FEM) is employed to numerically optimize the structural parameters, within the optimized configuration, the proposed sensor can detect the RI and T simultaneously, with a range of 1.18–1.34 and − 40 °C to 100 °C, achieving a maximum RI sensitivity of 19,000 nm/RIU when RI is 1.34 and an average T sensitivity of − 0.5 nm/℃. Then, combining the sensitivity matrix decoupling method with the segmented fitting method, the detection accuracy of dual-parameter sensing has been effectively improved. Finally, a fabrication tolerance analysis confirms that a ± 10% variation in structural parameters exerts negligible influence on the resonance wavelength, demonstrating excellent robustness. Comparative analysis with previously reported dual-parameter sensors demonstrates that the proposed sensor provides competitive advantages in terms of wide low-RI detection range, broad temperature detecting range especially for low-temperature environment, and structural simplicity, making it highly promising for applications in gas sensing, chemical analysis, biomedicine, and low-temperature industrial monitoring.

The speckle based classification of Laguerre-Gauss (LG) beams using the convolution neural network (CNN) has gained a significant interest in view of their applications towards free space optical communication. For the LG beams with higher azimuthal index or topological charge (TC), the classification becomes extremely difficult due to the presence of similarities or structural correlations between the speckle patterns corresponding to the various orders. Here, we propose two deep learning techniques based on CNN and a Vision Transformer (ViT) for classifying the LG beams with higher TC up to 50 using the speckle patterns generated by scattering these modes through a rough surface. The obtained results show that the ViT based model achieves a better mean classification accuracy of 98.1%, outperforming the CNN based model by approximately 3.3%. Additionally, we validated the robustness of the proposed models by accurately detecting TC over three independent test runs and we are also able to determine the sign of TC as well. This classification was performed using a small patch (224 (times ) 224) of speckle patterns, approximately 1% of the recorded speckle pattern. The robustness of the proposed method for high topological charges after perturbing using the scattering environments enables more reliable, high capacity optical systems for real world use.

We present a comprehensive study of the Doppler broadened (4S rightarrow 5P) transitions in natural abundance potassium vapor, focusing on both the (hbox {D}_1) and (hbox {D}_2) lines. Using a home-built 405 nm extended-cavity diode laser and a 10 cm heated vapor cell, we measure the transmission spectra across a wide range of probe intensities and vapor temperatures. Weak-probe measurements confirm the applicability of the ElecSus model, which accurately reproduces the observed absorption profiles and allows the quantitative extraction of atomic temperatures with high precision. The (hbox {D}_2) transition exhibits stronger absorption and greater sensitivity to temperature changes, while the (hbox {D}_1) line provides a robust test of the weak-probe regime. By fitting the Doppler broadened spectra with ElecSus, we demonstrate a spectroscopic thermometry technique capable of resolving small temperature variations, even in the presence of thermal gradients and electronic noise. Our results establish ElecSus as a reliable tool for the precise characterization of hot atomic vapors, demonstrating excellent agreement with models using experimentally determined 4S (rightarrow ) 5P isotope-shift values.

High-precision detection of methane is highly valuable in many fields, such as natural gas pipeline leakage detection, greenhouse gas monitoring, and noninvasive disease diagnosis. However, significant technical challenges remain in further improving the sensitivity and portability of methane sensors. Multipass cells (MPCs) adopted in existing sensors based on tunable diode laser absorption spectroscopy (TDLAS) generally suffer from limited optical path length and poor beam quality under the constraint of a small volume, which restricts the overall detection performance of the sensors. To address the above technical difficulties, in this study, a high-precision methane sensor based on a novel multipass cell named the triple-switching matrix multipass cell (TSMC) was designed and developed. The TSMC can realize triple switching of the reflection mode of the Pickett Bradley White Cell (PBWC) in three directions and has the remarkable advantages of excellent beam quality, a high ratio of optical path length to volume (RLV), and separation of incident and exit positions. The experimental results reveal that the TSMC can achieve an effective optical path length of 53.2 m with an effective optical volume of only 268.27 mL, and the RLV reaches 19.83 cm^− 2, demonstrating excellent miniaturization potential. Allan deviation analysis indicates that the sensor has a minimum detection limit of 12.6 ppb at an average time of 9.4 s, indicating high detection sensitivity. The sensor design proposed in this study provides a new approach for improving the high-precision detection of trace gases.

A computer-generated hologram (CGH) is optimized in an optical system. The in-system CGH optimization method automatically generates a shaped beam that compensates for the imperfections in the optical system. A new in-system CGH optimization method using luminescence induced by holographically generated parallel beams was developed. In contrast to previously demonstrated in-system CGH optimization methods that directly measure the laser beam with an imager in a different space from the sample, the developed method measures the beam state passing through the objective lens on a sample under a laser fluence at the same level as that used in laser processing. Therefore, the developed method enables CGH design that reflects the state of the laser beam on the sample and is effective for femtosecond laser processing at high laser fluences.