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A high-resolution absorption spectrum of the S1–S0 transition of free-base phthalocyanine was observed and analyzed with improved reliability. The spectrum, with a partially resolved rotational structure, was obtained by using the buffer-gas cooling technique and a single-mode tunable laser. Our new analysis reveals that the S 1 ← S 0 0 0 0 band belongs to the a-type transition, where the electronic transition moment aligns parallel to the NH–HN direction, allowing the assignment of the S1 state to 1B3u. These results agree with a prior study using supersonic expansion and are well supported by theoretical calculations. Interestingly, the rotational constant B in the S1 state, which is often smaller than that in the ground state for typical molecules, was found to be slightly larger than that in the S01Ag state. This suggests a change in the character of π bonds with the electronic excitation. [ABSTRACT FROM AUTHOR]

Precise and reproducible current generation is the key to realizing quantum current standards in metrology. A promising candidate is a tunable-barrier single-charge pump, which can accurately transfer single charges one by one with an error rate below the ppm level. Although several measurements have shown such levels of accuracy, it is necessary to further pursue the possibility of high-precision operation toward reproducible generation of the pumping current in many devices. Here, we investigated silicon single-hole pumps, which may have the potential to outperform single-electron pumps because of the heavy effective mass of holes. Measurements on the temperature dependence of the current generated by the single-hole pump revealed that the tunnel barrier had high energy selectivity, which is a critical parameter for high-accuracy operation. In addition, we applied the dynamic gate-compensation technique to the single-hole pump and confirmed that it yielded a further performance improvement. Finally, we demonstrated gigahertz operation of a single-hole pump in which the estimated lower bound of the pump error rate was around 0.01 ppm. These results imply that single-hole pumps in silicon are capable of high-accuracy, high-speed, and stable single-charge pumping in metrological and quantum-device applications. [ABSTRACT FROM AUTHOR]

This article presents methodological advances in the state-of-the-art for making time-dependent, thermochemical measurements within kilogram-scale explosive post-detonation fireballs utilizing tunable laser absorption spectroscopy. This measurement capability is critical for validating multi-scale, multi-physics models of post-detonation dynamics. The technique is based on hardened gauges built around rapidly-tunable lasers and custom post-processing algorithms that provide quantitative thermochemical data interior to large and opaque explosive fireballs. The authors present a holistic overview of the technique including gauge design, the laser absorption diagnostic, and the custom data processing algorithms. Additionally, fielding high-bandwidth laser absorption probes at stand-off ranges presents new challenges in data processing that must compensate for long distance signal transmission effects. We highlight representative data from a hardened gauge measurement at 0.81 m stand-off from a 2.78 kg LX-14 explosive charge detonated in an outdoor test arena. We discuss progress in all-optical measurement of temperature, pressure, and water vapor number density at a 100 kHz repetition rate during the first 10 ms of the fireball evolution. We conclude the article with a brief discussion on our current approach for comparing hardened gauge measurements with computational fluid dynamic simulations. [ABSTRACT FROM AUTHOR]

Over the past six decades, dye laser technology has improved substantially. The usage of solid matrices containing laser dyes is becoming more and more popular as a viable substitute for the traditional liquid dye solutions. In this research, we designed and built a tunable solid dye laser device which consists of three main parts. (i) The active material producing the laser beam, which is a polymer rod [glycidyl methacrylate (GMA) polymer] doped with the laser dyes pyrromethene 597 (PM-597) or Nile blue (Nb-690). (ii) The pumping source is xenon linear flashlamp driven by a homemade power supply. (iii) Two broadband mirrors providing the optical feedback stable optical resonator. The properties of the gain medium and hence the laser emission characteristics were determined, and the results showed the efficiency of the homemade tunable solid dye laser device, that is suitable for a variety of applications. [ABSTRACT FROM AUTHOR]

In this paper, a novel two-stage hydrocarbon fuel heating and delivery system was used to more accurately simulate the state of fuel in the cooling channel of the regenerative cooling engine. Mildly cracked (<10%) experiments of n-dodecane were conducted by using the new heating system and pyrolysis products were sampled and analyzed. The ignition delay time and laminar flame speed of the cracked products were different from those of the surrogate gas fuel. When the temperature is 1100 K, the ignition delay time of the cracked products are ~50% higher than that of surrogate gas fuel. The laminar flame speed of the cracked products is about 30% lower than that of surrogate gas fuel at the equivalence ratio of 1.0. The supersonic combustion experiments of mildly cracked and different equivalence ratios n-dodecane were also employed at Mach number 3.0 of the isolator entrance, with total temperature ~1550 K and total pressure ~1.5 MPa. The static pressure along the axis of combustor was measured, while the velocity and temperature of the airflow at the exit of combustor were also measured by the tunable diode laser absorption spectroscopy (TDLAS). The distributions of airflow velocity, static temperature, and total temperature along the combustor at different conditions were analyzed through a one-dimensional analysis method. The deviations of velocity and temperature of the airflow are less than 4% and −6%, respectively, which indicates the calculation is in great accordance with the TDLAS detections. The experimental data will be helpful for validating the simulation of supersonic combustion, and studying the influence of mildly cracked fuel on supersonic combustion characteristics. [ABSTRACT FROM AUTHOR]

Nonlinear optical (NLO) crystals that have the capability of achieving tunable lasers through frequency conversion are of great importance in solid‐state lasers. Herein, three new deep‐UV transparent alkali‐scandium sulfates with strong second‐harmonic generation (SHG) response and high thermal stability, A3Sc(SO4)3 (A = K, Rb, Cs), are successfully obtained. It is interesting that the slightly distorted [ScO6] octahedrons are in an ordered/oriented arrangement, which is in favor of the generation of SHG response. Especially, K3Sc(SO4)3 not only exhibits a strong phase‐matching SHG response of 1.6 × KDP, a short deep‐ultraviolet (deep‐UV) cutoff edge of 190 nm, a wide transparent window covering from the deep‐UV region to mid IR, a high laser‐induced damage threshold of 165 MW cm−2 and easy growth of large single crystal, but also behaves thermal stable up to 900 °C under an air atmosphere. Crystal structures and theoretical calculations reveal that their strong SHG responses mainly stem from the slightly distorted [ScO6] octahedron. This study highlights A3Sc(SO4)3 (A = K, Rb, Cs) as promising deep‐UV transparent NLO crystals and offers a new approach for the pursuit of high‐performance NLO crystals. [ABSTRACT FROM AUTHOR]

Photonic integrated circuits (PICs) offer ultra‐broad optical bandwidths that enable unprecedented data throughputs for signal processing applications. Dynamic reconfigurability enables compensation of fabrication flaws and fluctuating external environments, tuning for adaptive equalization and training of optical neural networks. The initial step in PIC reconfiguration entails measuring its dynamic performance, often described by its frequency response. While measuring the amplitude response is straightforward, e.g. using a tunable laser and optical power meter, measuring the phase response presents challenges due to various factors, including phase variations in test connections and instrumentation limitations. To address these challenges, a universal and robust characterization technique is proposed, which uses an on‐chip reference path coupled to the signal processing core (SPC), with a delay gap larger than the total delay across the signal processing paths. A Fourier transform of the chip's power response reveals the SPC's impulse response. The method is more robust against low reference‐path power and imprecise delays. Experiments using a finite‐impulse‐response (FIR) structure demonstrate rapid SPC training, overcoming thermal crosstalk and device imperfections. This approach offers a promising solution for PIC characterization, facilitating expedited physical parameter training for advanced applications in communications and optical neural networks. [ABSTRACT FROM AUTHOR]

In this study, we developed and validated an improved tuning method for a nonlinear scanning laser diode in methane gas sensing, using wavelength modulation spectroscopy - tunable diode laser absorption spectroscopy (WMS-TDLAS). This approach combines cubic nonlinear scanning with high-frequency modulation to precisely target the methane absorption peak at 6046.96 cm−1. The method enhances absorption time, thus increasing system sensitivity and stability. Our results show that this nonlinear scanning, optimized for peak absorption, significantly improves the slope of the methane concentration relationship curve by 76.9%. It also reduces the standard deviation by 61.1% and the limit of detection by 60% compared to linear scanning. The limit of detection the system is 4.2 ppm, and the optimal integration time is 48s. This novel modulation strategy significantly enhances the WMS-TDLAS system's efficiency, offering significant benefits for various applications. • Cubic nonlinear scanning method (CNSM) was proposed and CH 4 detection was verified. • Scanning central wavenumber of LD obtained a longer absorption time. • CNSM significantly improved the slope of the methane concentration curve. • The sensing sensitivity and stability were improved by 45% and 49%, respectively. [ABSTRACT FROM AUTHOR]

A new optical diagnostic method that predicts the global fuel–air equivalence ratio of a swirl combustor using absorption spectra from only three optical paths is proposed here. Under normal operation, the global equivalence ratio and total flow rate determine the temperature and concentration fields of the combustor, which subsequently determine the absorption spectra of any combustion species. Therefore, spectra, as the fingerprint for a produced combustion field, were employed to predict the global equivalence ratio, one of the key operational parameters, in this study. Specifically, absorption spectra of water vapor at wavenumbers around 7444.36, 7185.6, and 6805.6 cm–1 measured at three different downstream locations of the combustor were used to predict the global equivalence ratio. As it is difficult to find analytical relationships between the spectra and produced combustion fields, a predictive model was a data-driven acquisition. The absorption spectra as an input were first feature-extracted through stacked convolutional autoencoders and then a dense neural network was used for regression prediction between the feature scores and the global equivalence ratio. The model could predict the equivalence ratio with an absolute error of ±0.025 with a probability of 96%, and a gradient-weighted regression activation mapping analysis revealed that the model leverages not only the peak intensities but also the variations in the shape of absorption lines for its predictions. [ABSTRACT FROM AUTHOR]

Micro/nanoscale lasers that span the entire visible spectrum, especially those in red, green, and blue colors, are not only essential for a variety of optical devices but also have important applications in visible color communication, multi‐color fluorescence sensing, and wavelength division multiplexing. Despite the great efforts made to achieve multi‐color lasing using a variety of approaches, on‐chip white light emission and even red, green, and blue multi‐color lasers still suffer from considerable challenges in micro‐nano structures. Here, CdSxSe1−x, CdS, and ZnS micro‐tripod structures are successfully prepared using chemical vapor deposition approaches. The micro‐photoluminescence (µ‐PL) spectra and PL‐mapping of these micro‐tripods reveal various emissions at 630, 508, and 460 nm, respectively. Additionally, white‐light emissions based on these composition‐tunable tripods are realized through an end‐coupling structure system. Moreover, room‐temperature modes tunable lasers are observed clearly from three legs of these micro‐tripods, with a low threshold of ≈48.39 µJ cm−2 and a high quality factor of 1227.3. The realization of micro‐tripods‐based lasers may provide an innovative way for highly integrated photonic circuits and communications. [ABSTRACT FROM AUTHOR]

We describe a new version of the Chicago Water Isotope Spectrometer (ChiWIS), designed for airborne measurements of vapor-phase water isotopologues in the dry upper troposphere and lower stratosphere (UTLS) aboard research aircraft. This version of the instrument is a tunable diode laser (TDL), off-axis integrated cavity output spectrometer (OA-ICOS). The instrument was designed to measure the HDO/H2O ratio in the 2017 Asian Summer Monsoon flight aboard the M-55 Geophysica during the StratoClim campaign, and so far has also flown aboard the WB-57F in the 2021 and 2022 ACCLIP campaigns. The spectrometer scans absorption lines of both H2O and HDO near 2.647 μm wavelength in a single current sweep, and has an effective path length of 7.5 km under optimal conditions. The instrument utilizes a novel non-axially-symmetric optical component which increases the signal-to-noise ratio by a factor of 3. Ultra-polished, 4-inch diameter cavity mirrors suppress scattering losses, maximize mirror reflectivity, and yield optical fringing significantly below typical electrical noise levels. In laboratory conditions, the instrument has demonstrated a 5-second measurement precision of 3.6 ppbv and 82 pptv in H2O and HDO, respectively. [ABSTRACT FROM AUTHOR]

Thin film lithium niobate (TFLN) has become a promising material platform for large scale photonic integrated circuits (PICs). As an indispensable component in PICs, on-chip electrically tunable narrow-linewidth lasers have attracted widespread attention in recent years due to their significant applications in high-speed optical communication, coherent detection, precision metrology, laser cooling, coherent transmission systems, light detection and ranging (LiDAR). However, research on electrically driven, high-power, and narrow-linewidth laser sources on TFLN platforms is still in its infancy. This review summarizes the recent progress on the narrow-linewidth compact laser sources boosted by hybrid TFLN/III-V semiconductor integration techniques, which will offer an alternative solution for on-chip high performance lasers for the future TFLN PIC industry and cutting-edge sciences. The review begins with a brief introduction of the current status of compact external cavity semiconductor lasers (ECSLs) and recently developed TFLN photonics. The following section presents various ECSLs based on TFLN photonic chips with different photonic structures to construct external cavity for on-chip optical feedback. Some conclusions and future perspectives are provided. [ABSTRACT FROM AUTHOR]

We present the design, fabrication, and characterization results of a compact, widely tunable laser realized on an indium phosphide membrane-on-silicon (IMOS) platform. The laser features a compact Mach–Zehnder interferometric structure as the wavelength-selective intracavity filter with a footprint of 0.13 mm2. The filter design is optimized to ensure narrow filter transmission and high side-mode-to-main-mode-ratio, enabling single-mode operation for the laser. The high optical confinement on the IMOS platform can support tight waveguide bends. Leveraging this, the laser achieves a short cavity length, further enhancing the single-mode operation. Measurement results indicate a threshold current of 29 mA and a maximum on-chip output power of approximately 3.6 dBm and wall plug efficiency of 1.8%. The side-mode suppression ratio ranges from 30 to 44 dB, with a tuning range spanning 40 nm, from 1555 to 1595 nm. A complete tuning lookup table is generated via an automated setup incorporating a stochastic search algorithm. [ABSTRACT FROM AUTHOR]

The micro‐ring resonator (MRR) is a crucial element in integrated photonics. It allows for the realization of various optical devices, including integrated filters, modulators, micro‐ring laser cavities, detectors, optical logic switches, and nonlinear optical devices. The development of integrated photonics has enabled the integration of these devices with other components into a photonic chip to perform specific functions. For instance, widely tunable MRR‐based lasers have been integrated with optical phased arrays (OPAs), detectors, and other elements to form a fully integrated LiDAR engine. Advances in nanofabrication have facilitated the miniaturization of nanophotonic phased arrays and on‐chip lasers. MRR‐based integrated external cavity lasers (ECLs) and integrated optical frequency combs (OFCs) have significantly enhanced the scanning and detection capabilities of LiDAR recently. Additionally, their size and power consumption have been continuously reduced, rendering them suitable for chip‐scale integration. This paper systematically reviews the major advancements of MRR‐based integrated lasers for chip‐scale LiDAR. [ABSTRACT FROM AUTHOR]

Dynamic tuning of light properties by external stimuli is at the core of various applications such as electro‐optical modulators, beam steering, and spatial light modulators. The conventional mechanism involves fine‐tuning the eigenmode of an optical system through adjusting the effective refractive index. However, the weak nonlinearity results in low modulation efficiency, leading to devices with poor performance and large size. Polarization topological charge is a significant concept that has facilitated the development of innovative optical devices like low‐threshold lasers and vortex generators. But the devices reported so far are static in nature. Here, a method for dynamically controlling light by actively manipulating the evolution of topological charges in momentum space is first presented. By switching between integer and half‐integer states of topological charges, the device's radiation properties undergo a significant transformation. The beam direction can be tuned up to 160°, which, to the best of the authors' knowledge, is the largest tuning angle among similar beam steering devices. Furthermore, the device demonstrates high radiation efficiency while maintaining a compact device size. This light controlling method can be applied in various fields, including optical communication, tunable lasers, and light detection and ranging. [ABSTRACT FROM AUTHOR]

In this paper, we have developed a theoretical formalism to investigate the nonlinear absorption of high-power Hermite cosh-Gaussian laser beam in arrays of cylindrical carbon nanotubes in the presence of an external static magnetic field. As the incident Hermite cosh-Gaussian laser beam interacts with the carbon nanotubes, the atoms associated with this medium get excited and became ionized resulting to form the preformed plasma. The field of laser beam radiation causes to displace the electrons cylinder with respect to ion cylinder. Due to the displacement of electron cylinder, an electrostatic restoration force will be taken in to consideration and this causes the nonlinearity. An analytic expression of effective nonlinear absorption coefficient of Hermite cosh-Gaussian laser beam is calculated. The maximum nonlinear effective absorption is achieved as the laser beam frequency approaches near the 1 / 2 times the electron plasmon frequency. The absorption process of laser beam on the surface of carbon nanotube enhanced in the presence of static magnetic field. The absorption of laser beam is strongly dependent on various parameters such as electron cyclotron frequency, decentered parameter, electron–ion collisional frequency, transverse propagation distance, laser mode index, and beam width parameter. This tunable and enhanced laser beam absorption on the surface of carbon nanotube might be applicable in electron heating and high harmonic generation. [ABSTRACT FROM AUTHOR]

Indium seals have been used extensively in ultra-high vacuum and cryogenic applications. Typically, these seals use indium alongside or in place of other metal gaskets in stainless-steel vacuum flanges, with some custom applications for flanges sealing directly with glass (optics or tubes). Here, we present the design and performance of three pressed indium seals (99.99% In) between aluminum and 0.5 in. diameter sapphire optics and aluminum and gold coated Kovar semiconductor packages (TO-66 and TO-39). Test fixtures were designed to mimic those of future tunable diode laser spectrometers for Earth, planetary, and manned spaceflight environmental monitoring applications. Successful high-hermeticity seals [<10−10 atm cc/s (He)] were achieved for all seals formed with sufficient pressure applied to allow indium to flow between mating surfaces. The hermeticity of the seals was maintained after temperature cycling (−10 to +80 °C, 20 cycles), with the optical seals surviving extended duration tests (−55 to +85 °C, per MIL-STD-883). Semiconductor packages (TO-39) subjected to these extended tests saw a moderate increase in leak rate [∼5 × 10−9 atm cc/s (He)]; however, further testing showed that either the glass-metal package seals or the indium were affected (the sample size was too small to draw firm conclusions for future applications). Overall, these results suggest long-term survivability of indium seals for Kovar–aluminum and sapphire–aluminum interfaces [>10 years at 10−10 atm cc/s (He)], where the coefficient of thermal expansion differs by approximately four times. [ABSTRACT FROM AUTHOR]

The VUv Laser for Considering Astrophysical and Isolated Molecules (VULCAIMs) setup [Harper et al., Phys. Chem. Chem. Phys. 24, 2777 (2022)] integrates a narrow-bandwidth tunable vacuum ultraviolet (VUV) and extreme ultraviolet (XUV) nanosecond-pulsed laser system (6–16 eV) and a photoelectron spectrometer, designed for recording high-spectral-resolution (rotationally resolved) photoelectron spectra of gas-phase free radicals. This approach usually needs beforehand medium-resolution synchrotron data to guide the selection of specific spectral regions to be investigated at higher resolution with the VULCAIM setup. We present an upgraded version of the VUV laser system integrating an optical parametric oscillator for continuously scanned medium-resolution measurements (< 3 cm−1) across the whole VUV and XUV spectral ranges. This innovation enables broader coverage without the need to access synchrotron facilities. Furthermore, rapid mode switching allows for maintaining optimized radical production conditions from mid-resolution to high-resolution operation mode, enhancing spectroscopy capabilities significantly. The new capabilities of the VULCAIM setup are illustrated on two showcases of photoionization studies: the nitric oxide (NO) stable molecular species and the benzyl (C6H5CH2) free radical produced by pyrolysis. [ABSTRACT FROM AUTHOR]

Cryogenic ion vibrational predissociation (CIVP) spectroscopy is an established and valuable technique for molecular elucidation in the gas phase. CIVP relies on tunable lasers, wherein among typical laser schemes, the application of mid-infrared continuous-wave quantum cascade laser (cw-QCL) is the most robust and elegant solution, as we have recently demonstrated. However, potential challenges arise from an inhomogeneous character across laser power tuning curves. A large laser power output could have undesired consequences, such as multiphoton absorption or saturation effects. Significant variations in laser power tuning curves could potentially alter the shape of the investigated band, particularly for diffuse bands. In this study, we have developed and introduced an automatic variable laser power attenuator designed to keep the laser power output uniform at a user-defined value across the entire available spectral range. We demonstrated the application of this attenuator in obtaining CIVP spectra of a model compound with a diffuse N–H–N band. This approach enhances the reliability of measuring diffuse bands and overall applicability of cw-QCL. [ABSTRACT FROM AUTHOR]

We demonstrated a wavelength discretely tunable laser with outputs at 572.6 nm, 589.1 nm, 603.8 nm, and 611.8 nm using external cavity Ng-cut KGW Raman conversion and intracavity frequency mixing. The multiple visible wavelengths were generated by utilizing this specially cut KGW crystal and the g[mm]g Raman configuration, as well as the temperature-dependent phase-matching characteristic of the LBO crystal. The maximum output powers at 572.6 nm, 589.1 nm, 603.8 nm, and 611.8 nm were measured to be 227 mW, 175 mW, 258 mW, and 103 mW respectively, with nearly fundamental Gaussian beam profiles. This work provides a monolithic setup capable of producing tunable yellow-red laser outputs through Raman and χ 2 frequency conversions for various applications such as air detection, optogenetics, biomedicine, and chemical analysis. [ABSTRACT FROM AUTHOR]

Indium selenide (InSe) crystal, as an emerging Van der Waals semiconductor, showed great potential in optical and optoelectronic devices due to its remarkable electron mobility, flexible direct bandgap, and photoresponsivity. Especially, the bulk single‐crystalline InSe shows superplastic deformability with the aid of the interlayer gliding and cross‐layer dislocation slip. The extraordinary mechanical behavior brings significant opportunities and possibilities to pressure‐modulated optical devices, which, however, have not been explored sufficiently. Additionally, InSe crystals are challenged in the rapid large‐scale preparation and reproduction due to limitations, such as phase heterogeneity, dissociation risks, and chemical instability. Herein, we proposed a fiber drawing technique to create single‐crystalline InSe by the molten core drawing method combined with CO2 laser‐induced recrystallization. The fabricated InSe fiber, spanning several meters in length, shows a uniform and directional single‐crystalline InSe core encased within a 450 µm thick protective borosilicate glass cladding. The inherent core‐cladding configuration of the InSe fibers not only provides robust protection to the InSe crystals but also seamlessly creates a whispering‐gallery‐mode microcavity. Utilizing a 532 nm nanosecond pulsed laser as a pump, a tunable lasing from 1076.2 to 1111.8 nm was achieved, dictated by the controlled pressure within the InSe fiber. We present a simple and effective method for single‐crystalline InSe fabrication and first achieve a pressure‐modulated laser in a near‐infrared band based on InSe fiber, heralding an advancement for the scalable, efficient generation of tunable lasers. [ABSTRACT FROM AUTHOR]

High-sensitivity solutions for real-time and on-line multi-component gas detection are required in coal mine safety, petrochemical and storage and transportation industries, atmospheric and environmental monitoring, combustion diagnosis, and industrial process monitoring and control. In optical technology, Tunable Diode Laser Absorption Spectroscopy (TDLAS) has been proven to be an advanced technology for detection of multi-component gas, which benefits from the high sensitivity, good selectivity, fast response and anti-disturbance of TDLAS sensors. In detection of multi-component gas, the presence of non-negligible cross-interference, cost and performance requirements become a primary concern in the design of many detection schemes. In this review article, the application of TDLAS technology in multi-component gas detection will be discussed in two categories: (I) Multiple lasers for detection of multi-component gas without cross-interference. (II) A single laser for detection of multi-component gas. This article outlines the advantages and disadvantages of applying various schemes in the two categories when TDLAS is applied to multi-component detection, with the aiming of providing ready-made guidance for multi-component detection under various applications. [ABSTRACT FROM AUTHOR]

As global internet traffic continues to increase, technologies for generating high-frequency signals, such as sub-terahertz (sub-THz) bands, through photonics are gaining attention. In this study, we demonstrate the generation of millimeter waves at approximately 17 GHz and sub-THz waves at approximately 300 GHz by converting the frequency difference of a two-wavelength tunable laser, fabricated using silicon photonics, into an optical–electrical signal. This device is expected to be used as a compact and low power consumption, two-wavelength tunable light source for THz wave transceivers. [ABSTRACT FROM AUTHOR]

In order to improve the detection accuracy of CO2 and other gases in the flue gas emitted from thermal power plants, a CO2 concentration detection model based on tunable semiconductor laser absorption spectroscopy was proposed. First, a variational mode decomposition model was used to filter the harmonic signal after removing the outliers to reduce the influence of noise on the detection results. Suitable absorption lines and concentration characteristics were then selected according to the gas absorption properties and correlation theory. Finally, the CO2 concentration inversion was completed using long short-term memory networks, and a Bayesian optimization algorithm was introduced to optimize the hyperparameters of the network. The experimental results showed that the R 2 and RMSE of the test set were 0.998 84 and 0.116 08, respectively, in the concentration range of 1%–12%. In addition, the Allan analysis of variance revealed that the maximum measurement error of CO2 was only 0.005 619% when the integration time was 38 s. Compared to the traditional CO2 detection schemes, the detection accuracy and stability are significantly improved, which provides a feasible scheme for flue gas detection in thermal power plants. [ABSTRACT FROM AUTHOR]

We describe a novel ultrahigh vacuum state-to-state molecule/surface scattering apparatus with quantum state preparation of the incident molecular beam and angle-resolved quantum state detection of the scattered molecules. State-resolved detection is accomplished using a tunable mid-infrared laser source combined with a cryogenic bolometer detector and is applicable to any molecule with an infrared-active vibrational transition. Results on rotationally inelastic scattering of CH4 methane from a Ni(111) surface and NiO(111)/Ni(111) oxide film, obtained by the new apparatus, are presented. Molecules scattering from the oxidized surface, compared to those scattering from the bare nickel surface, are more highly excited rotationally and scatter into a broader distribution of angles. The internal alignment of molecular rotation is in addition found to be stronger in molecules scattering from the bare surface. Furthermore, the maxima of the state-resolved angular distributions shift toward and away from surface normal with increasing rotational quantum number J for the oxidized and bare surface, respectively. Finally, the rotational state populations produced in scattering from the oxidized surface are well-described by a Boltzmann distribution, while those produced in scattering from the bare surface exhibit large deviations from their best-fit Boltzmann distributions. These results point toward a marked enhancement in molecule–surface collisional energy exchange induced by oxidation of the nickel surface. [ABSTRACT FROM AUTHOR]

Due to the significant advantages of methane sensitivity and area-type leakage detection, tunable diode laser absorption spectroscopy (TDLAS) gas detection has been promoted as an effective method for microleakage monitoring in oil and gas stations. However, the output of the TDLAS detector is the integral concentration (IC). Based on the relevant research, the alarm threshold and risk are assessed via the gas concentration rather than the IC. How to evaluate the concentration of leaked gas clouds based on IC from TDLAS detectors is still a challenge. To address this problem, the characteristics of IC and the influence of the laser path, wind speed and leakage rate were studied via computational fluid dynamics (CFD). A neural network classification model (NNCM) was proposed to obtain the probability distribution of the maximal concentration along the laser path (Cmax). The results indicated that the IC is strongly correlated with the Cmax. Considering the accuracy and operability, the NNCM with input features of IC, wind speed and angle of laser path was selected. Field tests showed that the developed model achieved the concentration evaluation of leaked gas clouds. Additionally, the NNCM can also quantify the uncertainty of the results, which avoids misjudgments caused by deviations. [ABSTRACT FROM AUTHOR]

Bismuth (Bi)‐doped glass fibers are being developed into next‐generation broadband amplifiers and tunable lasers. Yet, the well‐developed Bi‐doped fiber devices only realize silica‐based optical fibers prepared by the modified chemical vapor deposition method, which faces challenges such as low doping concentration, high cost, intricate device structure, and high preparation difficulty. Here, a novel highly Bi‐doped multicomponent phosphate glass was developed. The high ion solubility of this phosphate glass facilitates achieving a Bi doping concentration of 8 mol%. The introduction of aluminum nitride (a new reducing agent) can create a local reducing environment, further increasing the concentration of low‐valence near‐infrared (NIR) active Bi ions. Furthermore, the resulting enhanced highly Bi‐doped multicomponent phosphate glass with efficient 900–1600 nm NIR emission can be drawn into corresponding optical fibers by a rod‐in‐tube method. Broadband NIR amplified spontaneous emission with a 3 dB bandwidth of 275 nm was achieved in this new fiber. As far as we know, this is the first successful preparation of Bi‐doped multicomponent phosphate glass fiber. Our results indicate that this fiber will be a powerful alternative to Bi‐doped silica‐based fibers for the preparation of related Bi‐doped fiber devices. [ABSTRACT FROM AUTHOR]

The target gas concentration in TDLAS (Tunable Diode Laser Absorption Spectroscopy) gas detection is nonlinearly related to the photodetector output signal, which is effortlessly disturbed by the temperature and pressure inside the gas absorption cell, and it will decrease the detection accuracy of the system. This paper proposes an SA-PSO-SVM (Simulate Anneal-Particle Swarm Optimization-Support Vector Machine) based temperature-pressure error compensation approach to improve the accuracy of TDLAS gas detection. First, an experimental system concerning the effects of gas temperature-pressure on the absorption line distortion was built. Meanwhile, a method for processing and exploring normalized data about the temperature and pressure effects on the gas concentration was proposed. Second, an SA-PSO-SVM-based model of temperature-pressure compensation for TDLAS gas detection was constructed, and a multi-point correction was performed. Finally, the developed model was compared with traditional BP (Back Propagation), SVM, and PSO-SVM-based temperature-pressure compensation models for TDLAS gas detection, to verify the effectiveness in terms of mean absolute error, mean relative error, and mean square error, which could significantly improve the detection accuracy of TDLAS system. The results provide guidance and lay a technical foundation for the study of multi-component gas cross-aliasing absorption line separation. [ABSTRACT FROM AUTHOR]

A systematic spectroscopic study of the dicarbon molecule C2 has important applications in various research fields, such as astrochemistry and combustion. In the short vacuum ultraviolet (VUV) wavelength region, recent theoretical calculations have predicted many absorption band systems of C2, but only few of them have been verified experimentally yet. In this work, we employed a tunable VUV laser radiation source based on the two-photon resonance-enhanced four-wave mixing method and a time-of-flight mass spectrometer to investigate the absorption bands of C2 in the VUV range of 64 000–66 000 cm−1. The electronic transition 23Σg−(v′)–a3Πu(v″) of C2 has been observed and identified experimentally for the first time. The term value Te for the 23Σg− state is determined to be 66 389.9 ± 0.5 cm−1 above the ground state X1Σg+, and the vibrational and rotational constants are also determined. The experimentally measured spectroscopic parameters in this study are in excellent agreement with the theoretical results based on high-level ab initio calculations. [ABSTRACT FROM AUTHOR]

Because the work function (WF) of the thermionic cathode of an electron gun is the primary factor for determining emission current density, the precise monitoring of the WF in actual operational conditions under high temperatures is essential for tracking the status of the cathode surface. We adjusted a theoretical model of quantum efficiency under high temperatures as proposed by R. H. Fowler to include the final energy state in the photoexcitation process. This modification is necessary for estimating the WF precisely with the measurement results obtained by photoelectron yield spectroscopy using a tunable pulsed laser in the wavelength range from 410 to 709 nm. As the first step of the investigation, we conducted WF measurements of a single-crystal CeB6 cathode, which is employed for the SACLA x-ray free-electron laser. The WF of the unused fresh CeB6 was 2.44 ± 0.02 eV at a high temperature of 836 °C. Our future plans for WF measurements are also discussed below. [ABSTRACT FROM AUTHOR]

In order to address the 'capacity crisis' caused by the narrow bandwidth of the current C band and the demand for wide-spectrum sensing sources and tunable fiber lasers, a broadband luminescence covering the C + L bands using Er3+/Yb3+ co-doped fluorotellurite glass fiber is investigated in this paper. The optimal doping concentrations in the glass host were determined based on the intensity, lifetime, and full width at half maximum (FWHM) of the fluorescence centered at 1.5 µm, which were found to be 1.5 mol% Er2O3 and 3 mol% Yb2O3. We also systematically investigated this in terms of optical absorption spectra, absorption and emission cross-sections, gain coefficients, Judd–Ofelt parameters, and up-conversion fluorescence. The energy transfer (ET) mechanism between the high concentrations of Er3+ and Yb3+ was summarized. In addition, a step-indexed fiber was prepared based on these fluorotellurite glasses, and a wide bandwidth of ~112.5 nm (covering the C + L bands from 1505.1 to 1617.6 nm) at 3 dB for the amplified spontaneous emission (ASE) spectra has been observed at a fiber length of 0.57 m, which is the widest bandwidth among all the reports based on tellurite glass. Therefore, this kind of Er3+/Yb3+ co-doped fluorotellurite glass fiber has great potential for developing broadband C + L band amplifiers, ultra-wide fiber sources for sensing, and tunable fiber lasers. [ABSTRACT FROM AUTHOR]

We explore the principles, implementation details, and performance characteristics of a lensless multi-spectral digital holographic sensor and demonstrate its potential for quality assurance in semiconductor manufacturing. The method is based on capturing multi-spectral digital holograms, which are subsequently utilized to evaluate the shape of a reflective test object. It allows for a compact setup satisfying high demands regarding robustness against mechanical vibrations and thus overcomes limitations associated with conventional optical inspection setups associated with lens-based white light interferometry. Additionally, the tunable laser source enhances the versatility of the system and enables adaptation to various sample characteristics. Experimental results based on a wafer test specimen demonstrate the effectiveness of the method. The axial resolution of the sensor is ± 2.5 nm, corresponding to 1σ. [ABSTRACT FROM AUTHOR]

This work demonstrates the use of an erbium-doped fiber amplifier (EDFA), a tunable bandpass filter (TBF), and a tunable delay line interferometer (TDLI) to form a ring laser that produces multi-format, multi-wavelength laser beams. The TDLI serves as the core of the proposed laser generation system. TDLI harnesses the weak Fabry–Pérot (FP) interferences generated by its built-in 50/50 beamsplitter (BS) with unalterable filtering characteristics and the interferences with free spectral range (FSR) adjustable from each of its two outputs with nearly complementary phases to superpose and generate a variable interference standing wave. The interferometric standing wave and weak FP interferences are used to form a spatial-hole burning to promote the excitation of multi-format and multi-wavelength lasers. The proposed system enables dual-wavelength spacing ranging from 0.3 nm to 3.35 nm, with a switchable wavelength position at approximately 1527 nm to 1535 nm, providing flexible tunability. [ABSTRACT FROM AUTHOR]

Continuously manipulating the resonant wavelength of lasing modes within a large spectral range is of great significance for expanding device functionality. Here, to synthesize a single CsPbClxBr3‐x perovskite microwire with the energy bandgap gradient spanning from 2.33 to 2.83 eV along the length direction defined as the axis by using the vapor‐phase anion exchange method is proposed. A high‐quality (≈103), wide range (≈60 nm), and continuously tunable single‐mode laser is achieved in as‐prepared perovskite microwire alloy, which provides both gain media and microresonator. Simultaneously, the exciton recombination dynamics and atomic‐scale interdiffusion mechanisms at different components are clarified through time‐resolved photoluminescence (PL) spectra and theoretical calculations. The vacancy defects have a significant impact on the interdiffusion of halogen anions, excitonic recombination lifetime, and fluorescence quantum efficiency. The work provides a new strategy for the construction of new‐type broadband tunable lasers and high‐precision microspectrometers. [ABSTRACT FROM AUTHOR]

In this paper, a Raman laser is proposed based on a high-Q silica microcavity, and arbitrary wavelength tuning is realized by fine and coarse tuning. The mechanical tuning range is extended to 5.5 GHz by designing the geometry of the microcavity. Axial discrete tuning in a wideband range is achieved by coupling different transverse modes. The wavelength of the Raman laser is tuned arbitrary in a range of 38.6 nm with the tuning accuracy about 50 MHz. This study extends the application of microcavity Raman laser to the field of tunable lasers and high dynamic range sensors. [ABSTRACT FROM AUTHOR]

Ten to 25 μm‐long‐wavelength mid‐infrared laser has broad application prospects in fields such as military, industrial, medical, and spectroscopy. Currently, lasers capable of emitting 10–25 μm‐long‐wavelength mid‐infrared radiation include CO2 lasers, solid‐state lasers, quantum cascade lasers, infrared supercontinuum light sources, and free electron lasers. This review summarizes the development status of these five types of lasers, providing a detailed analysis and summary of their operating principles, advantages, limitations, and future outlooks. The aim is to offer valuable reference information for related research and to promote the further development and application of tunable mid‐infrared laser at 10–25 μm. [ABSTRACT FROM AUTHOR]

Tunable diode laser spectroscopy (TDLS) is a measurement technique with high spectral resolution. It is based on tuning the emission wavelength of a semiconductor laser by altering its current and/or its temperature. However, adjusting the wavelength leads to a change in emission intensity. For applications that rely on modulated radiation, the challenge is to isolate the true spectrum from the influence of extraneous instrumental contributions, particularly residual intensity and wavelength modulation. We present a novel approach combining TDLS with interferometric techniques, exemplified by the use of a Mach–Zehnder interferometer, to enable the separation of intensity and wavelength modulation. With interferometrically enhanced intensity modulation, we reduced the residual wavelength modulation by 83%, and with interferometrically enhanced wavelength modulation, we almost completely removed the residual derivative of the signal. A reduction in residual wavelength modulation enhances the spectral resolution of intensity-modulated measurements, whereas a reduction in residual intensity modulation improves the signal-to-noise ratio and the sensitivity of wavelength-modulated measurements. [ABSTRACT FROM AUTHOR]

Significant progress has been made in the research of all-optical neural networks in recent years. In this paper, we theoretically explore the properties of a neural system composed of semiconductor ring lasers (SRLs). Our study demonstrates that external optical signals generated by a tunable laser (TL) are injected into the first semiconductor ring laser photonic neuron (SRL1). Subsequently, the responses of SRL1 in the clockwise (CW) and counterclockwise (CCW) directions are unidirectionally injected into the CW and CCW directions of the second semiconductor ring laser photonic neuron (SRL2), respectively, which then exhibits similar spiking inhibition behaviors. Numerical simulations reveal that the spiking inhibition behavior of the SRL response can be precisely controlled by adjusting the perturbation time and intensity of the external injection signal, and this behavior is highly repeatable. Most importantly, we successfully achieve the stable transmission of these responses between the two SRL photonic neurons. These inhibition behaviors are analogous to those of biological neurons, but with a response speed reaching the sub-nanosecond level. Additionally, we indicate that SRL photonic neurons undergo a refractory-period-like phenomenon when subjected to two consecutive perturbations. These findings highlight the immense potential for the design and implementation of future all-optical neural networks, providing critical theoretical foundations and support for them. [ABSTRACT FROM AUTHOR]

We introduce a broadband tunable femtosecond laser source in the long-wave infrared (LWIR) band, covering the range of 5–13.5 μm, based on the integration of optical parametric amplification and difference frequency generation techniques. We utilize a dual-stage tuning method, combined with the high nonlinear coefficient and broadband phase matching range of the BaGa4Se7 crystal, to facilitate significant improvements in spectral coverage and energy efficiency. The laser yields a peak output energy of 43 μJ and maintains energies above 10 μJ across the entire tuning range, with an average power output exceeding 10 mW. The pulse duration at the central wavelength of 8.3 μm is measured at 72 fs full width at half-maximum using the electro-optic sampling method. This LWIR femtosecond laser can be used in many applications, such as molecular fingerprint spectral analysis, ultrafast chemical reaction spectral analysis, materials science, and ultrafast physics research, providing an important research basis for the generation and application of mid-infrared ultrafast laser sources. [ABSTRACT FROM AUTHOR]

The rapid development of emerging technologies observed in recent years, such as artificial intelligence, machine learning, mobile internet, big data, cloud computing, and the Internet of Everything, are generating escalating demands for expanding the capacity density, and speed in next‐generation optical communications. This poses a significant challenge to existing communication techniques. Within this context, the integration of near‐infrared broadband, tunable, and high‐gain luminescent materials into silicon optical circuits or fiber architectures to transmit and modulate light shows enormous potential for advancing next‐generation communication techniques. Here, this review provides an overview of the recent breakthroughs in near‐infrared luminescent epitaxial/colloidal quantum dots, and metal‐active‐center‐doped materials for broadband optical amplifiers and tunable lasers. We also expound on efforts to enhance the bandwidth and gain of these materials‐based amplifiers and lasers, exploring the challenges associate with developing ultra‐broadband and high‐speed optical communication systems. Additionally, the potential applications in Fifth Generation Fixed Networks, integration with 5G and 6G wireless networks, compensation for current Si electronic based CMOS for high computing capability, and the prospects of these light sources for next‐generation optoelectronic devices are discussed. [ABSTRACT FROM AUTHOR]

Silver nanoparticle (Ag NP) pastes become a potential die-attachment material with the increased electronic power density. However, the weakness of bonding interface between sintered Ag NPs and bare Cu substrate limits the applications of the Ag NPs paste, thereby reducing the shear strength of the sintered joint. In this work, ultrafast laser processing is utilized to enhance the bonding strength of the sintered Ag joint by fabricating a microstructure interface. The microstructure dimensions are tunable by controlling laser parameters, and then high-strength joints could be obtained. Different substrate microstructures were constructed, and the enhanced bonding mechanism was analyzed by characterizing the cross section and fracture surface morphologies of joints. The ultrafast laser processing could increase the surface energy of Cu substrates to form a more reliable connection with Ag NPs and more energy required for crack extension with the increasing connection area, thereby resulting in a significant improvement in the shear strength of the Ag NP joints. The patterned microstructures on the Cu substrate using this technique showed improved surface energy and increased number of connection areas on the substrate, showing potential for the use in third-generation semiconductors for highly reliable packaging. [ABSTRACT FROM AUTHOR]

The intrinsic non‐Hermiticity of photonic devices with tunable optical gain and loss makes them excellent platforms to explore and implement applications based on parity‐time symmetry, and a notable example is mode management in micro‐cavity lasers. Thus far, parity‐time‐symmetric lasers are fabricated via conventional top‐down etching processes, which are known to cause cavity sidewall roughness that is potentially detrimental to laser performance. Bottom‐up fabrication of parity‐time‐symmetric lasers, however, has seen limited success due to strict requirements on the uniformity of cavity morphology. Here, parity‐time‐symmetric lasing is demonstrated in coupled InP micro‐ring cavities grown directly by selective area epitaxy. With a facet engineering technique, ring laser cavities with a highly deterministic morphology are realized, enabling parity‐time‐symmetric laser designs. Furthermore, benefiting from the versatility and controllability of this bottom‐up process, lasing mode selectivity can be enhanced through coupling gap tuning and cavity shape engineering, leading to single‐mode lasing with a peak side mode suppression ratio exceeding 17 dB and a threefold single‐mode brightness enhancement compared to a single, uncoupled laser cavity. This work unlocks a lasing mode management strategy that is previously unavailable to bottom‐up laser cavities, which is a major step toward the realization of on‐chip, low‐loss, and single‐mode micro‐cavity lasers. [ABSTRACT FROM AUTHOR]

We realize the electromagnetic regulation of nonreciprocal microwave transmission by introducing a pump-induced magnon mode (PIM) into a cavity magnonic device with dissipative photon–magnon coupling. As a peculiar spin wave, the PIM's dynamic properties, including its spin number and resonant frequency, can be easily tuned by the microwave pump. Hence, it facilitates the precise control of the coupling process between the PIM and the cavity magnonic device by regulating the pump signal. Along with these manipulations, the nonreciprocal microwave transmission produced by the dissipative photon–magnon coupling is regulated without reconfiguring the system. In the experiment, we achieve a pump-controlled nonreciprocal bandwidth of 16 MHz and a pump-tunable isolation range of up to 40 dB. Our work demonstrates the control of a microwave with another microwave. It has a great potential in the design of fast microwave switches and programmable isolators for information processing. [ABSTRACT FROM AUTHOR]

Direct laser writing method is a promising technique for the large‐scale and cost‐effective fabrication of periodic nanostructure arrays exciting hybrid lattice plasmons. This type of electromagnetic mode manifests a narrow and deep resonance peak with a high dispersion whose precise controllability is crucial for practical applications in photonic devices. Here, the formation of differently shaped gold nanostructures using the direct laser writing method on Au layers of different thicknesses is presented. The resonance peak is demonstrated to be highly dependent on the shape of the structures in the array, thus its position in the spectra, as well as the quality, can be additionally modulated by changing the morphology. The shape of the structure and the resonance itself pertain not only on the laser pulse energy but also on the grating period. This overlapping effect occurring at distances smaller than the diameter of the focused laser beam is studied in detail. By taking advantage of the highly controllable plasmonic resonance, the fabricated gratings open up new opportunities for applications in sensing. [ABSTRACT FROM AUTHOR]

Tunable laser spectroscopy (TLS) with infrared (IR) imaging is a powerful tool for gas leak detection. This study focuses on direct absorption spectroscopy (DAS) that utilizes wavelength modulation to extract gas information. A tunable interband cascade laser (ICL) with an optical power of 5 mW is periodically modulated by a sawtooth injection current at 10 Hz across the methane absorption around 3271 nm. A fast and sensitive thermal imaging camera for the mid-infrared range between 3 and 5.7 µm is operated at a frame rate of 470 Hz. Offline processing of image stacks is performed using different algorithms (DAS-F, DAS-f and DAS-2f) based on the Lambert–Beer law and the HITRAN database. These algorithms analyze various features of gas absorption, such as area (F), peak (f) and second derivative (2f) of the absorbance. The methane concentration in ppm*m is determined on a pixel-by-pixel analysis without calibration. Leak localization for methane leak rates as low as 0.25 mL/min is accurately displayed in a single concentration image with pixelwise sensitivities of approximately 1 ppm*m in a laboratory environment. Concentration image sequences represent the spatiotemporal dynamics of a gas plume with high contrast. The DAS-2f concept demonstrates promising characteristics, including accuracy, precision, 1/f noise rejection, simplicity and computational efficiency, expanding the applications of DAS. [ABSTRACT FROM AUTHOR]

Adaptive photonic films in response to external stimuli have broad applications in optical communications, sensing, and anticounterfeiting. Yet, it remains challenging to develop photonic structures with tailorable fine patterns that display broadband color changing under ambient conditions. Here a hydrogen‐bonded supramolecular cholesteric liquid crystalline polymer (CLCP) that is mediated by a binary solvent consisting of citric acid (CA) and water is presented. The incorporation of CA not only improves the long‐range order of CLCPs through evaporation‐induced self‐assembly but is also capable of tuning their helical pitch across the entire visible spectrum. The extent of hydration‐induced pitch expansion can be further manipulated by thermal crosslinking, enabling a unique patterning strategy based on mask‐free, programmable laser inscription. High‐precision photonic patterns are created onto CLCPs, which reveal themselves upon hydration in high visual contrast. The study offers a feasible and up‐scaling route toward tailoring environmentally adaptive CLCPs with tunable broadband coloration and highly programmable photonic patterns. [ABSTRACT FROM AUTHOR]

Many optical applications, including free-space optical communications, lidar, and astronomical measurements, are impacted by the presence of light-scattering particles also known as obscurants. Scattering from particles consisting of sand, dust, dirt, and other substances can significantly degrade optical signals. For many obscurants, the index of refraction is dependent on the wavelength of light, and there exists a Christiansen wavelength (λc) at which scattering is at a minimum. At λc the index of refraction of the scattering particles (ns) matches that of the surrounding medium, in this case air (with refractive index na). This condition makes the scattering particulates almost invisible to the propagating light, minimizing scattering and increasing transmission at λc. Previously, the authors showed a technique for measuring the index of refraction n (λ) and the extinction coefficient k (λ) using spectroscopic ellipsometry for various sand samples. Spectroscopic measurements on static sand samples demonstrated good agreement with the predicted spectral properties and highlighted the presence of a Christiansen feature near 8 µm. However, in outdoor environments, the scattering particles are never stationary but in a constant state of motion. In this work, spectroscopic measurements on dynamic sand samples (sand that is falling through the optical beam path) show two Christiansen features seen previously in predicted and observed static sand measurements. Additionally, we characterize, for the first time, transmission around a Christiansen feature using a tunable laser and show results consistent with other spectroscopic measurements. [ABSTRACT FROM AUTHOR]

Wavelength‐tuneable lasers can play a crucial role across many fields, and their applications continue to expand with advancements. Cholesteric liquid crystal (CLC) devices are developed, enabling good wavelength‐tuneable lasers. In CLC devices, pitch control is required to tune the desired wavelength. However, pitch control takes a long time and is difficult, which limits the speed and range of laser wavelength tuning. Here, by applying a voltage once and forming an electric field gradient along the wedge direction in the CLC wedge cell, we could realize CLC pitch gradient arrays that achieve continuous wavelength tuning over 500 nm in only 10 min through the electrothermal effect while continuously generating laser lines in the full visible spectrum. The laser wavelength could be rapidly tuned over the full visible spectrum by moving within the array in the wedge cell. The study of CLC cells for electric field application in real time is able to observe the dynamic process of pitch change directly. This new laser device and strategy has potential in many fields that require active laser tuning and is expected to lead to innovative changes in the development of future practical laser devices in the optical industry. [ABSTRACT FROM AUTHOR]