MXene, a new two-dimensional absorption material, holds promise in various fields of photovoltaic and energy storage, while high-performance MXene-based solar absorbers have rarely been implemented. In this study, by constructing a pagoda-shaped nanodisk structure, an average absorptivity of up to 99.83

Owing to their inherent high-speed acquisition, ultra-short pulses are the major provider of big data in communications, metrology, spectroscopy, imaging and sensing. This has paved the way for various artificial intelligence and machine learning applications. The development of ultra-fast optical time-domain transformation (UO-TDT) techniques — specifically, time-stretched dispersive Fourier transform, temporal imaging and time-stretch imaging — has revolutionized photonic information acquisition. Dispersion-based frequency-to-time transformation of ultra-short pulses enables optical signal spectra to be converted into the time domain for ultra-fast spectral characterization. High-speed acquisition of information encoded on optical spectra means that new measurement techniques can effectively converge photonic and digital technologies. This Primer introduces the fundamental concepts and experimental set-up of various UO-TDT techniques with the associated transformation mechanisms. Recent advances in UO-TDT for various applications, from spectroscopy to velocimetry, and novel breakthroughs in ultra-fast imaging or quantum science are focused on. A range of experimental results are discussed, alongside an examination of reproducibility and limitations of the methods. Finally, a perspective on promising emerging techniques is provided. Ultra-fast optical time-domain transformation (UO-TDT) techniques are used to encode spatial and spectral information from ultra-short pulses into the time domain. In this Primer, Zhang et al. cover the physical and mathematical basis of UO-TDT and its application in various imaging platforms, and conclude with an outlook on how these techniques could be applied to novel applications and imaging modalities.

Designing novel acetic acid gas sensors is highly imperative for human health. Two-dimensional (2D) layered MXene Ti3C2Tx is becoming an emerging and promising material in gas sensing. In this manuscript, the hydrothermal method was used to synthesize MXenes Ti3C2Tx/Nb2CTx supported NiCo2O4-MnO2 composites and pure materials. The structure, chemical composition and morphology of the samples were studied by SEM, EDS, TEM, HRTEM, XRD, BET, FTIR, UV-visible, XPS and Raman, justifying the successful synthesis of products. The layered structure of Ti3C2Tx enhanced the BET surface area and provided sufficient sites, which assisted the gas sensing improvement. The gas sensors were fabricated from synthesized products and were tested for different kinds of VOCs deeply. The results exposed that the gas sensor of Ti3C2Tx-NiCo2O4-MnO2 (5% of Ti3C2Tx=NCO-Mn-Ti-5) was highly sensitive to 20 ppm acetic acid and very less responsive to all other VOCs (acetone, TMA, ethanol, methanol, formaldehyde, acetaldehyde, acetylene and xylene) at room temperature. The response (Rg/Ra) to 20 ppm acetic acid was 12.5 and the lowest detection limit was 0.05 ppm. Additionally, the sensor of NCO-Mn-Ti-5 revealed great stability/reproducibility, short response/recovery times and linearity between acetic acid concentration and response. The idea of the novel sensor (NCO-Mn-Ti-5) could be potentially useful in the field of sensors.

Multiple resonant metasurfaces for mixture component detection by surface-enhanced infrared absorption in a broadband spectrum region are still highly desirable. This paper presents a polarization-sensitive multi-resonant broadband metasurface in the mid-infrared (MIR) region based on an odd symmetric folded antenna array. Six major resonant modes are effectively excited in 6-15 mu m covering the absorption peak of multiple gases and biomolecules. In addition, the calculation indicates the maximum electric field intensity enhancement reaches 3607 and the average electric enhancement factors are dominantly amplified to a maximum of 75.08. The excited multiple hotspots provide more spatial overlap with analytes and would be beneficial for label-free detection. Moreover, owing to the contributions of the intrinsic phonons and polaritons in the ENZ material, the spectroscopy features are relatively stable when changing the geometry parameters and the incident angles, allowing more fabrication tolerance and experiment flexibility. Furthermore, the multiple resonant metasurface is also effectively enabled under orthogonal circular polarizations. Finally, the sensing abilities of the designed metasurface for polyethylene terephthalate and isopropanol molecules have been computationally validated. These results indicate the potential for nanophotonic detection and mixture spectroscopy analysis.

In this paper, an agarose coated-fiber Bragg grating (AG-FBG) based respiratory rate (RR) sensor for evaluating respiratory function and health status by all-fiber strain-induced humidity-sensitive material is proposed and experimentally demonstrated. The variation of the environment humidity leads to changes in the film morphology of AG, i.e., expansion and contraction, which impose strains on FBG to dynamically shift the central wavelength. In the experiment, the average sensitivity of the proposed AG-FBG-based RR sensor is 23.6 pm/%RH in the range of 30 %RH-90 %RH. Under the monitoring of a commercial FBG demodulator with a wavelength resolution of 1 pm, rich human breathing information is collected. The RR monitoring under different human postures and breathing patterns are respectively measured. In one breathing cycle, the response time and recovery time are 778 ms and 762 ms, respectively. The proposed sensor is simple and low-cost, shows easy signal demodulation, stable signal wavelength variation, and high repeatability, which has potential applications in vital signs monitoring and medical treatment.

Ultrafast humidity interrogation is valuable in industrial production, healthcare, and environmental monitoring due to its fast response and high sensitivity. We present an ultrafast humidity sensor based on an agarose-coated fiber Bragg grating (AG-FBG) structure combined with a time-stretching method. Leveraging AG's ability to swell or contract in response to humidity variations results in changes to the effective grating period, which in turn causes a shift in the Bragg wavelength. Using a homemade high-power femtosecond all-polarization-maintaining fiber laser, the time-stretching technique can be achieved through a long dispersion-compensating fiber. According to wavelength-to-time mapping, the wavelength shift associated with humidity response can be converted into the time domain, resulting in a humidity sensitivity of -0.017 ns/% relative humidity (RH) at an interrogation speed of 76.75 MHz and a humidity resolution of 0.007%RH. The proposed humidity monitoring system has demonstrated high stability and repeatability. Therefore, with the advantages of high sensitivity, high accuracy, and ultrafast response time, our proposed humidity sensor shows significant potential in the field of real-time monitoring.

We have experimentally proposed an all-fiber temperature-insensitive twist sensor, which is fabricated by splicing a three-core fiber (TCF) section and two parts of single-mode fibers (SMFs). Because of the special structure of TCF, the fundamental core mode and cladding mode can interfere with each other. The experimental results show that the maximum twist sensitivity is-0.250 nm/(rad/m) in the range of 0-50 rad/m, and 0.109 nm/(rad/m) in the range of -50-0 rad/m. The sensor has features of all-fiber structure, low-cost, easy-fabri-cated, and temperature insensitive, which makes it a competitive candidate for harsh environment sensing application.

The occurrence of periodically oscillatory behaviors in various nonlinear dissipative systems is known as breathing dynamics, which holds potential applications in spectroscopy. Herein, we have employed both experimental and numerical methods to investigate the breathing dynamics of dissipative soliton induced by Q-switched modulation in a mode-locked fiber laser. The findings indicate that the dynamics of breathing significantly influences the buildup stage of both stationary and breathing dissipative solitons. The breathing rate, breathing duration and breathing period are quite sensitive to the pump power. Numerical simulations are employed to generate similar breathing dynamics within the context of a model that combines equations describing the population inversion in a mode-locked laser. These obtained results provide confirmation that the Q-switching modulation is responsible for the initiation of breathing oscillations, and present novel prospects for the development of different operational frameworks for ultrafast lasers.

MXene is promising in photothermal or photovoltaic conversion, while high-performance MXene metasurface solar absorbers based on simple and feasible structures are still lacking. This study aims to design a solar absorber with ultra-broadband absorption capability in the visible and near-infrared wavelength ranges based on the MXene nanoblock tetramer/silica film/MXene substrate structure. The average absorptivity of this proposed metasurface absorber is 96.9% in the wavelength range of 300-2500 nm covering the whole solar spectrum. The physics behind the high absorption results from multiple-mode hybridization in different resonant bands, including the coupling between the surface plasmons, cavity resonances, and guided-mode resonances. The broadband and high-absorption performance remains stable under large-angle incidence and structural parameter variations with the average absorption above 90% in the whole wavelength region of interest. The calculated energy absorption ratio of the AM1.5 solar radiation spectrum can reach up to 96.3%, indicating low solar energy loss and efficient solar energy capture. In summary, these results provide great application prospects in the fields of photothermal and photovoltaic conversion. (c) 2024 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.

Hydrogen sulfide (H2S) gas sensors with high response and minimum limit of detection at room temperature are of great importance to ensure the safety of humans and the environment. A series of samples such as MXene Ti3C2, Ti3AlC2, WS2, MoSe2/Zn2SnO4 and spherical Zn2SnO4 nanoparticles were synthesized via hydrothermal method in this paper. Different characterizations were performed on these samples to check crystal structures, morphologies, chemical states, etc. And gas sensing properties were also performed and analyzed deeply. A series of sensor sheets were fabricated via the spin coating method based on synthesized samples for gas sensing properties. MXene Ti3C2/Zn2SnO4-5 based gas sensor detected the highest response (110) towards 8 ppm H2S with excellent selectivity, the minimum limit of detection (0.01 ppm), mediate long-term stability and good reproducibility compared with other fabricated sensors at room temperature. Also, the sensor response was increased with the increase of H2S concentration. Experimental results showed that the highest response of Ti3C2/Zn2SnO4-5 based gas sensor was accredited to heterojunction, higher BET surface area, and increased oxygen species. The simple way synthesized nanocomposite and fabricated sensors throw a novel idea for tracing H2S at the lowest concentration to prevent different diseases.

The real-time interrogation of optical fiber sensors is highly desirable in a wide range of applications that require precise and dynamic measurements. Herein, a novel real-time bending interrogation based on a strong-coupled seven-core fiber (SC-SCF) structure and time-stretch technique has been experimentally demonstrated. The proposed SC-SCF-based Mach-Zehnder interferometer (MZI) plays a role of comb filter with the advantage of low insertion loss and highly sensitive bending response. The time-stretch technique is accomplished using a wideband and high-power femtosecond all-PM fiber laser source that based on the self-similar amplification method. Due to the mechanism of wavelength-to-time mapping, real-time spectrum-encoded waveform of bending response can be obtained at an interrogation speed of 79.23 MHz. By monitoring the time shift of bending response, a bending sensitivity of -2.19629 ns/m(-1) and a bending resolution of 56.914 mu m(-1) can be achieved. Furthermore, the compact size and high sensitivity of our real-time interrogation technique give it a competitive edge in the fields, where fast measurements are crucial.

An open-cavity optical fiber gas pressure sensor based on the Vernier effect is proposed. The sensor consists of two Fabry-Perot interferometers (FPIs) connected in parallel. As a sensing FPI, FPI-1 is an open-cavity FPI and composed of single-mode fiber (SMF), hollow core capillary (HCC), and twin-hole and dual-core fiber (THDCF). The open-cavity FPI structure is constructed using the air holes of THDCF. FPI-2 consists of SMF and HCC with a large inner diameter and works as a reference FPI. The manufacturing cost of the sensor is low and easy to manufacture. This study explored the relationship between the magnification factor and the cavity lengths of the sensing FPI and the referencing FPI. After optimizing, the sensor has a sensitivity of -168.82 nm/MPa in the gas pressure range of 0.1-1.6 MPa, which is 48.37 times the sensing sensitivity of single FPI. In addition, the temperature crosstalk of the sensor is 2.285 x 10(-4) MPa/degrees C. Repeated experiments have demonstrated that the sensor has good repeatability and stability. Therefore, the proposed sensor structure provides a new idea for the design and application of optical fiber gas pressure sensors with ultrahigh sensitivity and lower temperature cross sensitivity.

Respiratory rate monitoring has received widespread attention in medical field. Complex, expensive, and bulky commercial equipment limits the monitoring cost. To take full advantage of lab-on fiber technologies, we present a novel, compact size lab-on fiber breathing monitoring sensor based on a tapered no-core fiber (NCF) structure. The sensor is composed of humidity-sensitive polymer materials, single mode fiber (SMF)-NCF-SMF structure and monitors human respiratory rate by measuring exhaled humidity. The theoretical results prove that the self-imaging phenomenon can be induced by the changing of NCF diameter. The static relative humidity test results show a maximum relative humidity sensitivity of 0.0438 nm/%RH in the range of 35-58 %RH. Through the single-wavelength intensity fluctuation experiment, human respiratory information is collected. In the breathing process, a response time of 0.67 s can be achieved. The respiratory rate monitoring under different heart rates, breathing patterns, and human postures displays real-time tracking and high repeatability. Moreover, the compact size, low cost, and high sensitivity make our lab-on fiber sensor more competitive in the field of medical treatment and our daily life.

The utilization of the dispersive Fourier transformation approach has enabled comprehensive observation of the birth process of dissipative solitons in fiber lasers. However, there is still a dearth of deep understanding regarding the extinction process of dissipative solitons. In this study, we have utilized a combination of experimental and numerical techniques to thoroughly examine the breathing dynamics of dissipative solitons during the extinction process in an Er-doped mode-locked fiber laser. The results demonstrate that the transient breathing dynamics have a substantial impact on the extinction stage of both steady-state and breathing-state dissipative solitons. The duration of transient breathing exhibits a high degree of sensitivity to variations in pump power. Numerical simulations are utilized to produce analogous breathing dynamics within the framework of a model that integrates equations characterizing the population inversion in a mode-locked laser. These results corroborate the role of Q-switching instability in the onset of breathing oscillations. Furthermore, these findings offer new possibilities for the advancement of various operational frameworks for ultrafast lasers.

With the rapid development of physical ocean observation, high-performance pressure sensor integrated with stable mechanical strength, sufficient pressure sensitivity as well as negligible temperature crosstalk is highly desirable but still lack of research. This paper proposes an all-fiber optical sensor based on a Mach-Zehnder interferometer (MZI) for hydrostatic pressure measurement, utilizing a multi-mode fiber (MMF)-single-hole-dual-core fiber (SHDCF)-MMF structure. The MMF sections act as a beam splitter and coupler to enhance the Mach-Zehnder interference. The SHDCF serves as the pressure-sensing section. Under the action of the enclosed air hole in SHDCF, hydraulic variations cause changes in the structural parameters of the fiber, especially the refractive index change caused by the elasto-optical effect, which leads to the shift of the interference dips. The experimental results prove that the SHDCF-based MZI sensor has a higher hydrostatic pressure sensitivity than other hole-less silica fiber-based sensors. The max pressure sensitivity is-0.2358 nm/MPa in the range of 0.1 MPa to 16.1 MPa with good linearity and reliability. Moreover, the low temperature-pressure cross-sensitivity of 0.04368 MPa/celcius also proves our sensor more competitive in harsh environment applications.

In the paper, an optical fiber sensor based on a seven-core fiber composite structure is presented, which enables dual-parameter sensing of bending and temperature. The proposed structure is fabricated by combining the strongly-coupled seven-core fibers (SC-SCFs) and a weakly-coupled seven-core fiber (WC-SCF). The SC-SCF acts as a beam coupler and enhances the Mach-Zehnder interference, while the WC-SCF serves as the enhanced section of another Mach-Zehnder interference. Therefore, the spectrum response of the fiber structure mentioned above exhibits a superposition effect of two Mach-Zehnder interferometers (MZIs). Among them, two dips corresponding to different MZIs are used to measure bending and temperature. The experimental results show the bending sensitivity and temperature sensitivity of the two MZIs are −4.238 nm/m−1, −2.263 nm/m−1, 0.047 nm/°C, and 0.064 nm/°C, respectively. It proves that our sensor is very sensitive to bending. Through the dual-wavelength matrix method, the bending and temperature can be measured simultaneously. With the benefit of the composite structure, low cost, and ease of fabrication, the proposed sensor can be used in harsh environments.

Single-walled carbon nanotube-polyvinyl alcohol (SWNT-PVA) composite film, which can be used as a saturable absorber, has exhibited an excellent nonlinear optical property, especially for ultrafast photonics application. However, the main issue is its low damage threshold, which can be attributed to the agglomeration phenomenon of carbon nanotube (CNT) and the thermal cumulation effect. Herein, we proposed a simple thermal treatment method for enhancing the damage threshold and nonlinear optical property of SWNT-PVA composite film. By subjecting the composite film to appropriate heating, the surface area of the film is significantly enlarged, thereby reducing the agglomeration of CNTs. The thermal treatment procedure not only improves the surface characteristics but also ensures an enhancement of damage threshold with time and power, respectively. Within a specific temperature range, the modulation depth and saturable intensity exhibited enhancements, whereas the non-saturable loss decreases owing to the thermal treatment. This work not only presents a straightforward approach to improve the nonlinear optical property of CNT-PVA based saturable absorber for mode-locking fiber lasers, but also offers valuable insights for optimizing the performance of other low-dimensional nanomaterials based on carbon elements for the application of ultrafast photonics.

Ultrafast fiber laser, a vital tool in both science and industry, exhibits two distinct pulse states: the steady soliton (SS) and the breathing soliton (BS). While these states have been extensively studied individually, understanding the complex transition between them is crucial for controlling lasing states effectively. Herein, our experimental observations reveal an intermediate state that toggles between SS and BS, enabled by the dispersive Fourier transform technique. We find that energy hop and decaying breathing processes, driven respectively by the energy quantization effect and Q-switched modulation, govern this transition. Additionally, we observe that the transition between different BS states primarily involves a pure decaying breathing process. Numerical simulations are used to generate similar transition dynamics in a model that combines equations describing the population inversion in a mode-locked laser. This study sheds light on the transition dynamics in non-equilibrium systems, offering insights for intelligently manipulating lasing states.

To take full advantage of lab-on-fiber technologies for gas sensing, we propose a reflective Fabry-Perot interferometer (FPI) CO 2 gas sensor composed of the functional material (polyhexamethylene biguanide, PHMB) and a single-mode fiber (SMF)-hollow core capillary (HCC) structure. The theoretical analysis of the FPI cavity mode in the PHMB film is discussed. The experiment results show a maximum sensitivity of 286 pm/ppm in the low concentration region of 0-1000 ppm detection range and a high sensitivity of 6.88 pm/ppm in the atmosphere CO 2 concentration range (200-600 ppm). The response time of the CO 2 sensor is about 10 min. Repeated experiments show that the sensor has good sensing stability and a high selectivity for CO 2 gas. In addition, the sensitivity of our CO 2 sensor is stable to temperature changes. Moreover, the results of monitoring the real in-door atmosphere verify the repeatability and desirable sensing performance of our sensor. This reflective lab-on-fiber CO 2 sensor has a concise structure, low cost, and high sensitivity possessing more possibilities in various and complex applications.

Compressive spectral imaging (CSI) obtains spatial-spectral information from under-sampled measurements, which solves the contradiction between imaging speed and resolution in conventional scanning-based spectral imaging systems. The spatial-spectral modulation and the corresponding sparse prior in reconstruction algorithm jointly determine the reconstruction quality of CSI systems. In this paper, a compressive single-pixel spectral imaging system with the spatial-spectral modulation and the sparse prior simultaneously optimized by coherence minimization is proposed. By formulating the modulation of the single-pixel spectral imaging system and the sensing coherence in the differentiable matrix notation, the gradients of the modulation and the sparse prior with respect to the sensing coherence are derived for both spatial and spectral dimensions. The spatial-spectral modulation and the sparse prior are optimized via gradient descent to minimize the sensing coherence, for both the decoupled CSI systems with one dimension completely sampled and the coupled CSI system with both spatial-spectral dimensions compressively sampled. For the optimized spatial-spectral modulation, high relative mutual differences in the spectral dimension as well as low redundancy patterns in the spatial dimension are achieved, and by combining with the optimized spatial-spectral sparse prior, the enhancement of 5.7 dB for PSNR compared to the unoptimized system is realized.