Applied Physics B最新文献

Interferenceless Coded Aperture Correlation Holography (I-COACH) has emerged as a powerful computational imaging technique for retrieving three-dimensional information from an object without requiring two-beam interference. In this study, we propose and experimentally demonstrate an I-COACH system employing a Helico-Conical Vortex (HCV) mask. The HCV mask carries orbital angular momentum and features a phase profile with non-separable dependence on both azimuthal and radial coordinates. It is generated by combining helical and conical phase functions, resulting in a spiral-shaped intensity distribution at the focal plane. We compare the performance of I-COACH with the HCV mask against other coded masks (CMs), including random lens, ring lens, spiral axicon, axicon, and spiral lens. Additionally, we evaluate image reconstruction using four widely adopted algorithms: non-linear reconstruction (NLR), Lucy-Richardson algorithm (LRA), Lucy-Richardson-Rosen algorithm (LRRA), and non-linear LRA (NL-LRA). Quantitative analysis is conducted using figures of merit such as entropy, root mean squared error (RMSE), structural similarity index measure (SSIM), and peak signal-to-noise ratio (PSNR). The proposed approach holds promise for advancing incoherent holography and computational imaging applications.

In this paper, a model of a Cr:ZnSe laser amplifier at a wavelength of 2.4 μm is described. The effects of gain saturation and pump depletion in the gain medium are considered at different concentrations of active centers and the amplifier lengths. The effect of non-uniform distribution of Cr²⁺ ions on the overall gain and the cross-section intensity distribution of the signal beam is estimated.

While conventional Z-scan techniques using Gaussian beams have been extensively studied for nonlinear absorption measurements, structured beams like Mathieu beams offer unique advantages through their tunable intensity distributions and focusing characteristics, yet their analytical modeling for nonlinear absorption remains largely unexplored. Understanding how different beam parameters influence nonlinear optical interactions is crucial for optimizing experimental configurations and developing advanced nonlinear optical devices. This paper presents a comprehensive analytical model for nonlinear absorption in Z-scan measurements using Mathieu beams. Under the weak nonlinearity approximation, we derive closed-form expressions for transmitted optical intensity, power, and normalized transmittance, incorporating both linear and nonlinear absorption effects. Through extensive numerical simulations, we systematically investigate the influence of four key beam parameters: beam order, ellipticity parameter, Rayleigh range, and radial cutoff parameter on nonlinear absorption characteristics. The results demonstrate that higher-order Mathieu beams produce progressively stronger nonlinear absorption effects while simultaneously narrowing the Full Width at Half Maximum of Z-scan curves, indicating enhanced focusing characteristics. Increasing beam ellipticity enhances absorption strength while narrowing the interaction region, reflecting the direct relationship between ellipticity and intensity concentration. The Rayleigh range analysis confirms the fundamental trade-off between beam confinement and interaction length, where tighter focusing enhances peak intensities at the expense of interaction volume. The radial cutoff parameter investigation emphasizes the importance of spatial sampling in Z-scan measurements. These findings establish a comprehensive framework for understanding and optimizing nonlinear optical interactions with Mathieu beams, providing valuable insights for designing nonlinear optical devices and optimizing Z-scan experimental configurations, with unprecedented flexibility for applications in optical limiting, beam shaping, and nonlinear optical switching systems.

This paper compares the performance of axicon phase modulation and Bessel phase modulation in generating perfect optical vortices (POVs). Theoretical analysis and experimental verification show that the POVs generated by axicon phase modulation have energy concentrated on the ring, but the ring radius varies with changes in topological charge. The POVs generated by Bessel phase modulation have side lobes in addition to the main ring, but most of the energy is still focused on the main ring, and the ring radius is relatively stable. Suggestions for generating POVs in different scenarios are provided in the paper.

We present an experimental and theoretical study of anisotropy-induced changes in probe transmission at two-photon resonance for the D₂ line of ⁸⁷Rb in presence of a longitudinal magnetic field, exploring the effects of varying ellipticity of pump and probe fields. We theoretically investigate how a change in polarization leads to a population redistribution, thereby creating anisotropy that results in a conversion between transmission and absorption. Our theoretical investigation involves solving the density matrix equations comprising of 13 and 16 levels respectively. The 13-level model, which excludes the (F'=1) excited state, exhibits symmetric conversion between transmission and absorption for both positive and negative ellipticity. In contrast, the 16-level model, which includes the (F'=1) manifold, displays a loss of symmetry, with absorption observed only for positive ellipticity. These theoretical predictions are supported by experimental measurements. Our findings provide insight into how optical anisotropy at two-photon resonance can be engineered through atomic structure and light polarization.

Phase sensitive optical parametric amplification (PS-OPA) can realize noiseless amplification in theory. However, Raman effect affects the four-wave mixing (FWM) process and induces excess noise in FWM-based PS-OPA systems. To achieve a more precise description of the gain evolution within the PS-OPA and its associated noise characteristics, we modified a more comprehensive theoretical model to analyze the joint impact of the Raman effect and the FWM effect. It is found that the Raman effect competes with FWM effect, which can change the evolution trend of power and relative phase in a FWM process and affect gain and noise figure (NF) features of the PS-OPA. With different frequency shift, obvious asymmetry for both gain and NF spectra is induced by Raman scattering. This model can be applied to investigate single-pump frequency non-degenerate PS-OPA with third-order nonlinear media such as highly nonlinear optical fibers (HNLF) and silicon waveguides.

Volumetric multi-kHz laser irradiation can affect the measurements on soot particles. The severity of the influence causing local alteration of the particle and flame properties are investigated on a laminar diffusion flame. The aim is to identify a practical fluence threshold for which no or only minor influence on soot particles due to volumetric multiple laser irradiation occurs. To this end, the three quantities of interest (QoI) of particle temperature, soot volume fraction and primary particle size are measured and evaluated via 3D-two-color-pyrometry (3D-2CP) and 3D time-resolved laser-induced incandescence (3D-TiRe-LII). As laser source, a high repetition-rate burst-mode Nd:YAG pulse laser is employed. The progression of the three QoI is investigated for different fluences (up to ~ 160 mJ/cm²) and repetition rates (up to 5 kHz) for selected individual pulses within a pulse train. For the determination of practical fluence limits, a maximum change of 25% in soot volume fraction is applied as a threshold. The fluence limit is highly dependent on laser repetition rate and flow velocity of the object of interest. For irradiation with eight laser pulses with 5 kHz, a fluence limit of ~ 60 mJ/cm² is identified.

Einstein Telescope (ET) is expected to achieve sensitivity improvements exceeding an order of magnitude compared to current gravitational-wave detectors. The rigorous characterization in optical birefringence of materials and coatings has become a critical task for next-generation detectors, especially since this birefringence is generally spatially non-uniform. A highly sensitive optical polarimeter has been developed at the Department of Physics and Earth Sciences of the University of Ferrara and INFN - Ferrara Section, Italy, aimed at performing two-dimensional birefringence mapping of substrates. In this paper we describe the design and working principle of the system and present results for crystalline silicon, a candidate material for substrates in the low-frequency interferometers of ET. We find that the birefringence is (lesssim 10^{-7}) for commercially available samples and is position dependent in the silicon (100)-oriented samples, with variations in both magnitude and axis orientation. We also measure the intrinsic birefringence of the (110) surface: (Delta n^{(110)}=-(1.50pm 0.15)times 10^{-6}) @ (lambda =1550) nm. Implications for the performance of gravitational-wave interferometers are discussed.

This study employs Calibration-Free Laser Ablation Molecular Isotopic Spectrometry (CF-LAMIS), enhanced by the Molecular spectrA simulator under Born–OppenHeimer Approximation anD Boltzmann Equilibrium enVironment (MAHADEV) algorithm and its AWGN (Additive White Gaussian Noise) incorporated variant to directly analyse boron isotopes in natural boric acid samples and nuclear-grade boron carbide (B₄C) samples. The boric acid samples yielded accuracy and precision consistent with previous studies. Initial analyses of B₄C samples were challenged by the atomic and ionic interference lines. However, the BO:B-X (0–1) ro-vibrational band (RVB) was found to have fewer interferences, providing a clearer analytical window at longer acquisition delay times (t_d = 22 μs). This approach achieved reasonable accuracy and precision levels comparable to those obtained from pure boric acid samples, demonstrating the effectiveness of the CF-LAMIS method in handling complex matrices. The application of the AWGN–MAHADEV algorithm provided isotopic composition with significant (~ 50%) improvement in precision while maintaining comparable accuracy. Boric acid samples achieved an accuracy and precision of 0.3% and 0.8% respectively, while natural and ¹⁰B-enriched B₄C samples achieved an accuracy of 0.7% and 0.2% and a precision of 0.9% and 0.6% respectively. This study highlights the potential of noise integration in calibration-free methods to achieve more precise and accurate results.

A deep understanding of soot production in flames is mandatory to control this pollutant emission and design zero emission combustors. In this context, we propose to combine Auto-Compensated Laser-Induced Incandescence (AC-LII) and Separated-Pulse LII (SP-LII) approaches to measure three relevant soot quantities using a single experimental setup: soot volume fraction (f_v), absorption function (E(m_lambda )), and gas temperature (T_0). In theory, this combination is quite simple as it requires measuring the LII signal at two wavelengths for a reduced number of laser fluences. However, in practice, this combination is not trivial since each LII approach has its own limitations. Specifically, the SP-LII approach requires low laser fluences to guarantee a linear relation between the peak laser-induced soot temperature (T_M) and laser fluence F, namely the linear regime. At the same time, (f_v) values estimated using the AC-LII technique strongly depend on the laser fluence, a well-know behaviour referred to as the ‘(f_v) anomaly’, especially at low fluences. Thus, in this work, we establish a procedure to reduce the ‘(f_v) anomaly’ when performing in-flame measurements at low laser fluences so to allow a combination of AC-LII and SP-LII methods. For this, this work mainly focuses on the contribution of the background subtraction on the ‘(f_v) anomaly’ when considering measurements in the linear regime observed at low fluences. First, we theoretically quantify the error on the estimation of (T_M) and (f_v) due to background subtraction. Then, a two-loop iterative procedure is proposed to correctly estimate (T_M), (T_0) and (E(m_lambda )). This is necessary to reduce the ‘(f_v) anomaly’ and to correctly predict (f_v) in the linear LII regime. Finally, the accuracy of the combined AC-LII/SP-LII approach is evaluated by comparing the obtained results with reference state-of-the-art data for (f_v), (T_0), and (E(m_lambda )). This comparison demonstrates, for the first time, the feasibility of a combined AC-LII/SP-LII strategy to obtain (f_v), (T_0), and (E(m_lambda )) fields from a single LII setup, once the proposed procedure for background subtraction is implemented to partially correct the ‘(f_v) anomaly’.