Remote or standoff detection of greenhouse gases, air pollutants, and biological agents with innovative ultrafast laser technology attracts growing interests in recent years. Hybrid femtosecond/picosecond coherent Raman spectroscopy is considered as one of the most versatile techniques due to its great advantages in terms of detection sensitivity and chemical specificity. However, the simultaneous requirement for the femtosecond pump and the picosecond probe increases the complexity of optical system. Herein, we demonstrate that air lasing naturally created inside a filament can serve as an ideal light source to probe Raman coherence excited by the femtosecond pump, producing coherent Raman signal with molecular vibrational signatures. The combination of pulse self-compression effect and air lasing action during filamentation improves Raman excitation efficiency and greatly simplifies the experimental setup. The air-lasing-assisted Raman spectroscopy was applied to quantitatively detect greenhouse gases mixed in air, and it was found that the minimum detectable concentrations of CO2 and SF6 can reach 0.1% and 0.03%, respectively. The ingenious designs, especially the optimization of pump-seed delay and the choice of perpendicular polarization, ensure a high detection sensitivity and signal stability. Moreover, it is demonstrated that this method can be used for simultaneously measuring CO2 and SF6 gases and distinguishing 12CO2 and 13CO2. The developed scheme provides a new route for high-sensitivity standoff detection and combustion diagnosis.

https://downloads.spj.sciencemag.org/ultrafastscience/2022/9761458.pdf

High-Sensitivity Gas Detection with Air-Lasing-Assisted Coherent Raman Spectroscopy

Herein, inspired by the tactile perception architecture of human skin, including the epidermis and touch receptor, skin-like flexible pressure sensors were fabricated via the assembly of a sandwich structure composed of an encapsulation layer (bionic epidermis), a piezoresistive sensing layer (bionic touch receptor), and a transducer layer. The piezoresistive sensing layer composed of graphene oxide/ hydroxyl functionalized carbon nanotubes/bovine serum albumin (GO/CNTs/BSA) nanocomposites with wavy microstructures was fabricated using layer-by-layer (LBL) self-assembly technology. The mechanisms of hydrogen bond and electrostatic interactions may improve the adhesion efficacy of the textile substrate with conductive nanomaterials. Based on the skin-like architecture for pressure sensing, the sensor exhibited the sensing characteristics, including wide pressure-detecting range (0∼171 kPa), high sensitivity (2.456 kPa−1), rapid response/recovery time (225 ms and 50 ms), and outstanding durability (2000 cycles). Our work provide references for textile-based electronic devices that can be applied to intelligent robotics, wearable monitoring, and man–machine interaction. 

Bioinspired sandwich-structured pressure sensors based on graphene oxide/hydroxyl functionalized carbon nanotubes/bovine serum albumin nanocomposites for wearable textile electronics

Flexible piezoresistive pressure sensors have been attracted a lot of attention due to their simple mechanism, easy fabrication, and convenient signal acquisition and analysis. Herein, a new flexible piezoresistive sensor based on microstructured electrospun rough polyurethane (PU) nanofibers film is assembled. The microstructured PU film with tiny bumps is prepared in one step via electrospinning technology, which imparts a microstructured rough upper surface and a smooth lower surface. With this unique microstructure, we have made it possible for PU/Ag films to serve as sensing layers for piezoresistive sensors by introducing a silver conductive layer on the surface of electrospun PU film. The fabricated piezoresistive pressure sensor delivers high sensitivity (10.53 kPa−1 in the range of 0–5 kPa and 0.97 kPa−1 in the range of 6–15 kPa), fast response time (60 ms), fast recovery time (30 ms), and long-time stability (over 10,000 cycles). This study presents a fabrication strategy to prepare the microstructured PU nanofiber film using electrospinning technology directly, and the as-developed sensor shows promise in applications such as wrist pulse measurement, finger movement monitoring, etc., proving its great potential for monitoring human activities.

/pdf/flexible-piezoresistive-pressure-sensor-based-on-electrospun-35xfafj0.pdf

Flexible Piezoresistive Pressure Sensor Based on Electrospun Rough Polyurethane Nanofibers Film for Human Motion Monitoring

Femtosecond laser filamentation is a unique nonlinear optical phenomenon when high-power ultrafast laser propagation in all transparent optical media. During filamentation in the atmosphere, the ultrastrong field of 1013–1014 W/cm2 with a large distance ranging from meter to kilometers can effectively ionize, break, and excite the molecules and fragments, resulting in characteristic fingerprint emissions, which provide a great opportunity for investigating strong-field molecules interaction in complicated environments, especially remote sensing. Additionally, the ultrastrong intensity inside the filament can damage almost all the detectors and ignite various intricate higher order nonlinear optical effects. These extreme physical conditions and complicated phenomena make the sensing and controlling of filamentation challenging. This paper mainly focuses on recent research advances in sensing with femtosecond laser filamentation, including fundamental physics, sensing and manipulating methods, typical filament-based sensing techniques and application scenarios, opportunities, and challenges toward the filament-based remote sensing under different complicated conditions.

/pdf/sensing-with-femtosecond-laser-filamentation-2f41i3e2.pdf

Sensing with Femtosecond Laser Filamentation

Terahertz radiation with a Bessel beam profile is demonstrated experimentally from a two-color laser filament in air, which is induced by tailored femtosecond laser pulses with an axicon. The temporal and spatial distributions of Bessel rings of the terahertz radiation are retrieved after being collected in the far field. A theoretical model is proposed, which suggests that such Bessel terahertz pulses are produced due to the combined effects of the inhomogeneous superluminal filament structure and the phase change of the two-color laser components inside the plasma channel. These two effects lead to wavefront crossover and constructive/destructive interference of terahertz radiation from different plasma sources along the laser filament, respectively. Compared with other methods, our technique can support the generation of Bessel pulses with broad spectral bandwidth. Such Bessel pulses can propagate to the far field without significant spatial spreading, which shall provide new opportunities for terahertz applications.

https://downloads.spj.sciencemag.org/ultrafastscience/2022/9870325.pdf

Bessel Terahertz Pulses from Superluminal Laser Plasma Filaments

Laser ignition (LI) allows for precise manipulation of ignition timing and location and is promising for green combustion of automobile and rocket engines and aero-turbines under lean-fuel conditions with improved emission efficiency; however, achieving completely effective and reliable ignition is still a challenge. Here, we report the realization of igniting a lean methane/air mixture with a 100% success rate by an ultrashort femtosecond laser, which has long been regarded as an unsuitable fuel ignition source. We demonstrate that the minimum ignition energy can decrease to the sub-mJ level depending on the laser filamentation formation, and reveal that the resultant early OH radical yield significantly increases as the laser energy reaches the ignition threshold, showing a clear boundary for misfire and fire cases. Potential mechanisms for robust ultrashort LI are the filamentation-induced heating effect followed by exothermal chemical reactions, in combination with the line ignition effect along the filament. Our results pave the way toward robust and efficient ignition of lean-fuel engines by ultrashort-pulsed lasers.

/pdf/robust-and-ultralow-energy-threshold-ignition-of-a-lean-2pujot2p52.pdf

Robust and ultralow-energy-threshold ignition of a lean mixture by an ultrashort-pulsed laser in the filamentation regime

Highly sensitive and cost-effective flexible piezoresistive sensors are pursued as essential components of electronic skin (e-skin) for a variety of applications such as soft robotics and body prosthesis. A common strategy for achieving high sensitivity of these sensors is to fabricate their electrode material with complex 3-D microstructures, but manufacturing cost-effective micro-structured sensors with high reliability and reproducibility is a challenge. Herein, a robust approach for producing high-reproductive microstructured polydimethylsiloxane (PDMS) films composing the sensors is proposed, based on silicon molds with a honeycomb-like architecture fabricated rapidly and cost-effectively at a standoff distance by femtosecond laser pulses in the self-channeling regime. Single-walled carbon nanotubes (SWNTs) are embedded in the micro-structured PDMS films composing sandwich-structural flexible piezoresistive sensors. The sensing devices exhibit good performance with high sensitivity, fast response time, and excellent stability and are used with success for the detection of hand motion pressure and blood pressure at the wrist, thus showing a great potential as components of e-skin for detection of various human motions.

Piezoresistive Electronic-Skin Sensors Produced With Self-Channeling Laser Microstructured Silicon Molds

Engineering black titanium dioxide by femtosecond laser filament

We propose a simple pump-coupling-seed scheme to examine the optical X2Σg +-A2Πu coupling in N2 + lasing. We produce the N2 + lasing at 391 nm, corresponding to the B2Σu +(v'=0) -X2Σg +(v=0) transition, by externally seeding N2 + gain medium prepared by irradiation of N2 with an intense pump pulse. We then adopt a weak coupling pulse in between the pump and seed pulses, and show that the intensity of the 391-nm lasing can be efficiently modulated by varying the polarization direction of the coupling pulse with respect to that of the pump pulse. It is found that as the polarization directions of the pump and coupling pulses are perpendicular, the 391-nm lasing intensity is more sensitive to the coupling laser energy, which reflects the inherent nature of the perpendicular X2Σg +-A2Πu transition.

Wei Zhang

Papers

Robust and ultralow-energy-threshold ignition of a lean mixture by an ultrashort-pulsed laser in the filamentation regime

Piezoresistive Electronic-Skin Sensors Produced With Self-Channeling Laser Microstructured Silicon Molds

Engineering black titanium dioxide by femtosecond laser filament

Observation of the optical ${{\rm{X}}}^{2}{\Sigma }_{{\rm{g}}}^{+}$–A 2 Π u coupling in ${{\rm{N}}}_{2}^{+}$ lasing induced by intense laser field

Reliable laser ablation ignition of combustible gas mixtures by femtosecond filamentating laser