# Applied Physics B

Online ISSN: 1432-0649
Print ISSN: 0946-2171
Recent publications
Fourier domain mode-locked (FDML) lasers are frequency-swept lasers that operate in the near-infrared region and allow for the attainment of a large sweep-bandwidth, high sweep-rate, and a narrow instantaneous linewidth, all of which are usually quite desirable characteristics for a frequency-swept laser. They are used in various sensing and imaging applications but are most commonly noted for their practical use in optical coherence tomography (OCT). An FDML laser consists of three fundamental components, which are the semiconductor optical amplifier (SOA), optical fiber, and the wavelength-swept optical bandpass filter. Due to the complicated nonlinear dynamics of FDML lasers that stems from the coaction of these three components, often the output signal of an FDML laser is corrupted by frequent power-dips of varying depth and duration. The frequent recurrence of these dips in the FDML laser signal pattern lowers the quality of imaging and detection. This study examines the role of the linewidth enhancement factor (LWEF) of an SOA in reducing both the strength and the number of power-dips throughout the FDML laser operation. The results are obtained using numerical computations that are in agreement with experimental data. The study aims to show that using SOAs with low LWEFs, the number of power-dips can be reduced for a better detection and imaging quality.

We demonstrate the method of laser-induced plasma formation and characterization for achieving efficient high-order harmonics generation during the ablation of chromium and propagation of femtosecond pulses through the optimally adjusted plasma. The dynamics of chromium plasma spreading out from the target surface is correlated with the maximal harmonic yield for the non-resonant harmonics and resonance-enhanced harmonic. Plasma velocities at different regimes of ablation were correlated with the determination of the optimal delay between the heating pulses used for target ablation and femtosecond driving pulses propagating through this plasma. Laser-induced breakdown spectroscopy was used for the optimization of the fluency of heating pulses and determination of the parameters of plasma. The harmonics up to the forty order (in the case of 1300 nm driving pulses) and thirty order (in the case of 800 nm driving pulses) were obtained. The enhancement of the resonance-induced 29th harmonic of 800 nm radiation and 47th harmonic of 1300 nm radiation showed a direct relationship with the optimization of plasma characteristics.

A high-speed laser absorption technique is employed to resolve spectral transitions of CO $$_2$$ 2 in the mid-infrared at MHz rates to infer non-equilibrium populations/temperatures of translation, rotation and vibration in shock-heated CO $$_2$$ 2 - Ar mixtures. An interband cascade laser (DFB-ICL) resolves 4 transitions within the CO $$_2$$ 2 asymmetric stretch fundamental bands ( $$\Delta$$ Δ v $$_3$$ 3 = 1) near 4.19 $$\upmu \hbox {m}$$ μ m . The sensor probes a wide range of rotational energies as well as two vibrational states (00 $$^0$$ 0 0 and 01 $$^1$$ 1 0). The sensor is demonstrated on the UCLA high enthalpy shock tube, targeting temperatures between 1250 and 3100 K and sub-atmospheric pressures (up to 0.2 atm). The sensor is sensitive to multiple temperatures over a wide range of conditions relevant to Mars entry radiation. Vibrational relaxation times are resolved and compared to existing models of thermal non-equilibrium. Select conditions highlight the shortcomings of modeling CO $$_2$$ 2 non-equilibrium with a single vibrational temperature.

The present study discussed a theoretical model to simulate the interaction of an ultra-short femtosecond laser pulse with a slab of neutral Ar gas. The calculation was based on using the Particle in Cell-Monte Carlo Collision (PIC-MCC) method to investigate the possibility of infrared emission. It was found that besides several emissions that are inevitably obtained from a high-density Ar-based plasma medium, resonance radiation can be achieved at about 165 THz whenever the stimulated Raman scattering (SRS) based half-harmonic and rippled density wavenumber are involved in phase match condition with the fundamental laser frequency. Among several parameters examined to amplify the resonance emission, the symmetry breaking of electric field (SBEF) and turning up the input intensity to about 10¹⁸ W/cm² proved more efficient infrared emission. It turned out that compared to the SBEF effect, an almost three times stronger infrared intensity can be obtained using more intensive laser pulses. The transverse profile for half-harmonic emission and resonant radiation is presented. It is observed that, the infrared radiation has an off-axis profile. At an intensity of 10¹⁸ W/cm², the effects of hot electrons and the nonlinear Kerr refractive index have been presented. It was found that these two parameters limit the generation mechanism at the beginning of the plasma and at the end of the interaction medium, respectively.

A mid-infrared laser absorption technique is developed for sensing of temperature and carbon monoxide (CO) number density from 2000 K to above 9000 K. To resolve multiple rovibrational lines, a distributed-feedback quantum cascade laser (DFB-QCL) is modulated across 80% of its current range using a trapezoidal waveform via a bias-tee circuit. The laser attains a spectral scan depth of 1 cm$$^{-1}$$, at a scan frequency of 1 MHz, which allows for simultaneous measurements of four isolated CO transitions near 2011 cm$$^{-1}$$ (4.97 $$\mu$$m) with lower-state energies spanning 3,000 to 42,000 cm$$^{-1}$$. The number density and temperature are calculated using a Boltzmann regression of the four population fractions. This method leverages the information contained in each transition and yields a lower uncertainty than using a single line pair. The sensor is validated in shock tube experiments over a wide range of temperatures and pressures (2300–8100 K, 0.3–3 atm). Measurements behind reflected shock waves are compared to a kinetic model of CO dissociation up to 9310 K and are shown to recover equilibrium conditions. The high temperature range of the sensor is able to resolve rapid species and temperature evolution at near electronvolt conditions making it suitable for investigations of high-speed flows, plasma applications, and high-pressure detonations.

In this study, we present the dissociative single ionization of CO molecules in strong laser field by measuring the time-of-flight mass spectra of the dissociative fragment C+ with different laser intensities and polarization directions, respectively. The results indicate that the molecular ion CO+ produced by over-the-barrier ionization can be populated in not only the ground X2Σ+ state, but also the next two lower-lying electronic A2Π and B2Σ+ states. By evaluation of the potential energy curves, we assign the multi-peak structure of the time-of-flight spectra for ion fragment C+ to the dissociation of the vibrational states of two excited A2Π and B2Σ+ states via the dissociation pathways |A2Π-0ω〉→|D2Π-4ω〉, |B2Σ+-0ω〉→|32Π-3ω〉, and |B2Σ+-0ω〉→|32Σ+-3ω〉. The results imply that the dissociation pathway of molecular cations can be selected using pump-probe technology via accurately tuned laser parameters.

Based on cesium 6S1/2—6P3/2—6D5/2 (852 nm + 917 nm) ladder-type system, we simultaneously obtain the Doppler-free saturated absorption spectroscopy (SAS) between the 6S1/2 → 6P3/2 transition and optical–optical double resonance (OODR) spectroscopy between the 6P3/2 → 6D5/2 transition, when the frequency of 852 nm laser is scanned over the lower transition while keeping the 917 nm laser resonant on the upper transition. The SAS as a frequency scale is used to measure the frequency intervals of hyperfine splitting of the 6D5/2 excited state via the OODR spectra. With the measured values of hyperfine splitting, the hyperfine coupling constants of the 6D5/2 state are determined as magnetic dipole Ahfs = − 4.60 (5) MHz and electric quadrupole Bhfs = 0.23 (47) MHz, which are consistent with previous results. A simple and self-calibration method of measuring the unknown hyperfine structure of an excited state using its own known hyperfine splitting of another state is demonstrated without the aid of any extra frequency calibration tools.

We have proposed and designed a high-speed and high-performance optically controlled terahertz (THz) intensity modulator based on the free carrier modulation of a GaAs semiconductor. The device comprises a photonic crystal cavity–waveguide coupling structure for operation in the THz region. This modulator benefits from the strong interaction between the THz wave and the photoconductive substance to obtain a deep modulation with GHz speed, even with a low external optical power. The finite element method was used to calculate the most important properties of the modulator, such as the modulation depth, insertion loss, and modulation rate. The proposed modulator also demonstrates external optical power-dependent characteristics. The results indicate that the THz intensity can be modulated at a switching frequency of 1 GHz with high modulation depths of 83 and 90.3% under the continuous wave laser pumping of 50 W/cm² and 80 W/cm², respectively. In addition, this modulator exhibits efficient performance under the same pumping power with a switching frequency of up to 3 GHz. The device exhibits higher modulation depths with higher laser power intensities. The outstanding properties of the proposed structure are promising for the development of modulators and switches in THz communication systems.

Ultra-compact and -broadband integrated gold plasmonic half-wave and quarter-wave plates (HWP and QWP) based on the mode interference mechanism are proposed with applications in magneto-photonics, mode- and polarization-division multiplexing. The numerical simulations based on a finite element method reveals that the proposed HWP offers a maximum polarization conversion efficiency of 99.6% at a wavelength of λ0 = 1.55 μm with a 1 dB bandwidth of 140 nm (λ0 = 1.47–1.61 μm), covering 82% of the S to L telecommunication bands. In this wavelength range, HWP has an insertion loss below 2.3 dB and an extinction ratio between 9 and 24 dB, respectively. The presented QWP functions over the C telecommunication band (λ0 = 1.53–1.565 μm), where the transmission contrast between its transverse magnetic and electric modes is 0±0.04. Moreover, QWP offers a polarization rotation angle in the range of 45±5∘ over the same wavelength range. Finally, this paper illustrates that the proposed waveplates have a robust tolerance to fabrication errors.

We calculate the energy states and the optical absorption coefficient for electrons in a nanowire in the presence of the Rashba and the longitudinal-optical phonon interactions. The interplay of those interactions results in a splitting in the electron dispersion relation at zero wavevector that grows linearly for the ground state as the strength of the lateral quantum confinement increases. For higher states, the energy splitting increases more rapidly till the state of resonant polaron is reached, then it increases slowly due to the pinning effect. The frequency separation between the well-resolved absorption peaks and their number are greatly influenced by the state of the polarons.

This paper proposes a three-dimensional measurement method for small parts with large height and complex reflection characteristics based on microscopic fringe projection measurement technology. A composite microscopic system characterization model based on the traditional camera imaging model and the polynomial fitting model is proposed for the calibration of the measurement system, which is used to solve the complex problem of the microscopic imaging optical path and improve the measurement accuracy of the system. An adaptive multiple exposure measurement method is proposed. And the method solves the problems of overexposure and darkness caused by the optical properties of the micro-structure and the environment. The small depth of field limitation problem of traditional micro measurement technology is solved by the micro-imaging technology of super depth of field. Experiments with a prototype demonstrate the validation and accuracy of the proposed algorithm and system configuration. The root mean square of the reconstruction errors is less than 0.2μm\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$0.2\,\mu \hbox {m}$$\end{document} with a field of view of 2.4 mm×\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\times$$\end{document}3.2 mm and a measurement height of 600 μ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mu$$\end{document}m.

We study the possibility to fabricate an arbitrary phase mask in a one-step laser-writing process inside the volume of an optical glass substrate. We derive the phase mask from a Gerchberg–Saxton-type algorithm as an array and create each individual phase shift using a refractive index modification of variable axial length. We realize the variable axial length by superimposing refractive index modifications induced by an ultra-short pulsed laser at different focusing depth. Each single modification is created by applying 1000 pulses with 15 $$\upmu$$ μ J pulse energy at 100 kHz to a fixed spot of 25 $$\upmu$$ μ m diameter and the focus is then shifted axially in steps of 10 $$\upmu$$ μ m. With several proof-of-principle examples, we show the feasibility of our method. In particular, we identify the induced refractive index change to about a value of $$\varDelta n = 1.5 \cdot 10^{-3}$$ Δ n = 1.5 · 10 - 3 . We also determine our current limitations by calculating the overlap in the form of a scalar product and we discuss possible future improvements.

The influence of semi-spherical cavities of different sizes was investigated for the enhancement of propulsion parameters in conjunction with pulsed Nd:YAG laser operating at 1064 nm. The 10 mm, 11.5 mm, 15 mm, 18.5 mm and 20 mm cavities of iron were used to confine the laser-induced plasma. Aluminum (Al), Silver (Ag), and Gold (Au) samples in form of foils were used as propellants. A significant enhancement in momentum coupling coefficient (Cm) and specific impulse (Isp) was observed when cavities were used to confine the laser-induced plasma. From the measurements obtained for various sizes of cavities, it was observed that Cm and Isp recorded the highest values for the 10 mm cavity. Therefore, the confinement effect was most significant for the smallest cavity. In confinement geometry the value of Cm increased from 16.9 to 39.6 (dyne/W) for Al, from 13.1 to 30.6 (dyne/W) for Ag and from 12.9 to 34.5 (dyne/W) for Au target respectively, while the value of Isp increased to nearly 40,000 s with the confined geometry, as compared with 3000 s for direct propulsion. Results demonstrated that due to uniform and balanced compression of shockwaves in all directions, semi-spherical cavities played a significant role in the enhancement of laser ablation propulsion parameters. This may introduce new avenues to tune the specific impulse and thrust in micropropulsion applications.

We report on the dependence of the frequency-to-intensity noise conversion in the locking of an ultrafast laser against a high-finesse optical enhancement resonator from the carrier envelope offset frequency. By a proper combination of the cavity finesse and laser carrier envelope offset frequency, it is possible to optimize the signal-to-noise ratio of the laser intensity trapped into the optical resonator. In this paper, we describe the effect of the laser-enhancement cavity coupling on the intracavity power relative noise, and we demonstrate both theoretically and experimentally its reduction.

This paper presents a method for the fabrication of cross-scale structures using the controllable Moiré effect to break through the scale barriers caused by the spatial distribution in a laser interference lithography (LIL) system. The formation principle of macroscopic Moiré gratings in the LIL system was analyzed as partial wavefront interference introduced by optical components. In this work, an additional lens was used in the improved two-beam LIL system to precisely control the size of Moiré gratings, combined with the intrinsic period in the LIL system to form a cross-scale distribution of light intensity. This method provides a way for the fabrication of cross-scale surface structures by single exposure. Through the double-exposure technology, the fabrication of isotropic and anisotropic structures can be achieved flexibly for different applications, such as photonic crystals, self-cleaning surfaces, structural color elements and anti-counterfeiting labels.

Monolayer MoS2 has excellent optical properties, but its low optical absorption hinders its application in optical absorbers. To improve its absorption performance, we design an aperiodic multilayer structure with monolayer MoS2, the dielectric materials Si and SiO2 arranged by Kolakoski sequence, and achieve the perfect absorption in the visible wavelength range because of its omnidirectional reflectivity. By optimizing the thicknesses of SiO2 (db), Si (dc) and the repetition number (N) of the unit cell, the absorption of the aperiodic multilayer structure can be greatly improved at normal incidence. When N is 3, this aperiodic multilayer structure achieves high absorption of up to 98% in the wavelength range from 350 to 450 nm. As N increases to 5 or 7, the average absorption of this aperiodic multilayer structure exceeds 90% in the 350–600 nm wavelength range. In addition, we verify that the broadband and high absorption obtained in this aperiodic multilayer structure is robust against the angle of incidence and polarization of the incident wave. Our results suggest an alternative configuration with monolayer MoS2 to realize the broadband and perfect absorption, which is useful for future applications in solar cell or photodetectors.

We demonstrate high conversion efficiency towards the third harmonic (TH) of the 900–1700 nm, 150 fs laser in the thin (40–120 nm) films containing quantum dots (HgTe, HgSe, and PbS) and exfoliated nanoparticles (Bi2Te3) deposited on the glass substrates. The ratio of TH conversion efficiencies in the films and glasses of the same thickness was estimated to be > 10⁴. The intensity, polarization, and spectral dependencies of this process in thin films are reported. We discuss the relation between the TH process and the low-order nonlinear optical properties of these quantum dots and nanoparticles.

With diamond crystals as Raman media, picosecond synchronously pumped solid-state Raman laser is theoretically studied in detail for the first time. High efficient working point and effective pulse compression working point are investigated. For both 532 nm and 1064 nm pumping, high Raman conversion efficiency can be achieved for negative cavity length detuning (∆x) and diamond crystal length of 5 mm. The higher efficiency can be obtained with longer Raman crystal, longer pumping pulse width and higher pumping power. For 532 nm pumping, effective pulse width compression can be realized for ∆x = 0 nearby and diamond crystal length of 10 mm. Shorter pulse width and higher peak power of 1st Stokes laser can be achieved with longer Raman crystal, shorter pumping pulse width and higher pumping power. The findings can contribute to the design and optimization of picosecond synchronously pumped diamond Raman lasers.

Due to the pixel limit of the polarization imaging detector and the object detection conditions, the spatial resolution of the object polarization image in actual imaging detection applications is generally low. Convolutional neural networks (CNNs) have been introduced to image super-resolution (SR). However, these methods rarely explore the internal connections between local spatial components and multi-scale feature maps in different receptive fields. To this end, we propose a multi-scale adaptive weighted network (MSAWN) for polarization computational imaging super-resolution to gain superior reconstruction performance. Computational imaging methods centered on information acquisition and interpretation can obtain high-resolution images that are superior to imaging systems. Specifically, we use a limited amount of memory and computational power even with multi-scale and multi-level polarization information. Second, a spatial pyramid structure based on the space-channel attention mechanism is designed to effectively adjust the feature weight of polarization information. Third, we adopt an adaptive weight unit to reduce redundant network branches and parameters. Particularly, we design an innovative reconstruction layer with inputs coming from multiple paths by means of sub-pixel convolution. The experimental results show that the proposed method achieves better reconstruction accuracy and visual effect, and the objective evaluation indexes such as PSNR and SSIM are significantly improved.

Edge detection has been widely applied and plays an important role in security and medical diagnosis. The existing edge detection based on ghost imaging mainly focuses on the computational ghost imaging using space light modulation equipment, but there is little research on the edge detection of pseudo-thermal ghost imaging, which also has great application prospect in remote sensing and three-dimensional imaging. Hence, in this paper, we have proposed and demonstrated a joint iteration edge detection for pseudo-thermal ghost imaging method. It makes up for the deficiency that the existing edge detection method based on ghost imaging can only be used to calculate ghost imaging of space light modulation equipment, but can not be applied to ghost imaging of pseudo-thermal light. Experiments verify that the proposed method can obtain higher imaging quality and edge detection quality for transmitted, reflected, and three-dimensional target objects in less measurement time. This method has great application prospect in feature extraction and target recognition.

Entanglement between a spin-wave qubit (memory qubit) and a photonic qubit is a basic building block for quantum repeaters. Duan-Lukin-Cirac-Zoller (DLCZ) scheme, which generates spin waves via spontaneous Raman scattering (SRS) of Stokes photons in atomic ensemble, provides a promising way to generate such entanglement. In a recent work [arXiv: 2006.05631, accepted by communications physics], DLCZ-like quantum memory that generates long-lived atom-photon entanglement has been experimentally demonstrated, where magnetic-field-insensitive (MFI) coherence is used to store spin waves. For realizing such MFI spin-wave storage, the atoms have to be initially prepared in a specific Zeeman sublevel, which is achieved by applying optical pumping lasers. Here, we demonstrate the memory lifetimes for the cases that the atoms are perfectly and imperfectly prepared in the specific Zeeman level, respectively. The experimental results show that the spin waves associated with magnetic-field-sensitive (MFS) and MFI coherences will be simultaneously created for the case that the atoms are imperfectly prepared in the Zeeman sublevel. Thus, the read outs will experience decay oscillations due to interferences between the two spin waves and the memory lifetime will be shorten due to dephasing of MFS coherence. A detailed theoretical analysis has been developed for explaining the experimental results. The present work will help one to understand decoherence of spin waves (SWs) and then enable one to obtain optimal lifetime of the entanglement storage in the cold atoms.

The interface polarization effect of the electron blocking layer (EBL) hinders the hole transmission efficiency, and the traditional EBL cannot effectively suppress the electron leakage and reduce the hole energy loss. To solve this problem, in this study, a tapered p-cladding layer (p-CL) is proposed to increase the accumulation of holes, and a triangular EBL is proposed to reduce the leakage of electrons and reduce the energy loss of holes during transmission. The results show that tapered p-CL and triangular EBL can effectively reduce electron leakage and increase hole energy. Owing to the improvement of the carrier density, the output power of the laser increases by 27.09%, and the threshold current decreases by 38.24%. Thus, for a 267 nm deep-ultraviolet laser diode, the proposed structure has an optical confinement factor of up to 43.59%, which is increased by 117% and the radiation recombination rate is increased by about 113%.

An underwater propulsion microsystem is proposed in this work, which employs a nanosecond laser pulse out from the tapered fiber tip. Noteworthily, the system can generate a directional shock wave (or plasma) to propel the polystyrene (PS) microsphere. Through simulation, the shock wave propagation characteristics and the bubble dynamic are investigated. Experimentally, high-speed photography method is employed to obtain the motion image of microsphere. The results show that the propulsion efficiency is dependent on the laser energy. Meanwhile, we explain the role of the bubble dynamic process in propelling microsphere, and find that the bubble diameter increases with the laser energy. In addition, an experiment is performed to separate and remove the PS microsphere clusters in water at fixed point. Compared with conventional technology, this new method has advantages of high controllability, directional and non-contact, and can be used for directional manipulation of underwater microstructures and removal of contaminated microspheres in water environments.

Magnetooptical properties of one-dimensional aperiodic structures formed by stacking together magnetic and nonmagnetic layers according to the Kolakoski self-generation scheme are studied theoretically using the 4 $$\times$$ × 4 transfer matrix method. The effect of the generation stage of the sequence, and the helicity and direction of light propagation through the magneto-photonic crystals on the transmission/reflection spectra as well as Faraday and ellipticity rotations, have been investigated. Our results reveal that this kind of aperiodic magneto-photonic crystals can be used for the fabrication of multifrequency laser cavities, and optical filters/sensors.

This roadmap article focuses on spatially incoherent digital holography (IDH). Representative IDH methods such as optical scanning holography (OSH), Fresnel incoherent correlation holography (FINCH), coded aperture correlation holography (COACH), IDH with a Fresnel zone aperture, and IDH with an interferometer along with a state-of-the-art optical device are introduced as modern IDH methods. We describe these IDH techniques with applications of three-dimensional (3D) imagers, 3D thermography, and 3D microscopy

In this study, a laboratory laser-induced breakdown spectroscopy (LIBS) system was used to measure the gadolinium (Gd) content in thorium (Th) oxide matrix to investigate and assess the ability of the LIBS technique for nuclear material analysis. Five different Gd concentrations were quantitatively spiked into the Th oxide powder and analyzed by the LIBS system. Although a spectrometer with relatively limited resolution was used in the experiment, the characteristic emission lines of Gd were identified and the unresolved Gd II 348 nm-band with no interference with Th lines was selected for Gd quantitative analysis. Results show that the mathematical technique of multi-component Lorentzian Fit can be well utilized to deconvolve the unresolved spectra. Univariate calibration curves for Gd element were constructed using peak area and peak intensity methods. The best calibration curve was obtained using the peak area of the Gd 348.18 nm and yielded the lowest limit of detection (LOD) of 0.60%. The leave-one-out method was employed to further investigate the analytical prediction skills of LIBS for each reference samples. For Gd concentration ≥\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\ge$$\end{document} 4.33%, using the peak area of 348.18 nm can provide higher accuracy on the predicted concentrations. These results presented here demonstrate that the Gd element can be qualitatively and quantitatively monitored in Th oxide matrix using LIBS with a relatively limited resolution spectrometer.

Dynamic laser speckle and its biological version (biospeckle laser) have been used in many areas of knowledge. Its non-invasive approach allows the application in advantage regarding those that need contact or damage the analyzed sample. However, one needs the sharp adjust of the image acquiring and processing. In this article, we show how the variation of sampling rate in a dynamic speckle analysis affects the value of dynamic speckle indexes concerning the absolute value of the differences index, the temporal speckle standard deviation index, and the temporal speckle mean index. We show that the dynamic speckle index value changes its maximum excursion with the variation of sampling rate, affected directly by the camera's time integration (time of exposure). We highlight the importance of knowing the frequency band of the analyzed phenomenon and its signal to choose the appropriate sampling rate, with the recommendation of using the lowest sampling rate possible—without compromise the speckle grains—to obtain an acceptable maximum excursion and an illumination level with a good signal–noise ratio. The results will help those who work with the phenomenon/technique to enhance their analysis tailoring the set up and yielding reliable results, since the optical method demands a rigorous bias of the image acquiring and processing.

We have demonstrated the dual-wavelength domain-wall pulse operation in an Er-doped mode-locked fiber laser based on single-mode fiber (SMF)–graded index multimode fiber (GIMF)–SMF as the saturable absorber for the first time. The domain-wall dark–bright pulse pairs exhibited two peaks, which were located at 1604.6 and 1606 nm. The pulse repetition rate was maintained at 16.5 MHz throughout the dark–bright pulse pairs operation. Besides, dark–bright pulse pairs operation was demonstrated at a relative stability with the signal to noise ratio of 55.3 dB. Moreover, the dark–bright pulse pairs could be converted into bright–dark pulse pairs by adjusting the polarization state, which has potential advantages for the development of wavelength division multiplexing and optical fiber sensing technology.

We performed a detailed numerical uncertainty quantification of the fixed-and scanned-wavelength direct absorption spectroscopy method (DA) at high temperature. A transition near 2551 nm, relevant to water vapor sensing in a shock tube facility, was considered. The uncertainty quantification for this transition was based on a critical evaluation of the uncertainty associated to each parameter required for DA modeling. The results show a non-negligible uncertainty on the water concentration with a standard deviation on the order of 7% when perturbing all the parameters separately at their maximum uncertainty. This uncertainty originates mainly from the uncertainty on temperature, pressure, line strength, and lower-state energy. The non-negligible error induced by finite scanning time in scanned-DA was also discussed and a local optical Damköhler number was introduced to characterize such a phenomenon. The uncertainty for practical temperature measurement using the two-color DA approach was also estimated. Depending on the selected absorption line pair, the minimum uncertainty can be as low as 5.1% or as high as 18%.

We report on an all-solid-state light source for nanosecond (ns) laser pulses, with broad tunability in the mid-infrared, spectral bandwidth close to the Fourier transform limit, and pulse energy in the mJ regime. To this end, we extend a tunable, continuous wave (cw) singly resonant optical parametric oscillator by an optical parametric pre-amplifier with a periodically poled lithium niobate crystal and a power amplifier stage with two bulk lithium niobate crystals. We demonstrate pulse energies beyond 1 mJ in a tuning range between 3.3 and 3.8 μm\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upmu \mathrm {m}$$\end{document} center wavelength, with options for even larger output pulse energy and tuning range. The total conversion efficiency in the power amplifier reaches 20%. From measurements of absorption spectroscopy, we determine a very narrow linewidth of 108 MHz (full width at half maximum, FWHM), which is only a factor of 1.4 above the Fourier limit. We demonstrate the applicability and versatility of the laser system for nonlinear spectroscopy by resonantly enhanced third harmonic generation and sum frequency mixing in a gas sample of HCl molecules.

In this paper, a simple approach is presented to generate the dual-wavelength second harmonic(SH) with flexible conversion efficiency ratio and wavelength interval in nested quasi-periodic optical superlattice(NQOS) structure. The characteristics of dual-wavelength second-harmonic generation(SHG) is studied with a 10-mm-long NQOS devices base on 5% mol MgO:LiNbO3 crystal. The simulation results show that the ratio of the conversion efficiency of two wave peaks can be adjusted by altering the incident position of NQOS devices. Using the combination of type-0(o + o → o) and type-I(e + e → o) quasi-phase matching(QPM), the dual-wavelength SHG around 1.55 μm is achieved. By adjusting the operating temperature, the interval of two wave peaks in dual-wavelength SHG devices can continuously adjusted with the range of 0–352 nm. Furthermore, the central fundamental wavelength and the maximum interval range of dual-wavelength SHG devices depend on the period of the NQOS.

An efficient intracavity frequency-doubled diamond Raman laser output at 620 nm wavelength is demonstrated. Laser output power of 750 mW at central wavelength of 620.1 nm was achieved successfully at a repetition rate of 2 kHz. The optical conversion efficiency was up to 11.0%. The laser beam M2 factors were less than 1.6 and 1.3 in the x and y directions, respectively. This intracavity diamond Raman laser offers a new way towards generating efficient pulsed Raman red laser.

The non-negativity of the measurement matrix in the traditional compressed ghost imaging system and iterative optimization process leads to low imaging quality and slow reconstruction speed. This paper proposes a singular value decomposition compressed ghost imaging method based on deep unfolding. The measurement matrix and training data pairs are generated through numerical simulation to reduces the cost of data acquisition. This paper adds a preprocessing layer to the network, which performs singular value decomposition on the measurement matrix to simultaneously obtain an optimized semi-orthogonal measurement matrix and optimized measurements. Then, iterative shrinking threshold algorithm network (ISTA-Net +) is used to learn the mapping between the measurements to the original signal from the training data set. Finally, the trained deep neural network can achieve non-iterative real-time reconstruction of high-quality images from low sampling rate measurements. Numerical experiments demonstrate that our proposed method has good reconstruction performance and good anti-noise performance at low sampling rates.

Atmospheric pressure discharge plasma is widely utilized in industry and science. However, due to the spatiotemporal uncertainty of the natural discharge, it is difficult to measure the discharge plasma spectra with a high spatiotemporal resolution. This prevents the accurate investigation of discharge plasma evolution and limits further applications. Here, we harnessed a femtosecond laser filament to trigger and guide a high-voltage discharge, i.e., the discharge plasma channel is rigorously controlled by the filament in both space and time. Therefore, the spectra of the plasma channel with a high spatiotemporal resolution could be measured using an imaging spectrometer. The spectra of the whole process of femtosecond laser filament-triggered discharge plasma are thoroughly studied. According to the spectral emission features, the whole process is divided into three stages: femtosecond laser filamentation, streamer propagation, and discharge. The spectral emissions at different stages can be utilized as required according to the spectral emission features. Based on the spatiotemporally resolved spectra of the streamer, the streamer propagation velocity is calculated to be about 3 × 105 m/s. In addition, atomic emissions from a discharge plasma triggered by femtosecond laser filament can be used for one-dimensional component measurements of flow fields.

In Appl. Opt. 55, 4720-4728 (2016), authors demonstrated the vulnerability of Linear Canonical Transform (LCT)-based optical encryption system. One of the primary reasons for this is the predictable nature of the security keys (i.e., simulated random keys) used in the encryption process. To alleviate, in this work, we are presenting a Physically Unclonable Function (PUF) for producing a robust encryption key for the digital implementations of any optical encoding systems. We note a correlation function of the scattered perfect optical vortex (POV) beams is utilized to generate the encryption keys. To the best of our knowledge, this is the first report on properly utilizing a scattered POV for the optical encryption systems. To validate the generated keys, the standard Linear Canonical Transform-based Double Random Phase Encoding (LCT-DRPE) technique is used. Experimental and simulation result validates the proposed key generation method as an effective alternative to the digital encryption keys.

Unlike the traditional cylindrical vector beams, the power-exponent azimuthal-variant (PEAV) vector beams possess the non-axisymmetric polarization state distribution. We find the Taiji-like symmetric splitting of the opposite spin and orbital angular momentum occurs in the tight focusing of the PEAV vector beams, it can be considered as the manifestation of the spin-orbit coupling induced by its non-axisymmetric polarization state distribution. More interestingly, when the PEAV vector beam possesses the topological charge, the spin angular momentum is no longer the Taiji-like symmetric distribution, it will be converged as the increase of the topological charge; meanwhile, the pattern of the orbital angular momentum will evolve into the annular distribution as the increase of the topological charge. The evolution properties of the spin and orbital angular momentum are completely determined by the topological charge of incident PEAV vector beam. Our results provide the potential applications of optical manipulation, particles trapping, and optical shaping.

An extension to the quantum-mechanical laser master equation for the density operator is derived to incorporate the beam-splitter effect caused by typical dielectric laser output couplers. This effect gives rise to a significant change in the photon statistical distribution of the part of the laser light reflected back into the resonator and, therefore, may have an influence on the total laser output photon statistics. Different cases without and with additional intra-cavity losses were discussed and their influence on the expected laser photon statistics was deduced. As a result, it was found that the well-known Poisson distribution of laser light is in most cases the result of additional losses or absorption, which act uncorrelatedly on single photons. In a laser with negligible additional losses where outcoupling is dominated by the beam-splitter effect, the photon statistics reveal to be mainly non-Poisson. A Poisson distribution would only occur for very low outcoupling rates, i.e., high finesse cavities. It is found that in the limit of strong outcoupling even the distribution of thermal light can result.

In this paper, a scheme of optical encryption system for color code is proposed which is based on double random polarization encryption and exclusive-OR operation. First, an original image is generated into a color code according to the rules of color code generation, then the color code is encoded by exclusive-OR operation, finally, the coded color code is used as the input image for double-random polarization encryption. The original image can be recovered only if all the keys are correct. If the ciphertext is tampered or cut, the image cannot be recovered or even displayed on the computer. This method can apply the double random polarization encryption method used for binary image encryption to color code encryption, and the exclusive-OR operation can improve the security of the system. Numerical experimental results show the feasibility and effectiveness of this method.

In this paper, the evolution and interaction of the Pearcey–Gaussian beam in nonlinear Kerr medium are numerically studied. The results show that the self-focusing effect can suppress the inversion of the Pearcey–Gaussian beam and cause the main lobe and side lobes of Pearcey–Gaussian beam to separate, forming solitons during propagation, while the defocusing effect can retain partially the beam inversion and accelerate its spatial expansion after the inversion. As for the interaction between the Pearcey–Gaussian beams, in self-focusing medium, except for the elastic interaction between the main lobes, the in-phase between them can result in the pattern of periodic collision around the center of coordinate caused by the side lobes, while the out-of-phase can suppress its occurrence. In defocusing medium, the interactions of the in-phase and out-of-phase Pearcey–Gaussian beams show the constructive and destructive interference, respectively. Finally, the influence of the width of Gaussian function in Pearcey–Gaussian beam on the interaction of the two Pearcey–Gaussian beams is studied.

We have addressed the characteristic distinguishability of coherent states in the temporal domain from a directly modulated quantum well-based gain-switched laser diode. We identify adjustable parameters to generate indistinguishable coherent states from an electrically pumped semiconductor laser using small-signal and large-signal models. The experiment confirms the generation of indistinguishable signal and decoy coherent states as predicted by the numerical simulation. In addition, the potential for indistinguishability has been explored in different types of coherent states.

We report Second Harmonic Generation (SHG) and Third Harmonic Generation (THG) energy conversion efficiencies up to 59% and 27%, respectively, for laser pulses simultaneously delivering high peak power in the sub-TW range and average powers in the sub-kW range. No damage or efficiency decrease is observed after more than 100 h operation time. The resulting high-energy visible and near-UV pulses are suitable for applications, such as lightning control, material analysis and machining, or OPCPA pumping.

An elliptic spiral forked plate (ESFP) is proposed to generate two off-axis elliptic optical vortices, whose topological charges are identified by the numbers of lines in the vortices. Moreover, the number of lines minus one is equal to the absolute value of the topological charge. a and c are the vertical ellipticity factors, which adjust the lengths of the vertical axes for the elliptical vortices generated by φ and ϕ, respectively. b and d are the horizontal ellipticity factors, which adjust the lengths of the horizontal axes for the elliptical vortices generated by φ and ϕ, respectively. φ and ϕ present the elliptic spiral phase plates for the elliptic forked grating and elliptic spiral zone plate, respectively. When ellipticity factors (a, b, c, d) satisfy a = c = 1 and b = d = 0.5, the topological charge identification is the most obvious. At this time, the elliptic vortices are vertical. On the contrary, i.e., a = c = 0.5 and b = d = 1, the elliptic vortices are horizontal. The elliptical vortex can rotate by adjusting the horizontal modulation parameter g. For the vertical and horizontal elliptical vortices, the slopes of the long axes are equal to − 1/g and g, respectively. The ESFP consists of the elliptic forked grating and elliptic spiral zone plate with the topological charges l1 and l2 respectively. The left and right elliptic vortices have the topological charges of l2 – l1 and l2 + l1, respectively. The method of constructing the ESFP is illustrated. It is proved in the simulations and experiments that the topological charges of the two elliptic vortices can be identified by the numbers of the lines. The proposed zone plate is used for optical trapping, optical communication and optical imaging.

Mid-infrared laser polarization spectroscopy (IRPS) is a powerful diagnostic technique suitable for the quantitative measurement of species concentration and temperature. IRPS is well suited to reactive multiphase flows and is not affected by the presence of particles and other nanostructures. It has the potential to detect species that have no accessible single-photon electronic transition in the UV/Visible range. In this review, an overview of the various contributions that have been devoted to the development and application of IRPS are summarized and discussed. The basic theoretical framework is presented and the nature of the IRPS experiments is described. Then, several types of IRPS applications are reviewed, including the measurement of the concentration of minor species and the temperature, the study of molecular dynamics and collision energy transfer, and the feasibility of 2D imaging with high spatial resolution. Finally, future prospects, required improvements, and potential application fields of IRPS are provided.

The geometrical configuration of the dual-beam photothermal lens (PTL) experiment has a major role in the PTL sensitivity. This work examines the mode-matched configuration in terms of temporal evolution of PTL signal amplitude. The main results concern the improvement of the PTL signal under specific conditions with the mode-match arrangement considered in several previous works as less sensitive, but less influenced by the aberration effects. The amplitude of the obtained PTL signal is considerably increased to be of the same order of magnitude as that given by the mode-mismatched configuration. This occurred by reducing the probe beam waist at the sample to probe a part of temperature gradient area having a photothermal thin lens behavior. The experimental PTL signal is positive and presents low optical aberrations and a linear dependence with the excitation power. For the purpose of verifying the reliability of this experimental setup, the thermal diffusivities and the absorption coefficients of paraffin oil and ethanol were measured and compared to the results reported in literature.

This work reports the development and validation of a new imaging sensor arrangement optimization method for volumetric tomography, namely effective voxel corrections maximization (EVCM). Different from past optimization approaches that only studied the influence of sensor orientations on the reconstruction accuracy, the EVCM method considers the impact of both sensor orientations and measured target distribution. Combining all recorded target projections, the EVCM first determines the number of effective voxels, indicating the voxels that participate in tomographic calculation. The method then calculates the number of corrections that modifies the value of effective voxels within a single reconstruction iteration step. The ratio between numbers of corrections and effective voxels (RE) is established as a new criterion for the sensor arrangement optimization to achieve improved reconstruction accuracy. Both numerical simulations on signal phantoms and controlled experiments on lab-scale flames are employed to validate the EVCM. Comparisons between EVCM and other sensor arrangement optimization methods are also performed based on the 3D reconstructions both numerically and experimentally. Results show that EVCM turns out to be a more accurate way to decide the optimal arrangements specified for different 3D targets.

We present a method for determining the temporal and spatial evolution of a gas jet generated by a pulsed nozzle using high-order harmonics of a titanium–sapphire laser. This radiation in the extreme ultraviolet spectral range (17–40 nm) is transmitted through the gas jet and becomes partially absorbed depending on its wavelength and the gas density. If the absorption in this spectral range shows a sufficiently strong dependence on the wavelength, as is the case for many gases including the noble gases argon, neon, and helium, it is possible to select a proper harmonic exhibiting an absorption strong enough to generate a detectable decrease of the transmitted light but still weak enough to allow a significant amount of radiation to be transmitted through the gas jet. In the case of radial symmetry the density profile can be reconstructed by means of the Abel inversion. We show that this method allows for the determination of argon neutral densities as low as $$10^{17}$$ 10 17 cm $$^{-3}$$ - 3 and is also suited for other gases, such as neon and helium.

We report a high-performance self-powered, flexible, and single aluminum nitride nanowire ultraviolet photodetector (AlNNW-UVPD) with an excellent detectivity and responsivity of 2.62 × 1012 cm Hz1/2 W−1 and 362.5 mA/W, respectively, under 254 nm UV light exposure. We fabricated a very small size, flexible nanoscale photodetector via a very cost-efficient hot-contact method, achieving very good speed with the rise time and fall time of 50 ms and 251.5 ms, respectively. The AlNNW-UVPD device selectivity was demonstrated by two different UV light sources of 254 nm and 302 nm at a bias voltage of 0 V. Furthermore, the device showed very good durability to vigorous bending test. In addition to being very small, cost-effective, flexible, high performing, and durable, our self-powered AlNNW-UV photodetector can offer innovative solutions and insights for portable, sensitive, small, and flexible electronics and photonics.

An attosecond water window pulse generated from high harmonics spectrum is chirped, and needs to be compressed to transform limited pulse by chirp compensation. In this paper, a Cr/Sc-stacked multilayer with chirped structure is proposed for attosecond pulse compression. Three stacked multilayer mirrors are designed to compress chirped attosecond pulses covering 300–400 eV water window spectrum. Nearly, transform limited attosecond pulses are all obtained after reflected by the stacked multilayer mirrors, which exhibit 0.27%, 0.41%, and 0.58% average reflectivity and − 2086, − 3481 or − 5644 as² average group delay dispersion in the 300–400 eV region, respectively. The research proposes a stacked multilayer mirror for chirp compensation and pulse compression, which could be used for pulse shaping of an attosecond water window pulse.

The mathematical and numerical analysis of the Graphene infrared tunable frequency selective surface (FSS) for far-infrared spectrum has been clarified. The proposed FSS shows different properties of transmittance, reflectance, polarization variation for the different range of fermi voltage which can be tuned by external biasing. Angular changes in the graphene plane demonstrate different frequency responses of reflected wave polarization. Proposed structures are also computed at normal as well as the complementary condition of the same design. The proposed structure also computed the variation in the many parameters with the rotation change of the frequency selective surface structure. It can observe more than 60% of the overall transmittance for the wide range of the frequency. The ultrathin and easy to fabricate structure opens up the door for various applications such as reflector, absorber, polarizer, a modulator for the THz regime.

Top-cited authors
• Physikalisch-Technische Bundesanstalt
• University of Duisburg-Essen
• University of Manitoba
• Indiana University of Pennsylvania
• China University of Petroleum