Search

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.

While the efficiency of quantum dot sensitized solar cells (QDSSCs) has been long limited by photogenerated electron extraction of photoanode, we propose here an alternative approach for boosting the light harvesting efficiency of QDSSCs, which is based on the modifications of the TiO2 photoanode absorbance and impedance by irradiating crystalline TiO2 nanoparticles with intense femtosecond laser pulses propagating in the filamentation regime. We demonstrate that a two-fold enhancement of the power conversion efficiency in CdSe/CdS-cosensitized solar cells is unambiguously achieved with the produced “black” TiO2 nanoparticles as photoanode, which allows for the light absorption in the spectral range of 240-2500 nm and enables possible facilitation of electron transport. Our results offer a viable route to boosting QDSSCs efficiency that is normally realized by optimizing the photoanode material morphology and structure as well as quantum dot species in QDSSCs.

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.

Cavity-free air lasing offers a promising route towards the realization of atmospheric lasers for various applications such as remote sensing and standoff spectroscopy; however, achieving efficient generation and control of air lasing in ambient air is still a challenge. Here we show the experimental realization of a giant lasing enhancement by three to four orders of magnitude in ambient air for the self-seeded N 2 + lasing at 428 nm, assigned to the B 2 Σ u + ( ν ′ = 0 ) and X 2 Σ g + ( ν ′ ′ = 1 ) emission, by modulating the spatiotemporal overlap of ultrashort near-infrared control-pump pulses in a filamentary plasma grating; meanwhile, the spontaneous emission from the same transition is only enhanced by three to four times. We find that this enhancement is sensitive to the relative polarization and interference time of the two pulses, and reveal that the formation of the plasma grating induces different population variations in the B 2 Σ u + ( ν ′ = 0 ) and X 2 Σ g + ( ν ′ ′ = 1 ) levels, resulting in an enormous population inversion between the two levels, thereby a higher gain for the giant enhancement of N 2 + lasing in ambient air.

We experimentally demonstrate an asymmetric enhancement of the N2+ lasing at 391 nm for the transition between the B2Σ u + (v = 0) and X2Σ g + (v” = 0) states in an intense laser field with the ellipticity, ε , modulated by a 7-order quarter-wave plate (7-QWP). It is found that when the 7-QWP is rotated from α = 0 to 90°, where α is the angle between the polarization direction of the input laser and the slow axis of the 7-QWP, the intensity of the 391-nm lasing is optimized at ε ∼ 0.3 with α ∼ 10°-20° and 70°-80° respectively, but the optimization intensity at α ∼ 10°-20° is about 4 times smaller than that at α ∼ 70°-80°. We interpret the asymmetric enhancement based on a post-ionization coupling model, in which the birefringence of the 7-QWP induces an opposite change in the relative amplitudes of the ordinary ( Eo ) and extraordinary ( Ee ) electric components under the two conditions, so that the same temporal separation of Eo and Ee leads to a remarkably different coupling strength for the population transfer from the X2Σ g + (v ”=0) to A2Πu (v ’=2) states.

We propose a simple pump-coupling-seed scheme to examine the optical X 2 Σ g + –A2Πu coupling in N 2 + lasing. We produce the N 2 + lasing at 391 nm, corresponding to the B 2 Σ u + (v′ = 0)– X 2 Σ g + (v = 0) transition, by externally seeding the N 2 + 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 when 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 X 2 Σ g + –A2Πu transition.